Sub-micron Optoelectronic Properties of Polycrystalline Solar Cell Materials

Abstract

Generation, transport and collection of carriers in polycrystalline (PX) solar cells and their constituent materials are poorly understood, and significantly different than in their single-crystal counterparts. Recent theoretical and experimental results have put forth the expectation that grain boundaries in PX-solar cell materials such as CdTe and CuInGaSe₂, either as-grown or after appropriate post-growth treatment, may have electronic properties which are advantageous to charge separation and solar cell operation[1-3]. However, a microscopic picture of the spatial variations in the optoelectronic properties of these materials is, for the most part, still lacking. The goal of the work reported here is to explore the optoelectronic and spectroscopic properties of grain-boundaries in these materials at the nanometer length-scale, via novel, high-resolution optical techniques. Towards this end, a significant enhancement in photo-response near grain boundaries in CdTe solar cells, consistent with models put forth in reference 2, was observed via near-field Optical Beam Induced Current (n-OBIC) [4]. A systematic μ-PL study of the effect of CdC12-treatment on recombination in CdTe/CdS solar cell structures of varying thickness directly examined the variation in optoelectronic properties at grain-boundaries in this material, revealing the grain-boundary and surface passivation effects of this important post-growth processing step. For comparison, we also studied the effects of SiN_x post-growth treatment and annealing on the photo-response of PX-silicon solar cells using n-OBIC. These results and our most-recent n-OBIC measurements in CdTe and CuInGaSe₂ solar cells are discussed.