Bipolar transport polymers have been developed as host materials for electroluminescent devices by incorporating both electron-transporting and hole-transporting functionalities into copolymers. Two different copolymers having the same molecular weight (M-n similar to 30 kg/mol) and the same fraction of electron-transporting monomers (f(OXA) = 0.50) have been synthesized in the form of random and diblock copolymers, respectively. The effect of molecular structure and film morphology of these bipolar polymers on device performance has been studied. For the diblock copolymers,pronounced phase segregation forming different nanomorphologies has been observed by modern microscopic techniques, which is not observed for the random counterparts under the same thin film preparation conditions. The results of single-layer polymer light emitting diodes (PLEDs) show that the nanophase separation morphology of diblock copolymers has a significant effect on device performance: lowering charge transport and facilitating the hole-electron recombination leads to a much higher quantum efficiency. Applying this high triplet block copolymer as host, a high external quantum efficiency of 5.4% at the brightness of 900 cd/m(2) was achieved for single-layer PLEDs with a green-emitting complex dopant.