Deciphering the NH4PbI3 Intermediate Phase for Simultaneous Improvement on Nucleation and Crystal Growth of Perovskite

The NH4PbI3-based phase transformation is realized by simply adding NH4I additive, in order to simultaneously control perovskite nucleation and crystal growth. Regarding the nucleation process, the NH4+ with small ionic radius preferentially diffuses into the [PbI6](4-) octahedral layer to form NH4PbI3, which compensates the lack of CH3NH3I (MAI) precipitation. The generation of NH4PbI3 intermediate phase results in extra heterogeneous nucleation sites and reduces the defects derived from the absence of MA(+). Regarding the crystal growth process, the cation exchange process between MA(+) and NH4+, instead of the MAs directly entering, successfully retards the crystal growth. Such NH4PbI3 consumption process slows down the crystal growth, which effectively improves the perovskite quality with lowered defect density. The cooperation of these two effects eventually leads to the high-quality perovskite with enlarged grain size, prolonged photoluminescence lifetime, lowered defect density, and increased carrier concentration, as well as the finally enhanced photovoltaic performance. Moreover, NH3 as a byproduct further facilitates the proposed transformation process and no external residue remains even without any post-treatment. Such methodology of introducing a novel phase transformation to simultaneously control nucleation and crystal growth processes is of universal significance for further devotion in the foreseeable perovskite solar cells (PSCs) evolution.