Effect of Hole Extraction Layer on the Composition of Thermally Evaporated Formamidinium-Based Mixed Halide Perovskites

In the quest for optimizing the stability and efficiency of perovskite-based devices, the preparation and composition of the perovskite layer as well as the choice of adjacent charge extraction layers play a crucial role. While most commonly solution processing is used to produce the perovskite films, thermal evaporation is emerging as an important alternative, in particular, with respect to future commercialization. This study investigates the film crystallinity as well as film composition of the thermally evaporated mixed halide perovskite FAPbI₁Br₂ on different hole extraction layers (HELs). Despite using PbBr₂ as the bromide source, measurements by X-ray diffraction and photoelectron spectroscopy reveal that significant bromide losses can occur at the interface, depending on the choice of HEL. Our results indicate that this preferential loss of bromide is linked to the strong affinity of iodide to bind to the metal oxide surfaces as well as the phosphonic and sulfonic acid moieties of some of the chosen HELs. Notably, interfacial layers that work well for solution-processed films, such as self-assembled monolayers, turned out to be the most problematic for the formation and crystallinity of the mixed halide perovskite when thermal evaporation is employed. Additionally, we are able to show that the insertion of a thin layer of PbI₂ at the interface to the HEL can, in some cases, hinder this formation of volatile Br components by suppressing the formation of iodide surface bonds, thereby stabilizing the bromide content and improving the mixed halide film formation. These findings underscore the importance of substrate selection and passivation in achieving chemically inert interfaces, which is necessary for the formation of absorber films of high crystallinity with the desired halide composition.