Scanning Light-Spot Measurements on Grain Boundaries in Polycrystalline-Silicon Solar Cells
Esman, Ronald D.
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The theoretical electrical effects of grain boundaries in polycrystalline' silicon are discussed. Zook's model for the electrical effects of a grain boundary is reviewed. Then experimental techniques, with which to test and apply the model, are discussed. A microprocessor-controlled XY table was used to generate scans of a light spot across grain boundaries in polycrystalline-silicon solar cells. An interface was designed to duplicate the induced-current response on an XY-recorder display. Zook's model was then checked for validity by the following measurements. Grain-boundary scans were performed at two laser wavelengths (.6328µm and 1.152µm) and with a filtered, collimnated tungsten source. The grain-boundary surface recombination velocity, s. was determined to be (2.3 ± 0.2) x 10 3 cm/sec. The intragranular minority-carrier diffusion length, L. was also determined: L = (200 ± 25)µm for Monsanto 1mm grain-size material. After passivation, s was reduced to (3.5 ± 0.1) x 10 2 cm/sec. and L was reduced to (140 ± 15µm. The L measurement was observed to be independent of the wavelength of the incident light. which suggest that Zook's model of the intragranular material is valid. However. the observed dependence of s on the wavelength of the incident light does not support Zook's grain-boundary model, but may be explained within the model.