1. Comparative study of processes for CdTe and CIGS thin-film solar cell technologies 5070 term paper presentation FENG Zhuoqun Dec. 3, 2014

2. Outline Introduction Materials and structures Process and module Conclusion

3. Thin Film Solar Cell Industry Thin film solar panels Comparison of thin-film photovoltaic market share between amorphous Silicon, CdTe and CIGS Advantages: Simple production Flexibility Less volume Drawbacks: Efficiency Longevity PV industry consists of 10% of thin-film Share

4. Materials IBIIBIIIAIVAVAVIA Cadmium Telluride Copper Indium Gallium Selenide

5. Materials a-SiCdTeCIGS Best cell efficiency13.4%21.0%21.7% Best module efficiency8.5%17.0%~17% Bandgap1.7eV1.5eV1.0eV (CIS)-1.7eV (CGS) Thin-film market share32%43%25% Major manufacturersSharpFirst SolarSolar Frontier Advantages Mature technology; Small device suitable Low cost High efficiency; Flexible substrate Disadvantages Low efficiency; High cost Rigid substrate; Toxic raw materials Costly traditional process; complex Comparison between materials used for thin film solar cells

6. Structure Similarity: Form hetrojunction with n-type CdS Vertical stacks Difference: Sequence of deposition (Substrate and Superstrate ) Thickness

7. CdTe: VTD Vapor Transport Deposition Growing rate: 0.1-1 μm/min Carrier gas: Ar, N 2, He Temperature: source >800°C substrate < 600°C Moving substrage: Extreme success in industrial production given by First Solar

8. CdTe: CSS Close Space Sublimation Reduce re-evaporation and increase sticking coefficient Temperature control: ΔT= ~100 K Pressure: 10 torr Carrier gas: Same as VTD Close space: ~1 mm Growth rate: ~1 μm/min

9. CIGS: Co-evaporation Element sources evaporate and condensate at the substrate surface Typical evaporation Temperature: Cu 1300-1400°C In 1000-1100°C Ga 1150-1250°C Se 250-350°C Composition is fixed if there is sufficient Se: high sticking coefficient of Cu, In and Ga Flux control:

10. CIGS: Precursor reaction Two step process Step 1: Precursor deposition contains Cu, In and Ga Various methods can be used: Sputtering Electrodeposition Spray Step 2: Selenization (Se annealing) H 2 Se at 400-500°C for 30-60 minutes Se vapor reaction Advantages: Well established technique for precursor deposition In process uniformity measurement within two processes Drawbacks: Vertical control of composition (Compensated by subsequent Sulfidation)

11. Device formation CdS deposition: (~100 nm) Chamical Bath Deposition, CBD (preferred) Vacuum evaporation Sputtering Atomic Layer Deposition, ALD Metal and TCO deposition: Sputtering (Mo, TCO) CVD (TCO) Cd(NH 3 ) 4 2+ + SC(NH 2 ) 2 + 2OH − → CdS + H 2 NCN + 4NH 3 + 2H 2 O CBD: ALD:

12. Methods CdTe Evaporation VTD CSS Sputter Electrodeposition MOCVD Spray Screen-print CIGS Coevaporation Reactive/Hybrid sputter CSS CBD Precursor reactions Spray Surface reaction and condensation Reduction of ions Precursor reaction

13. Module Top Contact Bottom contact Absorption layer Tandem structure of cells 3 laser scribing Monolithic integration

14. Conclusion Problems: Efficiency improvement Longevity Toxicity and other environmental concerns Future work to improve: CdTe Thinner film with high efficiency Flexible substrate CIGS New encapsulation methods with good impermeability Complexity of process

15. Thank you! Questions?