A research team led by Professor Shun-Wei Liu, the director of Organic Electronics Research Center and the vice dean of College of Engineering at Ming Chi University of Technology, Professor Ken-Tsung Wong from the Department of Chemistry at National Taiwan University, and Professor Bin Hu from the Department of Materials Science and Engineering at The University of Tennessee, Knoxville, have recently published a research article in Advanced Optical Materials journal. Based on the in-plane oriented Exitplex exciplex host [BCzPh:CN-T2T], the anisotropy dynamics of light-emitting dipoles were thoroughly investigated in both time and energy domains by time-resolved and steady-state photoluminescence anisotropy measurements.

The results discovered that designing in-plane oriented light-emitting dipoles is one of the key methods for developing high-efficiency OLEDs, which can significantly enhance light extraction efficiency compared to randomly oriented dipoles. However, the in-plane oriented dipoles must demonstrate sufficient polarization memory extended over the entire light emission lifetime in both time and energy domains, in order to maximize OLED’s light extraction efficiency. This study fully proves that such extended anisotropy dynamics is the necessary condition for developing high-efficiency OLEDs.

The research team constructed the emitting layer by doping the in-plane oriented Exitplex exciplex host [BCzPh:CN-T2T] with low concentration of red phosphorescent Ir(ppy)2(acac) molecules. It was found that the in-plane oriented exciplex host can significantly extend the anisotropy dynamics of phosphor’s dipoles from nanosecond to microsecond timescale, comparable to its phosphorescence lifetime, by suppressing the Coulomb scattering between host and guest. This satisfies the required time-domain condition for improving light outcoupling efficiency of the Ir(ppy)2(acac):exciplex system.

More importantly, the study discovered that under suppressed host-guest scattering, the high-energy emissive dipoles can still maintain extended anisotropy dynamics without randomization during the hot electron relaxation from high-energy states to LUMO. This is attributed to the polarization memory effect of exciplex host that influences not only the low-energy states, but also the in-plane orientation of high-energy emissive dipoles.

The comprehensive investigation in both time and energy domains reveals that extending anisotropy dynamics of light-emitting dipoles is the necessary condition for developing high-efficiency OLEDs. Eventually, adopting the extended anisotropy dynamics of Ir(ppy)2(acac):exciplex system successfully achieved high external quantum efficiency of 34.01%. This result provides important guidance for further improving OLED performance and promoting the development of efficient OLED display technology.

In this study, the team performed photoluminescence quantum yield (PLQY) measurements using the integrated spectrometer LQ-100X-PL designed and manufactured by Enlitech. The compact design and superior optomechanical integration of LQ-100X-PL solves the dilemma between sample size and sphere port size. It allows quick and accurate PLQY measurements for samples in various forms like thin films, powders and liquids. The overall results are excellent in precision and repeatability. In-situ time-resolved spectroscopy is also available. With comprehensive technical support from Enlitech, LQ-100X-PL enables focus on research while ensuring measurement quality for peer review.