We apply a hybrid finite element / transfer matrix solver to calculate generation rate spectra of thin film silicon solar cells with textured interfaces. Our focus lies on interfaces with statistical rough textures. Due to limited computational domain size the treatment of such textures requires a Monte Carlo sampling of texture representations to obtain a statistical average of integral target quantities. This contribution discusses our choice of synthetic rough interface generation, the Monte Carlo sampling and the need for an incorporation of the cell's substrate into optical simulation when illumination of the cell happens through the substrate. We present results of the numerical characterization and generation rates for a single junction cell layout. © 2011 SPIE.