Role of diffusion on SCLC transport in double injection devices

A theoretical study of SCLC transport in double injection insulators is presented. It will be demonstrated, that the inclusion of charge carrier diffusion, neglected in many previous studies of transport in organic light emitting diodes (OLEDs), is essential to obtain physical meaningful spatial charge carrier densities and field distributions. Only the knowledge of such correct spatial distributions enables one to compute the correct position of the charge carrier recombination zone. In previous calculations without diffusion the recombination process often takes place in the vicinity of both electrodes, even for equal mobilities of holes and electrons. In the present calculation including diffusion it is demonstrated that only one recombination zone exists. For equal mobilities of electrons and holes the recombination zone is found as expected in the centre of the device whereas for different mobility values it may be strongly shifted to one of the electrodes. The resulting I-V characteristics indicate that, in double injection devices, the well-known Mott-Gurney law holds only at sufficiently high voltages and only if recombination is taken into account. For small voltages, an ohmic-like behavior is observed in any case, however, if no recombination is assumed a transition to an I ∼ V3 law is obtained for higher voltages. Due to the inclusion of diffusion, all I-V characteristics exhibit temperature dependence.