The role of a reflecting interlayer in micromorph silicon thin-film solar cells is investigated from the optical point of view. Detailed optical modelling and simulation are used to study the effects of different interlayers on quantum efficiency and short-circuit current of the top, amorphous silicon, and bottom, microcrystalline silicon, solar cell. The role of refractive index of interlayers on quantum efficiency of the top and bottom cell is analysed. Critical issues, such as enhanced total reflection from the solar cell and decreased quantum efficiency of the bottom cell due to interlayer are studied. Besides the single interlayer concept, double and triple interlayer stacks are investigated and improvements in comparison to the single ZnO interlayer are demonstrated. Potential thickness reductions of the top amorphous silicon cell related to different interlayers are presented.