Application of alternating fluorene and thiophene copolymers in polymer light-emitting diodes

Four polyfluorene copolymers comprised of fluorene and thiophene or bithiophene moieties were investigated and applied for device fabrication. Relatively high photoluminescence quantum yields (23-40%), good optical and electrochemical properties have been demonstrated for this series of polymers. Single and double layer devices fabricated with the polymers in a conventional configuration appeared to have electrons as the majority carrier, and their performance was remarkably improved when poly(3,4-ethylenedioxythiophene) (PEDOT) was selected as the hole-injection/transporting layer. The best performance was demonstrated for a double layer green-emitting device, which exhibited luminescence of 356 cd/m² at a bias of 8 V with an external efficiency of 0.38%. The current-voltage and luminescence characteristics of the devices with PEDOT or poly(N-vinylcarbazole) (PVK) as the buffer layer were studied and discussed.