In this paper we report the results of an effort to develop a design tool to optimise any solar cell with thin-film structure. An optical admittance analysis which takes into account the interference effects in the multilayer thin-film system is discussed. This method provides a technique for calculating the optical properties of a thin-film device with a multilayer configuration. It also allows us to explore the nature and magnitude of optical losses in p-i-n type single or multi-junction a-Si:H solar cells, and to optimise the structure of such cells to utilise enhanced interference effects to obtain the maximum possible utilisation of incident solar radiation.