We present optical properties and microstructure analyses of hydrogenated silicon suboxide layers containing silicon nanocrystals (nc-SiOx:H). This material is especially adapted for the use as intermediate reflecting layer (IRL) in micromorph silicon tandem cells due to its low refractive index and relatively high transverse conductivity. The nc-SiOx:H is deposited by very high frequency plasma enhanced chemical vapor deposition from a SiH4/CO2/H2/PH3 gas mixture. We show the influence of H2/SiH4 and CO2/SiH4 gas ratios on the layer properties as well as on the micromorph cell when the nc-SiOx:H is used as IRL. The lowest refractive index achieved in a working micromoph cell is 1.71 and the highest initial micromoph efficiency with such an IRL is 13.3 %.