Enhanced green electrophosphorescence by using polyfluorene host via interfacial energy transfer from polyvinylcarbazole

For green-emitting Ir complex-doped polymer devices, high efficiencies have been achieved only when non-conjugated poly(N-vinylcarbazole) (PVK) was used as the host polymer. It is commonly believed that conjugated polyfluorenes (PFO) are not suitable for the use as host of green phosphorescent complexes due to the presence of the low-lying triplet state. In this paper we reported that despite very inefficient PL phosphorescent emission of Ir(Bu-PPy)3 in the PFO films, strong green electroluminescence (EL) and high device efficiencies have been observed for devices from such blend films when multilayer device with PVK was used as anode buffer. We analyze the PL spectra in steady state and transient response and found that the energy transfer via PVK interlayer plays important role in the efficient EL performance. This fact clearly indicates the efficient electrophosphorescence might be achieved even if the triplet energy of phosphorescence dye is higher than that of the host. These findings could significantly broaden our selection in polymer hosts for phosphorescent devices.