Inorganic Solid State Solar Cell with Ultra-thin Nanocomposite Absorber based on Nanoporous TiO2 and In2S3
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The increase of the diffusion length of a wet chemically prepared absorber by introducing a nanocomposite morphology and the resulting potential for reaching high energy conversion efficiencies were demonstrated. The basic processes of light absorption and photo-generation of charge carriers, charge transport and charge separation are separated in space in nanostructured solar cells. This opened up the opportunity to applymaterials with low diffusion length which can be produced at low temperatures and even under wet chemical conditions and raised the hope that production costs of solar cells can be strongly reduced by implementing nanostructured materials. In nanostructured solar cells the absorber layer has to be folded since the local absorber thickness is usually much less than the absorption length. For example, for dye-sensitized solar cells the internal surface area is increased by a factor of about 1000,1 for solar cells with extremely thin absorber a factor of about 10 is sufficient.2 The thickness of an inorganic absorber layer has to be much larger than the tunnelling length to avoid tunnelling recombination between electron and hole conducting regions. From this point of view the minimum thickness of inorganic absorber layers should be at least about 20–30 nm in systems with solid electron and hole contacts.2 However, the diffusion length of photo-generated charge carriers is usually of the same order or even less for wet chemically prepared absorber layers.3 In fact, the low diffusion length limits the energy conversion efficiency. We should mention that the diffusion length of a given absorber is quite fixed and can hardly be increased for a given deposition technique.