Effects of Grain Boundaries on Silicon Solar Cell Performances
The characteristics of single-crystal solar cells and large grain (~1mm size) polycrystalline silicon solar cells were compared to determine the effects grain boundaries had on solar cell performance. Using the van der Pauw-Hall technique, the mobility of the majority charge carriers in the base regions of the single-crystal cells was found to be 1400 cm/V-sec compared to 400 cm/V-sec for the polycrystalline cells. The van der pauw-Hall measurements for the emitter regions in both types of cells were very similar indicating that the grain boundaries had been passivated by heavy doping in this region. These measurements seemed to agree with J. W. Seto's carrier trapping model. Spectral response and surface photovoltage measurements showed that the effective minority-carrier diffusion length in the polycrystalline cells was less than the minority-carrier diffusion length in the single-crystal solar cells. Scanning laser-spot techniques were used to demonstrate the recombination effect at the grain boundaries. The p-n junction solar cell performance tests without anti-reflective coating showed that the efficiencies of the polycrystalline cells (about 6%) were less than the efficiencies of the single-crystal cells (about 9%). These measurements seem to indicate that grain boundaries had a significant effect on the performance of polycrystalline silicon solar cells.
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