Surface-Energy-Regulated Growth of α-Phase Cs 0.03 FA 0.97 PbI 3 for Highly Efficient and Stable Inverted Perovskite Solar Cells.

Methylammonium (MA)-free formamidinium (FA)-dominated Cs x FA 1-x PbI 3 is rising as the most promising candidate for highly efficient and stable perovskite solar cells. However, the growth of high-quality Cs x FA 1-x PbI 3 black-phase perovskite structure without ion doping in the lattice remains a challenge. Herein, propeller-shaped halogenated tertiary ammonium is synthesized, showing high binding energy on the perovskite surface and large steric hindrance. This molecule can significantly reduce the barrier of high surface energy that suppresses the growth of the α-phase Cs x FA 1-x PbI 3 structure. As a result, the α-phase structure can be formed at room temperature, which can further act as a seed for the growth of high-quality film. Solar cells based on the film show a record efficiency up to 23.6% for MA free Cs x FA 1- x PbI 3 solar cells with inverted structure and excellent stability at 85 °C over 200 h.