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Characterisation of low work function, air-stable electrodes for organic photovoltaics

Supervisor: Professor Ross Hatton

Summary: This PhD project will provide interdisciplinary training in a number of surface and thin film analytical techniques, applied to the characterisation of novel electrode materials for an emerging renewable energy application.

 Photovoltaics (PVs) based on conventional semiconductors such as silicon and cadmium telluride are set to be the dominant PV technologies for large scale electricity generation this century. However, there are a number of important applications in buildings and transportation for which they are unsuitable due to the inherent brittleness of these inorganic semiconductors, which restricts their utility to flat-plate, opaque designs. These semiconductors are also processed at high temperature (> 300oC) making them incompatible with low-cost plastic substrates, and they are limited in the extent to which their colour and transparency can be tuned. Consequently there is a need for colour-tuneable, flexible, low-cost PVs. PVs that can be printed at low temperature onto flexible substrates are of particular interest because they offer the possibility of rapid roll-to-roll production with a small energy outlay. Organic PVs (OPVs) offer all of these advantages, and have recently achieved power conversion efficiencies 16-17% which is sufficient for practical applications. However, for OPVs to achieve operational lifetimes of greater than ten years there is a need for low work function electrodes that are also air-stable, since ambient air inevitably ingresses into these device over time. The low work function electrode in an OPV device is needed to enable efficient extraction of photo-generated electrons to the external circuit, but is also invariably a source of device instability. This project will explore the potential of novel alternative low work function, air-stable electrodes for OPVs. It will involve the application of a number of analytical techniques to probe the electronic and morphological structure of these novel electrodes. X-ray photoelectron spectroscopy and Auger spectroscopy will be used to probe the chemical state of the elements, including the extent of doping and/or alloying and the depth distribution of the elements. Ultra-violet photo-electron spectroscopy (UPS) will be used to measure work function and the energy of the valence band edge. These measurements will be complimented by measurements of work function using a Kelvin probe and cathodoluminescence measurements performed using a scanning electron microscope (SEM) to determine the band gap of any ultra-thin surface oxide layers.