A comparative study on carbazole-based thermally activated delayed fluorescence emitters with different steric hindrance
Thermally activated delayed fluorescence (TADF) emitters based on carbazole group have been reported to realize different efficiencies in the devices. Here we report a couple of carbazole based TADF emitters, 2-(9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (CZ-TTR), with one free-rotation carbazole and 2,3-di(9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (DCZ-TTR), with two mutually restricted carbazole groups, studying the influence of steric hindrance in the molecules. Both the compounds employ exactly same constituent segments and connecting mode. However, due to steric hindrance between the two carbazole segements, DCZ-TTR exhibits a smaller singlet–triplet splitting of 0.03 eV comparing that of 0.10 eV for CZ-TTR. The device containing DCZ-TTR shows significantly higher efficiencies of 20.1% for external quantum efficicency (EQE), 58.5 lm/W for power efficiency (PE) and 59.6 cd/A for current efficiency (CE), which is significantly higher than that of the device based on CZ-TTR (EQE = 14.4%; PE = 32.9 lm/W; CE = 32.5 cd/A). These results clearly prove the necessity of introduction of suitable steric hindrance on designing highly efficient TADF emitters based on carbazole.