Electronic Structure of Interfaces with Conjugated Organic Materials

Norbert Koch

2.1 Introduction

The function and efficiency of most organic (opto)electronic devices, such as light-emitting diodes (OLEDs), thin-film transistors (OTFTs), and photovoltaic cells (OPVCs), are significantly depending on the electronic structure of the interfaces within the devices. For instance, in OLEDs and OTFTs, charges must be injected from electrodes into the organic semiconductor, which requires that energy barriers for charge injection must be minimized to achieve low operation voltages. Several layers of different organic semiconductors are used in OLEDs to separate charge injection/transport from the region of exciton formation and recombination, which necessitates proper adjustment of the energy levels at such organic heterojunctions to facilitate electron (hole) transport across the interface while holes (electrons) should be blocked simultaneously. The contact between an electron acceptor and an electron donor organic semiconductors represents the most important element of OPVCs because only at that very interface, the exciton dissociation can occur with high probability; once the electron and hole are separated, they can be transported to the respective electrodes. Consequently, the energy levels at these interfaces must be matched to allow optimal charge separation and thus energy conversion efficiency. The key electronic levels and energy parameters of such interfaces are summarized ...

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