Research on high efficiency and low cost thin film silicon solar cells. Xiaodan Zhang

Research on high efficiency and low cost thin film silicon solar cells Xiaodan Zhang 2013 China-America Frontiers of Engineering, May 15-17, Beijing, China

Institute Institute of of photo-electronics photo-electronics thin thin film film devices devices and and technique technique

1. Crystalline silicon solar cells Mono-crystalline and poly-crystalline solar cells 2. Thin film solar cells Silicon-based thin film solar cells Copper Indium Gallium Selenium solar cells (CIGS) CdTe solar cells Dye sensitized solar cell (DSC) 3. Concentrated solar cells

Monocrystalline silicon 11.5 GW Thin film solar cells: ~10% Amorphous/microcrystalline silicon 1.26 GW CdTe 2.05 GW Multicrystalline silicon 21.2 GW others 0.3 GW CIS 0.9 GW Source:Photon Int. 2011 Total Production : 37 GW

Thin film silicon solar cells Advantages: No materials limited Low deposited temperature (<200 ) Deposited on different substrates Disadvantages: Low conversion efficiency Light induced degradation efficiency

Thin film Silicon solar cell structures: substrate configuration 1. Initial 16.3% efficiency has been achieved using a-si:h/a-sige:h/ c-si:h structure. -----nip Baojie Yan et al. Appl. Phys. Lett. 2011. ITO p I (a-si: H) n p I(a-SiGe: H) n p I( c-si: H) n ZnO Ag/Al SS light

Thin film Silicon solar cell structures: superstrate configuration 2. Stable 13.44% efficiency has been achieved using a-si:h/ c-si:h / c-si:h structure.- ------pin Soohyun Kim et al. 27 th EU-PVSEC 2012.

Thin film silicon PV technology Target: Low cost and high conversion efficiency 1. Key issue: increase efficiency Multi-junction for full use of solar spectrum New materials Advanced light trapping New structures New concepts 2. Key issue: reduce cost High deposition rate High stable efficiency Single chamber deposition Suitable large area 3. Summary and outlook

EQE (%) 南开大学 1. Key issue: increase efficiency New materials: Amorphous and microcrystalline silicon alloys Deposition technique: For example: c-siox PECVD, VHF-PECVD Applications: Doped layers Intermediate reflectors Index matching layers... 100 80 60 40 20 0 400 600 800 1000 Wavelength (nm) c-siox top c-siox bot c-si top c-si bot

Light trapping: Trap light inside the absorber layer Approaches: Effective light in-coupling at the front side Nano-structured ARC Scattering at rough interfaces Broad band light scattering Reflection at the back side Random structure Periodic structure

ZnO NW :

Reflection (%) 南开大学 20 18 16 14 12 10 8 6 4 2 Texture Si/ZnO NW Texture Si 0 400 600 800 1000 Wavelength (nm) 20 18 16 14 12 10 8 6 4 2 0

Reflection (%) 南开大学 ZnO NW : amorphous silicon 35 30 Thin film a-si/ito/zno NW Thin film a-si/ito 35 30 25 20 15 10 5 25 20 15 10 5 Xiaoping Yang et al. to be submitted 0 400 600 Wavelength (nm) 0

Scattering at rough interfaces To scatter the incident light for effective light trapping To increase the short circuit current density (Jsc) in the solar cells.

Scattering at rough interfaces Broad band light scattering: double period structure--sputtering+ MOCVD S L G Sputtering + Wet etching MOCVD Combined large-period and small-period triangular type

Miocromorph tandem solar cells with the innovative front electrode: Looks black Low reflection High current density Innovative Traditional Susu Yang et al. RSC Advances. 2013, 3, 208-214

a-si/ c-si tandem solar cells Optimization of microcrystalline silicon solar cells Incorporation of n type SiO x Two-phase structure Front transparent conductive oxide 22

Reflection and haze (%) EQE (%) 南开大学 Reflection at the back side CWBR: conductive white back reflector Random structure Periodic structure 400 600 800 1000 1200 100 100 80 80 60 40 20 0 DR-CWBR TR-CWBR Haze-CWBR DR-TBR TR-TBR Haze-TBR 0 400 600 800 1000 1200 Wavelength (nm) Lisha Bai et al. to be submitted 80 60 40 20 60 40 20 0 (ma/cm 2 ) CWBR, 22.61 (0V) CWBR, 23.10 (-2V) TBR, 19.28 (0V) TBR, 21.12 (-2V) 400 600 800 1000 Wavelength (nm)

Reflection and haze in reflection (%) 南开大学 Reflection at the back side Random structure Periodic structure :Polystyrene spheres (PS) 100 80 60 TR DR Haze in reflection 40 20 Xuejiao Liang et al. to be submitted 0 400 600 800 1000 1200 1400 Wavelength (nm)

2. Key issue: reduce cost High deposition rate High stable efficiency Single chamber deposition Suitable large area

A: High deposition rate High pressure and high power (3Torr and 70W(powered electrode area: >100cm 2 ) Home made Lisha Bai et al. PVSEC-22. 2012 VHF-PECVD system

Microcrystalline silicon solar cell Deposition rate: 1.5nm/s; Conversion efficiency: 9.87% 27

Home made B: Single chamber P, I, N deposited in a chamber Advantages High electrode utilization High gas utilization Easy operation and less servicing. Disadvantages Contamination-doping gas Serious for microcrystalline silicon

Nankai Univeristy Results and discussion B: Single chamber Boron contamination: p/i interface p i n High Xc interface layer Improved short wavelength response Suitable thickness

Nankai Univeristy Results and discussion B: Single chamber Phosphorus contamination: n/p interface p i n p i n Improved short and long wavelength response The μc-si: H covering layer is the best method to reduce phosphorus contamination.

a-si/ c-si tandem solar cells Single chamber

Large area: In 2011, developed a VHF-PECVD system with the capacity of 2MW/year. Home made 8 chambers in this system; 40.68 MHz; substrate area is 2 feet by 4 feet; Each chamber simultaneously deposited two pieces.

33 Uniformity distribution of electric field

1.2m 1.2m 34

Film Thickness (A) 南开大学 4000 3000 1st row 2nd row 3rd row 2000 1000 Table 1 Film Thickness(Ǻ) 2335 2352 2348 2334 2285 2349 2350 2345 2341 2234 2364 2445 2338 2314 2095 0 position1 position2 position3 position4 position5 Un-uniformity for thickness: Position point Less than 3.85% was achieved at edge exclusion 2cm 35

Crystalline volume fraction (%) Less than 4.8% was achieved at edge exclusion 2cm 南开大学 100 80 1st row 2nd row 3rd row 60 40 20 0 Table 3 Thin Film Xc(%) 47.3 44.2 47.2 45.8 45.8 48.1 44.9 47.4 45.3 45.4 52.8 46.6 50.9 47.5 47.6 position1 position2 position3 position4 position5 Position point 36 Un-uniformity for Xc:

Dark or Photo Conductivity (s/cm) 南开大学 0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 1E-11 1E-12 1st row dark 2nd row dark 3rd row dark 1st row photo 2nd row photo 3rd row photo position1 position2 position3 position4 position5 Position point 37 Un-uniformity for electrical properties: Dark conductivity: 10-8 S/cm; Photosensitivity: ~1000

13KW a-si/a-sige/ c-si solar module PV station

Summary Improving efficiency New Materials Light management ZnO NW ARC Innovative front electrode Back reflector Low Cost technique High initial conversion efficiency 12.39% for a micromorph tandem cell; Single chamber-deposited tandem solar cells with an efficiency of 10.59%; Triple-junction module has achieved at 9.59% (NREL, 0.79m2) and increased by 20% compared to commercial module.

Future research directions----thin film silicon Light management: (Suitable for other solar cells) External ARC and low absorption TCOs 2-D or 3-D photonic crystals (PC) based flat substrates New materials : Silicon based alloys with O, C, and Ge Performance (ƞ>20%): Increase Voc (growth on flat substrates)

Acknowledgements!

Thank you for your attention!

Module production cost ($/W) 南开大学 10 1979 30 $/W a-si/micromorph modules 2003 3.03 $/W 2005 c-si shortage Production costs: C-Si (lower limit): 0.8 $/W 0.6 /W Eff 15 % CdTe (First Solar): 0.67 $/W 0.51 /W Eff=12.2 % 1 CdTe modules c-si modules 81% learning curve a-si:h/mcsi (Oerlikon,ThinFab14 0): 0.35 $/W 0.27 /W Eff=10.8 % FOSSIL FUEL COMPETITIVE LEVEL Forecast end 2012 0.7 $/W 0.1 1E-3 0.01 0.1 1 10 100 1000 10000 Cumulative production (GW) CIGS (Manz AG) 0.55 $/W 0.42 /W Eff=12.6% (Solar Frontier)