Metal halide perovskite solar cells have gained significant attention over the last decate due to their low-cost fabrication methods and high efficiency potential. Typically perovskite films are prepared by solution-based depositon techniques, which allow a rapid deposition and a wide variety of compositions.[1] However, conformal coverage of textured surfaces, higly relevant for monolithic perovskite/silicon tandem solar cells, or compositional gradients in the absorber material can only be realized with considerable effort with solution‑based deposition techniques. These limitations can be overcome by co‑evaporating the perovskite precursor materials.[2,3]

In contrast to solution-based deposition processes, where the stoichiometry is determined by the weight of the individual precursors in the solution, the composition in co-evaporation processes is controlled by the evaporation rates of the precursors, which are typically measured using quartz crystal microbalances (QCMs). However, determining the composition from these rates is not always straightforward and accurate, as the rates of the materials can overlap or the incorporation of precursors can change due to complex chemical reactions.[4] In this work, we analyze in detail the composition of perovskite films prepared via simultaneous co-evaporation of PbI₂, PbBr₂, formamidinium iodide (FAI) and CsI and correlate the film stochiometry to the optoelectronic properties of the resulting solar cells. We show that changes in the evaporation rates are not necessarily transferred proportionally into the final film composition.

Furthermore, we analyze how the composition of the perovskite films can be influenced by the choice of the hole transporting material for cells in p-i-n configuration. We observe that perovskites co-evaporated on spin-coated MeO‑2PACz ([2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) contain a significantly smaller amount of FAI if the MeO-2PACz layer is washed with ethanol before the perovskite depostion. The gained knowledge is used to tune the co-evaporation process to enable perovsktie solar cells with band gaps between 1.65 eV and 1.70 eV, optimized for monolithic perovskite/silicon tandem solar cells. Overall, our study provides valuable insights into the co-evaporation process and demonstrates the importance of composition control for achieving efficent perovskite solar cells.