We report the recent advances and key requirements for high-efficiency "micromorph" tandem thin-film silicon solar cells composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell. The impact of inserting a low-refractive-index silicon-oxide (SiOx) film as intermediate reflecting layer (IRL) is highlighted. We show that refractive indexes as low as 1.75 can be obtained for layers still conducting enough to be implemented in solar cells, and without no additional degradation. This allows for high top-cell current densities with thin top cells, enabling low degradation rates. A micromorph cell with a certified efficiency of 12.63% (short-circuit current density of 12.8 mA/cm(2)) is obtained for an optimized stack. Furthermore, short-circuit current densities as high as 15.9 mA/cm(2) are reported in the amorphous silicon top-cell of micromorph devices by combining a 150-nm- thick SiOx-based IRL and a textured antireflecting coating at the air-glass interface.