Browsing by Subject "Excitons"

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    Exciton Dynamics in Alternative Solar Cell Materials: Polymers, Nanocrystals, and Small Molecules
    (2014-07) Pundsack, Thomas
    To keep fossil fuel usage in 2040 even with 2010 usage, 50% of global energy will need to come from alternative sources such as solar cells. While the photovoltaic market is currently dominated by crystalline silicon, there are many low-cost solar cell materials such as conjugated polymers, semiconductor nanocrystals, and organic small molecules which could compete with fossil fuels. To create cost-competitive devices, understanding the excited state dynamics of these materials is necessary.The first section of this thesis looks at aggregation in poly(3-hexylthiophene) (P3HT) which is commonly used in organic photovoltaics. The amount of aggregation in P3HT thin films was controlled by using a mixture of regioregular and regiorandom P3HT. Even with few aggregates present, excited states were found to transfer from amorphous to aggregate domains in <50 fs which could indicate efficient long-range energy transfer.To further study P3HT aggregation, a triblock consisting of two P3HT chains with a coil polymer between them was investigated. By changing solvents, aggregation was induced in a stable and reversible manner allowing for spectroscopic studies of P3HT aggregates in solution. The polarity of the solvent was adjusted, and no change in excited state dynamics was observed implying the excited state has little charge-transfer character.Next, the conduction band density of states for copper zinc tin sulfide nanocrystals (CZTS NCs) was measured using pump-probe spectroscopy and found to be in agreement with theoretical results. The density of states shifted and dilated for smaller NCs indicative of quantum confinement. The excited state lifetime was found to be short (<20 ps) and independent of NC size which could limit the efficiency of CZTS photovoltaic devices.Finally, triplet-triplet annihilation (TTA) was studied in platinum octaethylporphyrin (PtOEP) thin films. By analyzing pump-probe spectra, the product of TTA in PtOEP thin films was assigned to a long-lived metal-centered state. To elucidate the mechanism of TTA, the annihilation dynamics were modeled using second order kinetics as well as Förster and Dexter energy transfer. Dexter energy transfer provided the best fits and the most reasonable fitting parameters.
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    Luminescent Probes of Emergent Physics from Organic Semiconductor Interfaces
    (2022-12) Concannon, Nolan
    To prevent the most harmful effects of the present climate crisis, development ofhigher performance energy conversion devices is needed to accelerate the adoption of renewable energy and energy efficiency technologies. Organic semiconductor materials have demonstrated exciting efficiency gains in a variety of emerging and in-production devices. These materials exhibit a variety of emergent material and device physics, requiring additional research to understand and design next-generation energy conversion technologies. Thin films of organic semiconductors, common in large-area optoelectronics such as consumer displays, present rich photophysics due to forming room-temperature stable excitons, unlike silicon or III-V semiconductors. A plethora of emergent phenomena of excitons at organic semiconductor interfaces requires a detailed understanding of such processes to optimally design devices such as energy-efficient lighting, flexible or transparent solar cells, photodetectors and displays. This dissertation focuses on investigating the novel optical physics of excited states at organic donor-acceptor interfaces through emission spectroscopy of organic mixtures and bilayer devices. In one study, exciplex diffusion is investigated in several donor-acceptor pairings toward an improved understanding of the mechanism of nanoscale energy transport in organic semiconductor mixtures. Additionally, the effect of electric field on exciplex emission spectra is studied to detail the effect of field on exciplex energy and electron-hole separation. Finally, preliminary data displaying the effect of binary dilution on exciplex energy in a two-component mixture is presented. All together, these findings present new insights into the behavior of key device properties such as exciton diffusion length and excited state energies to aid further study of device performance.