Clayton Mauldin, Amanda Murphy

In recent years, organic semiconductors have emerged as an enticing alternative for the development of low cost electronics. These materials have the potential to be solution processed which lowers production costs and increases substrate compatibility, permitting the use of flexible plastics or fabrics. There are several applications unique to these organic semiconducting materials, including disposable electronic devices such as radio frequency ID tags and sensors as well as large-area or flexible displays.

A major challenge left to address in this field lies in the effective integration of semiconducting materials into large-scale device fabrication. Current methods rely on thermal evaporation of organic semiconductors due to their low solubility. This technique is time-consuming and can only be done in small batches, which increases the cost dramatically. More economical methods for processing such as spin coating, stamping, and ink-jet printing require that these compounds dissolve in a solvent. Chemical modifications can be made to these materials to increase their solubility, but these modifications tend to decrease the crystallinity of the materials and dramatically reduce their semiconducting ability.

To eliminate these solubility problems, we have developed a novel method for temporarily modifying popular organic semiconducting materials in order to make them soluble in common solvents yet still retain high performance. We attach the solubilizing groups to the semiconductor core with a thermally labile ester linkage. This functionalization allows simple solution processing then a subsequent thermal annealing step induces a chemical reaction that releases the bulky side groups allowing them to evaporate away. The conjugated core of the molecules can then self-assemble due to p -stacking interactions into highly conductive films that are only a few nanometers thick.

We have been able to obtain charge mobilities through these films that are comparable with evaporated oligothiophene films, and further work is on-going to use this strategy with other oligomer cores to further increase the mobility or to improve the air stability.

Our group is also interested in reducing the size of electronic device components to nanometer and single molecule length scales. Recently it was demonstrated that nanometer-thick self-assembled monolayers (SAMs) are sufficient for use as a dielectric layer in OTFTs, and can significantly reduce the operating voltages of the devices. However, to date there have been no successful reports on also using SAMs as the semiconducting layer. Using the soluble oligothiophenes discussed above, we have developed specialized ink-jet printing procedures capable of depositing single monolayers of these semiconductors, and demonstrated that these monolayer films exhibit superior electrical performance compared to that of spun cast films. To expand on this technology, we are designing and testing new materials that can self-assemble into monolayers, and thereby eliminate the need for specialized deposition procedures. We also want to tune the interface between the semiconductor and the substrate through the introduction of a binding group to the end of the molecule. Covalent bonding of the molecules to the surface should increase the robustness of the films and aid in selective assembly on patterned substrates.

Selected Publications

• Murphy, A. R.; VanDyke, P.; Liu, J.; Fréchet, J. M. J.; Chang, C.; Subramanian, V.; DeLongchamp, D. M.; Sambasivan, S.; Fischer, D. A.; Lin, E. K. “Self-assembly, Molecular Ordering and Charge Mobility in Solution-Processed Ultrathin Oligothiophene Films.” Chem. Mater. 2005, 17 (24), 6033-6041.
• Murphy, A. R.; Liu, J.; Luscombe C.; Kavulak, D.; Fréchet, J. M. J.; Kline, J. R.; McGehee, M. D. “Synthesis, Characterization, and Field-Effect Transistor Performance of Carboxylate Functionalized Polythiophenes with Increased Air Stability.” Chem. Mater. 2005, 17 (20) , 4892-4899.
• DeLongchamp, D. M.; Sambasivan, S.; Fischer, D. A.; Lin, E. K.; Chang, P.; Murphy, A. R.; Fréchet, J. M. J.; Subramanian, V. “Direct Correlation of Organic Semiconductor Film Structure to Field-Effect Mobility.” Adv. Mater. 2005, 17 , 2340-2344 . (cover article) .
• Chang, P. C.; Lee, J.; Huang, D.; Subramanian, V.; Murphy, A. R.; Frechet, J. M. J. “ Film Morphology and Thin Film Transistor Performance of Solution-Processed Oligothiophenes.” Chem. Mater. 2004, 16 (23), 4783-4789.
• Murphy, A. R.; Frechet, J. M. J.; Chang, P.; Lee, J.; Subramanian, V. “ Organic Thin Film Transistors from a Soluble Oligothiophene Derivative Containing Thermally Removable Solubilizing Groups.” J. Am. Chem. Soc. 2004, 126 (6), 1596-1597.