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ABSTRACT: __________________________________________________________________________________ ABSTRACT ZnSe/CuGaSe 2 heterojunctions were prepared by growing CuGaSe 2 absorber layers onto the (110) surface of ZnSe single crystals by flash evaporation method and chemical vapor deposition. The morphology of the absorber layers was studied by scanning electron microscopy. Elemental distribution in the investigated thin films and along heterojunction cross-section was characterized using energy dispersive X-ray analysis. All prepared heterojunctions show high open circuit voltages of 0.92…1.12 V, which slightly depend on the preparation method. Currents through the ZnSe/CuGaSe 2 heterojunctions are limited by the presence of a high resistivity (ρ ~ 10 8 -10 9 Ω cm) i-ZnSe layer at the interface, which is highly compensated by Cu impurities.
Moldavian Journal of the Physical Sciences. ; 1.
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01/2011;
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S. Sadewasser,
D. Abou-Ras,
D. Azulay,
R. Baier,
I. Balberg,
D. Cahen,
S. Cohen,
K. Gartsman,
K. Ganesan,
J. Kavalakkatt,
W. Li,
O. Millo,
Th. Rissom,
Y. Rosenwaks,
H. -W. Schock,
A. Schwarzman,
T. Unold
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ABSTRACT: Despite many recent research efforts, the influence of grain boundaries (GBs) on device properties of CuIn1†-†xGaxSe2 solar cells is still not fully understood Here, we present a microscopic approach to characterizing GBs in polycrystalline CuIn1†-†xGaxSe2 films with x†=†0.33. On samples from the same deposition process we applied methods giving complementary information, i.e., electron backscatter diffraction (EBSD), electron-beam induced current measurements (EBIC), conductive atomic force microscopy (c-AFM), variable-temperature Kelvin probe force microscopy (KPFM), and scanning capacitance microscopy (SCM). By combining EBIC with EBSD, we find a decrease in charge-carrier collection for non-[summation operator]3 GBs, while [summation operator]†3 GBs exhibit no variation with respect to grain interiors. In contrast, a higher conductance of GBs compared to grain interiors was found by c-AFM at low bias and under illumination. By KPFM, we directly measured the band bending at GBs, finding a variation from -80 up to +†115†mV. Depletion and even inversion at GBs was confirmed by SCM. We comparatively discuss the apparent differences between the results obtained by various microscopic techniques.
Thin Solid Films. 01/2011; 519(21):7341-7346.
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ABSTRACT: The symmetry-dependence of electronic grain boundary (GB) properties in polycrystalline CuInSe2 thin films was investigated in a combined study applying scanning electron microscopy, electron backscatter diffraction, and Kelvin probe force microscopy. We find that highly symmetric R3 GBs have a higher probability to be charge neutral than lower symmetric non-R3 GBs. This symmetry-dependence can help to explain the large variations of electronic properties found for GBs in Cu(In,Ga)Se2.
Applied Phyiscs Letters. 01/2011; 99:172102.
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ABSTRACT: Topographical and electronic and properties of step bunched Si(111), prepared by electrochemical processing in alkaline solution, are analyzed. Tapping mode atomic force microscopy (TM AFM) analysis shows that one bunched step consists of about 15 atomic steps (each 0.314 nm in height) and that the (111) oriented terraces have widths that range from 150 to 250 nm. Scanning tunneling microscopy (STM) experiments show a corrugation of the (111) terraces with an rms roughness of 0.5–0.8 nm, correlated with etch pits in alkaline solution. Low energy electron diffraction (LEED) data show a splitting of the (10) and (01) spot from which a minimum terrace width of 4.8 nm have been calculated in good agreement with the TM AFM data. Kelvin probe force microscopy (KPFM) experiments show a decrease of the contact potential difference (CPD) at and near the edges of steps indicating a more negatively charged surface area. Synchrotron radiation photoelectron spectroscopy (SRPES) on electrochemically and purely chemically prepared step bunched surfaces is compared. From the Si 2p core level shift, and, in particular, from the onset of the valence band emission, an accumulation layer-type shift is observed on the electrochemically prepared sample that is absent for chemical preparation. The move of the Fermi level toward the conduction band minimum of the electrochemically conditioned samples is interpreted by H incorporation and discussed by a doping model that involves the mechanism of hydrogen evolution.
physica status solidi (b) 09/2010; 248(2):361 - 369. · 1.32 Impact Factor
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ABSTRACT: The unusual optoelectronic properties of chalcopyrite grain boundaries (GBs) have become the subject of an intense debate in recent years. In this work we investigate the defect density at GBs of Cu(In,Ga)Se2 by scanning tunneling spectroscopy. Contrary to our expectation, our results give evidence for a reduced density of deep level defects and point to an increased density of defect levels in resonance with the lower conduction band at GBs. Our findings imply low recombination activity at GBs, and thus can explain the low impact of GBs on the efficiency of chalcopyrite based solar cells.
Physical Review Letters 09/2010; 105(11):116802. · 7.37 Impact Factor
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ABSTRACT: Long-range electrostatic forces govern the imaging mechanism in electrostatic force microscopy as well as in Kelvin probe force microscopy. To improve the analysis of such images, simulations of the electrostatic field distribution have been performed in the past using a flat surface and a cone-shaped tip. However, the electrostatic field distribution between a tip and a sample depends strongly on the surface topography, which has been neglected in previous studies. It is therefore of general importance to study the influence of sample topography features on Kelvin probe force microscopy images, which we address here by performing finite element simulations. We show how the surface potential measurement is influenced by surface steps and surface grooves, considering potential variations in the form of a potential peak and a potential step. The influence of the topography on the measurement of the surface potential is found to be rather small compared to a typical experimental resolution. Surprisingly, in the case of a coinciding topography and potential step an improvement of the potential profile due to the inclusion of the topography is observed. Finally, based on the obtained results, suggestions for the realization of KPFM measurement are given.
Nanotechnology 12/2009; 20(50):505503. · 3.98 Impact Factor
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ABSTRACT: Surface photovoltage (SPV) spectroscopy is a common method for optoelectronic semiconductor characterization. Kelvin probe force microscopy has developed into a widely used tool for nanoscale characterization of semiconductors, metals, and insulators. We present here a setup for the measurement of local SPV spectra in a Kelvin probe force microscope operated under ultrahigh vacuum conditions. The atomic force microscope tip can be placed to any desired position with nanometer precision and the SPV can then be recorded as a function of the wavelength of the illuminating light. We introduce the realization of the setup and present the SPV spectra on two test systems, an epitaxially grown GaAs / CuGaSe <sub>2</sub> junction and a Zn-doped CuInS <sub>2</sub> polycrystalline thin film.
Review of Scientific Instruments 02/2009; · 1.37 Impact Factor
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ABSTRACT: In this work we present a combination of two spatially resolved experimental methods to analyse correlations be tween distributions of element concentrations and elec tronic structure of cross sections of thin film semiconductor devices. We applied these methods to a cross section of a thin film Cu In1 xGax S2 CdS ZnO solar cell. The electronic structure was analysed by measuring the work function of the cross section by Kelvin probe force microscopy. To determine the gallium concentration of the cross section of the Cu In1 xGax S2 layer we performed energy dispersive X ray analysis EDX . We were able to match the spatial coordinates of these measurements. By this we could observe a correlation between the gallium concentration of the Cu In1 xGax S2 layer and its work function. The EDX meas urements show that the Cu In1 xGax S2 layer features a two layer structure one layer with a low gallium concentration x amp; 8776; 0 and one at the back of the solar cell with a high gal lium concentration x amp; 8776; 1 . Furthermore we discuss the influ ence of the two layer structure of the Cu In1 xGax S2 absorber layer on the photovoltaic properties of the solar cell. The spectral quantum efficiency was measured on the same sam ple before the depth resolved data were taken. We observe an improvement of short circuit current and overall efficiency of the Cu In1 xGax S2 CdS ZnO solar cell compared to a gallium free CuInS2 CdS ZnO solar cell.
physica status solidi (a) 01/2009; 206:1017-1020. · 1.21 Impact Factor
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ABSTRACT: An extensive Kelvin probe force microscopy study in an ultrahigh vacuum has been undertaken to examine the influence of growth modifications of a few nm thick CdS buffer layers in thin film chalcopyrite solar cells. In regions around the grain boundaries of the polycrystalline Cu(In,Ga)Se(2) substrate a lowering of the work function extending to about 200 nm away from this vertical interface was observed. This electrical inhomogeneity depends strongly on the Cu(In,Ga)Se(2) surface condition and is interpreted by a diffusion process along the substrate grain boundaries. Our results contribute to the understanding of the crucial role of the several nm thick CdS layer for improving the photovoltaic performance of chalcopyrite thin film solar cells.
Nanotechnology 04/2008; 19(14):145705. · 3.98 Impact Factor
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Physical Review Letters 07/2007; 98(26):269701; discussion 269702. · 7.37 Impact Factor
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ABSTRACT: Cross-sectional Kelvin probe force microscopy (KPFM) in ultrahigh vacuum is used to characterize the electronic structure of In Ga As / In P quantum wells. The KPFM signal shows clear peaks at the position of the quantum wells and exhibits a systematic trend for different wells. It is demonstrated that KPFM is capable of detecting quantum wells as narrow as 5 nm . Evidence for carrier accumulation in the quantum wells is observed. A complete quantitative analysis of the quantum well properties is shown to be impeded by tip averaging effects and due to surface/interface states.
Applied Physics Letters 12/2004; · 3.84 Impact Factor
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ABSTRACT: Kelvin probe force microscopy in ultrahigh vacuum was used to image the electronic structure of thin-film solar cell cross sections based on as-grown Cu-rich Cu Ga Se <sub>2</sub> absorbers. We observe different secondary phases in the absorber film. A p -type degenerate Cu <sub>2-x</sub> Se phase is identified by a higher work function (Φ∼5.35 eV ) than Cu Ga Se <sub>2</sub> (Φ∼5.1 eV ) , allowing good contrast mappings of both phases within the absorber film. Besides entire Cu <sub>2-x</sub> Se crystallites we also observed this secondary phase segregated as an interfacial layer along Cu Ga Se <sub>2</sub> grain boundaries. An additional high-work function phase at the Cu Ga Se <sub>2</sub>/ window junction is attributed to the formation of an improper CuS buffer layer during chemical bath processing. The detrimental effect of these secondary phases on the solar cell performance is discussed.
Applied Physics Letters 11/2004; · 3.84 Impact Factor
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M. Bar,
M. Rusu,
J. Reiss,
T. Glatzel, S. Sadewasser,
W. Bohne,
E. Strub,
H.-J. Muffler,
S. Lindner,
J. Rohrich,
T.P. Niesen,
F. Karg,
M.C. Lux-Steiner,
C.-H. Fischer
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ABSTRACT: Uncapsulated CdS buffered chalcopyrite solar cells as well as devices where the buffer and the intrinsic part of the rf-sputtered ZnO window bi-layer is replaced by a ZnO window extension layer were exposed to damp-heat (DH) conditions. In order to investigate the degradation mechanisms they were characterized by J(V)-measurements under standard test conditions and at different temperatures (200-330 K) and illuminations (0.1-100 mW/cm/sup 2/) as well as by quantum efficiency, ultrahigh vacuum Kelvin probe force microscopy and elastic recoil detection analysis before and after DH. All measurements reveal a significant change of the corresponding characteristics induced by DH.
Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003
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ABSTRACT: Current high-efficiency chalcopyrite thin-film solar cells contain multilayer structures consisting of an absorber, a buffer and a window layer. The modeling and optimization of the structures have been hampered by the lack of characterization methods to assess the electrical potential and conduction band alignment in actual devices.In this work, Kelvin probe force microscopy (KPFM) under ultrahigh vacuum (UHV) conditions is used to directly image the electronic structure of a complete thin film solar cell based on Cu(In,Ga)(S,Se)2 absorber material. The potential distribution along different solar cells is directly measured by KPFM on polished and UHV-cleaned cross-sections. Due to the high-energy sensitivity together with a lateral resolution in the nanometer range, detailed information about the various interfaces within the heterostructure is obtained. In combination with simulations of the tip–sample interaction, the work function of the different layers and the built-in voltage of the heterostructure are deduced. In our previous work, we have demonstrated that the use of a Zn1−xMgxO alloy instead of the i-ZnO layer influences the conduction band offset between chalcopyrite absorber and window layer. This substitution enabled us to improve the solar cell performance from η=6.3% for the CdS-free solar cell with pure i-ZnO to η=12.5% for the cell with Zn1−xMgxO, which is comparable to that of the reference cells with a CdS buffer (η=13.2%). We present KPFM studies of comparable devices to illustrate the possibilities of our novel characterization method.The studies demonstrate that KPFM is an excellent tool for the characterization of heterostructures on a nanometer length scale. In chalcopyrite solar cells, the KPFM technique can lead to a direct correlation between the electronic structure and the solar cell performance.
Thin Solid Films.
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ABSTRACT: Cu phthalocyanine CuPc serves as a very efficient absorber molecule in organic solar cell devices. The authors have applied spatially resolved surface photovoltage spectroscopy in a Kelvin probe force microscope to investigate the absorption and exciton separation of CuPc molecules on Si 100 and ITO substrates. A positive surface photovoltage was observed for illumination energies above 1.5 eV, indicating absorption of the light in the CuPc molecules and subsequent separation of holes into the substrate
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ABSTRACT: We have grown epitaxial grain boundaries of CuGaSe2 by metal organic vapour phase epitaxy onto a GaAs substrate containing a S3 grain boundary. SEM micrographs show a dense grain boundary. TEM micrographs prove that the grain boundary in the film is the direct continuation of the grain boundary in the substrate. HRTEM shows that the grain boundary in the film is a twin as well and thus a S3 boundary. Thus, by using a S3 grain boundary in the cubic GaAs substrate as a template a S3 grain boundary is obtained in the tetragonal CuGaSe2 film. Kelvin Probe Force Microscopy gives no indication of a space charge around this grain boundary, while in Hall measurements a small barrier of a few 10 meV is evident. This is an experimental indication for the existence of neutral grain boundaries as predicted theoretically
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ABSTRACT: Single grain boundaries in CuGaSe2 have been grown epitaxially. Hall measurements indicate a barrier of 30 40 meV to majority carrier transport. Nevertheless, local surface potential measurements show the absence of space charge around the grain boundary, i.e. it is neutral. Theoretical calculations [Persson and Zunger, Phys. Rev. Lett. 91, 266401 2003 ] have predicted a neutral barrier for the present S3 grain boundary. Thus, we have experimentally shown the existence of a neutral grain boundary barrier, however, smaller than theoretically predicted
