Multi-source vacuum deposition of methylammonium-free perovskite solar cells

Halide perovskites of the form ABX3 have shown outstanding properties for solar cells. The highest reported compositions consist of mixtures of A-site cations methylammonium (MA), formamidinium (FA) and cesium and X-site iodide and bromide ions and are produced by solution processing. However, it is unclear whether solution processing will yield sufficient spatial performance uniformity for large-scale photovoltaic modules or compatibility with deposition of multi-layered tandem solar cell stacks. In addition, the volatile MA cation presents long-term stability issues. Here, we report the multi-source vacuum deposition of MA-free FA0.7Cs0.3Pb(I0.9Br0.1)3 perovskite thin films with high-quality morphological, structural and optoelectronic properties. We find that the controlled addition of excess PbI2 during the deposition is critical for achieving high performance and stability of the absorber material, and we fabricate p-i-n solar cells with stabilized power output of 18.2%. We also reveal the sensitivity of the deposition process to a range of parameters including substrate, annealing temperature, evaporation rates and source purity, providing a guide for further evaporation efforts. Our results demonstrate the enormous promise for MA-free perovskite solar cells employing industry-scalable multi-source evaporation processes.S.D.S. and M.A. acknowledge funding from the European Research Council (ERC) (grant agreement No. 756962 [HYPERION]) and the Marie Skłodowska-Curie actions (grant agreement No. 841386) under the European Union’s Horizon 2020 research and innovation programme. S.D.S acknowledges support from the Royal Society and Tata Group (UF150033). Y.-H. C. acknowledges funding from a Taiwan Cambridge Scholarship. Part of this work was undertaken using equipment facilities provided by the Henry Royce Institute, via the grant: Henry Royce Institute, Cambridge Equipment: EP/P024947/1. The authors acknowledge the Engineering and Physical Research Council (EPSRC) for funding (EP/R023980/1). We thank Tiarnan Doherty and Tim van de Goor for useful discussions