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An exact theory of spectral response of thin-film epitaxial solar cells is developed, and a new expression for the effective back surface recombination velocity is derived. On the basis of this theory, the spectral response as a function of relevant cell parameters, such as dopings, diffusion lengths, and thicknesses of the substrate and epitaxial layer regions and the dead layer, is calculated. It was found that two effects mainly control the spectral response of epitaxial solar cells: (i) the doping ratio of substrate to epitaxial layer and (ii) the dead layer thickness. The former markedly influences the long wavelength region spectral response, while the latter has a profound effect on the short wavelength response. The theory also predicts a significant contribution from the substrate to the quantum efficiency.