The internal quantum efficiency of organic light-emitting diodes ͑OLEDs͒ can reach values close to 100% if phosphorescent emitters to harvest triplet excitons are used; however, the fraction of light that is actually leaving the device is considerably less. Loss mechanisms are, for example, waveguiding in the organic layers and the substrate as well as the excitation of surface plasmon polaritons at metallic electrodes. Additionally, absorption in the organic layers and the electrodes can play a role. In this work we use numerical simulations to identify and quantify different loss mechanisms. Changing simulation parameters, for example, the distance of the emitter material to the cathode or thicknesses of the various layers, enables us to study their influence on the fraction of light leaving the OLED. An important parameter in these simulations and for the actual device is the radiative quantum efficiency q, which is defined as the efficiency of radiative exciton decay in an unbounded space filled by the emitting dye and its matrix. The simulations show that due to microcavity effects the radiative decay channel can be considerably changed in an OLED as compared to free space emission of a dipole. Thus the knowledge of the radiative quantum efficiency is crucial for the optimization of OLEDs. As an example, we present simulations of bottom-emitting OLEDs based on the well-known green emitter tris-͑8-hydroxyquinoline͒ aluminum with transparent indium tin oxide anode and a calcium/aluminum cathode.

A comprehensive opto-electronic device model for organic bulk-heterojunction solar cells is presented. First the optical in-coupling into a multilayer stack is calculated. From the photon absorption profile a charge transfer (CT) exciton profile is derived. In this study we consider the Onsager-Braun mechanism to calculate the dissociation of the CT-excitons into free charge carriers. These free charge carriers then migrate towards the electrodes under the influence of drift and diffusion. A general problem arising in computer simulations is the number of material and device parameters, which have to be determined by dedicated experiments and simulation-based parameter extraction. In this study we analyze measurements of the short-circuit current dependence on the active layer thickness and current-voltage curves in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based solar cells. We have identified a set of parameter values including dissociation parameters that describe the experimental data. The overall agreement of our model with experiment is good, however a discrepancy in the thickness dependence of the currentvoltage curve questions the influence of the electric field in the dissociation process. In addition transient simulations are analyzed which show that a measurement of the turn-off photocurrent can be useful for estimating charge carrier mobilities.

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