M. M. Al-Jassim

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

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Publications (473)

  • Harvey Guthrey · Steve Johnston · Dirk N. Weiss · [...] · Mowafak Al-Jassim
    [Show abstract] [Hide abstract] ABSTRACT: In this contribution, we demonstrate the value of using a multiscale multi-technique characterization approach to study the performance-limiting defects in multi-crystalline silicon (mc-Si) photovoltaic devices. The combination of dark lock-in thermography (DLIT) imaging, electron beam induced current imaging, and both transmission and scanning transmission electron microscopy (TEM/STEM) on the same location revealed the nanoscale origin of the optoelectronic properties of shunts visible at the device scale. Our site-specific correlative approach identified the shunt behavior to be a result of three-dimensional inversion channels around structural defects decorated with oxide precipitates. These inversion channels facilitate enhanced minority-carrier transport that results in the increased heating observed through DLIT imaging. The definitive connection between the nanoscale structure and chemistry of the type of shunt investigated here allows photovoltaic device manufacturers to immediately address the oxygen content of their mc-Si absorber material when such features are present, instead of engaging in costly characterization.
    Article · Oct 2016 · Solar Energy
  • Steven P. Harvey · Jeffery A. Aguiar · Peter Hacke · [...] · Mowafak Al-Jassim
    [Show abstract] [Hide abstract] ABSTRACT: We investigated potential-induced degradation (PID) in silicon mini-modules that were subjected to accelerated stressing to induce PID conditions. Shunted areas on the cells were identified with photoluminescence and dark lock-in thermography (DLIT) imaging. The identical shunted areas were then analyzed via time-of-flight secondary-ion mass spectrometry (TOF-SIMS) imaging, 3-D tomography, and high-resolution transmission electron microscopy. The TOF-SIMS imaging indicates a high concentration of sodium in the shunted areas, and 3-D tomography reveals that the sodium extends more than 2 μm from the surface below shunted regions. Transmission electron microscopy investigation reveals that a stacking fault is present at an area identified as shunted by DLIT imaging. After the removal of surface sodium, tomography reveals persistent sodium present around the junction depth of 300 nm and a drastic difference in sodium content at the junction when comparing shunted and nonshunted regions.
    Article · Sep 2016 · IEEE Journal of Photovoltaics
  • John Moseley · Mowafak M. Al-Jassim · Harvey L. Guthrey · [...] · Wyatt K. Metzger
    [Show abstract] [Hide abstract] ABSTRACT: We conducted 6 K cathodoluminescence (CL) spectrum imaging with a nanoscale electron beam on beveled surfaces of CdTe thin films at the critical stages of standard CdTe solar cell fabrication. We find that the through-thickness CL total intensity profiles are consistent with a reduction in grain-boundary recombination due to the CdCl2 treatment. The color-coded CL maps of the near-band-edge transitions indicate significant variations in the defect recombination activity at the micron and sub-micron scales within grains, from grain to grain, throughout the film depth, and between films with different processing histories. We estimated the grain-interior sulfur-alloying fraction in the interdiffused CdTe/CdS region of the CdCl2-treated films from a sample of 35 grains and found that it is not strongly correlated with CL intensity. A kinetic rate-equation model was used to simulate grain-boundary (GB) and grain-interior CL spectra. Simulations indicate that the large reduction in the exciton band intensity and relatively small decrease in the lower-energy band intensity at CdTe GBs or dislocations can be explained by an enhanced electron-hole non-radiative recombination rate at the deep GB or dislocation defects. Simulations also show that higher GB concentrations of donors and/or acceptors can increase the lower-energy band intensity, while slightly decreasing the exciton band intensity.
    Article · Sep 2016 · Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: Advancing CdTe solar cell efficiency requires improving the open-circuit voltage (VOC) above 900 mV. This requires long carrier lifetime, high hole density, and high-quality interfaces, where the interface recombination velocity is less than about 104 cm/s. Using CdTe single crystals as a model system, we report on CdTe/CdS electrical and structural interface properties in devices that produce open-circuit voltage exceeding 950 mV.
    Article · Aug 2016
  • Jeffery A. Aguiar · Adam Stokes · Chun-Sheng Jiang · [...] · Mowafak Al-Jassim
    [Show abstract] [Hide abstract] ABSTRACT: The effects of alkali post-deposition treatments and device properties for polycrystalline thin film Cu(In,Ga)Se2 have been investigated. It is reported that these surface treatments lead to differences in interface chemistry and device properties. The behavior of defects in the space charge region as a function of different growth parameters is investigated by correlative analytical microscopy. The latter combines electron microscopy based imaging, Kelvin probe force microscopy, and atom probe tomography. Alkali treatments lead to copper depletion and consequent sharpening of the compositional profiles, and the measured electric potential differences of exposed Cu(In1–x,Gax)Se2 surfaces. Measurable differences in resistivity and potential have also been observed, which are expected to relate to the improved open-circuit voltage, fill-factor, and device efficiency. This study frames one perspective as to why post-deposition alkaline treatments lead to copper depletion, a mildly n-type semiconductor interface, and higher efficiency for a Cu(In,Ga)Se2 thin-film photovoltaic device.
    Article · Jul 2016 · Advanced Materials Interfaces
  • [Show abstract] [Hide abstract] ABSTRACT: Organic-inorganic perovskites have emerged as an important class of next generation solar cells due to their remarkably low cost, band gap, and sub-900 nm absorption onset. Here, we show a series of in situ observations inside electron microscopes and X-ray diffractometers under device-relevant synthesis conditions focused on revealing the crystallization process of the formamidinium lead-triiodide perovskite at the optimum temperature of 175 °C. Direct in situ observations of the structure and chemistry over relevant spatial, temporal, and temperature scales enabled identification of key perovskite formation and degradation mechanisms related to grain evolution and interface chemistry. The lead composition was observed to fluctuate at grain boundaries, indicating a mobile lead-containing species, a process found to be partially reversible at a key temperature of 175 °C. Using low energy electron microscopy and valence electron energy loss spectroscopy, lead is found to be bonded in the grain interior with iodine in a tetrahedral configuration. At the grain boundaries, the binding energy associated with lead is consequently shifted by nearly 2 eV and a doublet peak is resolved due presumably to a greater degree of hybridization and the potential for several different bonding configurations. At the grain boundaries there is adsorption of hydrogen and OH⁻ ions as a result of residual water vapor trapped as a non-crystalline material during formation. Insights into the relevant formation and decomposition reactions of formamidinium lead iodide at low to high temperatures, observed metastabilities, and relationship with the photovoltaic performance were obtained and used to optimize device processing resulting in conversion efficiencies of up to 17.09% within the stability period of the devices.
    Article · Jul 2016 · Energy & Environmental Science
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    [Show abstract] [Hide abstract] ABSTRACT: Perovskite based solar cells are one of the emerging candidates for radically lower cost photovoltaics. Herein, we report on the synthesis and crystallization of organic inorganic formamidinium lead-triiodide perovskite films under controlled atmospheric and environmental conditions. Using in situ (scanning) transmission electron microscopy, we make observations of the crystallization process of these materials in nitrogen and oxygen gas with and without the presence of water vapor. Complementary planar samples were also fabricated in the presence of water vapor and characterized by in situ X-ray diffraction. Direct observations of the material structure and final morphology indicate the exposure to water vapor results in a porous film that is metastable, regardless of the presence argon, nitrogen, or oxygen. However, the optimal crystallization temperature of 175°C is unperturbed across conditions. Rapid modulation about the annealing temperature of 175°C in ±25°C steps (150°C to 200°C) promotes crystallization and significantly improves the film morphology by overcoming the presence of impregnated water trapped in the material. Following this processing protocol, we demonstrate substantial growth to micron-size grains via observation inside an environmentally controlled transmission electron microscope. Adapting this insight from our in situ microscopy, we are able to provide an informed materials protocol to control the structure and morphology of these organic inorganic semiconductors, which is readily applicable to benchtop device growth strategies.
    Full-text available · Article · May 2016 · ACS Energy Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.
    Full-text available · Article · May 2016 · Advanced Materials
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    File available · Dataset · Apr 2016
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    [Show abstract] [Hide abstract] ABSTRACT: Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a sustainable material for thin-film photovoltaics with device efficiencies greater than 12% have been demonstrated. Despite similar crystal structure and polycrystalline film microstructures, there is widespread evidence for larger-amplitude potential and bandgap fluctuations in CZTS than in the analogous Cu(In,Ga)Se2 (CIGSe) chalcopyrite material. This disorder is believed to account for a sizable part of the larger open-circuit voltage (VOC) deficit in CZTS devices, yet the detailed origins and length scales of these fluctuations have not been fully elucidated. Herein, we present a transmission electron microscopy study focusing on composition variation within bulk multicrystals of CZTS grown by the travelling heater method (THM). In these slow-cooled, solution grown crystals we find direct evidence for spatial composition fluctuations of amplitude <1at.% (∼5×1020cm-3) and thus, explainable by point defects. However, rather than being homogeneously-distributed we find a characteristic 20nm length scale for these fluctuations, which sets a definite length scale for band gap and potential fluctuations. At Σ3 grain boundaries, we find no evidence of composition variation compared to the bulk. The finding highlights such variations reported at grain boundaries in polycrystalline thin-films are direct consequences of processing methods and not intrinsic properties of CZTS itself.
    Full-text available · Article · Apr 2016
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    [Show abstract] [Hide abstract] ABSTRACT: CdTe solar cells have the potential to undercut the costs of electricity generated by other technologies, if the open-circuit voltage can be increased beyond 1 V without significant decreases in current. However, in the past decades, the open-circuit voltage has stagnated at around 800–900 mV. This is lower than in GaAs solar cells, even though GaAs has a smaller bandgap; this is because it is more difficult to achieve simultaneously high hole density and lifetime in II–VI materials than in III–V materials. Here, by doping the CdTe with a Group V element, we report lifetimes in single-crystal CdTe that are nearly radiatively limited and comparable to those in GaAs over a hole density range relevant for solar applications. Furthermore, the deposition on CdTe of nanocrystalline CdS layers that form non-ideal heterointerfaces with 10% lattice mismatch impart no damage to the CdTe surface and show excellent junction transport properties. These results enable the fabrication of CdTe solar cells with open-circuit voltage greater than 1 V.
    Full-text available · Article · Feb 2016
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    File available · Dataset · Jan 2016
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    File available · Dataset · Jan 2016
  • Adam Stokes · Mowafak Al-Jassim · David R. Diercks · [...] · Brian Gorman
    [Show abstract] [Hide abstract] ABSTRACT: This paper will discuss the findings of an ordered vacancy compound (OVC) phase that exists deep into the bulk of a high-efficiency Cu(In,Ga)Se2 (CIGS) absorber that is shown to be a result of many ordered defect pairs of 2VCu + (In,Ga)Cu as determined by atom probe tomography (APT). To date, literature has shown that absorbers grown with the three-step process exhibit the OVC Cu(In,Ga)3Se5 (135 phase) only within the first few nanometers from the CdS/CIGSe interface and at grain boundaries. In this contribution, we have found a small volume (100 nm × 100 nm × 300 nm) of an OVC phase to exist about 400 nm into the absorber. We show through concentration and density profiles that the concentration change from the stoichiometric Cu(In,Ga)Se2 to the OVC is indeed a result of many ordered defect pairs. We use this volume to perform point defect density distributions to give unique insight to the band structure at the nanoscale. © 2015 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
    Article · Jan 2016 · Acta Materialia
  • [Show abstract] [Hide abstract] ABSTRACT: Both tin oxide (SnO2) and fullerenes have been reported as electron selective layers (ESLs) for producing efficient lead halide perovskite solar cells. Here, we report that SnO2 and fullerenes can work cooperatively to further boost the performance of perovskite solar cells. We find that fullerenes can be redissolved during perovskite deposition, allowing ultra-thin fullerenes to be retained at the interface and some dissolved fullerenes infiltrate into perovskite grain boundaries. The SnO2 layer blocks holes effectively; whereas, the fullerenes promote electron transfer and passivate both the SnO2/perovskite interface and perovskite grain boundaries. With careful device optimization, the best-performing planar perovskite solar cell using a fullerene passivated SnO2 ESL has achieved a steady-state efficiency of 17.75% and a power conversion efficiency of 19.12% with an open circuit voltage of 1.12 V, a short-circuit current density of 22.61 mA cm⁻², and a fill factor of 75.8% when measured under reverse voltage scanning. We find that the partial dissolving of fullerenes during perovskite deposition is the key for fabricating high-performance perovskite solar cells based on metal oxide/fullerene ESLs.
    Article · Jan 2016 · Journal of Materials Chemistry A
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    Bradley West · Michael Stuckelberger · Harvey Guthrey · [...] · Mariana I Bertoni
    [Show abstract] [Hide abstract] ABSTRACT: It is well known that the addition of alkali elements such as Na and K during and after growth of Cu(In,Ga)Se2 (CIGS) has beneficial effects on the electronic properties of bulk material, improving device performance significantly. While the device level effects have been measured and reported, a direct observations of the localization of Na including its chemical nature are missing, and the impact of Na on elemental and phase segregation during CIGS growth is not fully understood. We investigate these aspects to shine light on the role of Na in CIGS solar cells with the ultimate goal of increasing their conversion efficiency. Utilizing a suite of synchrotron based x-ray characterization techniques, we discuss the challenges and advantages of these techniques for investigating segregation of main constituents of CIGS, Na distribution, chemical bonding of Na, and collection efficiency in CIGS as well as their correlations
    Full-text available · Conference Paper · Jan 2016
  • Pranab Sarker · Mowafak M. Al-Jassim · Muhammad N. Huda
    [Show abstract] [Hide abstract] ABSTRACT: A quaternary oxide, CuSnW2O8 (CTTO), has been predicted by density functional theory(DFT) to be a suitable material for sustainable photovoltaic applications. CTTO possesses band gaps of 1.25 eV (indirect) and 1.37 eV (direct), which were evaluated using the hybrid functional (HSE06) as a post-DFT method. The hole mobility of CTTO was higher than that of silicon. Further, optical absorption calculations demonstrate that CTTO is a better absorber of sunlight than Cu2ZnSnS4 and CuInxGa1−xSe2 (x = 0.5). In addition, CTTO exhibits rigorous thermodynamic stability comparable to WO3, as investigated by different thermodynamic approaches such as bonding cohesion, fragmentation tendency, and chemical potential analysis. Chemical potential analysis further revealed that CTTO can be synthesized at flexible experimental growth conditions, although the co-existence of at least one secondary phase is likely. Finally, like other Cu-based compounds, the formation of Cu vacancies is highly probable, even at Cu-rich growth condition, which could introduce p-type activity in CTTO.
    Article · Dec 2015 · Applied Physics Letters
  • Chuanxiao Xiao · Zhen Li · Harvey Guthrey · [...] · Mowafak M Al-Jassim
    [Show abstract] [Hide abstract] ABSTRACT: Electron-beam-induced damages in methylammonium lead triiodide (MAPbI(3)) perovskite thin films were studied by cathodoluminescence (CL) spectroscopy. We find that high-energy electron beams can significantly alter perovskite properties through two distinct mechanisms: (1) defect formation caused by irradiation damage and (2) phase transformation induced by electron-beam heating. The former mechanism causes quenching and broadening of the excitonic peaks in CL spectra, whereas the latter results in new peaks with higher emission photon energy. The electron-beam damage strongly depends on the electron-beam irradiation conditions. Although CL is a powerful technique for investigating the electronic properties of perovskite materials, irradiation conditions should be carefully controlled to avoid any significant beam damage. In general, reducing acceleration voltage and probing current, coupled with low-temperature cooling, is more favorable for CL characterization and potentially for other scanning electron-beam-based techniques as well. We have also shown that the stability of perovskite materials under electron-beam irradiation can be improved by reducing defects in the original thin films. In addition, we investigated effects of electron-beam irradiation on formamidinium lead triiodide (FAPbI(3)) and CsPbI3 thin films. FAPbI(3) shows similar behavior as MAPbI(3), whereas CsPbI3 displays higher resistance to electron-beam damage than its organic inorganic hybrid counterparts. Using CsPbI3 as a model material, we observed nonuniform luminescence in different grains of perovskite thin films. We also discovered that black-to-yellow phase transformation of CsPbI3 tends to start from the junctions at grain boundaries.
    Article · Nov 2015 · The Journal of Physical Chemistry C
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    Ana Kanevce · John Moseley · Mowafak Al-Jassim · Wyatt K. Metzger
    [Show abstract] [Hide abstract] ABSTRACT: We present a 2-D numerical model simulating cathodoluminescence (CL) measurements on CdTe. The model is used to analyze the impact of material parameters on the measured CL intensity to establish when grain-boundary (GB) recombination velocity SGB can be determined accurately from CL contrast. In addition to GB recombination, grain size and its ratio to the carrier diffusion length can impact CL measurements. Holding the grain interior and GB recombination rates constant, we find that as the grain size increases and exceeds the diffusion length, the observed CL contrast increases. For small-grain-size material, surface recombination lowers the overall intensity of the CL signal but does not significantly impact CL contrast. For large grains, high-surface recombination velocity can decrease the CL contrast. The model is combined with experimental results to quantify the SGB in polycrystalline CdTe before and after the CdCl2 treatment and to predict the impact of GB recombination on device performance.
    Full-text available · Article · Nov 2015 · IEEE Journal of Photovoltaics
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    Chun-Sheng Jiang · Mengjin Yang · Yuanyuan Zhou · [...] · Mowafak M. Al-Jassim
    [Show abstract] [Hide abstract] ABSTRACT: Organometal-halide perovskite solar cells have greatly improved in just a few years to a power conversion efficiency exceeding 20%. This technology shows unprecedented promise for terawatt-scale deployment of solar energy because of its low-cost, solution-based processing and earth-abundant materials. We have studied charge separation and transport in perovskite solar cells - which are the fundamental mechanisms of device operation and critical factors for power output - by determining the junction structure across the device using the nanoelectrical characterization technique of Kelvin probe force microscopy. The distribution of electrical potential across both planar and porous devices demonstrates p-n junction structure at the TiO 2 /perovskite interfaces and minority-carrier diffusion/drift operation of the devices, rather than the operation mechanism of either an excitonic cell or a p-i-n structure. Combining the potential profiling results with solar cell performance parameters measured on optimized and thickened devices, we find that carrier mobility is a main factor that needs to be improved for further gains in efficiency of the perovskite solar cells.
    Full-text available · Article · Sep 2015 · Nature Communications

Publication Stats

6k Citations


  • 1991-2010
    • National Renewable Energy Laboratory
      • National Center for Photovoltaics
      گلدن، کلرادو, Colorado, United States
    • Virginia Polytechnic Institute and State University
      Блэксбург, Virginia, 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
      Edo, Tōkyō, Japan