Global ETD Search

21	A Solid-state NMR Study of Tin and Phosphorus Containing Compounds Jamieson, Rebecca 22 August 2013 (has links) Various compounds were studied with solid-state 119Sn and 31P NMR spectroscopy and quantum chemical calculations. Connections were made between the shielding tensors and the geometric and electronic structures of the molecules. First, the 119Sn chemical shielding anisotropy of various para substituted tetraaryl tin compounds was shown to be dependent on the tilt angle of the phenyl rings. Tetrakis(o-tolyl) tin did not have the shielding anisotropy predicted by the tilt angle of the rings. It was suggested that ortho substitution distorts the structures of the phenyl rings causing the discrepancy. Analysis of the solid-state 31P NMR spectra of triphenylphosphorane ylides, Ph3P=CHC(O)R, determined that increasing the electron-donating effects of the R group decreased the δ33 component. Theoretical calculations showed that the component lay along the ylidic bond and was dependent on the difference in phosphorus-carbon bond lengths between the phenyl and ylidic bonds. Another study concerned the solid-state 31P NMR of the series of triphenylphosphine derivatives, PPh3-x(o-tolyl)x where x = 0 to 3. The addition of ortho methyl groups changed the position of the δ11 component which could be the result of the change in energy gap between the lone pair (HOMO) and σ* anti-bonding (LUMO). The solid-state 31P NMR spectra of deuterated piperazinium phosphonate and phosphonic acid were influenced by the shielding, dipolar and spin-spin interactions, as well as, second order quadrupolar effects. The spectrum of deuterated piperazinium phosphonate had a chemical shielding anisotropy of 130 ppm, an effective dipolar coupling of 2500 kHz and a one-bond phosphorus-deuterium J coupling of 90 Hz. The phosphorus-deuterium bond length was predicted to be 1.44(2) Å. A deuterium quadrupolar coupling constant of 104 kHz was obtained from the CP/MAS 2H spectrum. The non-axial symmetry of phosphonic acid complicated the analysis of the 31P spectrum. Phosphorus-deuterium bond lengths of 1.44(5) Å and 1.40(4) Å were obtained for the two inequivalent sites in the unit cell. Solid-state NMR Chemical Shift Anisotropy tin-119 phosphorus-31 tetraphenyl tin ylide triphenyl phosphine phosphonic acid Quantum chemical calculations molecular orbital
22	Analyse théorique de l'interaction d'échange magnétique : effets de solvant et décomposition Coulaud, Esther 05 October 2012 (has links) La Théorie de la Fonctionnelle de la Densité (DFT) combinée avec le formalisme de la Symétrie Brisée (BS) est aujourd'hui très utilisée dans le domaine du magnétisme moléculaire pour le calcul des constantes d'échange magnétique. Dans le but d'améliorer la compréhension des contributions qui participe à l'interaction d'échange caractérisant les systèmes magnétiques, nous avons mené une étude théorique systématique basée sur l'étude de deux types de modèles. Ces complexes binucléaires de cuivre(II) reliés par divers groupements pontant de type hydroxo, azoture, et chloro, ainsi que des bis-nitroxydes modèles constitués de deux groupements nitroxydes reliés par un système conjugué sont caractérisés par des couplages ferro- et anti-ferromagnétiques variés. Nous avons développé une approche théorique basée sur l'utilisation d'orbitales gelées permettant d'étudier la décomposition de l'interaction d'échange magnétique en différentes contributions telles que l'échange direct, l'échange cinétique et le mécanisme de polarisation de spin. Dans un second temps, nous avons étendu ce travail de rationalisation aux effets de solvant à l'aide de modèles implicites ou explicites. Grâce à une analyse basée sur l'approche de Hoffmann, nous avons pu déterminer les paramètres caractérisant dans les mécanismes d'échange direct et d'échange cinétique. / Density Functional Theory (DFT) combined with the Broken Symmetry (BS) method is today widely used in the field of molecular magnetism for the computation of magnetic exchange coupling constants. In order to improve the understanding of the contributions involved in the exchange phenomena characterizing some magnetic systems, we propose a study based on two types of models. Copper(II) binuclear complexes connected with various bridging groups like hydroxo, azide, and chlorine, as well as small bis-nitroxide composed of two nitroxide moieties linked by a conjugated system show various ferro- and anti-ferromagnetic couplings. We have developed a theoretical approach based on the use of frozen orbitals to decompose the magnetic exchange interaction in different contributions such as the direct exchange, the kinetic exchange and mechanism of spin polarization. In a second part, we have extended this work to rationalize the solvent effects using implicit or explicit models. Through an analysis based on the Hoffmann's approach, we have determined some parameters characterizing the mechanisms of direct exchange and kinetic exchange. Magnétisme moléculaire Interaction d’échange Symétrie brisée Orbitale magnétique Orbitale moléculaire Dimère de cuivre Bis-nitroxydes Density Functional Theory Molecular Magnetism Exchange Interaction Broken Symmetry Magnetic Orbital Molecular Orbital Copper Dimer Bis-nitroxides
23	Dinâmica molecular de sistemas iônicos poliatômicos: modelos polarizável e não-polarizável / Molecular dynamics of polyatomic ionic systems: Polarized models and not polarizable Costa, Marcello Ferreira da 01 April 2005 (has links) Simulações de sais de carbonato fundidos pelo método de Dinâmica Molecular (MD) foram efetuadas com o modelo polarizável de cargas flutuantes (FC). O modelo de cargas flutuantes implementa os efeitos de polarização pelo método de Lagrangiano estendido, onde as variáveis extras são as próprias cargas parciais do íon poliatômico. O modelo FC foi parametrizado por meio de cálculos ab inito, aplicado ao ânion carbonato. Cálculos de Química Quântica ab initio foram utilizados para corroborar o modelo proposto para o ânion carbonato. Os sistemas investigados consistem em misturas de carbonatos alcalinos fundidos, Li2CO3/K2CO3, os quais são utilizados como eletrólitos em células a combustível. As simulações MD foram utilizadas para verificar o efeito da polarização dos ânions sobre a estrutura e dinâmica do líquido. Estudamos o efeito da inclusão de polarização sobre a condutividade do eletrólito. / Simulations of molten carbonate salts by the method of Molecular Dynamics (MD) have been performed with the fluctuanting charge (FC) model. The FC model implements the effect of polarization by using method of extended Lagrangian, where the extra variables are the partial charges of the poliatomic ion. The FC model was parametrized by ab inito calculations os a single carbonate anion. Quantum Chemistry calculations bave been used to corroborate the model for the carbonate anion. The investigated systems consist of alkaline carbonate mixtures, Li2CO3/K2CO3, which are used as electrolytes in fuel cells. MD simulations have been used to verify polarization effects on structure and dynamics of the liquid. We study the effect of including anion polarization on the condutivity of the electrolyte. ab initio ab initio Alkaline mixtures Cargas flutuantes Dinâmica Molecular Electrochemistry Eletroquímica Física molecular Floating charges Misturas alcalinas Molecular dynamics Molecular orbital Molecular physics Orbital molecular Quantum chemistry Química quântica Química teórica Theoretical chemistry
24	Dinâmica molecular de sistemas iônicos poliatômicos: modelos polarizável e não-polarizável / Molecular dynamics of polyatomic ionic systems: Polarized models and not polarizable Marcello Ferreira da Costa 01 April 2005 (has links) Simulações de sais de carbonato fundidos pelo método de Dinâmica Molecular (MD) foram efetuadas com o modelo polarizável de cargas flutuantes (FC). O modelo de cargas flutuantes implementa os efeitos de polarização pelo método de Lagrangiano estendido, onde as variáveis extras são as próprias cargas parciais do íon poliatômico. O modelo FC foi parametrizado por meio de cálculos ab inito, aplicado ao ânion carbonato. Cálculos de Química Quântica ab initio foram utilizados para corroborar o modelo proposto para o ânion carbonato. Os sistemas investigados consistem em misturas de carbonatos alcalinos fundidos, Li2CO3/K2CO3, os quais são utilizados como eletrólitos em células a combustível. As simulações MD foram utilizadas para verificar o efeito da polarização dos ânions sobre a estrutura e dinâmica do líquido. Estudamos o efeito da inclusão de polarização sobre a condutividade do eletrólito. / Simulations of molten carbonate salts by the method of Molecular Dynamics (MD) have been performed with the fluctuanting charge (FC) model. The FC model implements the effect of polarization by using method of extended Lagrangian, where the extra variables are the partial charges of the poliatomic ion. The FC model was parametrized by ab inito calculations os a single carbonate anion. Quantum Chemistry calculations bave been used to corroborate the model for the carbonate anion. The investigated systems consist of alkaline carbonate mixtures, Li2CO3/K2CO3, which are used as electrolytes in fuel cells. MD simulations have been used to verify polarization effects on structure and dynamics of the liquid. We study the effect of including anion polarization on the condutivity of the electrolyte. ab initio Cargas flutuantes Dinâmica Molecular Eletroquímica Física molecular Misturas alcalinas Orbital molecular Química quântica Química teórica ab initio Alkaline mixtures Electrochemistry Floating charges Molecular dynamics Molecular orbital Molecular physics Quantum chemistry Theoretical chemistry
25	QM/MM Applications and Corrections for Chemical Reactions Bryant J Kim (15322279) 18 May 2023 (has links) <p>In this thesis, we present novel computational methods and frameworks to address the challenges associated with the determination of free energy profiles for condensed-phase chemical reactions using combined quantum mechanical and molecular mechanical (QM/MM) approaches. We focus on overcoming issues related to force matching, molecular polarizability, and convergence of free energy profiles. First, we introduce a method called Reaction Path-Force Matching in Collective Variables (RP-FM-CV) that efficiently carries out ab initio QM/MM free energy simulations through mean force fitting. This method provides accurate and robust simulations of solution-phase chemical reactions by significantly reducing deviations on the collective variables forces, thereby bringing simulated free energy profiles closer to experimental and benchmark AI/MM results. Second, we explore the role of pairwise repulsive correcting potentials in generating converged free energy profiles for chemical reactions using QM/MM simulations. We develop a free energy correcting model that sheds light on the behavior of repulsive pairwise potentials with large force deviations in collective variables. Our findings contribute to a deeper understanding of force matching models, paving the way for more accurate predictions of free energy profiles in chemical reactions. Next, we address the underpolarization problem in semiempirical (SE) molecular orbital methods by introducing a hybrid framework called doubly polarized QM/MM (dp-QM/MM). This framework improves the response property of SE/MM methods through high-level molecular polarizability fitting using machine learning (ML)-derived corrective polarizabilities, referred to as chaperone polarizabilities. We demonstrate the effectiveness of the dp-QM/MM method in simulating the Menshutkin reaction in water, showing that ML chaperones significantly reduce the error in solute molecular polarizability, bringing simulated free energy profiles closer to experimental results. In summary, this thesis presents a series of novel methods and frameworks that improve the accuracy and reliability of free energy profile estimations in condensed-phase chemical reactions using QM/MM simulations. By addressing the challenges of force matching, molecular polarizability, and convergence, these advancements have the potential to impact various fields, including computational chemistry, materials science, and drug design.</p> Computational chemistry Computational methods Free energy profiles Condensed-phase chemical reactions Force matching Mean force fitting Pairwise repulsive correcting potentials Free energy correcting model Semiempirical molecular orbital methods Doubly polarized QM/MM (dp-QM/MM) Chaperone polarizabilities Machine learning Computational chemistry
26	QM/MM Applications and Corrections for Chemical Reactions Kim, Bryant 05 1900 (has links) Indiana University-Purdue University Indianapolis (IUPUI) / In this thesis, we present novel computational methods and frameworks to address the challenges associated with the determination of free energy profiles for condensed-phase chemical reactions using combined quantum mechanical and molecular mechanical (QM/MM) approaches. We focus on overcoming issues related to force matching, molecular polarizability, and convergence of free energy profiles. First, we introduce a method called Reaction Path-Force Matching in Collective Variables (RP-FM-CV) that efficiently carries out ab initio QM/MM free energy simulations through mean force fitting. This method provides accurate and robust simulations of solution-phase chemical reactions by significantly reducing deviations on the collective variables forces, thereby bringing simulated free energy profiles closer to experimental and benchmark AI/MM results. Second, we explore the role of pairwise repulsive correcting potentials in generating converged free energy profiles for chemical reactions using QM/MM simulations. We develop a free energy correcting model that sheds light on the behavior of repulsive pairwise potentials with large force deviations in collective variables. Our findings contribute to a deeper understanding of force matching models, paving the way for more accurate predictions of free energy profiles in chemical reactions. Next, we address the underpolarization problem in semiempirical (SE) molecular orbital methods by introducing a hybrid framework called doubly polarized QM/MM (dp-QM/MM). This framework improves the response property of SE/MM methods through high-level molecular polarizability fitting using machine learning (ML)-derived corrective polarizabilities, referred to as chaperone polarizabilities. We demonstrate the effectiveness of the dp-QM/MM method in simulating the Menshutkin reaction in water, showing that ML chaperones significantly reduce the error in solute molecular polarizability, bringing simulated free energy profiles closer to experimental results. In summary, this thesis presents a series of novel methods and frameworks that improve the accuracy and reliability of free energy profile estimations in condensed-phase chemical reactions using QM/MM simulations. By addressing the challenges of force matching, molecular polarizability, and convergence, these advancements have the potential to impact various fields, including computational chemistry, materials science, and drug design. Computational Methods Free Energy Profiles Condensed-Phase Chemical Reactions Force Matching Mean Force Fitting Pairwise Repulsive Correcting Potentials Free Energy Correcting Model Sempiempirical Molecular Orbital Methods Doubly Polarized QM/MM (dp-QM/MM) Chaperone Polarizabilities Machine Learning Computational Chemistry
27	Charge transport and energy levels in organic semiconductors / Ladungstransport und Energieniveaus in organischen Halbleitern Widmer, Johannes 25 November 2014 (has links) (PDF) Organic semiconductors are a new key technology for large-area and flexible thin-film electronics. They are deposited as thin films (sub-nanometer to micrometer) on large-area substrates. The technologically most advanced applications are organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). For the improvement of performance and efficiency, correct modeling of the electronic processes in the devices is essential. Reliable characterization and validation of the electronic properties of the materials is simultaneously required for the successful optimization of devices. Furthermore, understanding the relations between material structures and their key characteristics opens the path for innovative material and device design. In this thesis, two material characterization methods are developed, respectively refined and applied: a novel technique for measuring the charge carrier mobility μ and a way to determine the ionization energy IE or the electron affinity EA of an organic semiconductor. For the mobility measurements, a new evaluation approach for space-charge limited current (SCLC) measurements in single carrier devices is developed. It is based on a layer thickness variation of the material under investigation. In the \"potential mapping\" (POEM) approach, the voltage as a function of the device thickness V(d) at a given current density is shown to coincide with the spatial distribution of the electric potential V(x) in the thickest device. On this basis, the mobility is directly obtained as function of the electric field F and the charge carrier density n. The evaluation is model-free, i.e. a model for μ(F, n) to fit the measurement data is not required, and the measurement is independent of a possible injection barrier or potential drop at non-optimal contacts. The obtained μ(F, n) function describes the effective average mobility of free and trapped charge carriers. This approach realistically describes charge transport in energetically disordered materials, where a clear differentiation between trapped and free charges is impossible or arbitrary. The measurement of IE and EA is performed by characterizing solar cells at varying temperature T. In suitably designed devices based on a bulk heterojunction (BHJ), the open-circuit voltage Voc is a linear function of T with negative slope in the whole measured range down to 180K. The extrapolation to temperature zero V0 = Voc(T → 0K) is confirmed to equal the effective gap Egeff, i.e. the difference between the EA of the acceptor and the IE of the donor. The successive variation of different components of the devices and testing their influence on V0 verifies the relation V0 = Egeff. On this basis, the IE or EA of a material can be determined in a BHJ with a material where the complementary value is known. The measurement is applied to a number of material combinations, confirming, refining, and complementing previously reported values from ultraviolet photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES). These measurements are applied to small molecule organic semiconductors, including mixed layers. In blends of zinc-phthalocyanine (ZnPc) and C60, the hole mobility is found to be thermally and field activated, as well as increasing with charge density. Varying the mixing ratio, the hole mobility is found to increase with increasing ZnPc content, while the effective gap stays unchanged. A number of further materials and material blends are characterized with respect to hole and electron mobility and the effective gap, including highly diluted donor blends, which have been little investigated before. In all materials, a pronounced field activation of the mobility is observed. The results enable an improved detailed description of the working principle of organic solar cells and support the future design of highly efficient and optimized devices. / Organische Halbleiter sind eine neue Schlüsseltechnologie für großflächige und flexible Dünnschichtelektronik. Sie werden als dünne Materialschichten (Sub-Nanometer bis Mikrometer) auf großflächige Substrate aufgebracht. Die technologisch am weitesten fortgeschrittenen Anwendungen sind organische Leuchtdioden (OLEDs) und organische Photovoltaik (OPV). Zur weiteren Steigerung von Leistungsfähigkeit und Effizienz ist die genaue Modellierung elektronischer Prozesse in den Bauteilen von grundlegender Bedeutung. Für die erfolgreiche Optimierung von Bauteilen ist eine zuverlässige Charakterisierung und Validierung der elektronischen Materialeigenschaften gleichermaßen erforderlich. Außerdem eröffnet das Verständnis der Zusammenhänge zwischen Materialstruktur und -eigenschaften einen Weg für innovative Material- und Bauteilentwicklung. Im Rahmen dieser Dissertation werden zwei Methoden für die Materialcharakterisierung entwickelt, verfeinert und angewandt: eine neuartige Methode zur Messung der Ladungsträgerbeweglichkeit μ und eine Möglichkeit zur Bestimmung der Ionisierungsenergie IE oder der Elektronenaffinität EA eines organischen Halbleiters. Für die Beweglichkeitsmessungen wird eine neue Auswertungsmethode für raumladungsbegrenzte Ströme (SCLC) in unipolaren Bauteilen entwickelt. Sie basiert auf einer Schichtdickenvariation des zu charakterisierenden Materials. In einem Ansatz zur räumlichen Abbildung des elektrischen Potentials (\"potential mapping\", POEM) wird gezeigt, dass das elektrische Potential als Funktion der Schichtdicke V(d) bei einer gegebenen Stromdichte dem räumlichen Verlauf des elektrischen Potentials V(x) im dicksten Bauteil entspricht. Daraus kann die Beweglichkeit als Funktion des elektrischen Felds F und der Ladungsträgerdichte n berechnet werden. Die Auswertung ist modellfrei, d.h. ein Modell zum Angleichen der Messdaten ist für die Berechnung von μ(F, n) nicht erforderlich. Die Messung ist außerdem unabhängig von einer möglichen Injektionsbarriere oder einer Potentialstufe an nicht-idealen Kontakten. Die gemessene Funktion μ(F, n) beschreibt die effektive durchschnittliche Beweglichkeit aller freien und in Fallenzuständen gefangenen Ladungsträger. Dieser Zugang beschreibt den Ladungstransport in energetisch ungeordneten Materialien realistisch, wo eine klare Unterscheidung zwischen freien und Fallenzuständen nicht möglich oder willkürlich ist. Die Messung von IE und EA wird mithilfe temperaturabhängiger Messungen an Solarzellen durchgeführt. In geeigneten Bauteilen mit einem Mischschicht-Heteroübergang (\"bulk heterojunction\" BHJ) ist die Leerlaufspannung Voc im gesamten Messbereich oberhalb 180K eine linear fallende Funktion der Temperatur T. Es kann bestätigt werden, dass die Extrapolation zum Temperaturnullpunkt V0 = Voc(T → 0K) mit der effektiven Energielücke Egeff , d.h. der Differenz zwischen EA des Akzeptor-Materials und IE des Donator-Materials, übereinstimmt. Die systematische schrittweise Variation einzelner Bestandteile der Solarzellen und die Überprüfung des Einflusses auf V0 bestätigen die Beziehung V0 = Egeff. Damit kann die IE oder EA eines Materials bestimmt werden, indem man es in einem BHJ mit einem Material kombiniert, dessen komplementärer Wert bekannt ist. Messungen per Ultraviolett-Photoelektronenspektroskopie (UPS) und inverser Photoelektronenspektroskopie (IPES) werden damit bestätigt, präzisiert und ergänzt. Die beiden entwickelten Messmethoden werden auf organische Halbleiter aus kleinen Molekülen einschließlich Mischschichten angewandt. In Mischschichten aus Zink-Phthalocyanin (ZnPc) und C60 wird eine Löcherbeweglichkeit gemessen, die sowohl thermisch als auch feld- und ladungsträgerdichteaktiviert ist. Wenn das Mischverhältnis variiert wird, steigt die Löcherbeweglichkeit mit zunehmendem ZnPc-Anteil, während die effektive Energielücke unverändert bleibt. Verschiedene weitere Materialien und Materialmischungen werden hinsichtlich Löcher- und Elektronenbeweglichkeit sowie ihrer Energielücke charakterisiert, einschließlich bisher wenig untersuchter hochverdünnter Donator-Systeme. In allen Materialien wird eine deutliche Feldaktivierung der Beweglichkeit beobachtet. Die Ergebnisse ermöglichen eine verbesserte Beschreibung der detaillierten Funktionsweise organischer Solarzellen und unterstützen die künftige Entwicklung hocheffizienter und optimierter Bauteile. Physik Festkörperphysik Festkörper-Physik Halbleiterphysik Halbleiter-Physik Organische Halbleiter Organische Elektronik Organische Solarzellen Organische Photovoltaik Organische Moleküle Kleine Moleküle Molekulare Elektronik Molekulare Halbleiter Amorphe Halbleiter Ladungstransport Ladungsbeweglichkeit Ladungs-Beweglichkeit Ladungsmobilität Ladungs-Mobilität Ladungsträgerbewegleichkeit Ladungsträger-Beweglichkeit Ladungsträgermobilität Ladungsträger-Mobilität Elektronenbeweglichkeit Elektronen-Beweglichkeit Elektronenmobilität Elektronen-Mobilität Löcherbeweglichkeit Löcher-Beweglichkeit Löchermobilität Löcher-Mobilität Raumladungsbegrenzte Ströme Raumladungsbegrenzter Strom Unipolare Bauteile Unipolares Bauteil Löchertransport Löcher-Transport Löcherleitfähigkeit Löcher-Leitfähigkeit Löcherleitung Elektronentransport Elektronen-Transport Elektronenleitfähigkeit Elektronen-Leitfähigkeit Elektronenleitung Hüpftransport Hüpf-Transport Poole-Frenkel-Transport Feldaktivierung Feld-Aktivierung Dichteaktivierung Dichte-Aktivierung Ladungsdichteaktivierung Ladungsdichte-Aktivierung Ladungsträgerdichteaktivierung Ladungsträgerdichte-Aktivierung Ladungsträger-Dichteaktivierung Ladungsträger-Dichte-Aktivierung Drifttransport Drift-Transport Temperaturaktivierung Temperatur-Aktivierung Ionisationsenergie Ionisations-Energie Ionisationspotential Ionisationspotenzial Ionisations-Potential Ionisations-Potenzial Elektronenaffinität Elektronen-Affinität Leitungsband Leitungs-Band Valenzband Valenz-Band Höchstes besetztes Molekülorbital Höchstes besetztes Molekül-Orbital Niedrigstes unbesetztes Molekülorbital Niedrigstes unbesetztes Molekül-Orbital Transportniveau Transport-Niveau Energieniveaus Energie-Niveaus Donator Akzeptor Donator-Akzeptor Effektive Lücke Effektive Bandlücke Bandlücke Energielücke Effektive Energielücke Mischschichtheteroübergang Mischschicht-Heteroübergang Flacher Heteroübergang Planarer Heteroübergang Dotierung Dotierte Transportschichten Dotierte Transport-Schichten Injektionsbarriere Injektions-Barriere Extraktionsbarriere Extraktions-Barriere Diffusion Ladungsdiffusion Ladungs-Diffusion Ladungsträgerdiffusion Ladungsträger-Diffusion Bandverbiegung Band-Verbiegung Dünnschichtelektronik Dünnschicht-Elektronik Physics Solid State Physics Semiconductor Physics Organic Semiconductors Organic Electronics Organic Solar Cells Organic Photovoltaics Organic Molecules Small Molecules Molecular Electronics Molecular Semiconductors Amorphous Semiconductors Charge Transport Charge Mobility Charge Carrier Mobility Electron Mobility Hole Mobility Space-Charge Limited Current SCLC Single Carrier Devices Hole-Only Devices Hole Conduction Hole Conductivity Hole Transport Electron-Only Devices Electron Conduction Electron Conductivity Electron Transport Hopping Transport Poole-Frenkel Transport Field Activation Density Activation Charge Density Activation Charge Carrier Density Activation Drift Transport Temperature Activation Ionization Energy Ionization Potential Electron Affinity Conduction Band Valence Band Highest Occupied Molecular Orbital HOMO Lowest Unoccupied Molecular Orbital LUMO Transport Level Energy Levels Donor Acceptor Donor-Acceptor Effective Gap Effective Band Gap Band Gap Energy Gap Effective Energy Gap Bulk Heterojunction BHJ Flat Heterojunction FHJ Planar Heterojunction PHJ Doping Doped Transport Layers Injection Barrier Extraction Barrier Diffusion Charge Diffusion Charge Carrier Diffusion Band Bending Thin Film Electronics ddc:530 rvk:UP 3130 Elektronik Polymerelektronik Halbleiterelektronik Festkörperelektronik Physik Angewandte Physik Festkörperphysik Amorpher Halbleiter p-Halbleiter n-Halbleiter Metall-Halbleiter-Kontakt Intrinsischer Halbleiter Dotierter Halbleiter Organischer Halbleiter Halbleiterschicht Ohm-Kontakt Organisches Halbleiterbauelement Fullerene Buckminsterfulleren Hopping-Leitfähigkeit Elektrische Leitfähigkeit Leitfähigkeit Dunkelleitfähigkeit Elektronenleitung Beweglichkeit <Physik> Ladungsträgerbeweglichkeit Elektronenbeweglichkeit Ionisationsenergie Elektronenaffinität
28	Charge transport and energy levels in organic semiconductors Widmer, Johannes 02 October 2014 (has links) Organic semiconductors are a new key technology for large-area and flexible thin-film electronics. They are deposited as thin films (sub-nanometer to micrometer) on large-area substrates. The technologically most advanced applications are organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). For the improvement of performance and efficiency, correct modeling of the electronic processes in the devices is essential. Reliable characterization and validation of the electronic properties of the materials is simultaneously required for the successful optimization of devices. Furthermore, understanding the relations between material structures and their key characteristics opens the path for innovative material and device design. In this thesis, two material characterization methods are developed, respectively refined and applied: a novel technique for measuring the charge carrier mobility μ and a way to determine the ionization energy IE or the electron affinity EA of an organic semiconductor. For the mobility measurements, a new evaluation approach for space-charge limited current (SCLC) measurements in single carrier devices is developed. It is based on a layer thickness variation of the material under investigation. In the \"potential mapping\" (POEM) approach, the voltage as a function of the device thickness V(d) at a given current density is shown to coincide with the spatial distribution of the electric potential V(x) in the thickest device. On this basis, the mobility is directly obtained as function of the electric field F and the charge carrier density n. The evaluation is model-free, i.e. a model for μ(F, n) to fit the measurement data is not required, and the measurement is independent of a possible injection barrier or potential drop at non-optimal contacts. The obtained μ(F, n) function describes the effective average mobility of free and trapped charge carriers. This approach realistically describes charge transport in energetically disordered materials, where a clear differentiation between trapped and free charges is impossible or arbitrary. The measurement of IE and EA is performed by characterizing solar cells at varying temperature T. In suitably designed devices based on a bulk heterojunction (BHJ), the open-circuit voltage Voc is a linear function of T with negative slope in the whole measured range down to 180K. The extrapolation to temperature zero V0 = Voc(T → 0K) is confirmed to equal the effective gap Egeff, i.e. the difference between the EA of the acceptor and the IE of the donor. The successive variation of different components of the devices and testing their influence on V0 verifies the relation V0 = Egeff. On this basis, the IE or EA of a material can be determined in a BHJ with a material where the complementary value is known. The measurement is applied to a number of material combinations, confirming, refining, and complementing previously reported values from ultraviolet photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES). These measurements are applied to small molecule organic semiconductors, including mixed layers. In blends of zinc-phthalocyanine (ZnPc) and C60, the hole mobility is found to be thermally and field activated, as well as increasing with charge density. Varying the mixing ratio, the hole mobility is found to increase with increasing ZnPc content, while the effective gap stays unchanged. A number of further materials and material blends are characterized with respect to hole and electron mobility and the effective gap, including highly diluted donor blends, which have been little investigated before. In all materials, a pronounced field activation of the mobility is observed. The results enable an improved detailed description of the working principle of organic solar cells and support the future design of highly efficient and optimized devices.:1. Introduction 2. Organic semiconductors and devices 2.1. Organic semiconductors 2.1.1. Conjugated π system 2.1.2. Small molecules and polymers 2.1.3. Disorder in amorphous materials 2.1.4. Polarons 2.1.5. Polaron hopping 2.1.6. Fermi-Dirac distribution and Fermi level 2.1.7. Quasi-Fermi levels 2.1.8. Trap states 2.1.9. Doping 2.1.10. Excitons 2.2. Interfaces and blend layers 2.2.1. Interface dipoles 2.2.2. Energy level bending 2.2.3. Injection from metal into semiconductor, and extraction 2.2.4. Excitons at interfaces 2.3. Charge transport and recombination in organic semiconductors 2.3.1. Drift transport 2.3.2. Charge carrier mobility 2.3.3. Thermally activated transport 2.3.4. Diffusion transport 2.3.5. Drift-diffusion transport 2.3.6. Space-charge limited current 2.3.7. Recombination 2.4. Mobility measurement 2.4.1. SCLC and TCLC 2.4.2. Time of flight 2.4.3. Organic field effect transistors 2.4.4. CELIV 2.5. Organic solar cells 2.5.1. Exciton diffusion towards the interface 2.5.2. Dissociation of CT states 2.5.3. CT recombination 2.5.4. Flat and bulk heterojunction 2.5.5. Transport layers 2.5.6. Thin film optics 2.5.7. Current-voltage characteristics and equivalent circuit 2.5.8. Solar cell efficiency 2.5.9. Limits of efficiency 2.5.10. Correct solar cell characterization 2.5.11. The \"O-Factor\" 3. Materials and experimental methods 3.1. Materials 3.2. Device fabrication and layout 3.2.1. Layer deposition 3.2.2. Encapsulation 3.2.3. Homogeneity of layer thickness on a wafer 3.2.4. Device layout 3.3. Characterization 3.3.1. Electrical characterization 3.3.2. Sample illumination 3.3.3. Temperature dependent characterization 3.3.4. UPS 4. Simulations 5.1. Design of single carrier devices 5.1.1. General design requirements 5.1.2. Single carrier devices for space-charge limited current 5.1.3. Ohmic regime 5.1.4. Design of injection and extraction layers 5.2. Advanced evaluation of SCLC – potential mapping 5.2.1. Potential mapping by thickness variation 5.2.2. Further evaluation of the transport profile 5.2.3. Injection into and extraction from single carrier devices 5.2.4. Majority carrier approximation 5.3. Proof of principle: POEM on simulated data 5.3.1. Constant mobility 5.3.2. Field dependent mobility 5.3.3. Field and charge density activated mobility 5.3.4. Conclusion 5.4. Application: Transport characterization in organic semiconductors 5.4.1. Hole transport in ZnPc:C60 5.4.2. Hole transport in ZnPc:C60 – temperature variation 5.4.3. Hole transport in ZnPc:C60 – blend ratio variation 5.4.4. Hole transport in ZnPc:C70 5.4.5. Hole transport in neat ZnPc 5.4.6. Hole transport in F4-ZnPc:C60 5.4.7. Hole transport in DCV-5T-Me33:C60 5.4.8. Electron transport in ZnPc:C60 5.4.9. Electron transport in neat Bis-HFl-NTCDI 5.5. Summary and discussion of the results 5.5.1. Phthalocyanine:C60 blends 5.5.2. DCV-5T-Me33:C60 5.5.3. Conclusion 6. Organic solar cell characteristics: the influence of temperature 6.1. ZnPc:C60 solar cells 6.1.1. Temperature variation 6.1.2. Illumination intensity variation 6.2. Voc in flat and bulk heterojunction organic solar cells 6.2.1. Qualitative difference in Voc(I, T) 6.2.2. Interpretation of Voc(I, T) 6.3. BHJ stoichiometry variation 6.3.1. Voc upon variation of stoichiometry and contact layer 6.3.2. V0 upon stoichiometry variation 6.3.3. Low donor content stoichiometry 6.3.4. Conclusion from stoichiometry variation 6.4. Transport material variation 6.4.1. HTM variation 6.4.2. ETM variation 6.5. Donor:acceptor material variation 6.5.1. Donor variation 6.5.2. Acceptor variation 6.6. Conclusion 7. Summary and outlook 7.1. Summary 7.2. Outlook A. Appendix A.1. Energy pay-back of this thesis A.2. Tables and registers / Organische Halbleiter sind eine neue Schlüsseltechnologie für großflächige und flexible Dünnschichtelektronik. Sie werden als dünne Materialschichten (Sub-Nanometer bis Mikrometer) auf großflächige Substrate aufgebracht. Die technologisch am weitesten fortgeschrittenen Anwendungen sind organische Leuchtdioden (OLEDs) und organische Photovoltaik (OPV). Zur weiteren Steigerung von Leistungsfähigkeit und Effizienz ist die genaue Modellierung elektronischer Prozesse in den Bauteilen von grundlegender Bedeutung. Für die erfolgreiche Optimierung von Bauteilen ist eine zuverlässige Charakterisierung und Validierung der elektronischen Materialeigenschaften gleichermaßen erforderlich. Außerdem eröffnet das Verständnis der Zusammenhänge zwischen Materialstruktur und -eigenschaften einen Weg für innovative Material- und Bauteilentwicklung. Im Rahmen dieser Dissertation werden zwei Methoden für die Materialcharakterisierung entwickelt, verfeinert und angewandt: eine neuartige Methode zur Messung der Ladungsträgerbeweglichkeit μ und eine Möglichkeit zur Bestimmung der Ionisierungsenergie IE oder der Elektronenaffinität EA eines organischen Halbleiters. Für die Beweglichkeitsmessungen wird eine neue Auswertungsmethode für raumladungsbegrenzte Ströme (SCLC) in unipolaren Bauteilen entwickelt. Sie basiert auf einer Schichtdickenvariation des zu charakterisierenden Materials. In einem Ansatz zur räumlichen Abbildung des elektrischen Potentials (\"potential mapping\", POEM) wird gezeigt, dass das elektrische Potential als Funktion der Schichtdicke V(d) bei einer gegebenen Stromdichte dem räumlichen Verlauf des elektrischen Potentials V(x) im dicksten Bauteil entspricht. Daraus kann die Beweglichkeit als Funktion des elektrischen Felds F und der Ladungsträgerdichte n berechnet werden. Die Auswertung ist modellfrei, d.h. ein Modell zum Angleichen der Messdaten ist für die Berechnung von μ(F, n) nicht erforderlich. Die Messung ist außerdem unabhängig von einer möglichen Injektionsbarriere oder einer Potentialstufe an nicht-idealen Kontakten. Die gemessene Funktion μ(F, n) beschreibt die effektive durchschnittliche Beweglichkeit aller freien und in Fallenzuständen gefangenen Ladungsträger. Dieser Zugang beschreibt den Ladungstransport in energetisch ungeordneten Materialien realistisch, wo eine klare Unterscheidung zwischen freien und Fallenzuständen nicht möglich oder willkürlich ist. Die Messung von IE und EA wird mithilfe temperaturabhängiger Messungen an Solarzellen durchgeführt. In geeigneten Bauteilen mit einem Mischschicht-Heteroübergang (\"bulk heterojunction\" BHJ) ist die Leerlaufspannung Voc im gesamten Messbereich oberhalb 180K eine linear fallende Funktion der Temperatur T. Es kann bestätigt werden, dass die Extrapolation zum Temperaturnullpunkt V0 = Voc(T → 0K) mit der effektiven Energielücke Egeff , d.h. der Differenz zwischen EA des Akzeptor-Materials und IE des Donator-Materials, übereinstimmt. Die systematische schrittweise Variation einzelner Bestandteile der Solarzellen und die Überprüfung des Einflusses auf V0 bestätigen die Beziehung V0 = Egeff. Damit kann die IE oder EA eines Materials bestimmt werden, indem man es in einem BHJ mit einem Material kombiniert, dessen komplementärer Wert bekannt ist. Messungen per Ultraviolett-Photoelektronenspektroskopie (UPS) und inverser Photoelektronenspektroskopie (IPES) werden damit bestätigt, präzisiert und ergänzt. Die beiden entwickelten Messmethoden werden auf organische Halbleiter aus kleinen Molekülen einschließlich Mischschichten angewandt. In Mischschichten aus Zink-Phthalocyanin (ZnPc) und C60 wird eine Löcherbeweglichkeit gemessen, die sowohl thermisch als auch feld- und ladungsträgerdichteaktiviert ist. Wenn das Mischverhältnis variiert wird, steigt die Löcherbeweglichkeit mit zunehmendem ZnPc-Anteil, während die effektive Energielücke unverändert bleibt. Verschiedene weitere Materialien und Materialmischungen werden hinsichtlich Löcher- und Elektronenbeweglichkeit sowie ihrer Energielücke charakterisiert, einschließlich bisher wenig untersuchter hochverdünnter Donator-Systeme. In allen Materialien wird eine deutliche Feldaktivierung der Beweglichkeit beobachtet. Die Ergebnisse ermöglichen eine verbesserte Beschreibung der detaillierten Funktionsweise organischer Solarzellen und unterstützen die künftige Entwicklung hocheffizienter und optimierter Bauteile.:1. Introduction 2. Organic semiconductors and devices 2.1. Organic semiconductors 2.1.1. Conjugated π system 2.1.2. Small molecules and polymers 2.1.3. Disorder in amorphous materials 2.1.4. Polarons 2.1.5. Polaron hopping 2.1.6. Fermi-Dirac distribution and Fermi level 2.1.7. Quasi-Fermi levels 2.1.8. Trap states 2.1.9. Doping 2.1.10. Excitons 2.2. Interfaces and blend layers 2.2.1. Interface dipoles 2.2.2. Energy level bending 2.2.3. Injection from metal into semiconductor, and extraction 2.2.4. Excitons at interfaces 2.3. Charge transport and recombination in organic semiconductors 2.3.1. Drift transport 2.3.2. Charge carrier mobility 2.3.3. Thermally activated transport 2.3.4. Diffusion transport 2.3.5. Drift-diffusion transport 2.3.6. Space-charge limited current 2.3.7. Recombination 2.4. Mobility measurement 2.4.1. SCLC and TCLC 2.4.2. Time of flight 2.4.3. Organic field effect transistors 2.4.4. CELIV 2.5. Organic solar cells 2.5.1. Exciton diffusion towards the interface 2.5.2. Dissociation of CT states 2.5.3. CT recombination 2.5.4. Flat and bulk heterojunction 2.5.5. Transport layers 2.5.6. Thin film optics 2.5.7. Current-voltage characteristics and equivalent circuit 2.5.8. Solar cell efficiency 2.5.9. Limits of efficiency 2.5.10. Correct solar cell characterization 2.5.11. The \"O-Factor\" 3. Materials and experimental methods 3.1. Materials 3.2. Device fabrication and layout 3.2.1. Layer deposition 3.2.2. Encapsulation 3.2.3. Homogeneity of layer thickness on a wafer 3.2.4. Device layout 3.3. Characterization 3.3.1. Electrical characterization 3.3.2. Sample illumination 3.3.3. Temperature dependent characterization 3.3.4. UPS 4. Simulations 5.1. Design of single carrier devices 5.1.1. General design requirements 5.1.2. Single carrier devices for space-charge limited current 5.1.3. Ohmic regime 5.1.4. Design of injection and extraction layers 5.2. Advanced evaluation of SCLC – potential mapping 5.2.1. Potential mapping by thickness variation 5.2.2. Further evaluation of the transport profile 5.2.3. Injection into and extraction from single carrier devices 5.2.4. Majority carrier approximation 5.3. Proof of principle: POEM on simulated data 5.3.1. Constant mobility 5.3.2. Field dependent mobility 5.3.3. Field and charge density activated mobility 5.3.4. Conclusion 5.4. Application: Transport characterization in organic semiconductors 5.4.1. Hole transport in ZnPc:C60 5.4.2. Hole transport in ZnPc:C60 – temperature variation 5.4.3. Hole transport in ZnPc:C60 – blend ratio variation 5.4.4. Hole transport in ZnPc:C70 5.4.5. Hole transport in neat ZnPc 5.4.6. Hole transport in F4-ZnPc:C60 5.4.7. Hole transport in DCV-5T-Me33:C60 5.4.8. Electron transport in ZnPc:C60 5.4.9. Electron transport in neat Bis-HFl-NTCDI 5.5. Summary and discussion of the results 5.5.1. Phthalocyanine:C60 blends 5.5.2. DCV-5T-Me33:C60 5.5.3. Conclusion 6. Organic solar cell characteristics: the influence of temperature 6.1. ZnPc:C60 solar cells 6.1.1. Temperature variation 6.1.2. Illumination intensity variation 6.2. Voc in flat and bulk heterojunction organic solar cells 6.2.1. Qualitative difference in Voc(I, T) 6.2.2. Interpretation of Voc(I, T) 6.3. BHJ stoichiometry variation 6.3.1. Voc upon variation of stoichiometry and contact layer 6.3.2. V0 upon stoichiometry variation 6.3.3. Low donor content stoichiometry 6.3.4. Conclusion from stoichiometry variation 6.4. Transport material variation 6.4.1. HTM variation 6.4.2. ETM variation 6.5. Donor:acceptor material variation 6.5.1. Donor variation 6.5.2. Acceptor variation 6.6. Conclusion 7. Summary and outlook 7.1. Summary 7.2. Outlook A. Appendix A.1. Energy pay-back of this thesis A.2. Tables and registers info:eu-repo/classification/ddc/530 ddc:530

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