TY - GEN A1 - Bär, Marcus A1 - Schubert, Björn-Arvid A1 - Marsen, Björn A1 - Wilks, Regan G. A1 - Pookpanratana, Sujitra A1 - Blum, Monika A1 - Krause, Stefan A1 - Unold, Thomas A1 - Yang, Wan Li A1 - Weinhardt, Lothar A1 - Heske, Clemens A1 - Schock, Hans-Werner T1 - Cliff-like conduction band offset and KCN-induced recombination barrier enhancement at the CdS/Cu2ZnSnS4 thin-film solar cell heterojunction N2 - The electronic structure of the CdS/Cu2ZnSnS4 (CZTS) heterojunction was investigated by direct and inverse photoemission. The effects of a KCN etch of the CZTS absorber prior to CdS deposition on the band alignment at the respective interface were studied. We find a “cliff”-like conduction band offset at the CdS/CZTS interface independent of absorber pretreatment and a significant etch-induced enhancement of the energetic barrier for charge carrier recombination across the CdS/CZTS interface. KW - kesterite KW - thin-film solar cells KW - energy band alignment Y1 - 2011 ER - TY - GEN A1 - Bär, Marcus A1 - Schubert, Björn-Arvid A1 - Marsen, Björn A1 - Krause, Stefan A1 - Pookpanratana, Sujitra A1 - Unold, Thomas A1 - Weinhardt, Lothar A1 - Heske, Clemens A1 - Schock, Hans-Werner T1 - Impact of KCN etching on the chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers N2 - The chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers has been investigated by direct and inverse photoemission. Particular emphasis was placed on the impact of KCN etching, which significantly alters the surface composition and is best explained by a preferred etching of Cu and, to a lesser degree, Sn. As a consequence the surface band gap increased from (1.53 +/- 0.15) eV, which agrees with optically derived bulk band gap values, to (1.91 +/- 0.15) ev. KW - kesterite KW - thin-film solar cells KW - electronic surface structure Y1 - 2011 ER - 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 - 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 - 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 - 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 - TY - GEN A1 - Bär, Marcus A1 - Klaer, Joachim A1 - Weinhardt, Lothar A1 - Wilks, Regan G. A1 - Krause, Stefan A1 - Blum, Monika A1 - Yang, Wan Li A1 - Heske, Clemens A1 - Schock, Hans-Werner T1 - Cu 2−x S Surface Phases and Their Impact on the Electronic Structure of CuInS 2 Thin Films – A Hidden Parameter in Solar Cell Optimization T2 - Advanced Energy Materials N2 - The surface properties of CuInS 2 (CIS) thin-fi lm solar cell absorbers are investigated by a combination of electron and soft X-ray spectroscopies. Spatially separated regions of varying colors are observed and identifi ed to be dominated by either CuS or Cu 2 S surface phases. After their removal by KCN etching, the samples cannot be distinguished by eye and the CIS surface is found to be Cu-defi cient in both regions. However, a signifi cantly more pronounced off-stoichiometry in the region initially covered by Cu 2 S can be identifi ed. In this region, the resulting surface band gap is also signifi cantly larger than the E g Surf of the initially CuS-terminated region. Such variations may represent a hidden parameter which, if overlooked, induces irreproducibility and thus prevents systematic optimization efforts. Y1 - 2013 SN - 1614-6840 VL - 3 IS - 6 SP - 777 EP - 781 ER - TY - GEN A1 - Mezher, Michelle A1 - Garris, Rebekah A1 - Mansfield, Lorelle M. A1 - Blum, Monika A1 - Hauschild, Dirk A1 - Horsley, Kimberly A1 - Duncan, Douglas A. A1 - Yang, Wooyong A1 - Bär, Marcus A1 - Weinhardt, Lothar A1 - Ramanathan, Kannan A1 - Heske, Clemens T1 - Soft x-ray spectroscopy of a complex heterojunction in high-efficiency thin-film photovoltaics: Intermixing and Zn speciation at the Zn(O,S)/Cu(In,Ga)Se2 interface T2 - ACS Applied Materials & Interfaces Y1 - 2016 U6 - https://doi.org/10.1021/acsami.6b09245 SN - 1944-8244 VL - 8 IS - 48 SP - 33256 EP - 33263 ER -