Optimization of Transparent and Reflecting Electrodes for Amorphous Silicon Solar Cells: Final Technical Report

Transparent conducting fluorine-doped zinc oxide was deposited as thin films on soda-lime glass substrates by atmospheric pressure chemical vapor deposition (CVD) at substrate temperatures of 460 to 500 degrees C. The precursors diethylzinc, tetramethylethylenediamine and benzoyl fluoride were dissolved by xylene. This solution was nebulized ultrasonically and then flash vaporized by a carriergas of nitrogen preheated to 150 degrees C. Ethanol was vaporized separately, ant these vapors were then mixed to form a homogeneous vapor mixture. Good reproducibility was achieved using this new CVD method. Uniform thicknesses were obtained by moving the heated glass substrates through the deposition zone. The best electrical and optical properties were obtained when the precursor solution wasaged for more than a week before use. The films were polycrystalline and highly oriented with the c-axis perpendicular to the substrate. More than 90% of the incorporated fluorine atoms were electrically active as n-type dopants. The electrical resistivity of the films was a low as 5 x 10-4 Wcm. The mobility was about 45 cm2/Vs. The electron concentration was up to 3 x 1020/cm3. The opticalabsorption of the films was about 3%-4% at a sheet resistance of 7 ohms/square. The diffuse transmittance was about 10% at a wavelength of 650 nm. Amorphous silicon solar cells were deposited using the textured fluorine-doped zinc oxide films as the front electrode. The short-circuit current was increased over similar cells made with fluorine-doped tin oxide, but the open-circuit voltages andfill factors were reduced. The voltage was restored by overcoating the fluorine-doped zinc oxide with a thin layer of fluorine-doped tin oxide.