Imbalanced Surface Charge Induced Phase Segregation in Mixed Halide Perovskites

Alloying the metal halide perovskite composition has become a key strategy for engineering the optoelectronic properties for various applications. However, the difference in complexation between ions poses a threat to crystal stability. In particular, the phase segregation in wide-bandgap perovskites (WBG) with mixed halides is the major challenge in multi-junction photovoltaic devices. Here, a systematic study is reported on the origin of phase segregation in a prototypical mixed halide perovskite (MHP) MAPbI₂Br. It is found that the crystallinity and surface morphology are not associated with the lattice stability. Using highly sensitive external quantum efficiency (s-EQE) measurement, it is found that the defects in the MHP are mainly associated with the Pb²⁺ and I⁻/Br⁻ ions. Consistently, density-function-theory (DFT) calculation confirms that the MAPbI₂Br surface is dominated by the metal-halide components, and the imbalanced surface charge is responsible for the lattice instability. The role of physical and chemical passivation approaches in MHP is further investigated. The former limits the ion migration and allows dissociated ions to recover to their corresponding defective sites, whereas the latter lowers the surface energy by neutralizing the charge density on the lattice surface. This work brings insight into the phase segregation and passivation mechanisms in MHP.