In general, mixed cations and anions containing formamidinium (FA), methylammonium (MA), caesium, iodine, and bromine ions are used to stabilize the black alpha-phase of the FA-based lead triiodide (FAPbI(3)) in perovskite solar cells. However, additives such as MA, caesium, and bromine widen its bandgap and reduce the thermal stability. We stabilized the alpha-FAPbI(3) phase by doping with methylenediammonium dichloride (MDACl(2)) and achieved a certified short-circuit current density of between 26.1 and 26.7 milliamperes per square centimeter. With certified power conversion efficiencies (PCEs) of 23.7%, more than 90% of the initial efficiency was maintained after 600 hours of operation with maximum power point tracking under full sunlight illumination in ambient conditions including ultraviolet light. Unencapsulated devices retained more than 90% of their initial PCE even after annealing for 20 hours at 150 degrees C in air and exhibited superior thermal and humidity stability over a control device in which FAPbI(3) was stabilized by MAPbBr(3).