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