Implementation of High-Color Purity and High-Efficiency OLED Based on Solution Processing Using Eco-Friendly Solvent

Development of innovative multilayer device manufacturing technology

KU’s research group published the article in Chemical Engineering Journal, a renowned international journal in chemical engineering

▲ (From left) (Top) Dr. Park Su-hong (first author); Dr. Kwon Na-yeon (first author), Dr. Koh Chang-woo (first author); (bottom) Research Professor Cho Min-ju (corresponding author), Professor Park Sung-nam (corresponding author), and Professor Choi Dong-hoon (corresponding author)

A research group of Professor Choi Dong-hoon and Professor Park Sung-nam of the Department of Chemistry developed a high-efficiency solution-processed organic light-emitting diode (OLED) that exhibits excellent color purity, using the world’s first boron-based light-emitting material and an eco-friendly solvent.

As the innovativeness of the novel light-emitting material and the eco-friendly process were recognized, the results were published online in Chemical Engineering Journal [IF: 15.1, citation rank (JCR, Journal Citation Reports): top 3.2%], which is an internationally renowned journal in chemical engineering, on December 31, 2023. The research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF).

In general, fabricating multilayer structures in solution-processed organic light-emitting diodes (OLEDs) presents challenges owing to the potential degradation of the initial film during subsequent processing steps. OLED devices have a multilayer structure including the cathode, electron injection layer (EIL), electron transport layer (ETL), emissive layer (EML), hole transport layer (HTL), hole injection layer (HIL), and anode. In the solution process, various fabrication methods for each organic film layer include spin-coating, blade coating, and injection printing techniques. However, many studies in fabricating multilayer organic films could not be conducted due to the issue of the solution forming an upper layer degrading or dissolving the film of a previously formed lower layer. Therefore, researchers have introduced the orthogonal solvent system and crosslinking treatments for the films to address these issues and effectively prevent solvent-related deterioration of the lower layers. Since such a special structure had to be used, the diversity of the materials comprising the individual layers was highly limited.

Until now, in the fabrication of organic semiconductor-based photoelectric devices (organic photovoltaics (OPVs) and OLEDs), aromatic or halogenated solvents are typically used, because the solubility of the conjugated organic semiconductor materials in the organic solvents should be taken into consideration. However, since these solvents are harmful to health and may have negative effects on the surrounding environment, their use remains an important problem that should be solved when commercializing organic semiconductors for solution processes.

To fabricate eco-friendly OLED devices without using any aromatic or halogenated solvents, the research group developed an innovative light-emitting material in the form of macromolecules that are dissolved in an aliphatic solvent. Furthermore, the research group newly introduced a charge transfer layer organic semiconductor polymer, which is also dissolved in an aliphatic organic solvent. The OLEDs fabricated using these constituents exhibited a very narrow photoluminescence spectrum, successfully achieving high color purity.

In the study, for each layer of the thermally activated delayed fluorescence (TADF) OLED, water was used in the HIL, cyclohexanone in the HTL, and ethyl acetate in the EML. In particular, for the study, BCzBN-3tCz was used as an emitter, which is a novel multi-resonance TADF emitter formed by integrating BCzBN into the dendritic molecular structure of 3tCz. The BCzBN-3tCz shows excellent film-forming properties and a narrow PL spectrum with a bandwidth of 22 nm. As a result, the eco-friendly solution-processed OLED with clearly observed interfaces between individual layers, achieved a maximum external quantum efficiency (EQEmax) of 14.0 % and a narrow bandwidth of 30 nm. Furthermore, the utilization of a TADF sensitizer enhanced the EQEmax by over 15 % and significantly improved the efficiency roll-off at high luminance.

Professor Choi said, “Despite the many advantages, the solution process-based technology for fabricating laminated devices has not been studied much compared to the vacuum process-based device fabrication technologies. However, I’m sure that together with the vacuum process-based device fabrication technologies, the solution process-based technology will be considered as an essential technology in the future, and so various material technologies need to be established.” Professor Choi also added, “Particularly, the eco-friendly technology for fabricating laminated devices, free of any halogenated or aromatic solvents, will be an efficient and promising strategy considering performance and the possibility of commercialization in not only OLEDs but also ‘electronic and photoelectronic device technologies’ using solution-processed organic semiconductors.”

The research group anticipates that the eco-friendly process will be extensively applied to the development of various types of smart wearable devices and thus are conducting the relevant studies.

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Picture description: The structure of an OLED device fabricated using an eco-friendly solvent and an actual cross-sectional image thereof. Images showing the external quantum efficiency, emission wavelength, and OLED device operation.