Impacts of dopant concentration on the carrier transport and recombination dynamics in organic light emitting diodes

The carrier transport and recombination dynamics of organic light emitting diodes as a function of dopant concentration were studied. In the lightly-doped sample, a higher carrier mobility and better device performance were observed. Due to the aggregations in the highly-doped sample, carrier quenching as well as nonradiative recombination degraded the device performance. In addition, the observed decay rates and luminescence efficiencies of the doped samples could be used to calculate the radiative decay rate (κr) and nonradiative decay rate (κnr). With a higher dopant concentration, κr became lower and κnr was enhanced. It was found that the applied voltages, corresponding to equal κr and κnr, decreased with the dopant concentration. These demonstrate that the lightly-doped sample exhibits better luminescence efficiency than the highly-doped samples at all applied voltages and that all the doped samples exhibit peak luminescence efficiency at relatively low applied voltage, with luminescence efficiency decreasing for all the doped samples as the applied voltage is increased. The resulting recombination dynamics are correlated with the device characteristics and performance of the doped samples.