Operation mechanism of TADF/hyperfluorescence OLEDs aimed for high stability
Through the extensive R&D of organic light-emitting diodes (OLEDs) for more than 30 years, plenty of well-elaborated novel organic optoelectronic materials and device architectures have been extensively developed, resulted in the unique commercial utilization of OLEDs for cutting-edge smartphones, large-area TVs,and further new future display applications by taking advantage of light-weight and flexibility. From the aspect of materials science, the creation of novel light-emitting materials in OLEDs has been the central issue aimed for high electroluminescence quantum efficiency (EQE). Starting from the development of conventional fluorescence materials (1st generation) during 1990-2000th, the room-temperature phosphorescence (2000-) (2nd generation) and thermally activated delayed fluorescence (TADF) (2012-) (3rd generation) continuously pioneered the novel possibilities of organic emitters, resulted in not only high-performance OLEDs but also enriched organic photochemistry. In recent days, there have been a wide variety of studies on TADF-OLEDs because of the unlimited possibilities of TADF molecular design. Further, hyper fluorescence (HP)-OLEDs have been developed since they canrealize the compatibility of high efficiency and narrow spectral width, which is ideal for practical display applications. Here we report our recent cutting-edge HP-OLEDs demonstrating high OLED performance by optimizing host,TADF, and terminal emitter (TE) molecules1-3). In particular, we focus on the blue-emission, which is capable of showing narrow FWHM and high ELquantum yield. Blue HP-OLEDs based on two new TEs are fabricated, resulting in high external quantum efficiency (EQE) of over 20%, high color purity, and high brightness. By analyzing the transient PL characteristics of the HP-OLEDs, we found that the presence of efficient FRET between TADF-assistant dopant(TADF-AD) and TE molecules. Further, transient EL analysis confirmed that a smaller EHOMO difference between TADF-AD and TE efficiently helps to decrease hole trapping inside the emitting layer, henceresulting in a lower efficiency roll off and a longer operational device lifetime. This report provides a designing principle for a TADF and TE inHP-OLEDs with well-matched energy levels, leading to efficient FRET and no significant carrier trapping.