Host materials are of great significance to achieve high-performance host–guest organic light-emitting diodes (OLEDs) through restraining the exciton annihilation in the emissive layer. Herein, four bipolar molecules bearing benzothienocarbazole (BTCz)/benzothienocarboline (BTCb) and benzimidazole (Bi) moieties were designed and synthesized. Using them as hosts, green phosphorescent OLEDs (PhOLEDs) with maximum current efficiency (CE_max) of 90.8 cd A⁻¹, power efficiency (PE_max) of 76.5 lm W⁻¹, and external quantum efficiency (EQE_max) of 27.9% were obtained. More importantly, it was found that these four molecules could be used as electron-donating materials to interact with electron-accepting triazine derivatives to form electroplexes. Employing these electroplexes as hosts, highly efficient green PhOLEDs with CE_max of 104.4 cd A⁻¹, PE_max of 85.1 lm W⁻¹, EQE_max of 29.9%, and extremely small efficiency roll-offs (EQE = 29.5%, 1.3% roll-off at 1,000 cd m⁻²; EQE = 28.3%, 5.4% roll-off at 5,000 cd m⁻²) were achieved. The excellent device performances fully exhibit the promising potential of electroplexes as hosts for high-performing OLEDs.