Vacuum deposited polycrystalline silicon films for solar cell applications. Final report, 29 September 1978-30 September 1979
The DOE goal of obtaining low-cost, 10% efficient solar cells was approached through the development of a potentially all-vacuum fabrication process incorporating vacuum deposited, thin-film, polycrystalline silicon. Experimental devices were constructed in layers as follows: alumina substrate/TiB/sub 2/ bottom electrode/p-type polycrystalline silicon film/diffused silicon n-region/Ti-Ag electrode. An essential feature of this design is the TiB/sub 2/ bottom electrode which is stable with respect to silicon up to 1200/sup 0/C and remains conducting after the silicon deposition. Grain growth, film composition and interfacial analyses, and photovoltaic characteristics were investigated using secondary-ion mass spectrometry (SIMS), X-ray diffraction and conventional electronics measurements techniques. Average grain diameters in the films parallel to the substrate were 5 ..mu..m. Phosphorus was diffused into the silicon film using standard techniques to create n-p junctions. Al, Ti, and Ti/Ag electrodes were used as top contacts to the n-type layer. Typical of the best photovoltaic responses from small grained devices without antireflection coating or electrode geometry optimization were V/sub oc/ = 0.26V, J/sub sc/ = 16 mA/cm/sup 2/, efficiency 2.2% and fill factor 0.54. The highest V/sub oc/ observed to date is 0.38V; the highest efficiency is 2.6%. Carrier concentrations were obtained by both Hall effect and SIMS measurements;more » diffusion lengths were determined by the short-circuit current vs incident light wavelength method. SIMS was used extensively in the examination of interactions at each interface. Results are presented and discussed.« less