NOVELTY - The tandem solar cell (100) comprises a substrate (102), a front electrode (104) disposed on the substrate, a back electrode (106) disposed opposite to the front electrode on the substrate, a first cell (101) that is disposed below the front electrode and includes a first buffer layer and a first light absorption layer, and a second cell that is disposed above the back electrode and includes a second light absorption layer and a second buffer layer. The first light absorption layer includes a copper gallium selenium sulfur (CuGaSeS) layer and a copper gallium selenide (CuGaSe) layer.    USE - Used as a solar cell such as substrate-type tandem solar cell, crystalline wafer-type solar cell and an amorphous or polycrystalline thin film-type solar cell.    ADVANTAGE - The solar cell can be effectively manufactured with improved light transmission and solar light utility efficiency and voltage characteristics.    DETAILED DESCRIPTION - The tandem solar cell (100) comprises a substrate (102), a front electrode (104) disposed on the substrate, a back electrode (106) disposed opposite to the front electrode on the substrate, a first cell (101) that is disposed below the front electrode and includes a first buffer layer and a first light absorption layer, and a second cell that is disposed above the back electrode and includes a second light absorption layer and a second buffer layer. The first light absorption layer includes a copper gallium selenium sulfur (CuGaSeS) layer and a copper gallium selenide (CuGaSe) layer. 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. The CuGaSeS layer includes sulfur, where a content of the sulfur is less than 0.3 parts by weight when a content of selenium in the CuGaSeS layer is 100 parts by weight. A concentration gradient of the sulfur in the CuGaSeS layer decreases along a direction from the front electrode toward the CuGaSe layer. A concentration of sulfur on first and second surfaces of the CuGaSeS layer have a concentration gradient of 1-0.01 atomic% through 1-0.3 atomic%. A ratio of a thickness of the CuGaSeS layer to a thickness of the CuGaSe layer is 1:10 to 1:2. A bandgap energy of the CuGaSeS layer is 1.6-1.9 electron volts. A bandgap energy of the CuGaS layer is 1.5-1.8 electron volts. A bandgap energy of the second light absorption layer is 1-1.4 electron volts. A bandgap energy difference between the first and second light absorption layers is 0.3-0.7 electron volts. The first and second cells are connected by a tunnel diode. An INDEPENDENT CLAIM is included for a method of manufacturing a tandem cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a partial cross-sectional view of a tandem solar cell.    Tandem solar cell (100)    First cell (101)    Substrate (102)    Front electrode (104)    Back electrode. (106)