Towards highly efficient thermally activated delayed fluorescence devices through a trap-assisted recombination mechanism and reduced interfacial exciton annihilation

摘要

In this contribution, two bipolar materials, 3-(3-(9H-carbazol-9-yl) phenyl)-5-(4-(tert-butyl) phenyl)-1,2,4-thiadiazole (3-CzTHZ) and 5-(3-(9H-carbazol-9-yl) phenyl)-3-(4-(tert-butyl) phenyl)-1,2,4-thiadiazole (5-CzTHZ), were developed as host matrices for the TADF emitter 1,2,3,4-tetrakis(carbazol-9-yl)-5,6-dicyanobenzene (4CzPN). By adjusting the host-guest compatibility, two types of emission mechanisms were realized in varied devices according to their different charge-trapping characteristics within the emissive layer (EML). Based on a systematic investigation on both photoluminescence (PL) and electroluminescence (EL) performances, we found that the trap-assisted recombination method could contribute to realizing the full potential of TADF emitters since this mechanism was beneficial to minimizing the energy loss during the host (sic) guest energy transfer (ET) processes. Moreover, the compatibilities between the electron transport layer (ETL) and EML were also fine-tuned to reduce the detriment of interfacial exciton annihilation. Consequently, an outstanding performance was achieved in a 3-CzTHZ-hosted device with the maximum external quantum efficiency (EQE) of 22.2% and power efficiency (PE) of 65.9 lm W-1.