Researchers at India’s Nirma University and Samastipur College, Samastipur have designed a cadmium-free thin-film solar cell featuring a copper indium selenide (CIS) absorber and an indium oxide (In₂O₃) electron transport layer (ETL).
They noted that, although CIS thin films are promising solar absorbers due to their direct bandgap of around 1.5 eV and high absorption coefficient, device performance is often limited by trap-assisted recombination and inefficient interfacial carrier collection.
“Historically, materials such as cadmium sulfide (CdS), titanium dioxide (TiO₂), zinc oxide (ZnO) and tin oxide (SnO₂) have been widely used as electron transport layers in thin-film solar cells,” corresponding author Shibu G. Pillai told pv magazine. “However, they present significant challenges for sustainable scaling. CdS raises serious environmental and toxicity concerns, while Cd-free alternatives also have drawbacks: TiO₂ suffers from UV-induced photocatalytic degradation and low electron mobility, ZnO shows chemical instability, and SnO₂ often requires high-temperature processing that can introduce interfacial trap states.”
“We selected In₂O₃ because it offers a unique combination of properties,” co-author Keyur Sangani added. “It provides high electron mobility, low resistivity, excellent optical transparency in the visible range, and strong chemical stability. These features enable efficient electron extraction from the CuInS₂ absorber while reducing interfacial recombination. In₂O₃ also avoids photocatalytic degradation and supports lower-temperature processing, making it suitable for flexible substrates and lower energy consumption.”
The proposed device structure consists of an aluminium (Al) front contact, a fluorine-doped tin oxide (FTO) substrate, an In₂O₃ ETL, a CuInS₂ absorber, an amorphous silicon (a-Si:H) hole transport layer, and a nickel back contact.
To evaluate practical feasibility and device robustness, the researchers conducted a comprehensive parametric sensitivity analysis. By systematically varying absorber thickness, doping concentration, and defect densities, they assessed tolerance under non-ideal conditions and identified an оптимal absorber thickness of around 1 μm. They also found that increased doping enhances open-circuit voltage and fill factor, while excessive defect density promotes Shockley–Read–Hall recombination and degrades performance. Maintaining low bulk and interface defect densities is therefore critical to preserving photovoltage and minimizing recombination losses.
Temperature-dependent simulations further showed that thermal effects significantly impact efficiency, underlining the need for effective thermal management to mitigate carrier lifetime degradation at elevated temperatures. In addition, Voc decreases with increasing absorber thickness due to higher bulk recombination and increased saturation current density, while FF remains relatively stable, indicating limited resistive losses.
The optimized device achieved a peak power conversion efficiency of 29.79% in SCAPS-1D simulations. However, this value is based on idealized defect assumptions and represents a theoretical upper limit. A detailed sensitivity analysis was therefore used to assess real-world applicability and device stability.
“Overall, combining CIS absorbers with In₂O₃ ETLs offers a clear, cost-effective and fully eco-friendly pathway for high-performance flexible thin-film photovoltaics,” co-author Ritesh Kumar Chourasia concluded.
The new cell concept was introduced in “Indium oxide as a high-performance ETL for CuInS₂ thin-film solar cells,” published in Next Materials.
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