Sequential Co-Deposition of Perovskite Film: An Effective Way of Tailoring Bandgap in All Vacuum Processed Perovskite Solar Cells

This study introduces a simple and effective way to control the bandgap of perovskite film by constructing bilayer film composed of methylammonium lead iodide (MAPbI₃) and formamidinium lead iodide (FAPbI₃) layers. The bilayer film is fabricated through the sequential co-deposition of methylammonium iodide (MAI) and lead iodide (PbI₂), followed by formamidinium iodide (FAI) and PbI₂. Interestingly, the bandgap of the bilayer film can be tuned from 1.60 to 1.51 eV by adjusting the thickness of each layer. X-ray photoelectron spectroscopy (XPS) indicates that the highly diffusive methylammonium ions (MA⁺) enables the formation of stable α-phase with formamidinium ions (FA⁺) in the bilayer perovskite film even without further thermal annealing. Bilayer film-based perovskite solar cells (PSCs) are fabricated through an all-vacuum deposition process. The bilayer PSC exhibits higher power conversion efficiency (PCE) of 17.0% compared to the single-layer PSC based on MAPbI₃ or FAPbI₃.