External heavy atom effect in brominated carbazole/oxadiazole hybrid hosts for enhanced efficiency in phosphorescence and TADF OLEDs

The influence of heavy atom effects in organic host materials has been rarely explored in organic light-emitting diodes (OLEDs). In this study, we present four novel carbazole/oxadiazole hybrid bipolar transport host materials with heavy bromine atom substitution: the asymmetric Cz-3BrCzOXD and Cz-4BrCzOXD, and the symmetric o-3BrCzOXD and o-4BrCzOXD. By introducing one or two bromine atoms at the 3- or 4-position of the carbazole units, all brominated hosts exhibited similar energy levels, while demonstrating enhanced hole transport and a slight reduction in electron transport compared to the bromine-free control material, o-CzOXD. When doped with either phosphorescent or thermally activated delayed fluorescence (TADF) dopants, the brominated hosts demonstrated enhanced intersystem crossing (ISC) rates, leading to more efficient exciton utilization. Consequently, compared to o-CzOXD, both phosphorescent and TADF OLEDs exhibited improved device efficiencies for all four brominated hosts. The maximum external quantum efficiencies increased from 20.9 % to 24.3 % for green phosphorescent (ppy)₂Ir(acac) OLEDs, from 20.3 % to 24.5 % for green TADF 4tCzDOXD OLEDs, and from 21.3 % to 24.9 % for light-blue TADF dCF₃5tCzOXD OLEDs. Our findings indicate that the external heavy atom effect offers an effective strategy for enhancing the efficiency of both phosphorescent and TADF OLEDs.