Electron transport in the dye sensitized nanocrystalline cell

Dye sensitised nanocrystalline solar cells (Gr\"{a}tzel cells) have achieved solar-to-electrical energy conversion efficiencies of 12% in diffuse daylight. The cell is based on a thin film of dye-sensitised nanocrystalline TiO$_2$ interpenetrated by a redox electrolyte. The high surface area of the TiO$_2$ and the spectral characteristics of the dye allow the device to harvest 46% of the solar energy flux. One of the puzzling features of dye-sensitised nano-crystalline solar cells is the slow electron transport in the titanium dioxide phase. The available experimental evidence as well as theoretical considerations suggest that the driving force for electron collection at the substrate contact arises primarily from the concentration gradient, ie the contribution of drift is negligible. The transport of electrons has been characterised by small amplitude pulse or intensity modulated illumination. Here, we show how the transport of electrons in the Gr\"{a}tzel cell can be described quantitatively using trap distributions obtained from a novel charge extraction method with a one-dimensional model based on solving the continuity equation for the electron density. For the first time in such a model, a back reaction with the I$_3^-$ ions in the electrolyte that is second order in the electron density has been included.Comment: 6 pages, 5 figures, invited talk at the workshop 'Nanostructures in Photovoltaics' to appear in Physica

Light induced oscillating reactions of silicon in ammonium fluoride solutions. Part 1. Simultaneous photocurent and excess microwave reflectivity measurements

For the first time, simultaneous photocurrent, I ph , and excess microwave reflectivity, m w , profiles are recorded during the light induced oscillatory behaviour of n Si 111 in aqueous ammonium fluoride solutions. Stationary oscillations in I ph , and m w are complementary to each other with maxima of I ph at minima of m w and vice versa. At higher positive potentials, irregular oscillatory behaviour is found and the complementarity of the signals is not fully maintained. For the initial time regime a model is suggested tentatively to determine electronic interface parameters from the simultaneous measurement of I ph and m w . For increased time, the oscillatory behaviour has also been evaluated with respect to surface recombination and charge transfer processes. In this time region, however, the application of the model suggested is problematic, as changes in band bending, trapping of charges and space charge layer recombination are not accounted for. For the observed lowest quantum yields gt; 0.2, it is assumed that the drift velocity is not the limiting facto