Perovskite solar cells (PSCs) with high efficiencies have been reported in recent years. Consequently, the main obstacle that hinders their commercialization is their poor thermal stability. Here, we describe the introduction of an A-site cation (2-choloroethylammonium) that affords an ABX(3) perovskite, which is stable at high temperatures (>80 degrees C) while achieving efficiencies > 19% in methylammonium lead iodide (MAPI)-based PSCs.