Provided in one embodiment is an article, comprising: a substrate comprising silicon; and a plurality of solar cells disposed over the substrate, wherein at least one of the plurality of the solar cells comprises one of: (i) a first semiconductor layer disposed over the substrate, the first layer comprising at least one semiconductor material; and (ii) a first Ge-containing layer disposed over the substrate, the first layer comprising a Ge-containing material, and a second layer disposed over the first layer, the second layer comprising at least one semiconductor material. At least some of the solar cells may comprise semiconductor materials of different bandgap values.