Unveiling the complexity of co-evaporation of perovskite: Why co-evaporation might not be the optimal choice

Co-evaporation has emerged as a promising deposition method for perovskite solar cells, offering several advantages such as solvent-free processing and scalability. However, in this work, we systematically report the challenges we faced during the thermal co-evaporation of organic–inorganic perovskite precursors to form a double-cation, double-halide wide bandgap perovskite with the composition (FA<inf>x</inf>Cs<inf>1−x</inf>Pb(I<inf>y</inf>Br<inf>1−y</inf>)<inf>3</inf>), which may hinder the deposition methods's transfer to industry. We demonstrate that the substrate material plays a crucial role in perovskite formation, where even minor surface treatments, such as annealing or washing (in the case of self-assembled molecules), can substantially influence film properties. More critically, we describe the difficulty in controlling the deposition rates of inorganic precursors in the co-evaporation method due to the non-linear evaporation of the organic component leading to inconsistencies in stoichiometry and irreproducible device performance. These inherent challenges limit the suitability of co-evaporation for systematic studies. Moreover, we show a direct relation between the amount of FAI incorporated in the perovskite film and the formation of a pure alpha phase.