Removing internal defects inside the perovskite nanocrystals

Abstract

In this study, we observed that structural defects, called Ruddlesden-Popper faults (RPFs), were introduced inside the perovskite nanocrystals (PNCs) in mixed-halide synthesis of PNCs. To further deliberate analysis of RPFs, halide intensity mapping near RPFs was conducted with annular dark field images of TEM. The intensity mapping of halide positions revealed the severe intensity drops near RPF. In specific, the halide intensity was dropped in 1st equatorial sites (EQS) and 2nd EQS. The intensity drops can be interpreted as two ways: chloride localization and halide vacancies. To understand the origin of intensity drops, formation energy of chloride substitutions and halide vacancies were calculated, revealing low formation energy of halide vacancies in 1st and 2nd EQS near RPF. Further, band structures of chloride localization and halide vacancies near RPF were calculated. While chloride localization did not introduce in-gap defect states, halide vacancies introduced the severe in-gap defect states. In summary, unwanted RPF structures were generated during mixed halide blue PNCs synthesis, deteriorating the optical properties of blue PNCs. Therefore, suppression of blue PNCs was further conducted to obtain highly efficient blue PNCs LEDs. At first, mixed-halide PNCs (MH-PNCs) were treated in post-synthetic halide and ligand exchange with didodecyldimethylammonium chloride (DDAC), expecting the suppression of RPFs. However, RPF was remained even after treatment process, indicating the post-synthetic procedures cannot recover the internal defects. Since the mono-halide CsPbBr3 PNCs (CPB-PNCs) did not show RPF structure, blue PNCs with post synthetic DDAC treatment toward CPB-PNCs was synthesized, called post-halide exchange PNCs (PHE-PNCs). As expected, PHE-PNCs did not introduce any RPF structures. I further compared optical properties of PHE-PNCs with MHS-PNCs in PLQY, showing average 60% of PLQY in PHE-PNCs and average 22% of PLQY in MHS-PNCs. Finally, the device fabrication of MHS-PNCs and PHE-PNCs were conducted, achieving highly efficient blue PNCs LEDs in PHE-PNCs with 2.12% of EQE.