@phdthesis{Rauh2013,
  author    = {Rauh, Daniel},
  title     = {Impact of Charge Carrier Density and Trap States on the Open Circuit Voltage and the Polaron Recombination in Organic Solar Cells},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-90083},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {The focus of this work is studying recombination mechanisms occurring in organic solar cells, as well as their impact on one of their most important parameters — the open circuit voltage Voc. Firstly, the relationship between Voc and the respective charge carrier density n in the active layer under open circuit conditions is analyzed. Therefor, a model after Shockley for the open circuit voltage is used, whose validity is proven with the aid of fits to the measured data. Thereby, it is emphasized that the equation is only valid under special conditions. In the used reference system P3HT:PC61BM the fits are in agreement with the measurement data only in the range of high temperatures (150 - 300 K), where Voc increases linearly with decreasing temperature. At lower temperatures (50 - 150 K), the experiment shows a saturation of Voc. This saturation cannot be explained with the model by the measured falling charge carrier density with decreasing temperatures. In this temperature range Voc is not directly related to the intrinsic properties of the active layer. Voc saturation is due to injection energy barriers at the contacts, which is ascertained by macroscopic simulations. Furthermore, it is observed that Voc in the case of saturation is equivalent to the so-called built-in potential. The difference between the built-in potential and the energy gap corresponds thereby to the sum of the energy barriers at both contacts. With the knowledge of the Voc(n) dependency for not contact limited solar cells, it is possible to investigate the recombination mechanisms of charge carriers in the active layer. For Langevin recombination the recombination rate is Rn2 (recombination order RO = 2), for Shockley-Read-Hall (SRH) Rn1 (RO=1); in various publications RO higher than two is reported with two main explanations. 1: Trap states for charge carriers exist in the respective separated phases, i.e. electrons in the acceptor phase and holes in the donor phase, which leads to a delayed recombination of the charge carriers at the interface of both phases and finally to an apparent recombination order higher than 2. 2: The enhanced R(n) dependency is attributed to the so called recombination prefactor, which again is dependent from n dependent mobility µ. It is shown that for the system P3HT:PC61BM at room temperature the µ(n) dependency does nearly completely explain the higher RO but not at lower temperatures which in this case supports the first explanation. In the material system PTB7:PC71BM the increased RO cannot be explained by the µ(n) dependency even at room temperature. To support the importance of trap states in combination with a phase separation for the explanation of the enhanced RO, additional trap states were incorporated in the solar cells to investigate their influence on the recombination mechanisms. To achieve this, P3HT:PC61BM solar cells were exposed to synthetic air (in the dark and under illumination) or TCNQ was added in small concentrations to the active layer which act as electron traps. For the oxygen degraded solar cell the recombination order is determined by a combination of open Voc-transients and Voc(n) measurements. Thereby, a continuous increase of the recombination order from 2.4 to more than 5 is observed with higher degradation times. By the evaluation of the ideality factor it can be shown that the impact of SRH recombination is increasing with higher trap concentration in relation to Langevin recombination. A similar picture is revealed for solar cells with TCNQ as extrinsic trap states. Finally, a phenomenon called s-shaped IV-curves is investigated, which can sometimes occur for solar cells under illumination. As course of this a reduced surface recombination velocity can be found. Experimentally, the solar cells were fabricated using a special plasma treatment of the ITO contact. The measured IV-curves of such solar cells are reproduced by macroscopic simulations, where the surface recombination velocity is reduced. Hereby, it has to be distinguished between the surface recombination of majority and minority charge carriers at the respective contacts. The theory can be experimentally confirmed by illumination level dependent IV-curves as well as short circuit current density and open circuit voltage transients.},
  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{Foertig2013,
  author    = {F{\"o}rtig, Alexander},
  title     = {Recombination Dynamics in Organic Solar Cells},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83895},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {Neben herk{\"o}mmlichen, konventionellen anorganischen Solarzellen — haupts{\"a}chlich auf Silizium basierend — ist die Organische Photovoltaik (OPV) auf dem besten Wege in naher Zukunft eine kosteng{\"u}nstige, umweltfreundliche, komplement{\"a}re Technolgie darzustellen. Die Produktionskosten, die Lebenszeit der Solarzellen sowie deren Wirkungsgrad m{\"u}ssen dabei weiter optimiert werden, um einen Markteintritt der OPV zu erm{\"o}glichen. Die vorliegende Arbeit befasst sich mit der Effizienz organischer Solarzellen und deren Limitierung durch die Rekombination von Ladungstr{\"a}gern. Um funktionsf{\"a}hige Zellen zu untersuchen, werden zeitaufgel{\"o}ste Experimente wie die Messung der transienten Photospannung (TPV), des transienten Photostroms (TPC), die Ladungsextraktion (CE) sowie die time delayed collection field (TDCF) Methode angewandt. Untersucht werden sowohl fl{\"u}ssig prozessierte als auch aufgedampfte Proben, unterschiedliche Materialzusammensetzungen und verschiedene Probengeometrien. Das Standardmaterialsystem der OPV, P3HT:PC61BM, wird bei verschiedenen emperaturen und Beleuchtungsst{\"a}rken auf die Lebenszeit und Dichte der photogenerierten Ladungstr{\"a}ger {\"u}berpr{\"u}ft. F{\"u}r den Fall spannungsunabh{\"a}ngiger Generation von Ladungstr{\"a}gern zeigt sich die Anwendbarkeit der Shockley-Gleichung auf organische Solarzellen. Des Weiteren wird ein konsistentes Modell erl{\"a}utert, welches den Idealtit{\"a}tsfaktor direkt mit der Rekombination von freien mit gefangenen, exponentiell verteilten Ladungstr{\"a}gern verkn{\"u}pft. Ein Ansatz, bekannt unter der Bezeichung j=V Rekonstruktion, erm{\"o}glicht es, den leistungslimitierenden Verlustmechanismus in unbehandelten und thermisch geheizten P3HT:PC61BM Solarzellen zu identifizieren. Dieses Verf ahren, welches TPV, CE und TDCF Messungen beinhaltet, wird auf Proben basierend auf dem neuartigen, low-band gap Polymer PTB7 in Verbindung mit dem Fulleren PC71BM ausgeweitet. W{\"a}hrend in der Zelle hergestellt aus reinem Chlorbenzol betr{\"a}chtliche geminale wie nichtgeminale Verluste zu beobachten sind, erleichtert die Zugabe eines L{\"o}sungsmittelzusatzes die Polaronenpaartrennung, was zu einer starken Reduktion geminaler Verluste f{\"u}hrt. In einer Kooperation mit dem IMEC Institut in Leuven, werden abschließend die beiden bedeutensten Probenarchitekturen organischer Solarzellen, die planare und die Misch{\"u}bergang Struktur, jeweils basierend auf CuPC und C60, bez{\"u}glich nichtgeminaler Rekombination und Ladungstr{\"a}gerverteilung miteinander verglichen. Neben den beiden experimentellen Techniken um TPV und CE werden makroskopische Simulationen herangezogen, um den Ursprung unterschiedlichen Voc vs. Lichtintensit{\"a}t-Verhaltens zu erkl{\"a}ren.},
  subject      = {Organische Solarzelle},
  language  = {en}
}
@phdthesis{Ruff2013,
  author    = {Ruff, Andreas},
  title     = {On the importance of electronic correlations in potassium-doped organic semiconductors},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83635},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {The present thesis is concerned with the impact of alkali metal-doping on the electronic structure of semiconducting organic thin films. The organic molecular systems which have been studied are the polycyclic aromatic hydrocarbons picene, pentacene, and coronene. Motivated by reports about exceptional behavior like superconductivity and electronic correlations of their alkali metal-doped compounds, high quality films fabricated from the above named molecules have been studied. The electronic structure of the pristine materials and their doped compounds has been investigated using photoelectron spectroscopy. Core level and valence band studies of undoped films yield excellent photoemission spectra agreeing with or even outperforming previously reported data from the literature. Alkali metal-doping manifests itself in a uniform manner in the electronic structure for all probed samples: Opposed to reports from the literature about metallicity and even superconductivity in alkali metal-doped picene, pentacene, and coronene, all films exhibit insulating nature with an energy gap of the order of one electron-volt. Remarkably, this is independent of the doping concentration and the type of dopant, i.e., potassium, cesium, or sodium. Based on the interplay between narrow bandwidths in organic semiconductors and sufficiently high on-molecule Coulomb repulsion, the non-metallicity is attributed to the strong influence of electronic correlations leading to the formation of a Mott insulator. In the case of picene, this is consolidated by calculations using a combination of density functional theory and dynamical mean-field theory. Beyond the extensive considerations regarding electronic correlations, further intriguing aspects have been observed. The deposition of thin picene films leads to the formation of a non-equilibrium situation between substrate and film surface. Here, the establishment of a homogeneous chemical potential is hampered due to the only weak van der Waals-interactions between the molecular layers in the films. Consequently, spectral weight is measurable above the reference chemical potential in photoemission. Furthermore, it has been found that the acceptance of additional electrons in pentacene is limited. While picene and coronene are able to host up to three extra electrons, in pentacene the limit is already reached for one electron. Finally, further extrinsic effects, coming along with alkali metal-doping, have been scrutinized. The oxidation of potassium atoms induced by the reaction with molecular oxygen in the residual gas of the ultra-high vacuum system turned out to significantly influence the electronic structure of alkali metal-doped picene and coronene. Moreover, also the applied X-ray and UV irradiation caused a certain impact on the photoemission spectra. Surprisingly, both effects did not play a role in the studies of potassium-doped pentacene.},
  subject      = {Organischer Halbleiter},
  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}
}
@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}
}