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.
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.
This project has dealt with organic semiconductors used as active layers in photovoltaic organic solar cells for harnessing the solar energy radiation that falls on Earth. This type of solar cell converts electromagnetic radiation into current density that can be used as a source of electrical power. The active layer is a blend of organic donors (usually polymers) and acceptors (usually fullerene molecules). Upon photon absorption a neutral excitation is generated, and separation into electron and hole charge carriers is necessary in order to form photocarriers. The dissociation process of the neutral excitation is not understood, and thus has been thoughly studied during the three years project. We found that a charge transfer complex (CTC) is formed in the boundary between the donor and acceptor grains that actively participates in the dissociation process. For this we have employed a myriad of linear and nonlinear optical techniques including ultrafast spectroscopy, steady state spectroscopy, electro-absorption, and optically detected magnetic resonance. We worked closely with a company that deals with organic photovoltaic application, namely Plextronics Ltd, Pittsburgh, PA. We studied their newly synthesized donor and acceptor materials, and provided feedback to the synthetic chemists. In addition we provided insight on the photogeneration and loss mechanism in the fabricated solar cells based on the synthesized materials. Four graduate students have participated in the reserach investigations; two of them graduated with PhD diploma, and moved on to assume postdoc positions elsewhere (LANL, and Cambridge UK). We have also collaborated with physics professors at the Technion in Israel and University of Arizona in Tucson, respectively. We published 22 journal articles in high impact periodicals, among them Nature Physics, Nature Photonics and Physical Review Letters; and also edited two books on Organic Lasers and Organic Spintronics, respectively. In addition we submitted two patents based on our investigations.
