Comparative study of processes for CdTe and CIGS thin-film solar cell technologies

5070 term paper presentationFENG 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 volumeDrawbacks:• Efficiency• Longevity

PV industry consists of 10% of thin-film Share

MaterialsIB IIB IIIA IVA VA VIA

• Cadmium Telluride

• Copper Indium Gallium Selenide

Materials

a-Si CdTe CIGS

Best cell efficiency 13.4% 21.0% 21.7%

Best module efficiency 8.5% 17.0% ~17%

Bandgap 1.7eV 1.5eV 1.0eV (CIS)-1.7eV (CGS)

Thin-film market share 32% 43% 25%

Major manufacturers Sharp First Solar Solar 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: 0.1-1 μm/min

Carrier gas: Ar, N2, He

Temperature: source >800°C substrate < 600°C

Moving substrage:

Extreme success in industrial production given by First Solar

CdTe: CSS

• Close Space SublimationReduce re-evaporation and increase sticking coefficientTemperature control: ΔT= ~100 K

Pressure: 10 torrCarrier gas: Same as VTDClose space: ~1 mm

Growth rate: ~1 μm/min

CIGS: Co-evaporationElement sources evaporate and condensate at the substrate surface

Typical evaporation Temperature:Cu 1300-1400°CIn 1000-1100°CGa 1150-1250°CSe 250-350°C

Composition is fixed if there is sufficient Se: high sticking coefficient of Cu, In and Ga

Flux control:

CIGS: Precursor reaction

Two step processStep 1:• Precursor deposition

contains Cu, In and Ga• Various methods can be used:

SputteringElectrodepositionSpray

Step 2:• Selenization (Se annealing)

H2Se 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)

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(NH3)42+ + SC(NH2)2 + 2OH− → CdS +

H2NCN + 4NH3 + 2H2O

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 cells3 laser scribing

• Monolithic integration

Conclusion

Problems:Efficiency improvementLongevityToxicity and other environmental concerns

Future work to improve:CdTeThinner film with high efficiencyFlexible substrateCIGSNew encapsulation methods with good impermeabilityComplexity of process

Thank you!Questions?