Theory of dye sensitized solar cells

Only a small fraction of the world energy usage (less than 0.05% in 2004) derives from solar cells because of the high cost of the current technology based on crystalline silicon. Dye Sensitized Solar Cells (DSSC or Grtzel cells) are one of the major alternatives to silicon photovoltaics and the subject of the proposed investigation. DSSC are relatively complex systems including a nanocrystalline titanium dioxide (TiO2) semiconductor, a dye adsorbed on it, a solution containing an electrolyte that can be reduced and oxidized and a transparent electrode. The efficiency in the conversion of light into electricity achievable in mass produced DSSC is around 7%, while a desirable target efficiency of 15% (or larger) would make this technology suitable for large scale electricity production. Current improvements of the DSSC rely on the development of new dyes and new electrolytes. There are currently no predictive theories that allow the systematic improvement of DSSC and the efficiency improvement is largely based on chemical intuition and systematic search. The aim of the proposed research is to develop a methodology to compute and predict the elementary rates of all microscopic (charge transfer) processes taking place in DSSC. The control and understanding of these elementary processes are the basis for the rational improvement of cell efficiency. The theoretical description of DSSC requires a cross-disciplinary approach involving elements of solid state physics, electrochemistry and quantum dynamics. An opportunity to find a common language among these disparate areas and to develop a coherent methodology is offered by the new field of Molecular Electronics (the study of electric transport properties of single molecules in contact with two electrodes), the research area where the applicant has the largest expertise. All the charge transfer processes in the solar cell will be described by the unified theoretical framework of molecular electronics. This formal theory will provide a bridge between the results of quantum chemical computations and the desired elementary rates. Through the systematic study of different dyes and electrolytes it will be possible to define the structure-property relation for these individual components of the system and to guide the synthesis of new and more efficient solar cells.