Physical characterisation of interfaces in organic devices

This thesis is concentrated on studies on organic interfaces in conjugated polymer devices. Both non-linear properties of devices, photo-physical experiments, surface morphology using scanning probes and conventional device characterisation techniques are used to characterise the structures and physical processes at the specific interfaces. In more detail, the thesis can be divided into two parts. The first part concerns the electrode/semiconductor interface of light-emitting diodes, where the injection layers are investigated both in terms of electrical and photo-physical characteristics, and also their influence on the electro-optical response of the device. The different injection layers studied include a cross-linked hole-injection layer, and a self-assembled monolayer (SAM) for enhanced electron injection. For the former, the additional cross-linking is shown to provide superior hole-injection due to a higher work function and a reduction of impurity states as quantified in electroabsorption experiments. For the latter, the induced dipole of the SAM is shown to reduce the injection barrier for electrons at the cathode. Through electroabsorption it is furthermore shown that the dipole of the SAM adsorbed on thin oxides is bias dependent and its dynamical reorganisation is characterised accordingly. The second part treats the charge/energy transfer properties at the organic/organic interfaces in binary blends both for application in light-emitting diodes (LEDs) and photovoltaic cells (PVs). In LEDs, the emphasis lies in creating non-resonant blends, i.e. to reduce energy transfer between the two materials, in order to achieve two-component electroluminescence that can be tuned to white light. In contrast, binary bulk heterojunction PVs are doped by metal complexes to introduce inter-system crossings to a triplet manifold with the goal to increase the exciton dissociation efficiency. It is demonstrated that the large portion that transfer into triplet states still dissociates and the subsequent free charges can be effectively extracted