Browsing by Subject "exciton"
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Item Exciton Dynamics in Organic Semiconducting Materials(2017-04) Goff, PhilipWorld energy consumption is projected to rise dramatically over the next three decades. Presently, over 80% of worldwide energy consumption is derived from nonrenewable sources. In order to meet these demands, increased utilization of solar energy will be required. Organic semiconducting materials are an attractive alternative to the traditional crystalline silicon devices which at present dominate the photovoltaic market. They possess a number of material and physical processes which may ultimately prove for photogeneration of electricity at a cost competitive with conventional nonrenewable fuel sources. However, the power conversion efficiencies of these devices at present limits their widespread adoption. In order to improve their efficiencies, it is important to understand the relationship between chemical and physical characteristics of the molcules with the exhibited excited state photophysics. In this dissertation, the excited state properties of various organic semiconducting materials, such as polymers and small molecules, will be examined. First, in Chapter 3 a series of statistical donor/acceptor copolymers were generated where the monomer unit composition was tuned in order to adjust the absorption properties of the polymer. Ultrafast pump-probe spectroscopy was employed to characterize the effect that changing polymer composition has on the exciton lifetime within these materials. It was found that by tuning the composition of the monomer units, the lifetime could be extended nearly 30 times over that of either neat donor or neat acceptor monomer units. Moreover, the lifetime reached a maximum at a ratio of approximately 1:4 donor to acceptor monomer units, suggesting that fine-tuning the ratio of the two may provide enhancements in OPV performance. In Chapters 4 and 5, ultrafast pump-probe spectroscopy was used to characterize exciton transport in thin films of Subphthalocyanine and Subnaphthalocyanine. Adjusting the film composition was found to have a substantial influence on the exciton diffusion length in the films. Importantly, exciton-annihilation induced heating of the films resulted in the manifestation of thermal signatures in the transient spectra. These signatures were not previously well appreciated in literature as manifesting on a sub-nanosecond timescale, and have been potentially erroneously assigned as electronic signatures from excited state species. A method is proposed to isolate these thermal signatures from the excitonic signatures, yielding accurate exciton decay dynamics. Finally, the excited state dynamics of a series of novel boron dipyrromethane derivatives will be investigated in Chapter 6. Particular attention will be made as to whether there is a potential for these materials to spontaneously form a spontaneously self-assembled supramolecular complex with fullerene in solution. The formation of such complexes is of considerable interest, however the results herein suggest that many of the previously reported complexes may actually be misinterpretation of photoluminescence extinguishing due to inner filter effects as opposed to quenching arising from energy and charge transfer.Item Kinetics of degradation and exciton quenching in organic light-emitting devices(2020-06) Myers-Bangsund, JohnOrganic light-emitting devices (OLEDs) are next-generation, thin film light sources which have significant advantages over conventional display and lighting technologies, including: high contrast, high power efficiency, tunable color, and compatibility with low-cost fabrication techniques on flexible substrates. These attributes have driven the rapid commercialization of OLEDs in mobile phone displays over the last two decades, but OLEDs have yet to gain traction in high brightness applications such as lighting, automotive head lights, imaging light sources, and lasers. In part, this is because OLED performance at high brightness tends to be limited by two processes: reversible efficiency roll-off (where efficiency decreases under high current) and irreversible degradation. Both of these processes are tied to excited state (or exciton) quenching reactions which dissipate energy non-radiatively and can drive chemical reactions within the active layers of an OLED. In this work, we seek to better understand these limiting phenomena so that they can ultimately be overcome. Our overarching strategy toward this end is to develop and apply combined electrical and optical analysis techniques to decouple efficiency loss pathways. Comparing measurements of electro- and photoluminescence (EL and PL), we unraveled how OLED lifetime depends on the spatial distribution of excited states and charge carriers. We found that multiple kinetic pathways determine the degradation rate, where the emitter radiative efficiency is deteriorated by exciton reactions in the emissive layer, while charge trapping and leakage are aggravated by reactions with charges at interfaces or outside the emissive layer. These methods have allowed us to identify design principles for mixed host emissive layers and molecular screening criteria to accelerate materials development. These techniques also revealed a surprising result which contradicted conventional models: luminescence quenching can occur even at low electrical bias levels, reducing peak efficiency by more than 20% in some cases. We connect this effect to preferred molecular orientation and net polarization of organic films, and we identify strategies to eliminate this loss pathway. Together, the findings in this work highlight the advantages and limitations of combined electrical and optical characterization of OLEDs. Wider application of these approaches may help researchers more quickly develop new materials and design strategies for efficient and durable OLEDs.