IMPROVEMENT OF N-SI PHOTOELECTROCHEMICAL CELLS BY THE USE OF ACETONE PLUS METHANOL MIXTURES AS ELECTROLYTE SOLVENTS

Unmodified n-type silicon (n-Si) electrodes immersed in an acetone solution containing a small amount of methanol generate a high open-circuit photovoltage V(oc) and a high short-circuit photocurrent J(sc). Consequently an energy conversion efficiency of over 10% is achieved despite the very simple solar cell structure. The work function of the n-Si as decreased by the adsorption of methanol, leading to the formation of a high energy barrier in the n-Si. Majority carrier thermionic emission is the dominant dark current for the n-Si cells in cases where the methanol concentration in the redox solution is less than 10 vol.%, while the minority carrier diffusion current becomes dominant at higher methanol concentration. V(oc) increases with the electrode illumination time for the following reasons: (i) the work function of the n-Si is decreased by the partially dissociative adsorption of methanol promoted by the incident light; (i) the n-Si surface is passivated with respect to electron-hole recombination because of the formation of a thin silicon oxide layer enhanced by photogenerated holes. J(sc) is also high in cases where the redox solution contains a small amount of methanol because of the enhanced probability of the separation of photogenerated electron-hole pairs due to the high energy barrier. The fill factor is improved by making the distance between the Si electrode and the counter-electrode very short (12.5 mum).