A Silane-Based Bipolar Host with High Triplet Energy for High Efficiency Deep-Blue Phosphorescent OLEDs with Improved Device Lifetime.

Abstract

A high triplet energy host is developed using a silane moiety, 9-(4-(triphenylsilyl)dibenzo[b,d]furan-2-yl)-9H-carbazole (SiDBFCz), is designed through extensive density functional theory (DFT) calculations to obtain appropriate hole and electron injection barriers. The chemical hardness and the charge transport characteristics are comprehensively investigated to realize a bipolar host with high triplet energy over 2.9 eV for deep blue phosphorescent organic light-emitting diodes (PHOLEDs). The synthesized SiDBFCz clearly exhibits the bipolar characteristics especially with emitter molecules doped. An external quantum efficiency over 19 % without any microcavity optimization is achieved thanks to the good charge balance in the SiDBFCz PHOLED. The device lifetime of the SiDBFCz PHOLED is improved more than 1000 %, compared to the unipolar control devices at an initial luminance of 500 cd m-2 . The dramatic enhancement of the operational stability of the deep blue PHOLED is also thoroughly investigated in terms of electrochemical stability of host molecules in charged or excited states. The results clearly indicate that the device lifetime is strongly correlated with the bond dissociation energy and the activation energy for the bond dissociation reaction in triplet excited state.