As a result of their attractive optoelectronic properties, metal halide APbI(3)perovskites employing formamidinium (FA(+)) as the A cation are the focus of research. The superior chemical and thermal stability of FA(+)cations makes alpha-FAPbI(3)more suitable for solar-cell applications than methylammonium lead iodide (MAPbI(3)). However, its spontaneous conversion into the yellow non-perovskite phase (delta-FAPbI(3)) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired alpha-FAPbI(3)perovskite phase by protecting it with a two-dimensional (2D) IBA(2)FAPb(2)I(7)(IBA=iso-butylammonium overlayer, formed via stepwise annealing. The alpha-FAPbI(3)/IBA(2)FAPb(2)I(7)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 degrees C over 500 h.