TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Solar Cell: The light absorbing layer comprises Cux(GayIn1-y)(SzSe1-z)2, and value of z gradually increases in the first direction within the second concentration region, where 0.75 is less than or equal to x is less than or equal to 1, 0.1 is less than or equal to y is less than or equal to 0.3, and 0.7 is less than or equal to z is less than 1. The solar cell further comprises a buffer layer disposed between the light absorbing layer and the transparent electrode layer. The light absorbing layer further includes a second concentration region in which the concentration of sulfur gradually increase in the first direction between the first concentration region and the buffer layer. The light absorbing layer further includes a third concentration region in which the concentration of sulfur is substantially constant between the first concentration region and the back electrode layer, the concentration of sulfur of the third concentration region corresponding to the maximum among the concentrations of sulfur of the first concentration region. The light absorbing layer includes a first compound layer, the first compound layer occupying 2.5-5% of a thickness of the light absorbing layer. Preferred Method: Forming of the light absorbing layer comprises: (a) forming a first precursor comprising Cu, Ga, and In on the back electrode layer; (b) forming a first compound layer by performing a first heat treatment to the first precursor using a sulfur containing gas; (c) forming a second precursor comprising Cu, Ga, and In on the first compound layer; and (d) forming the light absorbing layer by performing a second heat treatment to the first compound layer and the second precursor using a selenium containing gas. The method further comprises forming a buffer layer between the light absorbing layer and the transparent electrode layer. It further comprises performing a third heat treatment using sulfur containing gas, after the forming of the light absorbing layer. The light absorbing layer further includes a second concentration region in which the concentration of sulfur gradually increases in the first direction between the first concentration region and the buffer layer. The light absorbing layer further includes a third concentration region in which the concentration of sulfur is substantially constant between the first concentration region and the back electrode layer, the concentration of sulfur of the third concentration region corresponding to a maximum among the concentrations of sulfur of the first concentration region. The sulfur containing gas includes an inert gas and a hydrogen sulfide (H2S) gas, where the H2S gas in the sulfur containing gas has a flow ratio of 10-20%. The selenium containing gas includes an inert gas and a hydrogen selenide (H2Se) gas, where the H2Se gas in the selenium containing gas has a flow ratio of 3-10%. The first and second heat treatments are performed at a temperature of 500-600 degrees C.