46793-G10
Surface-Initiated n-Type Semiconducting Polymer Synthesis
Organic semiconducting polymers are an exciting class of materials with applications in many emerging modern technologies. In recent years, much progress has been made in using these materials for the development of light-weight, low cost, and flexible electronic devices such as thin-film field effect transistors, organic light emitting diodes (1), and photovoltaic devices (2). Currently, most semiconducting polymers operate as p-type semiconductors (primarily hole conductors), and for fully organic electronic devices, especially organic photovoltaics to be achieved, there is a great need to develop n-type semiconductors. A promising candidate for an n-type polymer is poly(benzimidazobenzophenanthroline) (BBL). It has been reported that by using this polymer as the active semiconductor in a field-effect transistor (FET), electron mobilities as high as 0.1 cm2V-1s-1 can be achieved (3), which is the highest reported for an organic semiconducting polymer. Unfortunately, present synthesizing methods of BBL have some drawbacks including processing difficulty due to its highly insoluble nature in common organic solvents. In this work, we aim to overcome the synthesis and processing difficulties by performing the reaction directly on the device substrate. By self-assembly method, the morphology is likely to be controlled. Self-assembly methods in this project consist of the following steps: growth of self-assembled monolayer; monomers containing anhydride react with monolayer; monomers with amines react with the first monomer; growth of main chain by repeating the previous two steps till desirable length of the monomer is achieved. Our work during the first year has focused on the first step, which is the monolayer formation on the device substrate, ITO. An aniline derivatized with a phosphonic acid was used as the monolayer molecule. The formed monolayers were analyzed using grazing angle FI-IR, water contact angle measurements, and AFM. After optimization, the following procedure was found to provide the most well-packed monolayer: 1. Wipe ITO substrate with cotton swab dipped in soapy water. 2. Rinse with DI water. 3. Immerse substrate in 1:1:5 H2O2/NH4OH/DI water at 70 °C for 1 h. 4. Rinse with large amount of DI water and dry the substrate in an oven at 120 °C for 4 h. 5. Sonicate in DI water, acetone and IPA. 6. Dry and treat substrate with oxygen plasma. 7. Immerse samples in self-assembly solution for 20 h, anneal and sonicate. The first reaction, which involves the reaction of the monolayer with the anhydride has also been investigated. This reaction proved to be harder than expected. However, with much optimization, we now have a working procedure in place as described below: 1. The anhydride monomer was dissolved in anhydrous DMAc to form a 5 mM solution. 2. The monolayer substrate was placed on a hotplate at 280 °C, and a drop of the monomer solution was placed onto the substrate. 3. The substrate was rinsed with DI water and CHCl3, sonicated in CHCl3. Using this procedure, contact angle and ellipsometry results confirmed that the reaction had successfully taken place on the surface. We had anticipated that the same conditions could be used for the reaction of the second monomer, however the second reaction did not occur. We are in the process of optimizing these conditions to identify the problem associated with this. During the first year of the ACS-PRF Type G Award, we have also embarked on a new project, which is also aimed at the surface-initiated synthesis of semiconducting polymers. The project discussed above, although attractive, is not ideal for the large-scale production of BBL since it involves the step-wise reaction of monomers, which can be tedious. As such, we have been developing the chain-growth polymerization of semiconducting polymers such poly(3-hexylthiophene) (P3HT). We have submitted one paper, and another is in preparation for this work (4, 5). The ACS-PRF Type G Award has had a profound impact on my career and the preliminary data obtained in the project has led to being awarded the NSF CAREER Award and the DARPA Young Faculty Award. The work has led to 4 invited presentations and 3 other presentations. References: 1. R. H. Friend et al., Nature 397, 121 (1999). 2. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 270, 1789 (1995). 3. A. Babel, S. A. Jenekhe, J. Am. Chem. Soc. 125, 13656 (2003). 4. N. Doubina, A. Ho, A. K. Y. Jen, C. K. Luscombe, Chem. Comm. Submitted (2008). 5. A. Ho, N. Doubina, M. Stoddard, A. K. Y. Jen, C. K. Luscombe, Manuscript in preparation (2008).
