Simulations of dye-sensitized solar cells

Abstract

Dye sensitized solar cells (DSSCs) are a novel design of solar cell which could be used as power producing windows or skylights. These nanocrystalline solar cells are currently the subject of intense research in the field of renewable energy as a low-cost photovoltaic device. The light adsorption occurs in dye molecules adsorbed on a highly porous structure of TiO2 porous film. The photo-conversion efficiencies of the DSSCs have been recently reported to reach 11%. Despite the progress in the efficiency and stability of these solar cells there are many fundamental aspects of their operation that are still unknown. One process, for which there is limited information, is the time taken to upload the dye on the TiO2 nanoporous film. Dye is adsorbed onto a TiO2 working electrode by dipping it into the dye solution for periods of several hours to several days. However, such long dipping times are not economic for industrial production of DSSCs. It has been shown recently that the time taken for dye uptake on the solar cell has an impact on its efficiency. The factors controlling this process are not yet fully understood. We develop a model based on the Langmuir isotherms to study and understand the diffusion and adsorption of the dye molecules in TiO2 nanotube films and compare our theoretical results to the experimental results. Our modelling results show that the adsorption of dye into the TiO2 nanotubes is controlled by the diffusion coefficient, the adsorption-desorption ratio and the initial dye concentration. The competition between the electron transport to the anode and the electron transfer to I − 3 ions in the electrolyte determines the efficiency of the collection of photoinjected electrons. The important parameter in this process is the electron diffusion length Ln, which is determined by the effective electron diffusion coefficient and life time. Efficient cells are characterized by a value of Ln that considerably exceeds the TiO2 film thickness. We introduce a new reliable and efficient approach to estimate the electron diffusion length in dye sensitized solar cells. This approach is based on the multiple trapping model which involves calculating the effective electron diffusion coefficient Dn and life time τn at the same quasi Fermi level. We show that in this context Ln = √Dnτn = L0, where Ln and L0 are the effective and free electron diffusion length respectively. Dn and τn are used to interpret the experimental intensity modulated photocurrent and photovoltage spectroscopic data. The good agreement between theory and experiment demonstrates that our model provides a powerful approach to estimate the diffusion length in terms of realistic devices.

The microwave technique is now a well established tool for contactless measurements of charge carriers in semiconductors. A 3D simulation model was developed for the study of photogenerated charge carriers by microwave reflectance techniques in DSSCs. This model is able to reproduce several features of the experimental results, and suggests that the microwave technique is a good measure of photoconductivity.

Date of Award	1 Dec 2010
Original language	English
Awarding Institution	University of Bath
Supervisor	Alison Walker (Supervisor)