Tuning emission colors from blue to green in polymeric light-emitting diodes fabricated using polyfluorene blends

The photo- and electroluminescent properties of single-layer two-component blends composed of one blue emitter polymer and one green emitter polymer were studied. The blue emitter, poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(9,9-di-{5'-pentanyl}-fluorenyl-2,7-diyl)] (PFOFPen), was used as the matrix, and the green emitter, poly[(9,9-dihexylfluorenyl-2,7-diyl)-alt-co-(bithiophene)] (F6T2), was used as the guest. The F6T2 content in the blends varied from 0.0075 wt % to 2.4 wt %. Remarkable differences were observed between the electroluminescent (EL) and photoluminescent (PL) spectra of these blends, which indicated that the mechanism for excited-state generation in the former process had a higher efficiency in the aggregated phase than in the nonaggregated phase. Blending these two polymers gradually tuned the emission color from blue (PFOFPen and blends with <0.75 wt % F6T2) to green (F6T2 and blends with >0.75 wt % F6T2). The photophysical processes involved in both EL and PL emission are also discussed.