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The investigation of suitable organic host materials is crucial for the development of high-efficiency triplet exciton-based organic light emitting diodes (OLEDs). In this study, two isomeric carbazole/1,3,5-triazine hybrid bipolar-transport host materials o-3CN-TRZ and o-4CN-TRZ are developed. The alteration of cyano substitution from the 3- to the 4-position of carbazole resulted in a slight reduction in the HOMO levels from −5.80 to −5.87 eV, accompanied by a gentle decline in the singlet/triplet energy levels. This was evidenced by a shift in the fluorescence emission peak from 450 to 474 nm and the observation of relatively high triplet energies of 2.79 and 2.77 eV for o-3CN-TRZ and o-4CN-TRZ, respectively. Notably, compared to the bare carbazole-based model compound o-Cz-TRZ, the introduction of a cyano group at either the 3- or the 4-position led to a notable enhancement in the electron-transport properties. However, the hole-transport behavior of o-3CN-TRZ was evidently inferior to that of o-4CN-TRZ. Therefore, when employed as host materials for both phosphorescence and thermally activated delayed fluorescence OLEDs, o-4CN-TRZ hosted devices all exhibited higher efficiencies than o-3CN-TRZ. Maximum external quantum efficiencies of 20.5, 16.8 and 16.9% versus 19.6, 12.5 and 13.8% were achieved for green phosphorescent (ppy)2Ir(acac), greenish-yellow TADF 4tCzDOXD and 4tCzCNPy-based devices, respectively. [ABSTRACT FROM AUTHOR]