Abstract High efficiency stable red light-emitting diodes have been realized employing red copolymer (PFO-DHTBT15) from 9,9-dioctylfluorene (DOF) and 4,7-di(3-hexylthien-2-yl)-2,1,3-benzothiadiazole (DHTBT) blending into green copolymer (PFO-BT15) from 9,9-dioctylfluorene and 2,1,3-benzothiadiazole (BT) as a novel fluorescent emitting layer. The external quantum and luminous efficiency of device from blend film (PFO-DHTBT15:PFO-BT15 = 10:90) reached 5.2% and 3.16 cd/A at the current density of 35 mA/cm 2, respectively. The corresponding Commission Internationale de l’Eclairage coordinates is (0.64, 0.36). In comparison with the devices from phenyl-substituted poly [ p-phenylene vinylene] derivative (P-PPV) as host of PFO-DHTBT15, the device from PFO-DHTBT15/PFO-BT15 blend film shows higher luminous efficiency and better stability under high current density at the same blend weight ratio. The improved device performance is mainly attributed to the effective energy transfer from PFO-BT15 copolymer to PFO-DHTBT15 copolymer and better carrier confinement.