Diffusion-driven currents in organic-semiconductor diodes

Abstract

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 doublecarrier 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. © 2014 Nature Publishing Group All rights reserved.