Electric field evolution of charge and energy transfer in molecule-doped polymer light-emitting diodes

The electroluminescence (EL) from tetramethyl ester of perylene-3,4,9,10-tetracarboxylic acid (TMEP)-doped poly(vinylcarbazole) (PVK) light-emitting diodes and their current-voltage characteristics were investigated. The results indicated that field-emission tunneling injection predominated in the ITO/TMEP:PVK/Al diodes at high applied voltages. With increasing the TMEP dopant density, the injection barrier for holes was slightly reduced. Under injection-limited conditions, the possible kinetic processes were discussed in view of the field evolution of the EL spectra and the energy level alignment in the diode configuration. With increasing the field intensity, different emission components varying their intensities in EL spectra indicates that both charge transfer and energy transfer occurred in TMEP-doped PVK layer. These transfer processes are influenced by varying the concentration of TMEP dopants as electron traps. The results indicate that the dynamic equilibrium among carrier recombination, charge transfer and energy transfer, can be adjusted by dopant concentration and external bias. This information helps to establish a design framework for host–dopant systems.