Nanotech 2010

Next-Generation Photovoltaics Research at the Colorado School of Mines (invited presentation)

K.R. Williams
Colorado School of Mines, US

Keywords: photovoltaics, silicon, nanowires, nanostructures


Meeting the world’s energy needs is one of the most significant challenges we face in the coming century. The National Science Foundation sponsored Renewable Energy Materials Science and Engineering Center (REMRSEC) at the Colorado School of Mines, in collaboration with the National Renewable Energy Laboratory, is focused on transformative materials advances and educational directions that greatly impact emerging renewable energy technologies. Interactions with numerous companies provide practical input into research directions. Our photovoltaics research engages a multidisciplinary team of scientist and engineers in an integration of unique materials synthesis, novel characterization and computational interrogation. This effort is directed at developing a fundamental understanding of advanced materials, nanoscale architectures, and novel energy harvesting concepts with the potential for substantial photovoltaic cost reductions and efficiency improvements. This presentation will focus on the synthesis and characterization of nanomaterials and nanostructures for next-generation photovoltaics. Group IV elements are of particular interest and have been synthesized as nanocrystalline silicon films, isolated silicon nanocrystals (NC), nanowires (NW), and NW arrays. SiNCs with diameters less than 5 nm have demonstrated a size-dependent tunable band gap, visible photoluminescence, and, in reports from other research groups, multiexciton generation. These properties make them a promising material for third-generation photovoltaic devices. Semiconductor NW arrays have also demonstrated a potential for tunable properties through radial quantum confinement effects and large surface-to-volume ratios and in this case, while maintaining a topology consistent with efficient carrier transport and collection. SiNCs and SiNWs have been synthesized using plasma enhanced chemical vapor deposition (PECVD) in a SiH4 plasma. For NCs, the advantage over solution synthesis is that these plasma synthesized nanoparticles do not agglomerate as they acquire a negative charge in the discharge. With both NC and NWs, plasma synthesis allows novel heterostructures in which nanostructures are embedded in an alternative matrix material. Experiments are underway to understand the PECVD process parameters that control the size, size distribution, and aspect ratios of these NCs and NWs. Furthermore, surface passivation of these nanostructures and the use of different nanoseeds as catalysts to effect vapor-liquid-solid growth of SiNWs are key elements of this research. Initial studies have shown that SiNWs grown from Sn seeds are thicker and less tapered than those grown from the commonly used Au seeds. The third aspect of our synthesis work takes a partial solution synthesis approach to the fabrication of SiNW arrays. Highly ordered mesoporous (~4 nm pores) silica templates are initially formed by electro-assisted self assembly of surfactant micelles. PECVD is used to grow SiNWs within the nanopores and the silica template is subsequently removed to yield the SiNW array. Extensive characterization studies are conducted on these nanoscale materials including electron microscopy, infrared and Raman spectroscopy, photoluminescence, small angle Xray scattering, time resolved studies, and electron spin resonance. These results, when integrated with computational interrogation, offer the real possibility of achieving higher efficiency and lower cost production of photovoltaic devices. Support from NSF-DMR 0820518 and the Center for Revolutionary Solar Photoconversion is gratefully acknowledged.
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