Thecatalyticeffectofiron(III)ontheetchingofZnO:Alfront

contactsforthin‐filmsiliconsolarcells

Sascha E. Pust*, Janine Worbs, Gabrielle Jost, Jürgen Hüpkes

Forschungszentrum Jülich GmbH, IEK5 – Photovoltaik, 52425 Jülich, Germany

* Corresponding author:

Dr. Sascha E. Pust

E-mail: s.pust@fz-juelich.de

Phone: +49 2461 61 5740

Fax: +49 2461 61 8355

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Abstract

Sputter-deposited ZnO:Al thin films, used as front contact in thin-film Si solar cells,

were etched in diluted HCl containing catalytic amounts of Fe(III) salts. The Fe(III)

effectively catalyzes the HCl-based etch process, leading to a crater-like morphology that is

qualitatively similar to the one generated by the uncatalyzed etching process at the same HCl

concentrations. Utilizing this catalyzed process, an increase of the etching rate by a factor of

approximately 1.8-2.6 was observed. This allows for a well-controllable tuning of the etch

duration without changing HCl concentration or temperature. The process has been evaluated

with a selection of Fe(III) salts at different concentrations of the acid and of the catalyst.

Optical, electrical and scanning force microscopic characterization of such catalytically

etched ZnO:Al films has shown that the catalytical process leads to slightly smaller

morphological features on the film compared to the uncatalyzed etching, accompanied by a

shift of light scattering intensity to higher angles. This etching behavior may be used

beneficially for light trapping in thin-film Si solar cells. In addition to these application-

oriented aspects, this approach provides a deeper insight into the mechanistical details of the

ZnO thin-film dissolution.

Keywords: Catalysis; Etching; Thin-film Si solar cells; Transparent conductive oxide (TCO);

ZnO thin films

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1. Introduction

The front contact in thin-film Si solar cells has to fulfil three criteria: (i) it has to be

highly transparent, (ii) a low series resistance is necessary, and (iii) it has to provide a certain

roughness for light management in the device, thus helping to trap the light within the

absorber layer stack by total internal reflection. The first two aspects are given for many

doped, transparent conductive oxides (TCO) like SnO2:F, In2O3:SnO2, ZnO:Al, ZnO:B, or

ZnO:Ga [1-6]. The latter criterion, however, necessitates further texturing in the case of some

TCO materials that are smooth in the as-deposited state. Sputter-deposited, polycrystalline

ZnO:Al, for example, may be artificially roughened by an etching step in diluted HCl [7-10]

(e.g., leading to the 'standard Jülich' type material [11, 12]), HF [13], NH4Cl [14-16], other

acids in liquid [7-10, 17, 18], gaseous [19], or vaporized state [20], by electrochemical

means [21], or by a combination of such processes [13, 22-26]. The variety of possible

etching methods together with a model for the etching of polycrystalline ZnO:Al has been

addressed in a recent review [27].

Besides the texture, one parameter of significant industrial relevance is the duration of

the etching step. For inclusion of such a process into in-line production systems, adjustable

etch times are desirable. This can be done to a certain extent by changing the concentration of

the etchant or the temperature. However, the resulting morphology is changed significantly by

such alterations as well [22, 28]. Readily applicable methods for the adjustment of the etch

time are hence not at hand.

One general option for accelerating chemical reactions is the utilization of catalytic

processes. It is well known that catalysis is a concept of utmost relevance for chemical

industry because of its ability to initiate, accelerate, or steer chemical reactions. Although the

exact nature of the interfacial processes during acidic etching of ZnO:Al thin films is still not

completely understood [27], it can be expected that this process is accelerated by suitable,

catalytically active agents, similarly to what has been known for In2O3:SnO2 thin films since

many years [29].

In this study, we evaluated the potential of Fe(III) salts to catalyze and accelerate the

etching of sputter-deposited ZnO:Al thin films in diluted HCl. A catalytic mechanism is

proposed that is strongly supported by experimental evidence. Electrical measurements,

angle-resolved scattering (ARS), and scanning force microscopy (SFM) were used to quantify

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the surface morphologies resulting from the catalyzed etching. This new process renders very

suitable morphologies for light scattering at increased etch rates, and their suitability for thin-

film Si photovoltaic devices is demonstrated.

2. Experimental

2.1 ZnO:Al thin-film preparation

Approximately 800-850 nm thick, polycrystalline ZnO:Al films were deposited on a

(1010) cm2 glass substrate (Corning Eagle XG) using radio frequency magnetron sputtering

in a vertical in-line system (cf. Fig. 1 in Ref. [30]) from a ceramic target consisting of ZnO

with 1 w/w% Al2O3 (Cerac Inc., Milwaukee, WI, USA). The deposition was carried out at a

substrate temperature of 300°C, a discharge power density of 2 W cm-2, and an Ar pressure of

0.1 Pa. Details about the process and ZnO:Al film properties are reported elsewhere [12].

2.2 Etching process and chemicals

All solutions were prepared with deionised water (>16 M cm) from an Elix 10 water

purification system (Millipore Co., Schwalbach, Germany). HCl, FeCl3, Fe(NO3)3,

K3[Fe(CN)6], FeCl2 (all Merck KGaA, Darmstadt, Germany), and Fe2(SO4)3 (Sigma-Aldrich

Chemie GmbH, Munich, Germany) were of analytical grade and were used without further

purification. The temperature was kept constant at 250.5°C during all etching experiments.

After etching, substrates were rinsed with copious amounts of deionized water to remove salt

residues originating from the etching solution.

Special care has to be taken when an acid is added to a compound containing

hexacyanidoferrate because very toxic HCN may be generated. Such experiments therefore

have to be carried out with proper personal protective equipment in a closed fume hood and

with a very small amount of CN-containing compounds only.

2.3 Film characterization

Film thicknesses d have been measured with a Dektak 3030 surface profiler (Veeco

Instruments Ltd., Santa Barbara, CA, USA), taking into account those points where the films

have got the highest thickness. To correct for inhomogeneities in the initial measured film

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thickness, this initial thickness has been determined by taking the maximum measured d value

for each etch series as initial value instead of taking the measured values at each individual

etching spot. SFM images have been recorded utilizing a NANOStation 300 (S.I.S. GmbH,

Herzogenrath, Germany) in non-contact mode with NCHR-POINTPROBE Si cantilevers

(NanoWorld AG, Neuchâtel, Switzerland). Image post-treatment was performed with the

software SPIP, version 5.1.6 (Image Metrology A/S, Hørsholm, Denmark). A linear

background was subtracted from the images for removal of the sample tilt. A four-point probe

was utilized for determining the sheet resistance Rsh of the films. ARS measurements in

transmission were performed in a home-built system at an optical wavelength of =550 nm.

2.4 Solar cell preparation and characterization

Roughly 1 µm thick µc-Si:H films for single junction solar cells were deposited in a

(4040) cm2 in-line plasma-enhanced chemical vapor deposition (PECVD) system [31]. Si

films for 420 nm a-Si:H / 1.25 µm µc-Si:H tandem p-i-n solar cells were prepared by PECVD

in a (3030) cm2 reactor. Details of the Si PECVD process can be found elsewhere [32, 33].

The back contact consisted of sputter-deposited ZnO:Al and Ag from the same deposition

system as the front contact films. A lab-scale laser patterning system (ROFIN-SINAR Laser

GmbH, Hamburg, Germany) was used to determine a cell area of (11) cm2. Solar cells were

characterized with a Wacom WXS 140 S solar simulator (Wacom Electric Co., Saitama,

Japan) under standard test conditions (AM1.5 spectrum, 100 mW cm-2, 25°C). External

quantum efficiencies EQE were measured in a home-built system by differential spectral

response (DSR) at zero bias voltage. The crystallinity of the Si absorber was determined

from the back side of the cells via Raman measurements with an excitation wavelength of 647

nm which leads to information depths roughly of the thickness of the µc-Si:H absorber. In the

case of the tandem devices, the measured crystallinity of the bottom component cell is

also influenced by the top component cell. This is due to the penetration depth of the Raman

measurement and leads to an underestimation in compared to the real crystallinity of

the bottom component cell. To access the Si, the ZnO:Al/Ag back contact has been removed

at selected spots close to the measured cells with a wet-etching process. Details about the

Raman setup [34] and the determination of the crystallinity [35] can be found in the

respective references.

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3. Results and discussion

3.1 Mechanistical considerations

Van den Meerakker et al. [29] used Fe(III) species to catalyze the etching of In2O3:SnO2

thin films in halogen acids. Taking into account the nature of the wurtzite-type ZnO:Al

material, their proposed mechanism for the uncatalyzed [36] as well as for the catalyzed

etching of In2O3:SnO2 [29] may be adapted (Fig. 1). When etching ZnO in an aqueous,

diluted HCl solution (Fig. 1a), a Zn-O-Zn bond (1) is attacked by an HCl moiety. This leads

to the formation of one Zn-OH and one Zn-Cl bond. The resulting surface intermediate (2)

reacts with a second HCl moiety in a condensation reaction to form two Zn-Cl bonds (3) and

H2O. This process is repeated, yielding ZnCl2 and H2O as the reaction products.

Fig. 1. Proposed mechanism for the acidic dissolution of ZnO (a) without catalyst and (b) with Fe3+ as

a catalytically active species.

If Fe3+ is present in addition to HCl (Fig. 1b), the Fe3+ ion interacts with one electron pair

of the oxygen atom in 2, leading to a weakening of the Zn-O bond (2a). A chloride ion from

an HCl moiety is then able to attack the Zn atom. This leads to a heterolytic fission of the

Zn-O bond, yielding 3 and H2O, releasing the catalytically active Fe3+ ion and making it

available for further catalysis. The final reaction products are hence the same as in the

uncatalyzed reaction pathway (Fig. 1a).

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Van den Meerakker et al. [29] have shown for In2O3:SnO2 etching that in the catalyzed

case (Fig. 1b), the reaction of 1 to 2 is the rate-determining step, while the conversion of 2 to

3 is limiting the reaction rate if the process is not catalyzed (Fig. 1a). This means that, in the

case of acidic ZnO etching, an increase in the etch rate can be expected when adding Fe3+ to

the etching solution if the mechanism of In2O3:SnO2 etching [29, 36] is directly transferable

to the ZnO:Al thin film.

3.2 Initial tests

Before applying Fe(III) salts as catalysts in the acidic etching process of ZnO:Al, the

immediate effect of those salts on the thin film had to be evaluated because aqueous solutions

of FeCl3, Fe(NO3)3, Fe2(SO4)3, or similar are slightly acidic by themselves. For example,

taking into account that Fe(III) salts will exist in solution mainly as hydroxo and aqua

complexes with acid dissociation constants pKa in the range of 3.0-3.3 [37], a 0.5 w/w%

solution of FeCl3 will have a pH of approximately 2. This is about one pH unit higher than the

value of an equally concentrated HCl solution. This means that etching in a diluted FeCl3

solution should proceed slower than in similarly concentrated HCl if a possible catalytic

effect of the Fe3+ ions is not too pronounced.

To check this assumption, ZnO:Al substrates have been etched for 40 s in FeCl3 solutions

of 0.5, 0.05, and 0.005 w/w%, respectively, without addition of HCl. While the standard

Jülich method with 0.5 w/w% HCl proceeds at etch rates of a few nm s-1 (depending on the

material properties), the etch rates in FeCl3-containing solutions are indeed much lower: the

0.5 w/w% solution of FeCl3 (corresponding to 0.116 mol L-1) resulted in an etch rate of

roughly 0.9 nm s-1. This value can be ascribed partly to the pH of approximately 2, and partly

to a catalytic effect of the Fe3+ in this slightly acidic solution. Almost no removal (in the range

of few nm min-1) was observed for solutions with an even lower concentration. On the one

hand, this meets the expectations concerning the acidic strength of Fe3+ salts. On the other

hand, it is an important prerequisite for further experiments as this observation implies that

the addition of catalytic amounts of Fe3+ to an HCl solution will be insignificant in terms of a

pH decrease. Hence, Fe3+ would not increase the etch rate in an HCl-based etching

significantly if no catalytic reaction takes place.

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3.3 Quantification of the catalytic effect

To quantify the catalytic effect of Fe3+, several etch experiments on ZnO:Al substrates

were performed. While the HCl concentration was kept constant in all experiments at 0.5

w/w% (0.137 mol L-1), different catalytic amounts of iron salts were added to the HCl,

namely 1, 5, and 10 mol% with respect to the HCl concentration. Expressed as absolute iron

salt concentrations, these values correspond to 1.37, 6.85, and 13.7 mmol L-1, respectively.

The iron salt concentrations are hence at least about one order of magnitude smaller than in

the previously elucidated, initial experiments. FeCl3 was utilized as source for Fe3+.

Additionally, a cross-check was performed with HCl solutions containing FeCl2 as a salt that

does not contain Fe3+, and K3[Fe(CN)6] where the Fe3+ is tightly bound as the metal center of

a stable complex. The films were etched for 15, 30, 40, and 50 s with each catalyst

concentration and each iron salt. In addition to that, one reference substrate has been etched in

0.5 w/w% HCl without any iron compound to compare the catalyzed with the uncatalyzed

process.

The first visual impression was that all etch attempts lead to more or less pronounced,

hazy ZnO:Al surfaces. All etching solutions rendered visually homogeneous etch results. To

quantify the amount of material that is etched away in a given time, one may take into account

the film thickness d before and after etching, or the change in Rsh that will increase when the

film becomes thinner and more rough. Both parameters are good indicators for the change

that is introduced to the film via etching, but the change in film thickness d represents the

amount of removed material more exactly. Mainly due to the fact that the film is rough after

etching, the profilometric measurements will be afflicted with a certain overestimation of d

the rougher the films get [28]. For these experiments, we took into account only those points

where the films have got the highest thickness (namely crater rims instead of etch pits) in the

profilometric measurements to avoid a significant error originating from this overestimation.

Considering variations in the initial film thickness, we estimated the overall error for d to be

roughly 25 nm. The change in sheet resistance compared to the value before etching (Rsh)

has additionally been used to quantify the etch rates as the sheet resistance measurement is

straightforward and may advantageously be used in terms of process control. The according

Rsh values are presented in the supporting material (Fig. S1).

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Fig. 2. Decrease in film thickness d as a function of etch time t for etching in 0.5 w/w% HCl

containing different concentrations of (a) FeCl3, (b) FeCl2, (c) K3[Fe(CN)6]. As a reference, d of a

substrate etched in HCl without addition of a catalyst is plotted in gray. An estimated measurement

error for d of ±25 nm is included. Lines are solely meant to guide the eye.

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Indeed, an acceleration occurs if FeCl3 is used as a source for Fe3+. As Fig. 2a clearly

shows, the increase in d with etch time is higher compared to the reference (gray) whenever

FeCl3 is present. In all cases, the acceleration by far exceeds the result of an added-up acidity

of the Fe(III) salt and the HCl. A catalytic effect of the Fe3+ on the acidic dissolution of ZnO

(Fig. 1b) is thus clearly evident. In general, the extent of acceleration seems to be dependent

on the concentration of the catalyst (higher concentration leads to slightly stronger

acceleration) as well as on the anion in the respective Fe(III) salt. While addition of FeCl3

leads to a moderate, but well controllable acceleration of the etch process (Fig. 2a), the

addition of Fe(NO3)3 or Fe2(SO4)3 accelerates the etching to such an extent that the ZnO film

suffers an electrical breakdown after approximately 40 s of etching (not shown). Obviously,

the presence of sulfate and nitrate ions contributes more strongly to the acceleration than

chloride. A similar effect, especially concerning the sulfate influence, has been observed

earlier in the context of electrochemical etching of ZnO (cf. Fig. 3b in Ref. [25]), and that has

been ascribed to the strong interference of sulfate ions with Zn precipitation [38]. It is,

however, unclear why the same, pronounced acceleration is observed in the presence of

nitrate ions, as this was not observed in the aforementioned studies [25]. The effects when

using nitrate or sulfate salts may also be attributed to solubility issues concerning the reaction

products.

A cross-check experiment has been performed with FeCl2 where no Fe3+ is available in

the etch solution. An acceleration should thus not occur in this case. The differences in d

between etch attempts with and without the iron salt are indeed negligible and within the

measurement error (Fig. 2b). This is a clear evidence that Fe2+ cannot influence the ZnO

dissolution mechanism as postulated for Fe3+ (Fig. 1b), most probably due to its lower

oxidizing potential compared to Fe3+. In the case of K3[Fe(CN)6] where Fe3+ is bound inside

an octaedric complex ion, an acceleration indeed seems to occur at the utilized catalyst

concentrations (Fig. 2c). The degree of acceleration is similar to that observed under the

influence of FeCl3. Although no free Fe3+ should be available, the etch mechanism seems to

be influenced by the presence of the hexacyanidoferrate complex. We assume that, due to the

presence of HCl in the same solution, significant amounts of the complex are chemically

destroyed. This makes Fe3+ available in its ionic form to catalyze the etching process,

ultimately leading to an increase in etch rate that is similar to that under the influence of FeCl3

due to the presence of chloride ions originating from the HCl addition.

Further experiments have shown that the catalytic effect of Fe(III) is present at higher

HCl concentrations than 0.5 w/w%, although the reaction becomes less controllable as etch

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times for generating a feasible etch result are in the range of only a few seconds then.

Concentrations as low as approximately 0.1 w/w% are possible as well. For example, while

the etching of a substrate in 0.1 w/w% HCl for 40 s resulted in Rsh=2.1 □, the etch time to

reach a similar value (Rsh=1.9 □) could be reduced to 10 s when adding 5 mol% FeCl3 to

the etchant. In both cases, approximately 85 nm of the film were removed.

Below that concentration, the pH value of the solution is shifted into a range where many

Fe(III) salts are not soluble to an appropriate extent, making a catalytic acceleration

impossible. Over the whole utilizable concentration range of HCl, however, the extent of

acceleration is similar to the previously elucidated extents. This results in an overall

acceleration factor (concerning d) of approximately 1.8-2.6 for the application of FeCl3,

mostly depending on the catalyst concentration.

It has been furthermore observed that the catalytic effect is not limited to HCl. Similar

results have been achieved with other acids (e.g. HNO3 or H2SO4) at pH values of about 1

(not shown), although the degree of acceleration is different for all utilized compounds.

3.4 Influence on surface morphology

Besides the advantage of an accelerated etching, the resulting layers have to have suitable

morphologies for light management in thin-film Si solar cells. Concerning sputter-deposited

ZnO:Al films, it is well known that the standard Jülich reference material [11] is well suited

for this purpose, and this has also been confirmed by optical simulations [39]. It is hence

desirable to keep the morphology after etching similar, even if the process is accelerated

catalytically. This is an important issue, as Owen et al. [28] have pointed out recently that the

etching of sputter-deposited ZnO:Al thin films at different HCl concentrations and different

etchant temperatures influences not only the etch rate, but also the crater size after etching.

To check for this issue, SFM images have been recorded of substrates etched with and

without catalytic influence of FeCl3 to a similar d. Figure 3 shows representative

micrographs of one reference substrate etched without catalytic influence (Fig. 3a) and of one

substrate etched shorter down to a similar film thickness under catalytic influence of FeCl3

(Fig. 3b). Both etched films show qualitatively similar interfacial morphologies with craters

in the range of less than 1 µm in diameter. In general, the catalyst obviously does not seem to

alter the fundamental etch mechanism that has been postulated for such polycrystalline ZnO

thin films [27]. However, the morphology of both films does differ quantitatively: in the

catalytically etched case (Fig. 3b), medium crater diameters are lower compared to the

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reference substrate (Fig. 3a), while the medium crater depths and the root mean square (RMS)

roughnesses are similar. The respective quantitative values are given in Table I.

Fig. 3. (10×10) µm2 scanning force micrographs of ZnO:Al thin films etched in (a) 0.5 w/w% HCl for

50 s and (b) 0.5 w/w% HCl with 5 mol% FeCl3 for 30 s.

Table I. Statistical evaluation of the SFM-derived topographies of etched ZnO:Al thin films (cf. Fig. 3). The errors given are the standard deviations derived from the statistical evaluation of the crater dimensions.

etching conditions medium crater diameter [nm]

medium crater depth [nm]

RMS roughness [nm]

50 s in 0.5 w/w% HCl (reference)

887 ± 437 279 ± 140 114

30 s in 0.5 w/w% HCl + 5 mol% FeCl3

791 ± 375 275 ± 146 113

Interestingly, this trend to smaller craters is also observed when etching without catalytic

influence at elevated temperatures, i.e., higher etch rates, or lower concentrations of the acid,

i.e., lower etch rates [28]. This implies that the morphology is not directly related to the etch

rate, but the catalyst seems to lower the etching threshold for grain boundaries with a certain

etch potential [27], leading to an increased crater density and, as a consequence, a decreased

medium crater diameter. Hence, this new method is another tool in addition to the established

procedures basing on different acids and different etching conditions that allows us to tune the

size of surface features on etched ZnO:Al.

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While SFM is a local technique and can deliver representative topographic informations

of a microscopic fraction of the ZnO:Al surface, optical methods are suited to derive more

integral informations about the substrate on an area of about 1 mm2. Hence, we performed

ARS measurements in transmission at =550 nm (Fig. 4). While, in general, longer

wavelengths (>600 nm) are more relevant for scattering in thin-film Si solar cells, a of 550

nm has proven to resolve even small surface features properly. This is not the case any more

if red lasers are used as a light source. =550 nm is thus a compromise between an

appropriate sensitivity for small features and a relevance for light scattering.

Fig. 4. ARS data of ZnO:Al thin films etched for different durations in 0.5 w/w% HCl with 5 mol%

FeCl3. The angular intensity distribution of a substrate etched in HCl without addition of a catalyst is

plotted in gray (dashed line).

It had been reported earlier that ARS is a powerful tool for gaining a characteristic

surface fingerprint of etched ZnO:Al thin films [40-42]. Furthermore, the scattering intensity

to higher angles in the range of roughly 60° to the substrate normal correlates with the short-

circuit current density Jsc in thin-film Si solar cells, especially µc-Si:H single junction

devices [42]. The data presented in Fig. 4 show that the scattering is shifted from lower to

higher angles when utilizing an etching process based on the catalytical effect of FeCl3. It is

known that smaller features in general lead to a scattering into higher angles, while bigger

features scatter predominantly into lower angles [42]. Thus, the ARS data are coherent with

the SFM-based observation of smaller surface features compared to the standard Jülich

reference (Fig. 3 and Table I). A positive influence on Jsc could hence be expected for

µc-Si:H single junction solar cells on these catalytically etched films.

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3.5 Evaluation in solar cells

To check the catalytically etched films in terms of suitability for an application as front

contact, we co-deposited µc-Si:H p-i-n single junction solar cells onto films etched for

different durations in 0.5 w/w% HCl containing 5 mol% FeCl3. As a reference, a standard

Jülich-type ZnO etched for 40 s in 0.5 w/w% HCl was processed in the same deposition run.

Note that the utilized ZnO:Al thin films are the very same substrates that have been

characterized with ARS (Fig. 4). Table II shows the photovoltaic parameters of the prepared

solar cells, namely the initial conversion efficiency init, the fill factor FF, the open-circuit

voltage Voc, and the short-circuit current density Jsc.

Table II. Photovoltaic parameters and Raman crystallinity of µc-Si:H p-i-n single junction

solar cells on etched ZnO:Al thin films. The errors given are the standard errors of the mean of all measured, intact cells on one substrate.

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etchant etch duration

[s] init [%] FF [%] Voc [mV] Jsc [mA cm-2] RSCI [%]

without catalyst (reference)

40 8.0 ± 0.2 67.9 ± 0.7 500 ± 4 23.5 ± 0.3 59.5

15 7.4 ± 0.3 66.0 ± 0.9 496 ± 2 22.7 ± 0.6 59.7 25 7.6 ± 0.3 65.1 ± 1.5 500 ± 4 23.2 ± 0.5 55.7 with 5 mol%

FeCl3 30 8.0 ± 0.1 68.1 ± 0.9 513 ± 6 22.8 ± 0.4 55.3

Looking just at the overall efficiencies, the reference cells perform best in this

comparison, but the solar cells with a front contact etched catalytically deliver very similar

parameters, especially for longer etch durations. Increased etch times go along with an

increase in Voc and FF. We assume that the growth of the microcrystalline Si is responsible

for the Voc increase. The degree of crystallinity that is achieved during growth of µc-Si:H

strongly depends on factors such as the material properties and the topography of the

underlying front contact. Table II shows that of the Si absorber in the reference cell is at

59.5% and decreases from a similar value at 15 s of FeCl3-aided etching to 55.7 and 55.3% at

25 und 30 s, respectively. A decrease in crystallinity means an increase in the band gap of the

Si absorber that defines the theoretical limit of Voc [43]. Hence, the increase in Voc with etch

time is coherent with the measured values. The reasons for this may be (i) the smaller

feature sizes on the catalytically etched substrates and (ii) presumably residues of the iron salt

or its reaction products at the ZnO:Al surface that may influence the Si growth. In Jsc, there is

an optimum after an etch duration of 25 s, being within the standard error of the reference's

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Jsc. The cells after 30 s of FeCl3-catalyzed etching deliver the same init as the reference due

to a gain in Voc of roughly 10 mV.

Fig. 5. (a) EQE of representative µc-Si:H p-i-n single junction solar cells deposited on a ZnO:Al front

contact textured in 0.5 w/w% HCl with 5 mol% FeCl3 for different etch times. (b) Individual EQE,

including the individual Jsc values, of the two component cells in a representative a-Si:H/µc-Si:H

tandem p-i-n solar cell deposited on a ZnO:Al front contact textured in 0.5 w/w% HCl with 5 mol%

FeCl3 for 30 s. The sums of both component cells, being the overall EQE, are plotted as dashed lines.

The EQE of reference cells from the same absorber deposition processes on substrates etched in HCl

without addition of a catalyst are plotted in gray.

The EQE data of representative cells from this series (Fig. 5a) confirm the observation

that the reference cell (gray curve) as well as the cell with a front contact etched with addition

of FeCl3 for 25 s (red curve) perform similarly, especially at >600 nm. Especially the

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variation in Jsc for different etch times are almost within an error range that can occur just due

to inhomogeneities in the deposition. Nevertheless, there are small variations in the EQE for

different wavelength regions as a function of the etch time. Etching for only 15 s (black

curve) leads to a loss in the EQE over the whole spectral range compared to the reference,

while longer etching for 30 s (blue curve) results in a higher EQE at small wavelengths, but a

slight deterioration above 600 nm. On the one hand, this can be correlated to the tendency of

the cells to become more amorphous with increasing etch time (cf. Voc and in Table II).

On the other hand, the ZnO:Al thickness and the development of the morphology of course

plays a role as well. 15 s FeCl3-aided etching (black curve) means thick ZnO:Al and only

slight structure development, leading to losses over the whole spectral range. At 25 s (red

curve), the film is thinner and thus lets more light pass into the cell (short wavelength gain),

and the structures are further developed (long wavelength gain due to light trapping). 30 s of

etching (blue curve) mean that even more light passes through the very thin ZnO:Al (stronger

gain at <600 nm), but the craters reach the glass substrate and thus the structure depth is

reduced again (loss at long wavelengths due to deterioration of light trapping).

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Table III. Photovoltaic parameters and Raman crystallinity of a-Si:H/µc-Si:H tandem

p-i-n solar cells on etched ZnO:Al thin films (cf. Fig. 3). The errors given are the standard errors of the mean of all measured, intact cells on one substrate.

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Pretreatment

conditions init [%] FF [%] Voc [mV] Jsc [mA cm-2] RSbottomC,I [%]

40 s in 0.5 w/w% HCl (reference)

10.5 ± 0.2 68.2 ± 1.0 1410 ± 10 10.89 ± 0.13 40.6

30 s in 0.5 w/w% HCl + 5 mol% FeCl3

10.6 ± 0.1 69.7 ± 0.4 1379 ± 8 11.04 ± 0.14 40.9

As the FeCl3-based process has proven its suitability to etch ZnO:Al thin films for

µc-Si:H single junction devices, we also prepared a-Si:H/µc-Si:H tandem p-i-n solar cells on

FeCl3-etched ZnO:Al (Table III). This cell concept bears the advantage that the a-Si:H top

component cell (which is deposited directly onto the ZnO:Al thin film) is less sensitive in its

growth than µc-Si:H, meaning that the aforementioned crystallinity changes should not occur

in the tandem cell concept. In this specific cell configuration, a representative reference cell

deposited on ZnO:Al etched without influence of a catalyst delivered short-circuit current

densities for the top (Jsc,top) and bottom (Jsc,bottom) component cell of 10.9 and 11.7 mA cm-2,

respectively (Fig. 5b, gray curve). By adding FeCl3 to the etchant, we were able to improve

the light-trapping efficiency in both the top and bottom component cell which is seen as an

16

increase in Jsc,top of 0.2 mA cm-2 and in Jsc,bottom of 0.3 mA cm-2 (Fig. 5b, black curve),

improving init compared to the reference as well. This improvement is at the cost of a loss of

roughly 30 mV in Voc and at similar crystallinities of the bottom component cell absorber

(Table III).

In addition, µc-Si:H single junction as well as a-Si:H/µc-Si:H tandem p-i-n solar cells

have been prepared on some subtrates etched under the contribution of other Fe(III) salts.

Those cell results are collected in the supporting material (Tables SI and SII, Figs. S2 and S3)

and are generally in agreement with the expectations from the film characterization

experiments.

4. Conclusions and outlook

We have shown that the acidic etching of polycrystalline, sputter-deposited ZnO:Al thin

films may be accelerated efficiently and in a well-controllable manner by adding catalytic

amounts of an Fe(III) salt to the etchant. The Fe(III) acts as a catalyst in the etching

mechanism and alters it in favor of a faster overall reaction, leading to an approximate

doubling of the etch rate. The best and most homogeneous results have been achieved when

utilizing FeCl3 as Fe(III) source. As a result of the catalytic influence of Fe(III), the resulting

morphology of the etched ZnO:Al thin film is altered towards slightly smaller features

compared to non-catalytically etched substrates. This leads to a shift of the light scattering at

the film/air interface to larger angles which can be advantageous for the light trapping in a

thin-film Si solar cell if such ZnO:Al thin films are used as the front contact.

The degree of acceleration as well as the extent of the change in morphology may be

controlled by changing the catalyst concentration or the concentration of the etchant. Thus,

we have got a method at hand to decrease the etch duration of an industrially relevant etching

process with no adverse or, in some cases, even beneficial effects on the light trapping in the

device. Such an additional process parameter is of significant practical importance, e.g., for

the adjustment of process step times in an in-line production system, and it upvalues the

industrial applicability of the well-established ZnO etching process.

17

Acknowledgments

The authors thank Mengfei Wu for ARS measurements, Simone Bugdol, Joachim

Kirchhoff, Daniel Weigand, and Thomas Zimmermann for assistance in solar cell deposition

and characterization, Markus Hülsbeck for Raman measurements as well as Nicole Lühmann,

Matthias Meier, and Eerke Bunte (all Forschungszentrum Jülich GmbH) for fruitful

discussions. Financial support by the German Federal Environment Ministry (BMU, grants

0327693A and 0325299A) is gratefully acknowledged.

18

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Supporting material

The catalytic effect of iron(III) on the etching of ZnO:Al

front contacts for thin-film silicon solar cells

Sascha E. Pust*, Janine Worbs, Gabrielle Jost, Jürgen Hüpkes

Determination of sheet resistances

a

0 10 20 30 40 5

0

2

4

6

8

0

10 mol% FeCl3

05 mol% FeCl3

01 mol% FeCl3

00 no addition

incr

ease

in s

heet

res

ista

nce

R

sh [

]

etch time t [s]

b b

0 10 20 30 40 50

0

1

2

3

4 10 mol% FeCl

2

05 mol% FeCl2

01 mol% FeCl2

00 no addition

incr

ease

in s

heet

re

sist

ance

R

sh [

]

etch time t [s]

0 10 20 30 40 5

0

1

2

3

4

5

0

10 mol% K3[Fe(CN)

6]

05 mol% K3[Fe(CN)

6]

01 mol% K3[Fe(CN)

6]

00 no addition

incr

ease

in s

heet

res

ista

nce

R

sh [

]

etch time t [s]

Fig. S1. Increase in sheet resistance Rsh as a function of etch time t for etching in 0.5 w/w% HCl

containing different concentrations of (a) FeCl3, (b) FeCl2, (c) K3[Fe(CN)6]. As a reference, Rsh of a

substrate etched in HCl without addition of a catalyst is plotted in gray (dashed line). Lines are solely

meant to guide the eye.

* Corresponding author: Dr. Sascha E. Pust, s.pust@fz-juelich.de

1

In addition to the measured film thicknesses, the sheet resistances Rsh have also been

used to control and quantify the ZnO:Al thin film etching with and without catalytic

influence. The increases in sheet resistance (Rsh) as a function of the etch time t are

reproduced above for the experiments with FeCl3 and for the reference experiments with

FeCl2 and K3[Fe(CN)6] (Fig. S1). In general, the results of the determined trends in Rsh and

the thickness changes d (cf. Fig. 2 of the paper) are qualitatively coherent, although the

trends in the dependence on catalyst concentrations are not as clear as they have been in the

case of d.

2

Solar cells on ZnO:Al etched catalytically with Fe(NO3)3 and Fe2(SO4)3

Table SI. Photovoltaic parameters of µc-Si:H p-i-n single junction solar cells on etched ZnO:Al thin films. The errors given are the standard errors of the mean of all measured, intact cells on one substrate.

etchant etch

duration [s] init [%] FF [%] Voc [mV] Jsc [mA cm-2] RSCI [%]

without catalyst (reference)

40 8.0 ± 0.2 67.9 ± 0.7 500 ± 4 23.5 ± 0.3 59.5

15 7.5 ± 0.2 66.1 ± 0.4 501 ± 3 22.5 ± 0.3 52.7 with 5 mol% Fe(NO3)3 20 7.5 ± 0.1 66.9 ± 0.9 504 ± 2 22.2 ± 0.3 54.4

15 6.7 ± 0.3 64.3 ± 1.4 488 ± 9 21.3 ± 0.5 50.2 with 5 mol% Fe2(SO4)3 20 5.9 ± 0.2 60.1 ± 1.0 469 ± 9 20.8 ± 0.3 58.7

Table SII. Photovoltaic parameters of a-Si:H/µc-Si:H tandem p-i-n solar cells on etched ZnO:Al thin films (cf. Fig. 3 in the paper). The errors given are the standard errors of the mean of all measured, intact cells on one substrate.

Pretreatment conditions init [%] FF [%] Voc [mV] Jsc [mA cm-2] RSbottomC,I [%]

40 s in 0.5 w/w% HCl (reference) 10.5 ± 0.2 68.2 ± 1.0 1410 ± 10 10.89 ± 0.13 40.6

15 s in 0.5 w/w% HCl + 5 mol% Fe(NO3)3

10.3 ± 0.2 77.8 ± 0.8 1388 ± 3 9.52 ± 0.10 45.7

15 s in 0.5 w/w% HCl + 5 mol% Fe2(SO4)3

9.3 ± 0.1 72.5 ± 1.3 1381 ± 5 9.25 ± 0.15 43.9

a b

400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

23.7 mA cm-2

22.5 mA cm-2

400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

23.7 mA cm-2

21.5 mA cm-2

22.2 mA cm-2

no addition (40 s) 5 mol% Fe(NO

3)

3 (15 s)

5 mol% Fe(NO3)

3 (20 s)ex

tern

al q

uan

tum

effi

cie

ncy

EQ

E

wavelength [nm]

21.0 mA cm-2

no addition (40 s) 5 mol% Fe

2(SO

4)

3 (15 s)

5 mol% Fe2(SO

4)

3 (20 s)ex

tern

al q

uan

tum

effi

cie

ncy

EQ

E

wavelength [nm]

Fig. S2. EQE of a representative µc-Si:H p-i-n single junction solar cell deposited on a ZnO:Al front

contact textured in 0.5 w/w% HCl (a) with 5 mol% Fe(NO3)3, (b) with 5 mol% Fe2(SO4)3. The EQE of a

reference cell from the same absorber deposition process on a substrate etched in HCl without

addition of a catalyst is plotted in gray (dashed line).

3

a b

400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0 no addition (40 s) 5 mol% Fe(NO

3)

3 (15 s)

exte

rnal

quan

tum

effi

cienc

y E

QE

wavelength [nm]

9.9 mA cm-2

10.8 mA cm-2

11.7 mA cm-2

10.9 mA cm-2

400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0 no addition (40 s) 5 mol% Fe

2(SO

4)

3 (15 s)

exte

rnal

quan

tum

effi

cienc

y E

QE

wavelength [nm]

8.9 mA cm-2

10.1 mA cm-2

11.7 mA cm-2

10.9 mA cm-2

Fig. S3. Individual EQE, including the individual Jsc values, of the two component cells in a

representative a-Si:H/µc-Si:H tandem p-i-n solar cell deposited on a ZnO:Al front contact textured in

0.5 w/w% HCl (a) with 5 mol% Fe(NO3)3 for 15 s, (b) with 5 mol% Fe2(SO4)3 for 15 s. The EQE of a

reference cell from the same absorber deposition process on a substrate etched in HCl without

addition of a catalyst is plotted in gray. The sums of both component cells, being the overall EQE, are

plotted as dashed lines.

The data in Tables SI, SII, Figs. S2a and S3a clearly show that the cell parameters and

EQE deteriorate slightly over the whole spectral range when using Fe(NO3)3. In the case of

tandem cells, this is especially true for the bottom component cell. For the Fe2(SO4)3-treated

substrates (Figs. S2b and S3b), this effect is even stronger with losses of up 2.7 mA cm-2

compared to the reference cells. However, this should not surprise on account of the

inhomogeneities and salt residues that come with the Fe2(SO4)3 treatment. The Fe(NO3)3 and

Fe2(SO4)3-treated tandem cells are clearly bottom-limited and show increased bottom

component cell crystallinities , explaining elevated FF and decreased Voc values

(Table SII).

RSbottomC,I

4