Now in its 6th year, Dow recently hosted its Korea Award program honoring outstanding work in electronic materials to support the research activities of science and engineering students in Korea. The award is open to all graduate-level students at accredited four-year institutions as well as post-doctoral researchers based in Korea. Entries are reviewed by a panel of judges consisting of Dow Korea officials, as well as professors and researchers in the field of electronic materials. This year’s winners were recognized at a ceremony on April 21, 2016 and the winners each received a cash award.
The 2016 Dow Korea Award program received 238 entries, and, based on the opinions of five judges, six were selected as winners. We are proud to announce that this year’s first place award went to Kwon-Hyeon Kim, a Ph.D. degree candidate in the Department of Materials Science and Engineering at Seoul National University for his work titled “Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes.” The judging panel selected this work for its groundbreaking work on organic light-emitting diodes (OLED) efficiency, achieving demonstrated external quantum efficiencie (EQE) of 35.6% with horizontally oriented emitting dipoles.
Dow Korea Award Winner 2016. Author, Kwon-Hyeon Kim, Ph.D. degree candidate in the Department of Materials Science and Engineering, Seoul National University (L) with Andrew Ryu, Dow Korea Country Manager.
OLEDs are among the most promising organic semiconductor devices. Recent reports of OLEDs achieving EQE’s of 29–30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. However, the preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here, the use of quantum chemical calculations shows that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment.
The elongated supramolecules themselves or the preferred horizontal orientation of the host molecules resulted in the preferred horizontal triplet transition dipole moments of the HICs in the emitting layer. Understanding the origin of the preferred orientation of HICs will enable us to design dyes with highly oriented transition dipole moments. As a result, it will be possible to achieve EQEs much higher than the conventional limit of 30%, and this paper demonstrates an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.
To verify the effect of the orientation of the transition dipole moments on EQE, the simulation was extended to calculate the maximum achievable EQEs for red OLEDs using the emission spectrum of Ir(mphmq)2tmd with arbitrary values of PL quantum yield (qPL) and the ratio of horizontal emitting dipoles (Θ). The figure shows a contour plot of the maximum EQEs for the red OLEDs as a function of qPL and Θ assuming a perfect electron-hole balance without any electrical loss. The figure clearly shows that an EQE of over 45% is achievable if the emitter has qPL of 1 and horizontally oriented transition dipole moments. Practically, an EQE of 40% is possible with qPL of 0.95 and Θ of 0.95. The value is significantly higher than that achievable with randomly oriented emitting dipoles.
Figure 2: Optical simulation of the EQE of devices. (a) EQEs of the Ir(mphmq)2tmd-based OLEDs with varying ETL thickness (circles with dots). (b) Contour plot of the maximum EQEs achievable with Ir(mphmq)2tmd possessing a certain PL quantum yield (qPL) and ratio of the horizontal dipoles (Θ).