Effect of exciplex formation on organic light emitting diodes based on rare-earth complex

Abstract

An exciplex can be formed due to the charge transfer between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor. By introducing a mixing layer composed of [N,N′-diphenyl-N,N′bis (3-methylphenyl)-1,1′-diphenyl-4,4′-diamine] (TPD) and europium(dibenzoylmethanato)3(bathophenanthroline) [Eu(DBM)3bath] and a graded interface, elimination of light emission from the exciplex and significant luminescence enhancement of trivalent europium ions (Eu3+) in organic light emitting devices have been achieved. The elimination mechanism of exciplex emission based on the concept that an exciplex can be formed between LUMO of the acceptor (Eu complex) and HOMO of donor (TPD) was investigated. To comprehensively understand the mechanism, devices consisting of a Eu(DBM)3bath as the emitting material and the devices using other rare-earth (RE) complex [RE(DBM)3bath] as the emitting material were fabricated with the same device configuration. As a reference, four spin-coated films with the blend composed of TPD and the gadolinium complex [Gd(DBM)3bath] were also fabricated. The electroluminescence (EL) spectra from the devices and photoluminescence spectra from the spin-coating films were fully investigated. The results show that the exciplex was formed by the charge transfer from the donor TPD to the acceptor RE complex, the exciplex state that acted as a transient excited state can be controlled by altering the molecular ratio in the mixing films. The relation of the exciplex formation based on EL devices with the RE complex versus the variety of the RE ions is also discussed by manipulating the energy level of the excited state.