NOVELTY - The thin film solar cell comprises a transparent conducting substrate, an absorber layer formed on the transparent conducting substrate, a buffer layer formed on the absorber layer, a window layer arranged on the buffer layer, and an electrode arranged on the window layer. The absorber layer is formed by a solution method, and has a band-gap of 1.5 eV and a thickness of less than or equal to 1000 nm or 400 plus minus 20 nm, 800 plus minus 20 nm, and 1200 plus minus 20 nm. The thin film solar cell has an average transmittance of 10-20% in the wavelength of 600-750 nm as determined by UV-Vis spectroscopy.    USE - The thin film solar cell is useful in a window (claimed) such as building integrated photovoltaic window, where the thin film solar cell is useful a bifacial copper indium gallium selenide thin film solar cell.    ADVANTAGE - The thin film solar cell can be fabricated in a cost effective and safe manner with high throughput, power conversion efficiency, light transmission characteristics and stability.    DETAILED DESCRIPTION - The thin film solar cell comprises a transparent conducting substrate, an absorber layer formed on the transparent conducting substrate, a buffer layer formed on the absorber layer, a window layer arranged on the buffer layer, and an electrode arranged on the window layer. The absorber layer is formed by a solution method, and has a band-gap of 1.5 eV and a thickness of less than or equal to 1000 nm or 400 plus minus 20 nm, 800 plus minus 20 nm, and 1200 plus minus 20 nm. The thin film solar cell has an average transmittance of 10-20% in the wavelength of 600-750 nm as determined by UV-Vis spectroscopy. The band-gap is controlled by varying the ratio of indium/gallium in the absorber layer. The solution method is selected from printing, doctor blade coating, screen printing, spin coating, and/or ink-jet printing. The absorber layer is divided into two absorber layers comprising a first absorber layer and a second absorber layer. The thicknesses of the first absorber layer and the second absorber layer are identical to or different from each other and are each independently of 400 plus minus 20 nm, 800 plus minus 20 nm, and 1200 plus minus 20 nm. INDEPENDENT CLAIMS are included for:    (1) a method for manufacturing a thin film solar cell;    (2) a method for controlling a band-gap of a photoactive layer of a thin film solar cell; and    (3) a method for controlling the transmittance of a photoactive layer of a thin film solar cell.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of simultaneous generation of electricity from a semi-transparent, bifacial thin film solar cell for a window by outdoor sunlight and indoor illumination.