The optical absorption coefficient of silicon solar cell material grown by three low-cost growth methods was measured in the wavelength interval 0.8≤λ≤1.0 μm, the wavelength region of interest in surface photovoltage measurements of the minority carrier diffusion length. The square root of the absorption coefficient was found to vary linearly with photon energy over the wavelengths studied, and the measured data agree with a linear empirical fit to within 0.5% RMS. The absorption coefficients obtained are slightly lower than those reported by Runyan, with the greatest disagreement at long wavelengths. Minority carrier diffusion lengths computed using the present absorption coefficients are approximately 16% greater than those calculated using Runyan’s data. Excellent sample-to-sample agreement within and between lots indicates that for two of the growth methods studied, material quality as judged by optical properties has not been sacrificed by the use of low-cost growth methods. Samples grown by the third growth method studied showed measurably poorer optical quality.
One of the indicators which determine a material's potential for use as a solar cell is the minority carrier diffusion length (LD) of the material. To determine LD a surface photovoltage (SPV) technique is used.This method is dependent upon an accurate knowledge of the optical absorption coefficient as function of wavelength. Most of the work to date in this area has involved Czochralski grown material which has resulted in two empirical models for the absorption coefficients. However, with the advent of new growth processes, for example ribbon growth and cast silicon, it became necessary to measure the absorption coefficients of these materials before a SPV measurement could be performed.The results for the absorption coefficients for various types of silicon sheet material are compared to those previously used in the two models.The resultant effect upon the diffusion length is also discussed in detail.
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