This CAREER award is co-funded by the Electronic Materials Program in the Division of Materials Research and the Experimental Physical Chemistry Program in the Chemistry Division.
The project is to study charge carrier dynamics in organic semiconductors on the macroscopic and microscopic levels with the final goal of developing methodology for utilizing charge-transfer processes in single-molecule optoelectronic devices. Mechanisms of intermolecular charge transfer responsible for conductivity will be investigated using a combination of photoconductivity techniques: time-resolved luminescence, single-molecule fluorescence microscopy, and electric force microscopy. The project is to study functionalized polyacenes and anthradithiophenes. It aims (i) to establish mechanisms of charge photogeneration, transport and trapping and to model the photoconductive performance; (ii) to observe and characterize charge trapping and detrapping processes on the single molecule level; (iii) to measure charge carrier mobility on the microscopic scales and its dependence on local environment; and (iv) to design single-molecule devices based on charge trapping properties.
The project addresses basic research issues in a topical area of materials science with high technological relevance, and is expected to provide new scientific understanding of organic semiconductors, which have potential applications in thin-film transistors, light emitting diodes, solar cells, and lasers. The research component of the project is integrated with the educational component that will aim to achieve four main goals: (i) to provide learning and research experience to undergraduate and high-school students through their involvement in the research group activities; (ii) to introduce research context in the curriculum, to develop a novel course in advanced optical materials and a web-based tutorial illustrating physics and applications of organic optical materials; (iii) to provide exposure to modern science and technology for general public; and (iv) to attract women to careers in physics.
The scientific goal of this proposal was to understand charge carrier dynamics in high-performance organic semiconductors on the macroscopic and microscopic level and to develop methodology for utilizing charge-transfer processes in single-molecule optoelectronic devices. Organic semiconductors are of interest due to their low cost, easy fabrication, and tunable properties for applications in thin-film transistors, light-emitting diodes, solar cells, lasers, and many others. Since most of these applications rely on the conductive and photoconductive properties of the materials, it is critical to understand the physical picture of the charge carrier dynamics. In this project, the mechanisms of intermolecular charge transfer responsible for (photo)conductivity were investigated using combinations of time-resolved photoconductivity techniques with time-resolved luminescence and single-molecule fluorescence microscopy, as well as with numerical modeling. Intellectual Merit. During this CAREER project, the PI established a major research program in advanced optical materials and nanoscience at Oregon State University (OSU). The PI's group explored several classes of novel organic semiconductors, namely functionalized thiophene and acene derivatives, and established relationships between properties of individual molecules and optoelectronic response of thin-film devices composed of these molecules. In organic thin-film optoelectronic devices, we observed fast charge carrier photogeneration, high photoconductive gain, and photoluminescence that is dependent upon the degree of molecular disorder in the film. In order to understand photoexcited charge carrier dynamics, we developed a model that considers multiple pathways of charge photogeneration, charge trapping, detrapping, and three recombination pathways. The model enabled us to quantify contributions of each process to photoconductivity and to manipulate various contributions by creating composites of molecules of different types (such as donor and acceptor molecules). We established the conditions under which energy or charge transfer dominate in donor-acceptor composites (Figure 1) and determined that the efficiency of the charge transfer is dependent not only on the relative energies of the molecules but also on their packing at the donor-acceptor interface (Figure 2). The latter can be varied by varying the size of the side groups of the molecules and chosen to achieve an optimal donor-acceptor separation leading to highest photoconductivity. At nanoscales, we imaged organic semiconductor molecules, on the single-molecule level, using fluorescence microscopy, and studied their photophysics such as their brigthness, stability with respect to photodegradation, and blinking dynamics. We also quantified the molecular alignment depending on the local nanoenvironment and found that in organic polycrystalline hosts, the single guest molecules are considerably constrained, so that they assume an orientation of the transition dipole moment close to the substrate normal. In contrast, in polymer hosts, the guest molecules exhibit a much broader range of orientations ( Figure 3). Finally, we developed a model based on a Bessel process for a 3D Brownian motion to describe blinking statistics of single molecules in various hosts, which enabled us to extract relative measures of charge delocalization of the photoexcited molecule, depending on the molecule and on the host. Our research bridges fields of organic optoelectronics and single-molecule spectroscopy. Additionally, the research towards understanding of the charge carrier dynamics at both macroscopic and microscopic, down to a single-molecule, levels, is not only of fundamental interest, but also has the potential to lead to breakthroughs in material design and nanoscale electronics. Broader Impact: The highly integrated interdisciplinary research program that involves both state-of-the-art instrumentation (ultrafast lasers, high-speed electronics, high-resolution microscopy) and cutting-edge science (processes on a single-molecule level) provided excellent educational resources for graduate and undergraduate students who were involved in the project. Two Ph. D. theses (one female, one minority) and numerous B. Sc. theses (two female) were completed based on the project. Several book chapters, web-based tutorials, and novel course curricula were produced by the PI and her group. The PI's group also created web-based tutorials, available for a download at the PI's research webstite, on photorefractive polymeric materials and devices, on intermolecular interactions, and on donor-acceptor interactions in organic semiconductor composites. Additionally, the PI's group created several demonstrations based on the project including fluorescence of molecules depending on the molecular structure, microscopy of organic films, single molecule fluorescence microscopy, etc., for various student populations ranging between middle-school students and their parents, incoming physics and engineering freshmen, and senior undergraduate/graduate students enrolled in optics and optoelectronics courses across campus.