Lee Yu-bin, a master’s degree candidate in the Department of Chemistry at Kookmin University (President Jeong Seung Ryul), working as part of a research team led by Professor Kim Younghoon, has succeeded in developing high-efficiency perovskite light-emitting diodes (LEDs) without the need for a separate post-synthesis ligand exchange process, through a collaborative study with research teams led by Professor Lee Boram at Sungkyunkwan University and Professor Lee Tae Kyung at Hanyang University.

△ Photo: Schematic diagram of perovskite nanocrystal technology

The findings of this study are scheduled to be published in the international journal *Advanced Functional Materials*, a leading publication in the field of materials science, under the title “Direct Integration of Conductive, Emissive Perovskite Nanocrystals into Efficient Light-Emitting Diodes Without Post-Synthetic Ligand Exchange.” “Advanced Functional Materials” is an international journal covering research across the entire field of materials science, including functional materials, nanotechnology, chemistry, and physics. With an Impact Factor of 19.9, it ranks within the top 5% according to the JCR.
 
Perovskite nanocrystals (PNCs) are attracting attention as a key material for next-generation display light-emitting diodes due to their high luminous efficiency, excellent color purity, and solution processability. However, the surfaces of nanocrystals produced using conventional synthesis methods contained large amounts of organic ligands with long hydrocarbon chains, which hindered charge transport between nanocrystals. Consequently, a “post-synthetic ligand exchange” process—in which long ligands are replaced with short ones after synthesis—has generally been required to fabricate high-performance devices.
 
To overcome these limitations, the research team controlled the surface chemistry of the nanocrystals from the synthesis stage onward using a heat-up synthesis method based on an SN2 reaction. Through this approach, they significantly reduced the amount of oleate ligands on the surface of conventional nanocrystals and developed CsPbBr₃ perovskite nanocrystals with low ligand density, stabilized by an abundance of Br⁻ on the surface. In particular, they demonstrated through experiments and theoretical calculations that the OAM/Br⁻ surface combination forms more stable surface bonds than the conventional OAM/oleate combination, enabling the suppression of surface defects and the maintenance of excellent optical properties even at low organic ligand densities.
 
The developed perovskite nanocrystal thin film exhibited lower energy disorder and improved crystal alignment characteristics compared to nanocrystals synthesized using conventional methods, and the charge transport properties between nanocrystals were also significantly improved. Based on this, a maximum external quantum efficiency (EQE) of 11.4% was achieved in an LED where the nanocrystals were directly applied to the light-emitting layer without a separate post-synthesis ligand exchange process. This figure is approximately four times higher than the 2.7% achieved by devices using nanocrystals synthesized via conventional methods. The maximum luminance also improved more than fivefold, from 59 cd m⁻² to 314 cd m⁻², and the full width at half maximum (FWHM) of the emission spectrum decreased from 21 nm to 17 nm, ensuring high color purity.
 
This research is significant in that it simultaneously achieved excellent luminescence characteristics and electrical conductivity while omitting the post-synthesis ligand exchange process previously required for the fabrication of high-performance perovskite nanocrystal devices. By precisely controlling the surface ligand density and halide passivation of the nanocrystals during the synthesis stage, the fabrication process for light-emitting devices has been simplified, and it is expected to expand the potential for future applications in various optoelectronic devices.
 
This study is particularly noteworthy because Lee Yu-bin, a master’s degree candidate in the Department of Chemistry at Kookmin University, participated as a co-first author alongside Kim Ji-geon, a researcher at Kookmin University, and Seok Gyeong-eun, a researcher at Sungkyunkwan University. By participating in key research processes—including the analysis of the surface properties of perovskite nanocrystals and their application in light-emitting devices—Lee Yu-bin demonstrated her research capabilities in the field of next-generation optoelectronic materials and devices. Professor Lee Tae Kyung of Hanyang University, Professor Lee Boram of Sungkyunkwan University, and Professor Kim Younghoon of Kookmin University served as co-corresponding authors.