A team led by Prof. Tae-Woo Lee (Dept. of Materials Science and Engineering) at POSTECH have fabricated highly-efficient, solution-processed fluorescence organic light-emitting diodes (OLEDs) using pure-organic thermally-activated delayed-fluorescence (TADF) emitters. The research was published in Advanced Materials journal. This research is selected as a most significant paper and open to the public as a "Layman's abstract".

Conventional OLEDs use the phosphorescent emitters which have shown high internal quantum efficiency (IQE) of nearly 100%. However, they should incorporate precious heavy metals such as iridium and platinum into phosphorescent metal-complexes; these limit their commercialization. In order to overcome these disadvantages, the research team uses pure-organic TADF emitters which can show a very high IQE of nearly 100% without precious metals. Furthermore, TADF emitters have advantages of easy synthesis using pure-organic molecules and versatile molecular design, thus, reduce the synthesis cost.

They also introduced the inexpensive, simple solution-process to fabricate the TADF-OLEDs by solving fundamental problems which limit the high efficiency in solution processed TADF-OLEDs. A multi-functional buffer hole injection layer (Buf-HIL) that can increase the hole injection capability to the emitting layer (EML) due to its high work function, and also improve the luminescence efficiency of TADF-OLEDs by preventing exciton quenching at the HIL/EML interface was employed. Furthermore, new polar aprotic solvent improved the device efficiency by improving the solubility of pure-organic TADF emitters, reducing the surface roughness and the aggregation of dopants, and managing the exciton quenching in the emitting layer.

This improvement in solution processed TADF-OLEDs will remedy the disadvantages of a complex and expensive vacuum-deposition process and thus lower the production cost of the devices. It is of prime importance to reduce the production cost of the devices from the perspective of industrial mass-production of OLED displays and solid-state lightings.

Prof. Tae-Woo Lee mentioned, "This technology is a big leap toward the development of inexpensive and solution-processed OLED displays and solid-state lightings because this method uses only low-cost pure-organic molecules and simple solution process to realize the extremely high efficiency solution-processed OLEDs."

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP), and by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project.