8,736 research outputs found
Structural and elemental characterization of high efficiency Cu2ZnSnS4 solar cells
No full textWe have carried out detailed microstructural studies of phase separation and grain boundary composition in Cu2ZnSnS4 based solar cells. The absorber layer was fabricated by thermal evaporation followed by post high temperature annealing on hot plate. We show that inter-reactions between the bottom molybdenum and the Cu2ZnSnS4, besides triggering the formation of interfacial MoSx, results in the out-diffusion of Cu from the Cu2ZnSnS4 layer. Phase separation of Cu2ZnSnS4 into ZnS and a Cu-Sn-S compound is observed at the molybdenum-Cu2ZnSnS4 interface, perhaps as a result of the compositional out-diffusion. Additionally, grain boundaries within the thermally evaporated absorber layer are found to be either Cu-rich or at the expected bulk composition. Such interfacial compound formation and grain boundary chemistry likely contributes to the lower than expected open circuit voltages observed for the Cu2ZnSnS4 devices. (c) 2011 American Institute of Physics. [doi: 10.1063/1.3543621
An Interview with Tony David Sampson: Author of Virality: Contagion Theory in the Age of Networks
No full textTony D. Sampson is Reader in Digital Culture and Communication in the School of Arts and Digital Industries (ADI) at the University of East London, where he directs the EmotionUX lab, supervising research on the cognitive, emotional, and affective aspects of user experience. In 2013, he co-founded Club Critical Theory, an organization dedicated to the application of critical theory in everyday life in Southend-on-Sea, Essex. Tony is the author of Virality: Contagion Theory in the Age of Networks and The Assemblage Brain: Sense Making in Neuroculture, both from the University of Minnesota Press. He blogs at viralcontagion.wordpress.com.
The editors of this special NANO issue are delighted to have the opportunity to talk with Tony about how his work touches on issues of imitation and contagion—a loaded term unpacked within his 2012 book
Band alignment at the Cu2ZnSn(SxSe1−x)4/CdS interface
No full textEnergy band alignments between CdS and Cu2ZnSn(SxSe1-x)(4) (CZTSSe) grown via solution-based and vacuum-based deposition routes were studied as a function of the [S]/[S+Se] ratio with femtosecond laser ultraviolet photoelectron spectroscopy, photoluminescence, medium energy ion scattering, and secondary ion mass spectrometry. Band bending in the underlying CZTSSe layer was measured via pump/probe photovoltage shifts of the photoelectron spectra and offsets were determined with photoemission under flat band conditions. Increasing the S content of the CZTSSe films produces a valence edge shift to higher binding energy and increases the CZTSSe band gap. In all cases, the CdS conduction band offsets were spikes. (C) 2011 American Institute of Physics. [doi:10.1063/1.3600776
David Gregory
No full textPhotograph - David Gregory, member of the Book Sub-Committee, part of the Town of Athabasca 75th Anniversary Committee, Athabasca, Alberta. The Book Sub Committee produced the book "Athabasca Landing: An Illustrated History
David Audretsch: A Source of Inspiration, a Co-author, and a Friend
No full textIn this chapter, Enrico Santarelli discusses the profound impact that David had on his career. Beginning with a conference in Budapest, Santarelli and David bocame close friends and colleagues. They went on to collaborate on many papers and projects, several of which Santarelli highlights below
The integrated concurrent enterprise
No full textThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics; and, (S.M.)--Massachusetts Institute of Technology, Sloan School of Management; in conjunction with the Leaders for Manufacturing Program at MIT, 2003.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 177-180).by David B. Stagney.S.M
Discovery & Design of Complex Chalcogenide Semiconductors for Optical & Energy Conversion Applications
No full textMultinary chalcogenide semiconductors have long been a mainstay within optoelectronics industries. Chalcogenide materials consisting of at least four elements—i.e., quaternaries—have tunable structural, optical, and electronic properties, allowing the semiconductors to be tailored for specific applications. Recently, the I2-II-IV-X4 (I = Li, Cu, Ag; II = Ba, Sr, Pb, Eu; IV = Si, Ge, Sn; X = S, Se) family of materials has emerged as a source of promising semiconductors with applications in nonlinear optics and optoelectronics. A major challenge for these complex compounds is maintaining the ability to predictably control the desired properties. In this dissertation, solid-state chemistry methods are used to tackle three major goals: to investigate known I2-II-IV-X4-type compounds that have not been thoroughly explored; to develop predictable property trends within the wider family of materials; and to predict and make brand new semiconductors. The study of the quaternary semiconductor Cu2BaGeSe4 and the mixing of Ge and Sn within this compound (to make Cu2BaGe1-xSnxSe4) serves as the platform for branching into new compounds in the I2-II-IV-X4 family. Using the structural analysis established in the work on Cu2BaGe1-xSnxSe4, a structural tolerance factor is developed to predict the probable crystal structure of hypothetical compounds that fit into the family of the I2-II-IV-X4-type materials. Four new semiconductors (Cu2PbGeS4, Cu2SrSiS4, Ag2SrSiS4, and Ag2SrGeS4) were made and found to conform to the anticipated crystal structures based on the structural tolerance factor. The newly synthesized Cu2SrSiS4, Ag2SrSiS4, and Ag2SrGeS4 are potential nonlinear optical materials, while each of the four semiconductors may be used as buffer or n-type layers in thin film solar cells. In the pursuit of new I2-II-IV-X4-type compounds, a family of cubic compounds is found to either co-exist and compete with the synthesized materials (Ag2Sr3Si2S8 & Ag2Sr3Ge2S8) or to be more stable than the hypothetical I2-II-IV-X4 materials (Ag2Pb3Si2S8 & Ag2Sr3Sn2S8) with the same elemental makeup. Of these four cubic semiconductors, Ag2Sr3Si2S8 and Ag2Pb3Si2S8 have not been reported by others. The compounds within this family have predictable trends of optical properties and have applications as nonlinear optical materials if large single crystals can be synthesized.Finally, the solvothermal synthesis and properties of Ag2(NH4)AsS4 are explored, extending the scope of this dissertation from the I2-II-IV-X4 materials to those of the form Ag2-I’-V-X4 (I’ = NH4, K, Rb, Cs; V = As, Sb, Nb, Ta, V, P). Similar to the other studied Ag-based materials, Ag2(NH4)AsS4 has applications as a nonlinear optical material and as a buffer layer in solar cells. Understanding the Ag2(NH4)AsS4 synthesis technique allows future researchers to synthesize new Ag2-I’-V-X4-type semiconductors with the same methods or apply these principles to the fabrication of Ag2(NH4)AsS4 thin films. The work presented in this dissertation furthers the understanding of the synthesis and prediction of quaternary chalcogenide semiconductors and lays the foundation for future device and thin film studies using the semiconductors studied here.</p
Measurement of the ratio of branching fractions B(B0→K∗0γ )/B(B0s→φγ ) and the directCP asymmetry inB 0→K∗0γ
No full textThe ratio of branching fractions of the radiative B decays B0→K⁎0γ and B0s→ϕγ has been measured using an integrated luminosity of 1.0 fb−1 of pp collision data collected by the LHCb experiment at a centre-of-mass energy of s√=7TeV. The value obtained is
B(B0→K⁎0γ)B(B0s→ϕγ)=1.23±0.06(stat.)±0.04(syst.)±0.10(fs/fd),
where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is associated with the ratio of fragmentation fractions fs/fd. Using the world average value for B(B0→K⁎0γ), the branching fraction B(B0s→ϕγ) is measured to be (3.5±0.4)×10−5.
The direct CP asymmetry in B0→K⁎0γ decays has also been measured with the same data and found to be
ACP(B0→K⁎0γ)=(0.8±1.7(stat.)±0.9(syst.))%.
Both measurements are the most precise to date and are in agreement with the previous experimental results and theoretical expectations
Vacuum Deposition, Characterization and Property Engineering of Cu2BaGe1-xSnxSe4 Films and Their PV Applications
No full textKesterite Cu2ZnSnS4-xSex (CZTSSe) has once gained wide attention as a potential alternative to the CdTe and Cu(In,Ga)(S,Se)2 (CIGSSe) photovoltaic (PV) technologies, which are currently facing challenges in terms of scalability due to the use of the scarcity (Te, and In) and toxicity (Cd) of the elements. However, the similarities between the Cu and Zn atoms in terms of cation size and coordination environment result in the formation of a high density of Cu- and Zn-related anti-site defects and defect clusters in the CZTSSe lattice and limit the open-circuit voltage and efficiency of the CZTSSe solar cells. To target suppressing the formation of anti-site defects and related defect clusters, Cu2-II-IV-X4 (II = Sr, Ba; IV = Ge, Sn; X = S, Se) compounds, which have a significantly larger and chemically more differentiated group-2 element (i.e., Ba, and Sr) instead of Zn, have been introduced. Among Cu2-II-IV-X4 compounds, Cu2BaSnS4 (CBTS) and Cu2BaSnS4-xSex (CBTSSe) with trigonal structure (P31 space group) have been the first materials to have gained attention, and their thin-film deposition using both solution- and vacuum-based techniques, as well as their PV devices, have already been demonstrated. On the other hand, there are only a handful of studies on Cu2BaGeSe4 (CBGSe) and Sn-alloyed Cu2BaGe1-xSnxSe4 (CBGTSe) systems, which have the same crystal structure as CBTS and CBTSSe. In this dissertation, we explore film growth, material properties, property engineering methods, and PV application of this relatively unexplored system, CBGTSe, to get a better understanding of this compound as a potential PV material. To achieve this goal, the following studies are conducted and presented throughout this dissertation: (1) development of a deposition process for high-quality CBGTSe films, which yield functioning solar cells, and examination of the associated solar cell properties; (2) characterization of important optoelectronic properties of CBGSe and CBGTSe films and identification of major bottleneck for their solar cell performance; and (3) demonstration of two different film modification strategies (i.e., alloying and doping) for CBGTSe films and investigation of associated changes in the film properties.The first study demonstrates a high-quality CBGTSe film growth via sequential vacuum deposition (i.e., sputtering, and evaporation) of elemental layers (i.e., Cu, Ba, Ge, and Sn) followed by a selenization step to convert the metallic layer of Cu–Ba–Ge–Sn into CBGTSe compound. This work investigates film growth mechanisms via ex-situ analysis and reveals the critical process parameters (i.e., pre-annealing temperature, and Cu-content) for high-quality films. Additionally, functioning solar cell devices based on CBGTSe as light absorber are demonstrated for the first time. Second, the optoelectronic properties of CBGSe (Sn-free) films are also investigated in detail and compared with its isostructural CBTS using various analysis techniques, including temperature-dependent photoluminescence (PL), Hall effect, photoelectron spectroscopies, optical-pumped terahertz probe spectroscopy (OPTP), and open-cell time-resolved microwave conductivity (oc-TRMC), which reveal possible bottlenecks for the solar cell performance and possible directions for the improvement.
Next, two different modification approaches—i.e., 1) alloying with Ag, and 2) doping with group-1 (alkali metals) elements—, which have been used for the existing CIGSSe and CZTSSe technologies, are demonstrated to modify overall optoelectronic properties of the CBGTSe films. First, the partial substitution of Cu by Ag is examined as a potential film property modification strategy. The study reveal how much Cu can be substituted with Ag while maintaining its original trigonal crystal structure and how phase purity, morphology, charge carrier properties, band positions, and recombination properties, which are all critical for the PV and optoelectronic applications, change as a function of Ag-content. The intrinsic background carrier densities for CBGTSe films are relatively low (p = ~1012 cm-3) compared to other related chalcogenides (p = 1015–1017 cm-3 for CIGSSe, and CZTSSe), which can limit its applications as photovoltaic, thermoelectronic, and optoelectronic devices. Therefore, as prospective dopants for the CBGTSe films, alkali elements (Li, Na, K, and Rb) are evaluated to address the low hole carrier density and potentially to allow for property tunability. The study demonstrates orders of magnitude enhancement in hole carrier density via alkali-doping. The changes in other film properties (i.e., film morphology, carrier mobility, and minority carrier lifetime) with alkali-element doping are also examined.
Additionally, to address inappropriate band alignment within solar cells based on the Cu2-II-IV-X4 family (e.g., Cu2BaGe1-xSnxSe4, Cu2BaSnS4-xSex), which typically has shown noticeably lower electron affinity (EA) than conventional CdS/i-ZnO/ITO buffer/window stacks, we introduce Zn1-xCdxS/Zn1-xMgxO/ZnO:Al as an alternative low-EA buffer/window stack. The low-EA buffer and window layers contribute to improvement in the properties of CBTSSe solar cells and yield a maximum PCE of 6.5% (with MgF2 anti-reflection coating), which represents the current record PCE for CBTSSe-based solar cells. The study reveals that the alternative buffer/window stack improves overall recombination properties for the CBTSSe solar cells from band offset estimation using photoelectron spectroscopy, and recombination property analysis. In addition, device modeling and simulation results provide directions for further improvement of device performance. The works presented in this dissertation provide baseline understanding and knowledge on film synthesis, material properties, property engineering, and associated solar cells for the CBGTSe compound as well as for the relevant Cu2-II-IV-X4 multinary chalcogenide compounds.
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