Thermally evaporated methylammonium-free perovskite solar cells

Thermal evaporation is a well-established and versatile method for the deposition of large-area, uniform, high-quality semiconducting layers in a broad range of optoelectronic applications. Despite extensive investigation of deposition by thermal evaporation in the field of perovskite solar cells, solution-processed perovskite devices still significantly outperform those fabricated by thermal evaporation, particularly in the case of methylammonium-free perovskite compositions. Herein, we fabricate and investigate thermally evaporated Cs(0.1)FA(x)PbI(2+x)Br(0.1)perovskite solar cells and explore the effects of FAI deficiency or excess and that of post-annealing on the perovskite layer properties and device performance. We find that annealing can significantly improve the optical and structural properties of FAI-poor perovskite layers, resulting in a stark enhancement of their photovoltaic performance. While stoichiometric devices are also improved, albeit to a lesser degree, by post-annealing, this process is found to be detrimental for the FAI-rich devices, resulting in a drastic loss of performance. We show that annealed stoichiometric devices with an optimised active layer thickness result in power conversion efficiencies of up to 16.6%, approaching the performance of solution-processed devices of similar composition.