Cigs Material Composition Engineering

• Lattice type: Chalcopyrite (ratio c/a close to 2)
• Distortion: Tetragonal from differing Cu-Se vs In/Ga-Se bond strengths

Composition Window

• Cu/(In+Ga) ratio: 0.7 to nearly 1.0
• High efficiency possible across this entire range

Defect Physics Tolerance

Key defect complex:

• 2 Cu vacancies + In on Cu antisite
• Formation energy: Low
• Electrical activity: Inactive
• Result: Cu-poor/In-rich compositions acceptable without adverse effects

Grain Boundary Properties

• Inherently passive
• Efficient cells possible with grain sizes < 1μm
• No significant recombination at grain boundaries

Intrinsic Defects

Defect Types

1. Three vacancies: V_Cu, V_In/Ga, V_Se
2. Three interstitials: Cu_i, In/Ga_i, Se_i
3. Six antisites: Cu_In/Ga, In/Ga_Cu, Cu_Se, Se_Cu, In/Ga_Se, Se_In/Ga

Dominant Defects (Lowest Formation Energy)

• Copper vacancy (V_Cu)
• Indium or Gallium vacancy (V_In or V_Ga)
• Cu_In/Ga antisite (copper on indium/gallium site)
• Selenium vacancy (V_Se)

Special Defect Characteristics

Selenium Vacancy (V_Se)

• Type: Amphoteric defect
• Significance: Can explain metastable effects in solar cells

III_Cu Antisite (In_Cu or Ga_Cu)

• Related to: DX center
• Effect: Could limit doping in wide bandgap Cu(InGa)Se2

Shallow Defects

Identified by: Photoluminescence and Hall effect

• Number: Four dominant shallow defects
• Types: Three acceptors, one donor

Deep Defects

Identified by: Capacitance measurements

N2 Defect

• Energy level: 250-300 meV from valence band
• Effect: Does NOT act as recombination center

800 meV Defect

• Energy level: 800 meV from valence band
• Effect: May act as recombination center in high-Ga alloys

Band-gap Grading Strategy

Objective

Form band-gap gradients to separately reduce recombination and collection losses

Gradient Configuration

• Front (Cu(InGa)Se2/CdS interface): Wider band-gap
• Bulk (edge of space charge region): Narrower band-gap

Mechanism and Benefits

1. Reduced Recombination: Wider band-gap at interface increases Voc
2. Increased Collection: Smaller band-gap in bulk enables higher optical absorption and Jsc

Optical Properties

Absorption Coefficient

• α: > 3×10⁴ /cm for energies > 1.3 eV

Absorption Depth

• 95% of incident solar illumination absorbed in 1μm thickness3b:["$","$L43",null,{"content":"$44","frontMatter":{"name":"cigs-material-composition-engineering","description":"Use this skill when engineering Cu(InGa)Se2 material properties, optimizing composition for device performance, or analyzing defect physics. Covers bandgap engineering, composition tolerance, and intrinsic defects."}}]