Abstract
A strong red emission in bilayer polymer light emitting diodes made of poly(3-hexylthiophene)(P3HT) and polyfluorene copolymers was discussed. It was shown that the spectrum of the diode, peaked at 612 nm, was attributed to the exciplex state existed only at the heterojunction. The peak luminance of 3500 cd/m 2 with yield 1.3 cd/A was achieved and the efficiency of the junction emission was found to be very high for red polymer light-emitting diodes. It was suggested that by blending red-emitting polymers into polyfluorene to eliminate the small residual green emission, the emission color could be made more saturated.