CH₃NH₃PbI₃ films prepared by combining 1- and 2-step deposition: how crystal growth conditions affect properties

Perovskite solar cells continue to attract strong attention because of their unprecedented rate of power conversion efficiency increase. CH₃NH₃PbI₃ (MAPbI₃) is the most widely studied perovskite. Typically one-step (1-s) or two-step (2-s) deposition methods are used to prepare MAPbI₃ films. Here, we investigate a new MAPbI₃ film formation method that combines 1-s and 2-s deposition (termed 1 & 2-s) and uses systematic variation of the stoichiometric mole ratio (x) for the PbI₂ + xMAI solutions employed. The PbI₂ + xMAI solutions were used to deposit precursor films that were subsequently dipped in MAI solution as a second step to produce the final MAPbI₃ films. The morphologies of the 1 & 2-s MAPbI₃ films consisted of three crystal types: tree-like microcrystals (≫1 μm), cuboid meso-crystals (∼0.1-1 μm) and nanocrystals (∼50-80 nm). Each crystal type and their proportions were controlled by the value for x. The new 1 & 2-s deposition method produced MAPbI₃ films with tuneable optoelectronic properties that were related to those for the conventional 1-s and 2-s films. However, the 1 & 2-s film properties were not simply a combination of those for the 1-s and 2-s films. The 1 & 2-s films showed enhanced light scattering and the photoluminescence spectra displayed a morphologically-dependent red-shift. The unique morphologies for the 1 & 2-s films also strongly influenced PbI₂ conversion, power conversion efficiency, hysteresis and recombination. The trends for the performance parameters and hysteresis were compared for devices constructed using spiro-MeOTAD and P3HT and were similar. The 1 & 2-s method should apply to other perovskite formulations and the new insights concerning MAPbI₃ crystal growth conditions, morphology and material properties established in this study should also be transferable.