Formamidinium lead triiodide perovskites with improved structural stabilities and photovoltaic properties obtained by ultratrace dimethylamine substitution

Applications of organic–inorganic formamidinium (FA) lead triiodide (FAPbI₃) perovskites in high-efficiency solar cells often suffer from spontaneous α-to-δ phase transitions. However, current efforts to inhibit this phenomenon based on simple cation and anion alloying strategies continue to suffer from unintended consequences, such as unfavorable shifts in the bandgap energy and unwanted phase separation during operation. The present work compares the effects of bromine (Br) anion and dimethylamine (DMA) cation alloying on the structure and properties of FAPbI₃ perovskite in detail. DMA-incorporated FAPbI₃ perovskites show significantly improved structural stability and photovoltaic performance, while the inherent bandgap energy of the original material is maintained. Rigorous analyses demonstrate that the relatively large size and free isotropic motion of the incorporated DMA cations constrain the dynamic space of neighboring FA cations, which increases the degree to which the FA cations interact with the inorganic lattice and therefore stabilizes the PbI₆ lattice structure without significant lattice distortion. Hence, this work demonstrates an efficient method for improving the phase stability of FAPbI₃ perovskite materials while providing a plausible molecular mechanism for the stability engendered by the alloying of DMA and FA.