Doping of the emissive layer of organic light-emitting diodes has been mostly used to tune their emission colour and to increase the electroluminescence efficiency. In the first case, the process relies on the energy transfer from the host (which, in most cases, is the charge transporting material) to the guest molecules. In the second case, the increase of efficiency may result from an exciton confinement or due to the use of phosphorescent dopants in order to harvest the triplets. The presence of the guest or dopant has usually a detrimental effect on the charge transport, via charge localization or disorder, and leads also to an increase of the dopant emission with respect to its contribution to the photoluminescence spectrum, as a result of on-site exciton formation promoted by that charge localization.
In this study we use dopants with a wider gap than the host polymer, thereby avoiding energy transfer from the host to the guess and promoting instead the reverse energy transfer process. In this case, any on-guest formed exciton would be transferred to the host. The dopants were chosen to induce either hole or electron trapping.
In this particular case, we have selected TPD, PBD and OXD-7 as charge trapping dopants and PFO, F8BT and a blend of PFO/F8BT as host materials. TPD, having a ionization potential (Ip) of 5.4 eV will act as hole trap, while PBD (electron affinity, EA, of 2.6 eV) and OXD-7 (EA= 2.8 eV) are expected to act as electron traps. We have studied the concentration effect of these dopants on the current of hole-only devices and on the efficiency of the LEDs fabricated with such composites. For instance, in the case of the LEDs based on the PFO/F8BT blend, a significant increase of the efficiency occurs at low level concentration of both hole and electron trap dopants.