The main features of a recently developed semi-coherent optical model for a-Si:H thin film solar cells with rough interfaces are presented. In contrast to the previous optical models, the model takes into account also the interference fringes observed in measured wavelength-dependent characteristics of a-Si:H solar cells. The simulations of the quantum efficiencies of the cells with different intrinsic a-Si:H layer thicknesses and interface root mean square (rms) roughness of 40 nm are shown and compared with the measured data. (C) 2002 Elsevier Science B.V. All rights reserved.