In general, it is believed that the current in organic light-emitting diodes (OLEDs) occurs as a result of space charge-limited current (SCLC). On the other hand, some researchers have analyzed the current in OLEDs on the basis of the Schottky model. Although the current of OLEDs depends on temperature, i.e. it increases with increasing temperature, it is inferred that it is not controlled only by thermal emission since the charge transfer between molecules is thought to have a positive temperature dependence. The authors have investigated the necessary condition for the SCLC model: the dielectric relaxation time of a material is greater than the carrier drift time between both electrodes in the material. Carrier transport in OLEDs cannot be explained only by the unipolar carrier injection model; it also remains unclear whether hole injection is Ohmic or not. The authors estimated the apparent dielectric relaxation time, tau of OLEDs. Consequently, it was found that the necessary condition of SCLC is not satisfied in the high electric field region. (C) 2003 Elsevier Science B.V. All rights reserved.