Crystalline Silicon Solar Cells
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Most solar cells are fabricated from crystalline or semicrystalline silicon since they are relatively inexpensive starting materials and the resulting solar cells are very efficient. As a result, the optical properties of silicon are extremely important in many aspects of solar cell manufacture, and have been determined by many groups using several techniques. The most reliable values of the optical functions of silicon have been determined using spectroscopic ellipsometry, augmented by optical transmission measurements of the absorption coefficient for wavelengths greater than 700 nm. Obviously, these optical functions depend on the wavelength of light, but they also depend significantly on temperature and morphology. Several thin films are very important to solar cell manufacture, including silicon nitride, silicon dioxide, and aluminum oxide. While the optical properties of these thin films are strong functions of deposition conditions, spectroscopic ellipsometry is ideal for characterizing them. This work will present recent spectroscopic ellipsometry data and optical transmission data from which the optical functions of silicon are obtained. The optical transmission data have been fit from 700 to 1200 nm to a modification of Macfarlane’s et al. formula [Phys. Rev. 111, (1958) 759], resulting in a reduced χ2 of 0.84. This formulation is particularly valuable in that it gives the optical absorption coefficient from 700 to 1200 nm as a function of both wavelength and temperature.
GEJ acknowledges Oak Ridge National Laboratory for the use of facilities in the preparation of this manuscript. The work of PCJ was supported by the Department of Energy, Laboratory Directed Research and Development program at Oak Ridge National Laboratory, under contract DE-AC05-00OR22725.
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