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. The key factor determining the phase segregation in mixed halide perovskites (MHP) is the intrinsic imbalanced surface charges in the crystal lattice. MHP films with excellent crystallinity and surface morphology can suffer severe phase segregation. Effective passivation can be either achieved by creating a physical barrier to suppress the ion migration or chemically neutralizing the surface charges with additive materials. image