Charge Transport in Disordered Organic Field Effect Transistors

Charge transport in disorderedorganic field-effect transistors

Eduard Meijer

Charge transport in disorderedorganic field-effect transistors

PROEFSCHRIFT

ter verkrijging van de graad van doctoraan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof.dr.ir. J.T. Fokkema,voorzitter van het College voor Promoties,

in het openbaar te verdedigen op vrijdag 20 juni 2003 om 16.00 uur

door

Eduard Johannes MEIJER

natuurkundig ingenieurgeboren te Drachten.

Dit proefschrift is goedgekeurd door de promotor:

Prof.dr.ir. T.M. Klapwijk

Samenstelling promotiecommissie:

Rector Magnificus, voorzitterProf.dr.ir. T.M. Klapwijk, Technische Universiteit Delft, promotorProf.dr. P.W.M. Blom, Rijksuniversiteit GroningenProf.dr. L.D.A. Siebbeles, Technische Universiteit DelftProf.dr. G.G. Malliaras, Cornell University, Ithaca, New York, USAProf.dr. H. Bassler, Philipps Universitat, Marburg, DeutschlandProf.dr. D. Emin, University of New Mexico, Albuquerque, USADr. D.M. de Leeuw, Philips Research Laboratories, EindhovenProf.dr. S.W. de Leeuw, Technische Universiteit Delft, reservelid

This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie(FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek(NWO).

The work described in this thesis has primarily been carried out at the Philips Research Laboratories,Eindhoven, The Netherlands, as part of the Philips Research Programme.

Cover design by Henny Herps.

Meijer, Eduard JohannesCharge transport in disordered organic field-effect transistors / Eduard Johannes MeijerPh.D. thesis, Delft University of Technology. - With ref. - With summary in Dutch.ISBN 90-6734-306-4Keywords: Organic semiconductors / polymers / field-effect transistors / charge transport / doping

As we seek to improve our surrounding world, we should never forgethow miraculous nature already is.

Contents

1 General Introduction 11.1 Historical perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Charge transport in polymeric semiconductors . . . . . . . . . . . . . . . 3

1.2.1 Hopping transport . . . . . . . . . . . . . . . . . . . . . . . . . 61.2.2 Multiple trapping and release model . . . . . . . . . . . . . . . . 71.2.3 Polarons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.3 Scope of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.3.1 The field-effect transistor . . . . . . . . . . . . . . . . . . . . . . 9

1.4 Outline of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2 The switch-on voltage and the field-effect mobility in disordered organic field-effect transistors 152.1 The switch-on voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.1.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.1.3 The classical threshold voltage . . . . . . . . . . . . . . . . . . . 172.1.4 Definition of the switch-on voltage . . . . . . . . . . . . . . . . . 182.1.5 Modeling and discussion . . . . . . . . . . . . . . . . . . . . . . 182.1.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2 The field-effect mobility . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.2 Calculation of the charge distribution . . . . . . . . . . . . . . . 232.2.3 Modeling the mobility variation . . . . . . . . . . . . . . . . . . 242.2.4 Discussion of the field-effect mobility . . . . . . . . . . . . . . . 262.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.3 Unifying the charge transport in polymeric FETs with PLEDs . . . . . . 262.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262.3.2 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . 272.3.3 The mobility - charge density relation . . . . . . . . . . . . . . . 282.3.4 Unification of the LED and FET models . . . . . . . . . . . . . . 292.3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

viii Contents

3 The Meyer-Neldel rule in organic field-effect transistors 333.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343.3 Demonstration of the MNR . . . . . . . . . . . . . . . . . . . . . . . . . 353.4 Implications for the charge transport . . . . . . . . . . . . . . . . . . . . 373.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4 The isokinetic temperature in disordered organic semiconductors 414.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.3 Discussion of the isokinetic temperature . . . . . . . . . . . . . . . . . . 46

4.3.1 The field-dependent mobility . . . . . . . . . . . . . . . . . . . . 464.3.2 The Meyer-Neldel rule . . . . . . . . . . . . . . . . . . . . . . . 474.3.3 Comparison of the MNR with the field-dependent mobility . . . . 47

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5 Scaling behavior and parasitic series resistance in disordered organic field-effect transistors 515.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525.3 Scaling behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535.4 Parasitic series resistance determination . . . . . . . . . . . . . . . . . . 535.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6 Frequency behavior and the Mott-Schottky analysis in poly(3-hexyl thiophene)metal-insulator-semiconductor diodes 616.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 626.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 626.3 The Mott-Schottky analysis . . . . . . . . . . . . . . . . . . . . . . . . . 636.4 Equivalent circuit modelling . . . . . . . . . . . . . . . . . . . . . . . . 636.5 The relaxation time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

7 Photoimpedance spectroscopy of poly(3-hexyl thiophene) metal-insulator-semi-conductor diodes 717.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727.3 Flat-band shift under oxygen exposure . . . . . . . . . . . . . . . . . . . 737.4 Photoimpedance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 747.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Contents ix

8 Dopant density determination in disordered organic field-effect transistors 838.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 848.2 Motivation and Realization . . . . . . . . . . . . . . . . . . . . . . . . . 848.3 Interpretation and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 86

8.3.1 Determination of the dopant density . . . . . . . . . . . . . . . . 868.3.2 Determination of the bulk mobility . . . . . . . . . . . . . . . . . 89

8.4 Results for PTV and P3HT . . . . . . . . . . . . . . . . . . . . . . . . . 898.5 Shift of the switch-on voltage . . . . . . . . . . . . . . . . . . . . . . . . 918.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

9 Solution-processed ambipolar organic field-effect transistors 979.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 989.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1019.3 Ambipolar transistor operation . . . . . . . . . . . . . . . . . . . . . . . 1019.4 CMOS-like inverter operation . . . . . . . . . . . . . . . . . . . . . . . 1059.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

10 Ambipolar field-effect transistors based on a single organic semiconductor 10910.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11010.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11110.3 The PIF ambipolar transistor and inverter . . . . . . . . . . . . . . . . . 11110.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Summary 119

Samenvatting 123

List of Publications 127

Patent Application 128

Curriculum Vitae 129

Dankwoord 131

Chapter 1

General Introduction

1.1 Historical perspective

Solid-state electronics was founded by the invention of the bipolar transistor by Bardeen,Brattain and Shockley1 in 1947. Germanium, the original semiconducting material tofabricate diodes and transistors was soon replaced by single-crystalline silicon. The nextmajor development in the field of solid-state electronics was the invention of the first inte-grated circuit2 in 1960, which catapulted us into the information age. Today, the numberof devices manufactured on a chip has grown to over several hundreds of million and thefeature sizes have shrunk to submicron resolution. Our information, condensed into bitsand bytes, travels across the globe, processed by grains of sand, as silicon technology hasdominated integrated device manufacturing and will do so for the foreseeable future.

Plastics, or organic materials, are strong, lightweight, adaptable, and they can beproduced at low temperatures, giving them an economic and technological edge overcompeting materials, such as wood or ceramics, for the fabrication of packaging materials,furniture, domestic appliances, etc. In the 1960s the development of photo-conductiveorganic materials for xerographic applications [4] emerged. However it was the discoveryof the first highly conducting polymer, chemically doped polyacetylene3 in 1977, thatdemonstrated that polymers could be used as electrically active materials as well. Thisdiscovery resulted in a huge research effort on conjugated organic materials.

Combining the interesting properties of polymers with opto-electronic functional-ity, has demonstrated that organic materials, in their own right, can be the ”core” of awide range of new opto-electronic devices, such as polymeric light-emitting diodes, poly-meric solar cells, and organic integrated circuits. The polymers used in these applicationsare soluble in organic solvents and can therefore be processed from solution, using tech-niques such as spin coating, film casting or even inkjet printing. These techniques for filmdeposition allow large areas to be coated, which is important for the realization of large-

1The invention of the transistor won them the Nobel prize in physics in 1956 [1, 2]2which was awarded the Nobel prize in physics in 2000 to Alferov, Kroemer and Kilby [3]3This discovery won McDiarmid, Shirakawa and Heeger the Nobel prize in chemistry in 2000 [5–7]

2 General Introduction

Figure 1.1: The top left-hand photo shows a 64 by 64 polymer-dispersedliquid-crystal display driven by 4096 polymer TFTs, with solution-processed PTV as the semiconductor. An image containing 256grey levels is shown, while the display is refreshed at 50 Hz. The topright-hand photo shows a fully processed 150-mm wafer foil con-taining all-polymer transistors and integrated circuits. The bottomphoto shows a monochrome polymer LED segment display, whichis used as battery charge state indicator on a Philips shaver. This isthe first Polymer LED product that was launched on the consumermarket in June 2002. Photos: Philips Research

1.2 Charge transport in polymeric semiconductors 3

area displays as well as for high-volume production of integrated circuits [11]. Also,the mechanical toughness of polymers and the flexibility of polymeric thin films allowtheir use in flexible displays and flexible electronics (see Fig.1.1). Furthermore, polymersare usually associated with low-processing costs, allowing them to be used in disposableproducts.

The properties of polymers can be tuned by adding functionalizing sidegroups, orby building in elements such as sulfur or nitrogen. A lot of effort is being put in thesynthesis of new materials, with improved performance and novel properties. These ad-vantages make them interesting candidates for low-cost, flexible industrial applications.Understanding the optical and electronic properties of these materials on a microscopiclevel has turned out to be an intriguing challenge that merges the fields of physics andchemistry.

For the operation and performance of all these devices the charge transport throughthe polymer layer is the dominant factor. It is therefore crucial to gain insight into theircharge transport mechanisms. In the following section the most widely used descriptionsof charge transport in disordered organic semiconductors are presented. Emphasis is puton the presence of disorder. In the context of charge transport, the scope of this thesisis in the physical description of field-effect transistors based on disordered organic semi-conductors.

1.2 Charge transport in polymeric semiconductors

Conjugated polymers are intrinsically semiconducting materials. They lack intrinsic mo-bile charge, but are able to transport charge generated by light, injected by electrodes, orprovided by chemical dopants. The main constituent of conjugated polymers is the carbonatom. It is the nature of the bonds between the carbon atoms that gives the conjugatedpolymer its interesting physical and chemical properties. To understand the basics ofthese molecular bonds it is instructive to understand the shape of the electronic orbitals4

of the atoms participating in the molecular bond [8].Carbon, in the ground state, has four electrons in the outer electronic level. The or-

bitals of these electrons may mix, under creation of four chemical bonds, to form fourequivalent degenerate orbitals referred to as sp3 hybrid orbitals in a tetrahedral orienta-tion around the carbon atom, If only three chemical bonds are formed, they have threecoplanar sp2 hybridized orbitals which are at an angle of 120o with each other. Thesebonds are called σ -bonds, and are associated with a highly localized electron density inthe plane of the molecule. The one remaining free electron per carbon atom resides in thepz orbital, perpendicular to the plane of the sp2 hybridization. The pz orbitals on neigh-boring atoms overlap to form so-called π-bonds [9, 10]. A schematic representation ofthis hybridization is given for the simplest conjugated polymer, polyacetylene, in Fig.1.2.Molecules with σ and π-bonds are schematically represented by single and double alter-nating chemical bonds between the carbon atoms, and are called conjugated molecules.The π-bonds establish a delocalized electron density above and below the plane of the

4An atomic orbital is derived using the mathematical tools of quantum mechanics, and is a representation ofthe three-dimensional volume (i.e. the region in space) in which the probability of finding an electron is highest.


σ

π

σ

π

σ

π

Figure 1.2: (a) The moleculare structure of polyacetylene, for clarity, hydrogenatoms are not shown. The alternating double and single bonds in-dicate that the polymer is conjugated. (b) Schematic representationof the electronic bonds in polyacetylene. The pz-orbitals overlap toform π-bonds.

molecule. These delocalized π-electrons are largely responsible for the opto-electronicbehavior of conjugated polymers.

There are large differences between the three-dimensional crystal lattice of most in-organic semiconductors and the amorphous structure of conjugated polymers. Inorganicsemiconductor crystalline lattices, such as silicon and germanium, are characterized bylong range order and strongly coupled atoms. For silicon and germanium this results inthe formation of long-range delocalized energy bands separated by a forbidden energygap [12]. Charge carriers added to the semiconductor can move in these energy bandswith a relatively large mean free path. The limiting factor for this band transport is scat-tering of the carriers at thermal lattice vibrations, i.e. phonons [10, 12]. This is depictedschematically in Fig.1.3a. As the number of lattice vibrations decreases with decreasingtemperature, the mobility of the charge carriers increases with decreasing temperature.

In conjugated polymers the polymer chains are weakly bound by van der Waalsforces5. These polymers typically have narrow energy bands, the highest occupied molec-ular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which caneasily be disrupted by disorder. Although electric charge is delocalized along the π-conjugated segments of the polymer backbone, the length of such perfectly conjugatedsegments is typically limited to length scales of around 5 nm, separated by chemical de-fects, such as a nonconjugated sp3-hybridized carbon atom on the polymer backbone, orby structural defects, such as chain kinks or twists out of coplanarity. Due to the disorderthe semiconductor can not be regarded simply as having two delocalized energy bandsseparated by an energy gap. Instead, the charge transporting sites, which are the seg-ments of the main chain polymer, are subject to a Gaussian distribution of energies (seeFig.1.4b), implying that all states are localized [13]. The shape of the density of states(DOS) is suggested to be Gaussian, because of the observed Gaussian shape of the opticalspectra [13].

5We note that the carbon atoms in a polymer chain are strongly coupled.


Figure 1.3: Charge transport mechanisms in solids. (a) Band transport. In aperfect crystal, depicted as the straight line, a free carrier is delo-calized. There are always lattice vibrations that disrupt the crystalsymmetry. Carriers are scattered at these phonons, which limit thecharge carrier mobility. The mobility for band transport increaseswith decreasing temperature. (b) Hopping transport. If the carrieris localized due to defects, disorder or selflocalization, e.g. in thecase of polarons, the lattice vibrations are essential for a carrier tomove from one site to another. For hopping transport the mobilityincreases with increasing temperature. The figure is adapted fromref. [10]


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Figure 1.4: (a) Schematic view of polymer chains broken up in conjugated seg-ments, which are represented as charge transport sites, betweenwhich the charge carriers hop. (b) A representation is given of thesmeared out density of states, which is often approximated by aGaussian distribution for the HOMO and LUMO levels.

The specific shape of the DOS is rarely investigated, as it is difficult to determineexperimentally. Via chemical doping the DOS itself is often altered by the presence ofthe dopant counter ions, and via the field-effect no unique DOS can be determined. Theshape of the DOS, which is a manifestation of the disorder of the system, is however im-portant for the description of charge transport. Charge carriers move from localized site tolocalized site, on chain as well as between chains in order to percolate through a thin-filmdevice (see Fig.1.4a) [10, 13]. It can be concluded that the charge transport and semicon-ducting properties of polymeric semiconductors are sensitive to the morphology of thepolymer chains and the local structural order within the film. Structural and energeticdisorder in conjugated polymer systems are therefore of importance in the description ofcharge transport.

1.2.1 Hopping transport

Due to the disorder and the localization of charge, the motion of the charge carriers inorganic semiconductors is typically described by hopping transport, which is a phonon-assisted tunneling mechanism from site to site [14, 15]. This hopping transport takesplace around the Fermi level6. Many of the hopping models are based on the single-phonon jump rate description as proposed by Miller and Abrahams [16]. In this model

6The Fermi level is defined as the highest energy level occupied by charge at a temperature of 0 K. At finitetemperatures, some levels above the Fermi level are filled and some levels below are empty. The distribution offilled energy levels around the Fermi level is given by the Fermi-Dirac distribution function [12]. The positionof the Fermi level is determined by the charge neutrality condition of the system.


the hopping rate between an occupied site i and an adjacent unoccupied site j , which areseparated in energy by Ei − Ej and in distance by Rij, is described by:

νi j = ν0 exp(−2γ Rij

) exp(− Ei −E j

kB T

)Ei > E j ,

1 Ei < E j ,(1.1)

where γ −1 quantifies the wavefunction overlap between the sites, ν0 is a prefactor, and kB

is Boltzmanns constant. The Miller-Abrahams model addresses hopping rates at low tem-peratures between shallow three-dimensional impurity states, assuming that the electron-lattice coupling is weak. When the Miller-Abrahams model is applied to polymeric semi-conductors, it is assumed that the conjugated segments of the polymers play the role ofnearly isolated impurity states, and that Eq.1.1 is still valid at high temperatures [16].

Depending on the structural and energetic disorder of the system it can be energet-ically favorable to hop over a longer distance with a low activation energy (energy dif-ference between sites), than over a shorter distance with a higher activation energy. Thisextension to the Miller-Abrahams model is called variable range hopping [17]. Monroedeveloped a model describing hopping transport around the Fermi level in an exponentialdensity of states [18]. He found that this hopping description is analytically very simi-lar to a model in which charge carriers are thermally activated to a transport level. Forthe description of the temperature and gate voltage dependencies of organic field-effecttransistors Vissenberg and Matters [19] developed a percolation model based on variablerange hopping in an exponential density of states, which will be used throughout thisthesis.

1.2.2 Multiple trapping and release model

For polycrystalline organic semiconductor layers the temperature dependent transportdata is often interpreted in terms of a multiple trapping and release model [20]. In thismodel the organic semiconductor film consists of crystallites which are separated fromeach other by amorphous grain boundaries. In the crystallites the charge carriers canmove in delocalized bands, whereas in the grain boundaries they become trapped in lo-calized states. The trapping and release of carriers at these localized states results in athermally activated behavior of the field-effect mobility, which depends on the gate volt-age. The description of trapped, i.e. localized, charges which can be thermally activatedto a transport level, in this case the band, is very similar to hopping in an exponentialdensity of states [18, 19] as stated in the previous section. As the grain boundaries in apolycrystalline system determine the DC charge transport and typically an exponentialtrap distribution is used to model the experimental data, no clear distinction can be madebetween hopping transport and multiple trapping and release, on the basis of the temper-ature dependence of the mobility. One should be able to separate a trap-limited mobilityfrom a hopping mobility if the Hall mobility in a Hall effect experiment could be deter-mined, since in a magnetic field, there will be no Lorentz force on trapped charges [21].Unfortunately, due to the low charge carrier mobilities, the Hall effect is very difficult tomeasure experimentally.


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Figure 1.5: Molecular structures of the conjugated materials stud-ied in this thesis: (a) poly(2,5-thienylene vinylene) (PTV),(b) poly(3-hexyl thiophene) (P3HT), (c) poly([2-methoxy-5-(3’,7’-dimethylocyloxy)]-p-phenylene vinylene) (OC1C10-PPV), (d) poly([2,5-(3’,7’-dimethylocyloxy)]-p-phenylenevinylene) (OC10C10-PPV), (e) pentacene, (f) poly(3,9-di-tert-butylindeno[1,2-b] fluorene) (PIF), (g) [6,6]-phenyl C61-butyricacid methyl ester (PCBM).

1.3 Scope of this work 9

1.2.3 Polarons

Next to the disorder-induced localization of charge, the strong electron-phonon couplingin organic materials results in localization of charge. An excess charge carrier on a conju-gated polymer chain can minimize its energy by a local lattice deformation. This quasipar-ticle consisting of charge and a lattice deformation, or phonon cloud, is called a polaron.As polarons represent a local distortion of the lattice, the associated energy levels mustsplit off from the HOMO level and the LUMO level. These energy levels, which residein the energy gap, have often been observed in optical absorption experiments on chargedconjugated polymer films [22–24].

In polaronic charge transport, not only the charge moves under an applied electricfield but the lattice deformation moves with it. Typically, for hopping transport of po-larons, a description in terms of Miller-Abrahams hopping [16] is insufficient, as multi-phonon hopping rates need to be considered [25, 26].

1.3 Scope of this work

In this thesis we study charge transport in organic semiconductors. We do this by focusingon the physical characterization of disordered organic field-effect transistors. It will bemade clear that the disorder in the polymer films is crucial for the interpretation of thedata. The field-effect transistor geometry allows variation of the charge carrier density inthe semiconductor, without the presence of counter ions. Therefore, the transistor allowsa rather clean study of the charge transport in organic semiconductors as a function ofthe charge carrier density and temperature. In the following section the operation of thesilicon metal-oxide-semiconductor field-effect transistor (MOSFET) is described. In theexperiments we find that the organic transistors are in several respects not comparable tosilicon MOSFETs. Therefore, in this thesis we redefine and reevaluate basic transistorparameters, such as the threshold voltage, the field-effect mobility, the contact resistanceand the dopant density. Subsequently, we study the charge transport as a function ofcharge density and temperature, giving insight into the charge transport mechanisms. Andfinally, we investigate the stability of the polymer layer and discuss why typically onlyunipolar transistor behavior is observed experimentally. The conjugated organic materialsused throughout this thesis are given in Fig.1.5.

1.3.1 The field-effect transistor

The MOSFET can basically be considered as a parallel plate capacitor, where one con-ducting electrode, the gate electrode, is electrically insulated, via an insulating oxide layer,from the semiconductor layer (see Fig.1.6). Two electrodes, the source and the drain, arecontacted to the semiconductor layer. By applying a gate voltage, Vg , with respect tothe source electrode, charge carriers can electrostatically be accumulated or depleted inthe semiconductor at the semiconductor-insulator interface. Due to this field-effect thecharge carrier density in the semiconductor can be varied. Therefore, the resistivity of thesemiconductor, and hence the current through the semiconductor (upon application of asource-drain field), can be varied over orders of magnitude [27]. Since the MOSFET can


Figure 1.6: Schematic of a thin-film field-effect transistor.

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Figure 1.7: Energy level band diagram of an ideal metal-insulator-semiconductor diode structure with a p-type semiconductor:(a) flat-band condition, (b) accumulation, (c) depletion.

be switched between a conducting and a non-conducting state, it is widely used as thebasic building block of binary logic.

The band-bending diagrams of a p-type transistor in the different operating regimesare schematically given in Fig.1.7. In equilibrium the Fermi levels of the materials align,by charge carriers which move to or from the semiconductor-insulator interface. Whena bias is applied which is equal to the difference between the Fermi levels of the gatemetal and the semiconductor, no band bending will occur in the semiconductor at thesemiconductor-insulator interface, i.e. the energy bands in the semiconductor will be flat(see Fig.1.7a). This biasing condition is defined as the flat-band voltage. If the Fermilevel of the metal and semiconductor are similar this flat-band voltage will be 0. Fora p-type semiconductor, the application of a negative gate voltage will induce chargesat the semiconductor-insulator interface (these charges are supplied by the source anddrain contacts). In effect the Fermi level of the gate metal is varied with a value of qVg ,where q is the elementary charge, causing band bending in the semiconductor layer asis schematically represented in Fig.1.7b. For a positive applied Vg the energy bands inthe p-type semiconductor are bent downwards, and the mobile positive charge carriers aredepleted from the semiconductor-insulator interface. In this case the transistor is biased

1.4 Outline of this thesis 11

in the depletion mode (see Fig.1.7c).

1.4 Outline of this thesis

In Chapter 2 the switch-on voltage and the field-effect mobility in disordered organicfield-effect transistors are introduced. It is argued that the use of classical MOSFETtheory for the description of these transistors is not a priori justified. In fact a scruti-nizing look at the data of FETs based on three different organic semiconductors revealsthat MOSFET theory neglects basic properties of these materials, particularly disorder,and should therefore not be used. Alternative characterization parameters are introducedwhich will be used throughout the remainder of this thesis.

In the third section of chapter 2 the hole transport in light-emitting diodes and field-effect transistors is compared. The experimental hole mobilities extracted from both typesof devices, based on a single polymeric semiconductor, can easily differ by three ordersof magnitude. This apparent discrepancy in the charge transport description is resolvedby demonstrating that the hole mobility depends strongly on the charge carrier density indisordered organic semiconducting polymers.

In Chapter 3 the temperature dependence of the field-effect mobility is investigatedin two solution-processed disordered organic field-effect transistors (PTV and pentacene).We find thermally activated behavior, with an activation energy that depends on the in-duced charge density in the transistor. Upon extrapolation of the data to infinite temper-ature we find an empirical relation, termed the Meyer-Neldel rule, which states that themobility prefactor increases exponentially with the activation energy. From this analy-sis a characteristic temperature is extracted that does not vary much between differentmaterials. The possible implications of this observation in terms of charge transport aregiven.

As a follow-up to the previous chapter the field dependence of the in-plane conduc-tivity is investigated in PTV and P3HT in Chapter 4. Using an empirical relation forthis field-dependence we find an isokinetic temperature that is comparable to the valuesobtained from the Meyer-Neldel experiments of Chapter 3. Implications of these findingsare addressed.

In Chapter 5, we investigate the scaling behavior of FETs. It is demonstratedthat downsizing of the transistor channel does not automatically result in an improve-ment of integrated circuit performance. This is due to parasitic resistances at the metal-semiconductor contact. We find an empirical relation between the charge-carrier mobilityin the polymeric semiconductor and the parasitic resistance.

In Chapter 6 we investigate the dopant density in polymeric semiconductors inthe classical way by means of impedance spectroscopy of metal-insulator-semiconductordiodes. The diodes, based on poly(3-hexyl thiophene), are measured and analyzed as a


function of bias voltage, measurement frequency and temperature. We find that simpleMott-Schottky analysis can only be applied here if the relaxation time of the semicon-ducting polymer is taken into account. The long relaxation times are a direct result of thedisorder in the semiconducting polymer layer.

In Chapter 7 we investigate the instability of P3HT under oxygen exposure andillumination. P3HT forms a charge-transfer complex with molecular oxygen, which re-sults in an acceptor density increase with time. This acceptor density change with time,is investigated by means of photoimpedance measurements as a function of wavelength.The measurements show that the acceptor creation efficiency peaks upon excitation of themolecular oxygen-polythiophene contact charge transfer complex.

An alternative method for the determination and monitoring of the dopant densityis given in Chapter 8, where the dopant density is extracted directly from the transfercharacteristics of transistors based on PTV and P3HT. It is demonstrated that, due to thefact that in disordered semiconductors the charge carrier mobility depends on the chargedensity, the bulk current can be separated from the field-effect current in the total currentdensity. It is found that the morphology of the polymer film is crucial for the stability oftransistors that are prone to oxygen doping effects.

In the first 5 chapters we have looked at unipolar p-type organic field-effect tran-sistors. In Chapter 9 we use a blend of hole-conducting OC1C10-PPV and electron-conducting PCBM to construct a solution-processed ambipolar organic field-effect tran-sistor. The characteristics of this kind of transistor are measured and analyzed and CMOS-like inverter operation is demonstrated, paving the way for solution-processable CMOS-like logic circuitry.

As was demonstrated in the previous chapter a suitable choice of electrode and semi-conductor can result in ambipolar FETs. In Chapter 10 we show that even a singlesolution-processed organic semiconductor can be used in ambipolar transistors, demon-strating that in principle an intrinsic organic semiconductor can transport both polaritiesof charge. Important parameters for experimental observation of this ambipolar behaviorare extrinsic effects such as workfunction mismatch and material purity.

References

[1] http://www.nobel.se/physics/laureates/1956/press.html

[2] W. Shockley, Bell Syst. Techn. J. 28, 435 (1949).

[3] http://www.nobel.se/physics/laureates/2000/press.html

[4] D.M. Pai and B.E. Spingett, Rev. Mod. Phys. 65, 163 (1993).

[5] http://www.nobel.se/chemistry/laureates/2000/press.html

[6] C.K. Chiang, C.R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C.Gan, A.G. MacDiarmid, Phys. Rev. Lett. 39, 1098 (1977).

[7] C.K. Chiang, M.A. Druy, S.C. Gau, A.J. Heeger, E.J. Louis, A.G. McDiarmid, Y.W.Park and H. Shirakawa, J. Am. Chem. Soc. 100, 1013 (1977).

[8] P.W. Atkins, Physical Chemistry, Oxford University Press (1986).

[9] R.E. Peierls, Quantum theory of solids, Oxford University Press, London (1955).

[10] M. Pope and C.E. Swenberg, Electronic Processes in Organic Crystals and Poly-mers, Oxford University Press (1999).

[11] A.R. Brown, C.P. Jarrett, D.M. de Leeuw and M. Matters, Synth. Met. 88, 37 (1997).

[12] C. Kittel, Introduction to solid state physics, 6th edition (John Wiley & Sons, Inc1986).

[13] H. Bassler, Phys. Stat. Sol. B 175, 15 (1993).

[14] E.M. Conwell, Phys. Rev. 103, 51 (1956).

[15] N.F. Mott, Canadian J. Phys. 34, 1356 (1956).

[16] A. Miller and E. Abrahams, Phys. Rev. 120, 745 (1960).

[17] N.F. Mott and E.A. Davies, Electronic processes in non-crystalline materials, 2nd

Edition, Oxford University Press, London (1979).

[18] D. Monroe, Phys. Rev. Lett. 54, 146 (1985).

14 References

[19] M.C.J.M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998).

[20] G. Horowitz, R. Hajlaoui and P. Delannoy, J. Phys. III 5, 355 (1995).

[21] D. Emin, private communication, D. Emin, Phys. Today 35, 34 (1982).

[22] K.E. Ziemelis, A.T. Hussain, D.D.C. Bradley and R.H. Friend, Phys. Rev. Lett. 66,2231 (1991).

[23] P.A. Lane, X. Wei and Z.V. Vardeny, Phys. Rev. Lett. 77, 1544 (1996).

[24] R. Osterbacka, C.P. An, X.M. Jiang and Z.V. Vardeny, Science 287, 839 (2000).

[25] T. Holstein, Ann. Phys. 8, 325 (1959).

[26] D. Emin, Adv. Phys. 24, 305 (1975).

[27] S.M. Sze, Physics of semiconductor devices, 2nd edition, John Wiley & Sons, (NewYork, 1981).

Chapter 2

The switch-on voltage and thefield-effect mobility in disordered

organic field-effect transistors

Abstract

In this chapter we critically evaluate two characterization parameters of disordered or-ganic field-effect transistors.

• The switch-on voltage is defined as the flat-band voltage, and is used as character-ization parameter. The transfer characteristics of the solution processed organicsemiconductors pentacene, poly(2,5-thienylene vinylene) and poly(3-hexyl thio-phene) are modeled as a function of temperature and gate voltage with a hoppingmodel in an exponential density of states. The data can be described with reason-able values for the switch-on voltage, which is independent of temperature. Thisresult also demonstrates that the large threshold voltage shifts as a function of tem-perature reported in the literature constitute a fit-parameter without a clear physicalbasis.

• The experimentally determined field-effect mobility is compared to the local chargecarrier mobility, which takes into account the charge density distribution in the tran-sistor. It is demonstrated that the experimentally determined field-effect mobility isa reasonable estimate for the local mobility of the charge carriers at the interface.

In the last section of this chapter the transistor description is compared to the models usedfor polymeric light-emitting diodes and it is demonstrated that the two device descriptionscan be unified, when the large differences in charge densities in the two device geometriesare taken into account.

16 The switch-on voltage and the field-effect mobility

2.1 The switch-on voltage

2.1.1 Introduction

The charge transport in organic field-effect transistors (FETs) has been a subject of re-search for several years. It has become clear that disorder severely influences the chargetransport in these transistors [1, 2]. Studies on the effect of molecular order ultimatelyresulted in the observation of band transport in high quality organic single crystals [3].The electrical transport in these crystals is well described by monocrystalline inorganicsemiconductor physics [3, 4]. However, devices envisaged for low-cost integrated circuittechnology are typically deposited from solution [5, 6], resulting in amorphous or poly-crystalline films. In these solution-processed organic transistors the disorder in the filmsdominates the charge transport, due to the localization of the charge carriers. The disor-der is observed experimentally through the thermally activated field-effect mobility andits gate voltage dependency [7, 8]. These observations have thus far been modeled usingmultiple trapping and release [9], variable range hopping [8] and grain boundary charg-ing [10]. A further common feature of disordered organic field-effect transistors is thetemperature dependence of the threshold voltage, Vth [10].

In this paragraph, the temperature dependence of Vth in disordered organic field-effect transistors is addressed. It is argued that Vth , as used in literature for the descriptionof organic transistor operation, is a fit parameter with no clear physical basis. Instead, aswitch-on voltage, Vso, is defined for the transistor at flat-band. We model the experimen-tal data obtained on solution-processed pentacene, poly(2,5-thienylene vinylene) (PTV)and poly(3-hexyl thiophene) (P3HT), with a disorder model of variable-range hopping(VRH) in an exponential density of states [8]. The modeling shows that good agreementwith experiment can be obtained with reasonable values for the switch-on voltage, whichis independent of temperature. Furthermore, it is found that the shift of the Fermi-levelwith temperature has no influence on Vso.

2.1.2 Experimental

In the experiments we used heavily doped Si wafers as the gate electrode, with a 200 nmthick layer of thermally oxidized SiO2 as the gate-insulating layer. Using conventionallithography, gold source and drain contacts are defined with an interdigitated geometry.The SiO2 layer is treated with the primer hexamethyldisilazane (HMDS) to make thesurface hydrophobic. The samples are measured under high vacuum (10−7 mbar) in anOxford optistat CF-V flow cryostat, using a Hewlett-Packard 4156A semiconductor pa-rameter analyzer. The films of PTV are truly amorphous whereas the pentacene films arepolycrystalline. The P3HT films can be considered nanocrystalline, as ordered regionsof this regioregular polymer alternate with disordered regions [1]. We do not observeany hysteresis in the current-voltage characteristics and the curves are stable with time(in vacuum). The field-effect mobilities in the devices have been estimated from thetransconductance [7] at a gate voltage, Vg = −19 V at room temperature and are givenin Table 2.1. For the P3HT transistor described here the processing conditions were notoptimized to give the high mobilities reported in literature [1].

2.1 The switch-on voltage 17

Table 2.1: Values obtained by using Eq.2.6 to model the transfer characteris-tics of solution-processed pentacene, PTV and P3HT. TDOS repre-sents the width of the exponential density of states, σ0 is the conduc-tivity prefactor, α−1 the effective overlap parameter, Vso the switch-on voltage, and µRT the field-effect mobility at Vg = −19 V androom temperature.

TDOS[K] σ0[106S/m] α−1[A] Vso[V] µRT [cm2/V s]PTV 382 5.6 1.5 1 2 × 10−3

pentacene 385 3.5 3.1 1 2 × 10−2

P3HT 425 1.6 1.6 2.5 6 × 10−4

2.1.3 The classical threshold voltage

The difficulty of defining a threshold voltage in disordered organic transistors was alreadypointed out by Horowitz et al. [11]. The threshold voltage in inorganic field-effect tran-sistors is defined as the onset of strong inversion [4]. However, most organic transistorsonly operate in accumulation and no channel current in the inversion regime is observed.Nevertheless, classical metal-oxide-semiconductor field-effect (MOSFET) theory is oftenused to extract a Vth from the transfer characteristics of organic transistors in accumula-tion. The square root of the saturation current is then plotted against the gate voltage, Vg .This curve is fitted linearly and the intercept on the Vg-axis is defined as the Vth of thetransistor. For disordered transistors this method neglects the experimentally observed de-pendence of the field-effect mobility on the gate voltage [7,12]. In an attempt to take thisinto account in the parameter extraction several groups have used an empirical relation tofit the field-effect mobility [10, 13],

µ = K(Vg − Vth

)γ, (2.1)

where K , γ and Vth are fit parameters. Fitting of current-voltage characteristics of thetransistors, using either this empirical relation or the square root technique, has resultedin a temperature dependent Vth [10, 14]. The shift of Vth with temperature is as largeas 15 V in the temperature range of 300 K to 50 K [10]. However, for transistors basedon the same materials in the crystalline phase, for which MOSFET theory is valid, theshift of Vth with temperature is at most 0.5 V [4]. This observation raises the question:why, for a disordered system, the shift of Vth with temperature is so much larger thanin its crystalline counterpart. To answer this question, we first have to realize that, inboth types of analysis mentioned, the extracted Vth is a fit parameter. This fit parameterhas no direct relation with the original definition of the threshold voltage in MOSFETtheory. Also, depending on the range of Vg over which the data is fitted in disorderedtransistors, the value of the extracted Vth will be different. Therefore, we argue that Vth

as defined in MOSFET theory has no physical relevance in the description of the operationof disordered organic transistors. Despite these issues, some suggestions have been given


in literature to explain the large temperature dependence of the apparent Vth , such as awidening of the bandgap [14], and displacement [10] of the Fermi level with decreasingtemperature.

2.1.4 Definition of the switch-on voltage

Instead of Vth as characterization parameter, we will use the gate voltage at which there isno band-bending in the semiconductor, i.e. the flat-band condition (see Fig.1.7a). We callthis the switch-on voltage, Vso, of the transistor. Below Vso the variation of the channelcurrent with the gate voltage is zero, while the channel current increases with Vg aboveVso. For an unintentionally doped semiconductor layer, Vso is then only determined byfixed charges in the insulator layer or at the semiconductor/insulator interface. In that caseVg becomes Vg − Vso. Without these fixed charges Vso should in principle be zero [4].

2.1.5 Modeling and discussion

Here we will model the experimental DC transfer characteristics obtained on three differ-ent disordered organic field-effect transistors to estimate the temperature dependence ofVso. Because we are looking at disordered systems, we use the variable range hoppingmodel proposed by Vissenberg and Matters [8]. The charge transport in this model is gov-erned by hopping, i.e. thermally activated tunneling of carriers between localized statesaround the Fermi level, EF . The carrier may either hop over a small distance with a highactivation energy or over a long distance with a low activation energy. In the disorderedsemiconducting polymer the density of states (DOS) is described by a Gaussian distribu-tion [16]. For a system with both a negligible doping level compared to the gate-inducedcharge and at low gate-induced carrier densities the Fermi level is in the tail states of theGaussian, which is approximated by an exponential DOS [8](see also section 2.3):

g(E) = Nt

kB TDOSex p

(E

kB TDOS

)(2.2)

where Nt is the number of states per unit volume, kB is Boltzmann’s constant, and TDOS

is a parameter that indicates the width of the exponential distribution. The energy dis-tribution of the charge carriers is given by the Fermi-Dirac distribution. If a fraction,δ ∈ [0, 1], of the localized states is occupied by charge carriers, such that the density ofcarriers is δNt , then the position of the Fermi-level is fixed by the condition [8]:

δ = ex p

(EF

kB TDOS

)πT

TDOS sin(π T

TDOS

) . (2.3)

Using a percolation model of variable range hopping, an expression for the conductivityas a function of the charge carrier occupation δ and the temperature T is derived [8]:

σ (δ, T ) = σ0

δNt (TDOS/T )4 sin

(π T

TDOS

)(2α)3 Bc

TDOST

(2.4)


where σ0 is a prefactor of the conductivity, Bc is a critical number for the onset of percola-tion, which is 2.8 for three-dimensional amorphous systems [17], and α−1 is an effectiveoverlap parameter between localized states. To calculate the field-effect current we haveto take into account that in a field-effect transistor the charge density is not uniform. Usingthe gradual channel approximation (|Vg| |Vds|, where Vds is the source-drain voltage),we neglect the potential drop from source to drain electrode. To take into account thatthe charge-density decreases from the semiconductor-insulator interface to the bulk, weintegrate over the accumulation channel:

Ids = W Vds

L

∫0

t

dxσ [δ(x), T ] , (2.5)

where L, W , and t are the length, width and thickness of the channel, respectively. FromEqs. 2.4 and 2.5 we obtain the following expression for the field-effect current:

Ids = W Vdsεsemiε0σ0

Lq

(T

2TDOS − T

)√2kB TDOS

εsemiε0

×(

TDOST

)4sin(π T

TDOS

)(2α)3 Bc

TDOST

×[√

εsemiε0

2kBTDOS

(Ci(Vg − Vso

)εsemiε0

)] 2TDOST −1

(2.6)

where q is the elementary charge, ε0 is the permittivity of vacuum, εsemi the relative di-electric constant of the semiconductor, and Ci is the insulator capacitance per unit area.Eq.2.6 is used to model the transfer characteristics of solution processed PTV, pentacene,and P3HT as a function of Vg and T . The four parameters σ0, α−1, TDOS , and Vso wereused to model all the curves, with a value of Bc=2.8. After this initial fit, each curve wasindividually modeled with only Vso as variable parameter, with the other parameters fixed.From this modeling, no temperature dependence of Vso was observed. The results of themodeling are shown in Figs.2.1, 2.2, and 2.3. The fit parameters are given in Table 2.1.Good agreement is obtained for all three semiconductors. The single constant Vso forall temperatures accounts for any fixed charge in the oxide and/or at the semiconductor-insulator interface. Also, the low values obtained for Vso are realistic numbers. Becausethe measurement resolution in the low current regime is limited to 1-10 pA, the onset ofthe experimental curves in Figs.2.1, 2.2, and 2.3, seems to be shifting to more negativegate voltages with decreasing temperature. This is due to current decrease as a functionof temperature, as a result of the thermally activated field-effect mobility (see Chapter3) [18]. This effect does not translate in a temperature dependence of Vso. Analysis ofthe data with the square root technique yields an apparent threshold voltage shift withtemperature of 15 V for the PTV. Eq. 2.1 gives similar results. The values obtained forthe prefactor of the conductivity σ0 seem to be too high with respect to theoretical con-siderations [15]. The conductivity prefactor is discussed further in Chapters 3 and 4.


Figure 2.1: Ids versus Vg of a PTV thin-film field-effect transistor for severaltemperatures. The solid lines are modeled using Eq.2.6. W=2 cm,L=10 µm, Vds = −2 V . The inset shows the structure formula ofPTV.

Figure 2.2: Ids versus Vg of a pentacene thin-film field-effect transistor forseveral temperatures. The solid lines are modeled using Eq.2.6.W=2 cm, L=10 µm, Vds = −2 V . The inset shows the structureformula of pentacene.


Figure 2.3: Ids versus Vg of a P3HT thin-film field-effect transistor for severaltemperatures. The solid lines are modeled using Eq.2.6. W=2.5mm, L=10 µm, Vds = −2 V . The inset shows the structure formulaof P3HT.

)

).

Figure 2.4: Fermi-level displacement as a function of temperature, calculatedusing Eq.2.3 and the parameters of Table 2.1


We note that, the Fermi level shifting with decreasing temperature [10] has no effecton Vso. The Fermi level shift, which results from the Fermi-Dirac distribution of thecharge carriers in the exponential density of states, is calculated from Eq.2.3 and is foundto be 0.04 eV over a temperature range of 200 K (see Fig.2.4). This displacement doesnot result in a shift of Vso with temperature.

2.1.6 Conclusion

It was argued that the threshold voltage extracted from the transfer characteristics of dis-ordered organic transistors, using MOSFET theory or Eq.2.1, is only a fit parameter ifthe strong inversion regime is not observed in the transfer characteristics. The use of thisparameter in the physical description of organic field-effect transistors is therefore incor-rect. Instead, we have defined a switch-on voltage for unintentionally doped disorderedorganic field-effect transistors as the gate voltage that has to be applied to reach the flat-band condition. Using a disorder model of hopping in an exponential density of states,the experimental data of solution-processed PTV, pentacene and P3HT could be describedwith reasonable values for the switch-on voltage, which is temperature independent. Theuse of Vso as characterization parameter of disordered organic field-effect transistors isnot limited to the model described here, but is generally applicable.

2.2 The field-effect mobility

2.2.1 Introduction

As mentioned in the previous section the transport properties in disordered organic semi-conductors are dominated by localized states. For polymeric light-emitting diodes animportant consideration for device modelling is the non-uniform charge distribution inthe device. As stated above, the charge distribution in a FET is also non-uniform, it de-creases from the semiconductor/insulator (S/I) interface to the bulk, and it depends onthe applied Vg . Therefore, it is not trivial to assign one field-effect mobility to all carri-ers in the accumulation channel of a disordered organic field-effect transistor. Here, wecalculate the variation of the charge carrier mobility through the thickness of the accumu-lation channel. It is found that only a small error is made by assuming that all carriers aremoving at the semiconductor-insulator interface with the same mobility, at a given Vg .

In polymeric field-effect transistors (FETs) charge carriers are induced by a gateelectrode across an insulating layer. By applying a negative voltage at the gate electrodethe top of the valence band bends upward closer to the Fermi level (see Fig.1.7b). Thisband bending gives rise to a positive accumulation layer into the semiconductor next tothe interface. Applying a voltage Vds between the source and the drain contact gives riseto a current in the channel. In literature, typically this current in the linear regime of theFET is described by:

Ids = WCi VgµF EVds

L, (2.7)

with Ci Vg the total amount of accumulated charge carriers, Vds/L the electric field in thechannel, and µF E the field-effect mobility. It is important to note that the use of the total

2.2 The field-effect mobility 23

amount of induced charge Ci Vg in Eq.2.7 is only valid when all charge carriers have thesame mobility. In that case, the field-effect mobility is calculated from [7]:

µF E = L

WCi Vds

∂ Ids

∂Vg

∣∣∣∣Vds→0

. (2.8)

To check the validity of this equation for disordered organic FETs, we calculate thecharge distribution in the device from Poisson’s equation. Then we use the hopping modeldescribed in the previous section to calculate a local mobility, and compare the results withthe values obtained from Eq.2.8 for both P3HT and PTV field-effect transistors.

2.2.2 Calculation of the charge distribution

Again, an unintentionally doped system is considered. In the gradual channel approxima-tion the distribution of charge carriers has to be described only in the direction perpendic-ular to the S/I interface (x , see Fig.1.6). The distribution of the electric field, Fx , in theaccumulation layer perpendicular to the S/I interface is given by [12]:

Fx =[(

2

ε0εsemi

) ∣∣∣∣∫ V

0q(δNt

(V ′)) dV ′

∣∣∣∣]1/2

, (2.9)

where V ′ is the local potential, which varies from zero far away in the semiconductor bulkto V in the accumulation channel. The potential distribution as a function of the distancefrom the S/I interface, x , follows from the relation:

x =∫ V0

V

dV ′

Fx (V ′)(2.10)

where V0 is the surface potential of the S/I interface. From the variation of the gate-induced potential V (x) as a function of distance x the density of holes δNt (x) can becalculated. The induced charge per unit area Qind is related to the gate voltage as follows:

Vg = Qind

Ci+ Vso = ε0εsemi Fx (0)

Ci+ Vso, (2.11)

where Fx (0) = Fx (V = V0) is the electric field at the S/I interface. By increasing thegate voltage the surface potential increases resulting in an increase of charge carrier den-sity. Assuming Vso=0, δNt (x) is calculated for an undoped semiconductor with Ci =15.5nF/cm2 and εsemi =3. In Fig.2.5 the concentration of charge carriers as a function of dis-tance x is shown for gate voltages of Vg = −19 V and Vg = −10 V . At Vg = −19 Vthe charge carrier density decreases from 3.5·1019 cm−3 at the S/I interface (x = 0) to3·1016 cm−3 at a distance of 10 nm from the S/I interface. For Vg = −10 V the total in-duced charge is about half that of Vg = −19 V . This calculation is performed using clas-sical electrostatics, and we have neglected quantum effects close to the semiconductor-insulator interface. It should be noted that the calculated charge distribution is not specificfor organic semiconductors but is generally applicable to field-effect devices of undopedsemiconductors in the linear operating regime, since it only depends on Ci and εsemi .


δ

W

J

J

Figure 2.5: Numerical calculated distribution of charge carriers in the ac-cumulation channel perpendicular to the S/I interface for Vg =−19 V and Vg = −10 V , calculated for εsemi = 3 andCi =15.5 nF/cm2.

2.2.3 Modeling the mobility variation

In order to take into account the charge carrier dependent mobility we use again the hop-ping model described in the previous section. From Eq.2.4 an expression for the localmobility is derived:

µl = σ (δ, T )

qδNt= σ0

q

(TDOS/T )4 sin

(π T

TDOS

)(2α)3 Bc

TDOST

(δNt )TDOS/T −1 (2.12)

Using the parameters from Table 2.1 the local mobility is calculated as a functionof distance from the S/I interface and is plotted in Fig.2.6 for Vg = −19 V at roomtemperature for PTV and P3HT. For PTV the modeled local mobility varies from 2.1·10−3

cm2/Vs at the S/I interface to 8.4·10−4 cm2/Vs at a distance of 5·10−10 m from the S/Iinterface. For P3HT the local mobility varies from 6.6·10−4 cm2/Vs at the interface to1.2·10−4 cm2/Vs at a distance of 5·10−10 m from the interface. The calculations showthat in a polymeric FET even for moderate gate voltages, variations in the local mobilityare considerable. Thus, due to the inhomogeneous charge carrier density in a disorderedFET the local mobility demonstrates a strong variation in the active channel.

2.2 The field-effect mobility 25

O

O

Figure 2.6: The local charge carrier mobility as function of position in theaccumulation layer for (a) a PTV FET and (b) a P3HT FET atVg = −19 V . The extracted field-effect mobility as determinedfrom Eq.2.8 is given as the solid symbol in both figures.


2.2.4 Discussion of the field-effect mobility

For the interpretation of the charge transport in disordered FETs it is crucial to understandhow such a mobility distribution compares to the experimentally extracted field-effectmobility, using Eq.2.8. In Fig.2.6 the local mobility (Eq.2.12) and the field-effect mobility(Eq.2.8) are compared. With this modeling we find that the local mobility of the chargecarriers at the S/I interface at Vg = −19 V is 15% and 9% larger then the extracted field-effect mobility for PTV (Fig.2.6a) and P3HT (Fig.2.6b), respectively. The reason for thisrelatively small difference is that (as shown in Figs.2.5 and 2.6) not only a major partof the charge carriers is located close to the interface, but also that these charge carriershave the highest mobility. As a result the field-effect current is mainly determined by thecharge carriers at the interface. Consequently, the error due to the approximation usedin Eq.2.8, namely that all charge carriers have the same mobility, is relatively small andamounts typically to 10-15%.

2.2.5 Conclusion

In disordered organic field-effect transistors the dependence of the mobility with gatevoltage is determined by the charge carrier dependence of the local mobility. Taking intoaccount the distribution of the charge carrier density in the active channel perpendicularto the insulator the local mobility has been calculated as a function of position in theaccumulation layer. It is demonstrated that for disordered organic FETs, in spite of thestrong variations in the local mobility in the active channel, the experimentally determinedfield-effect mobility (Eq.2.8) is a reasonable estimate for the local mobility of the chargecarriers at the semiconductor-insulator interface.

2.3 Unifying the charge transport in polymeric FETs withPLEDs

2.3.1 Introduction

The experimental hole mobilities extracted from FETs can differ by three orders of mag-nitude from the hole mobilities extracted from polymeric light-emitting diodes (PLED),based on the same polymeric semiconductor. We resolve this apparent discrepancy byconsidering that the hole mobility depends strongly on the charge carrier density in dis-ordered semiconducting polymers. This is demonstrated in this section by a system-atic study of the hole mobility as a function of temperature and applied bias in PLEDsand FETs based on poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene)(OC1C10-PPV) and in PLEDs and FETs based on amorphous poly(3-hexyl thiophene)(P3HT). In contrast to the variable range hopping description in an exponential density ofstates for FETs described in the previous section, the transport in polymeric light-emittingdiodes (PLED) is typically described by hopping in a Gaussian density of states with long-range electronic correlations [16,19–21]. We unify the charge transport description in thetwo device geometries for both materials by demonstrating that in the energy range of

2.3 Unifying the charge transport in polymeric FETs with PLEDs 27

Figure 2.7: Temperature dependent current density versus voltage character-istics of a P3HT hole-only diode, with a thickness of 95 nm andactive area 10 mm2. The solid lines represents the description ofthe space-charge limited current model, incorporating the field de-pendence of the mobility (Eq.2.13). The inset shows the molecularstructure of P3HT.

interest the exponential density of states, which consistently describes the FET measure-ments at high carrier densities, is a good approximation for the Gaussian density of states,which consistently describes the PLED measurements at low carrier densities.

2.3.2 Experimental results

Field-effect transistors were fabricated using P3HT and OC1C10-PPV as the semicon-ductor (For experimental details see section 2.1.2) and were modeled with Eq.2.6. Themodeled parameters are given in Table 2.2. For P3HT and OC1C10-PPV we find a field-effect mobility at -19 V of respectively 6×10−4 cm2/Vs and 5×10−4 cm2/Vs.

Also PLEDs of P3HT and OC1C10-PPV were fabricated and the current density, J ,versus applied bias, V , is measured as a function of temperature. The J−V characteristicsof OC1C10-PPV PLEDs can accurately be described by space-charge limited currents,with a field- and temperature dependent mobility [19–21]. This mobility is well describedby a transport model based on hopping in a correlated Gaussian disordered system [16,22]:

µh = µ∞ exp

[−(

3σDOS

5kB T

)2

+ 0.78

((σDOS

kB T

) 32 − 2

)√qa F

σDOS

], (2.13)

with µ∞ the mobility in the limit T → ∞, σDOS the width of the Gaussian DOS, a theintersite spacing, and F the applied electric field. The J − V characteristics of a P3HT


Table 2.2: Values obtained by modeling the transfer characteristics ofOC1C10-PPV and P3HT FETs using Eq.2.6. The P3HT data istaken from Table 2.1. TDOS represents the width of the exponentialDOS, σ0 is the prefactor of the conductivity, α−1 is the effectiveoverlap parameter between localized states, and Vso is the switch-on voltage as defined in section 2.1. Also given are the values ob-tained using Eq.2.13 to model the J −V characteristics of the hole-only diodes. The OC1C10-PPV diode data are taken from [19].σDOS is the width of the Gaussian DOS and a is an average trans-port site separation.

TDOS[K ] σ0[106S/m] α−1[A] Vso[V ] σDOS[eV] a[nm]OC1C10PPV 540 31 1.4 0.5 0.112 1.4P3HT 425 1.6 1.6 2.5 0.098 1.7

PLED can also accurately be modeled with space-charge limited currents in combinationwith Eq.2.13, as is demonstrated in Fig.2.7. The modeled parameters from Eq.2.13 aregiven in Table 2.2. We find at low electric fields hole mobility values for P3HT andOC1C10-PPV of respectively 3×10−5 cm2/Vs and 5×10−7 cm2/Vs. These values areupto three orders of magnitude lower as compared to the mobility values obtained fromthe FETs.

2.3.3 The mobility - charge density relation

Experimentally, we find for both semiconductors large differences in mobility valueswhen measured in different device geometries. Because we study amorphous polymerfilms, we argue that anisotropy in the transport parallel or perpendicular to the substratesurface can not be the origin of this mobility difference. Instead, we propose to investi-gate the mobility versus volume charge density relation for both type of devices, as it iswell known that the charge carrier mobility in disordered organic semiconductors dependsstrongly on the charge carrier density.

For the diode, the mobility at low electric fields and at room temperature can directlybe obtained from the space-charge limited currents [19, 23]. The lowest charge carrierdensity, pl , in a diode, where the current is limited by space-charge, is found at the non-injecting contact and is given by [23]:

pl = 3

4

(ε0εsemi V

ql2

), (2.14)

where q is the elementary charge, and l is the semiconductor layer thickness. For theFET, the experimental field-effect mobility is determined as a function of gate bias usingEq.2.8 and we calculate the volume charge carrier density, p = δNt , at the semiconduc-tor/insulator interface as a function of gate voltage, as outlined in section 2.2.

2.3 Unifying the charge transport in polymeric FETs with PLEDs 29

µ K

'26

'26

Figure 2.8: Charge carrier mobility as a function of the hole density forP3HT and OC1C10-PPV determined in a hole-only diode (p <

1017 cm−3) and in a field-effect transistor (p > 1017 cm−3). Thedashed lines are a guide to the eye.

The hole mobility versus the volume charge density for P3HT and OC1C10-PPV aregiven in Fig.2.8, where the values at low charge density (p < 1017 cm−3) are derivedfrom the PLED data and the values at high charge density ( p > 1017 cm−3) are derivedfrom FET data. From Fig.2.8 we see that the mobility starts to increase rapidly withcharge carrier density when the charge density is larger than a certain minimum value.Fig.2.8 shows that when measured at the same high values of volume charge carrier den-sity the field-effect mobility of OC1C10-PPV is nearly equal to the field-effect mobility ofP3HT. Furthermore, the dependence of the field-effect mobility on charge carrier densityis stronger for OC1C10-PPV, which is likely due to the presence of stronger energetic dis-order in the OC1C10-PPV as compared to P3HT, as reflected by the larger value of TDOS

for OC1C10-PPV. The strong energetic disorder explains the low mobility values reportedfor OC1C10-PPV based light-emitting diodes [19], which operate at relatively low carrierdensities as compared to field-effect transistors. The large differences in mobility valuesobtained from diodes and FETs, based on a single semiconducting polymer, are directresults of the large difference in charge densities in these devices.

2.3.4 Unification of the LED and FET models

To further emphasize this point we compare the modeled parameters obtained from thePLED and the FET current voltage characteristics. In Fig.2.9 the obtained Gaussian den-sity of states for P3HT is plotted as a function of energy, in a semilogarithmic plot. For thetotal number of states per unit volume, Nt , we have used a value of 3·1020 cm−3, which


'26

!

Figure 2.9: The Gaussian DOS (dashed line), as obtained from the hole-onlydiode analysis and the exponential DOS (solid line), as obtainedfrom the field-effect transistors as a function of energy for P3HT.The exponential DOS is found to be a good approximation of theGaussian DOS.

roughly corresponds to 1/a3 (a=1.5 nm). Additionally, the exponential DOS of P3HT asobtained from the FET characteristics is shown, which is described by the characteristictemperature TDOS . For the charge carrier density range in which the P3HT FET operates,we find that the exponential distribution with TDOS=425 K is a good approximation of theGaussian DOS with σDOS=0.098 eV. This same analysis holds for the obtained OC1C10-PPV data [24]. This demonstrates that the two theoretical descriptions are consistent. Themobility description at high carrier densities, which employs an exponential DOS, is anaccurate representation of the mobility description at low carrier densities, which uses aGaussian DOS.

2.3.5 Conclusion

In conclusion, the large mobility differences reported for conjugated polymers used inPLEDs and FETs have been shown to originate from the strong dependence of the mo-bility on the charge carrier density. The exponential density of states, which consistentlydescribes the field-effect measurements, is shown to be a good approximation of the tailstates of the Gaussian density of states used in the description of PLEDs.

References

[1] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W.Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig and D.M.de Leeuw, Nature (London) 401, 685 (1999).

[2] S.F. Nelson, Y.-Y. Lin, D.J. Gundlach and T.N. Jackson, Appl. Phys. Lett. 72, 1854(1998).

[3] M. Pope and C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers,Oxford University Press (1999).

[4] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981)

[5] C.J. Drury, C.M.J. Mutsaers, C.M. Hart, M. Matters and D.M. de Leeuw, Appl. Phys.Lett. 73, 108, (1998).

[6] G.H. Gelinck, T.C.T. Geuns and D.M. de Leeuw, Appl. Phys. Lett. 77, 1487 (2000).


[8] M.C.J.M. Vissenberg and M. Matters, Phys. Rev. B, 57, 12964 (1998).

[9] G. Horowitz, R. Hajlaoui and P. Delannoy, J. Phys. III, 5, 355 (1995).

[10] G. Horowitz, M.E. Hajlaoui and R. Hajlaoui, J. Appl. Phys. 87, 4456 (2000).

[11] G. Horowitz, R. Hajlaoui, H. Bouchriha, R. Bourguiga and M. Hajlaoui, Adv. Mater.10, 923 (1998).

[12] M. Shur, M. Hack and J.G. Shaw, J. Appl. Phys. 66, 3371 (1989).

[13] N. Lustig, J. Kanicki, R. Wisnieff and J. Griffith, MRS Symp. Proc. 118, 267 (1988).

[14] B-S. Bae, D-H. Cho, J-H. Lee and C. Lee, MRS Symp. Proc. 149, 271 (1989).

[15] D. Emin, private communication.


[17] G.E. Pike and C.H. Seager, Phys. Rev. B 10, 1421 (1974).

32 References

[18] E.J. Meijer, M. Matters, P.T. Herwig, D.M. de Leeuw and T.M. Klapwijk, Appl.Phys. Lett. 76, 3433 (2000).

[19] P.W.M. Blom, M.J.M. de Jong and J.J.M. Vleggaar, Appl. Phys. Lett. 68, 3308(1996).

[20] P.W.M. Blom, M.J.M. de Jong and M.G. van Munster, Phys. Rev. B 55, R656 (1997).

[21] H. C. F. Martens, P. W. M. Blom and H. F. M. Schoo, Phys. Rev. B 61, 7489 (2000).

[22] S.V. Novikov, D.H. Dunlap, V.M. Kenkre, P.E. Parris and A.V. Vannikov, Phys. Rev.Lett. 81, 4472 (1998).

[23] M.A. Lampert and P. Mark, Current Injection in Solids (Academic, New York,1970).

[24] C. Tanase, E.J. Meijer, P.W.M. Blom and D.M. de Leeuw, submitted.

Chapter 3

The Meyer-Neldel Rule in organicfield-effect transistors

Abstract

We have measured and analyzed the temperature and gate voltage dependencies of thefield-effect mobility in organic field-effect transistors. We find that the mobility prefactorincreases exponentially with the activation energy in agreement with the Meyer-Neldelrule. This behavior is demonstrated in the mobility data of solution-processed pentaceneand poly(2,5-thienylene vinylene) and in mobility data reported in literature. Surprisingly,the characteristic Meyer-Neldel energy for all analyzed materials is close to 40 meV.Possible implications for the charge transport mechanism in these materials are discussed.

34 The Meyer-Neldel rule

3.1 Introduction

The temperature and gate voltage dependence of the charge carrier mobility, µF E , oforganic-based field-effect transistors (FET) have been the subject of research for someyears now [1–7]. However, the charge transport mechanisms in these organic devicesare still not fully understood. Reports vary from thermally activated behavior [2, 3, 5]to temperature independent transport [4]. Moreover, large variations in the experimentaldata on even nominally the same samples make it difficult to obtain an accurate picture ofthe transport mechanism [4, 6]. Band-like transport in extended states has been reportedin the past for high-purity single crystals [1]. It should be noted that in contrast to thishighly orderded system the carrier transport in disordered or partially ordered systemsis governed by localized states, which results in a different transport mechanism. Thetemperature and gate voltage dependencies of µF E in organic FETs have been describedin terms of multiple trapping [3], hopping [7] and Coulomb blockade [5]. A commonfactor in all these models is the gate voltage dependence of the activation energy, Ea .Whenever a property, say X , has a thermally activated behavior,

X = X0 exp

[−Ea

kB T

], (3.1)

and Ea is a variable, it is empirically found [8] that the prefactor, X0, increases exponen-tially with the activation energy:

X0 = X00 exp

[Ea

EM N

]. (3.2)

This relation between the prefactor X0 and Ea is known as the Meyer-Neldel rule (MNR) [8].Here kB is the Boltzman constant, T the absolute temperature, X00 is a constant prefactorand EM N is the so-called Meyer-Neldel energy. A combination of Eqs. 3.1 and 3.2 givesthe general form:

X = X00 exp

[−Ea

(1

kB T− 1

EM N

)], (3.3)

which implies a single crossing point for different activation energies at an isokinetictemperature determined by the Meyer-Neldel energy: T0 = EM N /kB . The MNR has beenobserved in a wide variety of physical, chemical and biological processes [9]. However,the microscopic origin of the MNR and therefore, the physical meaning of EM N , are stilla topic of discussion in literature [9].

In this work we demonstrate that the MNR applies to the field-effect mobility dataof solution-processed pentacene and poly(2,5-thienylene vinylene) (PTV) FETs, as wellas to mobility data reported in literature. We discuss the relation between the transportmechanism in organic FETs and the possible origin of the MNR.

3.2 Experimental

In the experiments we used heavily doped Si wafers as the gate electrode, with a 200nm thick layer of thermally oxidized SiO2 as the gate-insulating layer. Using conven-tional lithography, gold source and drain contacts were defined with a channel width

3.3 Demonstration of the MNR 35

GV

J

Figure 3.1: Ids vs. Vg for pentacene (Vds=-2 V (open squares) and -20 V (filledsquares)) and PTV (Vds=-2 V (open circles) and -40 V (filled cir-cles)) at 290 K in vacuum (10−7 mbar). Vg was swept from +2 to-30 V and back to +2 V.

W=2 cm and length L=10 µm. The SiO2 layer was treated with the primer hexamethyl-disilazane (HMDS) to make the surface hydrophobic. The films of both pentacene andPTV were deposited using a precursor-route process [2, 10, 11]. The obtained PTV filmsare truly amorphous [12]. The pentacene films are polycrystalline with a planar spacingof 14.3 ± 0.1 A [12], which corresponds to the bulk triclinic phase of pentacene [13, 14].The samples were measured under high vacuum (10−7 mbar) in an Oxford optistat CF-Vflow cryostat, using a Hewlett-Packard 4156A semiconductor parameter analyzer.

The source-drain current, Ids , is plotted as a function of gate voltage, Vg , in Fig. 3.1for both the pentacene and PTV transistors at 290 K. We do not observe any hysteresis inthe measurements and the curves are stable in time (in vacuum).

3.3 Demonstration of the MNR

The temperature dependence of µF E is evaluated in the linear regime [2] of the Ids − Vds

characteristics (at a low source-drain voltage Vds = −2 V ). The applied gate field inthis regime is much larger than the in-plane drift field, which results in an approximatelyuniform density of charge carriers in the active channel. We calculate µF E [2, 3] from

µF E = L

WCi Vds

∂ Ids

∂Vg, (3.4)

where Ci is the capacitance of the insulator per unit area. In Fig. 3.2 we plot µF E vs.T −1 [15] for pentacene.


J

J

J

J

J

)(

D

J

Figure 3.2: Temperature dependence of the field-effect mobility of pentacene.The inset shows the dependence of the activation energy on thegate voltage.

J

J

J

J

J

J

)(

D

J

Figure 3.3: Temperature dependence of the field-effect mobility of PTV. The in-set shows the dependence of the activation energy on the gate volt-age.

3.4 Implications for the charge transport 37

D

Figure 3.4: extrapolated prefactor, µ0, as a function of Ea for pentacene(filledsquares) and PTV(open circles).

We can fit the data [22] in Figs. 3.2 and 3.3 with

µF E = µ0 exp

[−Ea

kB T

](3.5)

and upon extrapolation to high T we clearly find a common crossing point of the curves.Plotting the prefactor, µ0, logarithmically as a function of Ea for both pentacene andPTV (see Fig. 3.4), results in a straight line. This demonstrates that the experimentalresults are in agreement with the MNR. From Fig. 3.4 we find EM N ≈ 38 meV andEM N ≈ 42 meV for pentacene and PTV respectively. When this analysis is also usedon the gate voltage dependent mobility data reported in literature, we find that the MNRalso holds for the mobility data on dihexyl-sexithiophene measured by Horowitz et al. [3](EM N ≈ 43 meV) and for the previous studies of pentacene (EM N ≈ 34 meV) andPTV (EM N ≈ 35 meV) [2]. Furthermore, for C60 FETs it was found that EM N ≈36 meV [23,24]. Surprisingly, for all these materials the value of EM N is close to 40 meV.

3.4 Implications for the charge transport

Now we will discuss the relation between the transport mechanism and the origin of theMNR. In inorganic amorphous semiconductors the MNR has been attributed to a dis-placement of the Fermi level in an exponential density of states (DOS) [16, 17, 25, 26].A consequence of this interpretation is, that in order to calculate a physically reasonablevalue for the prefactor (of conductivity or mobility) one has to assume physically unrea-sonable values for the attempt frequency for hopping, which can vary between 103 and


1028 s−1 [27–31]. Furthermore, Yelon et al. [29, 30] have argued that the MNR can notbe solely due to an exponential DOS, as the MNR is much more generally applicable.They attribute the difficulty of interpreting the prefactor values in the DOS model to theassumption that the excitation process involves only one phonon [32,33]. If the activationenergy is large compared to the typical phonon energies available, multiple excitations arerequired for a hopping event to occur. They show that a multiphonon process can explainthe MNR and that the large spread in values of the prefactor can be accounted for withreasonable values for the attempt frequencies [29, 30].

In organic FETs, the interpretation of the MNR in terms of a Fermi level shift wouldbe consistent with the hopping [7] and multiple trapping models [3]. In that case EM N isequal to the width of the DOS [27] (the TDOS parameter from Chapter 2) and no physicalmeaning is attributed to the prefactor µ0. We argue that the ubiquitous value of EM N

is more likely due to a characteristic transport mechanism in organic materials ratherthan to one general DOS. Whether the interpretation given by Yelon et al. [29, 30, 34] isapplicable to our results on organic FETs hinges on the question whether it is justifiedto describe the hopping of a charge carrier from one conjugated segment (or molecule)to the next, as a multiphonon process. We note, that if the charge carrier is a localizedpolaron [35,36], Emin argued that the transport should be a multiphonon process, even ifthe phonon energies are comparable to Ea [37, 38]. This interpretation would imply thatthe observed MNR is a direct consequence of the polaronic nature of the charge carriers.

3.5 Conclusions

In summary, we have shown the validity of the MNR in organic-based FETs. The ubiqui-tous value of EM N ≈ 40 meV is an indication of a common origin of the MNR in organicFETs. We have argued that the MNR is directly linked to the charge transport mechanismand possibly even to polaronic carriers.

References

[1] M. Pope and C. E. Swenberg, “Electronic processes in organic crystals”, Oxford Uni-versity Press, New York, (1982).

[2] A. R. Brown, C. P. Jarrett, D. M. de Leeuw and M. Matters, Synth. Metals 88, 37(1997).

[3] G. Horowitz, R. Hajlaoui and P. Delannoy, J. Phys III France 5, 355 (1995).

[4] S. F. Nelson, Y.-Y. Lin, D. J. Gundlach and T. N. Jackson, Appl. Phys. Lett. 72, 1854(1998).

[5] W. A. Schoonveld, J. Wildeman, D. Fichou, P. A. Bobbert, B. J. van Wees and T. M.Klapwijk, to be published in Nature.

[6] L. Torsi, A. Dodabalapur, L. J. Rothberg, A. W. P. Fung and H. E. Katz, Phys. Rev. B57, 2271 (1998).

[7] M. C. J. M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998).

[8] W. Meyer and H. Neldel, Z. Tech. Phys. 18, 588 (1937).

[9] For examples see references in [29, 30]

[10] A. R. Brown, A. Pomp, D. M. de Leeuw, D. B. M. Klaassen, E. E. Havinga, P. T.Herwig, K. Mullen, J. Appl. Phys. 79, 2136 (1996).

[11] P. T. Herwig and K. Mullen, Adv. Mater. 11, 480 (1999).

[12] A. R. Schlatmann, E. J. Meijer and D. M. de Leeuw, unpublished X-ray diffractionresults.

[13] R. B. Campbell and J. M. Robertson, Acta Cryst. 14, 705 (1961).

[14] I. P. M. Bouchoms, W. A. Schoonveld, J. Vrijmoeth and T. M. Klapwijk, Synth. Met.104, 177 (1999).

[15] We note that the MNR in inorganic FETs is sometimes evaluated by analyzing thesheet conductance of the channel [16,17]. However, the extraction of EM N , using thesheet conductance, may be inaccurate due to a decrease of the effective accumulationchannel thickness with increasing Vg [18, 19]. We therefore look at µF E instead, assuggested by Fortunato and co-workers [20, 21].

40 References

[16] R. Schumacher, P. Thomas, K. Weber and W. Fuhs, Sol. State. Comm. 62, 15 (1987).

[17] R. Schumacher, P. Thomas, K. Weber, W. Fuhs, F. Djamdji, P. G. Le Comber and R.E. I. Schropp, Phil. Mag. B 58, 389 (1988).

[18] M. Yamaguchi and H. Fritsche, J. Appl. Phys. 56, 2303 (1984).

[19] A. P. Gnadinger and H. E. Talley, Proc. IEEE, 916 (1970).

[20] G. Fortunato, L. Mariucci and C. Reita in “Amorphous and Microcrystalline Semi-conductor Devices” Volume 2, 355, Editor: J. Kanicki, Artech House, Norwood(1992).

[21] G. Fortunato, D. B. Meakin, P. Migliorato and P. G. Le Comber, Phil. Mag. B, 57,573 (1988).

[22] We note that the mobility curves of the PTV show a deviation from simple Arrheniusbehavior at low temperatures. This behavior has thus far been modelled with a specificdistribution of traps [3] and hopping in an exponential density of states [7].

[23] J. Paloheimo and H. Isotalo, Synth. Met. 55, 3185 (1993).

[24] J. C. Wang and Y. F. Chen, Appl. Phys. Let. 73, 948 (1998).

[25] H. Overhof, J. Non-Cryst. Sol. 97/98, 539 (1987).

[26] M. Ortuno and M. Pollak, Phil. Mag. B 47, L93 (1983).

[27] P. Irsigler, D. Wagner and D.J. Dunstan, J. Phys. C 16, 6605 (1983).

[28] B. Movaghar, J. Phys. Coll. C4, 73 (1981).

[29] A. Yelon and B. Movaghar, Phys. Rev. Lett. 65, 618 (1990).

[30] A. Yelon, B. Movaghar and H. M. Branz, Phys. Rev. B 46, 12244 (1992).

[31] A. M. Szpilka and P. Viscor, Phil. Mag. B 45, 485 (1982).


[33] N. F. Mott and E. A. Davis “Electronic Processes in Non-Crystalline Materials”,second edition, Clarendon Press Oxford (1979).

[34] H. M. Branz, A. Yelon and B. Movaghar, MRS Symp. Proc. 336, 159 (1994).


[36] A. J. Heeger, S. Kivelson, J. R. Schrieffer and W.-P. Su, Rev. Mod. Phys. 60, 781(1988).

[37] D. Emin, Phys. Rev. Lett. 32, 303 (1974).

[38] D. Emin, Adv. Phys. 24, 305 (1975).

Chapter 4

The isokinetic temperature indisordered organic semiconductors

Abstract

We have investigated the field dependence of the in-plane conductivity in poly(2,5- thieny-lene vinylene) and poly(3-hexyl thiophene) thin films. The conductivity is found to have asquare root dependence on the lateral electric field. The values for the characteristic tem-perature, obtained from the empirical field-dependent mobility relation are very similar tothe values found for the isokinetic temperature in Meyer-Neldel experiments on poly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) field-effect transistors. The possiblerelation between the field-dependent mobility and the Meyer-Neldel rule is discussed inthe context of charge transport in disordered organic semiconductors.

42 The isokinetic temperature

4.1 Introduction

Due to possible industrial applications, opto-electronic devices based on disordered or-ganic semiconducting layers are receiving much attention [1–4]. The disorder in the or-ganic films dominates the charge transport. Typically, low mobilities with a thermallyactivated behavior are observed. Transport is mostly described by hopping. Several inter-esting physical features are associated with transport through disordered materials, suchas the Meyer-Neldel rule (MNR) [5], which we demonstrated in the previous chapter.This rule states that the prefactor of the thermally activated mobility increases exponen-tially with the activation energy. For a number of materials we found the characteristicisokinetic temperature associated with the MNR to be in the range of 440-510 K [6]. Athigh electric fields in disordered systems the mobility becomes field dependent, whichcan be described by the empirical relation [7, 8]:

µ = µ0 exp

[ −

kB T+ γ

√F

](4.1)

with,

γ = B

(1

kB T− 1

kB T0

), (4.2)

where kB is Boltzmann’s constant, T the absolute temperature, F the applied electricfield and the low field activation energy. The field dependence as given in Eq.4.1 isobserved in a wide range of disordered materials, with typical values for the parametersof =0.5 eV, T0 ∼500-600 K and B=3·10−5eV(m/V)1/2 [9–11]. It has been suggestedthat the Meyer-Neldel rule and the field dependent mobility are related effects [11,12]. Toinvestigate this hypothesis we have studied both effects in two organic semiconductors,poly(2,5-thienylene vinylene) (PTV), and poly(3-hexyl thiophene) (P3HT).

4.2 Results

The in-plane conductivity, σ , was measured using interdigitated gold contacts on glassand the MNR using regular MISFET structures, as described in Chapter 2. X-ray ex-periments on the PTV films did not yield reflections [6], indicating that the PTV filmsare amorphous. The P3HT films are nanocrystalline. In Figs.4.1 and 4.2 σ is plottedas a function of F1/2 for various temperatures. We do not observe space-charge limitedcurrents in the range of electric fields shown in Figs.4.1 and 4.2. At low fields ohmicbehavior is observed in Fig.4.2.

Figs.4.3 and 4.2 show the temperature dependence of σ for different electric fields.The data are well described by Eq.4.1 [13] and the values of , B , and T0 fitted for bothPTV and P3HT are given in Table.4.1. The obtained values are in agreement with datapublished for other materials [9–11]. Fig.3.3 shows the field-effect mobility for a PTVtransistor as a function of T −1 for different gate voltages. The same is plotted for P3HTin Fig.4.5. The curves are fitted to Arrhenius behavior with a gate voltage dependentprefactor and activation energy, as was outlined in the previous chapter.

4.2 Results 43

6

Q

Figure 4.1: In-plane conductivity of PTV as a function of F1/2 for various tem-peratures. L=2 µm, W=2 cm. The lines are fits to Eq.4.1. Theinset shows the structure formula of PTV.

6

&+

Q

Figure 4.2: Measured in-plane conductivity of P3HT as a function of F1/2 forvarious temperatures. L=5 µm, W= 50cm. The lines are fits toEq.4.1. The inset shows the structure formula of P3HT.


Figure 4.3: In-plane conductivity of PTV as a function of T −1 for various elec-tric fields. The lines are Arrhenius fits.

The intersection of the fitted curves at high temperatures demonstrates the MNR[5,6] with a characteristic isokinetic temperature of 4.9·102 K for PTV and 4.6·102 K forP3HT. Interestingly, the characteristic temperature of the MNR in PTV and P3HT is closeto these values. This indicates that the origin of the T0 in the field dependence (Figs.4.3and Fig.4.4) could be the same as the isokinetic temperature observed in the Meyer-Neldelrule (Fig.3.3 and Fig.4.5) [11, 12].

Table 4.1: Values obtained by fitting the field dependence of the conductivityσ with Eq.4.1 for PTV and P3HT. is the low field activation en-ergy, B is the field dependent coefficient as given in Eq. 4.2, T0,σ

is the isokinetic temperature determined from the field-dependencedata of Figs.4.3 and 4.4, and T0,F E is the isokinetic temperaturedetermined from the field-effect experiments of Figs.3.3 and 4.5.

[eV] B[eV(m/V)1/2] T0,σ [K] T0,F E [K]PTV 0.46 2.3 · 10−5 5.2 · 102 4.9 · 102

P3HT 0.40 4.7 · 10−5 5.3 · 102 4.6. · 102

4.2 Results 45

Figure 4.4: In-plane conductivity of P3HT as a function of T −1 for variouselectric fields. The lines are Arrhenius fits.

)(

(D>H9@

9J>9@

Figure 4.5: Field effect mobility of a P3HT transistor as a function of T −1 fordifferent gate voltages. The lines are Arrhenius fits.The inset showsthe activation energy as a function of the gate voltage.


4.3 Discussion of the isokinetic temperature

An investigation into a possible relation between the isokinetic temperature observed inthe Meyer-Neldel experiments (see the previous chapter) and in the field-dependence ofthe mobility, requires a careful examination of the empirical relations from which theyoriginate, Eq.3.3 and Eq.4.1, and the underlying physical mechanisms.

4.3.1 The field-dependent mobility

Although Eq.4.1 gives a good description of the mobility as a function of temperature andelectric field, it still lacks theoretical justification. Other approaches to understand theln(µ) ∼ √

F relation have been proposed.Bassler argued that the observed field dependence of the mobility is related to the

intrinsic charge transport in disordered materials [14]. The disorder model developed byBassler and coworkers [14, 15] is based on a Gaussian distribution of localized states,and considers that hopping between sites is subject to both energetic and spatial disorder.Monte Carlo simulations of hopping transport in a Gaussian distribution of localized stateshas suggested that the field dependence of the mobility is a natural consequence of thepresence of disorder [14]. There is general agreement that the field-dependency observedat fields as low as 1 MV/m is related to the presence of correlations in site-energies in thedisordered material, and can be described by [16]:

µ = µ∞ exp

[−(

3σDOS

5kBT

)2

+ 0.78

((σDOS

kB T

) 32 − 2

)√qa F

σDOS

], (4.3)

with µ∞ the mobility in the limit T → ∞, σDOS the width of the Gaussian DOS, anda the intersite spacing. Various physical mechanisms for the origin of these correlationsin the Gaussian model have been suggested. Yu et al. considered molecular geometryfluctuations such as phenylene-ring torsion out of the plane of the molecule, which willinfluence the steric energy of neighbouring molecules, and results in long-range energycorrelations between molecular sites [17]. Rakhmanova and Conwell considered struc-tural disorder, where the morphology of the polymer is crucial, and the long-range cor-relations between sites arise due to local variations in order. Sites in ordered regions aresuggested to have lower site energies than those in amorphous regions [18], which againwill result in long-range energy correlations. Similar modeling based on the argumentof structural disorder was given by Vissenberg [19]. In principle any model of Gaussiandisorder that incorporates long-range energy correlations between sites will give rise to a√

F dependence of ln(µ).In addition to the different physical interpretations of the field-dependent mobility,

the Gaussian disorder model and the empirical relation of Eq.4.1 also predict that thetemperature dependence of the mobility should be distinct from each other. The zero fieldmobility is expected to vary as µ = µ0 exp(−/kB T ) from Eq.4.1, whereas it shouldfollow a much stronger variation as µ = µ0 exp(−[2σDOS/(3kBT )]2) from Eq.4.3. Anevaluation of the µF=0 as a function of temperature should therefore allow the applica-bility of the two models to be tested. Unfortunately, in polymer systems typically only a

4.4 Conclusions 47

small temperature range can be scanned experimentally, and therefore a clear distinctionbetween ln(µ) ∼ T −1 and ln(µ) ∼ T −2 cannot be made.

4.3.2 The Meyer-Neldel rule

As stated in the previous chapter, the Meyer-Neldel rule is also an empirical relation, thephysical justification of which is still heavily under debate. The discussion of the originof the MNR has focussed on multi-phonon hopping rates. Entropy change associated withthese hopping rates would lead to the MNR [20, 21](see the previous chapter).

4.3.3 Comparison of the MNR with the field-dependent mobility

It is instructive to compare the two physical pictures that emerge from the discussionaround the isokinetic temperature.

In the first picture, it is the environment in which the charge carriers move that isimportant: a manifold of localized states in a Gaussian energy distribution with long-range correlations [16]. The nature of the charge carriers themselves, or the specificsof the charge transfer from one localized state to the next, is taken as simple as possi-ble. Mostly Miller-Abrahams hopping rates [22] are considered, a single-phonon assistedtransfer process, for holes and electrons.

In the second picture, the nature of the carriers and the charge transfer description isof importance and the long-range environment is not: polaronic carriers, quasi particlesthat consist of charge accompanied by a lattice deformation, that require multiple phononsfrom their surroundings to move from a localized site to the next [21,23]. This requires amuch more complex mathematical treatment of the site to site hopping process.

4.4 Conclusions

The fact that polarons are the charge carrying species in these polymeric systems andthat disorder is tantamount for the description of charge transport, would suggest thata combination of the two interpretations arising from the discussion of the isokinetictemperature would give a more complete description of charge transport. This then resultsin a charge transport picture of polaronic charge carriers, that move with a multi-phonontransfer rate between localized states, which have a Gaussian energy distribution and long-range energy correlations.

References

[1] C.J. Drury, C.M.J. Mutsaers, C.M. Hart, M. Matters and D.M. de Leeuw, Appl. Phys.Lett. 73, 108 (1998).


[3] H. Sirringhaus, P.J. Brown, R.H. Friend, M. Nielsen, K. Bechgaard, B. Langeveld-Voss, A. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig and D.M. de Leeuw,Nature 401, 685 (1999).

[4] Z. Bao, A. Dodabalapur and A.J. Lovinger, Appl. Phys. Lett. 69, 4108 (1998).

[5] W. Meyer and H. Neldel, Z. Techn. Phys. (Leipzig), 18, 588 (1937).

[6] E.J. Meijer, M. Matters, P.T. Herwig, D.M. de Leeuw and T.M. Klapwijk, Appl. Phys.Lett. 76, 3433 (2000).

[7] D.M. Pai, J. Chem. Phys. 52, 2285 (1970).

[8] W.D. Gill, J. Appl. Phys. 43, 5033 (1972).

[9] P.W.M. Blom and M.C.J.M. Vissenberg, Mater. Scie and Engin. R27, 53 (2000) andreferences therein.

[10] P.W.M. Blom, M.J.M. de Jong and M.G. van Munster, Phys. Rev. B 55, R656 (1997).

[11] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins and T.M. Klapwijk, Mat. Res. Soc.Symp. Proc. 660 (2001).

[12] A. Peled and L. Schein, Phys. Scripta 44, 304 (1991).

[13] We assume that the field and temperature dependence of σ are due to the mobilityand not the charge density.


[15] M. Abkowitz, H. Bassler and M. Stolka, Phil. Mag. B 63, 201 (1991).

[16] S.V. Novikov, D.H. Dunlap, V. Kenkre, P. Parris and A. Vannikov, Phys. Rev. Lett.81, 4472 (1998).

50 References

[17] Z.G. Yu, D.L. Smith, A. Saxena, R.L. Martin and A.R. Bishop, Phys. Rev. Lett. 84,721 (2000).

[18] S. V. Rakhmanova and E.M. Conwell, Appl. Phys. Lett. 76, 3822 (2000).

[19] P.W.M. Blom and M.C.J.M. Vissenberg, Mater. Scie. Eng. R27, 53 (2000).

[20] A. Yelon and B. Movaghar, Phys. Rev. Lett. 65, 618 (1990).

[21] D. Emin, Phys. Rev. B 61, 14543 (2000).



Chapter 5

Scaling behavior and parasitic seriesresistance in disordered organic

field-effect transistors

Abstract

The scaling behavior of the transfer characteristics of solution-processed disordered or-ganic field-effect transistors with channel length is investigated. This is done for a varietyof organic semiconductors in combination with gold injecting electrodes. From the chan-nel length dependence of the transistor resistance in the conducting ON-state we deter-mine the field-effect mobility and the parasitic series resistance. The extracted parasiticresistance, typically in the M-range, depends on the applied gate voltage, and we findexperimentally that the parasitic resistance decreases with increasing field-effect mobility.

52 Scaling behavior and parasitic series resistance

5.1 Introduction

The interest in organic field-effect transistors has grown rapidly due to envisaged ap-plications such as integrated circuits [1] and active-matrix displays [2]. Research hasmainly been focused on improving the field-effect mobility, µF E [3–7], which is knownto depend on material purity and processing conditions. For transistors based on solution-processed organic semiconductors µF E typically ranges between 10−4 and 10−1 cm2/Vs.The switching speed of organic integrated circuits can be estimated from the performanceof the individual transistors and is roughly proportional to ∼ µF E/L2 [8], where L is thechannel length of the transistor. To reach higher switching speeds, the search for highermobility materials is therefore important, but it is also of great interest to downsize thetransistor geometries. In this work the scaling behavior of the transfer characteristicswith transistor channel length is investigated for a variety of solution-processable organicfield-effect transistors.

5.2 Experimental

In the experiments we use heavily doped Si wafers as the gate electrode, with a 200-nm-thick layer of thermally oxidized SiO2 as the gate-insulating layer. Using conventionallithography, gold source and drain contacts of 100 nm thick are defined with channelwidths ranging from 1 mm to 1 cm and channel lengths between 0.75 and 40 µm. Thestructures typically have an underetch of 0.5 µm, which we neglect in the following anal-ysis. A 10 nm layer of titanium acts as an adhesion layer for the gold on the SiO2.The SiO2 layer is treated with the primer hexamethyldisilazane to make the surface hy-drophobic. No special care is taken to clean the gold surface prior to deposition of thesemiconductor. Poly(2,5-thienylene vinylene) (PTV) films as semiconductor layer aredeposited using a precursor-route process [8]. We systematically varied the processingconditions for the conversion from precursor to PTV and we determined the average de-gree of conversion using the method described by Fuchigami et al. [9]. This enabled usto systematically study PTV transistors at various degrees of conversion ranging from60% to 100%, and consequently over a range of field-effect mobilities, between 10−4

and 10−3 cm2/Vs. Poly(3-hexyl thiophene) (P3HT) is spincoated from a 1 wt% chlo-roform solution [7]. Films of poly([2-methoxy-5-(3’,7’-dimethyloctyloxy)]-p-phenylenevinylene) (OC1C10-PPV) and poly([2,5-di-(3’,7’-dimethyloctyloxy)]-p-phenylene viny-lene) (OC10C10-PPV), are spun from a 0.5 wt% toluene solution. Pentacene thin filmsare deposited using a precursor-route process [4, 8]. The measurements are performedon freshly prepared samples in order to minimize external doping and degradation ef-fects [10]. The PTV, OC1C10-PPV, OC10C10-PPV and pentacene samples are measuredin air, whereas the P3HT samples are measured in vacuum and dark after a thermal dedopeprocedure [11]. The electrical characteristics are recorded using an HP4155B semicon-ductor parameter analyzer.

5.3 Scaling behavior 53

5.3 Scaling behavior

Typical source drain current, Ids , versus gate voltage, Vg , characteristics for solu-tion-processed PTV and P3HT are shown in Fig.5.1, for different channel lengths, where thechannel width, W , is kept constant. The position of the switch-on voltage, Vso (see Chap-ter 2) [12], which determines the onset of the field-effect and is defined as the flat-bandcondition of the transistor, does not vary much between the transistors with different chan-nel length. At low source-drain voltage, Vds = −2 V , where the in plane electric field ismuch smaller than the applied gate field (gradual channel approximation) [8], the field-effect mobility is evaluated using:

µF E(Vg) = L

WCi Vds

∂ Ids

∂Vg, (5.1)

where Ci is the capacitance of the insulating layer per unit area.The field-effect mobilities for both PTV and P3HT are found to depend on the chan-

nel length of the transistor, which can be seen from the insets of Fig.5.1. This means thatthe extracted µF E is a device parameter rather than a material parameter of the organicsemiconductor. By comparing the output characteristics multiplied by the channel length,i.e. at constant source-drain field, for short- and long channel transistors we clearly seean effective current decrease for shorter channel lengths, which is demonstrated for P3HTin Fig.5.2. Furthermore, at low drain voltages the output characteristics of the long tran-sistor show ohmic behavior [13], whereas for the short channel transistors, at low drainvoltages, superlinear output characteristics are observed. Because µF E is decreasing withreduced L, the reduction of the channel length will not result in the expected increase ofthe switching speed in circuits.

From amorphous silicon thin-film transistors it is well known that the presence ofsource and drain parasitic resistances, Rs and Rd respectively, can give rise to an apparentµF E that decreases with decreasing channel length [14, 15]. This is due to the fact thatin shorter channels, a relatively larger fraction of the applied source-drain voltage dropsover the parasitic resistance, as compared with the long channel transistors. To be ableto evaluate the performance of the organic semiconductor, a correction for the parasiticresistance, Rp = Rs + Rd , is required [16, 17]. A theoretical approach to this end waspresented by Horowitz et al. [18]. Experimentally, it has been demonstrated that theinfluence of Rp can be reduced by modifying the interface between the current injectingcontacts and the organic semiconductor [19, 20]. Kelvin probe force microscopy hasbeen employed for experimental evaluation of Rp [13]. Here, we investigate the scalingbehavior of the transistor current [15, 21, 22] to estimate Rp .

5.4 Parasitic series resistance determination

We plot the total device-resistance, RO N = Vds/Ids , as a function of the nominal channellength, L, for different gate voltages in Fig.5.3. In the linear operating regime of thetransistor the channel resistance varies linearly with the channel length. The parasiticresistance, Rp = Rs + Rd , at the source and drain contacts is assumed to be independent


GV

J

/

µPµPµPµP

µ)(FRUUHFWHGIRU5

S

µPµPµPµP

µ )(

J

G

V

J

/

µPµPµPµPµP

µ)(FRUUHFWHGIRU5

S

µ )(

J

µPµPµPµPµP

Figure 5.1: Ids vs Vg at Vds = −2 V , for different channel lengths for (a) PTV,converted at 80oC under 150 mbar of HCl partial pressure [8]. Thecharacteristics are measured in air at room temperature, W=1 mm.(b) P3HT, W=1 mm, in vacuum at room temperature after a thermaldedoping treatment [11]. The insets show the corresponding µF E -values derived from the gatesweeps by using Eq.5.1.

5.4 Parasitic series resistance determination 55

J

J

J

J

J

GV

GV

Figure 5.2: The normalized output characteristics for two P3HT transistorswith L=0.75 µm (closed circles) and L=40 µm (open squares).Clearly the current in the short transistor is more dominated by theparasitic series resistance as compared to the long transistor.

of L. The RO N can then be expressed as [15]:

RO N (L) = ∂Vds

∂ Ids

∣∣∣∣Vds→0,Vg

= Rch (L) + Rp. (5.2)

The experimental data are, in first order, well described by this equation, with RO N de-pending linearly on L (see Fig.5.3).

From the slopes of the plots in Fig.5.3 we find the channel resistance, Rch , the inverseof which, [RO N /L]−1, is the channel conductivity. From the derivative of the channelconductivity, the field-effect mobility, corrected for Rp can be obtained:

∂

([RON

L

]−1)

∂Vg= µF E

(Vg)

WCi (5.3)

The resulting corrected mobilities are plotted in the insets of figure 5.1. The correctedcurve yields a higher overall µF E

(Vg). From the insets of Fig.5.1b it is clear that for the

40 µm-channel, the influence of Rp is small, as µF E obtained from Eq.5.1 is close to thecorrected mobility. We note that any non-linearity of µF E with Vg in our samples cannota priori be attributed to the presence of an Rp [18], but is more probably the result ofa specific density of states in the semiconductor at the semiconductor/insulator interface(see Chapter 2). From the analysis with Eq.5.2 we find Rp in Fig.5.3 as the intercept


21

Ω

µ

9J 9

9J 9

9J 9

9J 9

9J 9

21

Ω

µ

9J 9

9J 9

9J 9

9J 9

9J 9

Figure 5.3: Total device resistance RO N , calculated with Eq.5.2 from the datain Fig.5.1, as a function of the mask channel length for various gatevoltages, for (a) PTV, (b) P3HT.

5.4 Parasitic series resistance determination 57

379PEDU+&OFRQYHUVLRQGHJUHHYDULHGEHWZHHQDQG



2&&

339

2&&

339

3+73+7IURPUHI>@SUHFXUVRUSHQWDFHQH

S

)(

Figure 5.4: The parasitic resistance times the channel width as a function ofthe effective field-effect mobility for a number of polymeric semi-conductors and pentacene.

of RO N at L = 0. This Rp , typically on the order of M for our devices, is found todecrease with increasing gate voltage, i.e. with increasing carrier density.

From the analysis with Eq.5.2 we can find Rp in Fig.5.3 as the intercept of RO N

at L = 0. This intercept depends on the applied gate voltage, which implies that Rp

decreases with increasing gate voltage, i.e. with increasing carrier density.We find experimentally that both the field-effect mobility and the parasitic resistance

depend on Vg . In Fig.5.4 we plot the experimentally determined Rp , multiplied with thechannel width, and effective µF E obtained from the scaling analysis of several organicsemiconductors. Also data from a P3HT study of Sirringhaus et al. [16] is included.An empirical relation is observed between the charge carrier mobility in the polymericsemiconductor and the parasitic resistance for the polymeric semiconductor in contactwith the gold/titanium stack. A possible reason for this empirical observation is thatthe density of localized states in the polymer is of importance for the charge injectionefficiency. The dependence of Rp on µF E is probed experimentally by accumulatingcharge in the semiconductor, by means of the field-effect, where we change the positionof the Fermi level in the density of states. Why this results in a very similar dependenceof µF E on Rp for different polymeric semiconductors is unclear at present. In literatureit has been demonstrated that injection-limited current into a disordered polymer can bedescribed by thermally assisted hopping from the electrode into the localized states of thepolymer, which are broadened due to disorder [23]. As a representation of this effect,the Rp for injection into a semiconducting polymer is found to depend on the chargecarrier mobility in the polymer [24]. For the molecular semiconductor pentacene the datais more scattered and does not follow the trend observed for the polymeric transistors(see Fig.5.4). We attribute this to the polycrystalline nature of the pentacene films, which


depends on the processing conditions.The origin of the observed parasitic series resistance, or injection-limited current,

can be due to a combination of effects. In general, geometrical or morphological contactproblems between the semiconductor film and the gold contacts can be of importance,which is indicated by the scattered pentacene data. However, the data for the polymers aremuch more consistent and suggest that the injection barrier is related to material parame-ters of electrode and semiconductor layer rather than processing variations. A mismatchbetween the workfunction of the gold, at 5.1 eV, and the highest occupied molecular or-bital (HOMO) level of the semiconductors (for the materials used here: around 5.2 eV)would lead to an injection barrier for holes. The width of this injection barrier can be nar-rowed by accumulating charge in the semiconductor, through the field-effect, by applyinga Vg [25]. For a small barrier height, in the order of the thermal energy kB T , this willresult in an ohmic parasitic resistance, whereas for higher barrier heights a non-ohmicparasitic resistance will be present at the electrode/semiconductor interface.

5.5 Conclusions

In conclusion, we have used channel length dependent measurements to experimentallydetermine the effective field-effect mobility, corrected for parasitic series resistance, ina variety of spin-coated organic field-effect transistors. The understanding and reduc-tion of parasitic series resistances is important for downsizing of the organic transistorgeometries, to be able to reach higher switching speeds for integrated circuits. For theinvestigated transistors we extract a parasitic series resistance which depends on Vg . Thisparasitic resistance is attributed to an injection barrier with a height in the order of afew times kB T , which results in an ohmic parasitic series resistance. Experimentally, wefind that the parasitic resistance decreases with increasing charge carrier mobility for theinvestigated polymeric field-effect transistors.

References

[1] G.H. Gelinck, T.C.T. Geuns, D.M. de Leeuw, Appl. Phys. Lett. 77, 1487 (2000).

[2] H.E.A. Huitema, G.H. Gelinck, J.B.P.H. van der Putten, K.E. Kuijk, C.M. Hart, E.Cantatore, P.T. Herwig, A.J.J.M. van Breemen, and D.M. de Leeuw, Nature (London)414, 599 (2001)

[3] D.J. Gundlach, Y.Y. Lin, T.N. Jackson, S.F. Nelson, and D.G. Schlom, IEEE Elec.Dev. Lett. 18, 87 (1997).

[4] P. T. Herwig and K. Mullen, Adv. Mater. 11, 480 (1999).

[5] H. Sirringhaus, R.J. Wilson, R.H. Friend, M. Inbasekaran, W.Wu, E.P. Woo, M. Grell,and D.D.C. Bradley, Appl. Phys. Lett. 77, 406 (2000).

[6] Z. N. Bao, Y. Feng, A. Dodabalapur, V. R. Raju, and A. J. Lovinger, Chem. Mat. 9,1299 (1997).

[7] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W.Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig, and D.M.de Leeuw, Nature (London) 401, 685 (1999).

[8] A. R. Brown, C. P. Jarrett, D. M. de Leeuw, and M. Matters, Synth. Metals 88, 37(1997).

[9] H. Fuchigami, A. Tsumura, and H. Koezuka, Appl. Phys. Lett. 63, 1372 (1993).

[10] M. Matters, D. M. de Leeuw, P. T. Herwig, and A. R. Brown, Synth. Met. 102, 998(1999).

[11] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins, and T.M. Klapwijk, Mat. Res.Soc. Symp. Proc. 660 JJ7.9.

[12] E.J. Meijer, C. Tanase, P.W.M. Blom, E. van Veenendaal, B.-H. Huisman, D.M. deLeeuw, and T.M. Klapwijk, Appl. Phys. Lett. 80, 3838 (2002).

[13] L. Burgi, H. Sirringhaus, and R.H. Friend, Appl. Phys. Lett 80, 2913 (2002).

[14] M. Shur and M. Hack, J. Appl. Phys. 55, 3831 (1984).

60 References

[15] S. Luan and W. Neudeck, J. Appl. Phys. 72, 766 (1992).

[16] H. Sirringhaus, N. Tessler, D.S. Thomas, P.J. Brown, and R.H. Friend, Festkorper-probleme 39, 101 (1999).

[17] L. Torsi, A. Dodabalapur, and H.E. Katz, J. Appl. Phys. 78, 1088 (1995).

[18] G. Horowitz, R. Hajlaoui, D. Fichou and A. El Kassmi, J. Appl. Phys. 6, 3202(1999).

[19] J. Wang, D.J. Gundlach, C.C. Kuo, and T.N. Jackson, 41st Electr. Mater. Conf. Di-gest, pg 16 (1999).

[20] Y.Y. Lin, D.J. Gundlach, and T.N. Jackson, Mat. Res. Soc. Symp. Proc. 413, 413(1996).

[21] K. Terada and H. Muta, Jap. J. Appl. Phys. 18, 953 (1979).

[22] J.G.J. Chern, P. Chang, R.F. Motta and N. Godinho, IEEE Elect. Dev. Lett. 1, 170(1980).

[23] T. van Woudenbergh, P.W.M. Blom, M.C.J.M. Vissenberg and J.N. Huiberts, Appl.Phys. Lett. 79, 1697 (2001).

[24] Y. Shen, M.W. Klein, D.B. Jacobs, J.C. Scott and G.G. Malliaras, Phys. Rev. Lett.86, 3867 (2001).

[25] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981).

Chapter 6

Frequency behavior and theMott-Schottky analysis inpoly(3-hexyl thiophene)

metal-insulator-semiconductor diodes

Abstract

Metal-insulator-semiconductor diodes with poly(3-hexyl thiophene) as the semiconductorwere characterized with impedance spectroscopy as a function of bias, frequency, andtemperature. We show that the standard Mott-Schottky analysis gives unrealistic valuesfor the dopant density in the semiconductor. From modeling of the data, we find thatthis is caused by the relaxation time of the semiconductor, which increases rapidly withdecreasing temperature due to the thermally activated conductivity of the poly(3-hexylthiophene).

62 Frequency behavior and the Mott-Schottky analysis

6.1 Introduction

Low-cost organic integrated circuits are being more and more recognised as a poten-tially interesting industrial application. This has increased the efforts to develop high-performance devices. The development of solution-processable high-mobility polymers[1, 2] and of technology for all-polymer integrated circuits [3, 4] is promising. However,from the application point of view the lifetime of the devices is an important issue. Thelimited lifetime of current devices is mainly determined by the increase in conductivity,σ , of the semiconductor upon doping in air and light [4, 5].

To study the doping effects in high-mobility polymeric semiconductors we usedmetal-insulator-semiconductor (MIS) diodes with poly(3-hexyl thiophene) (P3HT) as thesemiconductor. We measured the temperature and modulation frequency dependence inthese devices. The standard Mott-Schottky analysis to extract the dopant density, NA ,yields erroneous results for large frequency and temperature ranges. Analysis of the datawill show that this is due to the temperature dependence of the relaxation time of theP3HT. We model the data with a simple equivalent circuit and argue that the temperaturedependence of the semiconductor relaxation time is due to thermally activated conductiv-ity of P3HT.

6.2 Experimental

The MIS diodes were fabricated on glass, using patterned Indium-Tin-Oxide (ITO) con-tacts as gate electrode. A 300 nm insulating layer of novolak R© photoresist was spin-coated on top of the gate. Over the insulator a 200 nm thick P3HT film was spun froma 1 weight % chloroform solution. Finally, a 10 nm gold layer was evaporated through ashadowmask to form an ohmic contact with the P3HT layer. A cross-section of the de-vice is given in the inset of Fig.6.1. The capacitance, C , of the diode can be changed bydepleting or accumulating charge in the semiconductor at the interface with the insulator.The thickness of the insulator layer, dins , determines the maximum value of C:

Cins = εinsε0 A

dins, (6.1)

where εins is the relative dielectric constant of the insulator, ε0 the permittivity of vacuumand A the area of the device. The minimum value of C is determined by the relative di-electric constants of the insulator, εins , and the semiconductor, εsemi , and the total distancebetween the conductive layers. When the semiconductor layer is partially depleted, thedepletion layer acts as a capacitance in series with the insulator capacitance. We calculatethe total diode capacitance from the modulus of the impedance, Z , and its phase angle, ,using: C = − sin /(ω|Z |), with ω=2π fmod , where fmod is the modulation frequency.The capacitances of the MIS diodes scaled with the area of the devices, ranging from 9 to36 mm2, in the entire biasing regime. The results presented in this work represent typicaldata measured on more than 10 MIS diodes. All impedance measurements were donewith a Schlumberger 1260 Impedance Gain-Phase Analyzer in vacuum (<10−5 mbar) inan Oxford CV-flowcryostat.

6.3 The Mott-Schottky analysis 63

3+7

5HVLVW

*OD VV

*R OG

,72

J

Figure 6.1: Capacitance of the P3HT MIS diode at 137 Hz as a function of Vg

for different temperatures. The area of this device was 36 mm2.

6.3 The Mott-Schottky analysis

Typical capacitance vs gate bias, Vg , (C − Vg) curves at different temperatures are givenin Fig. 6.1 for fmod =137 Hz. The gate voltage is applied on the ITO contact, with thegold electrode at 0 V. The capacitance is clearly a function of Vg , and the semiconductorshows p-type behavior. When the temperature is lowered, we observe that the value ofthe accumulation capacitance becomes lower, whereas the depletion capacitance remainsconstant.

Using the standard Mott-Schottky analysis [6,7], we extract the dopant density fromthese curves:

∂

∂Vg

(C−2

)= 2

qεsemiε0 NA A2 , (6.2)

where q is the elementary charge. The derived dopant densities are plotted as a functionof temperature in Fig.6.2 for several values of fmod . At low temperatures the extractedNA drastically rises and seemingly depends on the measurement frequency. We considerthis behavior of NA to be physically unrealistic, and it is taken to indicate that the use ofEq.6.2 is not justified for the entire temperature and frequency range. We will show belowwhy this is the case and what the criteria are for extraction of NA .

6.4 Equivalent circuit modelling

In Fig. 6.3a we plot the accumulation capacitance, CA, at Vg = −20 V as a function oftemperature for several modulation frequencies. Fig. 6.3b gives the corresponding phase,


$

Figure 6.2: NA as a function of temperature for different frequencies, as de-rived from the curves in Fig.6.1 using Eq.6.2.

A, at Vg = −20 V . Clearly, both CA and A have a frequency dependence. Themaximum slope of CA with temperature coincides with the maximum in A. We findthat the maximum of A is thermally activated with an activation energy of 0.36 eV, asshown in the inset of Fig.6.3a. We reason that the observed thermally activated behav-ior is due to the conductivity of the semiconductor layer. A similar argumentation wasused by Stallinga et al. in a study of pn junctions based on a poly(phenylene vinylene)derivative [8]. Furthermore, this value compares well with E A = 0.36 eV obtained frombulk conductivity experiments on P3HT [9]. The activation energy is usually interpretedas the distance of the Fermi level in the bulk of the semiconductor to a certain transportlevel higher up in the density of states [9–11]. The assumption of the thermally activatedconductivity allows us to model the data in accumulation for different temperatures witha simple equivalent circuit which is given in the inset of Fig.6.4. We describe the semi-conductor layer by its geometric capacitance, Cs , in parallel with the layer resistance, Rs .The geometric insulator capacitance is Cins and we add a contact resistance, Rc, for theITO and the gold top contact. This simple model yields a good fit to the experimental data(see Fig.6.4). The fitted values are given in Table 6.1. The obtained values for Cins andCs correspond to their geometrical values, and the conductivity derived from Rs variesfrom 1.8·10−10 S/cm at 250 K to 4.3·10−9 S/cm at 330 K, which are values comparableto results of bulk conductivity experiments [9].

The expression for the phase angle as a function of frequency is derived as:

A = arctan

(− [

1 + ω2 R2s Cs (Cins + Cs)

]ωCins

[ω2 Rc R2

s C2s + (Rc + Rs)

])

. (6.3)

From the equivalent circuit analysis we find that Rc is small and can be neglected. The

6.4 Equivalent circuit modelling 65

+]+]+]+]

$

$

+]+]+]+]

IPD[>+]@

7>.

@

Figure 6.3: (a) Capacitance of the P3HT MIS diode at Vg = −20 V as a func-tion of temperature for different frequencies. The inset shows thefrequency at which the phase angle, A, is at its maximum vs re-ciprocal temperature (b) A of the MIS diode at Vg = −20 V fordifferent frequencies.


$

PRG

Figure 6.4: A (at Vg = −20 V ) vs frequency for different temperatures. Thelines are fits to the data, using the equivalent circuit from the inset.The fit results are given in table 6.1.

Table 6.1: Values obtained by fitting the frequency dependence of theimpedance and its phase angle (see Fig. 6.4) at Vg = −20 V fordifferent temperatures. The equivalent circuit is given in the insetof Fig. 6.4. Rc is the contact resistance, Cins the insulator capaci-tance and Rs and Cs the semiconductor resistance and capacitancerespectively.

T [K] Rc[] Cins [nF] Rs[k] Cs [nF]250 72 ± 2 3.99 ± 0.03 320 ± 10 3.54 ± 0.04264 67 ± 2 3.91 ± 0.03 120 ± 4 3.62 ± 0.04294 76 ± 3 3.93 ± 0.03 24 ± 1 3.59 ± 0.06330 71 ± 3 3.98 ± 0.03 12.8 ± 0.6 3.53 ± 0.07

6.5 The relaxation time 67

frequency at which A reaches its maximum is derived as [8]:

ωmax = 2π fmax = 1

Rs

1√Cs (Cins + Cs)

, (6.4)

with a maximum value of A:

max = arctan

[−2√

Cs (Cins + Cs)

Cins

]. (6.5)

Thus max , in accumulation, is purely dominated by the geometric capacitance of thedevice. Using this model, we find max ∼ −70, which is close to the experimental valueof -72 degrees. For devices with different geometries, max is well described by Eq.6.5.At low frequencies the fits in Fig.6.4 start to deviate from the experimental data. This isdue to leakage currents through the MIS diode and can be accurately modeled with anadded leakage resistance parallel to Cins . The leakage resistance is ∼100 M, whichmeans that for fmod >25 Hz we can neglect its influence on the determination of NA .We note that the inclusion of the thermally activated conductivity in the semiconductorresistance Rs combined with a very simple equivalent circuit is sufficient to model boththe temperature and frequency behavior of the MIS diode.

6.5 The relaxation time

The erroneous results obtained with the Mott-Schottky analysis are in fact due to therelaxation time or RC-time, τ = RsCs , of the P3HT. The relaxation time causes theincrease of NA , shown in Fig.6.2. When the condition

1/ω τ. (6.6)

is no longer satisfied, the charge carriers can not follow the AC voltage anymore. As aresult, one measures a smaller capacitance of the MIS diode with increasing frequency,as shown in Fig.6.3a. As Rs increases with decreasing temperature, this restricts thefrequency region over which Eq.6.6 is fulfilled even further for lower temperatures. Thisis why we observe an apparent temperature dependent capacitance as shown in Fig.6.1.

We use our understanding of the temperature and frequency behavior to apply Eq.6.2in a frequency range where these effects are negligible. In this case the lower limit wastaken 25 Hz, and we took the upper limit in frequency one decade lower than the inverseof τ , for each temperature. For the present device, we extract a dopant density of 5.4·1015

cm−3, comparable to the value of 1·1016 obtained on P3HT by Brown et al [12].

6.6 Conclusions

In summary, we have fabricated and analyzed MIS diodes based on P3HT. We find thatthe Mott-Schottky analysis may not be used for extraction of the dopant density, NA , overthe entire range of temperature and frequency. This is due to the relaxation time of the


P3HT, ranging from τ = 1 · 10−3 s at 250 K to τ = 5 · 10−5 s at 330 K. The observedthermally activated behavior related to the relaxation time is attributed to the conductivityof the P3HT.

References

[1] Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69, 4108 (1996).

[2] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W.Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig and D.M.de Leeuw, Nature 401, 685 (1999).

[3] C.J. Drury, C.M.J. Mutsaers, C.M. Hart, M. Matters and D.M. de Leeuw, Appl. Phys.Lett. 73, 108 (1998).


[5] M.S.A. Abdou, F. P. Orfino, Y. Son, S. Holdcroft, J. Am. Chem. Soc. 119, 4518(1997).

[6] E.H. Nicollian and J.R. Brews, “MOS (Metal Oxide Semiconductor) Physics andTechnology”, Wiley, New York (1982).

[7] S.M. Sze, “Physics of semiconductor devices”, Wiley, New York (1981).

[8] P. Stallinga, H.L. Gomes, H. Rost, A.B. Holmes, M.G. Harrison and R.H. Friend, J.Appl. Phys. 89, 1713 (2001).

[9] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins and T.M. Klapwijk, Mat. Res. SocSymp. Proc., 660 (2001).

[10] A.V. Gelatos and J. Kanicki, Appl. Phys. Lett. 56, 940 (1990).


[12] P.J.Brown, Charge Modulation Spectroscopy of Poly(3-alkylthiophene), PhD thesis,Cambridge (2000).

Chapter 7

Photoimpedance spectroscopy ofpoly(3-hexyl thiophene)

metal-insulator-semiconductor diodes

Abstract

Capacitance-voltage characteristics of metal-insulator-semiconductor diodes with poly(3-hexylthiophene) (P3HT) as p-type semiconductor were investigated as function of time,ambient, and illumination. P3HT is rapidly doped upon exposure to both oxygen andlight. Changes of the acceptor density profiles with time were determined by using Mott-Schottky analysis of the capacitance-voltage characteristics. The profiles were determinedto be constant over the P3HT film thickness. Wavelength dependent photoimpedancemeasurements show that the acceptor creation efficiency peaks upon excitation of themolecular oxygen-polythiophene contact charge transfer complex at (1.9±0.1) eV.

72 Photoimpedance spectroscopy of P3HT MIS diodes

7.1 Introduction

Thin-film field-effect transistors based on p-type organic thiophene-based semiconductorsusually are unstable under ambient conditions. The conductivity of the organic semicon-ductor often increases under exposure of oxygen, light, or a combination thereof [1,2]. Asa result, the current modulation, or on-off current ratio, of discrete transistors decreaseswith time, the gain of logic gates gets less than unity and, consequently, logic circuitsstop functioning. As is described in Chapter 8, the acceptor density can be estimatedfrom the pinch-off voltage of discrete ring-type transistors, where the pinch-off voltageis the applied gate voltage at which the depletion region in the semiconductor becomesequal to the thickness of the semiconductor layer [3]. This reported analysis implicitlyassumes that the acceptor density is constant over the semiconductor layer thickness. Inthis work we determine the acceptor profile in the polymeric semiconductor layer, us-ing the Mott-Schottky analysis on measured capacitance-voltage (C − Vg) characteristicsof polymer-based metal-insulator-semiconductor (MIS) diodes. Because we used semi-transparent MIS diodes, changes in the impedance could be investigated as a function oftime upon exposure to oxygen and/or light. As a typical example we investigated poly(3-hexyl thiophene) (P3HT) MIS diodes. In order to elucidate the doping mechanism, wecompared the wavelength dependent changes in the photoimpedance measurements withthe absorption spectra of molecular oxygen-polythiophene contact charge transfer com-plexes as reported by Abdou et al. [2].

7.2 Experimental

The MIS diodes are fabricated on glass, using patterned transparent indium-tin-oxide gateelectrodes. A 300 nm layer of novolak R© photoresist is spincoated on top of the contactsand subsequently cross-linked upon baking at 150 oC. On top of this insulator a 200 nmthick film of P3HT is spun from a 1 wt% chloroform solution. Finally, a semi-transparant10 nm gold electrode is evaporated through a shadow mask. The area of the diodes rangesfrom 9 to 36 mm2. A schematic cross-section of the MIS diode is given in the inset ofFig.7.1. After processing, the sample is inserted in an Oxford CF1204 optical flowcryo-stat, with a vacuum better than 10−5 mbar. Impedance measurements are done using aSchlumberger 1260 impedance gain-phase analyzer, with a modulation frequency, fmod

of 137 Hz. This frequency is low enough to prevent artifacts in the impedance data due tothe low bulk charge carrier mobility of P3HT, as described in the previous chapter [4]. Thediode capacitance, C , then follows from the modulus of the impedance, Z , and the phaseangle, , by C = −sin/(2π fmod |Z |). For the wavelength dependent photoimpedancemeasurements the light of an Oriel 66058 tungsten-filament lamp is fed into a Jobin YvonH25 PLE monochromator. The resulting light beam is focused on the sample in the cryo-stat. The wavelength of the light is varied between 400 nm and 1100 nm. The powerprofile of the light incident on the MIS diode is measured. The number of incident pho-tons is calculated from the calibrated power profile of the lamp in combination with themonochromator. The absorption spectrum of a thin film of P3HT is measured and takenequal to the absorption of the P3HT layer in the diode. In the analysis we have assumed

7.3 Flat-band shift under oxygen exposure 73

LQV

J !

LQGDUN

PEDU2 PEDU2

PEDU2 PEDU2

LQOLJKWλ QPYDFXXP

∆

)%

Figure 7.1: Normalized C − Vg curves of a P3HT MIS diode in vacuum and af-ter 82 minutes exposure to a 230 mbar dry oxygen atmosphere. Thearrow indicates the direction of the shift of the curve upon oxygenexposure. The inset on the left shows a schematic cross section ofthe MIS diode. The inset on the right shows the flat-band voltageshift as a function of time, for different oxygen pressures, as well asa measurement in vacuum where the MIS diode is exposed to light(wavelength of 700 nm).

that any dispersion in the absorption of the semi-transparant gold electrode can be disre-garded. After exposure to oxygen and/or light in the cryostat, the P3HT MIS diodes arededoped by annealing the diodes for several hours in vacuum at 150 oC. This process canbe repeated, without apparent degradation of the P3HT [5]. Due to this dedoping proce-dure there are some variations in the dopant density at the beginning of each experiment.

7.3 Flat-band shift under oxygen exposure

The capacitance as a function of gate voltage for an undoped P3HT MIS diode in vacuumand dark is presented in Fig.7.1. Accumulating or depleting charge in the p-type P3HTsemiconductor changes the capacitance. The maximum capacitance is obtained in accu-mulation at high negative gate bias and is determined by the thickness of the insulator,dins , as:

Cins = εinsε0 A

dins, (7.1)

where εins is the relative dielectric constant of the insulator, ε0 is the permittivity of vac-uum, and A is the area of the device. At positive gate bias the semiconductor layer is


partially depleted. The depletion layer then acts as a capacitance in series with the insu-lator capacitance. The minimum capacitance is obtained when the whole film is depletedand is determined by the dielectric constants of the insulator and the semiconductor, εsemi ,and by the total distance between the electrodes. The steepness of the C − Vg character-istics of an MIS diode, when biased in the gate voltage range where the semiconductorlayer is partially depleted, is the result of the ease with which the semiconductor can bedepleted and is related to the acceptor density and its profile as a function of the depthinside the film. The C − Vg characteristics of Fig.7.1 are measured in the dark and invacuum. The measurements do not change with time in vacuum and dark. The acceptordensity and depth profile therefore do not change; P3HT is stable in vacuum and dark.

Subsequently, the diodes are measured in dark upon exposure to dry oxygen. Thepartial oxygen pressure is varied between 0.8 and 230 mbar. The capacitance-voltagecharacteristics are recorded every four minutes. The final measurement is included inFig.7.1. These two measurements can be shifted over the voltage axis on top of each other.Apparently, the dominant effect is a shift of the flat-band voltage, VF B , the applied voltageat which there is no band bending in the semiconductor at the semiconductor/insulatorinterface. The flat-band voltage shift is indicated in the insert of Fig.7.1. The increaseof VF B with time is dependent on partial oxygen pressure. Upon exposure to light invacuum the MIS diode C − Vg characteristics also show a flat-band shift (see the inset ofFig.7.1.), which slowly decreases again when the light is turned off. The flat-band voltageshift is likely due to electrostatic charging of the semiconductor/insulator interface. Thisinterpretation agrees with reported shifts of the transfer characteristics of P3HT field-effect transistors (see Chapter 8) [3].

7.4 Photoimpedance spectroscopy

The constant shape of the measurements shown in Fig.7.1 indicate that on the time scaleof the measurements (80 minutes) P3HT is stable, with only a flat-band voltage shift,in vacuum and light, and in oxygen in the dark. P3HT is unstable however upon expo-sure to both oxygen and light. The shape of the capacitance-voltage measurements thengradually changes. Normalized capacitance-voltage curves in 230 mbar of oxygen duringillumination at a wavelength of light, λ of 700 nm are presented in Fig.7.2, which alsoshows that the flat-band voltage shifts. The flat-band shift is difficult to quantify becausethe slopes of the C − Vg curves change as well. Experimentally we find that the changesin the slopes depend on the wavelength of the illumination (not shown). The slope of theC − Vg characteristics depends on the density and the depth profile of the acceptors. Fora quantitative interpretation we use the Mott-Schottky analysis [6, 7]. The capacitance ata certain gate bias corresponds to the depletion depth, ddepl , in the semiconductor layerby:

ddepl(Vg) ( 1

C(Vg) − 1

Cins

)ε0εsemi A (7.2)

Eq.7.2 holds when the contribution to the capacitance of interface states and minoritycarriers can be disregarded [6]. The acceptor density, NA , at a certain depletion depth can

7.4 Photoimpedance spectroscopy 75

λ! "

# $ $# $ $

%&%

LQV

'J(')

Figure 7.2: Normalized C−Vg curves as a function of time in 230 mbar oxygenunder illumination of light with a wavelength of 700 nm.

now be determined using the Mott-Schottky relation:

∂

∂Vg

(C−2

)= 2

qεsemiε0 NA A2, (7.3)

where q is the elementary charge. Combination of Eqs.7.2 and 7.3 yields the acceptor den-sity as a function of depletion depth in the semiconductor layer. We note that the acceptordensity profile follows from the shape of the C − Vg characteristics only. It does not de-pend on the value of Vg . Hence, in this analysis flat-band voltage shifts have no influenceon the profile. This profiling technique probes the holes associated with the acceptors,rather than the acceptors themselves [8]. The evaluated profile of the hole concentrationfollows the acceptor profile unless the acceptor profile varies spatially over distances lessthan the Debije length. This is the distance where the electric field emanating from anelectric charge falls off by a factor of 1/e:

L D =√

εsemiε0kB T

q2 NA, (7.4)

where kB is Boltzmann’s constant and T is the absolute temperature. Under flat-bandconditions the number of ionized acceptors is equal to the number of mobile holes. Theapplication of a gate bias results in an additional charge in the semiconductor layer. Thiscauses a rearrangement of the mobile holes, which shield the bulk of the semiconduc-tor from the induced charge. The shielding distance, or band-bending region is on theorder of the Debije length. Experimentally, this means that the Debije length limitationprevents accurate profiling closer than 3L D from the interface of the semiconductor with


$

GHSO

λ !

Figure 7.3: Acceptor density profile extracted from the C − Vg data of Fig.7.2,using Eqs.7.2 and 7.3, for an undoped P3HT MIS diode in vacuumand dark (filled squares) and after 60 minutes of exposure to 230mbar dry oxygen and illumination with λ=700 nm (filled circles).

the insulator [6, 8]. A typical acceptor density of about 1016 cm−3 results in a Debijelength of about 20 nm at room temperature. The acceptor profile information can thenreliably be determined starting from a depletion depth of about 60 nm from the semicon-ductor/insulator interface.

Acceptor densities in the dark and after illumination at a wavelength of 700 nmin 230 mbar O2 are presented in Fig.7.3 as a function of depletion depth. In the darkNA is constant over the P3HT film thickness and amounts to about 3·1015 cm−3. Uponillumination NA increases, but in first order is constant over the layer thickness. There isonly a slight increase in acceptor density at the top contact. Similar profiles were obtainedfor illuminations at other wavelengths.

The profiles with time can now be calculated from the temporal C−Vg characteristicsof Fig.7.2. We take the values of NA at a depletion depth of 100 nm and plot them inFig.7.4 as a function of time on a double logarithmic scale. The acceptor density roughlyfollows a power law dependence under light exposure of 700 nm and O2 exposure of230 mbar with an exponent of about 0.3. We note that typically, the conductivity of avariety of oligo- and polythiophenes increases with time as tα with α between 0.2 and0.5, under ambient conditions [9]. This suggests that the increase of the conductivity onthe timescale of the measurements is dominated by an increase in acceptor density.

A clue for the doping mechanism can be obtained from the wavelength dependentphotoimpedance measurements, which are given in Fig.7.5. In these photoimpedanceexperiments the MIS diode is exposed for two minutes to light of a certain wavelength in230 mbar O2. Then the C − Vg characteristics were recorded in the dark, with the oxygen

7.4 Photoimpedance spectroscopy 77

$

GHSO

Figure 7.4: Acceptor density at 100 nm depletion depth in the semiconductorlayer as a function of oxygen exposure and/or illumination

LQV

J

λ

λλλ!λλ

Figure 7.5: Normalized C − Vg curves as a function of the wavelength incidenton the sample in 230 mbar oxygen. The curves were recorded inthe dark after 2 minutes of light exposure at each wavelength.


! "#

$%" !λ

Figure 7.6: Relative acceptor density creation efficiency as a function of the in-cident illumination, for P3HT in 230 mbar O2 at room temperature.The absorption spectrum of P3HT is given for comparison.

pressure constant at 230 mbar O2. This procedure was repeated at several wavelengthsof light going from λ=1000 nm to λ=600 nm, in a consecutive measurement. Due to thelow absorption in the P3HT layer from λ=1000 nm up to λ=700 nm, we assume that thelight is uniformly absorbed in the film. For shorter wavelengths the absorption profileof the P3HT film will not be homogeneous anymore, but will result in a depth profile ofabsorption. We neglect this absorption profile in the measurements up to 600 nm.

The acceptor density is determined from Fig.7.5 at a depletion depth of 100 nm asa function of the photon energy. The derivative yields the change in acceptor densitywith photon energy, where we have assumed that absolute value of NA at the beginningof each measurement has no influence on the increase of NA . This number is correctedfor the power profile of the light, by dividing with the incident photon flux. The relativeacceptor creation efficiency as a function of photon energy is then obtained by dividingwith the absorbance, and is plotted in Fig.7.6. This efficiency, or cross section, indicatesthe ease with which acceptors are formed in P3HT under illumination in an oxygen at-mosphere. The efficiency peaks at (1.9 ± 0.1) eV. This value corresponds roughly to thereported absorption of the contact charge transfer complex between molecular oxygenand polythiophene at 1.97 eV [2].

7.5 Conclusions

In summary, we have investigated the instability of P3HT using semitransparent MISdiodes. We have measured and analyzed the capacitance-voltage characteristics of these

7.5 Conclusions 79

MIS diodes as function of time, ambient and illumination. On the time scale of the mea-surements in vacuum and light, and in oxygen in the dark, the P3HT MIS diodes onlyshow a flat-band voltage shift. However, upon exposure of the P3HT to both oxygenand light, the capacitance-voltage data show a clear increase of the acceptor density, asis demonstrated using Mott-Schottky analysis. The acceptor density profile is in first ap-proximation constant over the semiconductor film thickness. The wavelength dependentphotoimpedance measurements show that the acceptor creation efficiency peaks upon ex-citation of the molecular oxygen-polythiophene contact charge transfer complex.

References


[2] M.S.A. Abdou, F. P. Orfino, Y. Son and S. Holdcroft, J. Am. Chem. Soc. 119, 4518(1997).

[3] E.J. Meijer, C. Detcheverry, P.J. Baesjou, E. van Veenendaal, D.M. de Leeuw andT.M. Klapwijk, J. Appl. Phys. 93, 4831 (2003).

[4] E.J. Meijer, A.V.G. Mangnus, C.M. Hart, D.M. de Leeuw, T.M. Klapwijk, Appl. Phys.Lett. 78, 3902 (2001).

[5] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins and T.M. Klapwijk, Mat. Res. Soc.Symp. Proc., 660, JJ7.9.1 (2001).

[6] E.H. Nicollian and J.R. Brews, “MOS (Metal Oxide Semiconductor) Physics andTechnology”, Wiley, New York (1982).

[7] S.M. Sze, “Physics of semiconductor devices”, Wiley, New York (1981).

[8] D.K. Schroder, “Semiconductor Material and Device Characterization”, Wiley(1990).

[9] L. Luer, H.-J. Egelhaaf and D. Oelkrug, Optical Materials 9, 454 (1998).

Chapter 8

Dopant density determination indisordered organic field-effect

transistors

Abstract

We demonstrate that, by using a concentric device geometry, the dopant density and thebulk charge carrier mobility can simultaneously be estimated from the transfer charac-teristics of a single disordered organic transistor. The technique has been applied to de-termine the relation between the mobility and the charge density in solution-processedpoly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) thin-film field-effect transis-tors. The observation that doping due to air exposure takes place already in the dark,demonstrates that photo induced oxygen doping is not the complete picture.

84 Dopant density determination in organic transistors

8.1 Introduction

For the development of polymeric integrated circuits the stability of the characteristics ofthe organic semiconductor layer is an important issue. One of the limiting mechanismsis an increase in p-type doping due to a charge-transfer reaction with ambient molecularoxygen (see chapter 7) [1]. An increase of doping leads to an increase of the conductivityof the bulk semiconductor, which reduces the current modulation, or on-off ratio, of thetransistor [2, 3]. Here we will demonstrate that, by using a ring-type transistor geometry,we can directly estimate the dopant density and the bulk mobility from the transfer char-acteristics of a single disordered organic field-effect transistor (FET). This allows moni-toring of the dopant density change in time. The disentanglement of the dopant densityand bulk mobility in the bulk conductivity will be crucial to understand and counteractthe instabilities observed in polymeric transistors. As an example we discuss the dopantdensity increase under the influence of oxygen exposure in poly(2,5-thienylene vinylene)(PTV) and poly(3-hexyl thiophene) (P3HT) field-effect transistors.

8.2 Motivation and Realization

The field-effect behavior of disordered organic FETs has been described in terms of hop-ping of charge carriers in an exponential density of localized states by Vissenberg etal. [4]. The relative position of the Fermi level, EF , in the density of states (DOS),which determines the charge carrier mobility [4,5], is then dominated by the gate inducedcharge carriers, Ci Vg , where Ci is the insulator capacitance per unit area. Similar con-cepts have been used to understand the superlinear increase of the bulk conductivity withdoping [6, 7], where the relative position of EF is dominated by the dopant density, NA .

Disordered organic FETs typically operate in accumulation mode. This means thatthere is no depletion layer, which is present in inversion layer devices [8], that isolates theconducting channel from the semiconductor bulk. As a consequence, a low conductivityin the bulk layer is required for a large on-off current ratio [5], defined as the ratio ofcurrents at Vg = 0 V and -20 V. When the bulk conductivity is not negligible, we ex-pect a clearly observable crossover from field-effect dominated current to bulk dominatedcurrent in the transfer characteristics of a doped accumulation mode disordered organictransistor [9].

Conventional devices with an unshielded drain electrode suffer from parasitic cur-rents outside the transistor area [10, 11], which may obscure the crossover from field-effect dominated current to bulk dominated current. These parasitic currents arise dueto the unpatterned semiconductor layer. This can be circumvented by using a ring-typetransistor geometry, where the source electrode forms a closed ring around the transistorchannel, with the drain electrode, at which the current is monitored, in the center (seeinset of Fig.8.1).

In the experiments we used heavily doped Si wafers as common gate electrode, witha 200-nm-thick layer of thermally oxidized SiO2 as the gate-insulating layer. The SiO2was treated with hexamethyldisilazane to make the surface hydrophobic. With conven-tional lithography gold source and drain electrodes were defined, with a channel length,

8.2 Motivation and Realization 85

GV

J

YDFXXP

VHFLQPEDUDLU

PLQLQPEDUDLU

PLQLQPEDUDLU

PLQLQPEDUDLU

Figure 8.1: PTV FET transfer characteristics as a function of time in 10 mbarair, in dark, L = 10 µm, W = 2.5 mm. Clearly visible is thecrossover from a bulk depletion transistor to an accumulation tran-sistor. The inset shows a topview of the source-drain geometry ofthe ring-type transistors.

L, varying between 10 and 20 µm, and channel widths, W , of 1 mm and 2.5 mm. As alast step a 200 nm thick semiconductor layer is spincoated over the contacts. The semi-conductors used, are poly(2,5-thienylene vinylene), which is applied as precursor froma 0.5 wt% chloroform solution and subsequently formed in-situ by conversion at 150oCin vacuum [12], and poly(3-hexyl thiophene) which was spun from a 1 wt% chloroformsolution. All transfer characteristics in this study were measured in the linear operatingregime of the transistor, at source-drain voltage, Vds = −2 V , at room temperature, in thedark.

Fig.8.1 shows the transfer characteristics of a PTV thin-film field-effect transistor,with a ring-geometry. The initial curve is measured in vacuum after a thermal dedopingprocedure [13]. After this measurement 10 mbar of air is let into the chamber. The time atwhich the valve is opened to admit the air is denoted as t = 0. Subsequently, the evolutionof the transfer characteristics in air is monitored as a function of time.

The initial curve shows a characteristic p-type semiconducting behavior. At nega-tive gate voltages, Vg , holes are accumulated in the semiconductor at the semiconductor-insulator interface. These accumulated charges move under the influence of the lateralsource to drain field, resulting in the source-drain current, Ids . At positive Vg the holes aredepleted from the semiconductor layer and no mobile charges are left to carry the current.Upon prolonged exposure to air, in dark, we observe two changes in the transfer char-acteristics. Firstly, in the depletion regime of the transistor (at positive gate voltages) anadditional current appears, which increases with oxygen-exposure and time. This feature


can also be found in transfer characteristics of conventional device geometries, publishedby other groups [14–16]. Secondly, the onset of the field-effect, i.e. the switch-on voltage,Vso, which was defined in Chapter 2 as the gate voltage at which the transistor is at theflat-band condition [17], is shifted slightly to more positive gate voltages with respect tothe initial curve.

8.3 Interpretation and Analysis

The features reported in Fig.8.1 are readily understood when the transistors are consideredas accumulation mode FETs. When the conductivity of the bulk layer increases, an addi-tional current will flow in the bulk that is not modulated by the field-effect (schematicallydepicted in Fig.8.2a). The magnitude of this bulk current compared to the channel currentdetermines whether the gate electrode can still switch the FET between the on and the offstate, at Vg = 0 V and -20 V. An increase of the dopant density in the semiconductor,results in an effective bulk depletion transistor (similar to a junction field-effect transis-tor) in parallel to the accumulation field-effect transistor. If this kind of FET is driven farenough into depletion, eventually the entire film will be depleted of mobile charge and nomore current will flow. The voltage at which this happens is called the pinch-off voltage,Vpinch (see Fig.8.2c).

We argue that the occurrence of the clearly observable crossover from field-effectbehavior to bulk behavior in Fig.8.1 is due to the fact that in disordered semiconductingpolymers the mobility depends on the charge density. In a system where the mobility isconstant and the same for both field-effect and the bulk, this crossover is not observed[18].

8.3.1 Determination of the dopant density

From Vpinch , we can directly determine the dopant density, NA , provided that we correctfor the experimentally observed shift of Vso, as described in Section 8.5 below. The initialcurve, obtained after the thermal dedoping procedure and measured in vacuum, is takenas reference measurement, and its Vso is estimated at a current level of 1 pA. We cannow determine Vpinch , which we also extract at a current level of 1 pA, with respect toVso from the initial transfer characteristics. In the following analysis we assume that thedopant density in the initial curves is negligible. The error introduced in the analysis bydetermining Vpinch at 1 pA is small as the current drop typically is steep at that currentlevel.

For a small source-drain field we assume that the depletion of the semiconductortakes place uniformly over the entire channel length (see Fig.8.2b) and that the dopantsare uniformly distributed throughout the semiconductor layer1. The depletion layer widthin a doped semiconductor is given by [8]:

Wdepl = ε0εsemi

Ci

√

1 + 2C2i

(Vg − Vso

)q NAε0εsemi

− 1

, (8.1)

1Dopant uniformity was investigated in chapter 7

8.3 Interpretation and Analysis 87

D

E

F

LQVXODWRUJDWH

VHPLFRQGXFWRU

Figure 8.2: Schematic of an accumulation-mode FET, showing a p-doped semi-conductor: + indicates a positive charge in the semiconductor indicates a negatively charged counterion. (a) The transistor in ac-cumulation, the current is composed of the field-effect current andthe bulk current, resulting from the dopant density. (b) Develop-ment of a depletion region when a positive gate bias is applied.The extent of the depletion region is indicated by the dotted line.Here the current only flows in the undepleted bulk. (c) The film isfully depleted, no more current flows beyond pinch-off.


$

Figure 8.3: dopant density, extracted using Eq.8.4, vs time for several air pres-sures, in dark.

where ε0 is the permittivity of vacuum, εsemi the relative dielectric constant of the semi-conductor, q the elementary charge and A the active transistor area (length times thewidth). By using the insulator capacitance per unit area,

Ci = ε0εins

dins, (8.2)

and the semiconductor layer capacitance,

Csemi = ε0εsemi A

dsemi, (8.3)

we can recalculate Eq.8.1 for the pinchoff condition, Vg − Vso = Vpinch , at which thedepletion layer width is equal to the semiconductor layer thickness, dsemi , to:

NA = 2Vpinchε0

q

(d2

semiεsemi

+ 2dsemi dinsεins

) , (8.4)

where dins is the film thickness of the insulator layer, and εins is the relative dielectricconstant of the insulator.

The dopant density, derived using Eq.8.4, as a function of time at different air pres-sures, is plotted in Fig.8.3. We observe that the initial increase of NA upon exposure tooxygen is the biggest difference between the measurements at different pressures. In PTVthis initial increase occurs within two minutes after exposure, in the dark. For the P3HTthis is a much slower process in the dark, as can be seen from Fig.8.3. In most models

8.4 Results for PTV and P3HT 89

the charge-transfer reaction with molecular oxygen, requires exposure to light [1]. Themeasurements in the dark presented here, demonstrate that further analysis into the dop-ing mechanisms is required, and that the mechanisms differ quantitatively for differentmaterials.

8.3.2 Determination of the bulk mobility

At Vg = Vso there is no band bending at the semiconductor-insulator interface, no ac-cumulation and no depletion (see Fig.1.7a). The measured current at Vso must then beflowing in the bulk, through the entire film-thickness. We find that the current values atVso in the doped films typically vary linearly with the applied source-drain voltage, in-dicative of bulk ohmic behavior. From the current flowing at Vso we can calculate thebulk charge carrier mobility:

µbulk = L Ids

NAqWdsemi Vds

∣∣∣∣Vg=Vso

(8.5)

Knowing NA from Eq.8.4, we can determine µbulk using Eq.8.5. We remark that theshape of the curves in depletion can be described by modeling the curves with

Ids = W Vds NAqµbulk

L

(dsemi − Wdepl

), (8.6)

using the values obtained from Eq.8.4 and Eq.8.5.

8.4 Results for PTV and P3HT

As a typical result, the bulk mobilities of PTV and P3HT as a function of the dopantdensity are given in Fig.8.4. For both materials the dependence of µbulk on the dopantdensity is found to be roughly µ ∼ N2.3

A . For comparison the field-effect mobilities,extracted from the linear operating regime of the transistors, using

µF E = L

WCi Vds

∂ Ids

∂Vg, (8.7)

are also given. The charge accumulated at the semiconductor-insulator interface is cal-culated using Poisson’s equation, and the charge distribution in the semiconductor is ne-glected [19].

We find that the dependence of mobility on charge density is not the same for thebulk and the field-effect [20]. The µbulk − NA relation obtained here was also foundin studies at dopant densities of 1019-1020 cm−3 [2]. These observations suggest thatchemical doping of the film not only influences the relative position of the Fermi levelin the DOS. In fact, the presence of the dopant counterions will alter the DOS itself, aphenomenon well known from studies on amorphous silicon [21].

The bulk mobility of P3HT is found to be much higher than the bulk mobility of PTV(see Fig.8.4). We argue that this is due to a more ordered film in the case of P3HT [22],


EXON

)(

$

LQGXFHG

EXON

EXON

)(

EXON

EXON

EXON

EXON

)(

Figure 8.4: The bulk charge carrier mobility in PTV and P3HT estimated usingEq.8.5 vs the dopant density for several air pressures, in dark. Forcomparison also the field-effect mobility as a function of inducedcharge at the semiconductor-insulator interface of a transistor invacuum, is given for both materials.

which can result in a higher bulk mobility. From studies of PTV deposited using a dif-ferent precursor-route [2], we initially did not find the bulk features in the dark and airas described above. Only after additional thermal treatment, which did not change thefield-effect behavior, the bulk feature appeared. As the polymer did not degenerate due tothis thermal treatment, we conclude that a morphology change in the bulk of the polymerresults in an increased bulk mobility, and therefore in an added bulk contribution to thecurrent. Morphological differences between the semiconductor bulk and the semiconduc-tor/insulator interface will result in different relative contributions of the bulk current andthe field-effect current to the total source-drain current. The bulk current increases withtime due to the doping with oxygen. For a system with a low mobility in the bulk incombination with a high field-effect mobility, the influence of this bulk current increaseon the total transistor current can be neglected for a longer period of time than in a systemwith a high bulk mobility. Study of the bulk morphological aspects of disordered organicsemiconductors, and their influence on the stability of devices is therefore of importance.We remark that reducing the thickness of the semiconductor layer will also significantlyreduce the bulk current, which we observed experimentally by preparing semiconductorlayers with a thickness in the order of 10 nm. The results in Fig.8.4 demonstrate the powerof our experimental technique, which allows the disentanglement of the dopant densityand the bulk mobility in the bulk conductivity.

8.5 Shift of the switch-on voltage 91

GV

J

VR

GV

J

VR

GV

J

Figure 8.5: The transfer characteristics of Fig.8.1 are shifted over the voltageaxis, such that the field-effect behavior of the curves coincide. Theinsets shows (a) the data of Fig.8.1 on a linear scale and (b) thesame data shifted over the voltage axis on a linear scale.

8.5 Shift of the switch-on voltage

The data of Fig.8.1 can be replotted, by shifting the transfer characteristics along the Vg

axis. In this way the field-effect part of the characteristics can be perfectly aligned ontop of each other, as demonstrated in Fig.8.5 and Fig.8.6. This suggests that the shiftingof the curves can be described as a pure shift of the switch-on voltage. Assuming thatthis is a correct representation of the data, we can proceed to estimate the dopant densityfrom Vpinch as outlined above. The various measured shifts as a function of exposedair pressure are plotted in Fig.8.7, from which we conclude that Vso depends on thepolymer used.

We find that the shift of Vso is not directly related to the increase in dopant density.This is illustrated in Fig.8.8, where a P3HT transistor after a substantial time in air (5.8days) shows both the bulk current in the depletion regime and a Vso. Upon evacuationof the sample the current in the depletion regime is not discernable anymore after half anhour at 10−5 mbar, whereas the shift in Vso is still present. The shift can only be removedwith longer pumping times (see the inset of Fig.8.8). This demonstrates that the dopantdensity changes and Vso are two separate processes in the dark, and that the suggestedrelation between dopant density and threshold voltage reported in the literature [5, 18]is not valid for our devices. However, the P3HT data in Figs.8.3 and 8.7 suggest thatthe increase of Vso and NA are time-scale related. Based on the results of Fig.8.8 this isexpected to be an indirect relation.

The shift of Vso can not be explained by charge stored in the insulator, as we use high-


GV

J

VR

GV

J

,GV

>m$@

9JD9

VR

>9@

,GV

>m$@

9J>9@

Figure 8.6: (a) P3HT FET transfer characteristics in 1 bar air, after some lightexposures from a lamp, L = 20 µm, W = 1 mm. The inset showsthe same data on a linear scale (b) The data are shifted over thevoltage axis in such a way that the field-effect part of the curvescoincide. The inset shows the same data on a linear scale.

8.5 Shift of the switch-on voltage 93

VR

Figure 8.7: Switch-on voltage shift vs time for several air pressures, in dark.For P3HT this clearly is a large effect.

DIWHUGD\VLQEDUDLUDQGGDUNDIWHUPLQXWHVLQYDFXXPDIWHUGD\VLQYDFXXPDIWHUGD\VLQYDFXXP

GV

J

VR

3+7YDFXXP

Figure 8.8: Evacuation of a P3HT FET as a function of time. L = 10 µm, W =2.5 mm. The pinchoff voltage, observed in the initial curve (closedsquares), is not present anymore after half an hour of pumping at10−5 mbar (closed circles), whereas the Vso shift remains. Afterseveral days of pumping Vso has shifted towards zero. The insetshows the reduction of Vso with time under vacuum and dark.


quality thermally grown SiO2 in the FETs, which is unlikely to incorporate charge uponoxygen exposure at room temperature [10, 18, 23]. A dopant profile in the semiconductorlayer can not explain the Vso shift in these organic devices in contrast to standard silicondevices [8], because here we are not dealing with the formation of an inversion layer[17]. We suggest that the observed shifts of Vso are due to interfacial charging at thesemiconductor-insulator interface [10]. This is supported by the fact that we observedthese shifts with the same magnitude also in the characteristics of transistors with verythin semiconductor layers. The origin and nature of these charges are unclear at present.

8.6 Conclusions

In conclusion, we have shown that the dopant density as well as the bulk charge carriermobility in disordered organic semiconductors can be simultaneously determined fromthe transfer characteristics of a single transistor. This is possible due to a concentric devicegeometry, which excludes parasitic currents outside the transistor area and a mobilitydependence on the charge density in the polymer. For two organic semiconductors, P3HTand PTV, it was demonstrated that field-effect and bulk influences could be separatedbecause of a clear crossover from an accumulation transistor to a bulk depletion modetransistor. These polymers already exhibit a dopant density increase upon air exposure inthe dark, which requires a re-evaluation of the doping mechanism in terms of a charge-transfer reaction with oxygen under light exposure. We have argued that the morphologyof the polymeric semiconductor is of importance for the stability of the transistors. Theability to analyse dopant influences directly from transistor measurements is crucial tostudy instabilities and lifetime issues of polymeric transistors.

References

[1] M.S.A. Abdou, F. P. Orfino, Y. Son, and S. Holdcroft, J. Am. Chem. Soc. 119, 4518(1997).

[2] A.R. Brown, C.P. Jarrett, D.M. de Leeuw, and M. Matters, Synth. Met. 88, 37 (1997).

[3] G.H. Gelinck, T.C.T. Geuns, and D.M. de Leeuw, Appl. Phys. Lett. 77, 1489 (2000).

[4] M. C. J. M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998).

[5] G. Horowitz, R. Hajlaoui, H. Bouchriha, R. Bourguiga, and M. Hajlaoui, Adv. Mater.10, 923 (1998).

[6] B. Maennig, M. Pfeiffer, A. Nollau, X. Zhou, K. Leo, and P. Simon Phys. Rev. B 64,195208 (2001).

[7] H.C.F. Martens, PhD thesis, Charge transport in conjugated polymers and polymerdevices, Leiden University (2000).

[8] S.M. Sze, Physics of Semiconductor Devices Wiley, New York, (1981).

[9] For band transport, where the mobility does not depend on the charge density, thiscrossover is not expected to occur in the transfer characteristics [18], in which casethe analysis given here can not be applied.

[10] Y.-Y. Lin, D. J. Gundlach, S.F. Nelson, and T.N. Jackson, IEEE Trans. Elec. Dev.44, 1325 (1997).

[11] We expect that patterning of the semiconductor layer will give similar results as thering-geometry.

[12] A.J.J.M. van Breemen, J.J.A.M. Bastiaansen, B.M.W. Langeveld, J. Sweelssen,J.A.E.H. van Haare, P.T. Herwig, K.T. Hoekerd, and H.F.M. Schoo, Int. Display Re-search Conf. Palm Beach, U.S.A. SID 20, 327 (2000).

[13] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins, and T.M. Klapwijk, Mat. Res.Soc. Symp. Proc., 660, JJ7.9.1 (2001).

[14] G. Horowitz, F. Garnier, A. Yassar, R. Hajlaoui, and F. Kouki, Adv. Mater. 8, 52(1996).

96 References

[15] G.C.R. Lloyd, N. Sedghi, M. Raja, R. Di Lucrezia, S. Higgins. and W. Eccleston,Mat. Res. Soc. Symp. Proc. 708, BB10.57 (2001).

[16] T.N. Jackson, C.D. Sheraw, J.A. Nichols, J.-R. Huang, D.J. Gundlach, H. Klauk, andM.G. Kane, Int. Display Research Conf. Palm Beach, U.S.A. SID 20, 411 (2000).

[17] E.J. Meijer, C. Tanase, P.W.M. Blom, E. van Veenendaal, B.-H. Huisman, D.M. deLeeuw, and T.M. Klapwijk, Appl. Phys. Lett. 80, 3838 (2002).

[18] S. Scheinert, G. Paasch, M Schrodner, H.-K. Roth, S. Sensfuß, and Th. Doll, J. Appl.Phys. 92, 330 (2002).

[19] C. Tanase, E.J. Meijer, P.W.M. Blom, and D.M. de Leeuw, submitted.

[20] H. Sirringhaus, N. Tessler, D.S. Thomas, P.J. Brown, and R.H. Friend, Adv. SolidState. Phys. 39, 101 (1999).

[21] R.A. Street, Hydrogenated amorphous silicon, Cambridge University Press (1991).

[22] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W.Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig, and D.M.de Leeuw, Nature (London) 401, 685 (1999).

[23] S.J. Zilker, C. Detcheverry, E. Cantatore, and D.M. de Leeuw, Appl. Phys. Lett. 79,1124 (2001).

Chapter 9

Solution-processed ambipolar organicfield-effect transistors

Abstract

Progress towards electronics based on organic semiconductors is strongly dependent onthe successful interplay between functional chemical units and the development of well-performing electronic components. There is ample evidence that organic field-effect tran-sistors analogous to standard metal-oxide-semiconductor (MOS) transistors have reacheda stage that they can be industrialized [1–3]. Currently organic semiconductors are di-vided into two classes, electron-transporters and hole-transporters, or n-type and p-typematerials. This distinction is important for the design of light-emitting diodes [4] andsolar cells [5] where both type of carriers are needed. Monocrystalline Si technology islargely based on complementary MOS (CMOS) structures which use both n-type and p-type transistor channels. This complementary technology has enabled the construction ofdigital circuits which operate with high robustness, a low power dissipation and a goodnoise margin. For organic integrated circuits, there is an urgent need to find ways to-wards organic semiconductors which can be ambipolar, i.e. are capable of transportingboth types of carriers, while maintaining the attractiveness of easy solution-processing.We report on the fabrication of solution-processed ambipolar field-effect transistors andinverters based on a blend of two suitable organic semiconductors.

98 Ambipolar organic field-effect transistors

9.1 Introduction

Charge transport through amorphous, or poly-crystalline organic semiconductors is notfully understood, but a number of aspects are clear. It is possible to blend an active com-pound with a non-active medium without spoiling the conductive properties, indicative ofthe role of percolating conducting paths [6]. Secondly, depending on the electrode work-functions and the nature of the organic semiconductors, the contacts play a crucial role.The electrodes need to have a workfunction that allows injection of holes into the highestoccupied molecular orbital (HOMO) of the semiconductor and/or injection of electronsin the lowest unoccupied molecular orbital (LUMO). Therefore usually other contact ma-terials are chosen for electron-transporters and hole-transporters [7]. Concomitantly, oneelectrode material can be used in combination with two organic semiconductors, whenone has its HOMO level and the other has its LUMO level aligned with the metal work-function. In the past, this concept was used to construct heterostructure devices by evap-orating an n-type semiconductor layer on top of a p-type semiconductor layer, whichresulted in both n-type and p-type transistor operation [8, 9]. In this stacked geometry,the gate field needs to deplete the lower layer first to achieve accumulation in the toplayer. Also separate n-type and p-type transistors have been evaporated, which could becombined to CMOS integrated circuits [10]. Instead of evaporating the organic semicon-ductors sequentially, one would like to deposit both semiconductors in one easy process-ing step, as was demonstrated by Tada et al. [11]. The fact that two active compoundscan be mixed together in solution, opens up the possibility to deposit simultaneously twostrongly interpenetrating networks of percolating conducting paths, with both hole- andelectron-transporting capabilities.

The interpenetrating networks used in this work, are composed of hole- transport-ing poly[2-methoxy-5-(3’,7’- dimethyloctyloxy)]-p-phenylene vinylene (OC1C10-PPV)and electron-transporting [6,6]-phenyl C61-butyric acid methyl ester (PCBM). This mix-ture is typically used for organic photovoltaic cells research [5]. The molecular struc-tures of these materials are given in Fig.9.1. Gold electrodes were used as injectingcontacts. The choice of the semiconducting materials in combination with the injectingelectrodes is crucial. The energy levels that come into play are schematically representedin Fig.9.2. For simplicity, the energy levels are drawn as straight lines, but band bendingoccurs at the semiconductor-insulator interface upon an applied gate bias. This shifts theFermi level in the semiconductor, which in turn can result in band bending at the elec-trode/semiconductor interface. The HOMO level of OC1C10-PPV, at 5.0 eV, is alignedwith the workfunction of gold, at 5.1 eV, which will result in an ohmic contact for holeinjection from gold into the OC1C10-PPV-network. Due to the large bandgap of OC1C10-PPV gold is a blocking contact for electrons into OC1C10-PPV [7]. The alignment of thegold workfunction with the LUMO level of the electron-transporter PCBM is not as good,and the mismatch in energy levels results in an injection barrier, φB, of 1.4 eV for electroninjection into the PCBM network. However, this injection barrier is significantly reducedto 0.76 eV, due to the formation of a strong interface dipole layer at the Au/PCBM in-terface [12]. Similar behavior has been observed for the Au/C60 interface by ultravioletphotoemission spectroscopy [13].

In general, the width of an injection barrier can be narrowed application of a large

9.1 Introduction 99

Figure 9.1: Schematic cross-section of the field-effect transistor geometry usedin this study, the molecular structures of PCBM and OC1C10-PPV,and an artist impression of the interpenetrating networks of the twosemiconductors.


Figure 9.2: Device band diagram of interpenetrating networks of OC1C10-PPVand PCBM in contact with Au electrodes, when no biases are ap-plied to the transistor. For simplicity the energy levels are drawnas straight lines. It should be noted that band bending due to anapplied gate voltage can reduce the barrier for electron injectioninto the PCBM network.

9.2 Experimental 101

source-drain field, or by accumulation of high charge densities in the semiconductor film,for instance through doping or by the field-effect [14]. A narrow injection barrier willallow tunnelling of charge carriers from the electrode to the semiconductor.

9.2 Experimental

In the experiments we used heavily doped Si wafers as the gate electrode, with a 200nm SiO2, grown via thermal oxidation, as the gate-insulating layer. Using conventionallithography, gold source and drain contacts were defined with a channel width W of 1 mmand length L of 40 µm. The SiO2 layer was treated with the primer hexamethyldisilazane,which makes the surface hydrophobic. The transistors were completed by spinning asolution of PCBM and OC1C10-PPV (4:1 by weight), with a 0.5% weight content inchlorobenzene. Prior to spincoating, the solution was stirred for one hour at 80oC. Thecompleted devices were annealed in vacuum for 15 hours at 90oC. The films were inves-tigated with atomic force microscopy, showing the same surface morphology as reportedby Shaheen et al. [15] for the same mixture. This indicates that the constituents are uni-formly mixed. The electrical measurements were performed at room temperature in avacuum of 10−5 mbar. A schematic cross-section of the transistors is given in Fig.9.1.The semiconductor layer of transistors with PCBM only was spincoated from a 1 wt%PCBM solution in chlorobenzene, while the semiconductor layer of OC1C10-PPV wasspun from a 0.4 wt% OC1C10-PPV solution in toluene or chlorobenzene. We note thatthe wetting behavior of the PCBM and the OC1C10-PPV on the substrates was very differ-ent, with the OC1C10-PPV solutions forming uniform films, whereas the PCBM solutionswhere difficult to deposit in a uniform film. The transfer characteristics of a transistorwith gold electrodes and PCBM as the semiconductor show good electrical performance,and a field-effect mobility was determined of 10−2 cm2/Vs at a gate voltage, Vg = 20 V ,demonstrating that the energy level mismatch between gold and PCBM can be overcomewith the field-effect.

9.3 Ambipolar transistor operation

Typical output characteristics of a field-effect transistor based on the OC1C10-PPV:PCBMblend (see Fig.9.3) demonstrate operation both in the hole-enhance-ment and electron-enhancement mode. For high negative gate voltages, Vg , the transistor is in the hole-enhancement mode and its performance is identical to a unipolar transistor based onOC1C10-PPV, with a field-effect mobility of 7·10−4 cm2/Vs at Vg = −20 V , which wasextracted from the transfer characteristics of Fig.9.4. For low gate voltages and high drainvoltages, Vds , the current shows a pronounced increase with Vds (see Fig.9.3a), whichis typical of an ambipolar transistor and not present in the unipolar transistor based onOC1C10-PPV. At positive Vg , the transistor operates in the electron-enhancement mode(Fig.9.3b), with a field-effect mobility of 3·10−5 cm2/Vs at Vg = 30 V , two orders ofmagnitude lower than the electron mobility in the PCBM-only transistor. At low drainvoltages we observe a non-linear current increase, indicating that there is a barrier for


9

9

9

9

9

9J 9

GV

GV

9

9

99

9

9

9J 9

GV

GV

Figure 9.3: Output characteristics of the OC1C10-PPV:PCBM ambipolar tran-sistor operating in (a) hole-enhancement, and in (b) electron-enhancement mode.

electron injection from gold into PCBM, which is much more pronounced than in a unipo-lar PCBM transistor, where the output characteristics look qualitatively similar to those ofFig.9.3a1. At low gate voltages and high drain voltages, we again observe a pronouncedincrease in current, typical of an ambipolar transistor, and which is not observed in theunipolar PCBM transistor.

The current increase can readily be understood, when considering that under certainbiasing conditions both holes and electrons are accumulated in the transistor channel,forming a pn-junction [16].

1The super-linear output characteristics at low Vds observed in the blend transistor, indicate an injectionproblem from the Au into the PCBM, which is probably due to the selective wetting of PCBM and OC1C10-PPV on gold, which can also account for the low electron mobility in the blend transistor as compared to thePCBM-only transistor.

9.3 Ambipolar transistor operation 103

9

9

9

9

9

9GV 9

GV

J

9

9

9

9GV 9

GV

J

Figure 9.4: The transfer characteristics of the OC1C10-PPV:PCBM ambipolartransistor. (a) For Vg < 0 V only the hole contribution is ob-served in the current, whereas for Vg > 0 V the electron currentis seen.(b) For Vg > 20 V only the electron contribution to thecurrent is observed. Depending on the value of Vds however, thehole current-contribution is already observed for Vg < 20 V . Theassymetry in electron and hole contributions to the total currentin (a) and (b) is due to the larger field-effect mobility for holes ascompared to the electrons.


K H

Figure 9.5: Schematic representation of the operation of the ambipolar transis-tor. a) Hole accumulation at the semiconductor insulator interfacefor a large negative Vg and small negative Vd. b) When the condi-tion Vd ≤ Vg−Vso is reached, an electron accumulation region willgrow at the drain electrode, while at the same time, holes are stillaccumulated around the source electrode. The two accumulationlayers will form a pn-junction in the channel.

This is schematically represented in Fig.9.5. For a large negative Vg and a small neg-ative Vds the transistor behaves as a unipolar hole accumulation transistor (Fig.9.5a)). Ifin a unipolar device Vds is increased beyond Vds = Vg − Vso, where Vso is the switch-onvoltage of the field-effect2, a depletion region around the drain will develop and the draincurrent saturates. However for an ambipolar transistor, electrons will start to accumulateat the drain electrode. This electron accumulation region forms a pn-junction in the chan-nel with the hole accumulation region at the source electrode (Fig.9.5b)). This electronaccumulation region is responsible for the observed current increase at high Vds . Whenthe transistor is biased in the hole-accumulation mode, the current can be described by amodel based on hopping of charge carriers in an exponential density of states (DOS) aswas described in chapter 2 [17, 18]:

Ids = Ah WCi

L

([[−Vg + Vso]]2 TDOS,h

T − [[−Vg + Vso + Vds]]2 TDOS,h

T

), (9.1)

where Ah is a prefactor for the hole current [17, 18], Ci the insulator capacitance per unitarea, TDOS,h is the width of the exponential DOS for holes [18], and [[x]] = 1

2 x + 12 |x |.

For a complete description of the electron accumulation mode, we should consider theinjection-limited current. This has recently been modelled by thermally assisted hop-ping from the electrode into the localized states of the organic semiconductor, which arebroadened due to disorder [20]. For the sake of simplicity, however, we will neglect the

2We defined the switch-on voltage as the gate voltage that needs to be applied to reach the flat-band condition(see Chapter 2) [17]

9.4 CMOS-like inverter operation 105

observed injection barrier for electrons here. Then, in the electron accumulation mode,the current can be described analogous to Eq.9.1:

Ids = −AeWCi

L

([[Vg − Vso

]]2 TDOS,eT − [[

Vg − Vso − Vds]]2 TDOS,e

T

), (9.2)

where Ae is a prefactor for the electron current, and TDOS,e is the width of the exponentialDOS for electrons. Under bias conditions where both holes and electrons accumulate inthe channel, the transistor can be described by two transistors in series, one of lengthLh where only holes accumulate and one of length Le where only electrons accumulate.The source-drain current can readily be calculated from the condition of current continuityacross the pn-junction and the relation Le + Lh = L. From these considerations it followsthat the total current is simply the sum of the source-drain currents in Eq.9.1 and Eq.9.2.The ambipolar transistor can be represented as a p-type and n-type transistor, both withlength L, connected in parallel. Here it is implicitly assumed that charge transfer acrossthe pn-junction is not a limiting factor in the device performance.

9.4 CMOS-like inverter operation

Due to the fact that both polarities of charge can be induced in the transistor, comple-mentary logic is possible. An inverter based on two identical ambipolar transistors wasconstructed, with a common gate as the input voltage, VI N . This device demonstratesCMOS-like inverter operation (see Fig.9.6). A high gain of 10 is easily achieved (thesteepness of the inverter characteristic), in combination with a good noise margin (theposition of the voltage switch in the inverter characteristic). For unipolar logic this com-bination is very difficult to achieve. Depending on the polarity of the supply voltage, VD D,the inverter works in the first or the third quadrant of Fig.9.6, which is a particular featureof the ambipolar transistor-based inverter, as unipolar logic works only in one quadrant.Furthermore, we see a small dependence of the output voltage, VOU T [21], at low andhigh values of the input voltage, VI N , which is normally not observed in CMOS-basedinverters. This change of VOU T is a direct result of the fact that both transistors that makeup the inverter can be considered as a parallel circuit of an n-type and a p-type transistor.

Using the sum of Eq.9.1 and Eq.9.2 for the ambipolar current, the inverter charac-teristics can already qualitatively be modelled, by using the various biasing conditions inthe inverter, and the ratio of the extracted field-effect mobilities for electrons and holes.From the modelling of the p-channel behavior with Eq.9.1 on the data of Fig.9.3, we findTDOS,h = 530 K , we assume TDOS,e = TDOS,h, and we neglect the injection-limitedcurrent observed for the n-channel. The inverter characteristics calculated in this way areplotted as solid lines in Fig.9.6. We note that, although we have taken a very simple modelfor the ambipolar transistor behavior, we already get a very reasonable description for theinverter characteristics, including the change in VOU T .


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Figure 9.6: Schematic representation and transfer characteristics of aCMOS-like inverter based on two identical ambipolar OC1C10-PPV:PCBM field-effect transistors. Depending on the polarity ofVD D, the inverter works in the first or the third quadrant. The fulldrawn lines are modeled on the basis of Eq.9.1 and Eq.9.2.

9.5 Conclusions

In conclusion, we have demonstrated ambipolar thin-film field-effect transistors, basedon interpenetrating networks of the solution-processed organic semiconductors OC1C10-PPV and PCBM. Complementary logic based on these ambipolar transistors has the po-tential to significantly simplify the design and improve the operation of organic integratedcircuits, while keeping the technology as simple as for unipolar logic.

References

[1] H.E.A. Huitema, G.H. Gelinck, J.B.P.H. van der Putten, K.E. Kuijk, C.M. Hart, E.Cantatore, P.T. Herwig, A.J.J.M van Breemen and D.M. de Leeuw, Nature (London)414, 599 (2001).

[2] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280, 1741 (1998).


[4] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend,P.L Burn and A.B. Holmes, Nature 347, 539 (1990).

[5] G. Yu, J. Gao, J.C. Hummelen, F. Wudl and A.J. Heeger, Science 270, 1789 (1995).

[6] C.Y. Yang, Y. Cao, P. Smith and A.J. Heeger, Synth. Met. 53, 293 (1993).

[7] P.W.M. Blom, M.J.M. de Jong and M.G. van Munster, Phys. Rev. B. 55, R656 (1997).

[8] A. Dodabalapur, H.E. Katz, L. Torsi and R.C. Haddon, Science 296, 1560 (1995).

[9] A.Dodabalapur, H.E. Katz, L. Torsi and R.C. Haddon, Appl. Phys. Lett. 68, 1108(1996).

[10] B. Crone, A. Dodabalapur, Y.-Y. Lin, R.W. Filas, Z. Bao, A. LaDuca, R. Sarpeshkar,H. E. Katz and W. Li, Nature 403, 521 (2000).

[11] K. Tada, H. Harada and K. Yoshino, Jpn. J. Appl. Phys. 35, L944 (1996).

[12] J.K.J. van Duren, V.D. Mihailetchi, P.W.M. Blom, T. van Woudenbergh, J.C. Hum-melen, M.T. Rispens, R.A.J. Janssen and M.M. Wienk, submitted.

[13] S.C. Veenstra, A. Heeres, G. Hadziioannou, G.A. Sawatzky and H.T. Jonkman,Appl. Phys. A, 75, 661 (2002).

[14] S.M. Sze, ”Physics of Semiconductor devices” (Wiley, New York, 1981).

[15] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz and J.C. Hum-melen, Appl. Phys. Lett. 78, 841 (2001).

[16] G.W. Neudeck, H.F. Bare and K.Y. Chung, IEEE Trans. Electr. Dev. 34, 344 (1987).

108 References

[17] E.J. Meijer, C. Tanase, P.W.M. Blom, E. van Veenendaal, B.-H. Huisman, D.M. deLeeuw and T.M. Klapwijk, Appl. Phys. Lett. 80, 3838 (2002).


[19] C. Detcheverry and M. Matters, Proc. ESSDERC, 328 (2000).

[20] T. van Woudenbergh, P.W.M. Blom, M.C.J.M. Vissenberg and J.N. Huiberts, Appl.Phys. Lett. 79, 1697 (2001).

[21] K.Y. Chung, G.W. Neudeck and H.F. Bare, IEEE J. Solid-State Circ. 23, 566 (1988).

Chapter 10

Ambipolar field-effect transistors basedon a single organic semiconductor

Abstract

With the coming of age of organic semiconductors, envisioned applications in the areaof light-emitting diodes, plastic solar cells and integrated circuits are becoming a reality.The promise of low-cost flexible integrated circuits for high volume applications such aselectronic paper, demands a flexible semiconductor layer, preferably a polymer, whichfor ease of processing should be deposited from solution, via for instance spincoatingor inkjet technology. For organic integrated circuits an important improvement in circuitperformance is the step from unipolar logic, which uses either p-type or n-type transistors,to complementary metal-oxide-semiconductor (CMOS) logic, which uses both n-type andp-type transistor channels. Here we expand the work on blends described in the previouschapter. We report on the fabrication of ambipolar field-effect transistors and invertersbased on the solution-processed organic semiconductor poly(3,9-di-tert-butylindeno[1,2-b] fluorene) (PIF).

110 Ambipolar transistors based on a single organic semiconductor

10.1 Introduction

Polymer electronics up to now is typically based on unipolar logic, mainly because ofthe choice of the electrodes in combination with a large band gap semiconductor. For theoperation of light-emitting diodes and solar cells based on organic semiconductors it isessential that both electrons and holes can flow through the semiconductor layer underan applied electric field, and that they can be collected in or emitted from the electrodes.This poses requirements on the choice of semiconductor layer in combination with theelectrode materials used for the anode and the cathode.

In organic solar cells, typically a blend of an electron-transporting and a hole-trans-porting semiconductor is used. Upon illumination, excitons are formed in the blend,which can be separated in electrons and holes by means of an applied electric field. Theelectron transporter and the hole transporter provide a conduction path for the electronsand holes to the electrodes, thus converting light into electrical current. In polymericlight-emitting diodes, a high workfunction metal is chosen as the anode for hole injectionbased on the alignment of its workfunction with the highest occupied molecular orbital(HOMO) of the organic semiconductor, and a low workfunction metal is chosen as thecathode for electron injection based on its workfunction alignment with the lowest un-occupied molecular orbital (LUMO) of the organic semiconductor. Due to the typicallylarge bandgap (>2 eV) of the polymers used in light-emitting diodes, which is requiredfor light emission in the visible range of the spectrum, the anode will be a blocking con-tact with an injection barrier for electrons, and the cathode a blocking contact for injectionof holes. Upon application of an electric field across the polymeric semiconductor, elec-trons and holes flow towards each other and form excitons, which can recombine underemission of light.

To fabricate ambipolar organic transistors, which operate as either n-channel or p-channel transistors, the concepts of the light-emitting diode and the solar cell are useful,and in this context we already demonstrated that a blend of suitable chosen n-type and a p-type semiconductors, deposited from solution, in combination with Au electrodes resultsin ambipolar transistor operation (see previous chapter) [2]. In that case two materialswere used where one has its HOMO level aligned with the Au workfunction and theother its LUMO level. The main difficulty in achieving ambipolar transistor operationin a single organic semiconductor is to inject both holes and electrons from the sameelectrode. A good contact for one polarity of charge typically results in an injection barrierfor the other polarity of charge. However, this injection barrier can be reduced by usinga material with a smaller energy gap. Furthermore, the width of an injection barrier canbe narrowed by applying a large source to drain field, or by accumulation of high chargecarrier densities in the semiconductor film by means of the field-effect [3]. For sufficientlyhigh amounts of accumulated charge, the injection barrier becomes small enough to allowtunneling from the electrode into the semiconductor. Next to the requirements of a smallband gap also the semiconductor purity is of importance in order to minimize trappingeffects.

10.2 Experimental 111

Figure 10.1: Schematic cross-section of the FET geometry used in this study.The molecular structure of PIF is given.

10.2 Experimental

As low band gap organic semiconductor in the ambipolar transistors we have used poly(3,9-di-tert-butylindeno[1,2-b] fluorene) (PIF) [4] which has a band gap of 1.55 eV. Gold, witha workfunction of 5.1 eV, is used for the source and drain injecting contacts. In the exper-iments we use heavily doped Si wafers as the gate electrode, with a 200-nm-thick-layer ofthermally oxidized SiO2 as the gate-insulating layer. Using conventional lithography, thegold source and drain interdigitated contacts are defined with a channel width W of 2 cmand length L of 10 µm. The SiO2 layer is treated with the primer hexamethyldisilazane.The transistors are completed by spinning a solution of 1wt% PIF in chloroform onto thesubstrate. The molecular structure of PIF and a schematic cross-section the transistor ge-ometry are given in Fig.10.1. The measurements are performed in a vacuum of 10−4 mbarat room temperature, after annealing the sample for an hour at 90 oC.

10.3 The PIF ambipolar transistor and inverter

Typical output characteristics of the PIF field-effect transistor (see Fig.10.2) demonstratethat the transistor operates both in the hole-enhancement and electron-enhancement mode.For high negative gate voltages, Vg , the transistor is in the hole accumulation mode(Fig.10.2a), with a small injection barrier for holes, which is visible via the non-linearoutput characteristics at low Vds . At high positive Vg electrons are accumulated in thesemiconductor at the semiconductor-insulator interface (Fig.10.2b). Also for the elec-trons we observe a small injection barrier from gold to PIF. From the transfer characteris-


J

GV

µ

GV

J

GV

µ

GV

Figure 10.2: The output characteristics of the PIF ambipolar transistor, oper-ating in (a) hole-enhancement, and in (b) electron-enhancementmode.

10.3 The PIF ambipolar transistor and inverter 113

GV

J

GV

GV

GV

J

GV

GV

Figure 10.3: The transfer characteristics of the PIF ambipolar transistor. (a)For Vg < −15 V only the hole contribution is observed in thecurrent, whereas for Vg > −15 V the electron current is seen. (b)For Vg > 10 V only the electron contribution to the current is ob-served. The hole current-contribution is observed for Vg < 10 V .


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Figure 10.4: Transfer characteristics of a CMOS-like inverter (see inset) basedon two identical ambipolar PIF-endcapped (see inset) field-effecttransistors. Depending on the polarity of the supply voltage, VD D,the inverter works in the first or the third quadrant.

tics in the linear operating regimes of the transistor (see Fig.3) we calculate the field-effectmobility using:

µF E(Vg) = L

WCi Vds

∂ Ids

∂Vg(10.1)

For the holes (at Vg = −30 V ) and electrons (at Vg = 30 V ) we find a µF E of4·10−5 cm2/Vs and 5·10−5 cm2/Vs respectively. We find that in the present transistor theonset of the field-effect for the electron accumulation was at Vg = 10 V , and for the holeaccumulation at Vg = −15 V (see Fig.10.3).

The combination of a single material for the source and drain electrodes and a singlematerial for the semiconductor layer, with evenly matched field-effect mobilities for theholes and electrons allows the most simple fabrication of a logic voltage inverter. Inverteroperation is observed for two identical PIF ambipolar transistors connected in accordancewith the schematic diagram given in the inset of Fig.10.4. The ambipolar inverter operatesin the first and the third quadrant, depending on the applied supply voltage, VD D. Thisbehavior was also observed for ambipolar inverters based on a blend of n-type and p-typesemiconductors (see previous Chapter) [2].

To demonstrate that the ambipolar behavior observed in FETs based on PIF, is notparticular to the semiconductor used here, but is more widely applicable to other semicon-ductors, we also performed measurements on a solution-processed pentacene film. Thesample was measured in vacuum after an anneal of 10 hours at 90oC, and showed electronenhancement at high applied positive Vg , as is shown in Fig.10.5. The electron current ishigher than the observed gate currents in the transistor. Also, the electron current is onlyobserved after thermal anneal in vacuum and is not observed anymore if the sample is

10.4 Conclusions 115

GV

J

GV

GV

Figure 10.5: Transfer characteristics of a pentacene FET in vacuum, demon-strating electron enhancement currents.

exposed to air. This demonstrates that there are organic semiconductors that can achieveambipolar transistor operation.

10.4 Conclusions

In conclusion, we have demonstrated ambipolar field-effect transistors based on the solution-processed organic semiconductor poly(3,9-di-tert-butylindeno[1,2-b] fluorene) and alsofor solution-processed pentacene. Both charge carrying moieties are injected into theorganic semiconductor from gold electrodes, and the semiconductor is processed fromsolution, which is an important prerequisite for low-cost polymer electronics. FunctionalCMOS-like inverters have been demonstrated. This result paves the way for CMOS tech-nology based on a single solution-processable organic semiconductor.

References

[1] H. Reisch, U. Wiesler, U. Scherf, N. Tuytuylkov, Macromolecules, 29, 8204 (1996).

[2] E.J. Meijer, D.M. de Leeuw, S. Setayesh, E. van Veenendaal, P.W.M. Blom, J.C.Hummelen, T.M. Klapwijk, submitted

[3] S.M. Sze ”Physics of Semiconductor devices”(Wiley, New York, 1981).

[4] H. Reisch, W. Wiesler, U. Scherf and N. Tuytuylkov, Macromolecules 29, 8204(1996).

Summary

Charge transport in disordered organicfield-effect transistors

Nowadays, plastics are used in a large variety of applications, ranging from packag-ing materials to toys, and from furniture to plastic bags. In recent times a lot of effort isput also in plastics with electronic and optical functionality. This paves the way for “plas-tic electronics”, such as flexible, flat displays, or cheap electronic barcodes. The plastics,or organic polymers, that are being used for such applications need to be able to conductcurrent, i.e. facilitate charge carrier motion. In this thesis we address the question of howcharge carriers move in an organic semiconducting polymer film. The experimental studyis performed on field-effect transistor structures, where the semiconducting polymer liesbetween two electrodes, and on top of an insulating layer. By applying a current betweenthe two electrodes, current flows through the organic semiconductor. This current can bemodulated by a third electrode, which lies under the insulating layer. this third electrode,termed “gate”, can modulate the current by changing the density of charge carriers in thesemiconducting polymer. Therefore, the charge transport can be studied as a function ofthe charge carrier density in the field-effect transistor.

Of importance for the charge transport is the disorder in the semiconducting polymerlayer. This disorder is described in theoretical models by a spatial and energetical distri-bution of energy states. Furthermore, the charge carriers themselves give rise to locallattice deformations, through which they localize in the polymer film. The charge carrierscan move in the polymer film under the influence of an applied electric field, by hoppingbetween two neighbouring energy sites. The energy required to hop is obtained from thelattice vibrations, i.e. phonons, of the polymer.

In chapter 2 two parameters, that are used for the description of the field-effect tran-sistor, are critically evaluated. It is argued that the characterization parameters used instandard silicon technology, are not useable for disordered organic transistors. Therefore,we define a new physical parameter, the “switch-on” voltage of the transistor, as the volt-age that needs to be applied to the gate electrode to have exactly no charge accumulationin the semiconductor. To test the suitability of this parameter, we measured the current-voltage characteristics of transistors, based on a variety of organic semiconductors, as afunction of temperature. These characteristics are theoretically modeled using a descrip-tion based on hopping between sites in the polymer. From the modelling we conclude thatthe “switch-on” voltage is a useful physical parameter for the description of organic tran-sistors. In the second paragraph, the mobility of the charge carriers, which is a measureof how easy the carriers can move in the polymer film, is analyzed and determined as afunction of the charge density distribution in the transistor. We conclude that the average

120 Summary

mobility of charges directly at the interface with the insulator layer is a useful parameterin the description of the transistors. In the last paragraph of chapter 2 we link the mobilityobtained from the transistors to the mobility as it is determined in light-emitting diodes.We conclude that the large differences in mobility obtained for a single polymeric semi-conductor in these two devices is mainly due to a large difference in charge carrier densityin the two devices.

In chapter 3 the mobility of the charge carriers is measured and analyzed as a func-tion of temperature. From these data we observe an empirical rule, termed the Meyer-Neldel rule. Associated with this rule is a characteristic energy, which has a very similarvalue for all the studied materials, which implies a common origin of this energy in thecharge transport mechanism. It is argued that a possible origin of the characteristic energyis due to the fact that a charge carrier can require multiple phonons to hop in the polymerfilm, rather than just one phonon. In chapter 4 this empirical rule is also related to theexperimentally observed electric field dependence of the mobility.

For practical applications, it is not only of importance to understand the charge trans-port through the polymer film, but also to gain insight in how the charge carriers are in-jected into the polymer film. The electrodes that we used in this study consist of gold. Thecharges do not go from the gold to the polymer layer without resistance. To get an ideaof the resistance involved in this injection process, in chapter 5 we looked at series oftransistors, where the length of the transistor is systematically varied. In this way we canseparate the charge transport process in the polymer film from the charge injection processat the electrodes. We find a correlation between the injection resistance and the mobilityin the polymer itself and interpret this by considering injection from the electrode into anenergetical distribution of states in the polymer.

Certain polymers, with promising properties for applications, also have their draw-backs in terms of air and light stability. In chapters 6 and 7 we look at the polymerpoly(3-hexyl thiophene), which has a conductivity that increases with time under the in-fluence of light and oxygen exposure. This is the result of a doping process where anincreasing amount of charge carriers is present in the polymer layer, that contribute tothe conductivity. In chapter 6 we look first at impedance analysis techniques applied tometal-insulator-semiconductor diodes (MIS diodes), to be able to determine the amountof dopants in the polymer. We find that the measurement frequency used in the impedancetechnique can not be too high, because otherwise the low mobility of the charge carriersin the polymer will dominate the measurement results instead of the dopant density. Inchapter 7 this technique is applied to determine how efficient the dopant charges form un-der the influence of light and oxygen. In chapter 8 we demonstrate that the dopant densitychanges can also directly be determined from field-effect transistor measurements. Fur-thermore we are able to relate the mobility of charge carriers in the polymer bulk to thedopant density.

In the previous chapters we have mainly looked at unipolar p-type transistors, whichmeans: transistors in which predominantly positively charged carriers move. In chapter9 we show that a suitable combination of the electrode material with a mixture of a p-typeand an n-type semiconductor can be used to construct ambipolar field-effect transistors.These transistors enable a simplification of the circuit design of integrated circuits. At thesame time the circuit performance may improve. Logic gates (such as a voltage inverter,

Summary 121

as is demonstrated in chapter 9) switch in a better defined region of the voltage axis anda have a steeper crossover in voltage upon going from high to low voltage as comparedto unipolar logic gates. In chapter 10 we demonstrate that ambipolar transistor operationcan also be realized on the basis of a single semiconducting polymer. These findingscontribute to develop organic electronics more towards complementary logic, by usingthe properties of the semiconductors as optimally as possible for electronic circuitry.

Samenvatting

Ladingstransport in wanordelijke organischeveldeffecttransistoren

Tegenwoordig worden plastics veelvuldig gebruikt in toepassingen, varierend vanverpakkingsmateriaal tot speelgoed en van meubels tot plastic tassen. Er wordt de laatstetijd zelfs gewerkt aan plastics die elektronische en/of optische functies kunnen vervullen.Dit opent de deur naar de “plastic elektronica”, zoals bijvoorbeeld flexibele, dunne dis-plays, of goedkope elektronische streepjescodes. De plastics, of organische polymeren,die gebruikt kunnen worden in deze toepassingen dienen stroom te kunnen geleiden, endus moeten ze ladingsdragers kunnen transporteren. In dit proefschrift wordt ingegaan opde vraag hoe deze ladingsdragers bewegen in een organische halfgeleidende polymeer-laag. De experimentele studie is gedaan aan de hand van de veldeffecttransistor, waarhet halfgeleidende polymeer zich tussen twee elektrodes bevindt op een isolerende on-dergrond. Door een elektrische spanning aan te leggen tussen de twee elektrodes, loopter een stroom door de organische halfgeleider. Deze stroom kan gereguleerd worden meteen derde elektrode, die zich onder de isolerende laag bevindt. Deze derde elektrode (diemen “gate” noemt) reguleert de elektrische stroom door de hoeveelheid ladingsdragersin het polymeer te veranderen. Zo kan het ladingstransport in organische halfgeleidersbekeken worden als functie van de ladingsdichtheid.

Van belang voor dit ladingstransport is de wanorde in de polymeer laag. Dezewanorde wordt in theoretische modellen vaak beschreven met een ruimtelijke en ener-getische verdeling van transporttoestanden. Ladingsdragers in zo’n wanordelijke poly-meerlaag zorgen zelf voor een lokale roostervervorming, waardoor zij zich lokaliseren opeen plek in de polymere film. Deze ladingen kunnen bewegen in de polymeerfilm on-der invloed van een aangelegd elektrisch veld, door te springen tussen twee nabijgelegentransporttoestanden. De energie om zo’n sprong te kunnen maken halen ze uit roostervi-braties (phononen) van het polymeer.

In hoofdstuk 2 worden allereerst twee parameters, die gebruikt worden om de werk-ing van de veldeffecttransistoren te beschrijven, kritisch belicht. We beargumenteren datde parameters die gebruikt worden in de standaard siliciumtechnologie, niet bruikbaarzijn voor wanordelijke organische transistoren. Daarom definieren we de startspanning(switch-on voltage) van de transistor als de spanning die op de gate-elektrode aangelegdmoet worden zodat er net geen ladingen in de halfgeleiderlaag meer zitten. Om de bruik-baarheid van deze parameter te toetsen hebben we de stroom-spannings karakteristiekenvan transistoren, die gebaseerd zijn op allerlei verschillende polymeren, doorgemeten alsfunctie van de temperatuur. Deze karakteristieken worden theoretisch beschreven meteen model gebaseerd op het springen tussen de energetische toestanden in het polymeer.

124 Samenvatting

Uit het modelleren concluderen we dat de startspanning een bruikbare fysische param-eter is voor de beschrijving van organische transistoren. Voorts wordt de mobiliteit vande ladingsdragers, welke aangeeft hoe gemakkelijk ladingen bewegen door de polymeerlaag heen, experimenteel bepaald als functie van de ladingsdichtheid in de transistor. Weconcluderen dat de gemiddelde mobiliteit van ladingen direct aan het oppervlak met deisolator een goede parameter is voor de beschrijving van de transistoren. In de laatsteparagraaf van hoofdstuk 2 wordt deze ladingsdichtheidsafhankelijke mobiliteit in veld-effecttransistoren gelinkt met de mobiliteit zoals die bepaald wordt uit lichtemitterendediodes. We concluderen dat de grote verschillen in mobiliteit, die waargenomen wor-den voor een enkel polymeer in de twee verschillende structuren, voor een groot deelhet gevolg zijn van een aanzienlijk verschil in ladingsdichtheid tussen de transistor en delichtemitterende diode.

In hoofdstuk 3 wordt de mobiliteit van de ladingsdragers bekeken als functie vande temperatuur. Aan de hand van deze data, nemen we een empirische regel, getiteld deMeyer-Neldel regel, waar. Aan deze regel is een karakteristieke energie verbonden dievoor de bestudeerde materialen steeds eenzelfde waarde oplevert. Dit feit duidt mogelijkop een gemeenschappelijke oorsprong van deze energie in het ladingstransport in de ver-schillende materialen. Er wordt beargumenteerd dat een mogelijke grondslag voor dezekarakteristieke energie gelegen is in het feit dat een lading voor een sprong tussen tweeplaatsen in het polymeernetwerk meerdere phononen per sprong nodig heeft, in plaats vanslechts een enkel phonon. In hoofdstuk 4 wordt deze empirische regel tevens gekoppeldaan de experimenteel waargenomen veldafhankelijkheid van de mobiliteit.

Voor praktische toepassingen is niet alleen het begrip van het ladingstransport belan-grijk, maar ook hoe ladingen geınjecteerd worden in de polymeerlaag. De elektrodes, diewe hiervoor gebruikt hebben, bestaan uit goud. De ladingen gaan niet zonder enige weer-stand de polymeerlaag in vanuit het goud. Om een idee te krijgen van de weerstandendie hierbij belangrijk zijn, hebben we in hoofdstuk 5 gekeken naar series van transistorenwaarbij de lengte van de transistor systematisch veranderd wordt. Zo kunnen we het lad-ingstransport in de polymeerlaag loskoppelen van de ladingsinjectie van het goud naarde polymeerlaag. We vinden een correlatie tussen de injectieweerstand en de mobiliteitin het polymeer zelf en interpreteren dit in de context van de verdeling van energetischetoestanden in het polymeer.

Bepaalde polymeren met veelbelovende eigenschappen hebben ook nadelen ten aan-zien van hun lucht- en lichtgevoeligheid. In hoofdstukken 6 en 7 kijken we naar hetpolymeer poly(3-hexyl thiofeen), waarvan de geleiding in de tijd toeneemt onder invloedvan zuurstof en licht uit de omgeving. Dit is het gevolg van een doteringsproces waar-bij er steeds meer ladingsdragers in het polymeer aanwezig zijn, die bijdragen tot degeleiding. In hoofdstuk 6 wordt allereerst gekeken naar de impedantieanalysetechniektoegepast op metaal-isolator-halfgeleider diodes (MIS diodes), voor het bepalen van dehoeveelheid dotering in het polymeer. We vinden dat de meetfrequentie waarbij de bepal-ing wordt gedaan niet te hoog mag zijn omdat anders de mobiliteit van de ladingsdragersin het polymeer de meting domineert in plaats van de hoeveelheid dotering. In hoofd-stuk 7 wordt deze meettechniek toegepast om te bepalen hoe efficient doteringsladingengevormd worden onder invloed van zuurstof en licht. In hoofdstuk 8 laten we zien datde doteringsveranderingen ook direct aan de hand van veldeffecttransistoren bepaald kun-

Samenvatting 125

nen worden. Vervolgens kunnen we tevens de bulk mobiliteit van de doteringsladingenrelateren aan de hoeveelheid dotering.

In de voorgaande hoofdstukken hebben we vooral gekeken naar unipolaire p-typetransistoren, dat wil zeggen transistoren waarin slechts positieve ladingen bewegen. Inhoofdstuk 9 laten we zien dat een geschikte combinatie van elektrodemateriaal en eenmengsel van een p-type halgeleider met een n-type halfgeleider gebruikt kan worden omambipolaire veldeffecttransistoren te maken. Deze transistoren maken het mogelijk hetontwerp van geıntegreerde circuits te vereenvoudigen. Tegelijkertijd verbetert de werk-ing van de circuits. De logische poortjes (zoals een spanningsomkeerder), die gebaseerdzijn op ambipolaire transistoren, schakelen meer symmetrisch en hebben een steilere om-schakeling van hoge spanning naar lage spanning, dan de unipolaire logische poortjes. Inhoofdstuk 10 tonen we aan dat ambipolaire transistoroperatie ook gerealiseerd kan wor-den op basis van een enkel halfgeleidend polymeer. Deze vindingen dragen ertoe bij deorganische elektronica verder te laten ontwikkelen naar complementaire logica.

List of Publications

1. The Meyer-Neldel rule in organic thin-film transistors,E.J. Meijer, M. Matters, P.T. Herwig, D.M. de Leeuw and T.M. Klapwijk, Appl.Phys. Lett 76, 3433 (2000). [Chapter 3]

2. The isokinetic temperature in disordered organic semiconductors,E.J. Meijer, D.B.A. Rep, D.M. de Leeuw, M. Matters, P.T. Herwig and T.M. Klap-wijk, Synth. Met. 121, 1351 (2001). [Chapter 4]

3. Charge-transport in partially-ordered regioregular poly(3-hexylthiophene) studiedas a function of the charge density,D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins and T.M. Klapwijk, Mat. Res.Soc. Symp. Proc. 660, JJ7.9.1 (2001). [Chapter 4]

4. Frequency behavior and the Mott-Schottky analysis in poly(3-hexyl thiophene) metal-insulator-semiconductor diodes,E.J. Meijer, A.V.G. Mangnus, C.M. Hart, D.M. de Leeuw and T.M. Klapwijk, Appl.Phys. Lett. 78, 3902 (2001). [Chapter 6]

5. Switch-on voltage in disordered organic field-effect transistors,E.J. Meijer, C. Tanase, P.W.M. Blom, E. van Veenendaal, B.-H. Huisman, D.M. deLeeuw and T.M. Klapwijk, Appl. Phys. Lett. 80, 3838 (2002). [Chapter 2]

6. Charge transport in disordered organic field-effect transistors,C. Tanase, P.W.M. Blom, E.J. Meijer and D.M. de Leeuw, Mat. Res. Soc. Symp.Proc. 725, P10.9.1 (2002). [Chapter 2]

7. Dopant density determination in disordered organic field-effect transistors,E.J. Meijer, C. Detcheverry, P.J. Baesjou, E. van Veenendaal, D.M. de Leeuw andT.M. Klapwijk, J. Appl. Phys. 93, 4831 (2003). [Chapter 8]

8. Scaling behavior and parasitic series resistance in disordered organic field-effecttransistors,E.J. Meijer, G.H. Gelinck, E. van Veenendaal, B.-H. Huisman, D.M. de Leeuw andT.M. Klapwijk, accepted for publication in Appl. Phys. Lett. [Chapter 5]

9. Local charge carrier mobility in disordered organic field-effect transistors,C. Tanase, E.J. Meijer, P.W.M. Blom and D.M. de Leeuw, submitted. [Chapter 2]

10. Unification of the hole transport theories in light-emitting polymeric diodes andfield-effect transistors,C. Tanase, E.J. Meijer, P.W.M. Blom and D.M. de Leeuw, submitted. [Chapter 2]

128 List of Publications

11. Photoimpedance spectroscopy of poly(3-hexyl thiophene) metal-insulator-semicon-ductor diodes,E.J. Meijer, A.V.G. Mangnus, B.-H. Huisman, G.W. ’t Hooft, D.M. de Leeuw andT.M. Klapwijk, submitted. [Chapter 7]

12. Solution-processed ambipolar organic field-effect transistors and inverters,E.J. Meijer, D.M. de Leeuw, S. Setayesh, E. van Veenendaal, B.-H. Huisman,P.W.M. Blom, J.C. Hummelen, U. Scherf, and T.M. Klapwijk, submitted. [Chapters9 and 10]

Patent Application

Solution-processed ambipolar organic field-effect transistorsD.M. de Leeuw, E.J. Meijer, S. SetayeshEuropean Patent Application EP 03100177.9

Curriculum Vitae

Eduard Johannes Meijer

April 5th, 1975 Born in Drachten, The Netherlands.

1987 - 1993 Gymnasium at the Ichthus College in Drachten.

1993 - 1998 M.Sc. Applied Physics at the University of Groningen. Graduateresearch in the Thin-Film Physics group of prof.dr.ir. T.M. Klap-wijk. Subject: Charge trapping instabilities in quaterthiophenethin-film transistors: experimental observations.

1998 Visiting student at Xerox Palo Alto Research Center, Palo Alto,USA, in collaboration with dr. R.B. Apte. Subject: Morphologystudy of anodized porous aluminum oxide.

1998 - 2003 Ph.D research at Delft University of Technology in thenanophysics group of prof.dr.ir. T.M. Klapwijk. This work wasperformed at the Philips Research Laboratories in collaborationwith dr. D.M. de Leeuw. Subject: Charge transport in disor-dered organic field-effect transistors.

april 2003 Research scientist at Philips Research Laboratories, Eindhoven.

Dankwoord

Hoe mooi is het om de culminatie van mijn promotieonderzoek hier gebundeld tehebben en terug te kunnen lezen. Voor de lezer zal dit boek overkomen als een verza-meling van droge feiten, een cijfermatig geheel waarin claims worden gelegd en physicawordt bedreven. Voor mij is het dat al lang niet meer. Het is een klein geschiedenis-boek waarin ik de verschillende perioden en fasen, pieken en dalen, van de afgelopen vieren een half jaar nog weer eens de revue kan laten passeren. Het geheel is meer dan desom der delen en hoewel dit dankwoord maar twee pagina’s beslaat, zijn de mensen dieme op allerlei fronten terzijde hebben gestaan ten aanzien van mijzelf en mijn werk, devoedingsbodem geweest om het werk dat beschreven staat in de voorgaande pagina’s totuitvoer te kunnen brengen.

Teun Klapwijk wil ik hartelijk danken voor de kans die hij me gegeven heeft en degok die hij met me nam door me aan te nemen op het moment dat de verhuizing naarDelft voor de deur stond. Teun, ik heb veel geleerd van je kritische en heldere manier vanwetenschappelijk discussieren. Ik heb je input en steun altijd zeer gewaardeerd.

Bij het Natuurkundig Laboratorium van Philips, waar ik vier en een half jaar vertoefdheb wil ik graag als eerste Dago de Leeuw bedanken voor de stimulerende en enthousiastesamenwerking. Dago, je vermogen de oppervlakte van de wetenschap af te schuimen opzoek naar de interessante en relevante onderwerpen heeft op mij zeer stimulerend gewerkt.En om dan vervolgens ook nog de diepte in te kunnen gaan door met “boerenverstand” dejuiste vragen op te werpen, is iets wat ik als een zeer vruchtbare werkwijze heb ervaren.Voorts, wil ik Bart-Hendrik Huisman bedanken voor de innemende sfeer die hij met zichmeebracht naar de laatste kamer van WB 6. Ik heb met veel plezier de kamer met jegedeeld. Estrella Mena-Benito, voor haar voortdurende steun, luisterend oor en haar nietaflatende spraakwaterval, van achter de stepper in de fitnessruimte. Eugenio Cantatore,voor zijn enthousiasme en onze aangename gesprekken over relaties en het leven. MarcoMatters voor zijn begeleiding tijdens de beginfase van mijn promotie. En tevens de restvan het polymere elektronica cluster voor de plezierige samenwerking: Patrick Baesjou,Monique Beenhakkers, Celine Detcheverry, Gerwin Gelinck, Tom Geuns, Ben Giesbers,Kees Hart, Edzer Huitema, Karel Kuijk, Albert Mangnus, Alwin Marsman, Andre Mon-tree, Bas van der Putten, Laurens Schrijnemakers, Sepas Setayesh, Fred Touwslager, Erikvan Veenendaal en Stephan Zilker. Jan Verhoeven voor het maken van zovele standaard-substraten. Verder wil ik Hans Hofstraat en Gerjan van de Walle en tevens de mensenvan de groepen “Polymers and Organic Chemistry” en “Integrated Device Technologies”,in het bijzonder Elize Harmelink en Isabella Geuens, bedanken voor hun gastvrijheid enplezierige werksfeer in de afgelopen jaren. Henny Herps voor het design van de kaft ende gesprekken over creativiteit.

Aan de Technische Universiteit Delft was ik altijd meer gast dan groepslid van degroep “Nanophysics”. Niettemin heb ik het altijd prima naar mijn zin gehad aldaar, tijdens

132 Dankwoord

mijn tweewekelijkse bezoeken, waarvoor hartelijk dank aan de groepsleden. DiederikRep, die op velerlei vlak een parallel pad heeft gevolgd aan het mijne, wil ik graag dankenvoor de mooie tijd die we samen in maar ook buiten polymerenland hebben doorgebracht.Diederik, ik spreek nogmaals de wens uit, nu gezien vanuit het perspectief van mijn pro-motie, dat we ondanks divergerende wegen binnen elkaars cirkel zullen blijven. Verderde andere mensen verbonden aan het organische halfgeleider werk: Ruth de Boer, JosKoonen, Hon Tin Man en Alberto Morpurgo. Van het secretariaat Monique Vernhout,Maria Roodenburg-van Dijk en Margaret van Fessem voor alle administratieve hulp dieze me geboden hebben de afgelopen jaren.

Aan de Rijksuniversiteit Groningen wil ik graag de plezierige en vruchtbare samen-werking met Paul Blom en Cristina Tanase vermelden. Jan Anton, ik wens je veel succesmet jouw promotie. Verder heb ik veel baat gehad bij de technische hulp van Minte Mul-der en Bernard Wolfs in de beginfase van mijn promotie. Alex Schoonveld bedankt voorzijn hulp bij het begin van mijn promotie, die eerste maand in Groningen.

Mijn vrienden met wie ik door heel Nederland mooie tijden heb beleefd en nog zalbeleven.

Mijn ouders en mijn broers voor hun steun tijdens het promotiewerk. Arnoud enRichard, ik vind het heel bijzonder dat jullie mijn paranimfen willen zijn.

Pimm, bedankt voor je warmte, liefde en steun in alles wat ik doe.

Eduard

Charge Transport in Disordered Organic Field Effect Transistors

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