Lu, H; Liu, Y; Ahlawat, P; Mishra, A; Tress, WR; Eickemeyer, FT; Yang, Y; Fu, F; Wang, Z; Avalos, CE; Carlsen, BI; Agarwalla, A; Zhang, X; Li, X; Zhan, Y; Zakeeruddin, SM; Emsley, L; Rothlisberger, U; Zheng, L; Hagfeldt, A; Grätzel, M; ,
Mixtures of cations or halides with FAPbI3 (where FA is formamidinium) lead to high efficiency in perovskite solar cells (PSCs) but also to blue-shifted absorption and long-term stability issues caused by loss of volatile methylammonium (MA) and phase segregation. We report a deposition method using MA thiocyanate (MASCN) or FASCN vapor treatment to convert yellow δ-FAPbI3 perovskite films to the desired pure α-phase. NMR quantifies MA incorporation into the framework. Molecular dynamics simulations show that SCN- anions promote the formation and stabilization of α-FAPbI3 below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI3 films to make PSCs with >23% power-conversion efficiency and long-term operational and thermal stability, as well as a low (330 millivolts) open-circuit voltage loss and a low (0.75 volt) turn-on voltage of electroluminescence.