This report describes the research undertaken to increase the efficiency of thin-film solar cells based on amorphous silicon in the so-called 'superstrate structure.' The TCO layer must meet many requirements: high optical transparency in the wavelength region from about 350 to 900 nm low electrical sheet resistance stability during handling and deposition of the subsequent layers and during use a textured (rough) surface to enhance optical absorption of red and near-infrared light and low-resistance electrical contact to the amorphous silicon p-layer. Fluorine-doped tin oxide has been the TCO used in most commercial superstrate amorphous silicon cells. Fluorine-doped zinc oxide (ZnO:F) was later shown to be even more transparent than fluorine-doped tin oxide as well as being more resistant to the strongly reducing conditions encountered during the deposition of amorphous silicon. Solar cells based on ZnO:F showed the expected higher currents but the fill factors were lower than standard cells grown on tin oxide resulting in no consistent improvement in efficiency. This problem was recently mitigated by using a new proprietary p/buffer layer combination developed at BP Solar.