Molecular Dopant Induced Growth of Black Phase Cs <sub>x</sub>FA <sub>(1-x)</sub>PbI <sub>3</sub> for Highly Efficient and Stable Perovskite Solar Cells

The manipulation of growth progress to improve the quality of perovskite film significantly prompts the development of perovskite based optoelectronic devices. In recent years, CsxFA(1-x)PbI3 is rising as the most promising structure for the fabrication of highly efficient and stable perovskite solar cells, however, the growth of high quality black phase perovskite structure remains to be a challenge due to uncontrollable phase transition. Herein, we synthesized a halogenated tertiary ammonium salt that can reduce surface energy of black phase perovskite structure. This molecule accelerates the growth of black phase perovskite at room temperature and phase transition in annealing, leading to large grain size with less defect density. Solar cells based on the film shows much improved efficiency up to 23.6%, the highest efficiency for CsxFA(1-x)PbI3 solar cells with inverted structure. The device demonstrates excellent operation stability: it maintains 95% efficiency after over 2,200 hours of continuous operation in maximum power point, and the stable operation time at 85°C is over 250 hours, which is four times longer than the previous report.