Highly efficient solar cells using methylammonium lead iodide (MAPbI₃) have been reported by many researchers. Recently, other metal halide perovskites have been developed which promise better stability. For example replacing the organic cation methylammonium with formamidinium (FA) has been shown to improve the power conversion efficiency and stability of perovskite thin-films under certain conditions such as thermal stress [1]. Additionally, FAPbI₃ has a lower band-gap of 1.48eV, thus FAPbI₃ based solar cells have a higher short circuit current than cells based on MAPbI₃. Furthermore, mixed cation perovskites which alloy formamidinium, methylammonium and caesium have led to even greater improvements in stability and efficiency [2]. Until recently, thin films of perovskites with cations other than methylammonium have mainly be been fabricated using a solution processing route.

Here we demonstrate vacuum deposition of formamidinium lead halide absorber thin films [3]. We deposited the films by co-evaporating FAI and PbI₂. This vapour deposition method offers several advantages: the process takes place in vacuum, this reduces the influence of outside factors and increases the reproducibility; films can be vapour deposited on any substrate without the risk of solvent damage to the substrate layers; and finally, the resulting films are very uniform and pinhole-free over large areas. The good uniformity of this technique is particularly helpful for future upscaling of these solar cells. We characterised the FAPbI₃ films, using methods such as X-ray diffraction, visible- and infrared-absorption spectroscopy, scanning electron microscopy and cross-sectional transmission electron microscopy. These measurements confirm the formation of the perovskite phase of FAPbI₃.

Furthermore, we processed these co-evaporated films into planar perovskite solar cells. These solar cells reached high power conversion efficiencies, comparable to the efficiencies of MAPbI₃ control devices. As expected, the short circuit voltage was higher than for the MAPbI₃ devices while the open circuit voltage was slightly lower. The successful fabrication of vapour deposited and very uniform formamidinium lead iodide films is necessary for exact optical material characterisation. This work represents an important step towards realising large-area vapour deposited mixed cation perovskite solar cells.

References

[1] G. E. Eperon et al. Energy & Environmental Science (2014) 7, 982-988

[2] M. Saliba et al. Energy & Environmental Science 9.6 (2016) 1989-1997

[3] J. Borchert et al. ACS Energy Letters, (2017) 2, 2799-2804