Hybrid organic-inorganic perovskites (PVKs) offer exceptional optoelectronic performance, yet reproducible and scalable co-evaporation remains challenging. This study examines the interplay of factors affecting compositional control during three-source PVK deposition. We identify chamber pressure, precursor cross-contamination, and flux instability – especially from organic salts such as formamidinium iodide (FAI) – as major sources of variability. A critical influence is the occurrence of cross-reading, where omnidirectional evaporation of FAI contributes to the reading on the quartz crystal microbalance (QCM) sensors monitoring the inorganic precursors like caesium bromide (CsBr) and lead iodide (PbI₂) even though shielding is present. This effect, strongly dependent on FAI load, deposition rate, and QCM sensor position, erroneously inflates measured fluxes, leading to inaccurate rate control and unintentional compositional drift. Maintaining A-, B- and X-site stoichiometry therefore requires dynamic adjustment of precursor rates, particularly at higher deposition speeds where mean free path limitations come into play. We demonstrate the successful deposition of perovskite layers at a deposition speed of 27.8 nm min⁻¹ as the practical ceiling for the investigated Cs_xFA_1-xPb(I_1-xBr_x)₃ composition within our experimental framework. These findings highlight the delicate balance between deposition speed, precursor stability, and film quality, underscoring the need for improved delivery systems - such as continuous precursor feedthrough, multiple organic sources, alternative vapor transport or flash evaporation methods – to achieve reproducible, fast and large-scale fabrication of high-performance PVK films.

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.