NOVELTY - Method (M1) of manufacturing thin-film solar cell, comprises: (a) providing (111) a substrate; (b) forming a back electrode layer on substrate; (c) forming (114) a precursor layer on back electrode layer; (d) providing (115) an alkali layer on precursor layer; (e) providing a selenization process, so that precursor layer is selenized and an absorber layer having copper selenide (Cu(In, Ga)Se2) structure is formed; (f) forming (117) at least one buffer layer on absorber layer; and (g) forming (118) at least one front electrode layer on buffer layer.    USE - The method is useful for manufacturing thin-film solar cell.    ADVANTAGE - The method: has absorber layer having increased bandgap, increased open circuit voltage, and reduced bandgap discontinuity between the buffer layer and the absorber layer, thus upgrading conversion efficiency; and helps the crystal growth in the absorber layer to form crystals with relatively large grains, reduced grain boundaries, decreased resistivity, increased carrier density and minimizes defect density in the crystals.    DETAILED DESCRIPTION - Method (M1) of manufacturing thin-film solar cell, comprises: (a) providing (111) a substrate; (b) forming a back electrode layer on the substrate; (c) forming (114) a precursor layer on the back electrode layer, where the precursor layer includes at least one copper, indium and gallium; (d) providing (115) an alkali layer on the precursor layer; (e) providing a selenization process, so that the precursor layer is selenized and an absorber layer having a copper selenide (Cu(In, Ga)Se2) structure is formed; (f) forming (117) at least one buffer layer on the absorber layer; and (g) forming (118) at least one front electrode layer on the buffer layer, where an atomic percentage concentration of an alkali metal in the absorber layer is 0.01-10%, and the atomic percentage concentration of the alkali metal in the absorber layer closer to the back electrode layer is lower than the atomic percentage concentration of the alkali metal in the absorber layer closer to the buffer layer. INDEPENDENT CLAIMS are also included for:    (1) the thin-film solar cell, comprising: the substrate; the back electrode layer; the absorber layer, disposed on the back electrode layer, and formed by the precursor layer and the alkali layer via a selenization process, where the back electrode layer includes at least one copper, indium and gallium; the buffer layer; and the front electrode layer; and    (2) method (M2) of manufacturing thin-film solar cell, comprising: steps (a) and (b) as above per se; providing a co-evaporation process, so that the absorber layer having a chalcopyrite (Cu(In, Ga)Se2) structure is formed on the back electrode layer; providing an alkali metal sulfide for depositing on the absorber layer; providing a heat treatment process, so that a copper sulfide (Cu(In, Ga)S2) or copper sulfide/copper selenide (Cu(In, Ga)(Se,S)2) structure is formed on a surface of the absorber layer; and steps (f) and (g) as above per se.    DESCRIPTION OF DRAWING(S) - The figure shows a flowchart showing the steps included in a method of manufacturing thin-film solar cell.    Providing substrate (111)    Forming precursor layer on back electrode layer (114)    Providing alkali layer on surface of precursor layer (115)    Forming buffer layer on absorber layer (117)    Forming at least one front electrode layer on buffer layer (118)