Highly transparent electrodes with a well-tuned refractive index are essential for a wide range of optoelectronic devices, such as light emitting diodes and solar cells. Here, it is shown that the transparency of ZnO:Al can be improved and its refractive index can be reduced simultaneously by the addition of SiO2 into the layer. It is found that for low SiO2 concentrations, Si quenches oxygen vacancies and improves the layer transparency. At higher SiO2 concentrations a highly transparent amorphous compound of ZnxSiyO:Al forms, with a refractive index that scales down with the relative Si/Zn ratio. These layers are tested in Si heterojunction solar cells by inserting them between Si and the metallic rear contact of such devices. A consistent improvement is found in the cell short-circuit current density and external quantum efficiency with increasing Si incorporation. Our findings establish a general strategy to tune the optical properties of transparent conductive oxides for improved light management in solar cells.