Seed-Assisted Growth of Methylammonium-Free Perovskite for Efficient Inverted Perovskite Solar Cells.

The traditional way to stabilize α-phase formamidinium lead triiodide (FAPbI 3 ) perovskite often involves considerable additions of methylammonium (MA) and bromide into the perovskite lattice, leading to an enlarged bandgap and reduced thermal stability. This work shows a seed-assisted growth strategy to induce a bottom-up crystallization of MA-free perovskite, by introducing a small amount of α-CsPbBr 3 /DMSO (5%) as seeds into the pristine FAPbI 3 system. During the initial crystalization period, the typical hexagonal α-FAPbI 3 crystals (containing α-CsPbBr 3 seeds) are directly formed even at ambient temperature, as observed by laser scanning confocal microscopy. It indicates that these seeds can promote the formation and stabilization of α-FAPbI 3 below the thermodynamic phase-transition temperature. After annealing not beyond 100 °C, CsPbBr 3 seeds homogeneously diffused into the entire perovskite layer via an ions exchange process. This work demonstrates an efficiency of 22% with hysteresis-free inverted perovskite solar cells (PSCs), one of the highest performances for MA-free inverted PSCs. Despite absented passivation processes, open-circuit voltage is improved by 100 millivolts compared to the control devices with the same stoichiometry, and long-term operational stability retained 92% under continuous full sun illumination. Going MA-free and low-temperature processes are a new insight for compatibility with tandems or flexible PSCs.