This collaborative research project is to study the nucleation and growth of small-molecule crystalline semiconductors, to discern ways to precisely control crystal grain size, surface coverage, and donor/acceptor interfaces, with the ultimate goal to realize low-cost, highly efficient solar cells. High-purity, small-molecule organic materials promise significant advantages in photovoltaic performance. By using soluble small-molecule semiconductors, the creation of appropriate thin-film morphologies is expected to provide superior charge transport properties, potentially achieving high performance of photovoltaic devices. The project requires collaboration of mathematicians for modeling nucleation, crystal growth and phase separation of the blended semiconductors; chemists for the design, synthesis and tuning of appropriate donor and acceptor semiconductors; materials researchers for the analysis of films; and engineers for construction and evaluation of solar cells. The team will work synergistically to understand the mechanisms of grain growth, tailor donor and acceptor to maximize voltage and current, and improve surface treatments and deposition methods to allow the formation of large-area materials and devices. NON-TECHNICAL SUMMARY: An understanding of nucleation and growth of soluble small-molecule semiconductors is expected to have significant impact in an array of endeavors across the spectrum of electronics, including solar cells, solid-state lighting, flexible displays and radio-frequency identification tags. To maximize the training impact of this research project, the participating research groups at Kentucky and Princeton exchange researchers to enhance the cross-disciplinary training of participating students. This exchange introduces engineering researchers to organic synthesis, and allows mathematics graduate students to learn film growth and device fabrication techniques. The PIs plan to run an annual summer camp, to immerse all participants in a joint review of the project. The scientists will also continue outreach to local Schools and mentor high school students in research projects. This project is co-funded by the Divisions of Materials Research, Chemistry, and Mathematical Sciences.
Summary: Funding for this project supported a collaboration between researchers in the Chemical Engineeirng Department at Princeton University and in the Chemistry and Mathematics Departments at the University of Kentucky to understand and control the crystallization of molecular semiconductors for thin-film transistor and solar cell applications. By judiciously functionalizing electrically active molecules with bulky side groups, we can retard large-scale crystallization as these compounds are deposited as thin films. Subsequent thermal annealing or exposures to solvent vapors then allow controlled crystallization of these films. By decoupling film formation and structure development, we have been able to elucidate how structural heterogeneities - spanning many length scales - influece macroscopic charge transport. Such processing-structure-function relationships have informed the design and synthesis of new electrically-active compounds that, when incorporated into functional devices, exhibit improved device performance. Intellectual Merit: During this project, we elucidated how charge transport in molecular semiconductors is impacted by differences in molecular conformations; intermolecular packing leading to different crystal polymorphs; both in- and out-of-plane molecular orientations; and the presence of grain boundaries. This understanding has led to new guidelines and design rules, with which our collaborators have used to synthesize new-generation materials with enhanced electrical properties. It is through such iterative, feed-back and feed-forward cycles that we have garnered the detailed processing-structure-properties relationships of solution processable, crystallizable molecular semiconductors, with which we have been able to construct robust and reliable devices. To quantitatively determine structure, my group has also written a software code that allows extract of lattice parameters of a crystal structure whose x-ray diffraction pattern is consistent with the experimental x-ray diffraction pattern. This program is especially helpful for determining structures of molecules whose single crystals are difficult to obtain. The program is being accessed at the Cornell High Energy Synchrotron Source and by several research groups nationally. It is also available for download on our research website. Broader Impacts: This project is highly interdisciplinary and vertically integrated. Students on this project were exposed to organic chemical synthesis, quantitative structural characterization, as well as device fabrication and testing. The myriad of experience has prepared the students well for a career in science. Funding for this project supported two graduate students and numerous undergraduate researchers. One of the graduate students has just started her independent program as an assistant professor at a research institution. The project also supported numerous frequent student visits between the two institutions. Students supported on this project participated in K-12 education outreach activities with local middle and high schools. I leveraged my election as a Young Global Leader - and hence access to the international platform of the World Economic Forum - to inform world leaders, government officials, CEOs of companies about recent progress in my field more specifically and scientific developments and breakthroughs in energy technologies.
