Mixed or Segregated: Toward Efficient and Stable Mixed Halide Perovskite-Based Devices

Convenient modulation of bandgap for the mixed halide perovskites (MHPs) (e.g., CsPbBr I_1- ) through varying the halide composition (i.e., the ratio of bromide to iodide) allows for optimizing the light-harvesting properties in perovskite solar cells (PSCs) and emission color in perovskite light-emitting diodes (PeLEDs). Such MHPs, yet, severely suffered from the instability under light irradiation and electrical bias as a result of an intrinsic soft, ionic lattice and a high halide ion mobility. Understanding the halide ion migration (mediated through halide vacancies) and suppressing the halide ion segregation, thus, remain a significant challenge both in the field of PSCs and PeLEDs since it is directly linked to the long-term stability and performances of the corresponding devices. In this Mini-Review, we discuss the intrinsic instability of the MHPs arising from the ionic nature of perovskites. The liquid crystalline properties with the low formation energy of halide ion defects facilitate the defect-mediated halide ion migration. Several different mechanistic models are provided to explain the fundamental origin of the photo- or electric field-driven halide ion segregation based upon thermodynamics and kinetics. These reflect that lattice strains (internal or polaron-induced) and bandgap energy differences between parent mixed halide and iodide-rich domain serve as the thermodynamic driving forces for halide segregation. On the basis of the deeper understanding of the underpinning segregation mechanism mediated through hole trapping and accumulation at the iodide-rich sites, we further discuss the strategies to mitigate the detrimental halide segregation through composition-, defect-, dimension-, and interface-engineering. Finally, we provide a fundamental insight into designing perovskite-based photovoltaic and optoelectronic devices for the long-term operational stability.