Glenn Teeter

National Renewable Energy Laboratory, Golden, Colorado, United States

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Publications (54)121.7 Total impact

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    ABSTRACT: Selenium diffusion in polycrystalline thin-film Cu2ZnSn(S,Se)4 (CZTSe) on molybdenum-coated soda-lime glass substrates was investigated by in situ monitoring of the molybdenum back-contact resistance during high-temperature selenization treatments. In these measurements, selenium diffusion through the CZTSe layer results in conversion of the molybdenum layer to MoSe2, increasing the sheet resistance of the film stack. By monitoring the rate of MoSe2 formation as a function of annealing temperature, an activation energy of 0.5 ± 0.1 eV has been measured for selenium diffusion in CZTSe. The partial pressure dependence of chalcogen diffusion suggests that chalcogen vacancies are not the defect controlling chalcogen diffusion in thin-film CZTSe.
    Journal of Applied Physics 02/2015; 117(7):074902. DOI:10.1063/1.4907951 · 2.19 Impact Factor
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    ABSTRACT: Secondary phase segregation is hypothesized to have detrimental impacts on Cu2ZnSnS4 (CZTS) thin-film solar cells. In this study, we demonstrate the potential of using kinetic stabilization to inhibit phase decomposition in CZTS. By growing CZTS films at low temperature, we achieve a kinetically stabilized alloy with an expanded solid solution window in the pseudoternary CuS-ZnS-SnS phase diagram. Using X-ray absorption spectroscopy, we study the structural evolution and stability of this metastable alloy upon annealing. For near-stoichiometric samples, we observe a continuous emergence of short-range order toward crystalline CZTS that is nearly complete after a 1-min anneal at 450 °C. For Zn-rich samples, we detect precipitation of ZnS upon annealing, which suggests that the excess Zn exists as cation antisite defects in metastable CZTS.
    IEEE Journal of Photovoltaics 01/2015; 5(1):372-377. DOI:10.1109/JPHOTOV.2014.2360334 · 3.00 Impact Factor
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    ABSTRACT: The development of cuprous oxide (Cu 2O) photovoltaics (PVs) is limited by low device open-circuit voltages. A strong contributing factor to this underperformance is the conduction-band offset between Cu 2O and its n-type heterojunction partner or electron-selective contact. In the present work, a broad range of possible n-type materials is surveyed, including ZnO, ZnS, Zn(O,S), (Mg,Zn)O, TiO2, CdS, and Ga2O3. Band offsets are determined through X-ray photoelectron spectroscopy and optical bandgap measurements. A majority of these materials is identified as having a negative conduction-band offset with respect to Cu 2O; the detrimental impact of this on open-circuit voltage (V OC) is evaluated through 1-D device simulation. These results suggest that doping density of the n-type material is important as well, and that a poorly optimized heterojunction can easily mask changes in bulk minority carrier lifetime. Promising heterojunction candidates identified here include Zn(O,S) with [S]/[Zn] ratios >70%, and Ga2O3, which both demonstrate slightly positive conduction-band offsets and high V OC potential. This experimental protocol and modeling may be generalized to evaluate the efficiency potential of candidate heterojunction partners for other PV absorbers, and the materials identified herein may be promising for other absorbers with low electron affinities.
    Applied Physics Letters 12/2014; 105(26):263901. DOI:10.1063/1.4905180 · 3.52 Impact Factor
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    ABSTRACT: A key requirement for large-scale deployment of photovoltaic technologies is the development of highly functional materials with controllable opto-electronic properties. In this work, we report on the room-temperature synthesis of disordered alloys of the Earth-abundant, tetrahedrally coordinated semiconductors Cu2SnS3, Cu2ZnSnS4 (CZTS), and ZnS as (Cu2SnB3)(1-x)(ZnS)(x). The resulting disordered semiconductors are found to have continuously and independently tunable optical and electronic properties. Quasi-isovalent alloying on the cation sublattice allows the optical band gap to be varied continuously from 1.1 eV to 2.8 eV. Aliovalent alloying leads to independent control of carrier concentration over at least three orders of magnitude. A conceptual framework describing these disordered materials is presented, in which the structural disorder, constrained by local tetrahedral coordination of both anions and cations, leads to the observed high degree of tunability of the opto-electronic properties. These materials are not only independently interesting, but the developed framework also applies to the opto-electronic properties of kesterite CZTS materials as well as provides a basis for the development of new semiconductors.
    Solar Energy Materials and Solar Cells 10/2014; 129:124-131. DOI:10.1016/j.solmat.2014.05.024 · 5.03 Impact Factor
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    ABSTRACT: Co-evaporated Cu2ZnSnSe4 (CZTSe) is used to examine sensitivities to the device performance that originate from variations in Zn content very near the surface. While integral Zn content of the film is held approximately constant, the surface composition is manipulated via changes to the Zn flux at the end of the deposition. Surface composition, device performance, and open-circuit voltage extrapolated to zero temperature are measured as a function of deposition termination. Origins of the apparent reduction in surface recombination with increasing Zn are discussed.
    Thin Solid Films 09/2014; DOI:10.1016/j.tsf.2014.09.028 · 1.87 Impact Factor
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    ABSTRACT: Cu 2ZnSn(S,Se)4 (CZTSSe) is an earth-abundant semiconductor with potential for economical photovoltaic power generation at terawatt scales. In this work, we use Raman scattering to investigate phase coexistence in combinatorial CZTS thin films grown at 325 or 470 °C. The surface of the samples grown at 325 °C is rough except for a prominent specularly reflective band near and along the ZnS-Cu2 SnS 3 (CTS) tie line in the Cu-Zn-Sn-S quaternary phase diagram. All structurally incoherent secondary phases (SnS 2, CuS) exist only as surface phases or are embedded as separate grains, whereas the structurally coherent secondary phase CTS coexists with CZTS in the dense underlying film. In films grown at 325 °C, which are kinetically trapped by the low growth temperature, a change is observed in Cu and Sn site occupancy, evidenced by the shift from cubic-CTS in the Cu-rich region (Cu/Sn > 2) to more tetragonal-CTS in the Sn-rich region (Cu/Sn < 2). For CZTS samples grown at 470 °C, CTS is not observed and regions grown under excess Sn flux are more disordered than Cu-rich regions evidenced by broader CZTS A mode peaks. Therefore, increasing Sn chemical potential results in more CZTS lattice disorder, suggesting, with other evidence, the formation of Sn antisite defects. In contrast, the CZTS A mode breadth is insensitive to Zn richness suggesting that excess Zn does not induce significant disorder within the CZTS lattice. We postulate that initially growing CZTS films Cu-rich (Cu/Sn > 2) results in higher cation ordering meaning fewer antisite defects.
    Journal of Applied Physics 05/2014; 115(17):173503. DOI:10.1063/1.4871665 · 2.19 Impact Factor
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    ABSTRACT: For more than 25 years, the CdTe photovoltaic research and manufacturing communities have been subjecting CdTe materials to a CdCl2 treatment or activation step to improve performance. However, little work has been carried out using imaging to elucidate the spatial distribution of chlorine in the CdTe devices after this treatment. This work addresses fundamental questions about the spatial distribution of chlorine in the CdTe absorber material after a CdCl2 treatment comparable to industrial practices. We used a state-of-the-art, time-of-flight secondary ion mass spectrometer (ION-TOF GmbH) (Muenster, Germany) with a lateral resolution of about 80 nm to complete three-dimensional depth-profiling and imaging of two CdTe devices. The results clearly demonstrate enhanced chlorine concentration along grain boundaries, supporting the hypothesis that chlorine plays an important role in passivating grain boundaries in CdTe solar cells. The results are discussed in terms of possible passivation mechanisms, and the effect of chlorine on grain interiors and grain boundaries. The data are also used to estimate the free energy of segregation of chlorine to grain boundaries in CdTe. Copyright © 2014 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 04/2014; DOI:10.1002/pip.2498 · 7.71 Impact Factor
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    ABSTRACT: Cu 2ZnSnS4 is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325 °C) and high (475 °C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line in the quasi-ternary Cu 2S-ZnS-SnS2 phase system, and this effect is attributed to structural similarity between the Cu 2ZnSnS4, Cu 2SnS3, and ZnS crystalline phases. At 475 °C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line. Room-temperature photoluminescence imaging reveals a strong correlation for these samples between film composition and photoluminescence intensity, where film regions with Cu/Sn ratios greater than ∼2 show strong photoluminescence intensity, in comparison with much weaker photoluminescence in regions that received excess Sn flux during growth or subsequent processing. The observed photoluminescence quenching in regions that received excess Sn flux is attributed to the effects of Sn-related native point defects in Cu 2ZnSnS4 on non-radiative recombination processes. Implications for processing and performance of Cu 2ZnSnS4 solar cells are discussed.
    Journal of Applied Physics 01/2014; 115:173502. DOI:10.1063/1.4871664 · 2.19 Impact Factor
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    ABSTRACT: Copper (I) oxide (Cu 2 O) is a direct band gap semiconductor with p-type conductivity and is a potential candidate for multi-junction solar cells. In this work, incoherent light source based photo-assisted metal-organic chemical vapor deposition (MOCVD) was used to deposit high quality Cu 2 O thin films on n-type ,100. silicon and quartz substrates. X-ray diffraction studies reveal that crystalline Cu 2 O is deposited. UV-Vis-NIR spectroscopy results indicated a band gap of 2.44 eV for Cu 2 O thin films. Transmission electron spectroscopy results show that the Cu 2 O film grows in the form of three-dimensional islands composed of smaller nanocrystalline grains in the range of 10–20 nm. I–V measurements indicate that the Cu 2 O/n-Si device fabricated using the MOCVD process has a lower dark current density than other devices reported in the literature.
    Journal of Materials Research 06/2013; 28(13):1740. DOI:10.1557/jmr.2013.150 · 1.82 Impact Factor
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    ABSTRACT: Accuracy in composition control has been one of the top issues for fabricating high-performance kesterite (Cu2ZnSn(Se,S)4) solar cells. A detailed understanding of the effect of Zn excess on device performance has not yet been demonstrated. Thus, specific criteria for high-performance devices, in particular discriminating between the effects of Zn-rich features at the front versus the back of the absorber, are desired. In this study, we report that co-evaporated kesterite absorbers can demonstrate high device efficiency despite the presence of large quantities of ZnSe. However, the benign presence of ZnSe is found to be conditional. While large ZnSe grains on the back of the absorbers are not harmful to device performance, the ZnSe grains produced by excess Zn near the end of the deposition degrade the cell efficiency from 8% level to 6% level (without anti-reflection coatings). The other effect related to excess Zn on the front of absorber is the facilitation of breakdown in lower reverse bias. The breakdown indicated here occurs only under the illumination of blue photons, and to our best knowledge has not been reported before. The exact mechanism of the breakdown remains open, but it is demonstrated to be related to the photoconductivity of CdS, and is thus possibly a symptom of lateral defect issues in the absorber, caused by the overdose of Zn. The same type of issue contributing to the breakdown may also be responsible for part of the parasitic loses at the working voltage, and therefore warrants further research.
    Solar Energy Materials and Solar Cells 06/2013; 113(June 2013). DOI:10.1016/j.solmat.2013.02.015 · 5.03 Impact Factor
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    ABSTRACT: Recently, Teeter et al. at NREL have discovered that Cu2ZnSnS4 thin films, of interest for photovoltaics, are amorphous (a-CZTS) when grown at room temperature and the film resistivity can be tuned over a wide range by controlling the Cu:Sn ratio. Tetrahedrally-coordinated amorphous semiconductors belong to an interesting class of compounds that are predicted to have the ability of being doped both p- and n-type. The four-fold coordination plays a critical role in unpinning the Fermi level to allow effective control over doping levels in a disordered structure. We performed extended X-ray absorption fine structure spectroscopy at the K-edges of Cu, Zn and Sn to determine the extent of structural disorder and tetrahedral coordination in a-CZTS films grown with varying Cu:Sn content. All films exhibit a high degree of structural disorder beyond the cations' first coordination shell. Both Cu and Zn atoms have high degree of tetrahedral coordination with respect to S atoms while the average coordination number of Sn decreases with increasing Sn content, indicative of either the favorable formation of sulfur vacancies around Sn atoms or the presence of Sn-related secondary phase. We combine these results with conductivity measurements to understand the relationship between the structural and electrical properties of this new material.
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    ABSTRACT: Recent years have seen dramatic improvements in the performance of kesterite devices. The existence of devices of comparable performance, made by a number of different techniques, provides some new perspective on what characteristics are likely fundamental to the material. Here, we review progress in kesterite device fabrication, aspects of the film characteristics that have yet to be understood, and challenges in device development that remain for kesterites to contribute significantly to photovoltaic manufacturing. Performance goals, as well as characteristics of midgap defect density, free carrier density, surfaces, grain boundaries, grain-to-grain uniformity, and bandgap alloying are discussed.
    IEEE Journal of Photovoltaics 01/2013; 3(1):439-445. DOI:10.1109/JPHOTOV.2012.2215842 · 3.00 Impact Factor
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    ABSTRACT: A polycrystalline Cu2ZnSnS4 thin film was deposited on fused quartz by co-evaporation. The selected thickness was ~100 nm to avoid artifacts in its optical properties caused by thicker as-grown films. The composition and phase of the film were checked with x-ray fluorescence, Raman shift spectroscopy, scanning transmission electron microscopy, and energy dispersive x-ray spectroscopy. An improved spectroscopic ellipsometry methodology with two-side measurement geometries was applied to extract the complex dielectric function ε = ε1 + iε2 of the Cu2ZnSnS4 thin film between 0.73 and 6.5 eV. Five critical points were observed, at 1.32 (fundamental band gap), 2.92, 3.92, 4.96, and 5.62 eV, respectively. The ε spectra are in reasonable agreement with those from theoretical calculations.
    Optics Express 03/2012; 20 Suppl 2(6):A327-32. DOI:10.1364/OE.20.00A327 · 3.53 Impact Factor
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    ABSTRACT: Kesterite thin films (i.e., CuâZnSn(S,Se)â and related alloys) have been the subject of recent interest for use as an absorber layer for thin film photovoltaics due to their high absorption coefficient (> 10⁴ cm⁻¹), their similarity to successful chalcopyrites (like CuInxGa{sub 1-x}Seâ) in structure, and their earth-abundance. The process window for growing a single-phase kesterite film is narrow. In this work, we have documented, for our 9.15%-efficient kesterite co-evaporation process, (1) how appearance of certain undesirable phases are controlled via choice of processing conditions, (2) several techniques for identification of phases in these films with resolution adequate to discern changes that are important to device performance, and (3) reference measurements for those performing such phase identification. Data from x-ray diffraction, x-ray fluorescence, Raman scattering, scanning electron microscopy, energy dispersive spectroscopy, and current-voltage characterization are presented.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2012; 30(5). DOI:10.1116/1.4732529 · 2.14 Impact Factor
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    ABSTRACT: In this contribution, we present a comparative study of the luminescence of the kesterites Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) and their related chalcopyrite Cu(In,Ga)Se2 (CIGSe). Luminescence spectroscopy suggests that the electronic properties of Zn-rich, Cu-poor kesterites (both CZTS and CZTSe) and Cu-poor CIGSe are dictated by fluctuations of the electrostatic and chemical potentials. The large redshift in the luminescence of grain boundaries in CIGSe, associated with the formation of a neutral barrier is clearly observed in CZTSe, and, to some extent, in CZTS. Kesterites can therefore replicate the fundamental electronic properties of CIGSe.
    Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE; 01/2012
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    ABSTRACT: The kesterite Cu2ZnSnS4 (CZTS) is attracting considerable interest because first-principles calculations predict that its electronic properties must be similar to their associated chalcopyrite Cu(In,Ga)Se2 (CIGS) compounds [Chen , Phys. Rev. BPLRBAQ0163-182910.1103/PhysRevB.81.245204 81, 245204 (2010)]. Here, the authors report on first experimental evidence of the close resemblance in the luminescence of Cu-poor kesterites and Cu-poor chalcopyrites used in photovoltaic applications. Microluminescence measurements suggest that even the very distinct electronic structure of grain boundaries in CIGS is present, to some extent, in CZTS. The similarities between CIGS and CZTS become more pronounced as the efficiency of the CZTS solar cells gradually increases. The implications of these results for the future development of CZTS solar cells are discussed.
    Physical review. B, Condensed matter 10/2011; 84. DOI:10.1103/PhysRevB.84.165324 · 3.66 Impact Factor
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    ABSTRACT: In this report, we investigate the electrical and optical properties of thin conducting films of SWNTs after treatment with small molecule and polymeric amines. Among those tested, we find hydrazine to be the most effective n-type dopant. We use absorbance, Raman, X-ray photoelectron, and nuclear magnetic resonance spectroscopies on thin conducting films and opaque buckypapers treated with hydrazine to study fundamental properties and spectroscopic signatures of n-type SWNTs and compare them to SWNTs treated with nitric acid, a well-characterized p-type dopant. We find that hydrazine physisorbs to the surface of semiconducting and metallic SWNTs and injects large electron concentrations, raising the Fermi level as much as 0.7 eV above that of intrinsic SWNTs. Hydrazine-treated transparent SWNT films display sheet resistances nearly as low as p-type nitric-acid-treated films at similar optical transmittances, demonstrating their potential for use in photovoltaic devices as low work function transparent electron-collecting electrodes.
    ACS Nano 03/2011; 5(5):3714-23. DOI:10.1021/nn200076r · 12.03 Impact Factor
  • Heli Wang, Glenn Teeter, John A. Turner
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    ABSTRACT: Nitridation is widely used to modify the materials surface. An electrochemical nitridation procedure, simple to handle at room temperature, is introduced for the surface modification of alloys. A nitrogen-incorporated oxide film was formed on the stainless steel's surface in a nitrate-bearing solution using the electrochemical method.
    Journal of Materials Chemistry 02/2011; 21(7):2064-2066. DOI:10.1039/C0JM03585H · 6.63 Impact Factor
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    ABSTRACT: Chalcopyrite solar cells based on CuInSe2 and associated alloys have demonstrated high efficiencies, with current annual shipments in the hundreds of megawatts (MW) range and increasing. Largely due to concern over possible indium (In) scarcity, a related set of materials, the kesterites, which comprise Cu2ZnSnS4 and associated alloys, has received increasing attention. Similarities and differences between kesterites and chalcopyrites are discussed as drawn from theory, depositions, and materials characterization. In particular, we discuss predictions from density functional theory, results from vacuum co-evaporation, and characterization via x-ray diffraction, scanning electron microscopy, electron beam-induced current, quantum efficiency, secondary ion mass spectroscopy, and luminescence.
    MRS Online Proceeding Library 01/2011; 1324. DOI:10.1557/opl.2011.844
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    ABSTRACT: Copper zinc tin sulfide (CZTS) is a promising Earth-abundant thin-film solar cell material; it has an appropriate band gap of ∼1.45 eV and a high absorption coefficient. The most efficient CZTS cells tend to be slightly Zn-rich and Cu-poor. However, growing Zn-rich CZTS films can sometimes result in phase decomposition of CZTS into ZnS and Cu2SnS3, which is generally deleterious to solar cell performance. Cubic ZnS is difficult to detect by XRD, due to a similar diffraction pattern. We hypothesize that synchrotron-based extended X-ray absorption fine structure (EXAFS), which is sensitive to local chemical environment, may be able to determine the quantity of ZnS phase in CZTS films by detecting differences in the second-nearest neighbor shell of the Zn atoms. Films of varying stoichiometries, from Zn-rich to Cu-rich (Zn-poor) were examined using the EXAFS technique. Differences in the spectra as a function of Cu/Zn ratio are detected. Linear combination analysis suggests increasing ZnS signal as the CZTS films become more Zn-rich. We demonstrate that the sensitive technique of EXAFS could be used to quantify the amount of ZnS present and provide a guide to crystal growth of highly phase pure films.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2011; DOI:10.1109/PVSC.2011.6186455

Publication Stats

592 Citations
121.70 Total Impact Points

Institutions

  • 2003–2014
    • National Renewable Energy Laboratory
      • National Center for Photovoltaics
      Golden, Colorado, United States
  • 2005
    • Colorado State University
      • Department of Physics
      Fort Collins, Colorado, United States