NOVELTY - Solar cell (C1) comprises: a first layer (22) having a first-layer lattice parameter; a second layer (24) having a second-layer lattice parameter different from the first-layer lattice parameter, where the second layer includes a photoactive second-layer material; a third layer (26) having a third-layer lattice parameter different from the second-layer lattice parameter, where the third layer includes a photoactive third-layer material; a graded first buffer layer (42) extending between and contacting the first layer and the second layer; and a transparent graded second buffer layer (46).    USE - Used as a solar cell.    ADVANTAGE - The solar cell: exhibits improved power output and performance, and thus exhibits improved efficiency; and is cost-effective to manufacture.    DETAILED DESCRIPTION - Solar cell (C1) comprises: a first layer (22) having a first-layer lattice parameter; a second layer (24) having a second-layer lattice parameter different from the first-layer lattice parameter, where the second layer includes a photoactive second-layer material; a third layer (26) having a third-layer lattice parameter different from the second-layer lattice parameter, where the third layer includes a photoactive third-layer material; a graded first buffer layer (42) extending between and contacting the first layer and the second layer and having a first-buffer-layer lattice parameter that increases with increasing distance from the first layer toward the second layer; and a transparent graded second buffer layer (46) extending between and contacting the second layer and the third layer and having a second-buffer-layer lattice parameter that decreases with increasing distance from the second layer toward the third layer. INDEPENDENT CLAIMS are also included for:    (1) a solar cell (C2) comprising: the first layer; the second layer; the third layer; a graded first buffer layer extending between and contacting the first layer and the second layer and having a first-buffer-layer lattice parameter that decreases with increasing distance from the first layer toward the second layer; and a transparent graded second buffer layer extending between and contacting the second layer and the third layer and having a second-buffer-layer lattice parameter that increases with increasing distance from the second layer toward the third layer, where the second buffer layer is of a different composition than the first buffer layer;    (2) a solar cell (C3) comprising: a first layer having a first-layer lattice parameter; a second layer having a second-layer lattice parameter different from the first-layer lattice parameter, where the second layer includes a photoactive second-layer material; and a buffer layer extending between and contacting the first layer and the second layer and having a buffer-layer lattice parameter that increases with increasing distance from the first layer toward the second layer; and    (3) a solar cell (C4) comprising: a first layer having a first-layer lattice parameter; a second layer having a second-layer lattice parameter different from the first-layer lattice parameter, where the second layer includes a photoactive second-layer material; and a graded buffer layer extending between and contacting the first layer and the second layer and having a buffer-layer lattice parameter that decreases with increasing distance from the first layer toward the second layer.    DESCRIPTION OF DRAWING(S) - The figure illustrates a schematic elevational view of a solar cell.    First layer (22)    Second layer (24)    Third layer (26)    First buffer layer (42)    Second buffer layer (44)