π–π stacking: a strategy to improve the electron mobilities of bipolar hosts for TADF and phosphorescent devices with low efficiency roll-off
A series of D–π–A type bipolar hosts based on triphenylene/carbazole were designed and synthesized. π–π stacking of the triphenylene units between two adjacent molecules renders these hosts high electron mobilities above 1 × 10−4 cm2 V−1 s−1, and their electron and hole mobilities can be regulated through varying the connection position and π moieties. Due to more balanced charge mobilities, a D1-based green thermally activated delayed fluorescence (TADF) device achieved the highest external quantum efficiency (EQE) of 16.3%, and its EQE could still maintain 15.3% and 13.8% at the high luminance of 5000 cd m−2 and 10000 cd m−2, respectively. D1-based green phosphorescent devices also exhibited the highest EQE of 18.6% with a reduced roll-off to 17.6% at 5000 cd m−2 and 16.2% at 10000 cd m−2.