NOVELTY - A polycrystalline thin film tandem solar cell structure comprises a polycrystalline thin film absorber layer (42) with a bottom surface and a top surface (40) through which light enters the absorber layer; and a semi-transparent conductive layer (43) including at least one of a ruthenium oxide, an osmium oxide and an iridium oxide, where the semi-transparent conductive layer makes physical contact with the bottom surface of the absorber layer.    USE - As a polycrystalline thin film solar cell structure or a tandem solar cell structure for converting sunlight directly into electrical power.    ADVANTAGE - The polycrystalline thin film solar cell structure is semi-transparent and allows a predetermined wavelength range of light to pass through the cell. The ruthenium-oxide (RuO2) is inert, therefore can withstand corrosion in a selenium, sulfur or tellurium atmosphere at high temperatures of 400-600degreesC; and is highly conductive and transparent. The property of oxides of Ru, Os and Ir is that they have metallic conductivity, resistivity values of 10-5-10-6 ohm-cm range. Therefore, very thin transparent layers of such materials provide reasonably low sheet resistance. Additionally, RuO2 does not chemically react with the constituents of solar cell absorber materials to any appreciable level. Therefore, a Group IBIIIAVIA material or a Group IIBVIA material can be grown on a surface comprising RuO2, without adversely affecting the interface or ohmic contact between the growing absorber layer and the surface comprising RuO2.    DETAILED DESCRIPTION - A polycrystalline thin film tandem solar cell structure comprises a polycrystalline thin film absorber layer (42) with a bottom surface and a top surface (40) through which light enters the absorber layer; and a semi-transparent conductive layer (43) including at least one of a ruthenium oxide, an osmium oxide and an iridium oxide, where the semi-transparent conductive layer makes physical contact with the bottom surface of the absorber layer. The polycrystalline thin film solar cell structure is semi-transparent. The absorber layer thickness is less than 10 microns. The semi-transparent conductive layer includes at least two sub-layers.    DESCRIPTION OF DRAWING(S) - The figure shows the thin film solar cell structure using at least one contact including ruthenium oxide.    top surface (40)    buffer layer (41)    polycrystalline thin film absorber layer (42)    semi-transparent conductive layer (43)    transparent substrate. (44)