Band Gap and Defect Engineering for High‐Performance Cadmium‐free Sb 2 (S,Se) 3 Solar Cells and Modules

High‐efficiency antimony selenosulfide (Sb 2 (S,Se) 3 ) solar cells are often fabricated by hydrothermal deposition and also comprise a CdS buffer layer. Whereas the use of toxic materials such as cadmium compounds should be avoided, both of these issues hinder scaling up to large areas and market access. For this reason, co‐sublimation is studied as a manufacturing process for the active layer as well as the use of Cd‐free buffer layers. To further improve the power conversion efficiency (PCE), a graded bandgap profile is designed for the absorber layer. A V‐shaped graded bandgap in the Sb 2 (S,Se) 3 absorber layer is produced on a TiO 2 substrate by co‐sublimation of a controlled varying molar ratio of Sb 2 Se 3 and Sb 2 S 3 . Moreover, increasing the Se/S ratio improves the grain size and favorable (hk1) orientations, reduces the detrimental bulk defects in Sb 2 (S,Se) 3 films. Consequently, the optimized Sb 2 (S,Se) 3 solar cells reach a PCE of 9.02%, which is a record value for Cd‐free Sb‐based solar cells. A PCE of 7.15% is further demonstrated for a Sb 2 (S,Se) 3 monolithically interconnected minimodule with an active area of 12.32 cm 2 . This co‐sublimation graded bandgap technique provides a useful guidance for the optimization of a range of solar cells based on alloy compounds.