Modeling morphology evolution during solvent-based fabrication of organic solar cells

Solvent-based techniques usually involve preparing dilute blends of electron-donor and electron-acceptor materials dissolved in a volatile solvent. After some form of coating onto a substrate, the solvent evaporates. An initially homogeneous mixture separates into electron-acceptor rich and electron-donor rich regions as the solvent evaporates. Depending on the specifics of the blend and processing conditions different morphologies are typically formed. Experimental evidence consistently confirms that the morphology critically affects device performance. A computational framework that can predict morphology evolution can significantly augment experimental analysis. Such a framework will also allow high throughput analysis of the large phase space of processing parameters, thus yielding insight into the process-structure-property relationships. In this paper, we formulate a computational framework to predict evolution of morphology during solvent-based fabrication of organic thin films. This is accomplished by developing a phase field-based model of evaporation-induced and substrate-induced phase-separation in ternary systems. This formulation allows all the important physical phenomena affecting morphology evolution during fabrication to be naturally incorporated. We discuss the various numerical and computational challenges associated with a three dimensional, finite-element based, massively parallel implementation of this framework. This formulation allows, for the first time, to model 3D morphology evolution over large time spans on device scale domains. We illustrate this framework by investigating and quantifying the effect of various process and system variables on morphology evolution. We explore ways to control the morphology evolution by investigating different evaporation rates, blend ratios and interaction parameters between components

Thin Films Photovoltaics

Thin film photovoltaic-based solar modules produce power at a low cost per watt. They are ideal candidates for large-scale solar farms as well as building-integrated photovoltaic applications. They can generate consistent power, not only at elevated temperatures but also on cloudy, overcast days and at low sun angles.Thin film photovoltaics are second-generation solar cells produced by depositing one or more thin layers, or thin films, of photosensitive material on a suitable substrate such as glass, polymer, or metal. Thin film solar cells are based on various materials such as cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous thin film silicon (a-Si, TF-Si) are commercially used in several conventional and advanced technologies

Computational methods to engineer process-structure-property relationships in organic electronics: The case of organic photovoltaics

Ever since the Nobel prize winning work by Heeger and his colleagues, organic electronics enjoyed increasing attention from researchers all over the world. While there is a large potential for organic electronics in areas of transistors, solar cells, diodes, flexible displays, RFIDs, smart textiles, smart tattoos, artificial skin, bio-electronics, medical devices and many more, there have been very few applications that reached the market. Organic photovoltaics especially can utilize large market of untapped solar power -- portable and affordable solar conversion devices. While there are several reasons for their unavailability, a major one is the challenge of controlling device morphology at several scales, simultaneously. The morphology is intricately related to the processing of the device and strongly influences performance. Added to this is the unending development of new polymeric materials in search of high power conversion efficiencies. Fully understanding this intricate relationship between materials, processing conditions and power conversion is highly resource and time intensive. The goal of this work is to provide tightly coupled computational routes to these expensive experiments, and demonstrate process control using in-silico experiments. This goal is achieved in multiple stages and is commonly called the process-structure-property loop in material science community. We leverage recent advances in high performance computing (HPC) and high throughput computing (HTC) towards this end. Two open-source software packages were developed: GRATE and PARyOpt. GRATE provides a means to reliably and repeatably quantify TEM images for identifying transport characteristics. It solves the problem of manually quantifying large number of large images with fine details. PARyOpt is a Gaussian process based optimization library that is especially useful for optimizing expensive phenomena. Both these are highly modular and designed to be easily integrated with existing software. It is anticipated that the organic electronics community will use these tools to accelerate discovery and development of new-age devices

Light-to-Energy Conversion in Organic Solar Cells and Molecular Motors

In recent decades, remarkable research efforts have been dedicated to developing artificial material systems that convert light into other desirable forms of energy, be they electrical, mechanical, or chemical. This thesis focuses on the first two classes of such systems: organic solar cells and light-driven molecular motors.Organic solar cells based on organic materials convert sunlight into electricity. They hold great potential because of their unique properties, such as flexibility, semitransparency and low weight. However, understanding the light-to-electricity conversion process is crucial to improve their performance further. This is the focus of the first part of the thesis.Light-driven molecular motors exhibiting autonomous motion in a unidirectional manner have shown great potential in materials science, catalyst and biology. However, they typically operate under ultraviolet–blue light, which may be detrimental to the surrounding (bio)environment. In addition, for practical applications, it is highly desirable to track the motor’s location with photoluminescence microscopy. In the second part ofthis thesis, the functionality of several molecular motors operating under red or nearinfrared light and capable of photoluminescence is demonstrated using ultrafast optical spectroscopy.Overall, this thesis provides new insight into light-to-electricity conversion in highperformance organic solar cells and opens up exciting prospects for using artificial molecular motors in biological settings and soft material

Optical, Excitonic, and Electronic Properties of CH3NH3PbI3 Thin Films and Their Application in Photovoltaics

In the past two years, the highest power conversion efficiency of perovskite absorber (PA)–based photovoltaics has been 20.2%. The PA can be fabricated on flat substrates (for example, ZnO, TiO2, and PEDOT:PSS) using solution processes, which have a low-cost advantage in terms of industry production. In this report, the recent advances of PA-based photovoltaics will be mentioned. Then, the optoelectronic properties of PA, material fabrication, and photovoltaic performance will be discussed. On the other hand, we used scanning electron microscopy, two-dimensional X-ray diffractometer, and photoluminescence spectroscopy to investigate the fundamental properties of CH3NH3PbI3 thin films fabricated with and without toluene washing treatment, which provides an assessment of the development potential of PA-based photovoltaics

Characterisation and optimisation of hybrid polymer/metal oxide photovoltaic devices

Imperial Users onl

Photoluminescence of 2D and quasi-2D perovskites

In den letzten Jahren wurden Perowskite, insbesondere 2D- und Quasi-2D-Perowskite, in Bereichen wie Photovoltaik, Leuchtdioden und Lasertechnik intensiv erforscht. Diese Arbeit soll einen Einblick in die grundlegende Photophysik der angeregten Zustände dieser Materialien nach einer Anregung durch Laserlicht geben und als Anleitung zur Interpretation der emittierten Photolumineszenzspektren dienen. Zunächst werden reine 2D-Perowskite in der Ruddlesden-Popper-Phase unter Verwendung von Phenethylamin (PEA) als Spacermolekül ($PEA_{2}PbI_{4}$) und der Dion-Jacobson-Phase unter Verwendung von 1,4-Phenylendimethanamin bei verschiedenen Temperaturen und mit einer gepulsten und einer Dauerstrich-Laserquelle bei unterschiedlichen Anregungsintensitäten untersucht. Während bei beiden Materialien die exzitonische Emission bei allen gemessenen Temperaturen dominiert, werden für Temperaturen unterhalb von 140 K zusätzliche Peaks in den Emissionsspektren beobachtet. Im Ruddlesden-Popper-Material werden bei 5 K zwei Peaks mit einem Energieabstand von 40,3 meV beobachtet, wobei der Peak mit der höheren Energie den freien Exzitonen und der mit der niedrigeren Energie den gebundenen Exzitonen (gebunden durch ein Fehlstelle) zugeschrieben wird. Bei hohen Anregungsintensitäten dominiert die Emission der gebundenen Exzitonen die Gesamtemission. Dies ist darauf zurückzuführen, dass die Exziton-Exziton-Annihilierung die Emission der freien Exzitonen stark unterdrückt, während die Emission der gebundenen Exzitonen kaum beeinträchtigt wird. Dieser Effekt ist im Dion-Jacobson-Perowskit weniger ausgeprägt. In beiden Materialien kann die Emission von $PbI_{2}$-Einschlüssen bei niedrigen Temperaturen beobachtet werden. Anschließend werden die Emissionseigenschaften verschiedener Quasi-2D-Materialien untersucht. Im Gegensatz zu 2D-Perowskiten, deren angeregte Zustände nur aus Exzitonen bestehen, zeigen Quasi-2D-Perowskite Emission von Exzitonen, freien Ladungsträgern oder sogar von beiden. Durch Analyse der zeitaufgelösten Photolumineszenz, der Quanteneffizienz und der anfänglichen Photonenemissionsdichte kann die Mischung der angeregten Zustände bestimmt werden. Die Emissionseffizienz von freien Ladungsträgern nimmt mit der Anregunsintensität zu, während die von Exzitonen abnimmt. Es wird ein einfaches Modell von zwei nicht wechselwirkenden Populationen von Exzitonen und Ladungsträgern in getrennten Teilvolumina des Films vorgestellt, das alle Beobachtungen in dieser Arbeit beschreibt. Die Emissionscharakteristiken hängen stark von dem in der Perowskitlösung verwendeten Spacer-Molekül und dessen Konzentration ab. Hohe Konzentrationen von Butylamin führen zu 100 % Emission von Exzitonen, die auf 7 % Exzitonenemission und 93 % freie Ladungsträgeremission bei Perowskiten mit niedrigen Konzentrationen von 1-Naphthylmethylamin Spacermolekülen zurückgeht. Schließlich wird auf der Grundlage dieser Beobachtungen eine Messtechnik eingeführt, bei der zwei Anregungspulse mit steuerbarer Verzögerung und ein USB-Spektrometer, das die zeitintegrierten Spektren aufzeichnet, verwendet werden, um die Geschwindigkeitskonstanten sowie den Anteil der Emission von Exzitonen und freien Ladungsträgern zu ermitteln. Diese Methode ist wesentlich schneller und kostengünstiger als herkömmliche Zeitaufgelöste-Messmethoden der Photolumineszenz. Die Robustheit und Schnelligkeit dieser Methode ist potenziell sehr interessant für den Einsatz in einer Fertigungslinie für photovoltaische 3D-Perowskit-Absorberschichten und ebnet so den Weg von der Forschung zur Anwendung

Single-source pulsed laser deposition of hybrid halide perovskites for solar cells

The world is rapidly shifting towards renewable and sustainable energy as we face concerns about climate change. In such times, the abundant energy from the sun is crucial in aiding this transition. The devices responsible for the conversion of solar energy into electricity are termed solar cells. Nowadays, the well-established photovoltaic (PV) industry belongs to silicon PV. Nevertheless, new materials are being researched to complement silicon PV technologies. Metal halide perovskites (MHPs) are one of the emerging solar cell technologies that have fascinated researchers due to their versatility in terms of both composition and fabrication methods, delivering power conversation efficiencies in pair-to-crystalline silicon cells, making them one of the best candidates for the next generation of photovoltaics. The construction of these emerging solar cell devices involves heterostructures containing an absorber material sandwiched between carrier-selective layers and electrodes. Challenges remain regarding upscalable fabrication methods compatible with integrating complex perovskite materials within heterostructures. Therefore, one of the main challenges addressed by the research within this PhD is the demonstration of an alternative physical vapor deposition (PVD) method known as pulsed laser deposition (PLD) for the growth of MHPs. The main motivation to employ PLD for growing MHPs is its unique capability to transfer highly complex chemical compositions from a single-source target to the substrate or a partial solar cell stack. In this thesis, we have demonstrated the utility of PLD as an alternative PVD method for depositing complex MHP thin films with precise stoichiometry. Notably, this method exhibits compatibility with heterostructures and potential for scalability. This compatibility can be further enhanced by improving the hardware configuration of the PLD for wafer-scale area coatings and superior deposition rates. Additionally, we demonstrate PLD as an appealing deposition method for studying the growth of low- and wide-bandgap MHP. These materials pose challenges with alternative deposition methods due to constraints regarding the solubility of different precursors, varying solvent evaporation rates, or difficulties in reproducibility arising from the need to control four or more sublimation sources with significant differences in volatilities. <br/