Suppressed phase and structural evolution of CH3NH3PbBr3 microwires to (CH3)2NH2PbBr3 by addition of hydrazine bromide

Organic acids, adopted in the conventional synthetic methods of metal halide perovskite (MHP) nanocrystals (NCs), have shown to accelerate the formation of (CH3)2NH2± cations in the colloidal solution, resulting in the morphological and phase transition of CH3NH3PbBr3 to (CH3)2NH2PbBr3 via a reversible acid-base mediated decomposition. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) analysis demonstrated the formation of (CH3)2NH2± and the complete removal of CH3NH3± cations during the aging process of the colloidal MHP NCs when oleic acid was introduced as a common capping agent. Our results propose a dominant degradation pathway via the deprotonation of CH3NH3± and the subsequent reaction with the residual CH3NH3± by methyl group transfer, leading to the formation of (CH3)2NH2±. Meanwhile, their phase and compositional stability could be significantly improved by introduction of hydrazinium bromide that provides a strong Lewis base suppressing the deprotonation of CH3NH3± cations. Furthermore, the excess Br- anions enable to passivate Br- ion vacancies at the MHP NC surface, which enhances their optoelectronic properties as well as photostability. Thus, these findings provide deep insight into the underlying chemistry associated with the degradation process in the colloidal MHP NCs, which is of fundamental importance for the use of highly stable and defect-free MHP NCs in the optoelectronic devices. © 2021