PVK-based organic light emitting diodes with low turn on voltage

Poly(N-vinylcarbazole) (PVK) thin films deposited by evaporation have been used in electroluminescent organic diodes. The turn on voltages of the current–voltage (I–V) and electroluminescence–voltage (EL–V) characteristics are not very different and they are smaller than the one obtained in such PVK-based devices, when the PVK film is obtained by spin coating. It is shown that the carrier injection process is dominated by tunneling field effect. During evaporation the chain length of the polymer is shortened and the PVK films contain a high density of dangling bonds. It is proposed that these dangling bonds introduce a broad distribution of localized states in the band gap. Therefore, the carrier injection occurs via hopping among an energetic distribution of localized states rather than by band motion, which is equivalent to a reduction of the energy barriers as experimentally measured.