TY - GEN A1 - Sullivan, J. T. A1 - Wilks, Regan G. A1 - Winkler, M. T. A1 - Weinhardt, Lothar A1 - Recht, D. A1 - Said, A. J. A1 - Newman, B. K. A1 - Zhang, Y. A1 - Blum, Monika A1 - Krause, S. A1 - Yang, Wan Li A1 - Heske, Clemens A1 - Aziz, M. J. A1 - Bär, Marcus A1 - Buonassisi, T. T1 - Soft x-ray emission spectroscopy studies of the electronic structure of silicon supersaturated with sulfur N2 - We apply soft x-ray emission spectroscopy (XES) to measure the electronic structure of crystalline silicon supersaturated with sulfur (up to 0.7 at. %), a candidate intermediate-band solar cell material. Si L2,3 emission features are observed above the conventional Si valence band maximum, with intensity scaling linearly with S concentration. The lineshape of the S-induced features change across the insulator-to-metal transition, indicating a significant modification of the local electronic structure concurrent with the change in macroscopic electronic behavior. The relationship between the Si L2,3 XES spectral features and the anomalously high sub-band gap infrared absorption is discussed. KW - black silicon KW - solar cells KW - electronic surface structure KW - x-ray emission spectroscopy Y1 - 2011 ER - TY - GEN A1 - Bär, Marcus A1 - Song, X. A1 - Caballero, R. A1 - Duarte, Roberto Felix A1 - Gerlach, Dominic A1 - Kaufmann, Christian A1 - Schock, Hans-Werner A1 - Wilks, Regan G. T1 - Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure T2 - Journal of Applied Physics N2 - Na has deliberately been incorporated into Cu(In,Ga)Se2 (“CIGSe”) chalcopyrite thin-film solar cell absorbers deposited on Mo-coated polyimide flexible substrates by adding differently thick layers of NaF in-between CIGSe absorber and Mo back contact. The impact of Na on the chemical and electronic surface structure of CIGSe absorbers with various Cu-contents deposited at comparatively low temperature (420 °C) has been studied using x-ray photoelectron and x-ray excited Auger electron spectroscopy. We observe a higher Nasurface content for the Cu-richer CIGSe samples and can distinguish between two different chemical Na environments, best described as selenide-like and oxidized Na species, respectively. Furthermore, we find a Cu-poor surface composition of the CIGSe samples independent of Na content and — for very high Na contents — indications for the formation of a (Cu,Na)–(In,Ga)–Se like compound. With increasing Nasurface content, also a shift of the photoemission lines to lower binding energies could be identified, which we interpret as a reduction of the downward band bending toward the CIGSe surface explained by the Na-induced elimination of InCu defects. Y1 - 2012 SN - 1089-7550 VL - 111 IS - 3 SP - 034903 ER - TY - GEN A1 - Bär, Marcus A1 - Schubert, Björn-Arvid A1 - Marsen, Björn A1 - Wilks, Regan G. A1 - Blum, M. A1 - Krause, S. A1 - Pookpanratana, Sujitra A1 - Zhang, Y. A1 - Unold, Thomas A1 - Yang, W. A1 - Weinhardt, Lothar A1 - Heske, Clemens A1 - Schock, Hans-Werner T1 - Cu2ZnSnS4 thin-film solar cell absorbers illuminated by soft x-rays T2 - Journal of Materials Research N2 - In view of the complexity of thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the chemical and electronic structure of these components. Because of the high complexity of the Cu2ZnSn(S,Se)4 compound semiconductor absorber material alone, this is particularly true for kesterite-based devices. Hence, this paper reviews our recent progress in the characterization of Cu2ZnSnS4 (CZTS) thin films. It is demonstrated that a combination of different soft x-ray spectroscopies is an extraordinarily powerful method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. The focus of the article will be on secondary impurity phases, electronic structure, native oxidation, and the CZTS surface composition. Y1 - 2012 U6 - https://doi.org/10.1557/jmr.2012.59 SN - 2044-5326 VL - 27 IS - 8 SP - 1097 EP - 1104 ER - TY - GEN A1 - Weinhardt, Lothar A1 - Benkert, Andreas A1 - Meyer, Frank A1 - Blum, Monika A1 - Wilks, Regan G. A1 - Yang, Wan Li A1 - Bär, Marcus A1 - Reinert, Friedrich A1 - Heske, Clemens T1 - Nuclear dynamics and spectator effects in resonant inelastic soft x-ray scattering of gas-phase water molecules T2 - The journal of chemical physics N2 - The electronic structure of gas-phase H2O and D2O molecules has been investigated using resonant inelastic soft x-ray scattering (RIXS). We observe spectator shifts for all valence orbitals when exciting into the lowest three absorption resonances. Strong changes of the relative valence orbital emission intensities are found when exciting into the different absorption resonances, which can be related to the angular anisotropy of the RIXS process. Furthermore, excitation into the 4a1 resonance leads to nuclear dynamics on the time scale of the RIXS process; we find evidence for vibrational coupling and molecular dissociation in both, the spectator and the participant emission. Y1 - 2012 SN - 1089-7690 VL - 136 IS - 14 SP - 144311 ER - TY - GEN A1 - Wilks, Regan G. A1 - Repins, Ingrid A1 - Contreras, M. A. A1 - Duarte, Roberto Felix A1 - Herrero-Albillos, J. A1 - Tati-Bismaths, L. A1 - Kronast, F. A1 - Noufi, R. A1 - Bär, Marcus T1 - Intergrain variations of the chemical and electronic surface structure of polycrystalline Cu(In,Ga)Se2 thin-film solar cell absorbers T2 - Applied Physics Letters N2 - The μm-scale spatial distribution of the elements of polycrystalline Cu(In1− x Ga x )Se2 absorber surfaces is examined using x-ray photoelectron emission microscopy. The chemical composition varies from grain to grain, and a direct, linear anticorrelation between the In 3d and Ga 2p photoemission line intensities is observed. The line intensities are interpreted in terms of a varying value of x= Ga/(In+ Ga); the band gaps calculated from the inferred compositions of the grains are shown to be normally distributed with a standard deviation of 40 meV. Y1 - 2012 SN - 0003-6951 VL - 101 IS - 10 SP - 103908-1 EP - 103908-4 ER - TY - GEN A1 - Blum, M. A1 - Odelius, Michael A1 - Weinhardt, Lothar A1 - Pookpanratana, Sujitra A1 - Bär, Marcus A1 - Zhang, Y. A1 - Fuchs, Oliver A1 - Yang, W. A1 - Umbach, Eberhard A1 - Heske, Clemens T1 - Ultrafast Proton Dynamics in Aqueous Amino Acid Solutions Studied by Resonant Inelastic Soft X‑ray Scattering T2 - The Journal of physical chemistry : B N2 - Resonant inelastic soft X-ray scattering (RIXS) has been used to study the electronic structure of glycine and lysine in aqueous solution. Upon variation of the pH value of the solution from acidic to basic, major changes of the nitrogen K edge RIXS data are observed for both amino acids, which are associated with the protonation and deprotonation of the amino groups. The experimental results are compared with simulations based on density functional theory, yielding a detailed understanding of the spectral changes, as well as insights into the ultrafast proton dynamics in the intermediate core-excited/ionized state of the RIXS process. Y1 - 2012 SN - 1520-5207 VL - 116 IS - 46 SP - 13757 EP - 13764 ER - TY - GEN A1 - Hofmann, Timo A1 - Yu, Ted H. A1 - Folse, Michael A1 - Weinhardt, Lothar A1 - Bär, Marcus A1 - Zhang, Yufeng A1 - Merinov, Boris V. A1 - Myers, Deborah J. A1 - Goddard, William A. A1 - Heske, Clemens T1 - Using Photoelectron Spectroscopy and Quantum Mechanics to Determine d‑Band Energies of Metals for Catalytic Applications T2 - The Journal of Physical Chemistry : C N2 - The valence band structures (VBS) of eight transition metals (Fe, Co, Ni, Cu, Pd, Ag, Pt, Au) were investigated by photoelectron spectroscopy (PES) using He I, He II, and monochromatized Al Kα excitation. The influence of final states, photoionization cross-section, and adsorption of residual gas molecules in an ultrahigh vacuum environment are discussed in terms of their impact on the VBS. We find that VBSs recorded with monochromatized Al Kα radiation are most closely comparable to the ground state density of states (DOS) derived from quantum mechanics calculations. We use the Al Kα-excited PES measurements to correct the energy scale of the calculated ground-state DOS to approximate the “true” ground-state d-band structure. Finally, we use this data to test the d-band center model commonly used to predict the electronicproperty/ catalytic-activity relationship of metals. We find that a simple continuous dependence of activity on d-band center position is not supported by our results (both experimentally and computationally). Y1 - 2012 SN - 1932-7455 VL - 116 IS - 45 SP - 24016 EP - 24026 ER - TY - GEN A1 - Gerlach, Dominic A1 - Wippler, David A1 - Wilks, Regan G. A1 - Wimmer, Manuel A1 - Lozac'h, Mickaël A1 - Duarte, Roberto Felix A1 - Ueda, Shigenori A1 - Yoshikawa, Hideki A1 - Lips, Klaus A1 - Rech, Bernd A1 - Sumiya, Masatomo A1 - Kobayashi, Kaoru A1 - Gorgoi, Mihaela A1 - Hüpkes, Jürgen A1 - Bär, Marcus T1 - p-Type a-Si:H/ZnO:Al and μc-Si:H/ZnO:Al Thin-Film Solar Cell Structures—A Comparative Hard X-Ray Photoelectron Spectroscopy Study T2 - IEEE journal of photovoltaics N2 - The chemical and electronic properties of a- Si:H(B)/ZnO:Al and μc-Si:H(B)/ZnO:Al thin-film solar cell structures are studied by hard X-ray photoelectron spectroscopy (HAXPES). Using a combination of different X-ray excitation energies and deliberate sample design, we were able to select the probed volume, i.e., the silicon capping layer only or the silicon and zinc oxide layer (including the buried interface). For the a-Si:H(B) material, we find a higher deposition rate and a smaller value for the modified Auger parameter than for μc-Si:H(B). In addition, we find indications of a pronounced band bending limited to the very surface of the a-Si:H(B) and the μc-Si:H(B) layers, which is more distinct in the latter case. Y1 - 2013 SN - 2156-3381 VL - 3 IS - 1 SP - 483 EP - 487 ER - TY - GEN A1 - Meyer, Frank A1 - Weinhardt, Lothar A1 - Blum, Mia A1 - Bär, Marcus A1 - Wilks, Regan G. A1 - Yang, Wooyong A1 - Heske, Clemens A1 - Reinert, F. T1 - Non-equivalent carbon atoms in the resonant inelastic soft X-ray scattering map of cysteine T2 - The journal of chemical physics N2 - Resonant inelastic soft x-ray scattering (RIXS) was used to study the electronic structure of solid cysteine films. A RIXS map approach, i.e., plotting the x-ray emission intensity as a function of excitation and emission energy, allows us to separate the contributions of the three chemically nonequivalent carbon atoms in cysteine. In particular, we can identify orbitals localized near the photoexcited atoms, as well as orbitals that are delocalized over the entire molecule. Y1 - 2013 SN - 1089-7690 VL - 138 IS - 3 SP - 034306 ER - TY - GEN A1 - Bär, Marcus A1 - Klaer, Joachim A1 - Duarte, Roberto Felix A1 - Barreau, Nicolas A1 - Weinhardt, Lothar A1 - Wilks, Regan G. A1 - Heske, Clemens A1 - Schock, Hans-Werner T1 - Surface Off-Stoichiometry of CuInS2 Thin-Film Solar Cell Absorbers T2 - IEEE Journal of Photovoltaics N2 - In this paper, X-ray photoelectron and X-ray-excited Auger electron spectroscopy was used to investigate the chemical surface structure of CuInS2 thin-film solar cell absorbers.We find that the [In]/[Cu] surface composition can vary between 1.6 (±0.4) and 3.7 (±0.7), depending on relatively minor variations in the absorber formation process and/or whether additional wet-chemical treatments are performed. These variations are primarily due to differences in the Cu surface concentration. The corresponding change of the modified In Auger parameter is interpreted as being indicative of a change in the chemical environment of In as a function of Cu off-stoichiometry. Y1 - 2013 UR - https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6380521 SN - 2156-3381 VL - 3 IS - 2 SP - 828 EP - 832 ER - TY - GEN A1 - Hofmann, Timo A1 - Yu, Ted H. A1 - Folse, Michael A1 - Weinhardt, Lothar A1 - Bär, Marcus A1 - Zhang, Yufeng A1 - Merinov, Boris V. A1 - Myers, Deborah J. A1 - Goddard, William A. A1 - Heske, Clemens T1 - Reply to "Comment on 'Using Photoelectron Spectroscopy and Quantum Mechanics to Determine d-Band Energies of Metals for Catalytic Applications'" T2 - Journal of Physical Chemistry : C Y1 - 2013 SN - 1932-7455 VL - 117 IS - 13 SP - 6916 EP - 6917 ER -