A single-junction germanium solar cell with a photonic crystal nanostructured surface has been developed. The solar cell comprises a Ge p-n junction and an InGaP window layer. The InGaP window layer has been nanopatterned with an extended photonic crystal structure consisting on a triangular lattice of holes with submicronic sizes. Enhancements of the external quantum efficiency of 22% for a wide range of wavelengths and up to a 46% for specific wavelengths have been measured, which implies an increase in photocurrent between 11% and 22%. A clear correlation with the area of photonic crystal patterned has been observed.