On the relevance of mass transport in thin layer nanocrystalline photoelectrochemical solar cells

A comprehensive analysis of the diffusion and migration processes in the steady state operation of mesoporous photoelectrochemical solar cells has been attempted. The dye sensitized TiO2 nanocrystalline solar cell utilizing the iodide/triiodide redox mediator serves as the system of reference. The porous nature of the semiconductor plays an important role in this process. Efficient design characteristics for such cells are obtained in order to minimize, e.g., the concentration overpotential, thus minimizing one of the sources of loss in such cells. The models developed illustrate operational aspects such as concentration profiles in the cell under the conditions relevant to existing systems, the limiting or maximum possible currents in the nanocrystalline PEC device, and the anticipated mass-transfer overpotential as a function of current density. The geometric and structural properties of the photoanode as well as the relative position of the counter-electrode with respect to the mesoporous film photoanode can be better exploited towards an efficient operation of the solar energy conversion device. The repercussions of the variation of solar cell design parameters are illustrated experimentally by the performance of practical application devices. These serve as evidence towards the plausibility and the validity of a mass transfer model for the electrolyte function in nanocrystalline PECs.