Ambipolar triplet hosts comprising 1,2,4-triazole and carbazole in ortho-positions have been developed. The blue PHOLED has a high current efficiency of 47.1 cd A(-1), power efficiency of 41.2 lm W-1, and low efficiency roll-off. The high efficiency was attributed to the successful control of pi-conjugation through orthogonal arrangement of the substituents so that a wide T-1-S-0 gap could be maintained.