This project provides a fundamental understanding the parameters that affect the nucleation and growth of organic semiconductors from vapor phase for the fabrication of large arrays of organic transistor devices with single crystal channels. Due to the polycrystalline nature of thin films, thin film devices rarely reach the high mobilities of single crystal devices due to trapping at grain boundaries. In order to significantly improve device performance, single crystals and single crystalline films are being provided to produce large arrays of single crystal devices for practical applications. Although technologically challenging, the realization of large arrays of single crystal devices will lead to dramatic performance improvement of organic semiconductor based devices. Detailed studies will be carried out to investigate effects of surface topology, chemical interactions between surface chemical functionalities and organic semiconductors, orientation of the chemical groups, ordering of the chemical groups, and distribution of chemical groups on the substrate surface.
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Organic semiconductors are promising candidates as the active elements in plastic circuits, particularly those using field-effect transistors (FETs) as switching or logic elements. Providing the mechanisms for patterning and investigating the numerous parameters that affect the nucleation and growth of organic semiconductors from vapor phase for the fabrication of large arrays of organic transistor devices with single crystal channels will advance an understanding of organic semiconductor nucleation and growth in general. This research will expose both under-represented graduate students and undergraduates to organic chemistry, surface chemistry, materials and thin film characterization, device fabrication, and device characterization. They will also learn the multidisciplinary approach to problem solving.
Intellectual merit: Our focus on controlling crystal growth morphology and the molecular packing of organic semiconductors has resulted in various high impact publications. We developed a solution coating technique known as solution shearing to reliably introduce strain into the crystal structure of various organic semiconductors, changing the molecular packing from equilibrium to a metastable state. This is a novel method of producing better electronic performance of organic semiconductors without having to change the underlying chemical structure. We achieved a record mobility for an organic semiconductor, TIPS-pentacene, to 4.6 cm2 V-1s-1.1 Solution shearing can also grow large area aligned crystals, at a speed relevant for industrial fabrication of organic electronics.2 We have also shown controlled equilibrium crystal growth of organic semiconductors, resulting in single crystalline, defect free organic semiconductors through a simple, solution based droplet pinned crystallization technique. This technique can grow highly aligned crystals that can cover large area substrates. As the crystal grown have a low defect density, charge transport in less hindered in these single crystals. Using the droplet pinned crystallization technique on an organic semiconductor, C60, yield electron charge transport mobilities as high as 11 cm2 V-1s-1, an order of magnitude higher improvement from the previously seen electron charge transport mobility through solution processing methods.3 This method was used to prepare a complementary inverter, which is an important building block for circuits.4 Probing the structure of the organic semiconductor thin films is important for understanding their charge transport properties. However, detailed information on the molecular packing is available only for bulk single crystals. We have developed a method to solve the detailed molecular packing structures in thin films. Using this method, we resolved the structure of various organic semiconductors5,6. This provided insights in understanding charge transport in organic semiconductors. We developed a new method for making a high quality crystalline self-assembled monolayers of organosilane compounds using a simple, spin-casting technique onto Si/SiO2 substrates.7 We found that organic semiconductors deposited onto this crystalline monolayer grow in a favorable two-dimensional layered growth manner which is generally preferred morphologically for high charge carrier transport. This technique has since been widely used for modifying the dielectric surface for obtaining high quality semiconductor-dielectric interface, which is critical for obtaining high electronic performance. New materials are typically discovered experimentally, with theoretical characterization providing only post-facto justification of the observed device performance. In our work, we utilized a computational screening procedure to guide chemical synthesis, which led to the discovery of a new high-performance semiconductor.8 This compound is one of the very few organic semiconductors that has exhibited mobility greater than 10 cm2 V-1 s-1. The study suggests that a computational screening approach can lead to the informed synthesis and characterization of novel organic materials for electronics applications. Broader Impact: This work resulted in a systematic understanding of nucleation and growth in solution processing of organic semiconductors. This is crucial for advancing the field of organic electronics as well as providing insights for future manufacturing of these devices. Our approach resulted in unprecedented performance of organic semiconductors. Since organic semiconductors are key components for organic light emitting diodes, organic solar cells, transistors and sensors, enhanced charge transport will result in better performance for these devices. We have been committed to work closely with our existing NSF centers and Office of Science Outreach on campus to reach out to a broad population ranging from K-12, community college, undergraduate, and graduate students as well as prepare the teachers of tomorrow for new areas of science and technology. This research trained graduate students, undergraduates and postdoctoral researchers to organic chemistry, surface chemistry, materials and thin film characterization, device fabrication and device characterization as well as a wide range of organic electronics technologies. Students received training on communicating project progress and directions by weekly meetings with the PI and presentations at group meetings and conferences. The students learned a multidisciplinary approach to problem solving, thus, obtaining an impressive combination of technical engineering, basic scientific understanding, and communication skills. In addition, various high school students, community college students participated in the research. Our work helped to broaden the participation in science and engineering. Selective publications: 1 Giri, G. et al. Nature 480, 504-508, (2011). 2 Becerril, H. A., Roberts, M. E., Liu, Z., Locklin, J. & Bao, Z. Advanced Materials 20, 2588, (2008). 3 Li, H. et al. Journal of the American Chemical Society 134, 2760-2765, (2012). 4 Li, H. et al. Advanced Materials 24, 2588-2591, (2012). 5 Mannsfeld, S. C. B., Virkar, A., Reese, C., Toney, M. F. & Bao, Z. Advanced Materials 21, 2294 (2009). 6 Mannsfeld, S. C. B., Tang, M. L. & Bao, Z. Advanced Materials 23, 127, (2011). 7 Ito, Y. et al. Journal of the American Chemical Society 131, 9396-9404, (2009). 8 Sokolov, A. N. et al. Nature Communications 2, doi:10.1038/ncomms1451 (2011).
