Thin film silicon technology. Cosimo Gerardi 3SUN R&D Tech. Coordinator

Transcription

1 Thin film silicon technology Cosimo Gerardi 3SUN R&D Tech. Coordinator 1

2 Outline Why thin film Si? Advantages of Si thin film Si thin film vs. other thin film Hydrogenated amorphous silicon Energy gap / band gap engineering Tandem junction: amorphous/microcrystalline Si Triple junction and multiple junctions Light trapping Technology roadmap

3 Solar Cell Technology: Why Thin Film Si? Solar cell Si raw material Efficiency Peak power Peak power c-si g/m 2 16% 160W/m W/g TF-Si 5 g/m 2 10% 100W/m 2 20W/g Large area on glass Flexible plastics Transparency

4 Air mass T. Watanabe, Sharp, 4th Saudi Solar Energy Forum, May 8-9t, Riyadh Saudi Arabia (2013)

5 Temperature coefficient 1.05 Normalized output amorphous c-si Temperature ( C) The output of thin-film silicon solar cell decreases by only 0.23% when temperature increases by one degree, while that of crystalline silicon cell decreases by 0.45% (Sharp@IEEE-IEDM 2008)

6 Robust structure: long term stability T. Watanabe, Sharp, 4th Saudi Solar Energy Forum, May 8-9t, Riyadh Saudi Arabia (2013)

7 Structure of glass-glass module High barrier encapsulant material

8 a-si:h distribution of density of allowed energy states for electrons Direct optical transitions are not forbidden in amorphous Si (because of disorder) Better light absorption than c-si Heavily hydrogenated network. 1-10% H2 content 8

9 Conventional p - n junction solar cell n p E C V p BSF E F Metal grid Sunlight Antireflective layer n-type layer E V V bi h + p-type layer e - hν > E G Electron For an abrupt p-n junction with constant doping on each side there are no electric field outside the depletion region. Photogenerated carriers in these regions are collected by a diffusion process while in the depletion region by drift Hole n p p + J L Back Metal Contact Current Density JM J SC Dark V M Light V OC Voltage

10 Amorphous a-si:h: p-i-n p i n Carriers are photogenerated in the intrinsic region and collected by drift p and n doped layers are very thin 15-20nm to allow all available photons to absorbed by the i-layer. The excess doping level induces many defects/traps in n and p layers that recombine the photo-generated carriers Typically the p-a-si:h layer is a p-a-sic:h layer (Eg~2eV) that The I a-si: layer must be below 300nm to reduce Staebler-Wronsky light induced degradation

11 Energy gap Thermalization losses One junction Light Energy=hυ (a) hυ>>eg (b) hυ>eg ~ Thermalization losses Absorbed light Multiple junction Light Eg1 Eg2 Eg3

12 Enhanced absorption: double junction/tandem Light intensity (kw/m 2 µm) spectrum splitting. Light glass TCO a-si:h Top cell µc-si:h Top cell Back reflector Wavelength (nm) ~3mm ~2µm Amorphous Eg= eV «High» absorption in the green-blue Microcrystalline Eg=1.1eV «High» absorption in the red-near I.R. Micromorph cell efficiency 11-14% Micromorph module efficiency %

13 Amorphous and Microcrystalline silicon Two materials with the same process: PECVD a-si:h Eg=1.8eV µc-si:h Eg=1.1 ev Typical process temperature: -180C a-si:h -150C µc-si:h Plasma conditions: MHz 10-15kW 13 Deposition rate ~ 0.5nm/s Cathode Plasma Anode Substrate

14 From Single to Multiple junctions Single Junction asi:h cell with enhanced light trapping TCO and Texturing Efficiency: 6 to 8% on module Double Junction / Tandem cell highest theoretical efficiency: combination of absorber materials having band gap 1.8 ev (a-si:h) for the top and 1.1 ev (µc-si:h) for the bottom cell. Efficiency 12.5% on cell 10% on large module Triple junction / Quadruple Junction a-sige:h middle absorber a-si:h/a-sige:h/ µc-si:h Efficiency: 14-15% on cell, 12% on large module Eg: eV glass textured TCO a-si:h top absorber Possible drawbacks of triple junction: Reduced throughput:~ 25% lower with respect to Tandem Eg: 1.45eV Eg: 1.1eV a-sige:h middle absorber µc-si:h bottom absorber Power stabilization weakness (light induced degradation) of a-sige:h (15% to 18% LID degradation factor) Quadruple Junction Approach: Higher Voc and improved LID degradation ZnO Ag

15 Light trapping Asahi VU APCVD (SnO 2 :F) Asahi W light glass TCO ~700nm p-i-n a-si:h ~250nm p-i-n uc-si:h ~1.6µm ZnO:B -MOCVD W text ZnO TCO ~50nm Back reflector Texturing causes light scattering, increasing the optical path of photons in silicon Natural texturing can be achieved during the CVD deposition process Double feature texture (possible both for SnO2 and ZnO): higher and smaller texturing shapes can be reached (but not ready for production!)

16 Impact of texturing and different TCO material Transmittance(%) TCO a-si:h Haze (%) µc-si:h BR low haze TCO high haze TCO ZnO has better transmittance at long wavelengths Higher Haze can be achieved with MOCVD Texturing (increases optical path) improves the currents generated in the top and bottom cells EXTERNAL QUANTUM EFFICIENCY ZnO:B SnO 2 :F a-si:h µc-si:h ZnO - H=20% ZnO - H=20% ZnO - H=20% SnO2-H=10% SnO2-H=10% SnO2-H=10% Wavelength (nm)

17 Thin Film Si Roadmap Improving absorber layers and cell structure Double J a-si:h / µc-si:h Triple J a-si:h / a-sige:h/µc-si:h Multi Junction Full spectrum TCO Eg1 Eg2 Eg3 Eg4 Back contact Multiple gap solar cell Efficiency: 9-10% 10-12% 12-14% 14-16% Improved light trapping SnO2:F U-Valley ZnO Double TCO Plasmonics

18 Long term research: beyond 16% efficiency Ultra-thin c-si 3D Structures Ag Nanowires in TCO Plasmonic resonators Si - nanowires solar cells EU Research Programs 18