M. M. Al-Jassim

National Renewable Energy Laboratory, گلدن، کلرادو, Colorado, United States

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Publications (408)711.68 Total impact

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    ABSTRACT: ZnSnN2 is an Earth-abundant analog to the III-Nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and the possibility to tune its bandgap through introducing disorder into the cation sublattice. Here we present a combinatorial study of ZnSnN2 that has achieved dense crystalline films with photovoltaic-relevant carrier concentration. We report evidence of a Burstein-Moss shift widening the apparent bandgap, and tunable carrier concentration with cation composition. These results reaffirm the potential of ZnSnN2 for energy-conversion applications.
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    ABSTRACT: Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells.
    Journal of Applied Physics 03/2015; 117(11):112807. DOI:10.1063/1.4913833 · 2.19 Impact Factor
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    ABSTRACT: We report microscopic characterization studies of wide-bandgap Cu(In,Ga)Se2 photovoltaic thin films using the nano-electrical probes of scanning Kelvin probe force microscopy and scanning spreading resistance microscopy. With increasing bandgap, the potential imaging shows significant increases in both the large potential features due to extended defects or defect aggregations and the potential fluctuation due to unresolvable point defects with single or a few charges. The resistance imaging shows increases in both overall resistance and resistance nonuniformity due to defects in the subsurface region. These defects are expected to affect open-circuit voltage after the surfaces are turned to junction upon device completion.
    Applied Physics Letters 01/2015; 106(4):043901. DOI:10.1063/1.4907165 · 3.52 Impact Factor
  • Pranab Sarker, Mowafak M. Al-Jassim, Muhammad N. Huda
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    ABSTRACT: The single-phase stability of Cu2ZnSnS4 (CZTS), after an intrinsic defect was incorporated in it, has been examined here for the first time based on ab initio calculations. The stability analysis of such a non-stoichiometric-defect incorporated CZTS shows that the single-phase formation is unlikely at thermodynamic equilibrium conditions. In addition, the effective growth condition of CZTS is determined and quantified for all the elements (Cu-poor, Zn-rich, Sn-poor, and S-rich) to extract maximum photovoltaic efficiency from CZTS. These conditions promote (i) spontaneous formation of Cu vacancy (V-Cu), which might benefit p-type conduction, and (ii) the co-existence of ZnS while suppressing other harmful defects and secondary phases. Further, the results presented here explain the unavailability of single-phase CZTS to date. (C) 2015 AIP Publishing LLC.
    Journal of Applied Physics 01/2015; 117(3):035702. DOI:10.1063/1.4906065 · 2.19 Impact Factor
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    ABSTRACT: We report on scanning spreading resistance microscopy on cross sections of thin-film CdTe devices. The results show the capability of identifying the multiple layers, the depletion region, and the nonuniform doping. We observe carrier injection and depletion region movement by laser illumination or by electrically biasing the device, directly revealing the underlying physics of the solar cell junction in real space with resolutions of nanometer scale.
    IEEE Journal of Photovoltaics 01/2015; 5(1):395-400. DOI:10.1109/JPHOTOV.2014.2363569 · 3.00 Impact Factor
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    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. DOI:10.1016/j.solmat.2014.08.046 · 5.03 Impact Factor
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    ABSTRACT: Both randomly oriented and highly (220) textured thin films of zinc phosphide (Zn3P2) were grown by the close-space sublimation method. The effect of deposition parameters, such as pressure and substrate temperature, on the texture evolution has been established. It was found that the deposition temperature plays a dominant role in determining the preferred orientation whereas the ambient pressure (below 10 Torr) does not greatly affect the film texture. We further found that the microstrain changes from tensile at lower deposition temperatures to compressive at higher deposition temperatures. The preferred orientation also had a strong impact on the electrical resistivity of the films. The results provide guidance on the selection of substrate and deposition parameters to grow Zn3P2 thin films with desirable properties.
    Journal of Electronic Materials 01/2015; DOI:10.1007/s11664-015-3699-3 · 1.68 Impact Factor
  • 12/2014; 1(1):12-18. DOI:10.15377/2410-2199.2014.01.01.2
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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; 98. DOI:10.1016/j.commatsci.2014.10.051 · 1.88 Impact Factor
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    ABSTRACT: We used low-temperature cathodoluminescence (CL) spectrum imaging (CLSI) with nanoscale spatial resolution to examine charge-carrier recombination and defects at grain boundaries (GBs) and grain interiors (GIs) in as-deposited and CdCl2-treated CdTe thin films. Supporting time-resolved photoluminescence, T = 4 K photoluminescence, secondary ion mass spectrometry, and electron backscatter diffraction measurements were conducted on the same films. Color-coded maps of the luminescence transition energies (photon energy maps) were used to analyze the qualitative characteristics of the CLSI data. We applied an image analysis algorithm to the pixels in grayscale CL intensity images to compare the luminescence intensities and spectra at the GIs and GBs quantitatively and with statistical relevance. Our results show that GBs in as-deposited films are active recombination centers and are thus harmful to solar cell operation. CL GB defect contrast is quantifiably reduced for the CdCl2-treated film, which is direct evidence of passivation of deep GB core states resulting from the treatment. However, the CdCl2 treatment is not a perfect fix for GB recombination, and GB recombination may still be limiting performance in CdCl2-treated devices.
    IEEE Journal of Photovoltaics 11/2014; 4(6):1671-1679. DOI:10.1109/JPHOTOV.2014.2359732 · 3.00 Impact Factor
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    ABSTRACT: At the CdTe/CdS interface, a significant Te-S interdiffusion has been found a few nanometers into the CdTe grain interiors with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy. This interdiffusion happens on both as-grown and CdCl2-treated CdTe. S substitution at Te sites has been directly resolved in CdTe with STEM Z-contrast images, which further confirms the S diffusion into CdTe grain interiors. Moreover, when a sufficient amount of S substitutes for Te, a structural transformation from zinc-blende to wurtzite has been observed. In the CdCl2-treated CdTe, Cl segregation has also been found at the interface. STEM electron-beam-induced current shows that the p-n junction occurs a few namometers into the CdTe grains, which is consistent with the S diffusion range we observe. The shift of the p-n junction suggests a buried homojunction which would help reduce nonradiative recombination at the junction. Meanwhile, long-range S diffusion in CdTe grain boundaries (GBs) has been detected, as has Te and Cl diffusion in CdS GBs.
    IEEE Journal of Photovoltaics 11/2014; 4(6):1636-1643. DOI:10.1109/JPHOTOV.2014.2351622 · 3.00 Impact Factor
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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; 16(43). DOI:10.1039/C4CE01431F · 3.86 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. DOI:10.1088/0022-3727/47/40/405301 · 2.52 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). DOI:10.1007/s11664-014-3259-2 · 1.68 Impact Factor
  • Microscopy and Microanalysis 08/2014; 20(S3):952-953. DOI:10.1017/S1431927614006485 · 1.76 Impact Factor
  • Microscopy and Microanalysis 08/2014; 20(S3):514-515. DOI:10.1017/S1431927614004292 · 1.76 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). DOI:10.1103/PhysRevLett.112.156103 · 7.73 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 02/2014; 104(6):063902-063902-5. DOI:10.1063/1.4864758 · 3.52 Impact Factor
  • Harvey Guthrey, Miguel Contreras, Mowafak Al-Jassim
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    ABSTRACT: The highest efficiency CuIn1-xGaxSe2 (CIGS) based solar cells have been produced from films with x∼0.3 which gives a value of Eg around 1.1-1.2eV. Increasing the Ga content of the CIGS absorber provides an increase in Voc, allows tuning of the band gap that can enhance performance under actual operating conditions, and potentially makes it possible to use CIGS films in multi-junction devices. However, champion cells have not yet been produced for values of x significantly greater than 0.3. This work focuses on how increased Ga content in CIGS films affects the recombination behavior of grain boundaries. Cathodoluminescence spectral imaging (CLSI) measurements on fully processed devices allow us to compare device properties with recombination behavior and optical properties of grain boundaries in films with different Ga content. Our data suggests that grain boundaries in high efficiency films with x∼0.3 exhibit a significant red shift in the CL spectra whereas grain boundaries in films with higher Ga content typically show either a small shift or none at all. This shift indicates band bending near the boundaries which could enhance charge separation and subsequent collection of carriers generated near grain boundaries. This is investigated statistically to identify trends in different regions of the films.
    MRS Online Proceeding Library 01/2014; 1670. DOI:10.1557/opl.2014.419

Publication Stats

3k Citations
711.68 Total Impact Points

Institutions

  • 1991–2015
    • National Renewable Energy Laboratory
      • National Center for Photovoltaics
      گلدن، کلرادو, Colorado, United States
    • Virginia Polytechnic Institute and State University
      Блэксбург, Virginia, United States
  • 2014
    • National Research Center (CO, USA)
      Boulder, Colorado, United States
  • 2005
    • University of Arkansas
      • Department of Electrical Engineering
      Fayetteville, Arkansas, United States
  • 2002
    • William Penn University
      Worcester, Massachusetts, United States
  • 1996
    • University of Delaware
      Ньюарк, Delaware, United States
  • 1995
    • Texas Instruments Inc.
      Dallas, Texas, United States
  • 1990
    • Palo Alto Research Center
      Palo Alto, California, United States
  • 1988
    • NTT DOCOMO
      Edo, Tōkyō, Japan