Synergistic effect on the efficiency of polymer light-emitting diodes upon blending of two green-emitting polymers

Light-emitting diodes based on blends of the two green-emitting polymers, poly (9,9-dioctylfluorene- alt-benzothiadiazole), F8BT, and poly (9,9-dioctylfluorene-alt-bithiophene), F8T2, show efficiencies that lie in between those of the devices based on the neat polymers (with a maximum efficiency of approximately 4 cd/A for the devices with magnesium cathodes based on F8BT), except for the blend with 5% by weight of F8T2, which is more efficient than the device based on neat F8BT (a maximum efficiency of approximately 5 cd/A is obtained). In view of the lower photoluminescence efficiency of F8T2, we attribute this improvement to the improved hole transport brought about by F8T2, though is surprising that 5% by weight, is enough to significantly improve the charge balance within the emissive layer. A detailed photophysics study was carried out for the neat polymers and their blends and no clear evidence for energy transfer between the components was found. This unanticipated devices performance improvement points to the need of a deeper screening of available conjugated luminescent polymers. (C) 2010 American Institute of Physics. [doi:10.1063/1.3456997]