Engineering topics
Publications and source records attributed to Robinson, P. H..
High-efficiency solar cells made in thin epitaxial films grown on low-cost commercial silicon substrates. Cost of cells is much less than if high-quality single-crystal silicon were used for substrates and performance of cells is almost as good.
Epitaxial deposition produces acceptable solar cells from metallurgicalgrade silicon. Instead of diffusing dopants into silicon to form pn junction, junction is formed by growing epitaxial layer--one having crystal structure continuous with that of substrate--on metallurgical-grade silicon. Less pure forms of silicon, such as metallurgical-grade, are less expensive than semiconductor-grade material and help to lower cost of solar cells.
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The epitaxial procedures, solar cell fabrication, and evaluation techniques are described. The development of baseline epitaxial solar cell structures grown on high quality conventional silicon substrates is discussed. Diagnostic layers and solar cells grown on four potentially low cost silicon substrates are considered. The crystallographic properties of such layers and the performance of epitaxially grown solar cells fabricated on these materials are described. An advanced epitaxial reactor, the rotary disc, is described along with the results of growing solar cell structures of the baseline type on low cost substrates. The add on cost for the epitaxial process is assessed and the economic advantages of the epitaxial process as they relate to silicon substrate selection are examined.
Growth and fabrication procedures for the baseline solar cells are described along with measured cell parameters, and the results. Reproducibility of these results was established and the direction to be taken for higher efficiency is identified.
This work is directed toward a low-cost approach to the fabrication of silicon solar cells and combines the advantages and flexibility of the epitaxial technique with the use of potentially low-cost silicon substrates. Epitaxial graded base n/p/p+ solar cell structures were grown and evaluated on various forms of polycrystalline and metallurgical grade silicon as well as single crystal substrates. X-ray topographic techniques have shown improvements in the crystallographic quality of the layer grown on polycrystalline material.
Dichlorosilane (SiH2Cl2) was used as the silicon source material in all of the epitaxial growths. Both n/p/p(+) and p/n/n(+) structures were studied. Correlations were made between the measured profiles and the solar cell parameters, especially cell open-circuit voltage. It was found that in order to obtain consistently high open-circuit voltage, the epitaxial techniques used to grow the surface layer must be altered to obtain very abrupt doping profiles in the vicinity of the junction. With these techniques, it was possible to grow reproducibly both p/n/n(+) and n/p/p(+) solar cell structures having open-circuit voltages in the 610- to 630-mV range, with fill-factors in excess of 0.80 and AM-1 efficiencies of about 13%. Combinations and comparisons of epitaxial and diffused surface layers were also made. Using such surface layers, we found that the blue response of epitaxial cells could be improved, resulting in AM-1 short-circuit current densities of about 30 mA/cm sq. The best cells fabricated in this manner had AM-1 efficiency of 14.1%.
P+/p/n/n+ epitaxial silicon solar-cell structures were fabricated with the n-base region intentionally graded over 2 orders of magnitude in a thickness of 35 microns. Solar cells made from these layers exhibited near-ideal I-V characteristics in the practical operating range, with an open-circuit voltage of 636 mV, fill factor of 0.79, and AM1 efficiency of 12.6%. A similar epitaxial structure containing no gradient in the n-base showed good solar-cell performance but lower open-circuit voltage (603 mV).
Silicon epitaxy has been studied for the fabrication of solar cell structures, with the intent of optimizing efficiency while maintaining suitability for space applications. SiH2CL2 yielded good quality layers and junctions with reproducible impurity profiles. Diode characteristics and lifetimes in the epitaxial layers were investigated as a function of epitaxial growth conditions and doping profile, as was the effect of substrates and epitaxial post-gettering on lifetime. The pyrolytic decomposition of SiH4 was also used in the epitaxial formation of highly doped junction layers on bulk Si wafers. The effects of junction layer thickness and bulk background doping level on cell performance, in particular, open-circuit voltage, were investigated. The most successful solar cells were fabricated with SiH2 CL2 to grow p/n layers on n(+) substrates. The best performance was obtained from a p(+)/p/n/n(+) structure grown with an exponential grade in the n-base layer.