Polymer blends have been used to improve light-emitting diodes, LEDs, efficiency by combining polymers with complementary charge transport ability and/or with efficient energy transfer. We explored the combination of two green emitting polymers, poly(9,9-dioctylfluorene-alt-benzothiadiazole), F8BT, and poly(9,9-dioctylfluorene-alt-bithiophene), F8T2, in light-emitting diodes, which was, at the very beginning, an unlikely combination if aiming for LEDs with improved efficiency. F8BT and F8T2 do have some complementarity in terms of the charge transport. F8BT is a good electron transporting polymer, with poorer hole mobility. Measurements in field-effect transistors revealed that electron-mobility in F8BT is similar to both electron- and hole-mobility of F8T2 (1). On the other hand, judging from absorption and emission spectra of both polymers, there is no significant tendency for energy transfer between them, emitting both in the same spectral range. Furthermore, F8T2 has a much lower photoluminescence efficiency than F8BT.
We found, however, that adding 5%, by weight of F8T2 to F8BT, an increase of the LEDs efficiency, with respect to that of LEDs based on neat F8BT occurs (2). Atomic force microscopy and time-resolved fluorescence studies of the neat materials and blends were carried out to get further insight into the origin of that improvement. A detailed discussion about the system characterisation and possible origins for that improvement will be presented.
Acknowledgments Financial support from FCT-Portugal under the contracts PTDC/QUI/65474/2006 and PTDC/FIS/72831/2006 is acknowledge.