Application of grain boundary function to develop new materials for solar cells with high performance

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The motivation of this work was to study the origin of grain boundary function suitable for achieving enhanced conversion efficiency of microcrystalline CdTe solar cells, and then to obtain a principle of grain boundary design for development of new materials for solar cells with high performance. We have found the potential barrier associated with grain boundaries in CdTe depends not only on grain boundary character but also on polarity at grain boundaries arising from its crystal structure. In addition, the CdCl2 treatment, which increases the conversion efficiency of CdTe solar cells, was found to increase the grain boundary potential barrier. These findings show that grain boundaries in CdTe can act as the built-in potential for separation of photogenerated hole - electron pairs, and that Cl segregation to grain boundaries after the CdCl2 treatment would result in the enhancement of the role of the built-in potential rather than the passivation of defect states in the band-gap.