Transparent Conductors and Barrier Layers for Thin Film Solar Cells: Final Technical Report, 15 June 2001

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' (glass front surface/transparent electrically conductive oxide (TCO)/pin amorphous silicon/metal back electrode). 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 zincoxide (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 inefficiency. This problem was recently mitigated by using a new proprietary p/buffer layer combination developed at BP Solar.