High Efficiency Hole‐Transport‐Layer‐Free Sb ₂ S ₃ Solar Cells via Platinum Back‐Surface Doping

ABSTRACT Owing to its low toxicity, excellent stability, and strong visible‐light absorption, Sb ₂ S ₃ has emerged as a highly promising candidate for thin‐film solar cells. However, Sb ₂ S ₃ devices employing conventional hole transport layers (HTLs) such as Spiro‐OMeTAD face critical stability challenges. In this work, we propose a Pt‐doping strategy on the back surface of Sb ₂ S ₃ to directly modulate its electrical properties, enabling the construction of an HTL‐free device architecture. Substitution of Pt at Sb sites introduces shallow acceptor states near the valence band maximum (VBM), optimizing energy band alignment at the rear interface and thereby enhancing hole extraction in Sb ₂ S ₃ solar cells. This reduces the hole transport barrier and forms a quasi‐Ohmic contact with the carbon electrode (work function ∼5.06 eV). Consequently, interfacial energetics are optimized, contact resistance is reduced, and the fill factor ( FF ) is significantly improved from 34.1% to 61.7%. As a result, a carbon‐based Sb ₂ S ₃ solar cell without HTL achieves a power conversion efficiency (PCE) of 6.86%, corresponding to a relative gain of 105.3%, representing the highest performance among HTL‐free Sb ₂ S ₃ solar cells to date. This work provides a novel paradigm and theoretical foundation for back surface engineering in low‐cost, highly stable carbon‐based Sb ₂ S ₃ thin‐film solar cells.