A Bipolar and Small Singlet‐Triplet Splitting Energy Host with Triplet Energy Lower Than a Blue Phosphor for Phosphorescent OLEDs in Panchromatic Range

A bipolar host material, 2,4,6-tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ), was synthesized according to reported method. Due to the higher triplet energy compared to green and red phosphors, TCPZ is suitable to host them in phosphorescent organic light-emitting diodes (PhOLEDs). Although the triplet of TCPZ is slightly lower than a common blue phosphor, good blue PhOLEDs using TCPZ as the host were successfully demonstrated in this work. By low temperature emission measurement, it was found that the energy splitting between the singlet and triplet of TCPZ is as small as 0.24 eV. Therefore, thermal activated energy transfer from triplet to singlet in the host TCPZ is expected to occur, which can be afterwards efficiently transferred to the blue phosphor, hence enabling it to host blue phosphor. As a result, TCPZ can be used as host for phosphors in panchromatic range. Additionally, single-carrier devices clearly prove its good bipolar transport feature, beneficial to device performance. By using TCPZ as a host, high performance deep-red, green and blue PhOLEDs have been achieved, with maximum efficiencies of 9.3 cd·A−1 (13.2%), 81.3 cd·A−1 (23.1%) and 17.03 cd·A−1 (10.4%), respectively.