Metal halide perovskites exhibit superior electrical and optical properties, including excellent photoluminescence quantum yield. Moreover, their optical bandgap can be tuned by altering their halide-ion composition. Due to these beneficial advantages, these perovskites have emerged as promising candidates for fabricating light-emitting diodes (PeLEDs) with remarkable performance. Post-annealing at higher temperatures is often necessary to promote the formation of stable α-phase by suppressing the δ-phase crystals, especially for red light emission based on cesium lead halide (CsPbX3). However, the high-temperature process results in rapid crystals growth, and the perovskite crystals thus grow with many surface defects. Unfortunately, the performance of PeLEDs is limited by the many surface defects induced by the halide vacancies. These defects often lead to longer diffusion length, rough surfaces, high current leakage, and non-radiative recombination, among other problems.
Mixed-halide perovskites have been extensively investigated as an effective approach for suppressing halide vacancies by facilitating the fabrication of stable α-phase perovskite crystals. However, ion migration associated with mixed-halide perovskite crystals, when subjected to electrical fields, deteriorates device performance by preventing the formation of stable α-phase crystals. Recently many other strategies, including the addition of ligands, have been proposed to suppress surface defects in PeLEDs. While ligands with amine groups have been observed to hinder phase segregation and the formation of large perovskite crystals, higher ligand concentration induces self-aggregation. Thus, there is a limit to the passivation of surface defects by increasing the concentration of the ligand. Nevertheless, using KBr as a passivation agent, the resulting PeLED exhibited improved luminance and external quantum efficiency. These developments have paved the way for developing high-performance PeLEDs for commercialization purposes.
To overcome these obstacles and improve device performance, a team of Yonsei University researchers: Dr. Do Hoon Kim, PhD candidate Hee Ju An, Mr. In Young Choi and led by Professor Jae-Min Myoung, supported by Samsung Research Funding & Incubation Center of Samsung Electronics, fabricated pure-red PeLEDs based on CsPbBrxI3-x–multi-ligands–KBr composite films. The limitations of adding ligands and KBr were also discussed. Different techniques like scanning electron microscopy were used to examine the surface morphology and structural properties of the produced PeLEDs. The research work is currently published in the Chemical Engineering Journal.
The authors showed that adding multi-ligands facilitated the recombination of excitons and subsequent formation of stable α-phase perovskite crystals. The interaction between the KBr with excess halide ions not only suppressed the formation of halide vacancies but also resulted in the full surface coverage of the composite films due to the significant decrease in crystal size and surface defects. The resulting PeLED with 70% multi-ligand concentration and 0.03M KBr exhibited a current efficiency, current density, luminance and external quantum efficiency of 3.2 cd/A, 5.1 mA/cm2, 743.2 cd/m2 and 10.2% at 3.5 V, respectively. In addition, a similar PeLED displayed a low turn-on voltage of about 1.6 V with pure red emission. Furthermore, the composite films exhibited a remarkable improvement in photoluminescence and EL intensities.
In summary, the study reported the successful fabrication of high-performance red PeLEDs based on CsPbBrxI3-x–multi-ligands–KBr composite films. The high performance was attributed to numerous factors, including the significant reduction in surface defects that were directly attributed to the positive effects of adding multi-ligand and KBr. Overall, the findings showed that the incorporation of multi-ligands and KBr is an ideal and promising strategy for suppressing the formation of halide vacancies in perovskite surfaces. In a statement to Advances in Engineering, Professor Jae-Min Myoung, the lead and corresponding author explained that their study provided valuable insights that would contribute to the design and fabrication of advanced and high-performance red PeLEDs for commercial applications.
Kim, D., An, H., Choi, I., & Myoung, J. (2022). High‐performance pure-red light‐emitting diodes based on CsPbBrxI3-x–multi-ligands–KBr composite films. Chemical Engineering Journal, 429, 132375.