Wide bandgap in window layers for efficient perovskite solar cells

Compositional gradient engineering has proven effective in enhancing Cu(In,Ga)Se$_2$ solar cell efficiency; however, its application in perovskite solar cells (PSCs) remains insufficiently explored. A gradient of composition is built with vapor processes here and compared with uniform composition. We found that co-vapor-deposition of CsBr induces a transition of PbI$_2$ from dual-orientation to single-orientation growth, and the latter facilitates the full diffusion of formamidinium iodide into the film. By tuning the spatial distribution of CsBr in vapor-processed PbI$_2$, specifically, depositing higher CsBr concentrations in the initial layers, we constructed wide-bandgap window layers (close to the side of incident light), which outperform uniform-composition counterparts. Using this strategy, we fabricated devices with bandgaps ranging from 1.58 to 2.03 eV. A flexible device exhibits a 19.57% efficiency, among the highest in vapor-processed ones. To gain a deeper understanding of the above observations, we carried out numerical simulations. The results show that wide-bandgap perovskites with compositional gradients generate expanded high-efficiency regions in efficiency–doping bandgap–doping ratio maps, revealing potential evolution pathways of device performance during phase segregation and compositional homogenization. ... mehrThis study elucidates the critical role of compositional gradients in improving the efficiency and stability of perovskite devices from both theoretical and experimental perspectives.