The effect of the dielectric nature of the transparent contact on charge injection and collection in organic optoelectronic devices

Transparent electrodes are common in organic-based optoelectronic devices because of the small diffusion length of charge carriers that require complete coverage by the electrode. The commonly reported drawback of transparent electrodes is their low conductivity, which leads to unwanted power dissipation. However, there is no reference to the influence of the dielectric nature on the contact properties. The effect of the dielectric nature is introduced by modifying the expression for the image potential and the resulting barrier lowering at the transparent contact using the Schottky model. The model is verified by fabricating and measuring simple unipolar devices, which show excellent agreement with the proposed modification. Without modification, there should have been unrealistic values for some parameters, such as a dielectric constant of 23 for the organic layer to have a good fitting. The modified expression is then inspected in numerical simulations to study its nonlinear influence on the device. We find that using our model with typical device parameters, the predicted current density of the device is around half of the value calculated for a metallic electrode. Similar results are obtained by reducing the mobility value while maintaining the injection condition of a metallic electrode. This explains why this property has been overlooked and is an indication of the extreme care needed when fitting experimental results