Book Description

The metalorganic chemical vapor deposition (MO-CVD) technique has been applied to the growth of thin films of GaAs and GaAlAs on inexpensive polycrystalline or amorphous substrate materials (glasses, glass-ceramics, alumina ceramics, and metals) for use in fabrication of large-area low-cost photovoltaic device structures. Trimethylgallium (TMG), arsine (AsH3), and trimethylaluminum (TMAl) are mixed in appropriate concentrations at room temperature in the gaseous state and pyrolyzed at the substrate, which is heated in a vertical reactor chamber to temperature in the range 600 to 800°C, to produce the desired film composition and properties. Of ten candidate low-cost substrates initially identified for investigation, Corning Code 0317 glass and composites of CVD Ge/glass and sputtered Mo/glass were found to be the most satisfactory, the latter eventually serving as a reference substrate against which to compare the performance of other substrates. Single-crystal window-type solar cells, polycrystalline Schottky-barrier cells, and deposited-junction polycrystalline cells have been grown, fabricated, and characterized. Epitaxial GaAlAs/GaAs p-n junction cells with thin (~500Å) Ga02Al08As windows and GaAs:Zn - GaAs:Se junctions were made with AMO efficiencies as high as 12.8 percent with no AR coating, indicating the high quality of the films grown by the MO-C+VD process. Schottky barrier cells with efficiencies of 2.25 percent AMO (no AR coating) have been made on n/n polycrystalline GaAs structures on Mo/glass composite substrates, with short-circuit current densities up to 12.5 mA/cm2. Also, results of analyses of material and processing costs associated with fabrication of thin-film GaAlAs/GaAs solar cells by the MO-CVD process are discussed.