The transmittance spectra of thin film a-Si : H p-i-n solar cells and a combination of different layers were measured in the visible and near-infrared spectral regions. With the help of computer assisted optical spectroscopy, the spectra were analysed and the optical constants of the different layers of the layer stack systems were determined. The index of refraction, the energy gap; the layer thickness and the complex dielectric functions of the p- and n-doped layers and the intrinsic a-Si:H layer were calculated with the Forouhi and Bloomer model. This method allows a very fast and accurate optical characterisation of photovoltaic cells.