Van der Waals-Zeeman Institute, University of Amsterdam

“Twee halen - een betalen”

Si nano-photovoltaics

Tom Gregorkiewicz

Preferred solutions for energy• Use processes occurring in nature

- do not produce “new” components(nuclear waste, CFC, …)

- CO2, CO, SO2 do occur in nature but in small quantities (e.g. burning of wood)

• The scale needs to be “small” (best negligible) when compared to those occurring naturally

Absorption of solar energy is a natural process

PV “shapes” this natural process in the way useful to men, using only a (very)

small partVan der Waals-Zeeman Institute, University of Amsterdam

Calibrating the energy needs

Daily food consumption:2000 cal/day 100 W ~ 1 kW

Solar power: 120.000 TW

~0.02% of the total is enough to power our civilization!

2 kW pp 13 TW (2010) 28 TW (2050)

Van der Waals-Zeeman Institute, University of Amsterdam

– light low/high temperature heat

– light electricity

Main solar energy conversion options

Van der Waals-Zeeman Institute, University of Amsterdam

– light low/high temperature heat

– light electricity

– light chemical energy

(solar fuels, art. photosynthesis)

Main solar energy conversion options

Van der Waals-Zeeman Institute, University of Amsterdam

Jimmy Carter at SERI (now NREL) May 5, 1978

Oil crisis of the 1970’sDon’t worry Mr.

President, solar will be economical in 5

years!

I can’t believe he said that.

Van der Waals-Zeeman Institute, University of Amsterdam

“Global warming” crisis

Barack Obama at Nellis AFB May 2009Van der Waals-Zeeman Institute, University of Amsterdam

Van der Waals-Zeeman Institute, University of Amsterdam

Van der Waals-Zeeman Institute, University of Amsterdam

Solar electricity solutions

• Indirect conversion: light-high T heat-electricitySolar thermal energy: photons-to-phonons-to-electrons

- without energy storage- with energy storage

Van der Waals-Zeeman Institute, University of Amsterdam

Solar thermal power

Van der Waals-Zeeman Institute, University of Amsterdam

Solar thermal power

Van der Waals-Zeeman Institute, University of Amsterdam

Solar electricity solutions

• Indirect conversion: light-high T heat-electricitySolar thermal energy: photons-to-phonons-to-electrons

- without energy storage- with energy storage

• Direct conversion: light-to-electricityPhotovoltaics: photons-to-electrons

- without light concentration- with light concentration

Van der Waals-Zeeman Institute, University of Amsterdam

Van der Waals-Zeeman Institute, University of Amsterdam

load

topmetal

contact

bottommetal

contact

activematerial

(with asymmetryfor charges)

mobile negative chargemobile positive charge

Photovoltaic cell

Courtesy W. Sinke, ECN

Researchers at Bell Labs, N.J. (USA)1953, first photovoltaic solar cells based on silicon( 5%)

In 1954, the U.S. News & World Report wrote :…..one day such silicon strips……“may provide more

power than all the world’s coal, oil and uranium”

PV history

Van der Waals-Zeeman Institute, University of Amsterdam

17th March 1958: The Vanguard 1 satellite with solar panels - 0.1 watt peak power – is put onto orbit

PV history

Van der Waals-Zeeman Institute, University of Amsterdam

Van der Waals-Zeeman Institute, University of Amsterdam

Van der Waals-Zeeman Institute, University of Amsterdam

Polycrystalline silicon – a cheap & easy-to-make alternative

Van der Waals-Zeeman Institute, University of Amsterdam

PV application limits?

Van der Waals-Zeeman Institute, University of Amsterdam

Source: PhotonInternational

March 2010

Van der Waals-Zeeman Institute, University of Amsterdam

Thin film

1979

2009

wafer Si

silicon feedstockshortage

2007

2009

22% price decrease for everydoubling of cumulative production

Source: EPIA, October 2009

Price development

Van der Waals-Zeeman Institute, University of Amsterdam

• Over 90% of today’s PV modules are based on Crystalline Silicon

• Excellent performance modules: ~20% lab: up to ~25%

Current status PV

Van der Waals-Zeeman Institute, University of Amsterdam

Silicon for PV

• indirect bandgap• low emission/absorption rates (at low energies)

Van der Waals-Zeeman Institute, University of Amsterdam

Silicon and light

gap energy

heat generation

recombination

light

X

X

X

PV conversion – basic concept

Van der Waals-Zeeman Institute, University of Amsterdam

wavelength [nm]

1.6

1.2

0.8

0.4

400 800 1200 1600 2000 24000.0

available for conversion in crystalline Si

infraredvisibleUV

solar spectrum (Air Mass 1,5; 1000 W/m2)

po

we

r [W

/(m

2 .n

m)]

1100 nm 1.1 eV = Si bandgap

courtesy John Schermer, KUN

X

X

PV conversion loses

Van der Waals-Zeeman Institute, University of Amsterdam

Shockley-Queisser limit

Conversion efficiency maximum for single junction PV cell with Egap=1.1 eV (≈ 31 %)

Van der Waals-Zeeman Institute, University of Amsterdam

• Optimal bandgap energy • Abundant • Mechanically strong • High mobilities possible

Si for photovoltaics

Van der Waals-Zeeman Institute, University of Amsterdam

• Manipulate band-structure• Light management:

– waveguiding, cloaking, multiple reflection, dispersing

• Si nanowires• Si nanocrystals• Quantum cutting and pasting

“Smart” solutions for Si PV

Van der Waals-Zeeman Institute, University of Amsterdam

TGG

TGG

TGG

Si nanocrystals

Nanocrystals (NCs)

• Bandstructure modification induced by quantum confinement

• Bands → quantized energy levels• Relaxation of k-vector conservation for indirect

bandgap• Tuning optical properties

Silicon

4.3 nm

SiNC

Paillard et al., Tolouse

Si Nanocrystals in SiO2

Van der Waals-Zeeman Institute, University of Amsterdam

VB

CB

PL

SiNC

Van der Waals-Zeeman Institute, University of Amsterdam

Si NC photoluminescence

VB

CB

PL

SiNC

Van der Waals-Zeeman Institute, University of Amsterdam

Si NC photoluminescence

SiNC

Van der Waals-Zeeman Institute, University of Amsterdam

Si NC photoluminescence

VB

CB

PL

SiNC

Auger

Van der Waals-Zeeman Institute, University of Amsterdam

Si NC photoluminescence

Si NC PL saturation

Van der Waals-Zeeman Institute, University of Amsterdam

• photon convertors:size-tunable energy

• photon limitersonly one photon out

Van der Waals-Zeeman Institute, University of Amsterdam

Si nanocrystals

Hot electrons are not used!

Using “hot electrons”: Cutting

and emitting photons with Si-NCs

PL from SiNCs in SiO2

Van der Waals-Zeeman Institute, University of Amsterdam

λexc = 323 nmf = 3.8 MHz

MCMPMT

• 370 ≤ λdet ≤ 700 nm• τresolution ~25 ps

~2 psPL

O-related PL

Hot PL

Excitonic recombination

~μs~nsτ1 ≈ 25 psτ2 ≈ 100 ps

PL from SiNCs d=4.5 nm

Hot PL for all the samples

Van der Waals-Zeeman Institute, University of Amsterdam

3.32 eV 1.17 eV

Direct

Indirect

Si Nanocrystal

Theoretical model

Van der Waals-Zeeman Institute, University of Amsterdam

Pulsed vs. semi-cw excitation

1 – 10 ps

~μsNIR

~ns420 nm

Pulsed~2 ps

~5 ns

10 – 100 ps

Semi-cw

~μsNIR

~ns420 nm

Auger

cooling

<Nexc><1

<Nexc>>1

“hot” PL in Si NC 1000 stronger than in bulk Si

hot PL

s-PL≈ 5 hot PL

s-PL≈ 1

W.D.A.M. de Boer et al. Nature Nanotechnology 2010

Relative efficiency

enhanced emission and absorption in the visible

Van der Waals-Zeeman Institute, University of Amsterdam

Cutting photonswith Si NCs

Solid state sample: SiO2:Si-NCs

Colloidal sample: SiNCs in ethanol

• HF chemical etching: po-Si • suspended in ethanol

Experimental setup

Absolute QE of Si-NCs PL

Van der Waals-Zeeman Institute, University of Amsterdam

Q.E. for different wavelengths in visible and near UV

η is constant up to a photon energy

threshold of Ethreshold ≈ 2Egap

For larger photon energies a second excitation mechanism takes place

Definition relative quantum efficiency:

η =Nem

Nabs

Relative quantum efficiency

Van der Waals-Zeeman Institute, University of Amsterdam

Multi-excitongeneration

(MEG)

Space-separated quantum cutting

(SSQC)

Eexc ≥ 2Egap

Quantum cutting with Si-NCs

D. Timmerman et al., Nature Photonics (2008)

SSQC with SiNCs in SiO2

Eexc >2Egap

1 in → 2 out

Van der Waals-Zeeman Institute, University of Amsterdam

Quantum cutting with Si-NCs

QE is constant up to photon energy threshold of hν ≈ 2Eg

~100 % increase of initial value

Step-like behavior

Two types of Si-NC samples: Si-NCs in SiO2

po-Si in EtOh

In two different calibrated QE setups

Van der Waals-Zeeman Institute, University of Amsterdam

Shockley-Queisser limit

Conversion efficiency up to 44%!!!

D. Timmerman et al., under review Nature Materials

PV impact

Van der Waals-Zeeman Institute, University of Amsterdam

• XXIst century will begin“(Si) Solar Energy Age”

• Reaching ultimate PV cost and performance levels at sufficient sustainability critically depends on (Si) materials development

Conclusion

“you have seen nothing yet”

Van der Waals-Zeeman Institute, University of Amsterdam

TGG at WZI, UvA

Van der Waals-Zeeman Institute - UvA