We present a comprehensive scalar light-scattering model for the optical simulation of silicon thin film solar cells. The model integrates coherent light propagation in thin layers with a direct, noniterative treatment of light scattered at rough layer interfaces. The direct solution approach ensures computational efficiency, which is a key advantage for extensive calculations in the context of evaluation of different cell designs and parameter extraction. We validate the model with experimental external quantum efficiency spectra of state-of-the-art microcrystalline silicon solar cells. The simulations agree very well with measurements for cells deposited on both rough and flat substrates. The model is then applied to study the influence of the absorber layer thickness on the maximum achievable photocurrent for the two cell types. This efficient numerical framework will enable a quantitative model-based assessment of the optimization potential for light trapping in textured thin film silicon solar cells.