ABSTRACTThe optical properties of solar cell grade hydrogenated amorphous silicon (a-Si:H), silicon germanium (a-SiGe:H) and silicon carbon (a-SiC:H) alloy thin films have been investigated over a wide photon energy range (0.8–4.8 eV) using a combination of subgap photoconductivity, reflection and transmission, and spectroscopie ellipsometry techniques in order to obtain accurate optical functions for solar cell modelling. Studies on films with thicknesses ranging from a few hundred Å to a few microns show that the optical spectra obtained by the different techniques agree closely over the energy ranges of overlap and display no thickness dependence from the Urbach tail energies and above. Thus, the results appear to be free of measurement and sample related artifacts. Three different methods provide a common value for the optical gap within ±0.02 eV and the result for a-Si:H is ∼0.1 eV below the mobility gap.