Formamidinium–Cesium Lead Iodide Self-Doped Perovskite Films Characterized by Photoelectron Spectroscopies

We demonstrated intrinsic compositional self-doping in FA1–xCsxPbI3 perovskite films prepared by thermal co-evaporation under high vacuum. By systematically varying the (FAI + CsI)/PbI2 precursor ratio while keeping the FA/Cs ratio fixed, we induced a gradual Fermi level shift from n- to p-type, as confirmed by UPS/IPES. This doping control is achieved without external dopants or postdeposition treatments. A cesium content of x ≈ 0.20 enables room-temperature stabilization of the photoactive α-phase without annealing. These results provide a scalable, annealing-free route to engineer electronic properties in vacuum-deposited perovskites, offering a foundation for simplified device architectures such as homojunctions.