Seed Layers for Wide-Band Gap Coevaporated Perovskite Solar Cells: CsCl Regulates Band Gap and Reduces Process Variability

Coevaporation, an up-scalable deposition technique that allows for conformal coverage of textured industrial silicon bottom cells, is particularly suited for application in perovskite-silicon tandem solar cells (PSTs). However, research on coevaporated perovskites with an appropriate band gap for PSTs remains limited, with lower efficiency and reproducibility than solution-processed films. Here, we present a simple approach using a thin layer of a precursor material, namely, PbI2, PbCl2, CsI, or CsCl, as a seed layer on the hole-transporting layer/perovskite interface. We find CsCl to be the optimal seed layer for our system. Perovskite single junction cells prepared with CsCl seed layer exhibit 19.6% power conversion efficiency with a band gap of 1.69 eV and improved long-term stability. We attribute the observed enhancements to the more precise and consistent incorporation of the organic precursor into the perovskite lattice during the film growth. This work demonstrates that engineering the substrate surface is crucial for achieving well-controlled growth of efficient and stable coevaporated wide-band gap perovskite solar cells.