Professor Kim Tae-geun and Professor Choi Dong-hoon’s group developed a ‘next-generation solution-processed flexible OLED’ featuring excellent durability and mechanical stability.
The new OLED is expected to greatly contribute to the realization of various types of wearable devices and the improvement of their efficiency.

The research results were published in Nano Energy, a globally renowned journal, in January 2023.

▲ (From left) Professor Kim Tae-geun (corresponding author), Professor Dong Hoon Choi (corresponding author), Kim Na-hyun (integrated master-doctoral degree programs) (first author), and Dr. Hwang Jin-hyo (first author).

The research group led by Professor Kim Tae-geun of the School of Electrical Engineering and Professor Choi Dong-hoon of the Department of Chemistry developed a core technology that can drastically improve the optical efficiency and mechanical stability of next-generation flexible organic light-emitting diodes (OLED) using a flexible metal-doped mesh-structured transparent electrode. This allows the development of varied electronic and optoelectronic devices given the advancement of flexible electrode, organic semiconductor and solution process technologies.

As the importance of the study was recognized, the results were published in Nano Energy [IF:19.069, JCR (Journal Citation Reports): top 5.07%], a globally renowned journal, in January 2023. The study was supported by the Research Leader Program and the Priority Research Institute Program through the National Research Foundation of Korea funded by the Ministry of Science and ICT, and by LG Display.

- Title of article : Work-function-tunable metal-oxide mesh electrode and novel soluble bipolar host for high-performance solution-processed flexible TADF-OLED
- Authors: Nahyun Kim (first author), Jinhyo Hwang (first author), Ho Jin Lee, Na Yeon Kwon, Jin Young Park, Chang Woo Koh, Min Ju Cho, Sungnam Park, Dong Hoon Choi (corresponding author), and Tae Geun Kim (corresponding author).

Recently, in the field of flexible optoelectronic devices, OLEDs and solar cell devices have been miniaturized and require flexibility. Therefore, an electrode should possess both flexibility and mechanical durability, while maintaining transmittance and electric conductivity. In addition, new light-emitting materials should be developed. To achieve these goals, studies are being actively conducted in this field.

※ Indium tin oxide (ITO), having excellent conductivity and transparency, is commonly used as an electrode material for displays and other varied optoelectronic devices, but due to its intrinsic brittleness, it is easily damaged by repeated bending.

The research group employed an electrode made of a mesh structure to increase mechanical flexibility, and metal ions (Ni) were locally doped onto the electrode surface of indium zinc oxide (IZO), which is a conductive oxide, in consideration of electrical characteristics. In this way, sheet resistance was decreased while the high transmittance of the thin film was maintained, and the work function was increased to 5.3 eV to improve anode performance. Through this approach, the research group successfully developed a transparent electrode for OLED displays featuring superior flexibility and mechanical durability that can replace ITO, which is vulnerable to bending.

Figure 1) The structure of the newly developed green TADF OLED using the flexible and transparent electrode of mesh-structured Ni-doped indium zinc oxide (mNIZO) and CzCN-tCz host material (left); and the luminance reduction after the bending to a radius of 5 mm over 0 to 2000 bending cycles (right).

The electrode developed by the group allows control of the electric conductivity, transmittance, and work function through doping and the optimization of the flexible characteristics. The biggest advantage of the electrode is that the co-sputtering method simplifies the process, as it can be applied to independently control both the diffusion on the transparent electrode surface and the electric and optical properties of the film.

Furthermore, referring to the structure of mCBP-CN, which is a host material widely used in blue thermally activated delayed fluorescence (TADF) OLEDs fabricated by conventional vacuum deposition, the research group developed CzCN-tCz. It is, a new anodic host material featuring significantly higher solubility in organic solvents while retaining the unique characteristics of mCBP-CN.

Figure 2) Light emission by the OLED in a bent state.

The poly(ethylene 2,6-naphthalate)-based flexible TADF OLED fabricated using the newly developed flexible and transparent electrode of mesh-structured Ni-doped indium zinc oxide (mNIZO) and the CzCN-tCz host material exhibited a maximum external quantum efficiency of 20% or higher. In particular, the new OLED successfully demonstrated very high mechanical durability, by maintaining more than 80% of its initial luminance even after bending to a radius of 5 mm over 2000 bending cycles.

The results of this study are expected to allow the manufacturing of cost-effective high-efficiency flexible displays through the simple solution process and make further contributions to the fabrication of various types of wearable devices and the improvement of their efficiency.