Diffusion-driven currents in organic-semiconductor diodes

Gert-Jan A H Wetzelaer,Paul W M Blom

doi:10.1038/am.2014.41

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 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.

Highlights

Conjugated polymers and small molecules, as typically used in organic light-emitting diodes (OLEDs) or solar cells, are undoped semiconductors
IDENTIFYING THE NATURE OF RECOMBINATION FROM CHARGE-TRANSFER STATE ELECTROLUMINESCENCE In section 2, we demonstrated for OLEDs that the recombination mechanism, either bimolecular or trap-assisted recombination, is directly reflected in the ideality factor of the current and light output.[33]
It was demonstrated that a slight deviation of the apparent ideality factor from unity in single-carrier devices is characteristic of organic MIM diodes

Summary

INTRODUCTION

Conjugated polymers and small molecules, as typically used in organic light-emitting diodes (OLEDs) or solar cells, are undoped semiconductors. The current that flows through such an organic-semiconductor diode results from the drift and diffusion of charge carriers. We focus on diffusion-driven currents in several types of organicsemiconductor diodes Studying these diffusion currents provides important information on charge transport and recombination in organic semiconductors. To illustrate what determines the current flow in an organicsemiconductor diode, we will describe the case of a single-carrier device with an ohmic injecting contact and a non-ohmic collection contact. The collecting contact at x 1⁄4 L, where x denotes the position within the diode and L denotes the film thickness, is separated from the valence band by a barrier height, jb. When the bias is smaller than the built-in voltage, the electric field remains negative, pointing towards the injecting contact.

EF b φb

IDEALITY FACTOR AND THE EINSTEIN RELATION

IDEALITY FACTOR IN ORGANIC SOLAR CELLS

ORGANIC MIM DIODES

CONCLUSIONS