M. M. Al-Jassim

National Renewable Energy Laboratory, Golden, Colorado, United States

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Publications (367)625.19 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on a local potential and resistance mapping of Cu2ZnSnSe4 (CZTSe) films using nm-resolution electrical scanning probe microscopies of scanning Kelvin probe force microscopy and scanning spreading resistance microscopy. We have conducted a comparative study with high-performance Cu2(In,Ga)Se2 (CIGSe) film. Both CZTSe and CIGSe were deposited by co-evaporation of elements in vacuum. The results show that the microelectrical properties of the two polycrystalline materials are similar-higher potential and lower resistance on the grain boundaries (GBs) than on grain surfaces-suggesting inverted GB carrier polarity of these films. The consistent GB properties in contrast to the large difference in photovoltaic output of the two materials suggest that factors other than the GBs are responsible for the low photovoltaic output of CZTSe device.
    Solar Energy Materials and Solar Cells 01/2015; 132:342–347. · 5.03 Impact Factor
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    ABSTRACT: We present a detailed assessment of the structural and electronic properties of CdTe calculated by density functional theory (DFT) with on-site Coulomb self-interaction potentials (LDA+U /GGA+U ) on the Cd 4d4d band. We systematically calculate the lattice constants, bulk moduli, elastic constants, band structure, and density of states as a function of the U value, and compare the results with those calculated by using standard LDA/GGA and the hybrid functional (HSE06). Our study gives a more accurate account of the strong localization effect of Cd 4d4d electrons onto the overall electronic structure, in particular to the nature of localized Cd 4d4d derived bands and delocalized Te 5s5s derived bands and the coupling between them. We find that the s –d coupling is significant, which is underestimated within conventional DFT calculations (showing a single s -like peak, in disagreement with the experiments). LDA+U removes this discrepancy by shifting down the Cd-4d4d band closer to the Te-5s5s band, enhancing the s –d coupling, and leading to the appearance of two s -like peaks, which perfectly explains the so-called low intensity “shoulder” on the high-energy side of the Cd-4d4d peak in experimental spectra. Moreover, our results indicate LDA+U reveals an much more acceptable agreement with experiment at a adequate U than HSE06 does. A well balanced choice of U within LDA+U scheme is proposed to be at 7 eV.
    Computational Materials Science. 11/2014;
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    ABSTRACT: Crystalline carbon films are promising in many applications. However, traditional carbon film deposition methods are limited by involving ultra-high voltage or complicated equipment. In this study, we demonstrate a simple photocatalytic approach to the crystalline carbon fabrication. The crystalline carbon is selectively deposited on the anode side of a GaN diode chip immersed in a mixture of CH3OH and H2O2 with sunlight as the only energy source. Diamond and flat hexagonal crystallites are observed with scanning electron microscopy (SEM). Energy dispersive X-ray (EDX), Raman and Fourier transform infrared (FTIR) spectra further confirm that diamond and graphite crystallites are successfully prepared. A detailed theoretical analysis shows that both H2O2 and photon-generated holes play important roles in the crystalline carbon formation. Compared to traditional approaches, the new approach is easy to realize at low cost, has deposition selectivity at the anode side, and needs no strong electric fields. So our approach is novel and promising in preparing crystalline carbon.
    CrystEngComm 09/2014; · 3.88 Impact Factor
  • Muhammad N Huda, Yanfa Yan, John A Turner, Mowafak M Al-Jassim
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    ABSTRACT: Even though the quantum size effect on the electronic gap of nano-structures is well-understood, its implication on the optical absorption near band gap energies is still a challenging question, especially for metal-oxides. A unique class of highly stable, self-saturated and self-charge-compensated delafossite nanocrystals has been identified in this paper. The structural and electronic properties of these nanocrystalline Cu-based delafossites have been studied using density functional theory (DFT). To better estimate the electronic excitation energies, and consequently the optical gap, a time-dependent DFT has also been employed. The goal here is to study whether a nano-phase can enhance the optical absorption of near band gap energies of delafossite nano-structures compared to their bulk state to enable their application as a photocatalyst.
    Journal of Physics D Applied Physics 09/2014; 47(40):405301. · 2.53 Impact Factor
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    ABSTRACT: In this work, Cu2ZnSnS4 (CZTS) thin films were prepared by the sulfurization of metal precursors deposited sequentially via radio frequency magnetron sputtering on Mo-coated soda-lime glass. The stack order of the precursors was Mo/Zn/Sn/Cu. Sputtered precursors were annealed in sulfur atmosphere with nine different conditions to study the impact of sulfurization time and substrate temperature on the structural, morphological, and optical properties of the final CZTS films. X-ray fluorescence was used to determine the elemental composition ratio of the metal precursors. Final CZTS films were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). XRD and EDS were combined to investigate the films’ structure and to identify the presence of secondary phases. XRD analysis indicated an improvement in film crystallinity with an increase of the substrate temperature and annealing times. Also indicated was the minimization and/or elimination of secondary phases when the films experienced longer annealing time. EDS revealed slight Sn loss in films sulfurized at 550°C; however, an increase of the sulfurization temperature to 600°C did not confirm these results. SEM study showed that films treated with higher temperatures exhibited dense morphology, indicating the completion of the sulfurization process. The estimated absorption coefficient was on the order of 104 cm−1 for all CZTS films, and the values obtained for the optical bandgap energy of the films were between 1.33 eV and 1.52 eV.
    Journal of Electronic Materials 09/2014; 43(9). · 1.64 Impact Factor
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    ABSTRACT: When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells.
    Physical Review Letters 04/2014; 112(15):156103. · 7.73 Impact Factor
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    ABSTRACT: When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells.
    Physical Review Letters 03/2014; 112(15). · 7.73 Impact Factor
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    ABSTRACT: Through first-principle density functional theory (DFT) calculations, the atomic structure and electronic properties of intrinsic and passivated Σ3 (114) grain boundaries (GBs) in Cu2ZnSnSe4 (CZTSe) are studied. Intrinsic GBs in CZTSe create localized deep states within the band gap and thus act as Shockley-Read-Hall recombination centers, which are detrimental to cell performance. Defects, such as ZnSn (Zn atoms on Sn sites), Na+i (interstitial Na ions), and OSe (O atoms on Se sites), prefer to segregate into GBs in CZTSe. The segregation of these defects at GBs exhibit two beneficial effects: 1) eliminating the deep gap states via wrong bonds breaking or weakening at GBs, making GBs electrically benign; and 2) creating hole barriers and electron sinkers, promoting effective charge separation at GBs. The results suggest a unique chemical approach for engineering GBs in CZTSe to achieve improved cell performance.
    Advanced Energy Materials 01/2014; 4(1):1-7. · 14.39 Impact Factor
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    ABSTRACT: A set of neighboring multicrystalline silicon wafers has been processed through different steps of solar cell manufacturing and then images were collected for characterization. The imaging techniques include band-to-band photoluminescence (PL), defect-band or subbandgap PL (subPL), and dark lock-in thermography (DLIT). Defect regions can be tracked from as-cut wafers throughout processing to the finished cells. The finished cell's defect regions detected by band-to-band PL imaging correlate well to diffusion length and quantum efficiency maps. The most detrimental defect regions, type A, also correlate well to reverse-bias breakdown areas as shown in DLIT images. These type A defect regions appear dark in band-to-band PL images, and have subPL emissions. The subPL of type A defects shows strong correlations to poor cell performance and high reverse breakdown at the starting wafer steps (as-cut and textured), but the subPL becomes relatively weak after antireflection coating (ARC) and on the finished cell. Type B defects are regions that have lower defect density but still show detrimental cell performance. After ARC, type B defects emit more intense subPL than type A regions; consequently, type B subPL also shows better correlation to cell performance at the starting wafer steps rather than at the ARC process step and in the finished cell.
    IEEE Journal of Photovoltaics 01/2014; 4(1):348-354. · 3.00 Impact Factor
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    ABSTRACT: We report on direct imaging of current collection by performing conductive atomic force microscopy (C-AFM) measurement on a complete Cu(In,Ga)Se2 solar cell. The localized current was imaged by milling away the top conductive layer of the device by repeated C-AFM scans. The result exhibits enhanced photocurrent collection on grain boundaries (GBs) of CIGS films, consistent with the argument for electric-field-assisted carrier collection on the GBs.
    Applied Physics Letters 01/2014; 104(6):063902-063902-5. · 3.52 Impact Factor
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    ABSTRACT: Defect characterization in molecular beam epitaxial (MBE) compositionally-graded InxGa1-xAs layers on GaAs substrates consisting different thickness of overshooting (OS) layers was carried out using cathodoluminescence (CL) and transmission electron microscopy (TEM). We found that the thickness of the OS layer influences not only stress but also lattice defects generated in a top InGaAs layer. While the top InGaAs layer with a thin OS layer is under compression and has mainly threading dislocations, the top layer with a thick OS layer is under tension and exhibits inhomogeneous strain associating with phase separation. We will discuss the mechanisms of defect generation and their in-plane distribution based on strain relaxation at the top and OS layers. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 10/2013; 10:1640-1643.
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    ABSTRACT: Through the use of aberration corrected scanning transmission electron microscopy, the atomic configuration of CdTe intragrain Shockley partial dislocation pairs has been determined: Single Cd and Te columns are present at opposite ends of both intrinsic and extrinsic stacking faults. These columns have threefold and fivefold coordination, indicating the presence of dangling bonds. Counterintuitively, density-functional theory calculations show that these dislocation cores do not act as recombination centers; instead, they lead to local band bending that separates electrons and holes and reduces undesirable carrier recombination.
    Physical Review Letters 08/2013; 111(9):096403. · 7.73 Impact Factor
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    ABSTRACT: The atomic structure and electronic properties of aluminum (Al)-related defect complexes in silicon (Si) are investigated using first-principles calculations. Individual substitutional Al (AlSi), interstitial Al (Ali) and their possible complex configurations with oxygen (O) atoms are studied. We find a unique stable complex configuration consisting of an Ali and an oxygen dimer, Ali-2Oi, which introduces deep levels in the band gap of Si. The formation energies of the Ali-2Oi complexes could be lower than that of individual Ali atoms under oxygen-rich conditions. The formation of Ali-2Oi complexes may explain the experimental observation that the coexistence of Al and O results in reduced carrier lifetime in Si wafers.
    Journal of Applied Physics 08/2013; 114(6). · 2.21 Impact Factor
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    ABSTRACT: Aberration corrected scanning transmission electron microscopy (STEM) has been used to determine the structures of a variety of dislocation cores in CdTe, including 30° and 90° Shockley partial dislocations, positive and negative Frank sessile partial dislocations, and steps on twin boundaries. Structure models have been constructed from the images and electrical activity has been investigated with density functional calculations. An integrated electron energy loss spectroscopy, cathodoluminescence and electron beam induced current system has been designed and built to probe electrical and optical properties of individual defects. The first STEM-cathodoluminescence result shows strong impurity segregation between the CdTe and the glass. The correlation between the scanning electron microscopy-electron beam induced current and electron backscatter diffraction maps proves that the grain structures and boundaries dominate the electrical activity. After heat treatment in CdCl2, Cl is found to segregate to the grain boundaries, and they show higher efficiency than the bulk material.
    Ultramicroscopy 07/2013; · 2.47 Impact Factor
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    ABSTRACT: We report on a direct nm-resolution resistance mapping on the Cu(In,Ga)Se2 photovoltaic thin films, using scanning spreading resistance microcopy. We found a conductance channel along the grain boundaries (GBs) of the polycrystalline materials, which is consistent with the argument that carrier polarity of the GB and the space charge region around it is inverted. To minimize the probe/film contact resistance, so that the local spreading resistance beneath the probe is measured, the probe must be adequately indented to the film and a bias voltage larger than the onset value of the probe/film barrier should be applied.
    Applied Physics Letters 06/2013; 102(25). · 3.52 Impact Factor
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    ABSTRACT: We study the structural, chemical, and electronic properties of individual grain boundaries in Cu 2 ZnSnSe 4 (CZTSe) using combined electron microscopy techniques including scanning electron microscopy-based cathodoluminescence (CL)-spectrum imaging and scanning transmission electron microscopy-based Z-contrast imaging and energy-dispersive spectroscopy profiling. Two representative grain boundaries have been studied. We find that the grain boundary that exhibits a redshift in the CL spectrum image is found to link to a ZnSe second phase. The grain boundary showing no redshift in the CL spectrum image is not linked to any secondary phase. The stability of CZTSe cross-section samples with storage time is also discussed. Index Terms — Cu 2 ZnSnSe 4 , polycrystalline thin film, grain boundary, cathodoluminescence, electron microscopy.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC), Tampa Florida; 06/2013
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    ABSTRACT: Defect segregations at grain boundaries (GBs) of polycrystalline thin-film absorber are crucial to solar cell performance. The Σ3(114) GBs in CuInSe2 (CIS) have been studied through first-principle calculations. We reveal that the intrinsic CIS GBs produce deep gap states, which act as nonradiative recombination centers. However, the segregation of CuIn and OSe at GBs can clean the gap states and lead to electrically benign behavior. Our results suggest that the defect segregation at GBs could be an important feature for high efficiency CIS-based photovoltaic solar cells and it provides a general guidance for engineering GBs in other chalcogenide polycrystalline devices.
    Applied Physics Letters 05/2013; 102(19). · 3.52 Impact Factor
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    ABSTRACT: Mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with significantly reduced bandgaps were synthesized by using zinc oxide and gallium nitride target at 100 °C followed by post-deposition annealing at 500 °C in ammonia for 4 h. All the films were synthesized by RF magnetron sputtering on Fluorine-doped tin oxide-coated glass. We found that mixed zinc oxide and gallium nitride (ZnO:GaN) thin films exhibited significantly reduced bandgap, as a result showed improved PEC response, compared to ZnO thin film. Furthermore, mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with various bandgaps were realized by varying the O2 mass flow rate in mixed O2 and N2 chamber ambient.
    Applied Surface Science 04/2013; 270(1 April 2013). · 2.54 Impact Factor
  • Muhammad Huda, Yanfa Yan, John A. Turner, Mowafak M. Al-Jassim
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    ABSTRACT: A unique class of highly stabile, self-saturated and self-charge-compensated delafossite nanocrystals has been identified. The density functional theory (DFT) study of structural and electronic properties of these nano-crystalline CuYO2 will be presented. To have a better estimate of the electronic excitation energies, and consequently the optical gap, time dependent DFT has been employed as well. The goal is to show, first of all, that these unique set of nanocrystals exists, and to study whether the nano-phase can modify the electronic properties for enhanced optical absorption. It has potential application as photocatalysts for H2 production by water splitting.
    03/2013;
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    ABSTRACT: The electrostatic potential distribution across single, isolated, colloidal heterostructured nanorods (NR) with component materials expected to form a p-n junction within each NR has been measured using scanning Kelvin probe microscopy (SKPM). We compare CdS to bi-component CdS-CdSe, CdS-PbSe, and CdS-PbS NRs prepared via different synthetic approaches to corroborate the SKPM assignments. The CdS-PbS NRs show a sharp contrast in measured potential across the material interface. We find the measured built-in potential within an individual NR to be attenuated by long-range electrostatic forces between the sample substrate, cantilever and the measuring tip. Surface potential images were deconvoluted to yield built-in potentials ranging from 375 to 510 meV in the heterostructured NRs. We deduce the overall built-in potential as well as the charge distribution across each segment of the heterostructured NRs by combining SKPM data with simulations of the system.
    Nano Letters 02/2013; · 13.03 Impact Factor

Publication Stats

1k Citations
625.19 Total Impact Points

Institutions

  • 1992–2014
    • National Renewable Energy Laboratory
      • National Center for Photovoltaics
      Golden, Colorado, United States
  • 2013
    • Vanderbilt University
      Nashville, Michigan, United States
  • 2012
    • University of Texas at Arlington
      • Department of Physics
      Arlington, TX, United States
  • 2010
    • University of Electronic Science and Technology of China
      • Department of Applied Physics
      Chengdu, Sichuan Sheng, China
  • 2002
    • Pennsylvania State University
      • Department of Physics
      University Park, MD, United States
  • 1995
    • Texas Instruments Inc.
      Dallas, Texas, United States
  • 1990
    • Palo Alto Research Center
      Palo Alto, California, United States
  • 1989
    • Spire Corporation
      Bedford, Massachusetts, United States
  • 1988
    • NTT DOCOMO
      Edo, Tōkyō, Japan