Effect of exciplex forming co-hosts on the performance of phosphorescent dye doped organic light emitting diodes

Abstract

Exciplex is an excited state charge transfer complex formed between a donor and an acceptor molecules. Formation of exciplex in organic light emitting diodes (OLEDs) has been considered as an obstacle to increase the efficiency of OLEDs. However, high efficiency phosphorescent OLEDs was reported using an exciplex forming co-host recently. Unfortunately the effect of formation of exciplex in co-host systems on the performance of OLEDs has not been extensively studied yet. In this thesis, we studied the effect of the formation of exciplex in co-hosts on the performance of OLEDs. Three exciplex forming co-hosts and two non-exciplex forming co-hosts were selected for the purpose and used to fabricate OLEDs. It turns out that OLEDs with the exciplex forming co-hosts showed lower operating voltages than the OLEDs with the hosts not forming exciplex. We propose that in the exciplex forming co-host systems, electrons and holes recombine to form exciplex first followed by the energy transfer to the dopant to form excitons on the dopant molecules. In contrast, the direct recombination of electrons and holes on dopant molecule rather than the energy transfer from the host molecula is the dominent process, in non-exciplex forming co-host system. This process increases driving voltage.