Chen C.-H.Ding W.-C.Lin B.-Y.Huang J.-J.MAN-KIT LEUNGJIUN-HAW LEEChiu T.-L.2022-04-252022-04-25202121951071https://www.scopus.com/inward/record.uri?eid=2-s2.0-85113729045&doi=10.1002%2fadom.202100857&partnerID=40&md5=473efb819445bed17994aa69c25d67f9https://scholars.lib.ntu.edu.tw/handle/123456789/606913High-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 ?) 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. ? 2021 Wiley-VCH GmbHcarbazole moietyorganic light-emitting diodesthermally activated delayed fluorescenceDiffusionEnergy transferExcitonsFluorescenceSemiconductor dopingCarbazole DerivativesDexter energy transfersEfficient energy transferExternal quantum efficiencyHigh efficiency devicesOrganic light emitting diodes(OLEDs)Thermally activated delayed fluorescencesTransient electroluminescenceOrganic light emitting diodes (OLED)[SDGs]SDG7journal article10.1002/adom.2021008572-s2.0-85113729045