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Heiber, Michael C. "Dynamic Monte Carlo Modeling of Exciton Dissociation and Geminate Recombination in Organic Solar Cells." University of Akron / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=akron1353092083.
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Tamai, Yasunari. "Excited State Dynamics in Nanostructured Polymer Systems." 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/174961.
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Visnevski, Dmitri. "Collective dynamics of excitons and exciton-polaritons in nanoscale heterostructures." Phd thesis, Université Blaise Pascal - Clermont-Ferrand II, 2013. http://tel.archives-ouvertes.fr/tel-00914332.
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In my thesis I will discuss some aspects of collective dynamics of excitons and exciton-polaritons in nanoscale heterostructures. In the first Chapter I will make a brief introduction to the modern semiconductor physics and willdescribe the general elements and notions which will be used further. Other four chapters would be devoted to four works in which I participated, notably, in Chapter 2 I will speak about the coherent interactions between phonons and exciton orexciton-polariton condensates, in Chapter 3 I will discuss the quantum dots lasing and its amplification by an acoustic pulse. Chapter 4 and 5 will be devoted respectively to the polariton multistability and to the condensates of indirect excitons.
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Abbas, Chahine. "Optical spectroscopy of indirect excitons and electron spins in semiconductor nanostructures." Thesis, Montpellier, 2019. http://www.theses.fr/2019MONTS049.
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Ce travail porte sur l’étude optique de la dynamique de spin de deux systèmes: un gaz d’électrons dans des couches minces de CdTe d’une part et des excitons indirects dans un double puits quantique asymétrique en GaAs d’autre part. Des mesures de photoluminescence résolue en temps et en polarisation, et des mesures de spectroscopie pompe-sonde ont permis la détermination des temps de vie et des temps de relaxation de spin des excitons indirects. Le comportement général de la structure a été décrit, les contraintes techniques ont été mise en évidence et les meilleures conditions expérimentales ont été identifiées. En photoluminescence, nous avons mesuré des temps de vie de l’ordre de la quinzaine de ns et des temps de relaxation de spin de 5 ns dans le meilleur cas. L’utilisation d’un setup de spectroscopie pompe-sonde permettant d’étudier des délais très longs a démontré que des temps plus longs encore peuvent être atteints en séparant d’avantage deux impulsions lasers successives.Pour les électrons dans CdTe nous avons utilisé une autre méthode optique: la spectroscopie de bruit de spin qui s’est récemment imposée comme un outil de choix pour étudier la dynamique de spin dans les semi-conducteurs. Son principe consiste à sonder la dynamique d’un système de spins à travers ses fluctuations spontanées. Pour ce faire, ces fluctuations sont encodées dans le plan de polarisation d’un laser hors résonnant par l’intermédiaire de la rotation Faraday.Alors que les réalisations concrètes de cette technique se limitaient jusqu’à présent aux corrélations temporelles, nous proposons ici la première implémentation permettant d’accéder également aux corrélations spatiales du systèmes de spin. Cet accès à la dynamique spatiale est autorisé par une sélectivité en vecteur d’onde de la lumière diffusée venant de l’échantillon et nous offre l’opportunité de mesurer simultanément le temps de relaxation de spin et le coefficient de diffusion de spin. Ayant ainsi une vision complète de la dynamique de spin dans CdTe, nous avons pu confronter la physique du spin bien connue dans GaAs à nos observations dans CdTe. Contre toutes attentes, il semblerait que nos connaissances de GaAs ne soient pas directement transposables à CdTeThis work provides an optical study of spin dynamics in two different systems: electrons gas in n-doped CdTe thin layers, and indirect excitons in asymmetric GaAs coupled quantum wells. Time and polar resolved photoluminescence and pump-probe spectroscopy allowed the determination of both the lifetime and the relaxation time of indirect excitons.The global behaviour of the dedicated biased sample has been described, major technical constraints have been pointed out and optimal working conditions have been identified. In photoluminescence, we obtained a lifetime of 15 ns and a spin relaxation time of 5 ns. Pump-probe spectroscopy with an exceptional delay range shown that longer characteristic times could be obtained increasing the delay between two laser pulses.An other optical method has been used to study electrons in CdTe thin layers. Spin noise spectroscopy has recently emerged as an ideal tool to study dynamics of spin systems through their spontaneous fluctuations which are encoded in the polarisation state of a laser beam by means of Faraday rotation. Common spin noise setups provide only temporal fluctuations, spatial information being lost averaging the signal on the laser spot. Here, we demonstrate the first implementation of a spin noise setup providing both spatial and temporal spin correlations thanks to a wave vector selectivity of the scattered light. This gave us the opportunity to measure both the spin relaxation time and the spin diffusion coefficient. This complete vision of the spin dynamics in CdTe has been compared to our understanding of spin physics in GaAs. Against all odds, this knowledge seems not to be directly transposable from GaAs to CdTe
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Sajjad, Muhammad Tariq. "Exciton dynamics in carbon nanotubes." Thesis, University of Surrey, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.576127.
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ABSTRACT The confinement of excited charges in carbon nanotubes has significant effect on their optical and electronic properties. The absorption of light generates strongly correlated electron-hole pairs (excitons) in carbon nanotubes. We present a study of the decay of these photogenerated excitons in solutions of semiconducting SWNTs using the degenerate pump probe technique. Under specific experimental conditions, the exciton-exciton reactions on carbon nanotubes were found to correspond to an ideal ID coalescence-diffusion system with distinct regions of reaction-limited and diffusion-limited behaviours. We provided the first experimental evidence for such a system of 'universal behaviour' at longer times which exhibits a power law decay whose exponent and amplitude are independent of the initial population - one of the key characteristics of this reaction-diffusion system. We also show for the first time that exciton-exciton interactions are long-range, and further that the transition between reaction-limited and diffusion-limited regimes is much more abrupt than is predicted by existing theories. A modified theory incorporating a finite reaction length provided an excellent fit to the experimental data for a reaction length of -7 nm. We determined the reaction rate constant of k; = (3.76 ± 0.04) nmfps and the diffusion coefficient of D = (8.1 ± 0.4) nm2/ps from fitting of the asymptotic regimes with rate equations. We also provided the first experimental evidence of sub-diffusive transport of excitons in quasi ID SWNTs through studies of exciton annihilation dynamics in HiPco and CoMoCat SWNTs, where we observed that excitons in HiPco SWNTS exhibit normal diffusive transport where decay follows a Clal power law with decay exponent (a :::::: 0.5), whereas excitons in CoMoCat SWNTs decay more slowly with decay exponent (a :::::: 0.3) as result of sub-diffusive transport. We correlate this slow decay to a higher defect concentration in CoMoCat SWNTs as compared to HiPco measured using Raman spectroscopy and X-ray photoelectron spectroscopy. The experimental results were also compared to results from a Monte Carlo simulation of ID diffusion in a fluctuating potential landscape which shows good agreement with experiment and underlines the necessity to consider spatial aspects (separation of defects and interaction range) in theoretical approaches to the dynamics. The study of excitonic decay in single walled carbon nanotubes is very important in terms of understanding of fundamental photophysics of ID system and their possible application in electronics and photonic devices especially in light emitting devices and non-linear optics. 2
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Bouet, Louis. "Valley dynamics and excitonic properties in monolayer transition metal dichalcogenides." Thesis, Toulouse, INSA, 2015. http://www.theses.fr/2015ISAT0033/document.
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La possibilité de créer des monocouches de dichalcogenures à métaux de transition (MoS2, WSe2,MoSe2 pour ceux étudiés dans ce manuscrit) a été démontrée récemment (2005) et a ouvert la voie à l’étude de ces matériaux sous leur forme 2D. Il apparaît depuis que les propriétés de ces semi-conducteurs sous leur forme monocouche offrent des perspectives intéressantes à la fois du point de vue de la physique fondamentale et des potentielles applications qui peuvent en découler ; en plus de bénéficier d’un fort couplage avec la lumière, l’existence d’un gap important (situé dans le visible, 1.7-1.8 eV) permet entre autres de réaliser des transistors d’épaisseur mono-atomique. Par ailleurs, la physique de ces matériaux est prometteuse pour les applications dans le domaine de l’optoélectronique. En effet, lorsque le matériau est affiné jusqu’à la monocouche atomique, son gap optique devient direct et la brisure de symétrie d’inversion associée au fort couplage spin-orbite provoque l’apparition de règles de sélection optique originales qui relient directement la polarisation de la lumière émise ou absorbée à une des deux vallées non-équivalentes de l’espace réciproque. Cela ouvre la possibilité d’explorer une nouvelle physique, basée sur l’indice de vallée et intitulée en conséquence vallée-tronique, avec comme perspectives futures la manipulation de l’indice de vallée et l’exploitation d’effetsliés à cette relation originale entre propriétés optiques et électroniques (effet vallée-Hall par exemple). Cemanuscrit de thèse regroupe une série d’expériences réalisées dans le but de comprendre et caractériser les propriétés optoélectroniques de ces matériaux. Un premier chapitre introductif présente le contexte scientifique de ces travaux de recherche et démontre l’origine des propriétés électroniques et optiques de ces matériaux via un modèle théorique simple. Le second chapitre présente en détails les échantillons étudiés ainsi que le dispositif expérimental utilisé lors des mesures. Enfin les chapitres 3 à 6 détaillent les expériences menées et les résultats obtenus ; le lecteur y trouvera des mesures de photoluminescence apportant la démonstration expérimentale des règles de sélection optique, l’identification des différents raies spectrales d’émission pour les différentstypes d’échantillons mentionnés plus haut ainsi que des mesures de photoluminescence résolues en temps permettant d’extraire la dynamique des propriétés des porteurs photo-générés. Une part importante de ce manuscrit est consacrée à l’étude expérimentale des propriétés excitoniques de ces matériaux dont la structure de bande électronique est finalement sondée via des études de magnéto-spectroscopieThe possibility of isolating transition metal dichalcogenide monolayers by simple experimental means has been demonstrated in 2005, by the same technique used for graphene. This has sparked extremely diverse and active research by material scientists, physicists and chemists on these perfectly two-dimensional (2D) materials. Their physical properties inmonolayer formare appealing both fromthe point of view of fundamental science and for potential applications. Transition metal dichalcogenidemonolayers such asMoS2 have a direct optical bandgap in the visible and show strong absorption of the order of 10% per monolayer. For transistors based on single atomic layers, the presence of a gap allows to obtain high on/off ratios.In addition to potential applications in electronics and opto-electronics these 2D materials allow manipulating a new degree of freedom of electrons, in addition to the spin and the charge : Inversion symmetry breaking in addition to the strong spin-orbit coupling result in very original optical selection rules. The direct bandgap is situated at two non-equivalent valleys in k-space, K+ and K−. Using a specific laser polarization, carriers can be initialized either in the K+ or K− valley, allowing manipulating the valley index of the electronic states. This opens up an emerging research field termed "valleytronics". The present manuscript contains a set of experiments allowing understanding and characterizing the optoelectronic properties of these new materials. The first chapter is dedicated to the presentation of the scientific context. The original optical and electronic properties of monolayer transition metal dichalcogenides are demonstrated using a simple theoreticalmodel. The second chapter presents details of the samples and the experimental setup. Chapters 3 to 6 present details of the experiments carried out and the results obtained. We verify experimentally the optical selection rules. We identify the different emission peaks in the monolayer materials MoS2, WSe2 and MoSe2. In time resolved photoluminescence measurements we study the dynamics of photo-generated carriersand their polarization. An important part of this study is dedicated to experimental investigations of the properties of excitons, Coulomb bound electron-hole pairs. In the final experimental chapter, magneto-Photoluminescence allows us to probe the electronic band structure and to lift the valley degeneracy
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Brüggemann, Ben. "Theory of ultrafast exciton dynamics in photosynthetic antenna systems." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2004. http://dx.doi.org/10.18452/15037.
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Die Multiexzitonen-Theorie des Anregungsenergie-Transfers in Farbstoff-Protein-Komplexen und biologischen Antennensystemen wird um den Prozess der Exziton-Exziton-Vernichtung erweitert. Um eine mikroskopische Beschreibung zu erzielen, wird eine Herangehensweise benutzt, die auf der Internen Konversion der Anregungsenergie innerhalb der Farbstoffmoleküle basiert. Diese Interne Konversion führt zu nicht strahlenden Übergängen von höheren zu niedrigeren Exziton-Mannigfaltigkeiten. Neben der Einbeziehung der Exziton-Exziton-Vernichtung beinhaltet die hier verwendete Multiexziton-Dichtematrixtheorie auch die Kopplung zu niedrig-energetischen Schwingungs-Freiheitsgraden und dem elektrischen Feld. Für den Übergang von der Zwei- zu der Einexzitonen -Mannigfaltigkeit werden exakte und genäherte Ausdrücke hergeleitet. Die erste Anwendung der erweiterten Multiexziton-Dichtematrixtheorie ist die Berechnung von ultra-schnellen differentiellen Absoptionsspektren. Um den Prozess der Exziton-Exziton-Vernichtung in intensitätsabhängigen differenziellen Absorptionsspektren näher zu untersuchen, wird diese Herangehensweise auf den B850 Ring des LH2 von rhodobacter sphaeroides angewendet. Die Bedeutung der Exziton-Exziton-Vernichtung und der Einfluss von statischer Unordnung werden detailiert diskutiert. Die Simulationen der differentiellen Absorptionsspektren mit statischer Unordnung und Orientierungsmittelung zeigen gute Übereinstimmung mit experimentellen Beobachtngen. Durch die Veröffentlichung der Strukturdaten des Photosystem I (PS1) von Synechococcus elongatus wurde es zum ersten Mal möglich, ein Exziton-Modell für die 96 Chorophyllmoleküle einzuführen, die in die Proteinmatrix dieses Antennensystems eingebettet sind. Das Ziel dabei ist, sowohl die linearen Spektren in einem weiten Temperaturbereich, als auch die zeitaufgelöste Fluoreszenz zu reproduzieren. Die Kopplungen und die Dipolmomente der Chlorophyllmoleküle wurden den Strukturdaten entnommen. Da die Energien der einzelnen Farbstoffe stark von deren unmittelbarer Umgebung abhängt, werden diese bestimmt, indem simulierte Absorption, Lineardichroismus und Zirkulardichroismus bei niedrigen Temperaturen den experimentellen Spektren angepasst werden. Nachdem einige Chlorophyllmoleküle den Zuständen mit den niedrigsten Energien zugeordnet wurden, werden die Energien mit Hilfe eines evolutionären Algorithmus angepasst. Die Qualität des PS1 Modells wird durch die Berechnung der zeitabhängigen Fluoreszenz untermauert (mit zusätzlicher inhomogener Linienbreite), die Simulationen stimmen gut mit aktuellen experimentellen Resultaten überein. Die oben erwähnten Exziton-Modelle beschreiben die jeweiligen Experimente erfolgreich. Der nächste Schritt ist, diese Modelle zu nutzen, um einen neuen Typ von Experiment vorzuschlagen, das Exciton-Steuerungs-Experiment. Auf dem Exciton-Modell des FMO Komplexes von Prosthecochloris aestuarii und dem oben erwähnten PS1 Modell von Synechococcus elongatus aufbauend wird die Bildung von exzitonischen Wellenpaketen durch Laser-Anregung studiert. Diese stellen eine kohärente Überlagerung exzitonischer Zustände dar, ähnlich der bei Schwingungs-Wellenpaketen. Um die spezielle Form des Femtosekunden-Laserpulses zu bestimmen, der zu einer räumlichen Lokalisierung der Anregungsenergie führt, wird die Theorie der optimalen Steuerung verwendet. Die Möglichkeit, solch einen lokalisierten Zielzustand zu erreichen, wird aufgezeigt, auch unter dem Einfluss von energetischer Unordnung und Exziton-Exziton Vernichtung. Ferner wird gezeigt, dass die Effizienz der Lokalisierung und die Länge des optimalen Pulses stark von der Temperatur abhängen.The multi-exciton description of excitation energy transfer in chromophore complexes and biological light harvesting antenna systems is extended to include the exciton-exciton annihilation processes. To achieve a complete microscopic description the approach is based on intra--chromophore internal conversion processes which leads to non-radiative transitions from higher to lower lying exciton manifolds. Besides an inclusion of exciton-exciton annihilation the used multi-exciton density matrix theory also accounts for a coupling to low-frequency vibrational modes and the radiation field. Concentrating on transitions from the two- to the single-exciton manifold exact and approximate expressions for the annihilation rate are derived. A first application of the introduced extended multi-exciton density matrix theory is given by the computation of ultrafast transient absorption spectra. To elucidate the process of exciton-exciton annihilation in intensity dependent transient absorption data the approach is applied to the B850 ring of the LH2 found in rhodobacter sphaeroides. The signatures of exciton-exciton annihilation as well as the influence of static disorder are discussed in detail. The simulations of transient absorption including static disorder and orientational averaging are in good agreement with experimental data. The recently published structure of the Photosystem I (PS1) of Synechococcus elongatus made it for the first time possible to introduce an excitonic model for the 96 chlorophylls embedded in the protein matrix of that core-antenna system, as presented in this work. The challenge has been to reproduce linear frequency domain spectra in a wide temperature range as well as the time resolved fluorescence. The couplings and the dipole-moments of the chlorophylls are extracted from the x-ray crystal structure. Since the position of the energetic levels of the chlorophylls depend on the respective surrounding their determination is achieved by fitting low temperature absorption, linear dichroism and circular dichroism at the same time. After assigning some chromophores to the red-most states, an evolutionary algorithm is used to get the best fit. The quality of the resulting PS1 model (additionally accounting for inhomogeneous line broadening) is confirmed in calculating time dependent fluorescence spectra which show a good agreement with recent experimental results. The outlined method is also applicable to other photosynthetic antenna systems. The above described exciton models successfully explain the respective measurements. In a second step, they will be used to propose a new type of experiment, the exciton control experiment. Based on an exciton model for the FMO complex of Prosthecochloris aestuarii and the proposed PS1 model of Synechococcus elongatus one studies the laser pulse formation of excitonic wavepackets, i.e. a coherent superposition of excitonic states similar to vibrational wavepackets. Optimal Control theory is used to calculate the shape of femtosecond laser pulses that leads to a spatial localization of excitation energy. The possibility to populate such a localized target state is demonstrated, even in the presence of disorder or exciton-exciton annihilation, and it is shown that the efficiency of localization as well as the length the most suited pulses strongly depend on temperature.
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Nelson, Delene J. "Exciton operators, communication relations and dynamics." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp01/MQ33420.pdf.
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Lagoudakis, Pavlos G. "Exciton polariton dynamics in semiconductor microcavities." Thesis, University of Southampton, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.274583.
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Grevatt, Treena. "Exciton spin dynamics in quantum wells." Thesis, University of Southampton, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.242274.
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Wen, Patrick Ph D. Massachusetts Institute of Technology. "Correlated exciton dynamics in semiconductor nanostructures." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/82325.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2013.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 211-223).
The absorption and dissipation of energy in semiconductor nanostructures are often determined by excited electron dynamics. In semiconductors, one fundamentally important electronic state is an exciton, an excited electron bound to a positively charged vacancy. Excitons can become correlated with other excitons, mutually influencing one another and exhibiting collective properties. The focus of this dissertation concerns the origins, effects, and control of correlated excitons in semiconductor nanostructures. Correlated Coulomb interactions can occur between excitons, resulting in energy shifts and dephasing in each exciton. Two-dimensional Fourier-transform optical spectroscopy is a powerful tool to understand Coulomb correlations; the technique relates exciton dynamics during distinct time periods. However, the technique is still limited by weak spectral features. Using two-dimensional pulse shaping methods, waveforms of excitation fields were tailored to selectively amplify spectral features of correlated exciton states in gallium arsenide quantum wells. With the aid of theoretical models, 2D spectra of quantum wells revealed clear contributions of Coulomb correlations to the exciton dynamics. Time and power dependent properties of the 2D spectra indicate several mechanisms for exciton interactions that are neglected in commonly used theoretical models. If a semiconductor material is fabricated within a microcavity, optical fields can be trapped around the semiconductor, strongly distorting properties of the semiconductor excitons and forming new quasi-particles called exciton-polaritons. Theoretical work has suggested exciton-polariton Coulomb correlation strengths can be reduced compared to that of excitons. Using two-dimensional Fourier-transform optical spectroscopy, control of Coulomb correlations was demonstrated by varying the cavity structure. The cavity fields were also shown to induce high-order correlated interactions among exciton-polaritons. A macroscopic quantum degenerate system of exciton-polaritons can also become correlated, exhibiting long-range order typical of a Bose-Einstein condensate. However, unlike a Bose-Einstein condensate, exciton-polartions are not typically in thermal equilibrium. Using a sample with exciton-polariton lifetimes longer than previous samples, the macroscopic behavior of exciton-polaritons was investigated by imaging the exciton-polariton photoluminescence. Condensation depended significantly on spatially-varying potential energy surfaces. Using optically-induced harmonic potential barriers, thermal equilibrium among exciton-polaritons was achieved, with exciton-polaritons forming a Bose-Einstein distribution at densities above and below the condensation phase transition.
by Patrick Wen.
Ph.D.
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Tozer, Oliver. "Exciton dynamics in disordered conjugated polymers." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:1d899235-07fe-4a06-8146-d1b2bb113850.
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Conjugated polymers have been the subject of many experimental and theoretical studies in recent years, with the interesting behaviour of their low-energy excited states depending on both electron-electron and electron-nuclear interactions, as well as the disorder which is almost universally present in these materials. Their potential use in optoelectronic devices, such as LEDs and photovoltaic cells, adds to this interest, with the behaviour of the low energy excited states being primarily dictated by the p-conjugation. In this thesis we model the transport of energy in phenylene-based conjugated polymers, where the primary low energy excited states are Frenkel excitons that can be described by the disordered Frenkel-Holstein Hamiltonian. Initially, the eigenstates of this Hamiltonian are investigated using the density matrix renormalization group method. The results of this investigation show that the vibrational phonons of the polymer cause only a small change in the exciton wavefunction, while the relevant torsional normal modes are low enough in frequency that they can be treated classically. This allows the exciton states in the dynamics simulations to be calculated using the more numerically tractable Frenkel Hamiltonian with the coupling of the exciton to the vibrational modes being treated by the inclusion of appropriate polaron binding energies in the energy of the state and of Franck-Condon factors in the transition rates. The first dynamics simulations are an investigation into the intramolecular motion of excitons on polymer chains in solution, where it is the torsional motion of the polymer chain, modelled by a Langevin dynamics algorithm, that is the main driving force behind the exciton transport. The exciton motion in the solid state, where the exciton motion results mainly from intermolecular Förster resonant exciton transfer, is then investigated, with a Monte Carlo algorithm being employed to determine the exciton diffusion length for various polymer morphologies.
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Lee, Chee Kong Ph D. Massachusetts Institute of Technology. "Simulating exciton dynamics in organic semiconductors." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/121782.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages 105-116).
Organic semiconductors are carbon-based semiconductors with number of unique benefits over traditional semiconductors such as low production costs, versatile synthesis processes, and high portability. Unlike traditional crystalline semiconductors that exhibit high level of homogeneity, organic semiconductors are spatially and temporally heterogeneous due to the weak van der Waals intermolecular forces. In this thesis we utilize computational and theoretical methods to investigate how this heterogeneity affects the electronic properties in organic semiconductors. In particular, we focus on two microscopic processes fundamental to the performance of organic semiconductors: the transport of Frenkel exciton and dissociation of charge-transfer (CT) exciton. Frenkel excitons are tightly bound electron-hole pairs created upon photo-excitation of molecules and they carry the excess energy imparted by photons.
We employ theoretical approach that combines molecular dynamics and semi-empirical electronic structure calculations to reveal the effects of molecular disorder on Frenkel exciton transport in oligothiophene-based molecular semiconductors. Using this approach, we find that the magnitude and details of molecular disorder (i.e. spatial and temporal correlations) could have huge impact on exciton transport in this class of materials. CT excitons are electron-hole pairs partially separated across the donor-acceptor interface. To generate free charges, the oppositely charged electron and hole must overcome an electrostatic binding energy before they undergo ground state recombination. We explore the CT exciton dissociate mechanism and magnetic field effects through a model of quantum spin dynamics combined with a stochastic coarse-grained model of charge transport.
We demonstrate that simulations carried out on our model are capable of reproducing experimental results as well as generating theoretical predictions related to the efficiency of organic electronic materials. Next, we consider the effect of disorder in electronic energy levels on dissociation yield and demonstrate that it is maximized with a finite amount of disorder as a result of non-equilibrium effect.
by Chee Kong Lee.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Chemistry
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Nalitov, Anton. "Spin dynamics ande topological effects in physics of indirect excitons and microcavity polaritons." Thesis, Clermont-Ferrand 2, 2015. http://www.theses.fr/2015CLF22569/document.
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Cette thèse est consacrée à de nouveaux phénomènes en physique liées au spin et à la topologie des quasi-particules lumière-matière dans des hétérostructures. Elle est divisée en quatre parties. Chapitre 1 donne un fond nécessaire et introduit les propriétés fondamentales des polaritons et des excitons indirects dans des puits quantiques couplés. Chapitre 2 est concentré sur la dynamique de spin et sur formation de défauts topologiques dans des systèmes aux excitons indirects. Les 2 derniers chapitres considèrent les structures basées sur les microcavités. Chapitre 3 est consacré à la dynamique de spin des polaritons dans des oscillateurs paramétriques optiques. Finalement, chapitre 4 étudie les réseaux des microcavités en forme des piliers et introduit l’isolant topologique polaritoniqueThe present thesis manuscript is devoted to new phenomena in physics of light-matter quasiparticles in heterostructures, related to spin and topology. It is divided into four parts. Chapter 1 gives a necessary background, introducing basic properties of microcavity polaritons and indirect excitons in coupled quantum wells. Chapter 2 is focused on spin dynamics and topological defects formation in indirect exciton many-body systems. The last 2 chapters are related to microcavity-based structures. Chapter 3 is devoted to polariton spin dynamics in optical parametric oscillators. Finally, Chapter 4 studies pillar microcavity lattices and introduces the polariton topological insulator
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Daniel, C. "Exciton transfer dynamics in supramolecular semiconductor nanostructures." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598264.
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A simple model is developed to extract the intrinsic decay rates of excitonic states in chiral supramolecular assemblies of OPV derivatives and those rates are found to decrease significantly upon aggregation. In stacks of HBC derivatives, the photoluminescent anisotropy reveals the existence of two excited states with complex characteristics illustrating the subtle interplay between molecular structure, packing and optoelectronic properties. Ultrafast spectroscopy is used to probe the dynamics of excitonic states in a chiral supramolecular assembly of an OPV derivative. Fast exciton diffusion leads to diffusion to trap states, exciton depolarisation, dynamic photoluminescence red-shift and exciton-exciton annihilation. A Monte-Carlo model based on resonance energy transfer steps is presented and yields microscopic properties such as the diffusion coefficient. These excitonic properties are very similar to those of polymeric semiconductors and justify the term “supramolecular polymers”. The study of mixed assemblies of OPV derivatives gives evidence of efficient energy transfer from the short oligomers to longer ones. The results are analysed with a model based on one-step Förster transfers. The supramolecular assembly is shown to speed up dramatically the energy transfers. The transfer rate and dimensionality are controlled by the molecular configuration and external parameters such as the temperature. Thus, I show that the supramolecular assemblies possess polymeric properties while offering more control than organic polymers.
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Siddique, Sofia. "Exciton dynamics in polymer wrapped carbon nanotubes." Thesis, University of Surrey, 2015. http://epubs.surrey.ac.uk/807206/.
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This thesis describes an experimental study of exciton recombination in isolated semiconducting single-wall carbon nanotubes synthesised by different methods and wrapped in different polymers PFO (9,9-dioctylfluorenyl-2,7-diyl), PFO-BPy (9,9-Dioctyfluorenyl-2,7-diyl-Bipyridine) and P3HT (poly 3-hexylthiophene). We present a comprehensive study using femtosecond transient absorption measurements of the kinetics of exciton recombination, where diffusion of excitons in the confined one-dimensional system significantly affects their optical and electronic properties. In all studied samples of isolated nanotubes, an exciton-exciton process dominated the recombination under high excitation, and exhibited a distinct crossover to a diffusion-limited regime with anomalous kinetics at late times. We attribute the reaction-diffusion crossover to a finite reaction probability per exciton-exciton encounter. We have demonstrated a methodology to determine the microscopic parameters controlling reaction and diffusion processes, based on measurements at high initial density where the optical absorption is fully saturated. In studies of the same nanotube species synthesized by both HiPCO (high pressure catalytic decomposition of carbon monoxide) and CoMoCAT (cobalt and molybdenum catalysts based chemical vapour deposition technique) methods and wrapped by different polymers, the exciton reaction probability was approximately constant, corresponding to on average one in five exciton-exciton interactions resulting in an exciton recombination. On the other hand, there was significant variation in the diffusive hopping time for samples synthesized by different processes or subject to different processing. This is consistent with the wide range of values for the diffusion coefficient reported in the literature. We have found that excitons in nanotubes synthesized by the HiPCO are more mobile than CoMoCAT nanotubes, by a factor of ~2 for nanotubes wrapped in P3HT. This may be associated with a lower defect concentration in HiPCO nanotubes compared to CoMoCAT. The nanomaterials studied in this thesis are promising for nanotube-organic hybrids based light emitting and harvesting devices. Our findings will not only facilitate the selection of materials in these applications but also represent experimental data that challenges existing theoretical models for kinetics of non-equilibrium stochastic systems.
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Chatterjee, Sangam. "Exciton formation dynamics in semiconductor quantum wells." Diss., The University of Arizona, 2003. http://hdl.handle.net/10150/280403.
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Photoluminescence from direct-bandgap semiconductor quantum wells after non-resonant excitation is predominantly observed at energetic position of the 1s exciton resonance. The time evolution of the photoluminescence is generally interpreted as direct monitor of an excitonic population; a rise of the signal is interpreted as a buildup and the decrease as decay of the excitonic population. Recent microscopic calculations, however, have shown that even without an incoherent excitonic population, pure plasma decay yields photoluminescence peaked at the is exciton resonance. Experimental time-resolved photoluminescence spectra are taken across a large region of the parameter space of carrier density and lattice temperature. They are compared to the expected thermal equilibrium spectra, calculated from nonlinear absorption measurements taken under identical conditions. Under none of the experimentally explored parameters is the is emission as bright as expected for thermal equilibrium. To distinguish excitonic and plasma contributions, the deviations from thermal equilibrium at the is exciton resonance are then analyzed using a microscopic calculation. The dipole moment is adjusted to reproduce the excitonic binding energy and oscillator strength of the samples under investigation. The carrier densities and carrier temperatures are determined experimentally; no free fit parameters are necessary. The differences between experimental values and pure plasma calculation are explained with the presence of an incoherent excitonic population. Although at first the emission spectra under all conditions do not vary significantly, a more detailed analysis reveals that the sources of the photoluminescence can be either predominantly excitonic or plasma. For low temperatures and low densities the excitonic emission is extremely sensitive to even minute exciton populations making it possible to extract a phase diagram for incoherent excitonic populations. The maximum contribution of bright excitons is found at intermediate densities and low lattice temperatures; the absolute number of bright excitons is tiny, less than 0.04% of the total carrier density. However, it is not possible to determine the total number of bright and dark exciton by using photoluminescence.
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Cadirci, Musa. "Ultrafast charge dynamics in novel colloidal quantum dots." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/ultrafast-charge-dynamics-in-novel-colloidal-quantum-dots(865aba90-9d60-478d-8f49-ad4785516688).html.
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In this thesis ultrafast exciton dynamics of several colloidal quantum dots have been studied using visible transient absorption spectroscopy. The resultant transient decays and differential transmission spectra were analysed to determine the ultrafast relaxation channels, multiple exciton generation (MEG) efficiency and multi-exciton interactions in the observed materials. All QDs were preliminarily optically characterized using steady state absorption and photoluminescence spectroscopies. In addition, a high repetition infrared femtosecond pump probe experiment was designed and built to detect the picosecond intraband carrier relaxations in quantum dots. Picosecond carrier dynamics of type-II ZnTe/ZnSe and of CuInSe2 and CuInS2 type-I quantum dots were investigated. The common feature of these materials is that they are eco-friendly materials, being alternatives to the toxic Cd- and Pb- based materials. It was found that surface trapping occurred in both cases for electrons in the hot states, and in the minimum of the conduction band for ZnTe/ZnSe core/shell materials. Trion formation was observed in ZnTe/ZnSe core/shell dots at high power and unstirred conditions. The hot and cold electron trapping processes in type-II dots and CuInS2 and CuInSe2 dots shifted, distorted and moderately cancelled the bleach features. In addition, intra-gap hole trapping was observed in CuInS2 and CuInSe2 dots which results in a long decay feature in the recorded transients. MEG competes with Auger cooling, surface mediated relaxation and phonon emission. To enhance the MEG quantum yield, the rival mechanisms were suppressed in well-engineered CdSe/CdTe/CdS and CdTe/CdSe/CdS core/shell/shell and CdTe/CdS core/shell type-II quantum dots. The MEG slope efficiency and threshold for a range of different core size and shell thickness were found to be (142±9)%/Eg and (2.59±0.16)Eg, respectively. The observed threshold was consistent with the literature, whereas, the obtained slope efficiency was about three times higher than the previously reported values. The biexciton interaction energy of the dots stated in the previous paragraph was also studied. To date, time-resolved photoluminescence (TRPL) has been employed to study exciton interactions in type-II quantum dots and large repulsive biexciton interaction energy values between 50-100 meV have been reported. However, unlike the TRPL method, the TA experiment ensures that only two excitons remain in the band edge of the dot. Using this method, large attractive biexciton interaction energies up to ~-60 meV was observed. These results have promising implications regarding enhancing the MEG quantum yield.
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Roe, Mitchell Gregg. "Dynamics of photogenerated excitons and polarons in emeraldine base /." The Ohio State University, 1987. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487585645577477.
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Brand, Matthew Anthony. "Optical time resolved spin dynamics in III V semiconductor quantum wells." Thesis, University of Southampton, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.289510.
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Chow, Colin M., Hongyi Yu, Aaron M. Jones, John R. Schaibley, Michael Koehler, David G. Mandrus, R. Merlin, Wang Yao, and Xiaodong Xu. "Phonon-assisted oscillatory exciton dynamics in monolayer MoSe2." NATURE PUBLISHING GROUP, 2017. http://hdl.handle.net/10150/627085.
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In monolayer semiconductor transition metal dichalcogenides, the exciton-phonon interaction strongly affects the photocarrier dynamics. Here, we report on an unusual oscillatory enhancement of the neutral exciton photoluminescence with the excitation laser frequency in monolayer MoSe2. The frequency of oscillation matches that of the M-point longitudinal acoustic phonon, LA(M), suggesting the significance of zone-edge acoustic phonons and hence the deformation potential in exciton-phonon coupling in MoSe2. Moreover, oscillatory behavior is observed in the steady-state emission linewidth and in time-resolved PLE data, which reveals variation with excitation energy in the exciton lifetime. These results clearly expose the key role played by phonons in the exciton formation and relaxation dynamics of two-dimensional van der Waals semiconductors.
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Ma, Xuekai [Verfasser]. "Nonlinear dynamics of exciton-polariton condensates / Xuekai Ma." Paderborn : Universitätsbibliothek, 2017. http://d-nb.info/1137944048/34.
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Steiner, Colby Peyton. "Single and multi-exciton dynamics in nanoscale semiconductors." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/104977.
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Thesis: Ph. D. in Physical Chemistry, Massachusetts Institute of Technology, Department of Chemistry, 2016.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 205-225).
Recent advancements in the synthesis and fabrication of nanoscale semiconductors and optoelectronic devices are revolutionizing an array of industries. Nonlinear spectroscopy is a powerful tool for studying the unique optical and electronic properties of these semiconductors. This thesis describes experiments using a unique multi-dimensional spectrometer to study three semiconductors: double-walled J-aggregate nanotubes, cuprous oxide, and monolayer transition metal dichalcogenides. First, we use two-dimensional correlation spectroscopy and cryogenic tunneling electron microscopy to correlate the excitonic and structural properties of J-aggregate nanotubes in solution. We observe weak coupling between inner and outer wall excitons in both isolated and bundled nanotubes. We also use two-dimensional rephasing spectroscopy to measure the homogeneous and inhomogeneous linewidths in solution at 295 K and in a glass at 10 K and determine dynamic and static disorder dominate, respectively. In addition, we observe photo-induced damage and recovery of nanotubes in the solid state. Quantum process tomography is used to unambiguously elucidate the single-exciton dynamics of J-aggregate nanotubes in solution. Inversion of spectroscopic signals from eight transient grating experiments completely characterizes the time evolution of the single-exciton density matrix by determining the process matrix. We confirm the weak coupling of inner and outer wall excitons and observe no contributions from non-secular processes. Second, we use two-quantum spectroscopy to make the first experimental observation of two-exciton correlations (i.e. biexcitons) in cuprous oxide. The direct measurement of two-exciton correlations supports the proposed mechanism of biexciton-Auger recombination for the efficient suppression of the Bose-Einstein condensation of excitons. Third, we use two-dimensional rephasing spectroscopy to observe substantial inhomogeneous broadening due to large static disorder in monolayer transition metal dichalcogenides. We also use two-dimensional correlation spectroscopy to reveal interactions of excitons with in-plane optical and acoustic phonons. Lastly, we use one- and two-quantum spectroscopy to measure unprecedentedly large inorganic, biexciton binding energies due to reduced dielectric screening in the atomically thin limit. The exciton and phonon dynamics revealed in these experiments contribute to our understanding of the elementary excitations in organic and inorganic nanostructured semiconductors. It is our hope these insights will help guide the design of next generation devices utilizing nanoscale semiconductors.
by Colby Peyton Steiner.
Ph. D. in Physical Chemistry
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Brüggemann, Ben. "Theory of ultrafast exciton dynamics in photosynthetic antenna systems." [S.l. : s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=972544046.
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Schwab, Matthias [Verfasser]. "Exciton Dynamics in Semiconductor Quantum Dot Structures / Matthias Schwab." Aachen : Shaker, 2005. http://d-nb.info/118657657X/34.
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Al, Otaify Ali Abdullah. "Ultrafast charge dynamics in novel nanoparticles." Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/ultrafast-charge-dynamics-in-novel-nanoparticles(ec75ab4e-71cd-4051-8683-be3c724746c5).html.
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The ultrafast charge dynamics in a number of nanostructured materials relevant to the production of renewable energy are investigated using ultrafast transient absorption spectroscopy. The materials include mercury telluride and cadmium mercury telluride quantum dots, and gold nanoparticles loaded on titanium dioxide colloidal spheres. The analysis of the resultant pump-induced transmittance change spectra and transients allow the determination of charge relaxation routes including multiple exciton generation, trion formation and direct-surface trapping. The investigation of HgTe QDs passivated with thioglycerol, mercaptopropionic acid and dodecanethiol ligands suggests that mercaptopropionic acid ligand results in better passivation of HgTe QDs due to its carboxylic acid group. It allows more electron density donation to the QD surface to passivate the traps related with unsaturated Hg bonds and hence supresses the associated non-radiative processes. The decay lifetimes of the thioglycerol/dodecanethiol-capped QDs in addition to the photo-induced absorption feature in their spectra, are found to be consistent with surface charge trapping observed in CdSe QDs. In comparison, the transients obtained for mercaptopropionic acid passivated QDs coupled with the pump-induced transmittance change spectrum show no sign of any surface-related processes. Therefore, our analyses allow the determination of multiple exciton generation for the first time in these QDs with a quantum yield of 1.36 ± 0.04 when photo-exciting with photons of energy 3.1 times the band gap. Such result should turn researchers’ attention to those ligands which could improve the QD solar cell field. The study of exciton dynamics in CdxHg(1-x)Te alloy QDs is also presented here. Their pump-induced transmittance change spectrum show two bleaches: at the shoulder position of the steady state absorption and at the PL peak. The exciton dynamics of these materials are studied using four different wavelengths, two of them are above the MEG threshold. The resultant transmittance transients and the pump-induced transmittance change spectrum are free of any photo-induced absorption or long-lived surface trapping. Hence, the decay of the transients obtained above the MEG threshold for well-stirred samples at low pump fluences is attributed to biexciton recombination. The assessment of multiple exciton generation reveals a quantum yield value of 1.12 ± 0.01 when photo-exciting with 2.6 times the band gap. Finally, the investigation of the recovery of the plasmon bleach in TiO2 colloidal spheres decorated with different sizes of Au NPs is presented in this thesis. The pump-induced transmittance change spectra obtained for two different wavelengths show bleaches at the plasmon band maximum superimposed with two wings of absorption features at shorter and longer wavelengths. The resultant transmittance transients for these samples are well-described by bi-exponential decay with a very quick decline of a few ps associated with electron–phonon scattering, followed by a slower decay over a few 10s of ps associated with heat dissipation. Only the heat dissipation rate is found to be dependent on the size of the Au NPs as it rises from 49 ± 3 ps to 128 ± 6 ps when the diameter of the Au NPs is increased from 12.2 ± 2.2 nm to 24.5 ± 2.8 nm, respectively.
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Villamil, Franco Carolina. "Ultrafast dynamics of excitons and charge carriers in colloidal perovskite nanostructures studied by time-resolved optical spectroscopies." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASF012.
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Les matériaux semiconducteurs de pérovskite halogénée se sont révélés très prometteurs en raison de leurs propriétés exceptionnelles et de leur capacité d’élaboration à faible coût. Malgré les progrès réalisés, une meilleure compréhension des dynamiques de relaxation et de recombinaison dans ces matériaux photo-actifs est nécessaire afin d’améliorer les performances des dispositifs opto-électroniques. Cette thèse porte sur la relaxation des porteurs de charge/excitons « chauds » après excitation au-dessus de la bande interdite, et la recombinaison Auger advenant après une excitation de forte puissance ou à haute énergie. La spectroscopie de fluorescence résolue en temps et d'absorption transitoire (AT) femtoseconde sont utilisées afin d’étudier les effets du confinement et de la composition sur ces processus, en particulier dans les nanoplaquettes (NPLs) colloïdales de pérovskite bidimensionnelles (2D), dont la synthèse a été développée et optimisée au préalable. Les dynamiques de relaxation ont été étudiées par AT en utilisant une méthode d'analyse globale où l'évolution temporelle des différentes compositions spectrales est modélisée suivant une cinétique de réactions séquentielles. Cette méthode a pu être appliquée afin de décrire efficacement la relaxation progressive dans les nanostructures de FAPbI₃ (FA= formamidinium) faiblement confinées, et a permis de dissocier les processus de retardement de relaxation à haute puissance d’excitation dus aux effets de « hot phonon bottleneck » et de ré-excitation par recombinaison Auger (RA). Par ailleurs, l'analyse globale de l’évolution spectrale a été essentielle afin d’examiner les dynamiques de relaxation dans les NPLs 2D à fort confinement quantique et diélectrique, présentant des effets Stark importants et une transition de bord de bande excitonique discrète. Comme dans les systèmes faiblement confinés, le taux de relaxation dans les NPLs diminue avec la puissance d'excitation. Cependant, il est plus rapide dans les systèmes présentant un confinement plus important, mettant alors en évidence l'absence de ”phonon bottleneck” intrinsèque. La nature des cations internes (FA, MA=méthylammonium ou Cs) n’influe pas ce taux, ni son évolution en puissance. Toutefois, les mesures dans les films de pérovskite 2D présentant une épaisseur de puits quantique équivalente, suggèrent un rôle des ligands de surface dans la capacité à libérer l'excès d'énergie dans l'environnement. Ensuite, les recombinaisons multi-excitoniques dominées par la RA non radiative ont été étudiées dans les NPLs de pérovskite 2D fortement confinées. Dû à la géométrie asymétrique de ces nanostructures et de la délocalisation limitée de l'exciton, le temps de RA dépend fortement de la densité d'excitons via la distance inter-exciton moyenne : A faible puissance d’excitation, la RA est limitée par la diffusion de l'exciton dans le plan 2D et se produit sur plusieurs centaines de picosecondes (dépendant ainsi de la géométrie de l'échantillon). En revanche, une puissance d'excitation élevée produit des excitons dont les fonctions d’onde se recouvrent spatialement, entraînant des temps de RA inférieurs à 10 ps. Finalement, les dynamiques excitoniques dans les NPLs 2D ont été mesurées par AT après excitation dans l’ultraviolet afin d'observer le processus de multiplication d’excitons. Ce dernier implique la génération de plusieurs excitons “géminés”, et donc proches spatialement, suite à l'absorption d'un unique photon de haute énergieHalide perovskites have emerged as very promising photoactive materials due to their outstanding optoelectronic properties combined with low-cost processability. In spite of their successful implementation in photovoltaic or light-emitting devices, a deep understanding of the dynamics of relaxation and recombination is still missing in order to enhance the device performances. This thesis focuses on the study of two major fundamental processes occurring in colloidal halide perovskite nanostructures: the hot charge carrier/exciton relaxation (“cooling”), after excitation above the optical bandgap, and the non-radiative Auger recombination, taking place after high-fluence or high-photon energy excitation. In particular, time-resolved photoluminescence and femtosecond transient absorption spectroscopy were used to investigate the confinement and composition effects in strongly confined two-dimensional (2D) lead iodide perovskite nanoplatelets (NPLs), that were synthesized following the development and optimizations of colloidal methods. For the investigation of the cooling dynamics, a global analysis method based on single value decomposition was used, where the temporal evolution of the spectral lineshapes was modeled with a sequential kinetic scheme. This method was succesfully applied to effectively describe the continuous energy relaxation in weakly-confined thick FAPbI₃ nanoplates (FA=formamidinium) and allowed disantangleting the hot phonon bottleneck from the Auger reheating effects at high excitation fluence. Furthermore, the global analysis was essential to investigate the cooling dynamics in strongly confined 2D NPLs presenting large Stark effects and discrete excitonic band-edge transition far away from the continuum of states (exciton binding energy in several hundreds of meV). As in the weakly confined samples, the cooling rate of the NPLs decreases with the excitation fluence. However, it is faster in more-strongly confined samples, evidencing the absence of an intrinsic phonon bottleneck. Furthermore, the cooling rate and its evolution with the exciton density were found independent of the nature of the internal cations (FA, MA=methylammonium and Cs=cesium). However, when comparing with the rate measured in 2D layered perovskite thin film with equivalent quantum well thickness, the results strongly suggest a role of the surface ligands in the possibility to release the excess energy to the surrounding environment. This ligand-mediated relaxation mechanism becomes dominent in the thinner NPL samples with enhanced exciton/ligand vibrational mode coupling. Then, the multiple exciton recombination dominated by non-radiative Auger recombination (AR) was studied in the strongly-confined 2D perovskite NPLs. Due to the large asymmetric geometry and the limited exciton wavefunction delocalization, the AR rate strongly depends on the exciton density via the initial average inter-exciton distance. At low fluence, this distance is in several tens of nanometers such as the AR is limited by the exciton diffusion in the 2D plane. It thus occurs on a timescale of several hundreds of picoseconds and depends on the sample dimensionality (thickness and lateral sizes). In contrast, high excitation fluences produce “overlapping” excitons with inter-exciton distances of only a few times the exciton Bohr radius, resulting in AR times of less than 10 ps and independent of the NPL composition nor geometry. Finally, the exciton population dynamics of 2D NPLs after excitation in the ultraviolet was measured. The strong dependence of the AR with the inter-exciton distance allows the identification of multiple exciton generation (MEG), which involves the reaction of “geminate biexcitons” produced by the absorption of a single high-energy photon
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Wijesundara, Kushal Chinthaka. "Ultrafast Exciton Dynamics and Optical Control in Semiconductor Quantum Dots." Ohio University / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1336648375.
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Peckus, Domantas. "Ultrafast exciton and charge carrier dynamics in nanostructured molecular layers." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2013. http://vddb.library.lt/obj/LT-eLABa-0001:E.02~2013~D_20131220_150447-81409.
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Due to their unique properties organic semiconductors may be used for various applications in organic optoelectronic devices: light emitting devices, lasers, field-effect transistors, photovoltaic cells and etc. Despite high perspectives of organic semiconductors they are still upstaged by their inorganic counterparts. Development of organic electronics requires better understanding of electrooptical properties of organic semiconductors and relationships between their structure and functions.
The main goal of this thesis is a detailed investigation of ultrafast exciton and charge carrier processes in pure organic semiconductors and their blends with fullerene derivatives. Investigated organic or silicon organic semiconductors were poly-di-n-hexylsilane (PDHS), polyfluorenes F8BT and PSF-BT, merocyanine MD376. C60 fullerene and its derivative PCBM were used in blends. Ultrafast transient absorption, fluorescence, and integral mode photocurrent measurements were used for investigations.
The investigation of PDHS nanocomposites revealed that PDHS incorporation into nanopores can be used for improvement of fluorescence properties.
Formation of intramolecular charge transfer state was proposed for PSF-BT neat films. Charge transfer scheme of the formation of long-lived charge pair state in PSF-BT/PCBM blend was presented.
The charge pair and free charge carrier generation scheme in merocyanine blends with fullerene derivatives were discussed in detail.
Analysis of measurement... [to full text]Dėl savo unikalių savybių organiniai puslaidininkiai gali būti plačiai pritaikyti įvairiuose optoelektroniniuose prietaisuose: organiniuose šviestukuose, organiniuose lazeriuose, organiniuose tranzistoriuose ir organiniuose šviesos elementuose. Visi šie pritaikymai yra galimi dėl organinių molekulių laidumo. Nepaisant didelių organinių puslaidininkių perspektyvų, jie vis dar yra nukonkuruojami neorganinių puslaidininkių. Pagrindinis šių tezių tikslas yra detaliai ištirti eksitonų ir krūvininkų dinamikos procesus grynuose organiniuose puslaidininkiuose ir jų mišiniuose su fulereno dariniais. Buvo matuoti organiniai ir silicio organiniai puslaidininkiai: poli-di-n-heksilsilanas (PDHS), polifluoreno dariniai F8BT ir PSF-BT, merocianinas MD376. Mišiniuose naudoti fulerenai buvo C60 ir jo darinys PCBM. Tyrimams buvo naudoti ultraspartūs skirtuminės sugerties, fluorescencijos ir integralinės fotosrovės matavimai. PDHS tyrimai atskleidė, kad neorganinės matricos sumažina nespindulinį relaksacijos kanalą. PDHS nanokompozitai gali būti naudojami polimero fluorescencijos savybių: stabilumo, kvantinio našumo pagerinimui. Polifluorenų F8BT ir PSF-BT grynų plėvelių tyrimų metu nustatyti eksitonų-eksitonų anihiliacijos ir eksitonų migracijos skirtumai. Vidumolekulinės krūvio pernašos būsenos formavimasis buvo pasiūlytas PSF-BT grynoms plėvelėms. Pristatyta ilgi gyvuojančių krūvininkų porų formavimosi schema PSF-BT/PCBM mišiniuose. Krūvio pernašos būsenų formavimasis buvo ištirtas... [toliau žr. visą tekstą]
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Watkins, Pete. "Modelling of charge and exciton dynamics in organic solar cells." Thesis, University of Bath, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.421254.
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Chang, Wendi. "Modification of exciton energies and dynamics for thin film optoelectronics." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/109011.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 157-171).
Organic and nano-crystal thin film semiconductors have proven to be effective materials for a range of optoelectronic applications including light emitting diodes, visible wavelength lasers, photovoltaics, and sensors. Mediating photons and charges, excitons are responsible for electrical and optical properties in these energetically disordered media. To improve current technology and develop novel applications, a thorough understanding of exciton dynamics is crucial. This thesis focuses on modifying exciton dynamics in organic and nanocrystal thin films with spectroscopic monitoring for optoelectronic applications. First, we study local dielectric effects in doped organic thin films, which depends on material deposition and molecular density. An external pressure technique is used to modify molecular density, tuning exciton energies and dynamics as explained by solvation theory and exciton energy transfer. We extend this molecular density tuning method, coupled with other measurements, including optical imaging, magnetic field effect, and electric field effect to modify and monitor charge transfer (CT) exciton energy and dynamics. As intermediaries between excitons and free charges in organic photovoltaic donor-acceptor material systems, CT exciton state dynamics is still a subject of much debate. The effect of CT spin states on photocurrent generation is explored, revealing guidelines for material selection and molecule engineering with potential benefits in optimizing molecular density. Further investigation of low-energy CT exciton transport leads to evidence of spatial motion between interfacial states, driven by local energy disorder. These techniques and results directly apply to optimizing OPV devices, but may also be extended to other applications. This thesis also includes an investigate of ultrafast electric field effect on organic and colloidal quantum dot thin films, largely unexplored in previous works. We observe exciton luminescence and non-perturbative energy tuning of exciton states under terahertz electromagnetic radiation, developing a potential platform for detectors and cameras for coherent terahertz sensing. Lastly, we discuss a new fabrication technique to integrate organic thin films into micro-electro-mechanical system microcavity device designs. The demonstrated electrically tunable microcavity structures not only applicable as wavelength-tunable lasers and pressure sensors, but also presents a good example of fabrication flexibility and challenges of employing organic and nano-crystal thin films.
by Wendi Chang.
Ph. D.
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Kell, Adam. "Energy transfer and exciton dynamics in photosynthetic pigment–protein complexes." Diss., Kansas State University, 2016. http://hdl.handle.net/2097/32539.
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Doctor of PhilosophyChemistry
Ryszard J. Jankowiak
The structure-function relationships of natural pigment–protein complexes are of great interest, as the electronic properties of the pigments are tuned by the protein environment to achieve high quantum yields and photon utilization. Determination of electronic structure and exciton dynamics in protein complexes is complicated by static disorder and uncertainties in the properties of system-bath coupling. The latter is described by the phonon profile (or spectral density), whose shape can only be reliably measured experimentally for the lowest energy state. Low-temperature, laser-based spectroscopies are applied towards model pigment–protein complexes, i.e., the Fenna-Matthews-Olson (FMO) and water-soluble chlorophyll-binding (WSCP) complexes, in order to study system-bath coupling and energy transfer pathways. Site-selective techniques, e.g., hole burning (HB) and fluorescence line narrowing, are utilized to overcome static disorder and reveal details on homogeneous broadening. In addition, excitonic calculations with non-Markovian lineshapes provide information on electronic structure and exciton dynamics. A new lognormal functional form of the spectral density is recommended which appropriately defines electron-phonon parameters, i.e., Huang-Rhys factor and reorganization energy. Absorbance and fluorescence spectral shifts and HB spectra reveal that samples of FMO may contain a subpopulation of destabilized proteins with modified HB efficiencies. Simulations of spectra corresponding to intact proteins indicate that the entire trimer has to be taken into account in order to properly describe fluorescence and HB spectra. The redshifted fluorescence spectrum of WSCP is described by uncorrelated energy transfer as opposed to previous models of excited state protein relaxation. Also, based on nonconservative HB spectra measured for WSCP, a mechanism of electron transfer between chlorophylls and aromatic amino acids is proposed.
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Reynolds, Luke X. "Transient optical studies of exciton dynamics in organic solar cells." Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/10102.
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There is increasing evidence that the initially generated excited state species in bulk heterojunction solar cell photoactive layers are critical to device performance. At present however, an understanding of the nature and dynamics of such excited states still remains limited. This thesis presents a study of the ultrafast exciton dynamics in bulk heterojunction organic and hybrid organic-inorganic solar cells. Fluorescence upconversion is used to elucidate the dynamics of such transient species allowing internal properties of the blend systems to be probed including changes in film morphology and ultrafast energy loss mechanisms. An understanding of such processes is an important step forward in the evolution of molecular semiconductor based solar cells. The first chapter focuses on the main experimental technique, fluorescence upconversion, and how this can be employed to study excited states. In particular, this section addresses one of the main unanswered questions in the field and attempts to correlate the exciton dynamics with the structure of the common photoactive polymer poly(3-hexylthiophene) (P3HT). Three structural variations of P3HT are studied and their exciton dynamics associated with differing internal processes occurring within the polymers. These include self localisation, and different types of long-range energy transfer mechanisms. The following two chapters build upon the knowledge of exciton dynamics obtained from the first chapter. First, a study is made of amorphous polymers with different acceptors, all based on phenyl-C61-butyric acid methyl ester (PCBM). The distinct interactions of the PCBM-type molecules with the polymer results in different electron transfer dynamics, from which the exciton diffusion length of the polymer in real bulk heterojunction blends is extracted using a simple model. Second, the ultrafast excited state dynamics of a crystalline polymer with the same PCBM-type acceptors is studied. Correlation of these dynamics with thermal analysis of the blend films allows the morphology of the films to be extracted and allows two different mechanisms of microstructure development to be identified. In the final chapter, the effect of acceptor aggregation on exciton dynamics and charge generation yields in hybrid organic-inorganic blend films has been studied. Such aggregation has been shown to be essential for efficient charge generation in all-organic solar cells but has often been assumed to be less important in such inorganic hybrids. More aggregated acceptor nanoparticles are shown to not only result in greater than expected exciton quenching but are also shown to result in a greater yield of long-lived charges. This study is extended to show that in-situ grown inorganic nanoparticles exhibit superior performance to standard pre-synthesised inorganics.
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Zhu, Han Ph D. Massachusetts Institute of Technology. "Charge and exciton dynamics in quantum dot light-emitting diodes." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/123240.
Full textAbstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages [159]-174).
Colloidal quantum dot based light-emitting diodes (QD-LEDs) offer the possibility of bright, saturated, and tunable emission for the next generation of display and solid state lighting technologies. In this thesis, we study how the interplay of charges and excitons in a QD-LED affect its operational behavior. In order to construct a physical model of a QD-LED, we start by developing quantitative characterization methods that directly measure charge accumulation and electric field in an operating device. Comparison of measured internal device variables with observed electroluminescence and current density allows us to disentangle the deleterious effects of charge imbalance, electric field, and Joule heating on the external quantum efficiency. We also find that the magnitude of electron accumulation on the QD film is sensitive to its interface with the neighboring hole transport layer (HTL) and can reach nearly one electron per QD even in the best performing device. We next investigate how exciton formation is affected by the high charge density. Since the degree of electron charging of a nanocrystal shifts the energy barrier for hole injection, the kinetics of exciton formation are dependent on electron occupation statistics on the QD film. Using kinetic Monte Carlo simulations that explicitly incorporate both long and short range Coulomb interactions, we find that energetic disorder of the QD film strongly enhances the formation of negatively charged excitons by increasing the population of two-electron occupied QDs. Finally, we demonstrate that the photoluminescence yield of a QD film can be intentionally quenched by up to 99.5% in a QD-LED under reverse bias. This paves the way for a voltage-tuned optical down-conversion device using colloidal QDs.
by Han Zhu.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Physics
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Tanese, Dimitrii. "Non-linear dynamics of one-dimensional polaritons gases in semiconductor microcavities." Paris 6, 2013. http://www.theses.fr/2013PA066795.
Full textAbstract:
Cette thèse est consacrée à l’étude de gaz de polaritons dans des microcavités semiconductrices unidimensionnelles. Grâce au contrôle de la gravure de microstructures, plusieurs géométries ont été étudiées et une ingénierie du potentiel 1D a été possible. Nous avons analysé la propagation des condensats de polaritons dans des fils photoniques et nous avons mis en évidence une forte réduction de la diffusion par le désordre due aux interactions entre ces quasi-particules. La condensation dans un potentiel périodique unidimensionnel a également été explorée. Nous avons montré comment les interactions répulsives génèrent des états localisés dans la bande d’énergies interdites. La nature de ces états dépend de la force des interactions entre particules par rapport à celle des interactions avec le réservoir excitonique. Dans une expérience dynamique nous avons montré la transition dynamique d’un régime dominé par le réservoir à un régime où se forment des solitons de gap. De plus, le profil de potentiel induit par interactions avec le réservoir nous a permis de superposer un gradient d’énergie au réseau périodique. Dans cette configuration, nous avons mis en evidence les oscillations de Bloch de polaritons. Grâce à l'ingénierie du potentiel, nous avons aussi réalisé un potentiel quasi-périodique de type Fibonacci au sein duquel nous avons observé le caractère fractal du spectre polaritonique. Enfin, nous présentons un interféromètre à polaritons. Le contrôle optique de la phase d’un flux de particules nous permet de maîtriser l’intensité et la polarisation du faisceau à la sortie de l’interféromètreThis PhD thesis is dedicated to the study of one-dimensional polariton gases in semiconductor microcavities. Thanks to the control on the etching of the microstructures, a full engineering of the 1D potential has been obtained and several geometries have been investigated. We have studied the propagation of polaritons inside photonic wires and we evidenced strong reduction of the backscattering against disorder driven by interparticle interactions. Polariton condensation has been investigated in a one-dimensional periodic potential. We have shown how repulsive interactions create localized states inside the energy gap. The nature of these states depends on the relative strength between interparticle interactions and the interactions with the excitonic reservoir. In a time resolved-experiment we have shown the dynamical transition from a regime dominated by interactions with the reservoir to a regime where self-interactions give rise to the formation of gap solitons. Additionally, the potential induced by the interactions with the reservoir allowed us to combine an energy gradient with the periodic lattice. In this configuration, we have observed polaritons Bloch oscillations. Thanks to the potential engineering, we have also implemented a Fibonacci quasi-periodic potential. In this configuration, we have observed the fractal character of the polariton spectrum. Finally, we present a polariton interferometer. The optical control of the phase of the polariton flow allows controlling the intensity and the polarization of the beam transmitted through the device
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Rao, Akshay. "On the dynamics of excitons and charges in organic photovoltaics." Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609909.
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Swatton, Stewart Nathan Ridgley. "Dynamic excited state properties of organic dyes." Thesis, University of Exeter, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.244422.
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Kolata, Kolja [Verfasser], and Sangam [Akademischer Betreuer] Chatterjee. "Exciton Dynamics in Perfluoropentacene Single Crystals / Kolja Kolata. Betreuer: Sangam Chatterjee." Marburg : Philipps-Universität Marburg, 2015. http://d-nb.info/1068315571/34.
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Hofmann, Simone. "Exciton Dynamics in White Organic Light-Emitting Diodes comprising Triplet Harvesting." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-117447.
Full textAbstract:
This work comprises different approaches for the efficiency enhancement of white organic light-emitting diodes (OLEDs). In particular, diffusion and transfer processes of excited singlet and triplet states are investigated. Generation of white light is realized by using the so-called triplet harvesting method where the otherwise nonradiatively decaying triplets of a blue fluorescent emitter are transferred to a highly efficient phosphorescent emitter and result in additional emission at lower energies. Triplet harvesting significantly increases the internal quantum efficiency in OLEDs. First, the well-known blue emitter 4P-NPD is investigated as model case. Using time-resolved spectroscopy, triplet harvesting by a yellow and red phosphorescent emitter, respectively is directly proven. However, triplet harvesting by a green emitter is not possible due to the low triplet energy of 4P-NPD. Using quantum chemical calculations, two new emitter molecules, 8M-4P-NPD and 8M-4P-FPD, are synthesized with the aim to rise the triplet energy. Their properties and their ability to facilitate triplet harvesting by a green emitter are studied. For the first time, a white triplet harvesting OLED is demonstrated where triplet harvesting occurs directly from a blue emitter to a green and a red emitter. Furthermore, an additional singlet transfer is observed in the triplet harvesting OLEDs under investigation. Using the phosphorescent emitter as singlet sensor, this effect allows the determination of the singlet diffusion length in 4P-NPD. By varying the distance between singlet generation zone and singlet sensor, a singlet diffusion length of 4.6 nm is found. One further approach to increase the efficiency is the optimization of a tandem OLED which comprises two single OLED units stacked on top of each other. At a luminance of 1,000 cd/m², the white tandem OLED shows an external quantum efficiency of 25%, a luminous efficacy of 33 lm/W, a color rendering index (CRI) of 62, and Commission Internationale de l’Eclairage (CIE) color coordinates of (0.53/0.43). These efficiencies are comparable to state-of-the-art efficiencies of white OLEDs. Finally, the highly efficient white tandem structure is applied on an alternative electrode consisting of flattened silver nanowires. In comparison to the conventional OLED with indium-tin oxide (ITO) electrode, this OLED shows similarly high efficiencies as well as a superior color stability in terms of viewing angles. The color stability can be assigned to the light scattering properties of the nanowires. The OLED with silver nanowire electrode shows efficiencies of 24% and 30 lm/W at 1,000 cd/m² with a CRI of 69 and CIE coordinates of (0.49/0.47)In dieser Arbeit werden verschiedene Ansätze zur Effizienzsteigerung in weißen organischen lichtemittierenden Dioden (OLEDs) erforscht. Hierfür werden im Besonderen Diffusions- und Transferprozesse von angeregten Singulett- und Triplettzuständen untersucht. Zur Erzeugung von weißem Licht wird die sogenannte “triplet harvesting” Methode verwendet, bei der die sonst nicht zur Emission beitragenden Triplettzustände eines fluoreszenten blauen Emitters auf einen hocheffizienten phosphoreszenten Emitter übertragen werden. Dieser liefert dann zusätzliche Emission im niederenergetischen Spektralbereich. Durch triplet harvesting kann die interne Quantenausbeute in OLEDs beträchtlich gesteigert werden. Zunächst wird der bekannte blaue Emitter 4P-NPD als Modellbeispiel untersucht. Mittels zeitlich aufgelöster Spektroskopie kann triplet harvesting auf einen gelben bzw. roten Emitter direkt nachgewiesen werden. Allerdings ist auf Grund der niedrigen Triplettenergie triplet harvesting auf einen grünen Emitter nicht möglich. In Anbetracht dieser Tatsache werden unter Zuhilfenahme quantenchemischer Betrachtungen zwei neue Emittermoleküle, 8M-4P-NPD und 8M-4P-FPD, synthetisiert und auf ihre Eigenschaften und ihre Eignung für triplet harvesting untersucht. Dabei wird zum ersten Mal eine weiße OLED realisiert, in der triplet harvesting von einem blauen Emitter direkt auf einen grünen und einen roten Emitter erfolgt. Des Weiteren wird bei den untersuchten triplet harvesting OLEDs ein zusätzlicher Singulettübertrag auf den phosphoreszenten Emitter beobachtet. Dieser Effekt wird zur Bestimmung der Singulettdiffusionslänge in 4P-NPD genutzt. Der phosphoreszente Emitter dient dabei als Singulettsensor. Über eine Variation des Abstands zwischen Singulettgenerationszone und Sensor wird eine Singulettdiffusionslänge von 4,6 nm bestimmt. Ein weiterer Ansatz zur Effizienzsteigerung besteht in der Optimierung einer aus zwei OLEDs zusammengesetzten Tandem OLED. Bei einer Leuchtdichte von 1000 cd/m² erzielt diese weiße Tandem OLED eine externe Quanteneffizienz von 25% und eine Leistungseffizienz von 33 lm/W mit einem Farbwiedergabeindex (CRI) von 62 und Commission Internationale de l’Eclairage (CIE) Farbkoordinaten von (0,53/0,43). Diese Effizienzen sind vergleichbar mit dem aktuellen Forschungsstand weißer OLEDs. Schließlich wird diese hocheffiziente weiße Tandemstruktur auf eine alternative Elektrode bestehend aus flachgedrückten Silbernanodrähten aufgebracht. Im Vergleich zur konventionellen OLED mit Indiumzinnoxid (ITO) Elektrode erreicht diese ähnlich hohe Effizienzen sowie eine verbesserte Farbstabilität bezüglich des Betrachtungswinkels, was auf die Streueigenschaften der Nanodrähte zurückgeführt werden kann. Bei einer Leuchtdichte von 1000 cd/m² zeigt die OLED mit Silbernanodrahtelektrode Effizienzen von 24% und 30 lm/W bei einem CRI von 69 und CIE Koordinaten von (0,49/0,47)
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Ehrenreich, Philipp [Verfasser]. "Exciton Dynamics and Charge Separation in Polymer Thin Films / Philipp Ehrenreich." Konstanz : Bibliothek der Universität Konstanz, 2018. http://d-nb.info/1163537993/34.
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Birech, Zephania. "Exciton dynamics in tetracene single crystals studied using femtosecond laser spectroscopy." Thesis, Stellenbosch : Stellenbosch University, 2012. http://hdl.handle.net/10019.1/71699.
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Cameron, Alasdair R. "Exciton saturation dynamics and spin gratings in multiple quantum well semiconductors." Thesis, University of St Andrews, 1997. http://hdl.handle.net/10023/14661.
Full textAbstract:
The dynamics of exciton saturation in GaAs/AlGaAs multiple quantum wells are investigated using picosecond optical pulses. The main contributions to exciton saturation have been shown to be phase space filling. Coulomb screening and exciton lineshape broadening. The spin dependent nature of the phase space filling nonlinearity has allowed its separation from the effects due to Coulomb screening and broadening. The effect of lineshape broadening on exciton saturation has been investigated through its density dependence for a number of quantum well widths. The results show that the effects due to broadening are important in narrow wells of high quality. An investigation into electron spin relaxation at room temperature has been carried out as a function of the well width. The observed decrease in the electron spin relaxation time with decreasing well width is in good agreement with previously reported results and suggests the D'Yakonov-Perel mechanism is the dominant spin-flip mechanism for electrons in quantum wells at room temperature. The first demonstration of an electron spin grating in a quantum well semiconductor is reported by utilising the optical selection rules for quantum wells and optical polarisation gratings. Time resolved studies of the grating decay have allowed the first optical measurement of the in-plane electron diffusion coefficient in a quantum well semiconductor.
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Buller, Jakov. "Structure and Dynamics of Microcavity Exciton-Polaritons in Acoustic Square Lattices." Doctoral thesis, Humboldt-Universität zu Berlin, 2018. http://dx.doi.org/10.18452/19328.
Full textAbstract:
Exziton-Polaritonen in Mikrokavitäten sind Quasi-Teilchen, die unter bestimmten physikalischen Konditionen kondensieren und damit in einen energetisch gleichen, gemeinsamen makroskopischen Quantenzustand (MQZ) übergehen können. Exziton-Polariton-Kondensate können mithilfe von akustischen Oberflächenwellen moduliert werden, um ihre Eigenschaften zu verändern. Dies ist insbesondere von großer Relevanz für zukünftige Anwendungen.
In dieser Arbeit wurden die Struktur sowie die Dynamik der Exziton-Polariton-Kondensate in den durch die akustischen Oberflächenwellen erzeugten quadratischen Gittern untersucht. Es wurde dazu die Wellenfunktion der Exziton-Polariton-Kondensate im Rahmen der spektroskopischen und zeitaufgelösten Messungen im Orts- und Impulsraum abgebildet. Die MQZ wurden in einer optisch-parametrischen Oszillatorkonfiguration resonant angeregt.
Die spektroskopischen Messungen zeigten, dass Exziton-Polariton-Kondensate in akustischen quadratischen Gittern aus unterschiedlichen MQZ, nämlich aus einem zwei-dimensionalen Gap-Soliton (2D GS) umgeben von mehreren ein-dimensionalen MQZ, und einem inkohärenten Strahlungshintergrund zusammengesetzt sind.
Im Rahmen der zeitaufgelösten Experimente wurde die Dynamik der Wellenfunktion des
2D GS untersucht. Die zeitaufgelösten Ergebnisse zeigten, dass sowohl die Intensität
der von dem 2D GS emittierten Photolumineszenz (PL) als auch die Kohärenzlänge des
2D GS zeitlich oszillieren. Die Intensität der PL und die Kohärenzlänge hängen von
der Anregungsleistung, der Größe des Laserspots sowie von der relativen Position des
akustischen Gitters und dem Laserspot ab.
Im Ausblick dieser Arbeit wurde theoretisch die Anregung von Tamm-Plasmon/Exziton-
Polaritonen (TPEP) sowie deren Modulation mithilfe von akustischen Oberflächenwellen
diskutiert. TPEP entstehen durch die Superposition der in der Grenzschicht zwischen
Mikrokavität und Metall angeregten Tamm-Plasmonen und den in der Mikrokavität erzeugten
Exziton-Polaritonen.Microcavity (MC) exciton-polaritons can form condensates, i.e. macroscopic quantum states (MQSs), as well under a periodic potential modulation. The modulation by a surface acoustic wave (SAW) provides a powerful tool for the formation of tunable lattices of MQSs in semiconductor MC. In this work, fundamental aspects of the structure and dynamics of exciton-polariton condensate in acoustic square lattices were investigated by probing its wavefunction in real- and momentum space using spectral- and time-resolved studies. The MQSs were resonantly excited in an optical parametric oscillator configuration. The tomographic study revealed that the exciton-polariton condensate structure self-organises in a concentric structure, which consists of a single, two-dimensional gap soliton (2D GS) surrounded by one-dimensional MQSs and an incoherent background. 2D GS size tends to saturate with increasing particle density. The experimental results are supported by a theoretical model based on the variational solution of the Gross-Pitaevskii equation. Time-resolved studies showed the evolution of the 2D GS wavefunction at the acoustic velocity. Interestingly, the photoluminescence (PL) intensity emitted by the 2D GS as well as its coherence length oscillate with time. The PL oscillation amplitude depends on the intensity and the size of the exciting laser spot, and increases considerably for excitation intensities close to the optical threshold power for the formation of the MQS. In the outlook, the formation of Tamm-Plasmon/Exciton-Polariton (TPEP) hybrid states and their modulation by SAWs was theoretically discussed. Here, the upper DBR is partly replaced by a thin metal layer placed on top of the MC. In this case, TPEP form by the superposition of Tamm plasmons at the metal-semiconductor interface and the exciton-polaritons in the MC.
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Debnath, Arunangshu. "Dynamics and control of open quantum systems : applications to exciton dynamics in quantum dots and vibrational dynamics in carboxyhemoglobin." Toulouse 3, 2013. http://thesesups.ups-tlse.fr/2107/.
Full textAbstract:
Cette thèse présente certains aspects de la dynamique quantique dissipative, en relation avec la possibilité de contrôler la dynamique par des impulsions laser mises en forme. La première partie concerne le transfert de population dans des boîtes quantiques InAs/GaAs. Dans ces systèmes, les conséquences sur la dissipation de l'interaction avec des impulsions sont analysées, en relation avec le transfert de population adiabatique. En particulier, le problème de la création robuste d'excitons ou de biexcitons est adressé par une méthode de la matrice densité non Markovienne, qui a la spécificité de pouvoir décrire les systèmes dissipatifs en interaction avec des champs externes fortes. La dynamique dissipative en interaction avec des champs externes forts peut être interprétée sur la base de transitions dissipatives entre états habillés. De plus, des paramètres laser, qui permettent un transfert de population efficace, sont déterminés. La deuxième partie concerne le transfert de population vers des états vibrationnels élevés du mode d'élongation de CO dans la molécule de carboxyhémoglobine. Sur la base d'un modèle fluctuant, des spectres pompe-sonde sont modélisés par des propagations de paquets d'ondes quantiques. La méthode de contrôle local est appliquée pour déterminer des impulsions laser qui induisent les processus de " vibrational ladder climbing " ou qui mènent à l'excitation de niveaux vibrationnels spécifiques, malgré l'effet des fluctuations induites par l'environnement de la protéine. Ces résultats constituent des références pour des réalisations expérimentales futuresThis thesis work analyses aspects of dissipative quantum dynamics, with a view to look for further possibilities of controlling the dynamics with shaped laser pulses. The first part concerns the problem of efficient population transfer in mesoscopic zero-dimensional solid state systems - InAs quantum dots embedded in GaAs matrix. In these systems the consequences of laser driving induced dissipation of exciton dynamics are analyzed, in relation to adiabatic population transfer. Specifically, the problem of robust creation of exciton and biexciton states are addressed through numerical simulations and analytical approaches using a non-Markovian density matrix based analysis, suitable for dealing with dissipative quantum dynamics under strong laser fields. A physical picture describing phonon induced dissipation among dressed state has emerged as a consistent interpretation of the underlying dynamics. Furthermore, suitable parameter regimes where efficient population transfer can be achieved are proposed. The second part deals with the population transfer to high lying vibrational states of the CO stretching mode of carboxy-hemoglobin molecule in the native protein environment. On the basis of a fluctuating potential for the CO stretching mode, ultrafast pump-probe spectra are simulated using quantum wave packet propagation. To this end, Local Control Theory was employed to find design a set of laser pulses which accomplish the 'vibrational ladder climbing' and selective state preparation despite the detrimental fluctuations induced by the protein environment. These results will be providing benchmarks for the future experimental efforts
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Martelli, F. "WAVEPACKET APPROACHES TO DISSIPATIVE QUANTUM DYNAMICS." Doctoral thesis, Università degli Studi di Milano, 2010. http://hdl.handle.net/2434/150221.
Full textAbstract:
Many processes arising in Chemistry, Physics and Biology show quantum effects. Representative examples are
chemical reactions involving light atoms, energy transfer in complex environments, electronic and
geometrical properties of complex and bulk systems, tunneling phenomena etc. Theoretical Chemistry deals with such kind
of problems with the help of numerical solutions of the Time Dependent Schrodinger Equation (TDSE). On the other
hand, when classical effects are most relevant as, for example, in large dynamical systems like proteins at room temperature,
Theoretical Chemistry deals with this problems with the help of numerical solution of the Newton's equations.
Solving numerically the TDSE is much more expansive than solving the classical Newton's equations of motion.
Thus, often mixed descriptions are used treating the system at the exact -quantum- level and the environment
at the approximated classical level.
When a full quantum approach is needed both for the system and for the environment (as, for example, in
processes involving quantum objects, such as light atoms, energy transfer in small molecules interacting with
a thermal bath at low temperature, and so on), one faces with so called Open Quantum Systems,
and the corresponding dynamics is generally dissipative.
In open quantum systems the wavefunction description fails because of a lack of the complete knowledge of the
state. It is then necessary to switch to the density matrix description, which contains all the information.
The system dynamics is affected by the presence of the bath depending on the interactions between them.
If the coupling is small, than the system evolves in time without been affected by the environment. If the system
is initially prepared as a pure state, then the dynamics is coherent. On the other hand if system and
bath are strongly interacting, than the system dynamics is modified by the presence of the bath and described by
a mixture of states. Under the condition of strong coupling, the modified dynamics is incoherent,
also if the initial state of the system is pure.
Based on the Multi Configuration Time Dependent
Hartree (MCTDH) approach, which writes the wavefunction as linear combination of Hartree products,
the MCTDH program has been developed. This program is the only one allowing
to deal with large, dissipative problems (up to 100 degrees of freedom). Recently, variants of the MCTDH approach
have been developed in order to increase the number of degrees of freedom which can be considered. The GMCTDH
approach treats part of the MCTDH wavefunction as Gaussian functions, which are analytically known; the
LCSA approach, on the other hand, can be viewed as a GMCTDH approach with selected configurations.
In our project we have studied the effect of the environment on quantum systems. In particular, we have studied
the influence of the environment in simple chemical process both from a theoretical and from a computational point of view.
Specifically, we have developed a model to study excitoninc energy transfer in dissipative media,
performed numerical simulations
using the MCTDH approach, and obtained analytical expressions for the equations of motion in the so called
mixed quantum-classical hydrodynamics picture.
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Noller, Bastian M. "Excited-State Dynamics of Organic Intermediates." kostenfrei, 2009. http://www.opus-bayern.de/uni-wuerzburg/volltexte/2009/3607/.
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Brinne, Roos Johanna. "Reaction dynamics on highly excited states." Doctoral thesis, Stockholms universitet, Fysikum, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-27122.
Full textAbstract:
In this thesis I have performed theoretical studies on the reaction dynamics in few-atom molecules. In particular, I have looked at reaction processes in which highly excited resonant states are involved. When highly excited states are formed, the dynamics becomes complicated and approximations normally used in chemical reaction studies are no longer applicable.To calculate the potential energy curve for some of these states as a function of internuclear distance, a combination of structure calculations and scattering calculations have to be performed, and the reaction dynamics on the potentials has been studied using both time-independent and time-dependent methods.The processes that have been studied and which are discussed in this thesis are ion-pair formation in electron recombination with H3+, dissociative recombination and ion-pair formation of HF+, mutual neutralization in H++F- collisions and dissociative recombination of BeH+. Isotope effects in these reactions have also been investigated. Our calculated cross sections are compared with experimentally measured cross sections for these reactions.
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Volkov, Victor Vitorovich. "Bacteriorhodopsin excited state dynamics and photochemistry." Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/26308.
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Siller, Lidija. "Electronically excited states in surface dynamics." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.363321.
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Sills, Andrew Michael. "Excited state dynamics in semiconductor nanostructures." Thesis, University of Leeds, 2017. http://etheses.whiterose.ac.uk/18891/.
Full textAbstract:
Over the past two decades quantum-dot-based photovoltaic devices have been attracting a lot of attention due to their potential high efficiencies and low cost fabrication. Unlike conventional photovoltaic devices where the absorption of a single photon always produces a single electron hole pair (exciton), quantum-dot-based devices can generate multiple excitons from the absorption of just a single photon. Thanks to this process, which is referred to as either carrier multiplication or multiple excition generation, quantum-dot-based devices can potentially reach higher efficiencies breaking the Shockley-Queisser limit. In addition, the colloidal synthesis techniques used to fabricate these devices are potentially very cheap and scalable. Despite the intrinsic potential of these devices, they are not currently at a stage where they can compete with commercial photovoltaics. In this thesis various factors that effect the efficiency of carrier multiplication are investigated. In addition new analytical methods are developed to form a contribution to theoretical work in this field.