Using Neutron Scattering to Elucidate the Thermodynamics of Conjugated Polymer:Fullerene Nanocomposites

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TECHNICAL SUMMARY:In this research program, neutron scattering will be utilized to determine the miscibility, phase diagram, phase-separated structure, interfacial characteristics, and vertical phase separation of conjugated polymer:fullerene thin film mixtures as a function of polymer and fullerene structure, thermal processing, surface structure, and solvent. Conjugated polymers (CPs) are chosen as the focus of this study as they are a promising class of materials for use in the conversion of solar energy to electricity. Most studies create the CP nanocomposite, measure its photovoltaic (PV) properties and then attempt to ascertain the relationship between the PV activity and morphology based on the observed morphological features. There exists no a priori control of the morphology. The proposed research program is designed to supply thermodynamic information that will provide methods to control the formation of resultant CP:fullerene morphologies based on known surface and/or interfacial energies, and polymer:fullerene, polymer:solvent, and fullerene:solvent interactions. This knowledge will then be used to tune the interactions in the system to fabricate active layers with targeted structures. Correlation of PV activity to the fully characterized targeted morphology represents a paradigm shift in how the nanoscale morphology and photovoltaic activity are optimized in organic photovoltaics. Neutron scattering and reflectivity will be the primary tools in this project, as the large difference in neutron scattering length density between protonated polymers and fullerenes singularly allows the efficient and thorough characterization of the assembly, interfacial structure, morphology, and composition of polymer:fullerene systems. Moreover, the experiments are designed such that the experimental techniques, analyses, and interpretations will be applicable to polymer nanocomposites regardless of polymer structure and nanoparticle size, shape or constitution. Therefore, the completion of this research program will provide methods to develop an understanding of the fundamental thermodynamics and physics that govern the formation and structure of a broad range of polymer nanocomposites.NON-TECHNICAL SUMMARY: The direct conversion of solar energy to electricity is a promising method to solve the grand challenges facing the energy needs of the US and the world at large, as only solar energy can deliver the required power in an environmentally clean (i.e., zero carbon emission) process. However, the conversion of solar energy is currently 5-10 times more expensive than other commonly used energy sources. Major innovations made possible through fundamental, transformative research will be required to improve the efficiency of solar energy conversion and reduce its cost. The proposed research program is designed to meet this need, as its completion will provide critical fundamental information that is needed to rationally design and produce the next generation of more efficient and cost-effective photovoltaic cells. Additionally, broader impacts of this work will come from the experience of public High School students when they spend a summer in a university research lab contributing to this project obtaining hands-on laboratory experience and exceptional preparation for college. Further impact will result from the completion of experiments at the neutron facilities at ORNL and NIST where the students participating in this project will acquire hands-on experience in a multi-user facility to insure the continued health of these National facilities. This project will also further develop the sustainable research infrastructure in Tennessee, an EPSCOR state, and will be implemented to ensure the participation of underrepresented groups in this research.