Controlled orientation of perovskite films through mixed cations toward high performance perovskite solar cells

Solar cells based on organic inorganic hybrid metal halide perovskites have exhibited a rapid increase of power conversion efficiency (PCE). Perovskite solar cells involving mixed cations, especially recently reported Cs doping, have shown huge potential to improve PCE as well as device stability. However, when doping Cs into CH3NH3PbI3 (MAPbI3) and [HC(NH2)2]3PbI3 (FAPbI3), CsPbI3 could segregate from the perovskite phase, affecting the performance negatively. In addition, despite improved charge transfer was predicted for oriented film along <112>/<200> directions, the fabrication is still on the way and rarely reported. Herein, an ultra-smooth perovskite film oriented along <112>/<200> directions is created for the first time, with a homogeneous tetragonal phase of (MAPbI3)1−x(CsPbBr3)x. The preferentially precipitated heavily Cs-doped perovskite, and the lowered surface energy of (112) and (200) planes, verified by DFT calculations, are responsible for the orientation. The improved charge transfer and suppressed trap states in the oriented film substantially improved the performance. Upon an optimal doping ratio of 0.1, a PCE of 17.6% was achieved, together with remarkable improvements in stability under UV irradiance and in ambient atmosphere.