Interplay of diffusion and crystallization in sequentially evaporated lead tin perovskites

Abstract

Low bandgap (LBG) Pb–Sn perovskites (PVKs) are key absorber materials for high-efficiency all-perovskite tandem solar cells. Yet, their fabrication at device-relevant thicknesses with high structural and optoelectronic quality remains challenging. Here, we demonstrate the scalable deposition of formamidinium lead tin triiodide (FAPb_0.5Sn_0.5I₃) films up to 700 nm thick using a single cycle of sequential thermal evaporation (1c-sTE). The process relies on vacuum deposition of a PbSnI₄ alloy followed by FAI, without the need for additives typically used in solution processing. Optical spectroscopy, XRD, solid-state NMR, and TRMC measurements reveal that FAI readily diffuses into the inorganic precursor layer even at room temperature. Independent of the precursor mixing time, annealing is required to achieve films with increased local homogeneity, large grains, and high crystallinity, leading to reduced defect density, increased carrier mobility up to 80 cm² (Vs⁻¹), and lifetimes close to a microsecond. The resulting films are stoichiometrically uniform across their thickness and exhibit dense, columnar grain morphologies. A device with the architecture ITO/PEDOT:PSS/FAPb_0.5Sn_0.5I₃/C60/BCP/Ag, shows power conversion efficiencies of 10%, with voltage losses mainly at the PEDOT:PSS/PVK interface. This work demonstrates that 1c-sTE is a feasible method for producing LBG Pb–Sn PVK films suitable for PV applications.