Diffusion-driven currents in organic-semiconductor diodes

Organic light-emitting and photovoltaic diodes are attractive optoelectronic devices with organic semiconductors as the main component. Whereas the transport properties in the drift regime of such diodes have been extensively investigated, the diffusion-dominated regime has only lately attracted considerable attention. In this review, we discuss diffusion-driven currents in several types of organic diodes. It is demonstrated that the ideality factor of diffusion-driven currents can be regarded as an effective tool for studying the recombination mechanisms in organic light-emitting diodes (OLEDs) and solar cells. In double-carrier devices, such as OLEDs, the ideality factor of the current has a temperature-independent value of 2, which indicates that nonradiative trap-assisted recombination dominates the current. By contrast, the ideality factor of the light output approaches unity, indicating that luminescence is governed by bimolecular recombination. In a single-carrier device, in which recombination is absent, the ideality factor may deviate from unity because of a small number of deeply trapped carriers. Therefore, the ideality factor of a bulk-heterojunction solar cell can deviate from unity even in the absence of trap-assisted recombination. Finally, an analytical description of the diffusion current is derived that can be used to extract contact barriers and can explain slight deviations of the ideality from unity. Detailed knowledge of both drift and diffusion carrier transport processes is essential for the development of organic optoelectronics. Gert-Jan Wetzelaer from the University of Groningen in the Netherlands and Paul Blom from the Max Planck Institute for Polymer Research in Germany review diffusion-driven currents in organic light-emitting diodes (OLEDs) and organic photovoltaic diodes. The ideality factor—a measure of the current density-voltage slope characteristic that is equal to one for devices with ideal diode behavior—is a useful tool for studying carrier transport and recombination mechanisms in such devices, and the authors propose an analytical approach to describe the effect of device structure on diffusion current and ideality factor. In OLEDs, for example, trap-assisted recombination of charge carriers can result in a current ideality factor equal to two, whereas the ideality factor of the bimolecular light output approaches one. In this review, we discuss diffusion-driven currents in several types of organic-semiconductor diodes, including light-emitting diodes, solar cells and single-carrier devices. It is demonstrated that the ideality factor of the current and photon emission contains important information about charge transport and recombination in such devices. Most importantly, the effects of charge trapping and trap-assisted recombination can be explored. An analytical model is derived for diffusion-driven currents, which can explain the current–voltage behavior of organic diodes and allows determination of the injection barriers and built-in voltages.