Unveiling the role of dual grading in device optimization of HTL-free Sb₂(S, Se)₃ solar cells

Antimony Seleno-sulfide, Sb₂(S, Se)₃, is considered an extremely promising absorber for optoelectronic applications regarding its minor toxicity, phase stability, earth-abundancy besides its decent absorption capabilities. Moreover, it possesses tunable bandgaps which can be adjusted from 1.1 to 1.7 eV by varying the Se/S ratio in Sb₂(S, Se)₃ alloy films, making it an attractive candidate for efficient solar cells. This study proposes novel designs for hole-transporting layer (HTL)-free Sb₂(S, Se)₃ solar cells that enhance photo-generated carrier collection, thereby improving power conversion efficiency (PCE). Utilizing SCAPS-1D solar cell simulator, we first validate the device model by calibrating an experimental cell with the FTO/CdS/Sb₂(S, Se)₃/Au configuration. The primary objective is to optimize HTL-free Sb₂(S, Se)₃ solar cells using dual grading techniques in the electron-transport layer (ETL) and within the absorber. Our analysis reveals that the band offsets at both the ETL/absorber and ETL/front contact interfaces play competing roles in determining cell performance. To optimize these offsets, we replace the traditional CdS ETL with the tunable Cd₁₋ₓZₓS and gradually adjust the Zn fraction at both interfaces. We also conduct a comparative study of various bandgap grading (BGG) profiles (linear, logarithmic, and parabolic). A doping grading technique is also applied to the absorber to enhance charge carrier extraction. The simulation results indicate that dual grading techniques in the ETL and absorber, combined with optimized absorber thickness, can significantly improve device performance, yielding a simulated PCE of 21.15%. These findings provide valuable insights into the design and optimization of HTL-free Sb₂(S, Se)₃ solar cells.