This project will develop two new device systems which take advantage of the processing characteristics of organic field effect transistors (OFETs); a programmable logic cell (an organic PAL, a building block for creating an organic FPGA) and a piezoelectric film coupled OFET amplifier. The project will also investigate the interplay between the processing technologies used to fabricate and tune organic field effect transistors (OFETs) and circuit-level performance. This effort will develop a new tool to directly measure trap states in OFET devices to more thoroughly characterize that interplay. The proposed project will answer several long standing questions about the influence of processing techniques on shallow and deep trap states in these devices, spatially map both the trap density and electrically active grain distribution in OFETS, and lead to a deeper understanding of the role of processing and interfaces in OFETs. The fabrication and device platform and the tools developed for it will be integrated into both a capstone laboratory-based undergraduate course and modules for an outreach program which reaches underrepresented groups in the Harlem neighborhood of Manhattan.
Intellectual merit OFETs are used in a number of applications where their relatively straightforward fabrication, inherent mechanical flexibility, and low temperature processes are of benefit. This project will develop two new device architectures that take advantage of the low processing temperature and flexibility of OFETs to create a programmable logic cell and a piezoelectric sensor.
Most examination of pentacene-based OFETs has focused on measurements that examine aggregate properties (such as I-V characteristics) and often make assumptions inconsistent with trap-limited conduction. By using appropriate models and resolving both excitation and measurement in time, location, and energy, it will be possible to probe a variety of phenomena which are otherwise invisible. Of greatest interest are the effects of processing in a large-area compatible fashion. The tools developed will also be able to probe the internal states of the programmable logic cell and piezoelectric device fabricated in this program.
Broader impact This program will have significant impact beyond its immediate scientific and engineering output. A number of student populations will be engaged by this project including a high school student recruited through the NYAS SRTP program each summer, one undergraduate student each year, and one doctoral student per year. These students will contribute to the national supply of highly qualified personnel in this field. An additional outreach component will be through Columbia's GK12/TIP program administered through the School of Engineering and Applied Science. Two student years of outreach will be delivered by a second graduate student on the project who will work with high school teachers in Harlem to develop and deliver teaching modules based on this work. This outreach will have a significant impact on a traditionally underrepresented student population.
The device platforms developed will be integrated with the PI's previous work to create a new capstone undergraduate laboratory course at Columbia University on advanced display devices. Results from the research and educational components of the program will be disseminated through the literature and conferences in this field to other researchers and the public at large. On a societal scale, the development of OFET technology has tremendous potential. Wide availability of systems based on these devices could revolutionize the delivery of healthcare, IT, banking, commerce, and security, bringing significant benefits to both our society and the global community.
Organic semiconductors have the potential to create electronics that are mechanically flexible, large in area, and have optoelectronic properties that are not available in other thin film semiconductor systems. Organic semiconductors are held together with van der Waals bonds, which are weaker than covalent bonds, and can be disrupted using thermal processes or formulation of the material into a solution (e.g. for printing). This project examined two topics in organic field effect transistors (OFETs). OFETs can be used as amplifiers and switches in large area systems, and are one of the more promising devices based on organic semiconductors developed to date. One thrust in the project was the testing and behavior of OFETs to understand doping in OFET devices. The second was examining the integration of OFETs with electroactive polymer materials to make a new generation of sensors. In the characterization thrust, we looked at: Use of photocurrent to measure trap states, potential, and charge carriers in OFETs. We did this a few ways, including shining different wavelengths of light on the device to measure the level of trap states and moving a narrow bar of light across the device to measure the number of carriers and the potential in the channel. Use of x-ray spectroscopy to measure carrier injection at the contacts of OFETs. We used synchrotron-generated x-rays to demonstrate that treatments that are used on source and drain contacts of OFETs improve injection through two different mechanisms, and offer some guidance on which treatments are more effective Use of noise to determine the movement of carriers in a device. We deliberately doped devices and compared the noise in the current passing through to reach some conclusions about the energy states in the transistors. We also depopulated some traps selectively with flashes of light and were able to compare the noise with the traps empty and full. The electroactive polymer integration thrust focused more on the development of an active matrix strain sensor and microphone system that is monolithically integrated. Ultimately this allowed the creation of a 50 micron thick system that can effectively take a picture of sound and is roughly the same size as the wavelengths of sound in the acoustic range. We used a piezoelectric polymer sheet and took advantage of OFETs' low processing temperature to avoid damaging the sheet during the fabrication process. We also demonstrated that the microphone can be tensioned to make a mechanical signal processing system. The microphone's resonant modes can be measured to determine the frequency of incoming sound for a pure tone. Outcomes from the project include: 12 peer-reviewed journal publications (two more are currently under review) About 20 conference presentations Development of a monolithically integrated active matrix microphone based on piezoelectric polymers and using OFETs Development of a new techniques for examining doping in OFETs and related devices Development of a new lab-based course in displays, including lab modules and training materials for remote students. Training of about 15 students. 2 Ph.D. students were fully supported by the program, and 5 others were involved in the project. The project also engaged several MS-level, undergraduate, and high school students. The broader impact of this work includes: Training of students, who will form the next generation of engineers and scientists, including high school students and students recruited from underrepresented groups Development and dissemination of training materials using displays both as a topic and teaching object of interest for student learning A greater understanding of OFETs, their behavior, and their processing A greater understanding of the physics that underlies organic semiconductor performance Additional information and links to the publications from this work are available at http://kymissis.columbia.edu
