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Identifying the Electronic Properties Relevant to Improving the Performance of High Band-Gap Copper Based I-III-VI2 Chalcopyrite Thin Film Photovoltaic Devices: Final Subcontract Report, 27 April 2004-15 September 2007

Description: This report summarizes the development and evaluation of higher-bandgap absorbers in the CIS alloy system. The major effort focused on exploring suitable absorbers with significant sulfur alloying in collaboration with Shafarman's group at the Institute of Energy Conversion. Three series of samples were examined; first, a series of quaternary CuIn(SeS)2-based devices without Ga; second, a series of devices with pentenary Cu(InGa)(SeS)2 absorbers in which the Se-to-S and In-to-Ga ratios were chosen to keep the bandgap nearly constant, near 1.52 eV. Third, based on the most-promising samples in those two series, we examined a series of devices with pentenary Cu(InGa)(SeS)2 absorbers with roughly 25 at.% S/(Se+S) ratios and varying Ga fractions. We also characterized electronic properties of several wide-bandgap CuGaSe2 devices from both IEC and NREL. The electronic properties of these absorbers were examined using admittance spectroscopy, drive-level capacitance profiling, transient photocapacitance, and transient photocurrent optical spectroscopies. The sample devices whose absorbers had Ga fraction below 40 at.% and S fractions above 20 at.% but below 40% exhibited the best electronic properties and device performance.
Date: August 1, 2008
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Identifying electronic properties relevant to improving the performance and stability of amorphous silicon-based mid-gap and low-gap cells: Annual subcontract report, 16 January 1998--15 January 1999

Description: An overriding theme of the work described in this report has been the effect of partial crystallinity, or the approach to partial crystallinity, on the electronic properties of a-Si:H. This includes, of course, how degradation or the relative stability of these films is affected by the approach to, or onset of, microcrystallinity. The authors first discussed the results on a set of samples produced by dc reactive magnetron sputtering, obtained in collaboration with John Abelson's group at the University of Illinois, for which they demonstrated the existence of a small, but significant, microcrystalline component. For these films, the degradation kinetics was found to be quite unusual; however, it could be well accounted for by a model that postulated two phases of degrading material. One was a-Si:H host material of good quality and the other was a more defective component associated with boundary regions near the microcrystallites. The sub-band-gap photocapacitance measurements on these films also indicated the existence of a distinct feature (a ``shoulder'' with a threshold near 1.1 eV) that signaled the presence of the microcrystalline phase. The second set of samples investigated were produced by Uni-Solar, deposited under conditions of high hydrogen dilution, very close to but just below the microcrystalline phase boundary. Here they found that the defect density following light-induced degradation decreased as the film thickness increased. Corroborating their findings with X-ray diffraction results obtained by Don Williamson on sets of similar films, the authors concluded that the films were becoming more ordered and less defective just prior to the onset of a detectable microcrystalline component. Furthermore, they found that at almost exactly the conditions that Williamson found XRD evidence for the onset of microcrystallinity, they found the appearance of the distinctive ``shoulder'' in the sub-band-gap photocapacitance spectra. Third, they investigated two sets of samples where the ...
Date: March 31, 2000
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Identifying electronic properties relevant to improving stability in a-Si:H-based cells and overall performance in a-Si,Ge:H-based cells. Annual subcontract report, April 18, 1994--April 17, 1995

Description: This report describes work performed by the University of Oregon focusing on the characterization and evaluation of amorphous semiconductor materials produced by novel deposition conditions and/or methods. The results are based on a variety of junction capacitance techniques: admittance spectroscopy, transient photocapacitance (and photocurrent), and drive-level capacitance profiling. These methods allow the determination of deep defect densities and their energy distributions, Urbach bandtail energies, and, in some cases, {mu}{tau} products for hole transport. During this phase, the authors completed several tasks: (1) they carried out measurements on a-Si, Ge:H alloy samples produced at Harvard University by a cathodic glow discharge process, measurement indicated a smaller value of ({mu}{tau}){sub h} for these samples than would have been expected given their lower defect densities; (2) they characterized several hot-wire a-Si:H samples produced with varying hydrogen levels, studies indicate that hot-wire-produced a-Si:H, with H levels between 2--5 at.% should lead to mid-gap devices with superior properties; (3) they reported some results on a-Si:H glow discharge material grown under hydrogen dilution conditions. Preliminary studies point to film strain as playing a primary role for the observed differences in behavior.
Date: November 1, 1995
Creator: Cohen, J.D.
Partner: UNT Libraries Government Documents Department

Identifying Electronic Properties Relevant to Improving Stability in a-Si:H-Based Cells and Overall Performance in a-Si,Ge:H-Based Cells; final Subcontract Report, 18 April 1994-15 January 1998

Description: The work carried out by the University of Oregon Under this subcontract focused on the characterization and evaluation of low-gap (a-Si,Ge:H) alloy materials and on issues related to overall stability in the mid-gap (a-SiH) materials. First, researchers characterized an extensive series of Uni-Solar a-Si,Ge:H samples using drive-level capacitance profiling and the analysis of sub-band-gap photocapacitance and photocurrent spectra. Thus, several bands of deep defect transitions were identified. Researchers were able to verify that charged defects are responsible for the different observed defect bands in device-quality a-Si,Ge:H alloy material. Second, they reported results of their measurements on a-Si,Ge:H alloy ''cathodic'' samples produced at Harvard University; these samples were found to exhibit significantly lower defect densities in the high Ge composition range (>50at.% Ge) than alloy samples produced either by conventional glow discharge of photo-chemical vapor deposition. Third, they performed voltage pulse stimulated capacitance transient measurements on a-Si:H/a-Si,Ge:H heterostructure samples to look for carrier trapping states that might be associated with this interface; they found there was a clear signature of trapped hole emission extending over long times associated specifically with the interface itself in concentrations of roughly 10{sup 11} cm{sup -2}. Fourth, researchers reported the results on several hot-wire a-Si:H samples produced with varying hydrogen levels. Their studies indicate that hot-wire-produced a-Si:H, with H levels between 2-5at.%, should lead to mid-gap devices with superior properties. Finally, they discussed some results on glow-discharge material, as well electron-cyclotron-resonance-deposited a-Si:H grown under hydrogen dilution conditions, and confirmed that, in terms of deep-defect creation, such films exhibited improved stability compared to conventional glow-discharge material.
Date: November 16, 1998
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Identifying Electronic Properties Relevant to Improving the Performance and Stability of Amorphous Silicon Based Photovoltaic Cells: Final Subcontract Report, 27 November 2002--31 March 2005

Description: A major effort during this subcontract period has been to evaluate the microcrystalline Si material under development at United Solar Ovonics Corporation (USOC). This material is actually a hydrogenated nanocrystalline form of Si and it will be denoted in this report as nc-Si:H. Second, we continued our studies of the BP Solar high-growth samples. Third, we evaluated amorphous silicon-germanium alloys produced by the hot-wire chemical vapor deposition growth process. This method holds some potential for higher deposition rate Ge alloy materials with good electronic properties. In addition to these three major focus areas, we examined a couple of amorphous germanium (a-Ge:H) samples produced by the ECR method at Iowa State University. Our studies of the electron cyclotron resonance a-Ge:H indicated that the Iowa State a Ge:H material had quite superior electronic properties, both in terms of the drive-level capacitance profiling deduced defect densities, and the transient photocapacitance deduced Urbach energies. Also, we characterized several United Solar a Si:H samples deposited very close to the microcrystalline phase transition. These samples exhibited good electronic properties, with midgap defect densities slightly less than 1 x 1016 cm-3 in the fully light-degraded state.
Date: November 1, 2005
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Microscopic origins of metastable effects in a-Si:H and deep defect characterization in a-Si,Ge:H alloys. Final subcontract report, February 1, 1991--January 31, 1994

Description: This research supported by NREL Subcontract XG-1-10063-1 over the past three years has involved, first of all, a fairly complete characterization of a two series of a-Si{sub 1-x}Ge{sub x}:H samples: a series of 9 films grown at the University of Delaware by the photo-CVD method (for 0.29 {<=} {times} {<=} 0.62) and series of 6 films grown at U.S.S.C. by the glow discharge method (for 0.20 {<=} {times} {<=} 0.50). Both these series of samples seem to represent what is close to the {open_quotes}state-of-the-art{close_quotes} in current a-Si,Ge:H alloys. The authors detailed comparison of the properties of the glow discharge material with the photo-CVD samples show remarkable similarities rather than significant differences. In particular, measurements of these samples: (1) allowed the assignment of defect energy levels from a detailed analysis of transient sub-band-gap photocapacitance and photocurrent spectra. (2) The authors found the density of deep defects to increase exponentially with the germanium content. (3) The authors found that the trapping lifetimes related {mu}{tau} products for holes decrease in direct proportion to the density of midgap defects in these samples, at least up to Ge fractions of 50at.%. (4) The authors have also made significant progress toward identifying both the optical and thermal defect transitions in the a-Si,Ge:H alloys.
Date: September 1, 1994
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Investigations of the origins of metastable light-induced changes in hydrogenated amorphous silicon. Final subcontract report, April 1, 1988--March 31, 1991

Description: The work performed for this contract continued investigations of the origins of metastable effectS in a-Si:H through three kinds of studies: (1) the effect of carbon impurities in a-Si:H samples at low concentrations using drive-level capacitance profiling measurements on samples whose carbon content was intentionally modulated spatially during growth, (2) the characterization of metastable states in n-type doped a Si:H samples caused by quench cooling and by light-soaking with partial annealing, and (3) the use of depletion-width-modulated ESR spectroscopy together with junction capacitance spectroscopy to investigate deep defect states for various metastable states of a 10- and an 80-Vppm PH{sub 3}-doped a-Si:H sample.
Date: December 1, 1991
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Identifying Electronic Properties Relevant to Improving the Performance and Stability of Amorphous Silicon Based Mid-Gap and Low-Gap Cells: Final Subcontract Report, 16 January 1998-15 October 2001

Description: This report describes our experimental studies which have been concentrated in roughly five areas. Specifically: (1) We have examined a?Si:H grown very close to the microcrystalline phase boundary, so-called''edge material,'' to help understand why such material is more stable with respect to light-induced degradation; (2) We have also studied the electronic properties, and degradation characteristics of mixed phase material that is mostly a?Si:H, but which contains a significant microcrystalline component; (3) We have examined the electronic properties of high deposition rate material. These studies have included both moderately high deposition rate material (up to 6/s) produced by the PECVD growth method, and extremely high deposition rate material (up to 130/s) produced by the HWCVD growth method. (4) We have examined series of a-Si,Ge:H alloys from several sources. In one extensive series of studies we examined low Ge fraction alloys in an attempt to learn more about the fundamentals of degradation in general. In a couple other studies we evaluated the properties of a-Si,Ge:H alloys produced by methods we had not previously examined. (5) Finally, for three different types of samples we compared basic material properties with companion cell performance data. This was carried out in each case on series of samples for which one or more specific deposition parameters were varied systematically.
Date: July 1, 2002
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Tile-based Level of Detail for the Parallel Age

Description: Today's PCs incorporate multiple CPUs and GPUs and are easily arranged in clusters for high-performance, interactive graphics. We present an approach based on hierarchical, screen-space tiles to parallelizing rendering with level of detail. Adapt tiles, render tiles, and machine tiles are associated with CPUs, GPUs, and PCs, respectively, to efficiently parallelize the workload with good resource utilization. Adaptive tile sizes provide load balancing while our level of detail system allows total and independent management of the load on CPUs and GPUs. We demonstrate our approach on parallel configurations consisting of both single PCs and a cluster of PCs.
Date: August 15, 2007
Creator: Niski, K. & Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Microscopic origins of metastable effects in a-Si:H and deep defect characterization in a-Si,Ge:H alloys. Annual subcontract report, 1 February 1991--31 January 1992

Description: This report describes works to use transient photocapacitance and photocurrent measurements to determine the deep defect distribution and processes in low-band-gap a-Si,Ge:H alloys. Samples for these studies were produced by the photochemical vapor deposition (photo-CVD) growth method and were obtained through a collaboration with researchers at the University of Delaware. This report discusses how a detailed comparison between the photocapacitance and photocurrent spectra can be used to separately examine the majority and minority carrier processes. The results are as follows: (1) The midgap defect densities in the alloy regime near 1.3 eV can be as low as 5 {times} 10{sup 16} cm{sup {minus}3} in such photo-CVD samples. (2) There exists a second defect band roughly 0.4 eV below E{sub c} of a similar magnitude to the midgap defect density that exhibits significant lattice relaxation behavior in its electron trapping dynamics. (3) The hole {mu}{tau} products determined for the lowest defect sample are roughly 5 {times} 10{sup {minus}10} cm{sup 2}/V, comparable with the highest hole {mu}{tau} products reported in sandwich geometry measurements for alloys in this composition range. (4) The hole {mu}{tau} is found to be roughly inversely proportional to the midgap defect density for the samples studied. This is consistent with the fact that the effective minority carrier lifetime for such measurements is limited by the deep state trapping time.
Date: July 1, 1992
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Microscopic origins of metastable effects in a-Si:H and deep defect characterization in a-Si,Ge:H alloys. Annual subcontract report, 1 February 1992--31 January 1993

Description: This report describes work to evaluate low-mobility-gap a-Si,Ge:H alloy films. Results are based on junction capacitance techniques of admittance spectroscopy, transient photocapacitance (and photocurrent), and drive-level capacitance profiling. Eight a-Si,Ge:H alloy samples grown by photo-CVD encompassed the range of optical gaps from 1.3 to 1.6 eV, and corresponding Ge fractions from about 20 to 60 at%. We employed junction capacitance techniques to determine deep defect energies and densities, Urbach band-tail energies, and {mu}{tau} products for holes. Electron microprobe analysis provided accurate Ge fractions for our samples, thus enabling us to establish clear trends in measured electronic properties vs Ge fraction. We concluded that these photo-CVD samples exhibited equal or superior properties in terms of band-tail widths, and stable defect densities compared to any reported results on a-Si,Ge:H samples grown by glow discharge. By assigning defect energy levels from analysis of transient subband-gap photocapacitance and photocurrent spectra, we found clear evidence for two distinct defect subbands, one at roughly midgap and the other in the upper half of the gap. The trapping lifetime related {mu}{tau} products for holes decreased in direct proportion to the density of mid-gap defects in these samples. This appears to be the case regardless of whether we are dealing with stable defects or defects created by light-soaking.
Date: August 1, 1993
Creator: Cohen, J. D.
Partner: UNT Libraries Government Documents Department

Similarity-Guided Streamline Placement with Error Evaluation

Description: Most streamline generation algorithms either provide a particular density of streamlines across the domain or explicitly detect features, such as critical points, and follow customized rules to emphasize those features. However, the former generally includes many redundant streamlines, and the latter requires Boolean decisions on which points are features (and may thus suffer from robustness problems for real-world data). We take a new approach to adaptive streamline placement for steady vector fields in 2D and 3D. We define a metric for local similarity among streamlines and use this metric to grow streamlines from a dense set of candidate seed points. The metric considers not only Euclidean distance, but also a simple statistical measure of shape and directional similarity. Without explicit feature detection, our method produces streamlines that naturally accentuate regions of geometric interest. In conjunction with this method, we also propose a quantitative error metric for evaluating a streamline representation based on how well it preserves the information from the original vector field. This error metric reconstructs a vector field from points on the streamline representation and computes a difference of the reconstruction from the original vector field.
Date: August 15, 2007
Creator: Chen, Y.; Cohen, J. D. & Krolik, J. H.
Partner: UNT Libraries Government Documents Department

Investigations of the origins of metastable light-induced changes in hydrogenated amorphous silicon

Description: The work performed for this contract continued investigations of the origins of metastable effectS in a-Si:H through three kinds of studies: (1) the effect of carbon impurities in a-Si:H samples at low concentrations using drive-level capacitance profiling measurements on samples whose carbon content was intentionally modulated spatially during growth, (2) the characterization of metastable states in n-type doped a Si:H samples caused by quench cooling and by light-soaking with partial annealing, and (3) the use of depletion-width-modulated ESR spectroscopy together with junction capacitance spectroscopy to investigate deep defect states for various metastable states of a 10- and an 80-Vppm PH{sub 3}-doped a-Si:H sample.
Date: December 1, 1991
Creator: Cohen, J.D. (Oregon Univ., Eugene, OR (United States))
Partner: UNT Libraries Government Documents Department

Identifying electronic properties relevant to improving stability in a-Si:H-based cells and overall performance in a-Si,Ge:H-based cells. Annual subcontract report, 18 April 1995--17 April 1996

Description: The work done during this second phase of the University of Oregon`s NREL subcontract focused on degradation studies in both pure a-Si:H and a-Si,Ge:H alloys, as well as a detailed study of the interface between these two materials in a-Si:H/a-Si, Ge:H heterostructures. All samples discussed in this report were produced by the glow-discharge method and were obtained either in collaboration with United Solar Systems Corporation or with researchers at Lawrence Berkeley laboratory. First, the results from the a-Si, Ge:H degradation studies support the conclusion that considerable quantities of charged defects exist in nominally intrinsic material. Researchers found that on light-soaking, all the observed defect sub-bands increased; however, their ratios varied significantly. Second, researchers performed voltage pulse stimulated capacitance transient measurements on a-Si:H/a-Si, Ge:H heterostructure samples and found a clear signature of trapped hole emission extending over long times. Finally, researchers began comparison studies of the electronic properties of a-Si:H grown by glow discharge either with 100% silane, or with silane diluted in H{sub 2} or He gas. The results on these samples indicate that the films grown under high hydrogen dilution exhibit roughly a factor of 3 lower deep defect densities than those grown using pure silane.
Date: March 1, 1997
Creator: Cohen, J.D.
Partner: UNT Libraries Government Documents Department

Microscopic origins of metastable effects in a-Si:H and deep defect characterization in a-Si,Ge:H alloys

Description: This report describes works to use transient photocapacitance and photocurrent measurements to determine the deep defect distribution and processes in low-band-gap a-Si,Ge:H alloys. Samples for these studies were produced by the photochemical vapor deposition (photo-CVD) growth method and were obtained through a collaboration with researchers at the University of Delaware. This report discusses how a detailed comparison between the photocapacitance and photocurrent spectra can be used to separately examine the majority and minority carrier processes. The results are as follows: (1) The midgap defect densities in the alloy regime near 1.3 eV can be as low as 5 {times} 10{sup 16} cm{sup {minus}3} in such photo-CVD samples. (2) There exists a second defect band roughly 0.4 eV below E{sub c} of a similar magnitude to the midgap defect density that exhibits significant lattice relaxation behavior in its electron trapping dynamics. (3) The hole {mu}{tau} products determined for the lowest defect sample are roughly 5 {times} 10{sup {minus}10} cm{sup 2}/V, comparable with the highest hole {mu}{tau} products reported in sandwich geometry measurements for alloys in this composition range. (4) The hole {mu}{tau} is found to be roughly inversely proportional to the midgap defect density for the samples studied. This is consistent with the fact that the effective minority carrier lifetime for such measurements is limited by the deep state trapping time.
Date: July 1, 1992
Creator: Cohen, J. D. (Oregon Univ., Eugene, OR (United States))
Partner: UNT Libraries Government Documents Department

Correlation of Structural and Electronic Properties with Solar Cell Efficiency for Amorphous Silicon Deposited at Increasing Growth Rates

Description: This paper details the effects of increasing the growth rate of hydrogenated amorphous silicon (a-Si:H), deposited by dc plasma chemical vapor deposition, on the structural and electronic properties of the material in comparison with the performance of solar cells incorporating such layers. The hydrogen content exhibits the strongest correlation with the solar cell efficiency. The defect density measured by two different techniques, correlate poorly but when measured by a third technique, correlates well. On the other hand, the Urbach tail slope correlated well when measured by two different techniques but poorly when measured by a third one.
Date: June 1, 2003
Creator: Ganguly, G.; Han, D.; Williamson, D. L.; Nelson, B. P. & Cohen, J. D.
Partner: UNT Libraries Government Documents Department

a-SiGe:H Materials and Devices Deposited by Hot Wire CVD Using a Tantalum Filament Operated at Low Temperature

Description: We report the deposition of improved hydrogenated amorphous silicon germanium (a-SiGe:H) films by the hot wire CVD (HWCVD) technique using a tantalum filament operating at a low temperature. We gauge the material quality of the a-SiGe:H films by comparing infrared, small angle X-ray scattering (SAXS), photocapacitance, and conductivity measurements to earlier results, and fabricate single-junction n-i-p solar cell devices using these i-layers.
Date: February 1, 2005
Creator: Mahan, A. H.; Xu, Y.; Gedvilas, L. M.; Reedy, R. C.; Williamson, D. L.; Datta, S. et al.
Partner: UNT Libraries Government Documents Department