Efficient thermally evaporated γ-CsPbI${}_3$ perovskite solar cells

Thin-film deposition by thermal evaporation offers many advantages, yet in the field of perovskite photovoltaics solution-processed devices significantly outperform those fabricated by thermal evaporation. Here, high-quality γ-CsPbI${}_3$ perovskite layers by coevaporation of PbI${}_2$ and CsI with a small amount of phenylethylammonium iodide (PEAI) are deposited. It is demonstrated that the addition of PEAI into the perovskite layers leads to a preferred crystal orientation and a far improved microstructure, with columnar domains that protrude throughout the film's thickness. This is accompanied by a reduced density of defects as evidenced by the increase in photoluminescence and decrease in Urbach energy as compared to reference CsPbI${}_3$ films. Photovoltaic devices based on the PEAI containing perovskite layers reach up to 15% in power conversion efficiency, thus surpassing not only the performance of reference CsPbI${}_3$ devices, but also that of most solution-processed PEAI containing inorganic CsPbX${}_3$ (X = Cl, Br, I) perovskite solar cells. Importantly, encapsulated thermally evaporated perovskite devices maintain their performance for over 215 days, demonstrating the stabilizing effect of PEAI on thermally evaporated CsPbI${}_3$. (© 2021 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH)