We have developed the 3-dimensional optical model for thin film silicon solar cells (amorphous, microcrystalline, single or multijunction) with nanorough surfaces/interfaces. For these cells the external quantum efficiency, short circuit current, total reflectance and all absorption losses can be computed taking into account roughness, angular distribution of scattered light, thicknesses and experimentally determined optical constants of all layers. The model gives very good agreement with the experimentally determined data for micro-crystalline silicon and micromorph (amorphous-microcrystalline silicon tandem) solar cells. Based on the analysis of optical losses and possible improvements in the materials properties, a roadmap for over 15% stable efficiency of micromorph solar cell has been suggested.