Non-Linear I-V Characteristics of Double Schottky Barriers and Polycrystalline Semiconductors

An attempt to determine theoretically the highly non-linear current-voltage (I-V) characteristics of polycrystalline semiconductors, such as ZnO-based varistors, is made from the electrical properties of individual grain boundaries under dc bias. The role played by the fluctuations of double Schottky barrier heights at grain interfaces on driving electrical breakdown phenomena of macroscopic samples is pointed out in terms of a binary mixture model. An alternative trial form for the double Schottky barrier height is introduced to reproduce the breakdown voltage as well as the high non-linear coefficient alpha, where I propto V^{alpha}. ------------- Copies upon request to: [email protected]: CM-ICTP/92/1

Shorter Exciton Lifetimes via an External Heavy-Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light-Emitting Diodes

Multiexcited‐state phenomena are believed to be the root cause of two exigent challenges in organic light‐emitting diodes; namely, efficiency roll‐off and degradation. The development of novel strategies to reduce exciton densities under heavy load is therefore highly desirable. Here, it is shown that triplet exciton lifetimes of thermally activated delayed‐fluorescence‐emitter molecules can be manipulated in the solid state by exploiting intermolecular interactions. The external heavy‐atom effect of brominated host molecules leads to increased spin–orbit coupling, which in turn enhances intersystem crossing rates in the guest molecule. Wave function overlap between the host and the guest is confirmed by combined molecular dynamics and density functional theory calculations. Shorter triplet exciton lifetimes are observed, while high photoluminescence quantum yields and essentially unaltered emission spectra are maintained. A change in the intersystem crossing rate ratio due to increased dielectric constants leads to almost 50% lower triplet exciton densities in the emissive layer in the steady state and results in an improved onset of the photoluminescence quantum yield roll‐off at high excitation densities. Efficient organic light‐emitting diodes with better roll‐off behavior based on these novel hosts are fabricated, demonstrating the suitability of this concept for real‐world applications.United States. Department of Energy (Grant DE‐FG02‐07ER46474

Turbocharged molecular discovery of OLED emitters: from high-throughput quantum simulation to highly efficient TADF devices

Discovering new OLED emitters requires many experiments to synthesize candidates and test performance in devices. Large scale computer simulation can greatly speed this search process but the problem remains challenging enough that brute force application of massive computing power is not enough to successfully identify novel structures. We report a successful High Throughput Virtual Screening study that leveraged a range of methods to optimize the search process. The generation of candidate structures was constrained to contain combinatorial explosion. Simulations were tuned to the specific problem and calibrated with experimental results. Experimentalists and theorists actively collaborated such that experimental feedback was regularly utilized to update and shape the computational search. Supervised machine learning methods prioritized candidate structures prior to quantum chemistry simulation to prevent wasting compute on likely poor performers. With this combination of techniques, each multiplying the strength of the search, this effort managed to navigate an area of molecular space and identify hundreds of promising OLED candidate structures. An experimentally validated selection of this set shows emitters with external quantum efficiencies as high as 22%

Triplet-sensitization by lead halide perovskite thin films for near-infrared-to-visible upconversion

Lead halide-based perovskite thin films have attracted great attention due to the explosive increase in perovskite solar cell efficiencies. The same optoelectronic properties that make perovskites ideal absorber materials in solar cells are also beneficial in other light-harvesting applications and make them prime candidates as triplet sensitizers in upconversion via triplet-triplet annihilation in rubrene. In this contribution, we take advantage of long carrier lifetimes and carrier diffusion lengths in perovskite thin films, their high absorption cross sections throughout the visible spectrum, as well as the strong spin-orbit coupling owing to the abundance of heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite thin films as the absorber layer and spin-mixer in inorganic/organic heterojunction upconversion devices allows us to forego the additional tunneling barrier owing from the passivating ligands required for colloidal sensitizers. Our bilayer device exhibits an upconversion efficiency in excess of 3% under 785 nm illumination

A comparative analysis of system dynamics and agent-based modelling for health care reimbursement systems

Zsfassung in dt. SpracheSystem Dynamics und agentenbasierte Modellbildung sind zwei Methoden zur Modellierung dynamischer Systeme. Beide basieren auf dem Konzept, dass einfache Regeln und Zusammenhänge komplexes dynamisches Verhalten hervorbringen können. Trotzdem gibt es wesentliche Unterschiede: Agentenbasierte Modelle werden beispielsweise als flexibler angesehen, haben aber im Normalfall einen höheren Rechenaufwand. Ein zentrales Ziel der vorliegenden Studie ist daher, die beiden Modellierungsansätze hinsichtlich ihrer Gemeinsamkeiten und Unterschiede zu vergleichen. Darüber hinaus soll eine Antwort darauf gegeben werden, unter welchen Bedingungen eine der Methoden zu bevorzugen ist oder wie sich die beiden Methoden gegenseitig ergänzen können. Diese Fragestellungen wurden dabei von Modellierungsprojekten des Autors zum Vergleich von Bezahlungssystemen für Ärztinnen und Ärzte im niedergelassenen Bereich des Gesundheitssystems motiviert. Ein weiteres Ziel dieser Arbeit ist daher herauszuarbeiten, wie agentenbasierte Modellierung und System Dynamics in diesem Forschungsfeld angewandt werden können. Als gemeinsame systemtheoretische Basis für beide Methoden wird das Konzept eines stochastischen dynamischen Systems eingeführt. Es deckt Ansätze mit stochastischen Elementen, wie etwa agentenbasierte Modellierung, ab, enthält aber auch deterministische dynamische Systeme als Spezialfall. Tatsächlich definiert ein System-Dynamics-Modell über die Äquivalenz zu Differentialgleichungssystemen ein dynamisches System. Für agentenbasierte Modelle wird eine formale Definition auf der Basis der Stochastic Discrete Event System Specification (DEVS) gegeben. Anschließend werden beide Methoden anhand verschiedener Aspekte wie ihrer Eignung für die Modellierung von Heterogenität, der Konsequenzen von Aggregierung und der Abbildung von Feedback vergleichend analysiert. Es wird gezeigt, wie das Konzept einer Rate in einem gewissen Sinn äquivalent sowohl in System Dynamics als auch in der agentenbasierten Modellbildung für die Beschreibung von Veränderung verwendet wird. Die vergleichenden Analysen lassen darauf schließen, dass sich beide Methoden gut ergänzen, wenn im Modellierungsprozess zuerst die wichtigsten dynamischen Zusammenhänge mit einem System-Dynamics-Modell untersucht werden. Dieses muss dabei Aspekte, die mit System Dynamics schwer abzubilden sind, vorerst nicht berücksichtigen, kann aber später über eine Transformation in ein äquivalentes agentenbasiertes Modell übergeführt werden. Der Ansatz wird in der Arbeit anhand eines Modells der Bezahlung von Ärztinnen und Ärzten demonstriert, mit dem der Einfluss unterschiedlicher Bezahlungssysteme sowohl auf den Behandlungsstil der Ärztinnen und Ärzte als auch auf die Gesundheit der Bevölkerung untersucht werden kann und das jeweils in einer Version für beide Methoden vorgestellt wird. Die hier vorgestellten Modellstrukturen können als Grundlage für zukünftige Forschung in diesem Anwendungsfeld dienen, und der vorgeschlagene Modellbildungsprozess bietet die Möglichkeit, die Stärken von System Dynamics und agentenbasierter Modellbildung mit geringem Zusatzaufwand zu vereinen.System dynamics and agent-based modelling are two methods for modelling dynamical systems. While both emphasize that complex dynamics can evolve from simple rules and relationships, they have also important differences. Agent-based models are considered to be more flexible, but normally have a higher computational demand than system dynamics models. The central goal of this study is therefore to compare both approaches and investigate their similarities and differences. Moreover, it tries to answer the question of under which conditions one of the two methods is preferable or how they could complement each other. The motivation for these research questions comes from the author's modelling work on the study of reimbursement systems for physicians in extramural health care. A further goal is therefore to investigate how agent-based modelling and system dynamics could be applied to this field of research. The thesis puts both methods on a common system-theoretic basis as it introduces the concept of a stochastic dynamical system, which is able to cover approaches that include randomness, such as agent-based modelling, but also contains deterministic dynamical systems (in particular system dynamics models) as a special case. It is then shown that system dynamics defines a dynamical system via its equivalence to differential equation systems. For agent-based modelling, a formal definition based on the Stochastic Discrete Event System Specification (STDEVS) is given. The thesis then presents a comparative analysis of the two methods regarding several aspects such as their suitability for modelling heterogeneity, the consequences of aggregation, and the representation of feedback. It is shown that the concept of a rate is used to describe change equivalently both in system dynamics and agent-based modelling. The thesis argues that the two methods can complement each other beneficially if the most important dynamical relationships of a problem are first analysed with a system dynamics model, which does not have to incorporate aspects such as heterogeneity that are hard to capture in the method. On the contrary, a transformation can later lead to an equivalent agent-based model for further development. The approach is demonstrated for a physician reimbursement model, which allows for a comparison of the dynamic impact of different reimbursement systems both on physicians' treatment style and on the population's health and which is presented in both a system dynamics and an agent-based version. Future research in this application area can build upon the model structure developed here, and the proposed modelling process has the ability to combine the strengths of the two methods with little additional effort.16

Excitonic spin engineering for solar cells and organic light-emitting diodes

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2020Cataloged from PDF of thesis.Includes bibliographical references (pages 133-147).The last two decades have seen renewed interest in molecular organic semiconductors. Since these materials support the formation of excitons, their behavior differs considerably from their inorganic counterparts. This gives rise to a variety of novel properties that can be exploited to create entirely new or improve existing optoelectronic devices. In this thesis, we explore excitonic concepts for improving both organic-light emitting diodes (OLEDs) and silicon solar cells. OLEDs are already commercially successful. However they still suffer from several major drawbacks. Multiexcited-state phenomena are believed to be the root cause of challenges like efficiency roll-off and degradation. The development of novel strategies to reduce exciton densities under heavy load is therefore highly desirable.In this thesis, it is shown that triplet exciton lifetimes of thermally activated delayed fluorescence (TADF) emitter molecules can be manipulated in the solid state by exploiting intermolecular interactions. The external heavy-atom effect of brominated host molecules leads to increased spin-orbit coupling, which in turn enhances intersystem crossing rates in the guest molecule. Shorter triplet exciton lifetimes are observed, while high photoluminescence quantum yields (PLQYs) are maintained and emission spectra are essentially unaltered. A change in the intersystem crossing rate ratio due to increased dielectric constants leads to almost 50% lower triplet exciton densities in the emissive layer in the steady state and results in an improved onset of the PLQY roll-off at high excitation densities. Efficient OLEDs with better roll-off behavior based on these novel hosts are fabricated, demonstrating the suitability of this concept for real-world applications.In addition, efficient and stable blue emitters for OLEDs are urgently needed for next-generation display and lighting applications. This thesis presents a tunable series of TADF emitter molecules. After pairing the iminodibenzyl donor with the triazine acceptor via a phenylene linker, dihedral angle tuning is employed to regulate the difference between the energy levels of singlet and triplet excited states. Enhanced reverse intersystem crossing rates are observed in response to increased methylation at the phenylene linker. PLQYs of up to 98% are achieved upon doping into a solid-state matrix. When incorporated in devices, the maximum external quantum efficiency is 28.3% for the emitter with the most favorable trade-off between singlet-triplet splitting and fluorescent oscillator strength.This result highlights the general applicability of dihedral angle tuning, a molecular design strategy that can be used to improve the performance of donor-acceptor type TADF emitters without significantly changing their emission spectra. In contrast, contemporary solar cell technologies are dominated by silicon, an inorganic semiconductor. But when absorbing photons, silicon (like other semiconductors) wastes energy in excess of its bandgap. Reducing these thermalization losses is possible by sensitizing the silicon solar cell using singlet fission, a carrier multiplication phenomenon that occurs only in organic semiconductors. In this process, two triplet excitons are generated from a singlet exciton. In tetracene, those triplet excitons are energetically matched to the silicon bandgap. Transferring triplet excitons to silicon creates additional electron-hole pairs, promising to increase cell efficiencies from the single-junction limit of 29% to as high as 35%.In this thesis we reduce the thickness of the protective hafnium oxynitride layer at the surface of a silicon solar cell to just eight angstroms, using electric-field-effect passivation to enable the efficient energy transfer of triplet excitons formed in tetracene. The maximum combined yield of the fission in tetracene and the energy transfer to silicon is around 133%. The processes at the interface are investigated using photoluminescent and magnetic field effect experiments, revealing the impact of different interlayer thicknesses. Finally, the thesis presents the first example of a singlet-fission-enhanced silicon solar cell, a breakthrough that establishes the potential of singlet exciton fission to increase the efficiencies of silicon solar cells and reduce the cost of the energy that they generate.by Markus Einzinger.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Scienc