Simulation of organic light emitting diodes: influence of charges localized near the electrodes

In spite of experimental evidence for the formation of charged layers near the electrodes of organic light emitting diodes (OLED), the influence of such layers on the OLED performance has not yet been clarified. This article presents a simulation study of this subject, utilizing the drift-diffusion model. In order to understand the principal mechanism of the influence of such layers, only monolayer devices with unintentional low p-doping are considered. In this case, positively charged layers near the anode or the cathode modify the current voltage characteristics for areal charges above some critical value. Effectively, such areal charges create an additional barrier of a magnitude which depends on the applied bias. A fixed positive areal charge near the anode decreases the current. However, due to the additional bias-dependent barrier, with increasing forward bias one has then a rather strong increase of the current resembling trap assisted space charge limited current. A fixed positive areal charge near the cathode leads to an increase of the build-in potential compared to the ideal thin-layer value which is given by the work function difference of the electrodes. The possibility of compensating the effect of the fixed positive charge with the help of a p-doped layer is discussed.