Dual-Source Coevaporation of Low-Bandgap FA1-xCsxSn1-yPbyI3 Perovskites for Photovoltaics

Perovskite halides are well-suited to monolithic multijunction photovoltaics, promising low-cost solar-to-electrical power conversion. Critical to all-perovskite multijunction fabrication is the deposition of a low-bandgap absorber without damaging other device layers. Vapor deposition is thus an attractive method, obviating the need for optically lossy protective interlayers, but is challenging for multicomponent perovskites. Here, we demonstrate a method to dual-source coevaporate low-bandgap perovskite films and devices. We used mixtures formed by melting of metal halides as a single-crucible source of Cs, Pb, and Sn cations. Surprisingly, when this melt was coevaporated with formamidinium iodide (FM), uniform and dense perovskite films in the family( )FA(1-x)Cs(x)Sn(1-y)Pb(y)I(3 )were formed. Inclusion of SnF2 in the melt helped to regulate the perovskite's optoelectronic quality, leading to a steady-state power conversion efficiency of similar to 10% in a solar cell. This represents a new processing paradigm for evaporated perovskite alloys, which is an important step toward all-perovskite multijunction photovoltaics.