Application of Advanced Microstructural and Microchemical Microscopy Techniques to Chalcopyrite Solar Cells.
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Application of Advanced Microstructural and Microchemical Microscopy Techniques to Chalcopyrite Solar Cells. Changhui Lei, Chun-Ming Li, Angus Rockett, I. M. Robertson, and W. Shafarman+ Department of Materials Science and Engineering, University of Illinois, Urbana IL 61801 + Institute of Energy Conversion, University of Delaware, Newark DE 19716. ABSTRACT Analytical TEM techniques have been used to characterize the structure and chemistry of chalcopyrite solar cell devices produced by elemental evaporation using a bi-layer process at Ts=400°C and Ts=550°C. Dislocations, stacking faults, twins and voids exist in both materials but at a reduced density at the higher deposition temperature. The higher processing temperature also improved the density and structure of the grain boundaries and created a less rough surface. The CdS layer coats the surface conformally, although the roughness impacts the structure. CdS fills the grooves in the surface and infiltrates lower density grain boundaries to significant depths. The chemistry of the grain boundaries does not appear to be significantly impacted by the presence of the Cd and S. INTRODUCTION The demand for energy continues to increase. Recent forecast estimates suggest an average annual rate of increase of ~1.5 % through 2025. Photovoltaic devices are expected to see greater use, in part due to local mandates and more generally later as they become economically favorable in grid-connected applications. To achieve the needed advances, production, performance and reliability of photovoltaic devices must be optimized. The maximum efficiency of thin film solar cells based on Cu(In1–xGax)Se2 (CIGS) alloys is 19.2%.[1] While this is noteworthy, there is still a lack of understanding of the fundamental mechanisms limiting device efficiency. For example, the structure and chemistry at the nano-scale of the CIGS absorber layer and its surrounding grain boundaries, interfaces, and the heterojunction are not understood. In the last decade, there have been significant advances in analytical electron microscopy that present new opportunities to interrogate materials with improved spatial and chemical resolution. For example, the shift from a thermionic to a field emission electron source impacts chemical analysis through the spatial resolution, the minimum distance between two successive volumes from which an independent measurement can be obtained; the minimum mass fraction, the smallest concentration (wt. %) of an element measurable in the sample volume; and the minimum detectable mass, minimum number of atoms that can be detected in the sample volume. For a 10 nm thick sample and using a field emission source, a spatial resolution on the order of the probe diameter < 2 nm results in a corresponding minimum mass fraction of < 0.1 wt% and a minimum detectable mass on the order of 10-21g.[2] With a thermionic source, the
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probe diameter is 10nm, and the minimum mass fraction is ~1 wt%. Further improvements are possible using higher accelerating voltages, because o
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