@phdthesis{Fischer2023,
  author    = {Fischer, Mathias},
  title     = {Transient Phenomena and Ionic Kinetics in Hybrid Metal Halide Perovskite Solar Cells},
  doi       = {10.25972/OPUS-32220},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-322204},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {The fact that photovoltaics is a key technology for climate-neutral energy production can be taken as a given. The question to what extent perovskite will be used for photovoltaic technologies has not yet been fully answered. From a photophysical point of view, however, it has the potential to make a useful contribution to the energy sector. However, it remains to be seen whether perovskite-based modules will be able to compete with established technologies in terms of durability and cost efficiency. The additional aspect of ionic migration poses an additional challenge. In the present work, primarily the interaction between ionic redistribution, capacitive properties and recombination dynamics was investigated. This was done using impedance spectroscopy, OCVD and IV characteristics as well as extensive numerical drift-diffusion simulations. The combination of experimental and numerical methods proved to be very fruitful. A suitable model for the description of solar cells with respect to mobile ions was introduced in chapter 4.4. The formal mathematical description of the model was transferred by a non-dimensionalization and suitable numerically solvable form. The implementation took place in the Julia language. By intelligent use of structural properties of the sparse systems of equations, automatic differentiation and the use of efficient integration methods, the simulation tool is not only remarkably fast in finding the solution, but also scales quasi-linearly with the grid resolution. The software package was released under an open source license. In conventional semiconductor diodes, capacitance measurements are often used to determine the space charge density. In the first experimental chapter 5, it is shown that although this is also possible for the ionic migration present in perovskites, it cannot be directly understood as doping related, since the space charge distribution strongly depends on the preconditions and can be manipulated by an externally applied voltage. The exact form of this behavior depends on the perovskite composition. This shows, among other things, that experimental results can only be interpreted within the framework of conventional semiconductors to a very limited extent. Nevertheless, the built-in 99 potential of the solar cell can be determined if the experiments are carried out properly. A statement concerning the type and charge of the mobile ions is not possible without further effort, while their number can be determined. The simulations were applied to experimental data in chapter 6. Thus, it could be shown that mobile ions make a significant contribution to the OCVD of perovskite solar cells. j-V characteristics and OCVD transients measured as a function of temperature and illumination intensities could be quantitatively modeled simultaneously using a single global set of parameters. By the simulations it was further possible to derive a simple experimental procedure to determine the concentration and the diffusivity of the mobile ions. The possibility of describing different experiments in a uniform temperaturedependent manner strongly supports the model of mobile ions in perovskites. In summary, this work has made an important contribution to the elucidation of ionic contributions to the (photo)electrical properties of perovskite solar cells. Established experimental techniques for conventional semiconductors have been reinterpreted with respect to ionic mass transport and new methods have been proposed to draw conclusions on the properties for ionic transport. As a result, the published simulation tools can be used for a number of further studies.},
  subject      = {Simulation},
  language  = {en}
}
@phdthesis{Pfaff2016,
  author    = {Pfaff, Florian Georg},
  title     = {Spektroskopie und hochaufl{\"o}sende Mikroskopie zur Analyse der Grenzfl{\"a}cheneigenschaften in SrTiO\(_3\)-basierten Heterostrukturen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-145023},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {> In oxidischen Heterostrukturen kann es zur Ausbildung unerwarteter elektronischer und magnetischer Phasen kommen. Ein bekanntes Beispiel ist das Heterostruktursystem LaAlO\(_3\)/SrTiO\(_3\), an dessen Grenzfl{\"a}che ein zweidimensionalen Elektronensystem (2DES) entsteht, sofern die LaAlO\(_3\)-Filmdicke einen kritischen Wert von mindestens vier Einheitszellen aufweist. {\"A}hnliches Verhalten konnte an der Heterostruktur γ-Al\(_2\)O\(_3\)/SrTiO\(_3\) beobachtet werden. Die gemessenen Elektronenbeweglichkeiten und Fl{\"a}chenladungstr{\"a}gerdichten {\"u}bertreffen hierbei die in LaAlO\(_3\)/SrTiO\(_3\) um mehr als eine Gr{\"o}ßenordnung. Die vorliegende Arbeit besch{\"a}ftigt sich mit der Herstellung sowie der Analyse dieser beiden Heterostruktursysteme. Die Hauptaspekte sind dabei die Untersuchung der physikalischen Eigenschaften an der Grenzfl{\"a}che sowie das Verst{\"a}ndnis der zugrundeliegenden Mechanismen. > > Im Hinblick auf das Wachstum wird demonstriert, dass die f{\"u}r LaAlO\(_3\)/SrTiO\(_3\) etablierte Wachstumsroutine der gepulsten Laserablation sowie die zur {\"U}berwachung des Schichtwachstums verwendete Methode der Beugung hochenergetischer Elektronen in Reflexion (RHEED) f{\"u}r das γ-Al\(_2\)O\(_3\)-Wachstum modifiziert werden m{\"u}ssen. So kann gezeigt werden, dass durch eine geeignete Variation der Wachstumsgeometrie die Resonanz von Oberfl{\"a}chenwellen, welche im Falle des γ-Al\(_2\)O\(_3\)-Wachstums die Beobachtung von RHEED-Oszillationen erschwert, vermieden werden kann und somit auch hier die {\"U}berwachung des heteroepitaktischen Schichtwachstum mittels Elektronenbeugung m{\"o}glich wird. > > F{\"u}r die Ausbildung des 2DES in LaAlO\(_3\)/SrTiO\(_3\) wird das Szenario der elektronischen Rekonstruktion als m{\"o}gliche Ursache diskutiert, wonach das divergierende Potential innerhalb des polaren LaAlO\(_3\)-Films durch einen Ladungstransfer von der Probenoberfl{\"a}che in die obersten Atomlagen des unpolaren SrTiO\(_3\)-Substrats kompensiert wird. Zudem sind die Eigenschaften der Heterostruktur von den Wachstumsparametern abh{\"a}ngig. So wird in der vorliegenden Arbeit eine deutliche Zunahme der Ladungstr{\"a}gerkonzentration und der r{\"a}umliche Ausdehnung der leitf{\"a}higen Schicht insbesondere f{\"u}r Proben, welche bei sehr niedrigen Sauerstoffhintergrunddr{\"u}cken gewachsen wurden, gezeigt und auf die Erzeugung von Sauerstofffehlstellen innerhalb des Substrats zur{\"u}ckgef{\"u}hrt. Dar{\"u}ber hinaus wird erstmalig die Herstellung atomar scharfer Grenzfl{\"a}chen mit sehr geringer Defektdichte selbst bei sehr niedrigen Wachstumsdr{\"u}cken belegt und erstmals auch direkt elektronenmikroskopisch nachgewiesen. Es werden allenfalls vernachl{\"a}ssigbare Effekte der Sauerstoffkonzentration auf charakteristische, strukturelle Merkmale der Probe beobachtet. Desweiteren zeigt diese Arbeit erstmalig eine von den Wachstumsbedingungen abh{\"a}ngige Gitterverzerrung des Films, was in {\"U}bereinstimmung mit Rechnungen auf Basis der Dichtefunktionaltheorie einen Hinweis auf ein komplexes Zusammenspiel von elektronischer Rekonstruktion, Sauerstofffehlstellen an der LaAlO\(_3\)-Oberfl{\"a}che und einer Verzerrung der Kristallstruktur als Ursache f{\"u}r die Entstehung des 2DES in LaAlO\(_3\)/SrTiO\(_3\) liefert. > > Neben der mikroskopischen Analyse des 2DES in LaAlO\(_3\)/SrTiO\(_3\) wird die elektronische Struktur dieses Systems zudem mithilfe der resonanten inelastischen R{\"o}ntgenstreuung charakterisiert. Die vorliegende Dissertation zeigt dabei, neben dem Nachweis lokalisierter Ladungstr{\"a}ger vor dem Einsetzen metallischen Verhaltens ab einer kritischen Schichtdicke von vier Einheitszellen, die Existenz eines Raman- und eines fluoreszenzartigen Signals in Abh{\"a}ngigkeit der verwendeten Photonenenergie, was wiederum auf einen unterschiedlichen elektronischen Charakter im Zwischenzustand zur{\"u}ckgef{\"u}hrt werden kann. Gest{\"u}tzt wird diese Interpretation durch vergleichbare Messungen an γ- Al\(_2\)O\(_3\)/SrTiO\(_3\). In diesem System finden sich zudem ebenfalls Anzeichen lokalisierter Ladungstr{\"a}ger unterhalb der kritischen Schichtdicke f{\"u}r metallisches Verhalten, was ein Hinweis auf einen mit LaAlO\(_3\)/SrTiO\(_3\) vergleichbaren Grundzustand sein k{\"o}nnte. > > Weitere Messungen mithilfe der resonanten Photoelektronenspektroskopie erm{\"o}glichen zudem eine direkte Beobachtung und Analyse der Ti 3d-Valenzelektronen. Messungen an LaAlO\(_3\)/SrTiO\(_3\) und γ-Al\(_2\)O\(_3\)/SrTiO\(_3\) liefern dabei Hinweise auf verschiedene elektronische Ti 3d-artige Zust{\"a}nde. Diese werden zum einen den mobilen Ladungstr{\"a}gern des 2DES zugeschrieben, zum anderen als lokalisierte Elektronen in der N{\"a}he von Sauerstofffehlstellen identifiziert. Eine Analyse des Resonanzverhaltens sowie der spektralen Form der beobachteten Signale zeigt quantitative Unterschiede, was auf einen unterschiedlichen treibenden Mechanismus in beiden Systemen hindeutet und im Hin- blick auf den Einfluss von Sauerstofffehlstellen auf das System diskutiert wird. Zudem zeigen impulsaufgel{\"o}ste Messungen der Zust{\"a}nde am chemischen Potential eine unterschiedliche Intensit{\"a}tsverteilung im k -Raum. Dies wird im Zusammenhang mit Matrixelementeffekten diskutiert und kann vermutlich auf Photoelektronendiffraktion bedingt durch die unterschiedliche Kristallstruktur des Filmmaterials, zur{\"u}ckgef{\"u}hrt werden.},
  subject      = {{\"U}bergangsmetalloxide},
  language  = {de}
}
@phdthesis{Zapf2019,
  author    = {Zapf, Michael},
  title     = {Oxidische Perovskite mit Hoher Massenzahl Z: D{\"u}nnfilmdeposition und Spektroskopische Untersuchungen},
  doi       = {10.25972/OPUS-18537},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-185370},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Perovskite oxides are a very versatile material class with a large variety of outstanding physical properties. A subgroup of these compounds particularly tempting to investigate are oxides involving high-\(Z\) elements, where spin-orbit coupling is expected to give rise to new intriguing phases and potential application-relevant functionalities. This thesis deals with the preparation and characterization of two representatives of high-\(Z\) oxide sample systems based on KTaO\(_3\) and BaBiO\(_3\). KTaO\(_3\) is a band insulator with an electronic valence configuration of Ta 5\(d\)\(^0\) . It is shown that by pulsed laser deposition of a disordered LaAlO\(_3\) film on the KTaO\(_3\)(001) surface, through the creation of oxygen vacancies, a Ta 5\(d\)\(^{0+\(\delta\)}\) state is obtained in the upmost crystal layers of the substrate. In consequence a quasi two dimensional electron system (q2DES) with large spin-orbit coupling emerges at the heterointerface. Measurements of the Hall effect establish sheet carrier densities in the range of 0.1-1.2 10\(^{14}\) cm\(^2\), which can be controlled by the applied oxygen background pressure during deposition and the LaAlO\(_3\) film thickness. When compared to the prototypical oxide q2DESs based on SrTiO\(_3\) crystals, the investigated system exhibits exceptionally large carrier mobilities of up to 30 cm\(^2\)/Vs (7000 cm\(^2\)/Vs) at room temperature (below 10 K). Through a depth profiling by photoemission spectra of the Ta 4\(f\) core level it is shown that the majority of the Ta 5\(d\)\(^0\) charge carriers, consisting of mobile and localized electrons, is situated within 4 nm from the interface at low temperatures. Furthermore, the momentum-resolved electronic structure of the q2DES \(buried\) underneath the LaAlO\(_3\) film is probed by means of hard X-ray angle-resolved photoelectron spectroscopy. It is inferred that, due to a strong confinement potential of the electrons, the band structure of the system is altered compared to \(n\)-doped bulk KTO. Despite the constraint of the electron movement along one direction, the Fermi surface exhibits a clear three dimensional momentum dependence, which is related to a depth extension of the conduction channels of at least 1 nm. The second material, BaBiO\(_3\), is a charge-ordered insulator, which has recently been predicted to emerge as a large-gap topological insulator upon \(n\)-doping. This study reports on the thin film growth of pristine BaBiO\(_3\) on Nb:SrTiO\(_3\)(001) substrates by means of pulsed laser deposition. The mechanism is identified that facilitates the development of epitaxial order in the heterostructure despite the presence of an extraordinary large lattice mismatch of 12 \%. At the heterointerface, a structurally modified layer of about 1.7 nm thickness is formed that gradually relieves the in-plane strain and serves as the foundation of a relaxed BBO film. The thereupon formed lattice orders laterally in registry with the substrate with the orientation BaBiO\(_3\)(001)||SrTiO\(_3\)(001) by so-called domain matching, where 8 to 9 BaBiO\(_3\) unit cells align with 9 to 10 unit cells of the substrate. Through the optimization of the deposition conditions in regard to the cation stoichiometry and the structural lattice quality, BaBiO\(_3\) thin films with bulk-like electronic properties are obtained, as is inferred from a comparison of valence band spectra with density functional theory calculations. Finally, a spectroscopic survey of BaBiO\(_3\) samples of various thicknesses resolves that a recently discovered film thickness-controlled phase transition in BaBiO\(_3\) thin films can be traced back to the structural and concurrent stoichiometric modifications occuring in the initially formed lattice on top of the SrTiO\(_3\) substrate rather than being purely driven by the smaller spatial extent of the BBO lattice.},
  subject      = {Perowskit},
  language  = {en}
}
@phdthesis{Hoecker2022,
  author    = {H{\"o}cker, Julian Harald},
  title     = {High-quality Organolead Trihalide Perovskite Crystals: Growth, Characterisation, and Photovoltaic Applications},
  doi       = {10.25972/OPUS-25859},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-258590},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Overview of the Organolead Trihalide Perovskite Crystal Area Studies of perovskite single crystals with high crystallographic quality is an important technological area of the perovskite research, which enables to estimate their full optoelectronic potential, and thus to boost their future applications [26]. It was therefore essential to grow high-quality single crystals with lowest structural as well as chemical defect densities and with a stoichiometry relevant for their thin-film counterparts [26]. Optoelectronic devices, e.g. solar cells, are highly complex systems in which the properties of the active layer (absorber) are strongly influenced by the adjacent layers, so it is not always easy to define the targeted properties and elaborate the design rules for the active layer. Currently, organolead trihalide perovskite (OLTP) single crystals with the structure ABX3 are one of the most studied crystalline systems. These hybrid crystals are solids composed of an organic cation such as methylammonium (A = MA+) or formamidinium (A = FA+) to form a three-dimensional periodic lattice together with the lead cation (B = Pb2+) and a halogen anion such as chloride, bromide or iodide (X = Cl-, Br- or I-) [23]. Among them are methylammonium lead tribromide (MAPbBr3), methylammonium lead triiodide (MAPbI3), as well as methylammonium lead trichloride (MAPbCl3) [62, 63]. Important representatives with the larger cation FA+ are formamidinium lead tribromide (FAPbBr3) and formamidinium lead triiodide (FAPbI3) [23, 64]. Besides the exchange of cations as well as anions, it was possible to grow crystals containing two halogens to obtain mixed crystals with different proportions of chlorine to bromine and bromine to iodine, as it is shown in Figure 70. By varying the mixing ratio of the halogens, it was therefore possible to vary the colour and thus the absorption properties of the crystals [85], as it can be done with thin polycrystalline perovskite films. In addition, since a few years it is also doable to grow complex crystals that contain several cations as well as anions [26, 80, 81]. These include the perovskites double cation - double halide formamidinium lead triiodide - methylammonium lead tribromide (FAPbI3)0.9(MAPbBr3)0.1 (FAMA) [26, 80] and formamidinium lead triiodide - methylammonium lead tribromide - caesium lead tribromide (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 (CsFAMA) [81], which have made a significant contribution to increase the power conversion efficiency (PCE) in thin-film photovoltaics [47, 79, 182]. The growth of crystals to this day is performed exclusively from solution [23, 26, 56, 62]. Important preparation methods are the cooling acid-based precursor solution crystallisation [22], the inverse temperature crystallisation (ITC) [62], and the antisolvent vapour-assistant crystallisation (AVC) [137]. In the cooling crystallisation, the precursor salts AX and PbX2 are dissolved in an aqueous halogen-containing acid at high temperatures [56]. Controlled and slow cooling finally results in a supersaturated precursor solution, which leads to spontaneous nucleation of crystal nuclei, followed by subsequent crystal growth. The ITC method is based on the inverse or retrograde solubility of a dissociated perovskite in an organic solvent [23, 64]. With increasing temperature, the solubility of the perovskite decreases and mm-sized crystals can be grown within a few hours [23]. In the AVC method, the precursors are also dissolved in an organic solvent as well [137]. By slow evaporation of a so-called antisolvent [137], the solubility of the perovskite in the now present solvent mixture decreases and it finally precipitates. In addition, there are many other methods with the goal of growing high quality and large crystals in a short period of time [60, 61, 233, 310].},
  subject      = {Perowskit},
  language  = {en}
}
@phdthesis{Armer2023,
  author    = {Armer, Melina Brigitte Melanie},
  title     = {High-Quality Lead-Free Double Perovskite Single Crystals and their Optical Properties},
  doi       = {10.25972/OPUS-32750},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-327503},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {The presented thesis deals with the investigation of the characteristic physical properties of lead-free double perovskites. For this purpose lead-free double perovskite single crystals were grown from solution. In order to assess the influence of growth temperature on tail states in the material, the crystals were studied using Photoluminescence Excitation (PLE) and Transmission measurements. Additionally, lead-free double perovskite solar cells and thin films were investigated to address the correlation of precursor stoichiometry and solar cell efficiency. In a last step a new earth abundant lead-free double perovskite was introduced and its physical properties were studied by photoluminescene and absorptance. Like this it was possible to assess the suitability of this material for solar cell applications in the future.},
  subject      = {Perowskit},
  language  = {en}
}
@phdthesis{Schmitt2022,
  author    = {Schmitt, Matthias},
  title     = {High Energy Spin- and Momentum-Resolved Photoelectron Spectroscopy of Complex Oxides},
  doi       = {10.25972/OPUS-26475},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-264757},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Spin- and \(k\)-resolved hard X-ray photoelectron spectroscopy (HAXPES) is a powerful tool to probe bulk electronic properties of complex metal oxides. Due to the low efficiency of common spin detectors of about \(10^{-4}\), such experiments have been rarely performed within the hard X-ray regime since the notoriously low photoionization cross sections further lower the performance tremendously. This thesis is about a new type of spin detector, which employs an imaging spin-filter with multichannel electron recording. This increases the efficiency by a factor of \(10^4\) and makes spin- and \(k\)-resolved photoemission at high excitation energies possible. Two different technical approaches were pursued in this thesis: One using a hemispherical deflection analyzer (HDA) and a separate external spin detector chamber, the other one resorting to a momentum- or \(k\)-space microscope with time-of-flight (TOF) energy recording and an integrated spin-filter crystal. The latter exhibits significantly higher count rates and - since it was designed for this purpose from scratch - the integrated spin-filter option found out to be more viable than the subsequent upgrade of an existing setup with an HDA. This instrumental development is followed by the investigation of the complex metal oxides (CMOs) KTaO\(_3\) by angle-resolved HAXPES (HARPES) and Fe\(_3\)O\(_4\) by spin-resolved HAXPES (spin-HAXPES), respectively. KTaO\(_3\) (KTO) is a band insulator with a valence-electron configuration of Ta 5\(d^0\). By angle- and spin-integrated HAXPES it is shown that at the buried interface of LaAlO\(_3\)/KTO - by the generation of oxygen vacancies and hence effective electron doping - a conducting electron system forms in KTO. Further investigations using the momentum-resolution of the \(k\)-space TOF microscope show that these states are confined to the surface in KTO and intensity is only obtained from the center or the Gamma-point of each Brillouin zone (BZ). These BZs are furthermore square-like arranged reflecting the three-dimensional cubic crystal structure of KTO. However, from a comparison to calculations it is found that the band structure deviates from that of electron-doped bulk KTaO\(_3\) due to the confinement to the interface. There is broad consensus that Fe\(_3\)O\(_4\) is a promising material for spintronics applications due to its high degree of spin polarization at the Fermi level. However, previous attempts to measure the spin polarization by spin-resolved photoemission spectroscopy have been hampered by the use of low photon energies resulting in high surface sensitivity. The surfaces of magnetite, though, tend to reconstruct due to their polar nature, and thus their magnetic and electronic properties may strongly deviate from each other and from the bulk, dependent on their orientation and specific preparation. In this work, the intrinsic bulk spin polarization of magnetite at the Fermi level (\(E_F\)) by spin-resolved photoelectron spectroscopy, is determined by spin-HAXPES on (111)-oriented thin films, epitaxially grown on ZnO(0001) to be \(P(E_F) = -80^{+10}_{-20}\) \%.},
  subject      = {Elektronenkorrelation},
  language  = {en}
}