Photovoltaic cells

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An upper layer is an inorganic semiconductor with a relatively wide energy gap (eg TiO2, ZnO, ZnSe, ZnTe) and the lower layer 2 is an inorganic semiconductor with a relatively narrow energy gap (eg anatase TiO2 alloyed with Ga, Mn, Fe, Co, Zn, V, Cr, Cu, Ni, Ag and rare earth elements). The lower layer 2 absorbs irradiance of longer wavelengths to provide the layer 1 with photo-generated charges. In addition, higher energy solar irradiance can be directly absorbed by the upper layer 1 to avoid intra-band transitions. This enhances the performance of the photovoltaic cell with respect to longer wavelengths. Alternative device configurations include a graded bandgap region, nanowire active regions (fig 3) or a micro / nanocrystalline alloy of wide, narrow and intermediate bandgap semiconductor particulates (fig 6). The photovoltaic cell made of such inorganic semiconductor materials is chemically inert so as to provide prolonged service life.