HIGHLY EFFICIENT AND STABLE HYBRID WHITE ORGANIC LIGHT-EMITTNG DIODES BASED ON THE MIXED HOST SYSTEM

Abstract

In this thesis, we have investigated the design strategy of high-performance hybrid white organic-light emitting diodes (WOLEDs) and degradation mechanism of blue OLEDs based on thermally activated delayed fluorescent (TADF) emitter by using mixed host system. First, we demonstrated the high-performance hybrid WOLEDs with general fluorescent blue emitter having non-interlayer structure. The major feature of developed device is exciton manipulation using mixed host in blue emitting layer (EML). The developed non-interlayer hybrid WOLED exhibited enhanced performance compared to interlayer structure device and we investigated the electroluminescence (EL) process of non-interlayer device. However, non-interlayer device in this thesis has poor lifetime compared to that of interlayer device and it was due to the inevitable mutual exciton quenching between long wavelength phosphor and blue fluorophore which has low lying triplet energy level. From these results, we clearly see the necessity of blue fluorescent material having high triplet energy level. Thus, we adopted thermally activated fluorescent (TADF) emitter. Next, we investigated the degradation mechanisms of blue OLEDs based on TADF emitter and demonstrated that mixed host system can improve device stability. The main cause of degradation of OLEDs with TADF blue emitter was the host instability. Additionally, the electrochemical instability of molecules influences long-term OLED degradation. Also the formation of exciton quenchers and non-radiative recombination centers acts to reduce OLED luminance. The mixed host can be easily utilized and improve the device lifetime by enhancing stability on excited-state stress. Finally, we have investigated high-performance hybrid WOLEDs with superior efficiency, color stability, low efficiency roll-off, and device stability based on blue TADF emitter with mixed host. The resulting WOLED shows the maximum external quantum efficiency, current efficiency, and power efficiency of 22.1 %, 59.3 cd/A, and 50.3 lm/W, respectively. Moreover, the device exhibits extremely stable EL spectra with Commision Internationale de LEclairage (CIE) coordinates of (0.417, 0.422). We also characterized the exciton generation zone in the EML with versatile experimental and theoretical evidences. In addition, the investigated hybrid WOLED based on TADF emitter exhibited 2 times enhanced device lifetime compared to the device without mixed host system. In conclusion, the mixed host system has successfully utilized for achieving high-performance hybrid WOLEDs and stable blue OLEDs based on TADF emitter. The mixed host effectively broadens the exciton generation zone and improves the stability on excited-state stress. The investigated mechanisms and optimization procedure reported in this thesis can be anticipated as starting point for further research towards high performance stable hybrid WOLEDs.