Structural and Electronic Investigation of Strongly Correlated Transition Metal Oxide Perovskite Thin Films and Interfaces using In-situ Transmission Electron Microscopy
by Tobias Meyer
Date of Examination:2020-09-21
Date of issue:2021-09-09
Advisor:Prof. Dr. Michael Seibt
Referee:Prof. Dr. Michael Seibt
Referee:Prof. Dr. Christian Jooß
Referee:Prof. Dr. Joachim Mayer
Persistent Address:
http://dx.doi.org/10.53846/goediss-8815
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Abstract
English
Discussing the necessity as well as possible details of global strategies to reduce and eventually eliminate the anthropogenic climate change (ACC) is a delicate matter, which easily leads to statements based on ressentiments rather than on scientific facts. Indeed, public polls revealed the volatility of individual beliefs in the existence of ACC correlating with short-term weather phenomena [1] well after a scientific consensus about its impact was found [2–5]. Naturally, the models presented in the cited references do not cover all facets of the cybernetic global system at once and the assessment of resulting forecast uncertainties is part of the careful work of colleagues [6, 7]. However, the included and certainly possible scenario of turning the Earth in an increasingly hostile planet appears to be an unreasonably high stake when betting on the future. Consequently, in order to reduce the emission of greenhouse gases being indisputably linked to global warming [7, 8], progress in sustainable energy sources as well as their actual usage is indispensable. In essence, establishing a prevailing renewable energy supply is a threefold problem as primary conversions need to be followed by storage and transport in order to bridge temporal as well as spatial source heterogeneities [9]. One candidate to master the former challenge is solar energy conversion, which has recently gained a lot in global energy market share as module efficiencies resp. prices are constantly rising resp. falling [10, 11]. In detail, both the optimisation of established solar cells, i.e. most prominently silicon modules, as well as the inclusion of innovative concepts and materials is pursued. The latter approach is also referred to as third-generation photovoltaics and aims for solar cell efficiencies beyond the famous Shockley-Queisser limit [12], which describes the theoretical thermodynamic conversion limit under the assumption of transmission of sub-bandgap photons and complete thermalisation of hot charge carriers before power extraction. A particularly promising example, in which these assumptions are no longer valid, is given by the material class of organic halide perovskites showing an extraordinarily fast increase of related efficiencies over the past years [13–15]. In this work, the inorganic counterpart of transition metal oxide perovskites will be the main subject of study. Certainly, currently achieved solar energy conversion efficiencies in this material class are significantly lower compared with organic halides [16–18]. However, because of their rich phase diagrams emerging due to strong correlations [19–22] they serve as a well- suited model system to study underlying mechanisms (lifting the previously mentioned Shockley-Queisser limit) on a fundamental level. Importantly, this statement is not limited to the context of photovoltaics, but holds also for additional fields such as the study of catalysis [23–25] or resistive random access memory (RRAM) [26]. In more detail, this dissertation focuses on the structural and electronic investigation of transition metal oxide perovskite thin films, being typically the basis of technological devices [26]. It includes significant contributions to the phase diagram of Pr1−xCaxMnO3 epitaxial layers – grown on SrTi1−yNbyO3 substrates – in doping and temperature regimes where ordered phases occur due to correlative exchange interactions of lattice, orbital, and spin degrees of freedom [20]. Importantly, these ordered phases have been demonstrated to correlate with an enhanced photovoltaic acitvity [17, 18, 27, 28] emphasizing the importance of such studies in the context of solar energy conversion. In fact, as nicely described in the cited references, the underlying mechanism of the enhanced photovoltaic activity was found to be a prolonged lifetime of hot carriers due to phonon interactions and, thus, reaches beyond the assumptions of the Shockley-Queisser limit. Consequently, the materials in question are well-suited to explore fundamental processes in third generation photovoltaics. In order to study the mentioned phase transitions in thin films as well as electric properties relevant for solar energy conversion such as the excess charge carrier diffusion length, which happens to be located on the nanoscale [29], high-resolution techniques are needed. Therefore, the transmission electron microscope is employed enabling for versatile and highly- resolved real and reciprocal space signal extraction as well as a large variety of in-situ techniques, e.g. heating, cooling, biasing, and environmental control. In fact, facilitated by the outstanding advances in scientific instrumentation, such in-situ methods are ever-increasingly applied on the micro- and nanoscale and successfully correlated to macroscopic physical, chemical, and biological properties [30–33]. In this study, in-situ heating, cooling, biasing, and environmental control are combined with established techniques such as selective area electron diffraction [34] and electron energy loss spectroscopy [35] as well as with recently emerging methods like four-dimensional scanning transmission electron microscopy [36] and scanning transmission electron beam induced current [29]. Additionally, a substantial part of this thesis focuses on further developments of the latter techniques. Selected highlights are the successful extraction of ordering parameters as well as critical temperatures of phase transitions in Pr1−xCaxMnO3 during heating (in a gaseous environment) and cooling. The observed transitions, i.e. charge ordering for x = 0.34 and an orthorhombic to pseudo- tetragonal (or cubic) transition for x = 0.1, are discussed thoroughly in the context of correlation phenomena and photovoltaic activity and differences to the bulk such as decreased critical temperatures will be pointed out. Furthermore, a structural model is presented linking atomic configurations with the material’s lattice parameters. In addition, experimental and modelling advances in the field of scanning transmission electron beam induced current are demonstrated enabling the observation of diffusion and recombination properties of excess charge carriers in perovskites on the nanoscale as well mapping of a sub-0.1 ppm concentration line of boron in a textured silicon solar cell. Lastly, first interpretations of atomic modulations in electron beam induced current signals are presented.
Keywords: Perovskite; Photoltaics; Transmission electron microscopy; Correlated oxides; in-situ TEM