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

Outline Introduction Materials and structures Process and module Conclusion

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

Materials IBIIBIIIAIVAVAVIA Cadmium Telluride Copper Indium Gallium Selenide

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

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

CdTe: VTD Vapor Transport Deposition Growing rate: 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

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

CIGS: Co-evaporation Element sources evaporate and condensate at the substrate surface Typical evaporation Temperature: Cu °C In °C Ga °C Se °C Composition is fixed if there is sufficient Se: high sticking coefficient of Cu, In and Ga Flux control:

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 °C for 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)

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 ) SC(NH 2 ) 2 + 2OH − → CdS + H 2 NCN + 4NH 3 + 2H 2 O CBD: ALD:

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

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

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

Thank you! Questions?