Browsing by Subject "organic semiconductor"
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Item Overcoming Energy Losses for Efficient Organic Solar Cells(2019-09) Zhang, TaoOrganic photovoltaic cells (OPVs) are promising for low-cost solar energy harvesting as they are light-weight, mechanically flexible and compatible with large area fabrication methods. The power conversion efficiency of OPVs, however, still lags behind inorganic counterparts, which limits their widespread commercial application. This dissertation is focused on understanding the various energy loss mechanisms in OPVs and devising general design rules for minimizing these losses. Among all parasitic energy loss pathways in OPVs, charge recombination is a major source of inefficiency in state-of-the-art systems. It can take place at the donor-acceptor interface when charges are bound by electrostatic forces as charge-transfer (CT) states, or in bulk active layers when free charge carriers are in transit to electrodes. To develop a detailed understanding of recombination loss mechanisms, a novel technique based on transient photovoltage has been developed, which allows quantitative elucidation of the dominant recombination mechanisms (CT state vs. free charge carrier losses) in OPVs. Using the information obtained from photovoltage measurement, strategies have been developed to suppress charge recombination losses in various OPV systems, including optimizing thin film morphology that facilitates charge transport for dipolar donor materials system and devising advanced device architectures that can stabilize CT states and maximize charge collection for metal phthalocyanine-fullerene materials system. Efforts have also been devoted to understanding and engineering the transport of CT states, a potential strategy that reduces recombination losses in OPVs. Based on the detailed understanding of charge recombination losses, a device-based methodology has been developed to probe exciton losses in OPVs. It is the first method capable of probing the intrinsic active material exciton diffusion length, equally applicable to both luminescent and dark materials. With this novel technique, exciton transport has been investigated in various excitonic semiconductor systems, including dark small molecules, polymers, inorganic semiconductor quantum dots. Moving forward, topics like exploring long-range CT state migration and understanding singlet fission mechanisms are pathways towards enhanced device efficiency.Item A Systematic Study of Acene Derivatives: Synthesis, Crystal Structures, Optical and Electrochemical Characterizations(2018-08) Zhang, ZhuoranThis Thesis describes a systematic study towards a series of polycyclic aromatic hydrocarbons (PAHs). The objectives are to develop convenient and efficient synthetic approaches to unique aromatic frameworks in order to probe their optical and electronic properties. The ultimate goal for these studies is to establish structure−property relationships in organic electronic materials. Chapter 1 of this Thesis briefly reviewed the history and current challenge in organic electronic materials. In Chapter 2, the study of a benchmark p-type organic semiconductive material – rubrene is described. Fluorination, a strategy often used to tune the crystallographic and opto-electronic properties, is involved. Synthetic routes to partially and fully fluorinated rubrene derivatives are developed. The effect of fluorination is examined by optical and electrochemical measurements, and more importantly, by the influence on the crystal packing motifs. Fluorine-based intermolecular interactions are found to play an important role in the assembly of rubrene crystals. Chapter 3 elaborates on the work in developing a new type of PAH – dibenzo[g,s]rubicene, the structure of which contains five-membered rings that are embedded in a large conjugated core structure. The synthesis takes advantage of dehydro-Diels−Alder reactions as a powerful and highly efficient method to access the polycyclic conjugated system. The functionalized rubicene derivatives are then characterized by optical, electrochemical and computational methods. This work contributes to discovery of potential candidates for innovative n-type organic semiconductors. In chapter 4, synthetic studies toward a carbon nanobelt – [12]cyclacene are performed. Though previous reports have made significant achievements in the construction of a macrocyclic framework, however, late-stage modification to install the fully conjugated structure is proved to be challenging. In this work, a new strategy involving a thermally-driven cheletropic rearrangement is proposed to address the late-stage modification issue. In the effort towards the macrocycle synthesis, an advanced intermediate has been successfully obtained in a stereoselective manner via multiple stereoselective Diels−Alder reactions. The selectivity is attributed to the careful design of precursors, which afforded the desired products through a sterically favored reaction pathway. A detailed examination of the proposed synthetic route as well as the proposal of a more convenient route are illustrated in the end.Item Time-of-Flight Investigation of Charge Carrier Mobilities in Oligoacene Single Crystals(2017-08) Lidberg, RussellOrganic semiconductors remain an active area of research due to their unique mechanical and opto-electronic properties. The charge transport properties of organic semiconductors are dependent on their molecular packing structures. A fundamental understanding of the charge transport and device physics on a microscopic scale remains a central focus of discussion. Models and theories have been based on the understanding derived from inorganic systems, but these tend not to hold for organic semiconductors. Single crystals of small conjugated oligoacenes, with high chemical purity and molecular structural order, can be model systems in the study of the relationship between molecular packing and carrier charge transport. The ability to probe intrinsic charge transport, not influenced by environmental factors or measurement techniques, plays a fundamental role in gaining a deeper understanding of the factors affecting charge transport. Time of flight (TOF) is an experimental technique used for charge carrier mobility studies that minimizes the external factors affecting charge transport. TOF also has the potential to study both bulk (vertical) and surface (lateral) charge carrier transport in organic semiconductors. This work reports the charge carrier mobility in single crystals of tetracene and rubrene using vertical and lateral field TOF (LFTOF). TOF instrumentation was designed and constructed. Room temperature vertical TOF hole mobility results in the c-direction for tetracene single crystals were acquired as a function of electric field (µc ≈ 1.3 cm2/Vs at 296 K). Bulk TOF hole charge carrier in rubrene single crystal as a function of temperature and electric field were acquired with an average value of 0.29 cm2/Vs at 296 K increasing to 0.70 cm2/Vs at 180 K and demonstrated an inverse power law temperature dependence, ‘band-like’ transport, in the c-axis direction. The use of LFTOF to study transport on the surface of single crystal organic semiconductors was demonstrated. LFTOF hole mobilities of 0.8 cm2/Vs at 296 K were in the range of reported field effect transistor mobility results. An overview of organic semiconductors and traditional transport models along with emerging transport models for organic semiconductors is presented.Item Understanding and Engineering Molecular Order in Organic Semiconductors(2017-08) Fielitz, ThomasOrganic semiconductors often exist in disordered material phases which have sub-optimal optical and electrical properties. Bringing some degree of order to these materials with crystals and even oriented amorphous phases has been shown to be fruitful for many applications, but is challenging to achieve. This is largely because of the variability between different materials and poorly understood dynamics in device-relevant thin films. This thesis describes progress towards understanding and tuning crystallization and ordering in organic thin films to realize enhancement in parameters relevant to organic optoelectronic devices. In particular, this thesis demonstrates that in thin-film crystallization processes, the crystal structure, crystal shape, and growth type can be controlled most effectively with film thickness, temperature, and strategic incorporation of secondary additives within the film. These variables change the rate at which crystals grow as well as the crystal shape during growth by altering the ability of molecules to attach and conform to the growing crystal front. When films are heated to bring about these processes through increased molecular mobility, secondary processes may occur to transform the microscopic film morphology through the addition, subtraction, and long-range motion of material. This motion can be connected to substrate-film interactions and the material phase of the starting film. Material interactions within the film bulk can kinetically trap molecular conformations, with the extent of this trapping depending on interaction type and deposition conditions. These properties can be further exploited to produce useful and spontaneous structures within a thin film. Ultimately, the desired result of ordering an organic semiconductor is to produce more efficient and stable structures for devices. This is demonstrated here through engineering the motion of excited states with crystallization, then applying such techniques to different organic solar cell geometries to study how different crystallization methods affect device properties. First, the mobility of excited states in boron subphthalocyanine chloride (SubPc), an archetypal organic solar cell molecule, is shown to increase upon crystallization with rigorous calculations explaining the origin. Such an increase provides motivation to study the effects of induced crystallization via homoepitaxial growth using the well-studied transistor material rubrene in a solar cell geometry. This serves as a platform by which to generalize the study to mixed epitaxial growth in a material which is not intrinsically crystalline, showing marked morphological and electronic changes with changes in mixture composition. This is ultimately applied to heteroepitaxial growth of SubPc, which does not crystallize when deposited onto a template at ambient temperature. These projects explore crystallization techniques as a solution to improve the performance of organic solar cells, resulting in an improved fundamental understanding of such processes and avenues for future progress.