Tin/lead (Sn/Pb) iodide perovskites (PVKs) have emerged as promising absorber layer materials for high-efficiency tandem solar cells due to their low cost, high light absorption coefficients, and narrow band gaps. However, current solvent-based synthesis techniques offer poor scalability, in turn hindering commercialization.

This study introduces sequential thermal evaporation (STE) as a scalable method for producing narrow band gap Sn/Pb iodide PVK absorber layers for application in solar cells. The produced thin films show low doping densities, charge carrier mobilities close to 100 cm2/Vs, and charge carrier lifetimes of over 2 μs. Contrary to the established convention in solvent-based synthesis, no additives were required.

An alloy of precursors (PbSnI4) was used in the deposition, lowering the number of required
sources and increasing the production rate. Optimal annealing temperatures for FAPb0.5Sn0.5I3 and Cs0.05FA0.95Pb0.5Sn0.5I3 produced via STE were determined at 200 ◦C, showing significant improvements in charge carrier mobilities and lifetimes compared to lower annealing temperatures. The drastic increase in performance was ascribed to a recrystallization mechanism. Contrary to spin coating-based research, the introduction of cesium into the PVK structure led to reduced charge carrier mobility and lifetime. The underlying mechanism remains unclear. Addition of tin(II)fluoride (SnF2) led to reduced charge carrier mobilities and lifetimes, with slight improvement in morphology. However, its direct effects were uncertain, questioning its necessity in vacuum deposition methods for Sn-based PVK films.

This works demonstrates the significant potential of STE for the production of near-intrinsic high-performance Sn/Pb iodide PVKs for solar cell applications.