TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Parameter: In the thin film solar cell above, the first back reflection layer is formed of a material that has an absorption coefficient equal to or less than 400 cm-1 with respect to a solar light component having a wavelength equal to or greater than 700 nm. The first back reflection layer is formed of a material that has a refractive index of 1.5-2.5 at a wavelength of 800 nm, where the first intrinsic layer is formed of a material that has a refractive index of 3-5 at a wavelength of 800 nm. The back reflection layer further includes a second back reflection layer formed of a material that has an absorption coefficient equal to or greater than 400 cm-1 with respect to a solar light component having a wavelength equal to or greater than 700 nm.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The first intrinsic layer contains one of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon ( mu cSi:H). The first back reflection layer is formed of one of n-type or p-type hydrogenated microcrystalline silicon oxide ( mu c-SiOx:H), n-type or p-type hydrogenated microcrystalline silicon nitride ( mu cSiNx:H), and n-type or p-type hydrogenated microcrystalline silicon oxynitride ( mu c-SiOxNy:H). The second back reflection layer is formed of one of aluminum-doped zinc oxide (AZO) and boron-doped zinc oxide (BZO). The thin film solar cell further comprises a middle reflection layer between the first photoelectric conversion unit and the second photoelectric conversion unit, where the middle reflection layer includes a first middle reflection layer directly contacting the first intrinsic layer, and the first middle reflection layer is formed of one of n-type or p-type mu c-SiOx:H, n-type or p-type mu c-SiNx:H, and n-type or p-type mu cSiOxNy:H. The middle reflection layer further includes a second middle reflection layer directly contacting both the first middle reflection layer and the second intrinsic layer, and the second middle reflection layer is formed of one of n-type or p-type hydrogenated microcrystalline silicon oxide ( mu c-SiOx:H), n-type or p-type hydrogenated microcrystalline silicon nitride ( mu c-SiNx:H), and n-type or p-type hydrogenated microcrystalline silicon oxynitride ( mu c-SiOxNy:H). The first photoelectric conversion unit further includes a p-type doped layer, and the middle reflection layer includes a first middle reflection layer directly contacting the p-type doped layer of the first photoelectric conversion unit, and the first middle reflection layer is formed of one of AZO and BZO.