G. Teeter

National Renewable Energy Laboratory, Golden, Colorado, United States

Are you G. Teeter?

Claim your profile

Publications (49)104.25 Total impact

  • [Show abstract] [Hide abstract]
    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; · 7.71 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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 01/2014; 115:173503. · 2.21 Impact Factor
  • [Show abstract] [Hide abstract]
    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 01/2014; · 1.87 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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. · 2.21 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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. · 1.67 Impact Factor
  • [Show abstract] [Hide abstract]
    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). · 5.03 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    03/2013;
  • [Show abstract] [Hide abstract]
    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. · 3.00 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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:A327-32. · 3.55 Impact Factor
  • [Show abstract] [Hide abstract]
    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). · 1.43 Impact Factor
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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. · 12.03 Impact Factor
  • Heli Wang, Glenn Teeter, John A. Turner
    [Show abstract] [Hide abstract]
    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. · 6.63 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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.
  • Source
    [Show abstract] [Hide abstract]
    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 01/2011; 84. · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    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;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on recent advances in the development of nanoscale measurements of the surface photovoltage (SPV) based on scanning tunneling microscopy (STM) and its application to the kesterites Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe). One critical aspect of the electronic structure of Cu(In, Ga)Se2 (CIGS) that has yet to be determined in their related kesterite compounds is the character of the surface electronics. In CIGS, spontaneous deviations in the stoichiometry of the surface cause a depletion (or even a type inversion) region that reinforces the CIGS homojunction. First-principle calculations predict that this inversion region will be more difficult to form in CZTS. In this contribution, the characteristics of the surface space charge region for both CIGS and CZTS(e) are investigated by STM. The implications of the results of these measurements on the future development of CZTS solar cells will be discussed.
    Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE; 01/2011
  • Source
    01/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on studies related to the synthesis of thin-film Cu<sub>2</sub>ZnSnS<sub>4</sub> via sulfurization of metal-precursor thin films. Combinatorially graded thin-film Cu-Zn-Sn library samples spanning various regions of the ternary Cu-Zn-Sn phase diagram were deposited at temperatures below 375 K and subsequently sulfurized in a high-vacuum system equipped with a sulfur valved-cracking source at temperatures from 600 K to 675 K. Comparisons of x-ray fluorescence and x-ray diffraction data from pre- and post-sulfurization films have revealed correlations between processing conditions, film composition, and the crystalline phases present. We have also performed cathodoluminescence and photoluminescence measurements and identified emission features consistent with the formation of the Cu<sub>2</sub>ZnSnS<sub>4</sub>.
    Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on the effects of replacing both In <sub>2</sub> O <sub>3</sub>: Sn (ITO) and the hole transport layer (HTL) in organic photovoltaic (OPV) cells with single-walled carbon nanotube (SWNT) network transparent electrodes. We have produced an OPV device without an HTL exhibiting an NREL-certified efficiency of 2.65% and a short-circuit current density of 11.2 mA / cm <sup>2</sup> . Our results demonstrate that SWNT networks can be used to replace both ITO and the HTL in efficient OPV devices and that the HTL serves distinctly different roles in ITO- and SWNT-based devices.
    Applied Physics Letters 07/2010; · 3.52 Impact Factor

Publication Stats

191 Citations
104.25 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, CO, United States