The internal heavy-atom effect (IHAE) has garnered considerable attention as a promising approach for developing highly efficient emitters in organic light-emitting diodes (OLEDs). Nevertheless, the external heavy-atom effect (EHAE) in host materials, despite being equally important, has been largely overlooked. In this study, we introduce a selenium-doping strategy to unlock the potential of EHAE in host molecules. To demonstrate this approach, we developed a straightforward method for synthesizing structurally diverse pyridine-fused, selenium-containing heteroacenes via an intramolecular radical cyclization of 2-(arylselanyl)pyridin-3-amine derivatives. This method facilitates the rapid construction of a novel host molecule, DCz-BSeP, by incorporating two carbazole groups into the benzo[4,5]selenopheno[2,3-b]pyridine (BSeP) core. Compared to its oxygen-based (DCz-BFP) and sulfur-based (DCz-BTP) counterparts, the introduction of selenium in DCz-BSeP significantly enhances spin-orbit coupling and accelerates the reverse intersystem crossing rate of thermally activated delayed fluorescence (TADF) emitters by threefold, while also improving bipolar transport properties. These enhancements make DCz-BSeP an ideal bipolar host for high-performance, wide-color-gamut TADF-OLEDs, with notably reduced efficiency roll-off. Additionally, its successful application in phosphorescent OLEDs (Ph-OLEDs) and TADF-sensitized narrowband red fluorescence OLEDs (TSF-OLEDs) highlights its versatility in advancing OLED technologies.