Defect Engineering of Grain Boundaries in Lead-Free Halide Double Perovskites for Better Optoelectronic Performance

Halide double perovskites (HDPs) are promising lead-free perovskites for various optoelectronic applications. However, the device performances of HDPs are far below the optimized values, which open a critical question regarding the origin of low performance in these HDPs. In this article, using first-principles calculations, it is found that some types of grain boundaries (GBs) are easy to form in polycrystalline HDPs. Importantly, the existence of low-energy Σ5(310) GBs can induce harmful deep-level defect states within the bandgaps of type-I (e.g., Cs ₂ AgInCl ₆ ) and type-II (e.g., Cs ₂ AgBiCl ₆ ) HDPs, which may dramatically reduce the device performances. Interestingly, it is found that the formation of some intrinsic defects and defect complexes could effectively eliminate these deep-levels in type-II and type-I HDPs, respectively. Under some exactly predesigned growth conditions identified by utilizing thousands of chemicals through a potential screening process, these defects or defect complexes can spontaneously incorporate into the GB cores, meanwhile the harmful deep-level defects in the bulk can also be effectively eliminated. In addition, the self-passivated GBs could generate band bending, which may be beneficial for charge separation. The understanding of GB formation as well as the self-passivation mechanism in HDPs can provide a new viewpoint and guidance for designing polycrystalline perovskites with improved optoelectronic performance.