Abstract: A tandem solar cell includes: a substrate; a front electrode disposed on the substrate; a back electrode disposed opposite to the front electrode on the substrate; a first cell disposed below the front electrode and including a first buffer layer and a first light absorption layer; and a second cell disposed above the back electrode and including a second light absorption layer and a second buffer layer. The first light absorption layer includes a CuGaSeS layer and a CuGaSe layer, and the second light absorption layer includes a semiconductor compound selected from the group consisting of CuInSe2, CuInGaSe2, CuInSeS, CuInGaSeS and any combinations thereof. The CuGaSeS layer of the first light absorption layer is disposed closer than the CuGaSe layer of the first light absorption layer to the front electrode.