Low-refractive-index and high-transmittance silicon oxide (SiOx) with a mixed phase of n-type microcrystalline silicon was developed for intermediate reflector layers (IRLs) of high-efficiency amorphous Si and microcrystalline-Si tandem solar cells. The refractive index, crystalline fraction, and conductivity of the SiOx IRLs were characterized as functions of the deposition conditions. The SiOx films were prepared by plasma-enhanced chemical vapor deposition, at a high pressure of 9 Torr and high hydrogen dilution ratio of 400, and using a narrow electrode gap of 12 mm. The films showed a refractive index of 1.85, crystalline fraction of similar to 50%, and conductivity of similar to 10(-7) S/cm. Furthermore, the transmittance of the films was more than 90% at wavelengths between 600 nm and 1100 nm. We present here the procedure to optimize the SiOx IRLs, which is to compare the changes in the top and bottom cells' current densities of the tandem solar cells fabricated with 10-20% thinner absorption layers. A stabilized efficiency of 11.12% and light-induced degradation of -9.3% could be achieved in a tandem module containing an optimized IRL by enhancing the top cell current. The SiOx IRLs developed and the optimization procedure discussed in this paper can be very useful for the fabrication of high-efficiency thin-film Si tandem modules. (C) 2013 Elsevier B.V. All rights reserved.