Vertical recrystallization for highly efficient and stable formamidinium-based inverted-structure perovskite solar cells

Formamidinium (FA)-based perovskite materials show an extended absorption spectrum to 840 nm, which enables high power conversion efficiencies of over 20% compared with normal-structure perovskite solar cells (PSCs). However, it is rarely possible to obtain high performance in inverted-structure PSCs owing to the unbalanced electron-hole transport properties. To achieve desirable electronic qualities in a FA perovskite film, it is necessary to substantially improve the crystallinity of the perovskite films. A new perovskite growth method is presented here using methylammonium chloride (MACl) to assist vertical recrystallization in a formamidinium perovskite film. The obtained film consists of highly crystallized vertically-orientated grains, which minimize the vertical grain boundary and trap site in the films, and later contribute to a power conversion efficiency above 20% in inverted-structure PSCs. Most importantly, the highly crystalline, phase-pure morphology and low MA content in formamidinium perovskite films can contribute to the light-soaking stability and thermal stability up to 500 h in solar-cell devices.