Synergy

Nano-Tera Annual Plenary Meeting

Synergy Project 4th May 2015

JĂŠrĂŠmie Werner

jeremie.werner@epfl.ch

Goal of the project “Realizing photovoltaic energy harvesting systems based on tandem solar cells with efficiency beyond that achievable with state-of-the-art industrial single-junction cells” Record Efficiencies:  Silicon solar cells: 25.6% with SHJ  CIGS: 21.7 %

CdTe Perovskite

Close to practical limit: 26-27% for Si

Solution: Multi-junction solar cells Source: U. Sydney, updated values added

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Multi-junction solar cell concept Si

ďƒ˜ Potential for very high efficiency ďƒ˜ Careful choice of the top cell material is important!

Source: pveducation.org

Source: Empa, TFPV

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Top cell candidates GaAs nanowire arrays

Perovskite solar cells A = large cation (CH3NH3, Cs) B = small cation (Pb, Sn) X = halogen (I, Cl, Br) e.g.: CH3NH3PbI3

Fontcuberta et al., Nature Photon. (2013)

Green et al., Nature Photon. (2014)

Eg(GaAs) = 1.42 eV

Eg(MAPbI3) = 1.56 eV

Potentially beyond Schokley-Queisser limit, in single nanowire solar cell

Efficiency: up to 20.1 % (certified, not-stabilized)

jeremie.werner@epfl.ch

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GaAs nanowire arrays on Si

Eg(GaAs) = 1.42 eV Eg(Si) = 1.1 eV Fontcuberta et al., Nature Photon. (2013)

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GaAs nanowire arrays on Si: Embedding GaAs NWs solar cells in PDMS film Contacted by ITO from both sides, GaAs NWs can act as a top solar cell, and at the same time, transparent PDMS film provides high transmittance of the incoming light to the bottom solar cell.

1 µm

ITO PDMS NW ITO

Scheme of NW-PDMS composite with ITO contacts

1 cm NW-PDMS composite

10µm 1 µm

Top SEM view of as-grown sample

Embedding NWs into PDMS – process steps

1. As-grown sample

2. Spin-coating of PDMS solution on sample

3. Peeling of PDMSNW composite from substrate

Top SEM image of single NW

100100 nmnm embedded in PDMS and covered

by ITO

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GaAs nanowire arrays on Si: Optical properties of GaAs NWs embedded in PDMS Transmittance vs NW density, NWs in PDMS 100

1

Density of NWs in PDMS film

2

1 cm Transmittance

3

Transmittance, %

1

80

2

60 40

10µm

3

20 0 400

500

600

700

800

900 1000

Wavelength, nm

There is strong dependence of reflectance and transmittance of PDMS-NW composite from parameters of NW forest – density, diameter and length. After developing pilot devices, optimizing of size parameters of NW forest should improve effectivity of tandem solar cell.

Absorbance of light in NWs also depends on diameter as was shown in [1,2].

[1] Matteini, F. et al. Tailoring the diameter and density of self-catalyzed GaAs nanowires on silicon. Nanotechnology 26 (2015) 105603 [2] Krogstrup, P. et al. Single-nanowire solar cells beyond the Shockley–Queisser 100limit. nm Nat. Photonics 7 (2013) 306–10

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Perovskite solar cells: What are the challenges?

Green et al., Nature Photon. (2014)

Superstrate / encapsulation Transparent contact Perovskite top cell Transparent contact Lee et al. Science (2012)

Optical coupling/ interconnection Transparent contact Si or CIGS bottom cell Rear contact Substrate / encapsulation

• Electrodes with broadband transparency • Parasitic absorption in charge transport layers • Architecture and design • Low-temperature processing jeremie.werner@epfl.ch

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Low-temperature processing of mesoporous TiO2 A top cell on a-Si/c-Si heterojunction or on CIGS needs to be processed below 200°C. Higher temperatures would harm the performance of these two solar cell types.

ALD underlayer Al2O3: Low temperature TiO2: High temperature

JSC VOC 2 (mA/cm ) (mV)

FF (%)

ɳ (%)

Al2O3, low-T

19.44

1034 69.7

14.27

TiO2, high-T

20.38

1048

73

15.89

JSC VOC 2 (mA/cm ) (mV)

FF (%)

ɳ (%)

TiO2

ALD underlayer and TiO2 particles no sintering Difference between sintered and not sintered

sintering (500°C)

16.53

988

78

13.04

19.3

1022

75

15.25

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High transparency in the near-infrared: IZO/MoO3

Ne

M. Morales-Masis et al. Submitted IEEE JPV

ďƒ˜ Amorphous transparent conductive oxides with high transmittance (visible and NIR) ďƒ˜ High mobility and low free carrier absorption (low Ne)

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High transparency in the near-infrared: IZO/MoO3

J. Werner et al., submitted  Semitransparent: 10.3% efficiency. Opaque: 12.5%  No FF and Voc losses!  Sheet resistance of IZO low enough for cell size (6 mm x 6 mm)  Sputter damage completely avoided

 Jsc losses due to lack of rear reflector  IZO absorbs <3% in the NIR  FTO limits NIR transparency of perovskite cell

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High transparency in the near-infrared: ZnO:Al (AZO)/MoO3

JSC VOC 2 (mA/cm ) (mV)

FF (%)

ɳ (%)

Mpp (%)

Opaque cell

20.68

1053 65.2

14.2

8.6

NIR-transparent cell

17.5

969

10.6

7.4

62.3

 Good homogeneity  Voc, Jsc and FF decreased after ZnO:Al sputtering!

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Vacuum deposition combined with solution processing

• Pin-hole free perovskite layer • Flat and homogeneous over 5x5cm2 substrates • Controllable thickness and composition • High sub-bandgap transmittance jeremie.werner@epfl.ch 13

Some adaptations on the bottom cell Superstrate / encapsulation Transparent contact Perovskite top cell Transparent contact Optical coupling/ interconnection Transparent contact

â&#x20AC;˘ Optimization of both a-Si/c-Si heterojunction cell and CIGS cell â&#x20AC;˘ Rear reflector, to boost infrared quantum efficiency

Si or CIGS bottom cell Rear contact Substrate / encapsulation jeremie.werner@epfl.ch 14

SHJ bottom cell with high near-infrared quantum efficiency: Rear reflector with low refractive index

â&#x20AC;˘

SiO2 doped ZnO:Al leads to lower reflactive index TCOs, reducing plasmonic losses in the rear metal reflector.

â&#x20AC;˘

Enhanced optical absorption: steady increase in Jsc and in infrared EQE

High quantum efficiency in the NIR is essential for high-efficiency tandem cells

A. Dabirian et al., manuscript in preparation jeremie.werner@epfl.ch 15

Four-terminal tandem: a-Si/c-Si heterojunction solar cells

no ARC

FF (%) ɳ (%)

JSC (mA/cm2)

VOC (mV)

Top cell

17.5

870

68

10.36

Bottom cell

14.6

690

77.6

7.82

Tandem

Tandem cell performance limited by parasitic absorption

18.18

J. Werner et al., submitted jeremie.werner@epfl.ch 16

Four-terminal tandem: CIGS solar cells

JSC (mA/cm2)

VOC (mV)

FF (%)

ɳ (%)

CIGS

34.7

687

73.8

17.6

CIGS, filtered

11.9

637

69.3

5.3

Perovskite top cell

19.6

1003

51.4

10.1 (5.3)

Tandem

Tandem cell efficiency: 15.4% (reverse IV), 10.6% (mpp)

15.4 (10.6) jeremie.werner@epfl.ch 17

Perovskite mini-module ~5 cm2 ~0.2 cm2

~100 cm2 Standard lab cell size

Mini-module size

Typical 4-inch wafer size

Challenges: 1) Obtain uniform perovskite layer over full substrate size 2) Eliminate pinholes in perovskite and transport layers

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Perovskite mini-module: Approach • • • • •

Laser scribing 6 segments in serial connection < 16% dead area Active area: 5.02 cm2 => efficiency: 6.6 % Total aperture area: 6 cm2 => efficiency: 5.52 %

Soo-Jin Moon et al. IEEE JPV, 2015 jeremie.werner@epfl.ch 19

Conclusion and Overview  First embedded GaAs nanowires and optical study  Perovskite solar cells are promising candidates for tandem solar cells with c-Si or CIGS bottom cells  Transparent electrode was successfully developed  First tandem devices produced  Preliminary work on upscaling: 5x5cm2 perovskite mini-module  Promising steps towards optimized tandem devices, with potential efficiencies beyond 30%

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Prof. Christophe Ballif

Prof. Michael Graetzel

Prof. Anna Fontcuberta i Morral

Thank you for your attention!

Dr. Julien Bailat

Prof. Ayodhya Tiwari jeremie.werner@epfl.ch 21