TECHNOLOGY FOCUS - ELECTRONICS - Preferred Component: The back contact (42, 46) is transparent to optical radiation. A layer region of modified p-type semi-conductor material is included at a back surface of the p-type layer. A front layer of transparent conducting oxide is also included at a front surface of the n-type layer. A transparent substrate is at a front surface of the front layer of TCO.The photovoltaic device comprises a thin film solar cell and a light source for the optical radiation (40, 47) comprises a sun. The optical radiation comprises direct or indirect radiation. The photovoltaic device comprises a bi-facial cell for generating electricity when illuminated from both the front and back side simultaneously. A metal layer is on a back surface of the layer of TCO. A conductive paste is on a back surface of the layer of TCO. A metal grid is on the TCO layer.    TECHNOLOGY FOCUS - CERAMICS AND GLASS - Preferred Material: The substrate comprises a glass material.    TECHNOLOGY FOCUS - POLYMERS - Preferred Material: The substrate comprises a polymer material.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The p-type semi-conductor layer comprises a cadmium telluride thin film layer. The n-type semi-conductor layer comprises cadmium sulfide thin film layer. The p-type semiconductor layer comprises a ternary or quaternary II-VI compound with band gap energy of 0.9-1.8 eV. The n-type semiconductor layer comprises a ternary or quaternary compound with band gap energy of 2-3.4 eV. The TCO layers optionally comprise a doped tin oxide layer, an indium-tin oxide layer, doped zinc oxide layer, cadmium stannate layer, or zinc stannate layer.Preferred Method: The device is annealed via heat treatment optionally in vacuum, in air, in a gas, with cadmium chloride vapor, with chlorine vapor, or with hydrogen chloride vapor. The back surface of the p-type semiconductor is washed with a bromine methanol solution or nitric-phosphoric acid. It can be washed with deionized water. A 500 nm thick layer of cadmium sulfide is grown via vacuum evaporation. A 3000 nm thick layer of cadmium telluride is grown by vacuum evaporation. A 200 nm thick layer of cadmium chloride is evaporated on a back surface of the p-type semiconductor layer. An annealing step at 400 degreesC in air for 30 minutes is performed subsequent to the step of evaporation.