@phdthesis{Wirsing2023,
  author    = {Wirsing, Sara},
  title     = {Computational Spectroscopic Studies with Focus on Organic Semiconductor Systems},
  doi       = {10.25972/OPUS-28655},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-286552},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {This work presents excited state investigations on several systems with respect to experimental spectroscopic work. The majority of projects covers the temporal evolution of excitations in thin films of organic semiconductor materials. In the first chapters, thinfilm and interface systems are build from diindeno[1,2,3-cd:1',2',3'-lm]perylene (DIP) and N,N'-bis-(2-ethylhexyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIR-CN2) layers, in the third chapter bulk systems consist of 4,4',4"-tris[(3-methylphenyl)phenylamino] triphenylamine (m-MTDATA), 4,7-diphenyl-1,10-phenanthroline (BPhen) and tris-(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB). These were investigated by aggregate-based calculations. Careful selection of methods and incorporation of geometrical relaxation and environmental effects allows for a precise energetical assignment of excitations. The biggest issue was a proper description of charge-transfer excitations, which was resolved by the application of ionization potential tuning on aggregates. Subsequent characterization of excitations and their interplay condenses the picture. Therefore, we could assign important features of the experimental spectroscopic data and explain differences between systems. The last chapter in this work covers the analysis of single molecule spectroscopy on methylbismut. This poses different challenges for computations, such as multi-reference character of low-lying excitations and an intrinsic need for a relativistic description. We resolved this by combining complete active space self-consistent field based methods with scalarrelativistic density-functional theory. Thus we were able to confidently assign the spectroscopic features and explain underlying processes.},
  subject      = {Theoretische Chemie},
  language  = {en}
}
@phdthesis{Gruene2022,
  author    = {Gr{\"u}ne, Jeannine},
  title     = {Spin States and Efficiency-Limiting Pathways in Optoelectronic Materials and Devices},
  doi       = {10.25972/OPUS-29340},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-293405},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {This thesis addresses the identification and characterization of spin states in optoelectronic materials and devices using multiple spin-sensitive techniques. For this purpose, a systematic study focussing on triplet states as well as associated loss pathways and excited state kinetics was carried out. The research was based on comparing a range of donor:acceptor systems, reaching from organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) to organic photovoltaics (OPV) employing fullerene and multiple non-fullerene acceptors (NFAs). By developing new strategies, e.g., appropriate modeling, new magnetic resonance techniques and experimental frameworks, the influence of spin states in the fundamental processes of organic semiconductors has been investigated. Thereby, the combination of techniques based on the principle of electron paramagnetic resonance (EPR), in particular transient EPR (trEPR) and optically detected magnetic resonance (ODMR), with all-optical methods, such as transient electroluminescence (trEL) and transient absorption (TA), has been employed. As a result, excited spin states, especially molecular and charge transfer (CT) states, were investigated in terms of kinetic behavior and associated pathways, which revealed a significant impact of triplet states on efficiency-limiting processes in both optoelectronic applications.},
  subject      = {Elektronenspinresonanz},
  language  = {en}
}
@phdthesis{Hammer2021,
  author    = {Hammer, Sebastian Tobias},
  title     = {Influence of Crystal Structure on Excited States in Crystalline Organic Semiconductors},
  doi       = {10.25972/OPUS-24401},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-244019},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {This thesis focused on the influence of the underlying crystal structure and hence, of the mutual molecular orientation, on the excited states in ordered molecular aggregates. For this purpose, two model systems have been investigated. In the prototypical donor-acceptor complex pentacene-perfluoropentacene (PEN-PFP) the optical accessibility of the charge transfer state and the possibility to fabricate highly defined interfaces by means of single crystal templates enabled a deep understanding of the spatial anisotropy of the charge transfer state formation. Transferring the obtained insights to the design of prototypical donor-acceptor devices, the importance of interface control to minimize the occurrence of charge transfer traps and thereby, to improve the device performance, could be demonstrated. The use of zinc phthalocyanine (ZnPc) allowed for the examination of the influence of molecular packing on the excited electronic states without a change in molecular species by virtue of its inherent polymorphism. Combining structural investigations, optical absorption and emission spectroscopy, as well as Franck-Condon modeling of emission spectra revealed the nature of the optical excited state emission in relation to the structural \(\alpha \) and \(\beta \) phase over a wide temperature range from 4 K to 300 K. As a results, the phase transition kinetics of the first order \(\alpha \rightarrow \beta\) phase transition were characterized in depth and applied to the fabrication of prototypical dual luminescent OLEDs.},
  subject      = {Organischer Halbleiter},
  language  = {en}
}
@phdthesis{Vaeth2016,
  author    = {V{\"a}th, Stefan Kilian},
  title     = {On the Role of Spin States in Organic Semiconductor Devices},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-141894},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {The present work addressed the influence of spins on fundamental processes in organic semiconductors. In most cases, the role of spins in the conversion of sun light into electricity was of particular interest. However, also the reversed process, an electric current creating luminescence, was investigated by means of spin sensitive measurements. In this work, many material systems were probed with a variety of innovative detection techniques based on electron paramagnetic resonance spectroscopy. More precisely, the observable could be customized which resulted in the experimental techniques photoluminescence detected magnetic resonance (PLDMR), electrically detected magnetic resonance (EDMR), and electroluminescence detected magnetic resonance (ELDMR). Besides the commonly used continuous wave EPR spectroscopy, this selection of measurement methods yielded an access to almost all intermediate steps occurring in organic semiconductors during the conversion of light into electricity and vice versa. Special attention was paid to the fact that all results were applicable to realistic working conditions of the investigated devices, i.e. room temperature application and realistic illumination conditions.},
  subject      = {Organischer Halbleiter},
  language  = {en}
}
@phdthesis{Kraus2014,
  author    = {Kraus, Hannes},
  title     = {Optically Detected Magnetic Resonance on Organic and Inorganic Carbon-Based Semiconductors},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-106308},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {In dieser Arbeit werden drei verschiedene kohlenstoffbasierte Materialsysteme behandelt: (i) Organische Halbleiter und kleine Molek{\"u}le, in Kombination mit Fullerenen f{\"u}r Anwendungen in der organischen Photovoltaik (OPV), (ii) Halbleitende Einzelwand-Kohlenstoffnanor{\"o}hren und (iii) Siliziumkarbid (SiC), dessen Defekte erst seit kurzem als Kandidaten f{\"u}r Quantenapplikationen gehandelt werden. Alle Systeme wurden mit optisch detektierter Magnetresonanzspektroskopie (ODMR) untersucht. Im OPV-Kapitel, die intrinsischen Parameter und Orientierungen von Exzitonen mit hohem Spin wurden f{\"u}r die Materialsysteme P3HT, PTB7 und DIP untersucht. Speziell der Einfluss von Ordnung diesen organischen Systemen wurde diskutiert. Der zweite Teil des Kapitels besch{\"a}ftigt sich mit Triplettgeneration mittels Elektronenr{\"u}cktransfer im leistungsf{\"a}higen Materialsystem PTB7:PC71BM. Das Kohlenstoffnanor{\"o}hren-Kapitel zeigt zuert den ersten zweifelsfreien Nachweis von Triplettexzitonen in halbleitenden (6,5) Einzelwandkohlenstoffnanor{\"o}hren (SWNT), mittels ODMR-Spektroskopie. Ein Modell f{\"u}r die Anregungskinetik, die intrinsischen Parameter des Exzitons und Abh{\"a}ngigkeit von der Orientierung der R{\"o}hren wurden diskutiert. Der letzte Teil der Arbeit gilt Spinzentren in Siliziumkarbid. Nach einer kurzen Einf{\"u}hrung in das Materialsystem wird die Spinmultiplizit{\"a}t f{\"u}r die V2 und V3 Siliziumfehlstellen, sowie eines Frenkelpaars und eines noch nicht zugeordneten Defekts (UD) in 6H SiC, weiterhin f{\"u}r die V2 Fehlstelle und das Frenkelpaar in 4H SiC, durchg{\"a}ngig zu S=3/2 festgestellt. Das spinpolarisierte Bef{\"u}llen der 3/2-Zust{\"a}nde des Grundzustands der Siliziumfehlstellen erlaubt stimulierte Mikrowellenemission. Ausserdem wurde f{\"u}r UD und Frenkelpaar in 6H SiC eine große Temperaturabh{\"a}ngigkeit der Nullfeldparameter festgestellt, w{\"a}hrend die Siliziumfehlstellen temperaturunabh{\"a}ngig sind. Anwendung des UD und Frenkelpaars als Temperatursensor, und der Vakanzen als Vektormagnetometer wurden diskutiert.},
  subject      = {ODMR-Spektroskopie},
  language  = {en}
}
@phdthesis{Gorenflot2014,
  author    = {Gorenflot, Julien Fran{\c{c}}ois},
  title     = {Optical study of the excited states in the semiconducting polymer poly(3-hexylthiophene) for photovoltaic applications},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-116730},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {In the course of this dissertation, we have presented the interest of using spectroscopic methods to unravel the physics of polymer semiconductors in photovoltaic applications. Applying photoluminescence and photoinduced absorption spectroscopy to the reference system P3HT:PCBM has enabled us to study the major steps of photocurrent generation in organic bulk heterojunctions, from excitons generation to charges extraction and loss mechanisms and thus to improve the understanding of those mechanisms. The exciton binding energy, is the first obstacle to overcome for photocurrent generation in organic solar cell and the reason for the use of two materials, whose heterojunction act as a driving force for charge separation. We developed an original photoluminescence-detected field-induced exciton quenching method to investigate this energy. Absorption and photoluminescence spectra of pure P3HT show that, while both amorphous and crystalline domains participate in absorption, the energy is then transferred to the crystalline domains, from where the photoluminescence is exclusively originating. The field dependence of this photoluminescence showed that an energy of no less than 420 meV is necessary to split excitons into non photon-emitting species. Comparing those results with energy levels obtained by absorption and photoelectron spectroscopies, confirmed that the formation of those species is only a first step toward dissociation into free charges. Indeed, photoemission spectroscopy and the onset of photocurrent upon increasing the photon energy in a pure P3HT solar cell, concomitantly show that the energy level of a pair of free polarons is located 0.7 eV above the one of the exciton. The comprehensive analysis of those results originating from those different method enable us to draw a global picture of the states and energies involved in free polarons generation in pure material. This work has been widely acknowledged by the scientific community, published in Physical Review B in 2010 [1] and presented in national [2] and international [3] conferences. The spectroscopy of excited states is used to detect the presence of wanted species (charges) and potentially unwanted neutral species upon photoexcitation. As such, it offers us the possibility to qualify the efficiency of charge generation and, if any, identify the competing processes and the generation of unwanted species. In the frame of the European Marie Curie Research Network SolarNType,[4] this possibility was used - in combination with morphological, charge transport and devices characterizationsn - to study a number of new donor:acceptor blends. Thanks to those techniques, we were able to not only quantify the potential of those blends, but also to provide the chemist laboratories with a precious and detailed feedback on the strengths and weakness of the molecules, regarding charge generation, transport and extraction. The detailed study of terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for solar cells application was published in the peer review journal Synthetic Metals and was chosen to illustrate the cover page of the issue [5]. Finally, in the last chapter, we have used time resolved photoinduced absorption to improve the understanding of the charge carrier loss mechanisms in P3HT:PCBM active layers. This comprehension is of prime importance because, the fact that this recombination is far weaker than expected from the Langevin theory, enable polarons to travel further without recombining and thus to build thicker and more efficient devices. A comprehensive analysis of steady-state PIA spectra of pure P3HT, indicates that probing at 980 nm at a temperature between 140 and 250 K enables to monitor specifically polaron densities in both neat P3HT and P3HT:PCBM. Applying this finding to transient absorption enabled us to monitor, for the first time, the bimolecular recombination in pure P3HT, and to discover that - in sharp contrast with the blend - this recombination was in agreement with the Langevin theory. Moreover, it enables us to pinpoint the important role played by the existence of two materials and of energetical traps in the slow recombination and high recombination orders observed in the blend. This work has been published in the Journal of Applied Physics.[6] Those new insights in the photophysics of polymer:fullerene photoactive layers could have a strong impact on the future developement of those materials. Consistent measurements of the binding energy of excitons and intermediate species, would enable to clarify the role played by excess thermal energy in interfacial states dissociation. Better understanding of blends morphology and its influence on solar cells parameters and in particular on recombination could enable to reproduce the conditions of limited recombination on material systems offering some promising performances but with only limited active layer thicknesses. However, due to the number of parameters involved, further experimentation is required, before we can reach a quantitative modeling of bimolecular recombination. [1] Deibel et al., Phys. Rev. B, 81:085202, 2010 [2] Gorenflot et al., Deutsche Physikalische Gesellschaft Fr{\"u}hjahrstagung 2010, CPP20:10, Regensburg, Germany, 2010 [3] Gorenflot et al., International Conference of Synthetic Metals, 7Ax:05, Kyoto, Japan, 2010 [4] Marie-Curie RTN "SolarNTyp" Contract No. MRTN-CT-2006-035533 [5] Gorenflot et al., Synth. Met., 161(23{24):2669-2676, 2012 [6] Gorenflot et al., J. Appl. Phys., 115(14):144502, 2014},
  subject      = {Organische Solarzelle},
  language  = {en}
}
@phdthesis{Gieseking2014,
  author    = {Gieseking, Bj{\"o}rn},
  title     = {Excitation Dynamics and Charge Carrier Generation in Organic Semiconductors},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-101625},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {The transport of optically excited states, called excitons, as well as their conversion into charges define the two major steps allowing for the operation of organic photovoltaic (OPV) devices. Hence, a deep understanding of these processes, the involved mechanisms as well as possible loss channels is crucial for further improving the efficiency of organic solar cells. For studying the aforementioned processes spectroscopic methods like absorption and emission measurements are useful tools. As many of the processes take place on a sub-nanosecond (ns) timescale ultrafast spectroscopic methods are required. Due to this reason two experiments based on a femtosecond laser system were built and employed in this work, namely picosecond (ps) time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy. By analyzing the PL decay dynamics in the prototypical organic semiconductor rubrene, the feasibility of a new approach for improving the efficiency of organic solar cells by harvesting triplet excitons generated by singlet fission was examined. Singlet fission describes a process where two triplet excitons are generated via a photoexcited singlet exciton precursor state if the energy of the two triplets is comparable with the energy of the singlet. For this purpose the influence of characteristic length scales on the exciton dynamics in different rubrene morphologies exhibiting an increasing degree of confinement was analyzed. The results show that the quenching at interfacial states efficiently suppresses the desired fission process if these states are reached by excitons during migration. Since interfacial states are expected to play a significant role in thin film solar cells and are easily accessible for the migrating excitons, the results have to be considered for triplet-based OPV. While the aforementioned approach is only investigated for model systems so far, the efficiency of disordered organic bulk heterojunction (BHJ) solar cells could be significantly enhanced in the last couple of years by employing new and more complex copolymer donor materials. However, little is known about the photophysics and in particular the excitation dynamics of these systems. By carrying out a systematic optical study on the prominent copolymer PCDTBT and its building blocks we were able to identify the nature of the two characteristic absorption bands and the coupling mechanism between these levels. The latter mechanism is based on an intrachain partial charge transfer between two functional subunits and our time-resolved measurements indicate that this coupling governs the photophysical properties of solar cells based on these copolymers. The efficient coupling of functional subunits can be seen as a key aspect that guarantees for the success of the copolymer approach. Another important issue concerns the optimization of the morphology of BHJ solar cells. It arises from the discrepancy between the exciton diffusion length \mbox{(\$\approx\$ 10 nm)} and the absorption length of solar irradiation (\$\approx\$ 100 nm). Due to this reason, even for devices based on new copolymer materials, processing parameters affecting the morphology like annealing or employing processing additives are of major importance. In our combined optical, electrical and morphological study for solar cells based on the high-efficient copolymer PBDTTT-C we find a direct correlation between additive content and intermixing of the active layer. The observed maximum in device efficiency can be attributed to a morphology guaranteeing for an optimized balance between charge generation and transport. Our results highlight the importance of understanding the influence of processing parameters on the morphology of the BHJ and thus on the efficiency of the device.},
  subject      = {Organische Solarzelle},
  language  = {en}
}
@phdthesis{Wagenpfahl2013,
  author    = {Wagenpfahl, Alexander Johannes},
  title     = {Numerical simulations on limitations and optimization strategies of organic solar cells},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-90119},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {Continuously increasing energy prices have considerably influenced the cost of living over the last decades. At the same time increasingly extreme weather conditions, drought-filled summers as well as autumns and winters with heavier rainfall and worsening storms have been reported. These are possibly the harbingers of the expected approaching global climate change. Considering the depletability of fossil energy sources and a rising distrust in nuclear power, investigations into new and innovative renewable energy sources are necessary to prepare for the coming future. In addition to wind, hydro and biomass technologies, electricity generated by the direct conversion of incident sunlight is one of the most promising approaches. Since the syntheses and detailed studies of organic semiconducting polymers and fullerenes were intensified, a new kind of solar cell fabrication became conceivable. In addition to classical vacuum deposition techniques, organic cells were now also able to be processed from a solution, even on flexible substrates like plastic, fabric or paper. An organic solar cell represents a complex electrical device influenced for instance by light interference for charge carrier generation. Also charge carrier recombination and transport mechanisms are important to its performance. In accordance to Coulomb interaction, this results in a specific distribution of the charge carriers and the electric field, which finally yield the measured current-voltage characteristics. Changes of certain parameters result in a complex response in the investigated device due to interactions between the physical processes. Consequently, it is necessary to find a way to generally predict the response of such a device to temperature changes for example. In this work, a numerical, one-dimensional simulation has been developed based on the drift-diffusion equations for electrons, holes and excitons. The generation and recombination rates of the single species are defined according to a detailed balance approach. The Coulomb interaction between the single charge carriers is considered through the Poisson equation. An analytically non-solvable differential equation system is consequently set-up. With numerical approaches, valid solutions describing the macroscopic processes in organic solar cells can be found. An additional optical simulation is used to determine the spatially resolved charge carrier generation rates due to interference. Concepts regarding organic semiconductors and solar cells are introduced in the first part of this work. All chapters are based on previous ones and logically outline the basic physics, device architectures, models of charge carrier generation and recombination as well as the mathematic and numerical approaches to obtain valid simulation results. In the second part, the simulation is used to elaborate issues of current interest in organic solar cell research. This includes a basic understanding of how the open circuit voltage is generated and which processes limit its value. S-shaped current-voltage characteristics are explained assigning finite surface recombination velocities at metal electrodes piling-up local space charges. The power conversion efficiency is identified as a trade-off between charge carrier accumulation and charge extraction. This leads to an optimum of the power conversion efficiency at moderate to high charge carrier mobilities. Differences between recombination rates determined by different interpretations of identical experimental results are assigned to a spatially inhomogeneous recombination, relevant for almost all low mobility semiconductor devices.},
  subject      = {Organische Solarzelle},
  language  = {en}
}
@phdthesis{Gsaenger2013,
  author    = {Gs{\"a}nger, Marcel},
  title     = {Organic Thin-Film Transistors Based on Dipolar Squaraine Dyes},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-80588},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {In summary, it can be stated that the herein studied set of acceptor-substituted squaraine dyes can be seen as potent candidates for OTFTs. Furthermore, their transistor performance can be easily tuned to obtain hole mobilities up to 0.45 cm2/Vs from solution and 1.3 cm2/Vs from sublimation by choosing adequate deposition techniques. In the end, a probable structural model derived from studies of the thin-film morphology by methods such as optical spectroscopy, AFM and X-ray even facilitated the clarification of the observed charge transport behavior.},
  subject      = {Organische Chemie},
  language  = {en}
}
@phdthesis{Sperlich2013,
  author    = {Sperlich, Andreas},
  title     = {Electron Paramagnetic Resonance Spectroscopy of Conjugated Polymers and Fullerenes for Organic Photovoltaics},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-81244},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {In the presented thesis, the various excited states encountered in conjugated organic semiconductors are investigated with respect to their utilization in organic thin-film solar cells. Most of these states are spin-baring and can therefore be addressed by means of magnetic resonance spectroscopy. The primary singlet excitation (spin 0), as well as positive and negative polaronic charge carriers (spin 1/2) are discussed. Additionally, triplet excitons (spin 1) and charge transfer complexes are examined, focussing on their differing spin-spin interaction strength. For the investigation of these spin-baring states especially methods of electron paramagnetic resonance (EPR) are best suited. Therefore according experimental methods were implemented in the course of this work to study conjugated polymers, fullerenes and their blends with continuous wave as well as time-resolved EPR and optically detected magnetic resonance.},
  subject      = {Organische Solarzelle},
  language  = {en}
}