@misc{BaerSchubertMarsenetal., author = {B{\"a}r, Marcus and Schubert, Bj{\"o}rn-Arvid and Marsen, Bj{\"o}rn and Krause, Stefan and Pookpanratana, Sujitra and Unold, Thomas and Weinhardt, Lothar and Heske, Clemens and Schock, Hans-Werner}, title = {Impact of KCN etching on the chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers}, abstract = {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.}, language = {en} } @misc{SullivanWilksWinkleretal., author = {Sullivan, J. T. and Wilks, Regan G. and Winkler, M. T. and Weinhardt, Lothar and Recht, D. and Said, A. J. and Newman, B. K. and Zhang, Y. and Blum, Monika and Krause, S. and Yang, Wan Li and Heske, Clemens and Aziz, M. J. and B{\"a}r, Marcus and Buonassisi, T.}, title = {Soft x-ray emission spectroscopy studies of the electronic structure of silicon supersaturated with sulfur}, abstract = {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.}, language = {en} } @misc{BaerSongCaballeroetal., author = {B{\"a}r, Marcus and Song, X. and Caballero, R. and Duarte, Roberto Felix and Gerlach, Dominic and Kaufmann, Christian and Schock, Hans-Werner and Wilks, Regan G.}, title = {Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure}, series = {Journal of Applied Physics}, volume = {111}, journal = {Journal of Applied Physics}, number = {3}, issn = {1089-7550}, pages = {034903}, abstract = {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.}, language = {en} } @misc{BaerSchubertMarsenetal., author = {B{\"a}r, Marcus and Schubert, Bj{\"o}rn-Arvid and Marsen, Bj{\"o}rn and Wilks, Regan G. and Blum, M. and Krause, S. and Pookpanratana, Sujitra and Zhang, Y. and Unold, Thomas and Yang, W. and Weinhardt, Lothar and Heske, Clemens and Schock, Hans-Werner}, title = {Cu2ZnSnS4 thin-film solar cell absorbers illuminated by soft x-rays}, series = {Journal of Materials Research}, volume = {27}, journal = {Journal of Materials Research}, number = {8}, issn = {2044-5326}, doi = {10.1557/jmr.2012.59}, pages = {1097 -- 1104}, abstract = {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.}, language = {en} } @misc{WeinhardtBenkertMeyeretal., author = {Weinhardt, Lothar and Benkert, Andreas and Meyer, Frank and Blum, Monika and Wilks, Regan G. and Yang, Wan Li and B{\"a}r, Marcus and Reinert, Friedrich and Heske, Clemens}, title = {Nuclear dynamics and spectator effects in resonant inelastic soft x-ray scattering of gas-phase water molecules}, series = {The journal of chemical physics}, volume = {136}, journal = {The journal of chemical physics}, number = {14}, issn = {1089-7690}, pages = {144311}, abstract = {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.}, language = {en} } @misc{WilksRepinsContrerasetal., author = {Wilks, Regan G. and Repins, Ingrid and Contreras, M. A. and Duarte, Roberto Felix and Herrero-Albillos, J. and Tati-Bismaths, L. and Kronast, F. and Noufi, R. and B{\"a}r, Marcus}, title = {Intergrain variations of the chemical and electronic surface structure of polycrystalline Cu(In,Ga)Se2 thin-film solar cell absorbers}, series = {Applied Physics Letters}, volume = {101}, journal = {Applied Physics Letters}, number = {10}, issn = {0003-6951}, pages = {103908-1 -- 103908-4}, abstract = {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.}, language = {en} } @misc{BlumOdeliusWeinhardtetal., author = {Blum, M. and Odelius, Michael and Weinhardt, Lothar and Pookpanratana, Sujitra and B{\"a}r, Marcus and Zhang, Y. and Fuchs, Oliver and Yang, W. and Umbach, Eberhard and Heske, Clemens}, title = {Ultrafast Proton Dynamics in Aqueous Amino Acid Solutions Studied by Resonant Inelastic Soft X‑ray Scattering}, series = {The Journal of physical chemistry : B}, volume = {116}, journal = {The Journal of physical chemistry : B}, number = {46}, issn = {1520-5207}, pages = {13757 -- 13764}, abstract = {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.}, language = {en} } @misc{HofmannYuFolseetal., author = {Hofmann, Timo and Yu, Ted H. and Folse, Michael and Weinhardt, Lothar and B{\"a}r, Marcus and Zhang, Yufeng and Merinov, Boris V. and Myers, Deborah J. and Goddard, William A. and Heske, Clemens}, title = {Using Photoelectron Spectroscopy and Quantum Mechanics to Determine d‑Band Energies of Metals for Catalytic Applications}, series = {The Journal of Physical Chemistry : C}, volume = {116}, journal = {The Journal of Physical Chemistry : C}, number = {45}, issn = {1932-7455}, pages = {24016 -- 24026}, abstract = {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).}, language = {en} } @misc{GerlachWipplerWilksetal., author = {Gerlach, Dominic and Wippler, David and Wilks, Regan G. and Wimmer, Manuel and Lozac'h, Micka{\"e}l and Duarte, Roberto Felix and Ueda, Shigenori and Yoshikawa, Hideki and Lips, Klaus and Rech, Bernd and Sumiya, Masatomo and Kobayashi, Kaoru and Gorgoi, Mihaela and H{\"u}pkes, J{\"u}rgen and B{\"a}r, Marcus}, title = {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}, series = {IEEE journal of photovoltaics}, volume = {3}, journal = {IEEE journal of photovoltaics}, number = {1}, issn = {2156-3381}, pages = {483 -- 487}, abstract = {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.}, language = {en} } @misc{MeyerWeinhardtBlumetal., author = {Meyer, Frank and Weinhardt, Lothar and Blum, Mia and B{\"a}r, Marcus and Wilks, Regan G. and Yang, Wooyong and Heske, Clemens and Reinert, F.}, title = {Non-equivalent carbon atoms in the resonant inelastic soft X-ray scattering map of cysteine}, series = {The journal of chemical physics}, volume = {138}, journal = {The journal of chemical physics}, number = {3}, issn = {1089-7690}, pages = {034306}, abstract = {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.}, language = {en} } @misc{BaerKlaerDuarteetal., author = {B{\"a}r, Marcus and Klaer, Joachim and Duarte, Roberto Felix and Barreau, Nicolas and Weinhardt, Lothar and Wilks, Regan G. and Heske, Clemens and Schock, Hans-Werner}, title = {Surface Off-Stoichiometry of CuInS2 Thin-Film Solar Cell Absorbers}, series = {IEEE Journal of Photovoltaics}, volume = {3}, journal = {IEEE Journal of Photovoltaics}, number = {2}, issn = {2156-3381}, pages = {828 -- 832}, abstract = {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.}, language = {en} } @misc{HofmannYuFolseetal., author = {Hofmann, Timo and Yu, Ted H. and Folse, Michael and Weinhardt, Lothar and B{\"a}r, Marcus and Zhang, Yufeng and Merinov, Boris V. and Myers, Deborah J. and Goddard, William A. and Heske, Clemens}, title = {Reply to "Comment on 'Using Photoelectron Spectroscopy and Quantum Mechanics to Determine d-Band Energies of Metals for Catalytic Applications'"}, series = {Journal of Physical Chemistry : C}, volume = {117}, journal = {Journal of Physical Chemistry : C}, number = {13}, issn = {1932-7455}, pages = {6916 -- 6917}, language = {en} } @misc{BaerTheisenWilksetal., author = {B{\"a}r, Marcus and Theisen, J.-P. and Wilks, Regan G. and Erfurth, Felix and Duarte, Roberto Felix and Gerlach, Dominic and Haas, S. and Tati-Bismaths, L. and Reinert, F. and Kronast, F. and Niesen, T. P. and Palm, J{\"o}rg and Visbeck, Sven and Weinhardt, Lothar}, title = {Lateral inhomogeneity of the Mg/(Zn+Mg) composition at the (Zn,Mg)O/CuIn(S,Se)2 thin-film solar cell interface revealed by photoemission electron microscopy}, series = {Journal of Applied Physics}, volume = {113}, journal = {Journal of Applied Physics}, number = {19}, issn = {1520-8850}, doi = {10.1063/1.4804413}, pages = {193709}, abstract = {Spatial variations in the chemical composition of the (Zn,Mg)O/CuIn(S,Se)2 thin-film solar cell interface were studied by photoemission electron microscopy (PEEM). Energy filtered PEEM images indicate significant differences in the magnesium and zinc distribution. Local photoemission measurements reveal a relative difference in the derived Mg/(ZnþMg) composition of the (Zn,Mg)O material of up to (11.460.7)\%, which can be expected to induce band gap fluctuations of (60630) meV. Furthermore, local areas with significant accumulations of sodium could be observed.}, language = {en} } @misc{BaerKlaerWeinhardtetal., author = {B{\"a}r, Marcus and Klaer, Joachim and Weinhardt, Lothar and Wilks, Regan G. and Krause, Stefan and Blum, Monika and Yang, Wan Li and Heske, Clemens and Schock, Hans-Werner}, title = {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}, series = {Advanced Energy Materials}, volume = {3}, journal = {Advanced Energy Materials}, number = {6}, issn = {1614-6840}, pages = {777 -- 781}, abstract = {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.}, language = {en} } @misc{GorgoiGerlachWilksetal., author = {Gorgoi, Mihaela and Gerlach, Dominic and Wilks, Regan G. and Wippler, David and Wimmer, Manuel and Lozac'h, Micka{\"e}l and Duarte, Roberto Felix and M{\"u}ck, Leonie Anna and Meier, M. and Ueda, Shigenori and Yoshikawa, Hideki and Lips, Klaus and Rech, Bernd and Sumiya, Masatomo and H{\"u}pkes, J{\"u}rgen and Kobayashi, Kaoru and B{\"a}r, Marcus}, title = {The silicon/zinc oxide interface in amorphous silicon-based thin-film solar cells: Understanding an empirically optimized contact}, series = {Applied Physics Letters}, volume = {103}, journal = {Applied Physics Letters}, number = {2}, issn = {1077-3118}, pages = {023903}, abstract = {The electronic structure of the interface between the boron-doped oxygenated amorphous silicon "window layer" (a-SiOx:H(B)) and aluminum-doped zinc oxide (ZnO:Al) was investigated using hard x-ray photoelectron spectroscopy and compared to that of the boron-doped microcrystalline silicon (μc-Si:H(B))/ZnO:Al interface. The corresponding valence band offsets have been determined to be (-2.87 ± 0.27) eV and (-3.37 ± 0.27) eV, respectively. A lower tunnel junction barrier height at the μc-Si:H(B)/ZnO:Al interface compared to that at the a-SiOx:H(B)/ZnO:Al interface is found and linked to the higher device performances in cells where a μc-Si:H(B) buffer between the a-Si:H p-i-n absorber stack and the ZnO:Al contact is employed.}, language = {en} } @phdthesis{Gerlach, author = {Gerlach, Dominic}, title = {Surface and interface characterization of thin-film silicon solar cell structures}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-28254}, pages = {128}, language = {en} }