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

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 angstrom) 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.

Automated summary

The study demonstrates the fabrication of high-efficiency OLEDs with hosts doped with green TADF emitter and ultralow concentration of 4CzIPN in the EML, achieving a record-high external quantum efficiency of 31.8%. The long-distance triplet exciton diffusion ability of hosts exceeding the typical distance limitation of Dexter energy transfer plays a key role in obtaining high-efficiency OLEDs.