Optical, Electrical and Magnetic Studies of pi-Conjugated Polymer/Organic Acceptor Blends for Photovoltaic Applications

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Technical: This project aims for greater understanding of electronic states, charge transport, and spin phenomena in blends of PCP (p-conjugated polymers) with molecular acceptors that are used in bulk-heterojunction OPV (organic photovoltaic) devices. Processes/phenomena to be investigated in blends (films and devices) include: ultrafast exciton dissociation, charge generation mechanism, below-gap response, formation of charge transfer complex (CTC) of PCP chains and acceptor molecules, geminate recombination, spin-dependent transport and recombination, and morphology-dependent carrier mobility. These studies will be conducted in PCP-fullerene blends of different concentrations that show high OPV efficiency, as well as in novel PCP-acceptor blends that have not been used before. Outcomes are expected to contribute to basic understanding of physical processes that govern photovoltaic response in OPV devices, and have potential to increase their solar power conversion efficiency beyond the 6% record of the present time. The approach utilizes optical, electrical and magnetic experimental techniques: (i) ultrafast exciton and polaron dynamics in the blends will be investigated using a pump/probe photomodulation (PM) technique in the spectral range of 0.1 to 2.5 eV; (ii) the excitation in (i) is to vary from above to below-gap photon energy to explore the possibility of polaron generation by processes other than traditional charge transfer from the polymer chain onto the acceptor molecule; (iii) the possible formation of a CTC state in the blends will be studied by IR-photoluminescence, below-gap absorption, IR-electroabsorption, and photogenerated polaron action spectra; (iv) electrical admittance spectroscopy for obtaining the carrier mobilities; (v) magnetoresistance measurements of OPV devices for studying polaron recombination; and (vi) dynamics of optically-detected magnetic-resonance of spin ½ polarons for studying spin-dependent recombination, and spin-lattice relaxation rate in OPV devices. Non-technical: The project addresses basic research issues in a topical area of electronic/photonic materials science with high technological relevance. These studies may lead to OPV (organic photovoltaic) devices with enhanced solar power conversion efficiency, and deepen our understanding of the PCP-acceptor blends. In addition, the integration of experimental efforts, including polymer and small molecule synthesis, optics, magneto-transport, modeling, and device fabrication, processing and testing, will serve to educate graduate and undergraduate students, and a postdoctoral associate who will be involved in this interdisciplinary research project. A graduate course on ?Organic optoelectronics; basic studies and device applications? is also planned. An outreach program includes research participation of high school students and their teacher mentors during the summer, as well as engineering demonstrations for illustrating organic light emitting diodes and OPV devices to the public.