The minority carrier diffusion length, which is one of the most important parameters to evaluate qualities of solar cell materials, is strongly connected with surface photovoltage (SPV) appearing under light illumination because the SPV depends on how diffusive the photo-carriers are in those materials. We have already shown that the SPV is measurable by Kelvin probe force microscopy (KFM), which is based on atomic force microscopy (AFM), with high spatial resolution. In this paper, we report on the diffusion length measurements by KFM on a polycrystalline silicon solar cell fabricated on a p-type substrate with a phosphorus doped surface n-layer of approximately 500 nm in thickness.
Our KFM system is based on a commercial AFM system operating in a high vacuum at room temperature and in tapping mode with piezo-resistive cantilever. The sample surface beneath the AFM tip end was illuminated by a monochromatic light in a wavelength range between 890 and 990 nm from Ti:Al2O3 laser system, and the SPV was determined by a potential change under the light illumination from the dark condition. Then the minority carrier diffusion length was evaluated from numerical fitting of the dependence of the photovoltage on the incident wavelength experimentally obtained at a constant photon density.
As a result, the minority carrier diffusion lengths evaluated on a single grain of the polycrystalline silicon solar cell were 84, 92 and 93 µm at various photon densities (6.0, 7.2 and 12.0×1016 /cm2s), respectively, indicating that the reproducibility of the diffusion length measurements was better than ±5%. In addition, the derived values themselves are reasonable compared with a typical value of the diffusion length in the polycrystalline silicon solar cell. Therefore, the capability and reproducibility of our method have been confirmed.
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