Photovoltaic structures for the conversion of solar irradiance into electrical free energy. In particular implementations, the novel photovoltaic structures can be fabricated using low cost and scalable processes, such as magnetron sputtering. In a particular implementation, a photovoltaic cell includes a photoactive conversion layer comprising one or more granular semiconductor and oxide layers with nanometer-size semiconductor grains surrounded by a matrix of oxide. The semiconductor and oxide layer can be a disposed between electrode layers. In some implementations, multiple semiconductor and oxide layers can be deposited. These so-called semiconductor and oxide layers absorb sun light and convert solar irradiance into electrical free energy.