We focus on the use of mixed-phase phosphorus-doped SiOx (n-SiOx) as advanced n-layer in both the junctions of micromorph tandem solar cells. The material, deposited by plasma enhanced chemical vapor deposition at two pressure values, has been fully characterized. Direct evidence of phase separation into nanocrystalline Si and amorphous oxygen-rich phases is reported, accompanied by information on the optical properties of the material as a whole and the two phases, as gathered by spectroscopic ellispometry. The multifunctional role of the n-SiOx layer, providing for beneficial optical effects at both the intermediate and back reflecting stage in micromorph solar cells, is established. In particular superior performance (improved fill factor and enhanced bottom solar cell spectral response) is demonstrated with use at the rear side of the device in combination with a simple Ag back reflector, with respect to …