Regulating Crystallization for Pure-Iodide 1.68 eV Bandgap Perovskite Solar Cells with a Fill Factor over 86%

Mixed halide wide-bandgap (WBG) perovskites, widely used as a top-cell absorber in tandem solar cells, exhibit severe photoinduced halide phase segregation. A feasible solution is to exploit pure-iodide WBG perovskites, essentially increasing Cs content instead of Br to achieve bandgap widening. However, the efficiency of pure-iodine WBG perovskite solar cells (PSCs) reported so far has been inferior to that of the typical mixed halide WBG PSCs due to complex nucleation and phase transition processes, leading to poor crystallization quality and a high density of defect states in pure-iodine WBG perovskites. Here, by combining lead thiocyanate (Pb(SCN)₂) and oleylamine hydrochloride (OAmCl) with the Cs_0.3DMA_0.2MA_0.5PbI₃ perovskite precursor, a homogeneous phase distribution is obtained, resulting in enhanced crystallization and a reduction of excess lead source defects. With this approach, the resulting film quality is improved along with fewer surface-bulk defects as well as beneficial surface electronic properties. As a result, the pure-iodide WBG PSCs deliver a high efficiency of 21.55%, an extremely high fill factor of 86.03%, and superior photostability. The target film is fundamentally free of phase segregation under continuous light for 12 h (AM 1.5 G illumination, xenon lamp, 1 sun).