Heat revolution on photophysical properties and electroluminescent performance of Ir(ppy)3-doped bipolar host of oxadiazole derivatives attaching with inert group of tert-butyl moiety

Two bipolar materials, 2,5-bis(2-(9H-carbazole-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) and 2,5-bis(2-(3′,6′-di-tert-butyl-9H-carbazole-9-yl)phenyl)-1,3,4-oxadiazole (tBu-o-CzOXD), were synthesized according to reported methods. In parallel study, it was demonstrated that introduction of inert tert-butyl group improved material thermal stability, even though this modification only had a slight influence to the photophysical and electrochemical properties of these materials. A comparative study focusing on effects of heat treatment was carried out on the quartz glass substrates with vacuum deposited films containing one of the bipolar host doped with 6 wt% fac-tris(2-phenylpyridinato-N,C^2′)iridium (Ir(ppy)₃). Results show that when the two samples were heated, the absorption, emission, and photo images of the host:dopant system changed, with the o-CzOXD suffering more severe degradation under high temperature, which is consistent with their thermal stability. In addition, it was proved that the high temperature-annealed host:dopant system can enhance the emission of the dopant. This finding was used as a guideline to improve our device performance. We fabricated two types of phosphorescent organic light-emitting devices (PhOLEDs), one was based on o-CzOXD, the other was based on tBu-o-CzOXD. They had analogous structure. We investigated the effect of heat on device performance by selectively annealing. Although these two freshly prepared devices exhibited similar performance, when annealed at 90 °C for 10 minutes, the OLEDs based on tBu-o-CzOXD showed significant performance enhancement, which can be attributed to the observation that annealing Ir(ppy)₃ doped host can change film morphology and enhance the dopant emission. The maximum efficiencies of the freshly prepared tBu-o-CzOXD device were 25.8 cd A^?1, 23.1 lm W^?1, and 9.3%; whereas those for annealed device were 47.0 cd A^?1, 42.2 lm W^?1, and 13.4%.