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- D-1250-2010 (1)
This work revealed spin states that are involved in the light generation of organic light-emitting diodes (OLEDs) that are based on thermally activated delayed fluorescence (TADF). First, several donor:acceptor-based TADF systems forming exciplex states were investigated. Afterwards, a TADF emitter that shows intramolecular charge transfer states but also forms exciplex states with a proper donor molecule was studied. The primary experimental technique was electron paramagnetic resonance (EPR), in particular the advanced methods electroluminescence detected magnetic resonance (ELDMR), photoluminescence detected magnetic resonance (PLDMR) and electrically detected magnetic resonance (EDMR). Additional information was gathered from time-resolved and continuous wave photoluminescence measurements.
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 Rn2 (recombination order RO = 2), for Shockley-Read-Hall (SRH) Rn1 (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.
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.
In the field of organic photovoltaics, one of the most intensely researched topics to date is the charge carrier photogeneration in organic bulk heterojunction solar cells whose thorough understanding is crucial for achieving higher power conversion efficiencies. In particular, the mechanism of singlet exciton dissociation at the polymer–fullerene interface is still controversially debated.
This work addresses the dissociation pathway via relaxed charge transfer states (CTS) by investigating its field dependence for reference material systems consisting of MDMO-PPV and one of the fullerene derivatives PC61BM, bisPCBM and PC71BM. Field dependent photoluminescence (PL(F)) and transient absorption (TA(F)) measurements give insight into the recombination of charge transfer excitons (CTE) and the generation of polarons, respectively. Optically detected magnetic resonance and atomic force microscopy are used to characterize the morphology of the samples.
The comparison of the experimental field dependent exciton recombination recorded by PL(F) and the theoretical exciton dissociation probability given by the Onsager–Braun model yields the exciton binding energy as one of the key parameters determining the dissociation efficiency. The binding energies of both the singlet exciton in neat MDMO-PPV and the CTE in MDMO-PPV:PC61BM 1:1 are extracted, the latter turning out to be significantly reduced with respect to the one of the singlet exciton.
Based on these results, the field dependence of CTE dissociation is evaluated for MDMO-PPV:PC61BM blends with varying fullerene loads by PL(F) and TA(F). For higher PC61BM contents, the CTE binding energies decrease notably. This behavior is ascribed to a larger effective dielectric constant for well-intermixed blends and to an interplay between dielectric constant and CTE delocalization length for phase separated morphologies, emphasizing the importance of high dielectric constants for the charge carrier photogeneration process.
Finally, the CTE binding energies are determined for MDMO-PPV blends with different fullerene derivatives, focusing on the influence of the acceptor LUMO energy. Here, the experimental results suggest the latter having no or at least no significant impact on the binding energy of the CTE. Variations of this binding energy are rather related to different trap levels in the acceptors which seem to be involved in CTS formation.
Elektrooptische Transporteigenschaften und stochastisch aktivierte Prozesse Resonanter Tunneldioden
(2012)
Im Rahmen der vorliegenden Arbeit wurden elektrooptische Transporteigenschaften und stochastisch aktivierte Prozesse Resonanter Tunneldioden (RTDs) bei Raumtemperatur untersucht. Die RTDs wurden auf dem III-V Halbleitermaterialsystem AlGaAs/GaAs durch Molekularstrahlepitaxie, Elektronenstrahllithographie und trockenchemischen Ätztechniken hergestellt. Im Bereich des negativen differentiellen Leitwerts konnte bistabi-les Schalten und hierbei stochastisch aktivierte Dynamik nichtlinearer Systeme untersucht werden. Die Flächenabhängigkeit der Ätzrate konnte ausgenutzt werden, um RTDs mit einem Stamm und zwei Transportästen zu realisieren, welche hinsichtlich ihrer optischen und elektrischen Eigenschaften untersucht wurden. Im ersten experimentellen Abschnitt 3.1 werden die elektrischen Transporteigenschaften Resonanter Tunneldioden bei Raum-temperatur und die Flächenabhängigkeit des kohärenten und nicht-kohärenten Elektronen-transports analysiert. Die Realisierung universeller logischer Gatter (NOR und NAND) und deren Rekonfigurierbarkeit durch einen externen Kontrollparameter wird in Abschnitt 3.2 gezeigt. In Abschnitt 3.3 wird die Lichtsensitivität Resonanter Tunneldioden als Photode-tektoren für den sichtbaren Wellenlängenbereich und in Abschnitt 3.4 für die Telekommu-nikationswellenlänge bei λ = 1,3 µm demonstriert.
In this work the creation of silicon vacancy spin defects in silicon carbide with predictable properties is demonstrated. Neutron and electron irradiation was used to create silicon vacancy ensembles and proton beam writing to create isolated vacancies at a desired position. The coherence properties of the created silicon vacancies as a function of the emitter density were investigated and a power-law function established. Sample annealing was implemented to increase the coherence properties of existing silicon vacancies. Further, spectral hole burning was used to implement absolute dc-magnetometry.
Stationäre Gasturbinen können von großer Bedeutung für die Verlangsamung des Klima-wandels und bei der Bewältigung der Energiewende sein. Für die Weiterentwicklung von Gasturbinen zu höheren Betriebstemperaturen und damit einhergehend zu höheren Wirkungs-graden werden berührungslose Messverfahren zur Ermittlung der Oberflächentemperatur von Turbinenschaufeln und der Gastemperatur der heißen Verbrennungsgase während des Be-triebs benötigt. Im Rahmen dieser Arbeit werden daher Methoden der berührungslosen Tem-peraturmessung unter Verwendung von Infrarotstrahlung untersucht.
Die berührungslose Messung der Oberflächentemperatur moderner Turbinenschaufeln muss aufgrund derer infrarot-optischer Oberflächeneigenschaften im Wellenlängenbereich des mitt-leren Infrarots durchgeführt werden, in welchem die Turbinenbrenngase starke Absorptions-banden aufweisen. Zur Entwicklung eines adäquaten Strahlungsthermometers für diesen Zweck wurden im Rahmen dieser Arbeit daher durch Ermittlung von Transmissionsspektren von Kohlenstoffdioxid und Wasserdampf bei hohen Temperaturen und Drücken in einer ei-gens hierfür konstruierten Heißgas-Messzelle zunächst Wellenlängenbereiche identifiziert, in welchen die geplanten Messungen möglich sind. Anschließend wurde der Prototyp eines ent-sprechend konfigurierten Strahlungsthermometers im Zuge des Testlaufes einer vollskaligen Gasturbine erfolgreich erprobt.
Weiterhin wurden im Rahmen dieser Arbeit zwei mögliche Verfahren zur berührungslosen Gastemperaturmessung untersucht. Das erste untersuchte Verfahren setzt ebenfalls auf Strah-lungsthermometrie. Dieses Verfahren sieht vor, aufgrund der Temperaturabhängigkeit des spektralen Transmissionsgrades in den Randbereichen von gesättigten Absorptionsbanden von Gasen aus der in diesen Bereichen transmittierten spektralen Strahldichte auf die Gastempera-tur zu schließen. Im Rahmen dieser Arbeit wurden Voruntersuchungen für dieses Tempera-turmessverfahren durchgeführt. So konnten auf der Grundlage von experimentell ermittelten Transmissionsspektren von Kohlenstoffdioxid bei Drücken zwischen 5 kPa und 600 kPa und Gastemperaturen zwischen Raumtemperatur und 1073 K für das geplante Verfahren nutzbare Wellenlängenintervalle insbesondere im Bereich der Kohlenstoffdioxid-Bande bei 4,26 µm identifiziert werden.
Das zweite im Rahmen dieser Arbeit untersuchte Verfahren zur berührungslosen Gastem-peraturmessung basiert auf der Temperaturabhängigkeit der Wellenlängenposition der Trans-missionsminima der Absorptionsbanden von infrarot-aktiven Gasen. Im Hinblick darauf wur-de dieses Phänomen anhand von experimentell bestimmten hochaufgelösten Transmissions-spektren von Kohlenstoffdioxid überprüft. Weiterhin wurden mögliche Wellenlängenbereiche identifiziert und hinsichtlich ihrer Eignung für das geplante Verfahren charakterisiert. Als am vielversprechendsten erwiesen sich hierbei Teilbanden in den Bereichen um 2,7 µm und um 9,2 µm. Unter Beimischung von Stickstoff mit Partialdrücken von bis zu 390 kPa erwies sich zudem auch die Bande bei 4,26 µm als geeignet.
Die im Rahmen dieser Arbeit experimentell ermittelten Transmissionsspektren konnten dar-über hinaus schließlich durch Vergleich mit entsprechenden HITRAN-Simulationen verifiziert werden.
Der Ausbau der regenerativen Energiequellen führt vermehrt zu unvorhersehbaren Schwankungen der erzeugten Leistung, da Windkraft und Photovoltaik von natürlichen Bedingungen abhängen. Gerade Kurzzeitfluktuationen im Sekunden- bis Minutenbereich, die bei Solarzellen durch die Verschattung von vorüberziehenden Wolken zustande kommen, wird bislang wenig Beachtung geschenkt. Kurzzeitspeicher müssen eine hohe Zyklenstabilität aufweisen, um zur Glättung dieser Leistungsfluktuationen in Frage zu kommen. Im Rahmen der vorliegenden Dissertation wurden elektrochemische Doppelschichtkondensatoren für die Kopplung mit Siliziumsolarzellen und organischen Solarmodulen mit Hilfe von Simulationen und Messungen untersucht. Zusätzlich wurden grundlegende Fragestellungen zur Prozessierung und Alterung von Doppelschichtkondensatoren im Hinblick auf ein in der Literatur bereits diskutiertes System betrachtet, das beide Komponenten in einem Bauteil integriert - den sogenannten photocapacitor.
Um die Druckbarkeit des gesamten elektrochemischen Doppelschichtkondensators zu ermöglichen, wurde der konventionell verwendete Flüssigelektrolyt durch einen Polymer-Gel-Elektrolyten auf Basis von Polyvinylalkohol und einer Säure ersetzt. Durch eine Verbesserung der Prozessierung konnte ein größerer Anteil der spezifischen Fläche der porösen Kohlenstoffelektroden vom Elektrolyten benetzt und somit zur Speicherung genutzt werden. Die Untersuchungen zeigen, dass mit Polymer-Gel-Elektrolyten ähnliche Kapazitäten erreicht werden wie mit Flüssigelektrolyten. Im Hinblick auf die Anwendung im gekoppelten System muss der elektrochemische Doppelschichtkondensator den gleichen Umweltbedingungen hinsichtlich Temperatur und Luftfeuchte standhalten wie die Solarzelle. Hierzu wurden umfangreiche Alterungstests durchgeführt und festgestellt, dass die Kapazität zwar bei Austrocknung des wasserhaltigen Polymer-Gel-Elektrolyten sinkt, bei einer Wiederbefeuchtung aber auch eine Regeneration des Speichers erfolgt.
Zur passenden Auslegung des elektrochemischen Doppelschichtkondensators wurde eine detaillierte Analyse der Leistungsfluktuationen durchgeführt, die mit einem eigens entwickelten MPP-Messgerät an organischen Solarmodulen gemessen wurden. Anhand der Daten wurde analysiert, welche Energiemengen für welche Zeit im Kurzzeitspeicher zwischengespeichert werden müssen, um eine effiziente Glättung der ins Netz einzuspeisenden Leistung zu erreichen. Aus der Statistik der Fluktuationen wurde eine Kapazität berechnet, die als Richtwert in die Simulationen einging und dann mit anderen Kapazitäten verglichen wurde. Neben einem idealen MPP-Tracking für verschiedene Arten von Solarzellen und Beleuchtungsprofilen konnte die Simulation auch die Kopplung aus Solarzelle und elektrochemischem Doppelschichtkondensator mit zwei verschiedenen Betriebsstrategien nachbilden. Zum einen wurde ein fester Lastwiderstand genutzt, zum anderen eine Zielspannung für den Kurzzeitspeicher und somit auch die Solarzelle vorgegeben und der Lastwiderstand variabel so angepasst, dass die Zielspannung gehalten wird. Beide Betriebsmethoden haben einen Energieverlust gegenüber der MPP-getrackten Solarzelle zu verzeichnen, führen aber zu einer Glättung der Leistung des gekoppelten Systems. Die Simulation konnte für Siliziumsolarzellen mit einem Demonstratorversuch im Labor und für organische Solarzellen unter realen Bedingungen validiert werden.
Insgesamt ergibt sich eine vielversprechende Glättung der Leistungsfluktuationen von Solarzellen durch den Einsatz von elektrochemischen Doppelschichtkondensatoren.
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.
In this work, three different material systems comprising carbon were researched: (i) Organic polymers and small molecules, in conjunction with fullerene molecules for applications in organic photovoltaics (OPV), (ii) single walled semiconducting carbon nanotubes and (iii) silicon carbide (SiC), whose defect color centers are recently in the limelight as candidates for quantum applications. All systems were analyzed using the optically detected magnetic resonance (ODMR) spectroscopy.
In the OPV chapter, first the intrinsic parameters and orientations of high spin excitons were analyzed in the materials P3HT, PTB7 and DIP. Specifically the influence of ordering in these organic systems was adressed. The second part of the OPV chapter is concerned with triplet generation by electron back transfer in the high-efficiency OPV material combination PTB7:PC71BM.
The carbon nanotube chapter first shows the way to the first unambiguous proof of the existence of triplet excitons in semiconducting (6,5) single-walled carbon nanotubes (SWNT) by ODMR spectroscopy. A model for exciton kinetics, and also orientation and intrinsic parameters were propoesed.
The last part of this work is devoted to spin centers in silicon carbide (SiC). After a brief introduction, the spin multiplicity of the V2 and V3 silicon vacancies, and also of a Frenkel pair and an unassigned defect UD in 6H SiC, and of the V2 vacancy and the Frenkel pair in 4H SiC, was shown to be S=3/2. The spin polarized pumping of the 3/2 manifold of the quartet ground state of the silicon vacancies allows stimulated microwave emission. Furthermore, in 6H SiC, the UD and Frenkel pair were shown to have a large dependence of their intrinsic zero field interaction parameters on the temperature, while the vacancies are temperature independent. The application of the UD and Frenkel pair as temperature sensor, and of the vacancies as a vector magnetic field sensor is discussed.