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Microstructure of Amorphous-Silicon-Based Solar Cell Materials by Small-Angle X-Ray Scattering; Final Subcontract Report: 6 April 1994 - 30 June 1998

Description: This report describes work performed to provide details of the microstructure in high-quality hydrogenated amorphous silicon and related alloys for the nanometer size scale. The materials studied were prepared by current state-of-the-art deposition methods, as well as new and emerging deposition techniques. The purpose was to establish the role of microstructural features in controlling the opto-electronic and photovoltaic properties. The approach centered around the use of the uncommon technique of small-angle X-ray scattering (SAXS), which is highly sensitive to microvoids and columnar-like microstructure. Nanovoids of H-rich clusters with 1 to 4 nm sizes in a-Si:H at the 1 vol.% level correlate with poor solar-cell and opto-electronic behavior. Larger-scale features due either to surface roughness or residual columnar-like structures were found in present state-of-the-art device material. Ge alloying above about 10 to 20 at.% typically leads to significant increases in heterogeneity , and this has been shown to be due in part to non-uniform Ge distributions. Ge additions also cause columnar-like growth, but this can be reduced or eliminated by enhanced ion bombardment during growth. In contrast, C alloying typically induces a random nanostructure consisting of a narrow size distribution of 1-nm-sized objects with a high density, consistent with the notably poorer opto-electronic behavior of these alloys.
Date: December 8, 1998
Creator: Williamson, D.L. (Department of Physics: Colorado School of Mines)
Partner: UNT Libraries Government Documents Department

Device Physics of Thin-Film Polycrystalline Cells and Modules; Final Subcontract Report; 6 December 1993-15 March 1998

Description: This report describes work performed under this subcontract by Colorado State University (CSU). The results of the subcontract effort included progress in understanding CdTe and Cu(In1-xGax)Se2-based solar cells, in developing additional measurement and analysis techniques at the module level, and in strengthening collaboration within the thin-film polycrystalline solar-cell community. A major part of the CdTe work consisted of elevated-temperature stress tests to determine fabrication and operation conditions that minimize the possibility of long-term performance changes. Other CdTe studies included analysis of the back-contact junction, complete photon accounting, and the tradeoff with thin CdS between photocurrent gain and voltage loss. The Cu(In1-xGax)Se2 studies included work on the role of sodium in enhancing performance, the conditions under which conduction-band offsets affect cell performance, the transient effects of cycling between light and dark conditions, and detailed analysis of several individual series of cells. One aspect of thin-film module analysis has been addressing the differences in approach needed for relatively large individual cells made without grids. Most work, however, focused on analysis of laser-scanning data, including defect signatures, photocurrent/shunting separation, and the effects of forward bias or high-intensity light. Collaborations with other laboratories continued on an individual basis, and starting in 1994, collaboration was through the national R&D photovoltaic teams. CSU has been heavily involved in the structure and logistics of both the CdTe and CIS teams, as well as making frequent technical contributions in both areas.
Date: May 3, 1999
Creator: Sites, J. R. (Department of Physics, Colorado State University, Ft. Collins, Colorado)
Partner: UNT Libraries Government Documents Department

The Growth of Thin Epitaxial Copper Films on Ruthenium (0001)and Oxygen-Precovered Ruthenium (0001) as studied by x-rayphotoelectron diffraction. University of California, Davis, Department of Physics, Ph.D. Thesis

Description: In the first part of this dissertation, the variation of mean emitter depths with direction for core photoelectron emission from single crystals, including the effects of both isotropic inelastic scattering and single and multiple elastic scattering was theoretically studied. The mean emitter depth was found to vary by as much as &plusmn;30% with direction. In the second part of this dissertation, x-ray photoelectron diffraction (XPD) was used to study the structure and growth mechanisms of Cu films grown on a clean and an oxygen-precovered Ru(OOO1) surface. Experimental Cu 2p3/2 (E<sub>kin</sub> = 556 eV) and Ru 3d (E<sub>kin</sub> = 1205 eV) intensities were measured for Cu coverages from submonolayer up to several monolayer (ML) on the clean Ru(OOO1) surface. In addition, the O 1s (E<sub>kin</sub> = 958 eV) intensity was measured for Cu grown on oxygen precovered Ru(OOO1). These XPD intensities have been analyzed using single scattering cluster (SSC) and multiple scattering cluster (MSC) models. The first Cu layer has been found to grow pseudomorphically on the Ru(OOO1) surface in agreement with prior studies of the Cu/Ru(OOO 1) system. Thus, the initial growth is layer-by-layer. For higher coverages, XPD shows that the short-range structure of the Cu films is fcc Cu(l 11), but with significant interlayer relaxation (compared to bulk Cu(l 11)) that persists up to {ge}8 ML. When oxygen is preadsorbed on the Ru(OOO1) surface before Cu film growth (possibly to act as a surfactant promoting smoother growth), XPD shows that the first ~3 ML of Cu grow as 3-D islands. In addition, XPD shows that, during Cu growth, all of the oxygen "floats" on the CU surface, in contrast to prior studies which found that 30% of the oxygen remains at the Cu/Ru intetiace. XPD also indicates that the oxygen is highly disordered on the Cu overlayer surface. In ...
Date: June 1, 1997
Creator: Ruebush, Scott Daniel
Partner: UNT Libraries Government Documents Department

Research on High-Bandgap Materials and Amorphous Silicon-Based Solar Cells, Final Technical Report, 15 May 1994-15 January 1998

Description: This report describes work performed by Syracuse University under this subcontract. Researchers developed a technique based on electroabsorption measurements for obtaining quantitative estimates of the built-in potential Vbi in a-Si:H-based heterostructure solar cells incorporating microcrystalline or a-SiC:H p layers. Using this new electroabsorption technique, researchers confirmed previous estimates of Vbi {yields} 1.0 V in a-Si:H solar cells with ''conventional'' intrinsic layers and either microcrystalline or a-SiC:H p layers. Researchers also explored the recent claim that light-soaking of a-Si:H substantially changes the polarized electroabsorption associated with interband optical transitions (and hence, not defect transitions). Researchers confirmed measurements of improved (5') hole drift mobilities in some specially prepared a-Si:H samples. Disturbingly, solar cells made with such materials did not show improved efficiencies. Researchers significantly clarified the relationship of ambipolar diffusion-length measurements to hole drift mobilities in a-Si:H, and have shown that the photocapacitance measurements can be interpreted in terms of hole drift mobilities in amorphous silicon. They also completed a survey of thin BP:H and BPC:H films prepared by plasma deposition using phosphine, diborane, trimethylboron, and hydrogen as precursor gases.
Date: December 28, 1998
Creator: Schiff, E. A.; Gu, Q.; Jiang, L.; Lyou, J.; Nurdjaja, I. & Rao, P. (Department of Physics, Syracuse University)
Partner: UNT Libraries Government Documents Department

Analysis of Cu Diffusion in ZnTe-Based Contacts for Thin-Film CdS/CdTe Solar Cells

Description: Ohmic contacts to thin-film CdS/CdTe photovoltaic devices have been formed using a two-layer contact interface of undoped ZnTe (ZnTe) and Cu-doped ZnTe (ZnTe:Cu), followed by Ni or Ti as an outer metallization. Secondary ion mass spectroscopy (SIMS) is used to study Cu diffusion within this back-contact structure, and also, to monitor Cu diffusion from the contact into the CdTe. When Ni metallization is used, the ZnTe:Cu layer becomes increasingly depleted of Cu, and Ni diffusion into the ZnTe:Cu increases as the contact deposition temperature increases from 100 C to 300 C. Cu depletion is not observed when Ni is replaced with Ti. Diffusion of Cu from the ZnTe:Cu layer into the ZnTe layer also increases with contact deposition temperature, and produces a buildup of Cu at the ZnTe/CdTe interface. High-mass resolution SIMS indicates that, although Cu levels in the CdTe remain low, Cu diffusion from the contact proceeds into the CdTe layer and toward the CdTe/CdS junction region.
Date: October 27, 1998
Creator: Narayanswamy, C. (Department of Physics & Astronomy, University of Toledo) & Gessert, T. A. and Asher, S. E. (National Renewable Energy Laboratory)
Partner: UNT Libraries Government Documents Department