cigs-thin-film-fabrication - SKILL.md Agent Skill

name: cigs-thin-film-fabrication description: Use this skill when fabricating Cu(InGa)Se2 thin-film solar cells. Covers substrate selection, deposition methods, and TCO layer deposition for complete device 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

Block sodium from substrate with diffusion barrier (SiOx, Al2O3, SiN)
Direct supply: Deposit Na-containing precursor (NaF, ~10nm) onto Mo film
Co-deposition: Deposit Na with Cu(InGa)Se2
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:

Deposit precursor film containing Cu, In, and Ga
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