Long‐Distance Triplet Diffusion and Well‐Packing Hosts with Ultralow Dopant Concentration for Achieving High‐Efficiency TADF OLED

Abstract

Abstract High‐efficiency organic light emitting diodes (OLEDs) are fabricated using a group of benzimidazole‐carbazole derivatives with good molecular packing as hosts. The hosts are separately doped with a green thermally activated delayed fluorescence (TADF) emitter, 1,2,3,5‐tetrakis(carbazol‐9‐yl)‐4,6‐dicyanobenzene (4CzIPN). An ultralow concentration (0.5% volume ratio) of 4CzIPN is required in the emitting layer (EML) to achieve a record‐high external quantum efficiency of 31.8% by comparison with reported 4CzIPN‐relative devices. This result is attributed to efficient energy transfer, alleviation of concentration quench of 4CzIPN, long‐distance triplet exciton diffusion ability of host materials, and excellent horizontal molecular packing. With an increase in dopant concentration from 0.5% to 15%, the diodes exhibit a tiny variation in power efficiencies (82.9−78.7 lm W −1 ) and current efficiencies (92.2−87.5 cd A −1 ). In particular, the long‐distance triplet exciton diffusion ability of hosts exceeds the typical distance limitation (less than 10 Å) of Dexter energy transfer. Thus, high‐efficiency OLEDs are obtained with the scarce dopants . The aforementioned long‐distance triplet diffusion results are verified by the almost‐identical high‐efficiency device performances and the longer delayed emission lifetime in transient electroluminescence signals of a series of partially doped devices, conducting by separately doping low‐concentration TADF emitters in different EML regions.