We present our development of an n-mu c-SiOx:H intermediate layer with low refractive index for a highly efficient Si thin film solar module in a large area (1.1 x 1.3 m(2)). We try to increase the electron temperature (T-e) in the plasma to activate the oxygen reaction during the deposition of the n-mu c-SiOx:H intermediate layer. Thus, the T-e-related process conditions are controlled, such as the gap between the electrodes, the working pressure and the H-2 flow rate in the plasma-enhanced chemical vapor deposition (PECVD) system. An intermediate layer with a lower refractive index is fabricated in narrower gap between the electrodes, lower working pressure and lower H-2 flow rate. As a result, we obtain a superior intermediate layer with a refractive index of 1.73 on a large area substrate, vertically conductive enough for a favorable junction in the cell, as confirmed by conductive atomic force microscopy (C-AFM) analysis. This intermediate layer is applied between the middle and the bottom layers in the triple junction (a-Si:H/a-SiGe:H/mu c-Si:H) cell. A low refractive index leads to a higher current of the middle cell by the superior reflection. Our large area module gains 0.3% conversion efficiency, with an improvement in the open circuit voltage (V-oc) and the fill factor (FF) by the thinner middle absorbing layer owing to the intermediate layer with a lower refractive index. (C) 2013 Elsevier B.V. All rights reserved.