Performance Enhancement Mechanisms of Sb ₂ Se ₃ Thin‐Film Solar Cells via Reverse‐Hook Bandgap Engineering

Dual‐surface bandgap grading has emerged as an effective strategy for suppressing recombination losses in Sb ₂ Se ₃ thin‐film solar cells, yet the mechanisms by which front‐ and back‐surface grading influence device performance remain insufficiently understood. In this work, an HTL‐free Sb ₂ Se ₃ solar cell incorporating independently tunable front‐surface grading (FSG) and back‐surface grading (BSG) Sb ₂ (S, Se) ₃ layers was systematically investigated using SCAPS‐1D simulation. An asymmetric sulfur‐rich‐front reverse‐hook bandgap profile was identified as the optimal design, achieving a simulated power conversion efficiency (PCE) of 20.42%—an idealized upper‐limit prediction—compared with 16.17% for the ungraded baseline. Compared with conventional forward‐hook and symmetric V‐shaped grading structures, the reverse‐hook design provides superior carrier transport and recombination suppression. Mechanism analysis reveals that the FSG layer primarily improves front‐interface carrier collection and enhances V _oc and J _sc , whereas the BSG layer mainly suppresses back‐interface recombination and governs FF. The reverse‐hook band profile establishes favorable quasi‐electric fields that drive electrons toward the front contact and holes toward the back contact, thereby improving carrier separation and reducing recombination losses. These findings clarify the performance enhancement mechanisms of asymmetric dual grading and provide practical design guidelines for high‐efficiency Sb ₂ Se ₃ solar cells.