Impact of Diverse ETLs on the Efficiency of Tunable Band Gap Kesterite Based-Solar Cells

Open circuit voltages deficit (Voc,def) in kesterite solar cells remains the primary limitation to achieving performance parity with CIGS technology. One key strategy to address this issue is the optimization of the Kesterite/ETL interface. However, CdS remains the primary ETL material in second-generation solar cell technology, including kesterite. It has been proven that interface recombination at Kesterite/CdS interface is a significant limitation for performance enhancement. Therefore, integrating an effective Electron Transport Layer (ETL) is crucial for improving the performance of these technologies. A cadmium-free ETL is important not only for performance improvement but also for health and environmental reasons, enhancing market acceptance, promoting recycling, and stimulating innovation in the photovoltaic industry. However, significant progress is necessary to achieve higher efficiencies in kesterite devices with alternative ETLs.<br/>In this study we report the effectiveness of CdS as reference ETL layer and explore cadmium-free ETL alternatives. Thin-film In₂S₃, ZnS, ZnOS, and ZnSnO are promising ETL candidates due to their wider bandgap and good lattice match with kesterite absorbers. Several deposition methods for these ETL films have been explored, including Chemical Bath Deposition (CBD), Successive Ionic Layer Adsorption and Reaction (SILAR), and Atomic Layer Deposition (ALD). ALD is recognized for its precise and reproducible thin film deposition. The SILAR, employing a similar chemical principle, enables the formation of semiconductor films by immersing a substrate repeatedly in separated precursor solutions of cations and anions, achieving deposition at the nanometer scale. These methods offer potential to create optimal heterojunctions in solar cells using kesterite as the light-absorbing material. This study explores three growth techniques and emphasizes the utilization of environmentally safer and more abundant materials.<br/>For ALD and SILAR, different growth parameters such as precursor concentration, process temperature, time and number of cycles were studied to assess their influence on device performance. Characterization techniques like XRD, XPS, GDOES, XRF, and SEM were employed to analyze the thin film properties. The study focuses on the fabrication of CZTSSe-based solar cells in a substrate configuration, demonstrating the significant impact of ETL selection on the electrical performance. Currently, efficiencies higher than 8% have been achieved using a ZnSnO-ALD ETL nanolayer, and a relevant improvement in device efficiency is observed in relation to the reference CdS ETL one, likely related to its higher band gap and better band alignment with the kesterite aborbers. These results underscore the promising potential of alternative ETL materials to CdS, particularly in addressing environmental and health concerns associated with cadmium usage.