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