Polymer light-emitting diodes with saturated red emission were fabricated using the copolymer, poly　［2,7-(9,9-dioctyl)fluorene-co-5,5′-(4,7-diselenophenyl)-2,2′-yl-2,1,3-benzothiadiazole］ (PFO-SeBT). Special attention was paid to polymer/electrode interface modification. The highest electroluminescence external quantum efficiency of 1.79%, which was more than twice that of devices with low work function metal cathode Ba/Al, was achieved by incorporation of CsF/Al cathode with carefully optimized thickness of CsF, and insertion of PVK layer in the PFO-SeBT1/anode interface. Efficient improvement of electron injection of CsF/Al cathode and blocking effect of PVK to electrons were proposed to be responsible for the improved performances of the device.