The performance of all current polymer semiconductor devices, such as thin film transistors, photovoltaic cells, light-emitting transistors, light-emitting diodes, and photodetectors, is limited primarily by the charge carrier mobilities of current materials. A fundamental challenge to improving the performance of all these devices and moving them towards practical systems applications is thus to achieve higher charge carrier mobilities. Furthermore, investigation of n-type and ambipolar polymer semiconductors has lagged far behind p-type polymer semiconductors. In this project, fundamental insights into the structural factors that govern high-mobility electron and ambipolar charge transport in polymer semiconductors are sought. Novel organic solvent-soluble n-type polymer semiconductors will be synthesized and studied, including poly(anthrazoline)s, poly(pyrazinoquinoxalines)s, and ladder poly(pyrazinoquinoxaline)s whereas poly(bisindoloquinoline)s and other donor-acceptor copolymers will be explored as ambipolar semiconductors. The solid state morphology and molecular packing in thin films and nanowires of polymer semiconductors will be characterized by electron microscopy and X-ray diffraction techniques. Charge carrier mobilities of polymer semiconductor thin films and nanowires will be measured by using the field-effect transistor as a platform. The most promising materials will be explored in high-performance field-effect transistors, complementary inverters, and photovoltaic cells. Polymer semiconductors that combine air-stability with high electron mobility or ambipolar transport with high carrier mobilities will be useful for developing all-plastic complementary integrated circuits for logic and memory functions and for improving the efficiency of plastic solar cells. Indeed, the realization of ambipolar polymers with high carrier mobilities could revolutionize the design of organic solar cells, light-emitting transistors, and all organic electronic devices and systems.
NON-TECHNICAL SUMMARY: Electronic devices based on organic and polymer semiconductors, termed plastic electronics, are beginning to find many applications such as displays in cell phones, digital cameras, and car dashboards. Plastic electronics are also being tested for uses in applications ranging from flat-panel displays for computer and television screens, solid-state lighting, chemical- and bio-sensors, to low cost solar cells. This project will develop the basic knowledge for improving the performance of polymer semiconductors. Results from the project will lead to new materials and manufacturing technologies for plastic electronics and related applications in information technologies and renewable power sources. The project provides excellent opportunities for the training of scientists and engineers, including women and minorities, in the emerging interdisciplinary field of plastic electronics, which requires knowledge of chemistry, physics, materials science, and engineering. The principal investigator?s laboratory has several research collaborations with scientists in Greece, South Korea, Taiwan, Switzerland, and Japan in the general area of plastic electronics; it has hosted visits by senior scientists and students from some of these countries. This project will strengthen those international collaborations.
Organic and printed electronics based on organic semiconductors are rapidly emerging in applications ranging from full color displays, solid state lighting, and memories to low cost photovoltaics. However, a fundamental challenge common to all current organic electronic devices is to achieve higher charge carrier mobilities in order to substantially improve the performance of the devices and move them towards practical systems applications. In particular, investigation and development of electron-conducting (n-type) and ambipolar polymer semiconductors have lagged far behind hole-conducting (p-type) polymer semiconductors. The overall goal of the project was to synthesize and investigate the charge transport properties of n-type and ambipolar polymer semiconductors and to explore their applications in electronic and optoelectronic devices. The detailed scientific findings of this project are summarized in forty-one (41) published journal articles. Among the major findings and accomplishments of this research project are the followings. We synthesized and studied a series of new donor-acceptor copolymer semiconductors based on diketopyrrolopyrrole building block and found that they exhibit ambipolar charge transport with high carrier mobilities. Factors that lead to unipolar n-type versus ambipolar charge transport in polymer semiconductors were established by a systematic investigation of naphthalene diimide (NDI)-based donor-acceptor polymers. New n-type polymers having a field-effect electron mobility exceeding 0.1 cm2/Vs were realized. High performance ambipolar transistors based on donor-acceptor copolymers and related high-gain inverters were fabricated and demonstrated. The factors that govern long term (>4 years) stability and durability of n-channel polymer transistors in ambient air were established from the first of its kind study spanning over 4 years. We found that both orbital energy levels, which determine how facile electron injection from electrodes will be, as well as the close molecular packing in a highly crystalline film contribute to stability and durability. n-Type conjugated oligoquinolines were synthesized and found to be excellent electron transport materials, enabling the achievement of solution-processed high performance organic phosphorescent light emitting diodes (PhOLEDs). Dissemination of the results of this project and outreach activities have also been in the form of over 40 invited lectures and seminars presented by the PI at other Universities and at conferences in the US, Japan, China, Taiwan, South Korea, and Belgium. An important component of this research project is the training of graduate students in the areas of chemistry, chemical engineering, physics, and materials science and engineering as they relate to organic semiconductor-based electronics and optoelectronics. The PhD thesis research of 6 graduate students was partially supported during the period of this project. Five of these graduate students are US-born citizens and the remaining one is a permanent resident immigrant. A total of 5 graduate students have each graduated with a PhD and of these, 4 are female including an African-American. Three of the PhD graduates are currently employed in industry and the remaining two are postdoctoral research associates at universities. The STEM Bridge Program at the University of Washington is a four-week summer program designed to provide a summer bridge experience for incoming first year undergraduate minority students planning to study Science, Technology, Engineering, and Mathematics (STEM) disciplines. The Jenekhe group has hosted 8-12 students who are particularly interested in Chemical Engineering, in engineering design activities, during each summer. The students were engaged in the fabrication and characterization of solution-processed phosphorescent organic light-emitting diodes, a next generation display technology. During the design lab sessions, the students learned about the engineering profession and were introduced to the concept of an engineering system and the nature of design.
