A series of 1,2,4-thiadiazole core-based bipolar materials, 2,2'-(1,2,4-thiadiazole-3,5-diyl)bis(N,N-diphenylaniline) (o-TPATHZ), 3,3′-(1,2,4-thiadiazole-3,5-diyl)bis(N,N-diphenylaniline) (m-TPATHZ) and 4,4'-(1,2,4-thiadiazole-3,5-diyl)bis(N,N-diphenylaniline) (p-TPATHZ) were developed as the host matrixes for the deep red phosphorescent emitters tris(1-phenylisoqiunoline)iridium (Ir(piq)₃) and [bis(2-methyldibenzo-[f,h]-quinoxaline)Ir(III)(acetylacetonate)] (Ir(MDQ)₂(acac)). By systematic studying, we demonstrated that there are two types of charge-trapping effect within the emissive layers through adjusting the host-guest compatibility. And, it is revealed that a symmetric charge-trapping effect can contribute to realizing a stable charge-balance, which led to a mitigating efficiency roll-off at high current density. Consequently, a maximum external quantum efficiency (EQE) of 16.2% was achieved by an optimized device with p-TPATHZ-Ir(piq)₃ emissive layer. Remarkably, the EQE still remained as high as 15.7% at the high luminance of 1000 cd/m².