Abstract: A thin film solar cell, includes: a first electrode; a light absorption layer including a first light absorption layer including a group I element-group III element-group VI element compound, a second light absorption layer including a group I element-group III element-group VI element compound, and a third light absorption layer including a group I element-group III element-group VI element compound; and a second electrode, wherein the first light absorption layer has a band gap, which is less a band gap of the second light absorption layer, the band gap of the second light absorption layer is less than a band gap of the third light absorption layer, and the second light absorption layer has a band gap gradient, which increases in a direction from the first light absorption layer to the third light absorption layer.