A multi junction solar cell includes one or more upper cells and a thin-film, polycrystalline, low-bandgap bottom cell. A single-junction solar cell includes a polycrystalline semiconductor thin film, wherein a bandgap of the solar cell is greater than 1.2 eV or less than 1.2 eV, and the solar cell is configured to receive light through two surfaces, such that the bottom cell has bifacial operation.