Stabilization of Highly Efficient and Stable Phase‐Pure FAPbI ₃ Perovskite Solar Cells by Molecularly Tailored 2D‐Overlayers

Abstract

As a result of their attractive optoelectronic properties, metal halide APbI₃ perovskites employing formamidinium (FA⁺ ) as the A cation are the focus of research. The superior chemical and thermal stability of FA⁺ cations makes α-FAPbI₃ more suitable for solar-cell applications than methylammonium lead iodide (MAPbI₃ ). However, its spontaneous conversion into the yellow non-perovskite phase (δ-FAPbI₃ ) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α-FAPbI₃ perovskite phase by protecting it with a two-dimensional (2D) IBA₂ FAPb₂ I₇ (IBA=iso-butylammonium overlayer, formed via stepwise annealing. The α-FAPbI₃ /IBA₂ FAPb₂ I₇ based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.