ShaneLogic/cigs-thin-film-fabrication

CIGS Thin Film Fabrication

When to Use

• Fabricating Cu(InGa)Se2 thin-film solar cells
• Selecting substrates for CIGS deposition
• Choosing deposition methods for absorber layer
• Depositing transparent conducting oxide front contacts

General Requirements

• Low cost
• High deposition/processing rate
• High compositional uniformity over large areas
• Minimum thickness: 1 μm for light absorption

Substrate Selection

Soda-Lime Window Glass

Most commonly used substrate

Advantages:

• Available in large quantities at low cost
• Contains Na for diffusion into cell
• Used to make highest-efficiency devices

Thermal Properties:

• Deposition temperature: 350-750°C (at least 350°C required)
• Thermal expansion coefficient: 9×10⁻⁶/K
• Match with CIGS: CIGS coefficient = 9×10⁻⁶/K
• Result: Little stress during cool-down

Chemical Composition:

• Contains oxides: Na2O, K2O, CaO
• Provides alkali to Cu(InGa)Se2

Thermal Expansion Mismatch Effects

Lower Coefficient (e.g., borosilicate glass)

• Film under tensile stress during cool-down
• Results: Voids and micro-cracks

Higher Coefficient (e.g., polyimide)

• Film under compressive stress
• Results: Adhesion failures

Sodium Incorporation Effects

• Influences microstructure with larger grains
• Higher degree of orientation with (112) parallel to glass surface

Controlled Sodium Supply Methods

1. Block sodium from substrate with diffusion barrier (SiOx, Al2O3, SiN)
2. Direct supply: Deposit Na-containing precursor (NaF, ~10nm) onto Mo film
3. Co-deposition: Deposit Na with Cu(InGa)Se2
4. Post-deposition: Na treatment gives same performance increase

Metal Foil Substrates

• Can withstand higher temperatures
• Electrically conductive
• Stainless steel: Most commonly used, highest efficiency flexible cells

Deposition Methods

Method 1: Simultaneous Vapor Deposition (Co-evaporation)

Process: Simultaneous deposition of Cu, In, Ga, and Se onto substrate Temperature range: 450-600°C

Evaporation Temperatures:

• Cu: 1300-1400°C
• In: 1000-1100°C
• Ga: 1150-1250°C
• Se: 250-350°C

Growth Strategy:

• Bulk of film grown with Cu-rich overall composition
• Contains Cu_xSe phase in addition to Cu(InGa)Se2

Advantage: Flexibility to control film composition and band-gap Challenge: Difficulty controlling desired Cu-evaporation

Method 2: In-Line Process

Process: Substrate moves sequentially over constantly effusing sources

Control Methods:

• In situ flux measurement (electron impact, mass spectroscopy, atomic absorption)
• In situ film thickness measurement (quartz crystal, optical spectroscopy, XRF)
• Process monitoring for Cu-rich to Cu-poor transition (laser scattering, emissivity, IR transmission)

Method 3: Precursor Reaction Processes (Two-Step Process)

Process:

1. Deposit precursor film containing Cu, In, and Ga
2. React at high temperature to form Cu(InGa)Se2 (selenization)

Highest efficiency: 16.5%

Precursor Deposition Methods:

• Sputtering: Easily scalable, good uniformity, high rates
• Electro-deposition: High material utilization at low cost
• Particle ink/spray: High utilization and uniformity

Reaction Conditions:

• Agent: H2Se at 400-500°C
• Time: Up to 60 min
• Limitations: Poor adhesion at longer times, excessive MoSe2 formation
• Alternative: Diethyl selenium (less toxic)

TCO Materials and Deposition

Material Selection

• SnO2: Requires undesired high temperatures > 250°C
• ITO (In2O3:Sn): Can be used, ZnO often favored for lower cost
• ZnO: Preferred material for Cu(InGa)Se2 solar cells

Deposition Methods

ITO

• Method: Sputtering from ceramic ITO targets in Ar:O2 mixture
• Rate: 0.1 - 10 nm/s

ZnO:Al

• Method: rf magnetron sputtering from ceramic ZnO:Al2O3 targets (1-2 wt% Al2O3)
• Alternative: DC sputtering for higher deposition rates

Reactive DC Sputtering

• Targets: Al/Zn alloy targets
• Advantage: Lower costs
• Challenge: Precise process control due to hysteresis effect
• Rate: 5 - 10 nm/s

Chemical Vapor Deposition (CVD)

• Reaction: Water vapor and diethylzinc at atmospheric pressure
• Doping: Fluorine or boron