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

The NH 4 PbI 3 ‐based phase transformation is realized by simply adding NH 4 I additive, in order to simultaneously control perovskite nucleation and crystal growth. Regarding the nucleation process, the NH 4 + with small ionic radius preferentially diffuses into the PbI 6 4− octahedral layer to form NH 4 PbI 3 , which compensates the lack of CH 3 NH 3 I (MAI) precipitation. The generation of NH 4 PbI 3 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 NH 4 + , instead of the MAs directly entering, successfully retards the crystal growth. Such NH 4 PbI 3 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, NH 3 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.