Sub-micron Optoelectronic Properties of Polycrystalline Solar Cell Materials
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Sub-micron Optoelectronic Properties of Polycrystalline Solar Cell Materials S. Smith†, R. Dhere, T. Gessert, P. Stradins, T. Wang, K. Ramanathan, R. Noufi and A. Mascarenhas National Renewable Energy Laboratory 1617 Cole Blvd, Golden, CO 80401, USA † [email protected] 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 CdCl2-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 SiNx 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. INTRODUCTION Grain-boundary passivation is a key step in all thin-film photovoltaic technologies, as it substantially improves the efficiency and useful life of these devices. However, a microscopic picture of grain-boundary passivation is still lacking in most thin-film solar cell materials. It has been recently speculated that grain boundaries, after passivation, may actually improve solar cell performance [2]. The work reported here focuses on revealing the variation in optoelectronic properties of several important polycrystalline solar cell materials on the sub-micron length scale, and their affect on device performance. Two approaches are discussed: the first utilizes low-temperature μ-PL to assess the relative contributions of grain boundary and surface passivation in PX-CdTe/CdS solar cell material, the second, employs Near-Field Scanning Optical Microscopy (NSOM) to locally probe the photo-response directly at grain boundaries in PX-CdTe/CdS, PX-CuInGaSe2 and PX-Si solar cells. EXPERIMENT Using low-temperature, μ-PL, we investigate a series of CdTe/
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