Suppression of Defect Concentrated Planes, Ruddlesden-Popper Faults, via Post Halide Treatment for Blue Perovskite Light Emitting Diodes

The one of the critical issues in PeLEDs is low efficiency of blue LEDs. Even though the recent studies have been fabricated the highly efficient red and green perovskite LEDs above 20%, the blue perovskite LEDs still have low efficiency with ~5%. The major factors reducing the efficiency of blue LEDs are non-radiative defects in blue perovskite nanocrystals. Even if the colloidal PNC solutions are well known as the defect tolerant, the PNCs films are no more tolerant to the surface defects. Moreover, the chloride vacancies in mixed halide perovskite induce the deep trap states in PNCs band structure. These non-radiative defect states in blue PNCs films hinder the effective radiative recombination of blue PNCs LEDs during operation. The recent advances show the near unity photoluminescence quantum yield (PLQY) of as-synthesized Cl-based PNCs. Especially, CsPbCl₃ PNCs, expected to large number of deep trap states, also shows near unity PLQY, arranging short-order distance with adding nickel precursors during synthesis. However, efficiency of blue LEDs is still insufficient to follow up the green and red counterparts.<br/>In this study, I hypothesized that the mixed halide synthetic procedures induce several problems inside PNCs. To understand the atomic arrangement of mixed halide blue PNCs, I measured the high-resolution TEM images. In mixed-halide synthetic procedures, stacking faults, called Ruddlesden-Popper faults (RPFs), are introduced inside the PNCs. In contrast, mono-halide PNCs, CsPbBr₃, did not introduce the internal defects. 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. In here, low energy of halide vacancies in 1st and 2nd EQS near RPF were calculated, suggesting the severe defect concentration 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.<br/>Therefore, suppression of blue PNCs was further conducted to obtain highly efficient blue PNCs LEDs. At first, I tried to relieve the internal defects of mixed-halide PNCs (MH-PNCs) with post synthetic dual halide and ligand exchange. In here, MH-PNCs are defined as the blue mixed-halide PNCs synthesized with mixed halide precursors in hot injection and didodecyldimethylammonium chloride (DDAC) was applied to the MH-PNCs for halide and ligand sources, called MHS-PNCs. However, the internal defects were remained even after treating DDAC. That is, the post-synthetic procedures cannot recover the internal defects. I supposed that these kinds of internal defects might degrade the performance of blue MH-PNCs. Therefore, another blue synthetic procedure with post synthetic DDAC treatment toward mono-halide CsPbBr₃ PNCs (CPB-PNCs) was conducted, called post-halide exchange PNCs (PHE-PNCs). In the system, the chloride in DDAC acted as anion exchange precursors, making pure blue PNCs. As expected, PHE-PNCs did not introduce any internal defects. 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.