Modeling current transport in organic light-emitting devices (OLEDs)

There has been some substantial interest in studying current transport in organic light-emitting devices (OLEDs) because of their potential as solid-state light sources. While some of the physics remain to be resolved, there is in place a fair amount of understanding on the device operation to develop a working model for properties such as the J–V characteristics. This paper is an extension of an earlier publication [Solid-State Electron. 46 (2002) 645] to model mathematically the rate of exciton formation in OLEDs based on bimolecular recombination. The earlier work relied on knowing the spatial distribution of the carrier densities and this allowed one to study position dependent properties such as selective doping in OLEDs. Our current work extends the previous model to include space charge effect at low bias and suggests a possible mechanism bridging the transition between space charge limited current at low bias and recombination current at large bias. To validate our model, we compared our simulation results to data reported in the literature in the case of a ITO/TPA-PPV/Al device [Phys. Rev. 60 (1999) R8489]. In particular, effort was spent to explain the existence of a current peak at low bias in the J–V characteristics. Our overall result appeared to suggest that the operation of the OLEDs may well be accounted for by a simple model involving the capture of space charge injected holes at some unknown sites (possibly “trap” centers). This would be followed by “recombination”, which appeared to be strictly limited by the availability of the electrons within the active region. The transition from space charge limited current to recombination current was used to explain the existence of the current peak at low bias.