Interfaces in Organic Optoelectronics
The operation of organic electronic devices strongly depends on the charge injection or charge extraction properties of the metal contact. The interfaces between metal electrodes and organic semiconducting materials play a crucial role in these processes. The graphs (from left to right) below illustrate device architectures of organic light-emitting diodes (OLEDs), solar cells (OPVs) and field-effect transistors (OFETs), respectively. Generally speaking, good energetic alignment between the electrode work function (WF) and the conduction (valence) band of the electron (hole) conducting material is desired to maximize device performance. At the same time, the unique requirements of each application (photovoltaics, light-emitting diodes, and field-effect transistors) limit the range of suitable electrodes. For example, to produce electroluminescence in the PLED (left graph), electrons have to be injected from the top cathode to recombine with holes injected from the bottom anode. An opposite process occurs in the OPV (middle graph) where charges created in the active layer must be extracted at the electrodes to result in the photoconductivity. In the OFET (right graph), the current is formed via charges injected from the source electrode under the modulation of gate bias. In OLEDs and n-type or ambipolar FETs, for instance, efficient injection of electrons from a metal electrode to the LUMO of the organic material is necessary. A reactive, low WF metal electrode provides a suitable contact to the LUMO of organic materials but simultaneously raises a problem of air stability. Thus it is favorable to have stable materials with tunable WFs so that charge injection can be preferentially improved without complicating the device structure and fabrication procedure. An emerging alternative to a low WF metal is a solution processable interlayer between the organic film and the metal electrode that effectively modifies the electrode work function while maintaining its otherwise desirable characteristics such as stability and cost.
In reality, the charge injection efficiency is often limited by the choice of metal electrodes, undesirably affecting the ultimate device performance. To circumvent this shortage, interlayers are introduced to deal with charge injection issues. For instance, interlayers consisting of a thin organic film or DNA film have been incorporated in OLEDs, OFETs, or OPVs from solution processing. As a result the work function of the top electrodes is modified such that charge injection barriers are effectively reduced. This modification leads to improving the luminance efficiency (in the case of OLEDs) and charge carrier mobility in OFETs. To understand the impact of interlayers on the device performance, we employ a combination of techniques including ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), AFM, (scanning) Kelvin probe microscopy, electrostatic force microscopy, conducting AFM and contact angle measurements, etc. From these measurements, we aim to attain insights into the interfacial properties at the metal/organic interface.