The controlling influence of the Cu2S optical absorption coefficient on the short-circuit currents of Cu2S/CdS solar cells

Cu2S is a p-type defect semiconductor and is the main optical absorber-current generator in the Cu2S/CdS solar cell. This cell undergoes large reversible changes in its short-circuit current depending upon the ambient to which it is exposed. While a large number of mechanisms have been proposed for this effect, we find that it can be accounted for solely on the basis of changes in the absorption coefficient of the Cu2S, as controlled by the position of the Fermi level in the degenerate material. We have calculated the relation between the absorption coefficient and sheet resistance for degenerate Cu2S, and compared the results to existing experimental data on material prepared in the same way as solar cells. We find good agreement between the experimental data and our calculations if Cu2S is acting as a direct-gap semiconductor. Based upon the magnitude of the experimental changes in absorption coefficient with sheet resistance we have calculated the expected change in short-circuit current that would be produced in a typical Cu2S/CdS solar cell. The changes in short-circuit current calculated are on the order of 20-40 percent for ρ/d changing by an order of magaitude. These results are in agreement with the results on actual cells.