9% Certified Efficiency Record for Carbon‐Based Sb ₂ (S,Se) ₃ Solar Cells Enabled by Gradient‐Oxidized Treatment of CdS Electron Transport Layer

Abstract Cadmium sulfide (CdS) persists as the standard electron transport layer (ETL) in high‐efficiency Sb ₂ (S,Se) ₃ solar cells. However, its narrow bandgap imposes critical performance limitations through parasitic short‐wavelength absorption and interface mismatch. Here, this bottleneck is shattered through an innovative in situ ozone doping strategy. This approach revolutionizes CdS ETLs by simultaneously: 1) Widening the bandgap of CdS ETL to drastically suppress parasitic short‐wavelength photon loss; 2) Triggering a hexagonal‐to‐cubic phase transition that disrupts coherent Sb ₂ O ₃ impurity growth during Sb ₂ (S,Se) ₃ deposition; and 3) Engineering an oxygen‐rich, defect‐resistant FTO/CdS interface (DFT‐validated), thereby slashing recombination and enabling superior carrier extraction. As a result, a certified world‐record power conversion efficiency of 9.0% for carbon‐based Sb ₂ (S,Se) ₃ solar cell is achieved ( V _OC of 0.4908 V, J _SC of 26.88 mA cm ⁻² , and FF of 68.19%) using an FTO/CdS:O/Sb ₂ (S,Se) ₃ /PbS/carbon configuration. This work, beyond setting a new benchmark for carbon‐based Sb ₂ (S,Se) ₃ solar cells, shows that these devices demonstrate exceptional intrinsic stability under long‐term and extreme damp‐heat conditions without encapsulation, thus propelling Sb ₂ (S,Se) ₃ technology toward commercial viability.