Optimizing Conjugated Polymer Films for Desired Applications: Controlling Interchain, Interfacial and Intensity-Dependent Interactions

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9971842SchwartzThis research describes a series of experiments designed to improve our understanding of conjugated polymers in three areas which are key to exploiting these materials to their maximum potential in practical devices: determining the role of interactions between polymer chains, understanding the interactions between polymer excited states, and exploring the chemical nature of the metal/polymer interface. First, interchain interactions will be varied by casting conjugated polymer films from different solvents and/or annealing. Ultrafast laser studies on these samples will monitor the relationship between single-chain excited states (through their emission) and interchain excited states (through their absorption), and will correlate this relationship with how the chains are packed as established by X-ray diffraction and/or atomic force microscopy. Isolated and aligned polymer chains encapsulated in the nanoscale channels of a porous silica glass will also be studied. Second, experiments that systematically vary both the excitation geometry while monitoring the time evolution of the excited state absorption and stimulated emission will provide information about the interactions between excited states in conjugated polymers. Finally, surface-specific non-linear optical spectroscopies such as second harmonic and sum frequency generation will directly probe the chemical structure of the polymer/metal interface, and the results will be correlated with the behavior of such interfaces in practical polymer-based optoelectronic devices such as light-emitting diodes.Optoelectronic devices can be created from conjugated polymers due to their nature as plastic semiconductors. Unlike conventional inorganic semiconductors, conjugated polymers are lightweight and flexible, can be easily fabricated into desired shapes, and can be synthesized to have band gaps in any region of the visible spectrum. The ease of processing and desirable mechanical properties of conjugated polymers offer enormous advantages compared to the technologies presently used in LED's, displays, and other applications that are widespread in society. For example, conjugated polymers allow the construction of large-area, flexible light-emitting panels, and multi-color displays can be produced directly by ink-jet printing. The proposed work is aimed at learning how to optimize these materials for desired applications in commercial devices or displays. The results of the proposed experiments should provide a prescription for controlling the interactions between polymer chains to enhance desired electrical or optical properties; the understanding of excitations at high intensity may open the possibility for producing plastic diode laser; and the detailed knowledge of the polymer/electrode interface obtained from the experiments could lead directly to improvements in device performance.