Organic materials for photovoltaic devices
This thesis is a contribution towards the understanding of the operation of bilayer solar cells, and the development of a theoretical basis for the selection of suitable pairs of materials for the fabrication of such cells. The work is divided into two main areas: (a) theoretical calculations on materials used in solar cells (b) fabrication of devices to test the calculations. Practically, many devices were made using some previously untried materials, the most successful of which was formed from dibromoanthanthrone and titanyl phthalocyanine. This sample was 0.30% efficient under incident white light of intensity 20mW/cm2 and had an open circuit voltage of 0.39V. Measurements of the response time of the sample were also recorded which provided information on the quality of the device made. Theoretically, calculations were performed using the extended Huckel method on potential materials for photovoltaic devices. Initially, these provided information on the variation of bandwidth with inter-ring separation for cofacially stacked phthalocyanines. They were also used to predict the position of the HOMO/LUMO for different materials. Then by deducing the position of the Fermi level, it is possible to simulate the junction formed between the two materials. Predicted behaviour for the phthalocyanine/perylene interface agreed with that found experimentally from UPS and optical absorption measurements of the ionisation potentials, work functions and band gaps for a similar junction. The calculations have also demonstrated how substituting or changing the two layers alters the performance of the device. This allowed a set of criteria to be established that should enable a more systematic approach to choosing potential pairs and then optimising their performance in future solar cells.