Diffusion‐Driven Targeted Passivation of Selenium Vacancies via an I‐Doped CdS Buffer Layer for Efficient Sb ₂ Se ₃ Solar Cells

ABSTRACT Antimony selenide (Sb ₂ Se ₃ ) is an emerging photovoltaic absorber with attractive optoelectronic properties, but its device performance is largely constrained by deep‐level defects, particularly selenium vacancies (V _Se ), which induce severe non‐radiative recombination. Herein, we report a targeted anion‐defect passivation strategy by incorporating iodine into the CdS buffer layer (CdS:I). Iodine doping not only promotes the preferential (100)‐oriented growth of CdS and passivates its intrinsic sulfur vacancies but also facilitates the spontaneous diffusion of iodine into the Sb ₂ Se ₃ absorber during annealing. Density functional theory (DFT) and deep‐level transient spectroscopy (DLTS) analyses confirm that the diffused iodine atoms preferentially occupy V _Se sites, forming charge‐neutral I _Se defects with low formation energy. This results in a remarkable reduction in the defect capture cross‐section to 4.88 × 10 ⁻²² cm ² . The optimized Sb ₂ Se ₃ solar cell achieves a high open‐circuit voltage of 489.36 mV and a power conversion efficiency of 10.07%. This work provides an effective and generalizable defect‐passivation route for enhancing the performance of Sb ₂ Se ₃ and related polycrystalline photovoltaic devices.