The Solar Cell Revolution You Can Print
Solution Processing for High-Efficiency CuIn(S,Se)â‚‚ Solar Cells
August 12, 2025
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Solar energy's next big leap isn't happening in sprawling vacuum chambers or billion-dollar fabrication plants. It's brewing in labs where scientists are crafting high-performance solar cells using techniques reminiscent of inkjet printing.
Welcome to the world of solution-processed CuIn(S,Se)₂ (CISSe) solar cells – a technology promising to deliver efficient, affordable, and flexible solar power.
Why Solution Processing? The Allure of Simplicity
Traditional methods for making top-tier solar cells like silicon or vacuum-deposited Cu(In,Ga)Se₂ (CIGS) involve high temperatures, complex equipment, and significant material waste. Solution processing flips this script. Imagine dissolving metal salts in specially formulated solvents and then depositing the solar absorber layer like paint or ink onto surfaces – even flexible ones.
For CISSe – a material prized for its near-perfect bandgap (~1.0 eV) for absorbing infrared light – solution processing is particularly promising. It enables precise control over composition and microstructure, critical for high efficiency. Recent breakthroughs have pushed solution-processed CISSe efficiencies past 16.5% for single junctions and over 20% in tandem configurations, rivaling vacuum methods 2 3 6 .
Efficiency Progress
Solution-processed CISSe efficiency milestones over time
Traditional vs Solution Processing
- Equipment Cost High Low
- Temperature >600°C 400-550°C
- Material Waste 30-50% <5%
- Flexibility Rigid Flexible
The Science Behind the Ink: Key Concepts Unpacked
The Bandgap Sweet Spot
CISSe belongs to the chalcopyrite family of semiconductors. Its magic lies in its tunable bandgap. By adjusting sulfur-selenium ratios, scientists can target ~1.0 eV – the ideal bandgap for the bottom cell in tandem solar stacks (e.g., paired with perovskite top cells). This allows efficient harvesting of low-energy infrared light missed by conventional cells 3 5 .
The Solution Processing Advantage
Unlike vacuum deposition, solution processing involves:
- Precursor Dissolution: Metal salts (e.g., CuCl, InCl₃) and chalcogens (S, Se) are dissolved in solvents (amines, thiols).
- Coating: The "solar ink" is deposited via spraying, spin-coating, or blade-coating.
- Thermal Treatment (Selenization): The coated film is heated (400–550°C) in a controlled atmosphere (e.g., Se vapor), transforming it into crystalline CISSe 1 6 8 .
Tackling the Efficiency Killers
Early solution-processed CISSe cells suffered from:
- Poor Crystallinity: Small grains riddled with defects.
- Elemental Loss: Volatile elements like indium evaporating during heating.
- Secondary Phases: Unwanted impurities (e.g., Cuâ‚‚â‚‹â‚“Se) acting as recombination centers.
Recent innovations target these issues through doping, composition grading, and selenization engineering 2 6 .
Solution processing of solar cells in a research lab setting
Inside the Lab: The Breakthrough Experiment That Engineered Perfection
A 2024 study published in Nature Communications (Vol. 15, Art. 10365) achieved a record 20.26% efficiency in a narrow-bandgap CIGS cell – a landmark for solution-compatible processing 3 . This experiment showcased how nanoscale control over elemental distribution could overcome voltage limitations in CISSe.
Methodology: The Three-Stage Control Strategy
The team crafted a CISSe absorber with a meticulously engineered "U-shaped" gallium (Ga) profile:
Stage 1: Back Ga-Grading
- A high-Ga layer (~1.6 eV bandgap) was pre-deposited near the back contact (Mo).
- Function: Creates an electric field pushing electrons away from the lossy back interface.
Stage 2: Low-Ga Notch Region
- Standard In+Se/Cu+Se layers deposited, forming the main absorber with minimal Ga (bandgap ~1.01 eV).
- Key Innovation: 15% excess copper was added during growth. This suppressed Ga/In interdiffusion, preserving the narrow bandgap.
Stage 3: Front Ga-Grading
- A thin Ga-rich layer (~1.1–1.2 eV) was added at the very front (near the CdS buffer).
- Function: Boosts voltage by reducing interface recombination.
| Parameter | Value | Significance |
|---|---|---|
| Efficiency (PCE) | 20.26% | Certified record for narrow-bandgap CIGS |
| Open-Circuit Voltage (Voc) | 642 mV | 30 mV gain from front grading |
| Bandgap Minimum | 1.01 eV | Optimal for IR absorption in tandems |
| Voltage Deficit | 368 mV | Lowest reported for CISSe-type cells |
Results & Analysis: Why the "U" Shape Wins
- The U-shaped Ga profile (high Ga at back/front, low Ga in the middle) delivered a 44 mV Voc boost compared to a single back grade.
- Excess copper was crucial: It enlarged grains and pinned Ga in place, preventing bandgap widening.
- Quantum efficiency confirmed enhanced infrared response (>800 nm) due to the preserved 1.01 eV notch.
- This holistic approach demonstrated that strategic elemental grading + growth control could overcome the traditional voltage-efficiency trade-off in CISSe.
The Scientist's Toolkit: Essential "Ingredients" for High-Efficiency CISSe
Creating champion solution-processed CISSe cells relies on specialized reagents and materials. Here's what's in the virtual lab cart:
| Material/Reagent | Role in Solution Processing | Impact on Performance |
|---|---|---|
| Amine-Thiol Solvents (e.g., Ethylenediamine + 1,2-Ethanedithiol) | Dissolve Cu/In/Ga salts without toxic hydrazine | Enables stable, eco-friendly precursor inks 1 |
| Ag-Se Co-selenization Source (Ag powder + Se powder) | Forms low-melting Agâ‚‚Se liquid phase during annealing | Accelerates Se uptake & grain growth; boosts efficiency to 16.48% 6 |
| Cu-Gradient Precursors (Layered Cu-deficient solutions) | Controls Cu distribution before selenization | Prevents secondary phases; enables 13.35% efficiency without KCN etching 2 |
| D-Homoserine Lactone Hydrochloride (D-HLH) | Additive for perovskite top cells in tandems | Improves perovskite coverage on rough CISSe; passivates defects |
| 2-Thiopheneethylammonium Iodide (2-TEAI) | Surface passivator for perovskite | Suppresses interface recombination in tandems |
| CZTS (Cuâ‚‚ZnSnSâ‚„) Hole Transport Layer | Replaces Mo at back contact | Enhances hole extraction; simulated >27% efficiency in CMTS cells 7 |
Material Composition
Key Chemical Reactions
Precursor Formation:
CuCl + InCl₃ + 2(Se) + 4(EDT) → CuIn(SEdt)₄ + 4HCl
Selenization:
CuIn(SEdt)₄ + Se(vapor) → CuInSe₂ + 4EDT↑
The Future: Tandems, Flexibility, and Greener Processing
Solution-processed CISSe is rapidly evolving beyond single junctions:
Perovskite/CISSe Tandems
CISSe's infrared harvesting pairs perfectly with perovskite's visible light capture. Solution methods enable monolithic stacking. Recent work hit 24.6% efficiency using novel crystallization control and interface passivation .
Ultra-Thin Flexible Cells
Deposited on bendable ultra-thin glass (<200 µm), CISSe achieves minority carrier lifetimes nearing 100 ns – rivaling rigid cells. This unlocks applications in vehicle-integrated PV and portable electronics 4 .
| Cell Type | Processing Method | Efficiency | Key Advantage |
|---|---|---|---|
| CISSe (Single Junction) | Ag-Se Co-selenization | 16.48% | Highest for non-hydrazine solutions 6 |
| CIGS (Single Junction) | Cu-Gradient Solution | 13.35% | No toxic etching required 2 |
| CIGS (Single Junction) | Vacuum (Co-evaporation) | 23.64% | Current lab record (rigid) 5 |
| Perovskite/CISSe (Tandem) | Solution + Sputtering | 24.6% | All-thin-film tandem record |
Flexible solar cell applications in building integration
Conclusion: Painting a Brighter Energy Future
Solution processing has transformed CISSe solar cells from lab curiosities into serious contenders for the next generation of photovoltaics. By mastering ink chemistry, nanoscale grading, and defect control, researchers are closing the efficiency gap with vacuum techniques while unlocking unprecedented cost and form-factor advantages. As tandem architectures mature and flexible modules roll toward commercialization, these "solar inks" may soon power everything from smart windows to electric vehicles – proving that high efficiency doesn't have to come at a high cost. The future of solar isn't just bright; it's printable, bendable, and incredibly versatile.