A method of manufacturing a solar cell includes forming a buffer layer between an optical absorption layer and a window electrode layer. Forming the buffer layer includes depositing a metal material on the optical absorption layer, supplying a non-metal material on the optical absorption layer, supplying a gas material including oxygen atoms on the optical absorption layer, and reacting the metal material with the non-metal material. The gas material reacts with the metal material and the non-metal material to form a metal sulfur oxide on the optical absorption layer.