(351aw) The Effect of Extensional Flow on the Evaporative Assembly of Donor-Acceptor Semiconducting Polymers

Authors: 
Wu, H. C., Stanford University
Wang, G. J. N., Stanford University
Yan, H., SLAC National Accelerator Laboratory
Toney, M., SLAC National Accelerator Laboratory
Bao, Z., Stanford University
Schneider, S. A., Stanford University
Because they are lightweight, mechanically compliant, and easily tunable through chemical synthesis, polymer semiconductors (PSCs) are excellent candidates for use in electronics with functionalities currently underserved by conventional inorganic semiconductors. The ability of PSCs to be easily deposited over large area at near ambient conditions makes them particularly promising for low-cost large area electronic applications. Given the sensitivity of charge carrier mobility in PSCs to the nature of their deposition from solution, a mechanistic understanding of deposition processes is essential for enhancing and controlling a PSC’s electrical performance. Polymer chain alignment has been largely posited as a strategy for improving electrical performance of PSCs. In this work the ability of a micropillar shearing blade to align polymer chains of a DPP-based polymer, poly(2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)diketopyrrolo[3,4-c]pyrrole1,4-dione-alt-thieno[3,2-b]thiophen) (DPP-TT), during solution shearing by increasing extensional flow is investigated. A sufficiently strong extensional flow field is found to be capable of aligning polymer chains and inducing crystallization. Field effect transistors (FETs) are fabricated and tested to explore the relationship between induced alignment/crystallization and charge transport. The effect on charge carrier mobility is hypothesized to be a function of deposition parameters. The semiconducting characteristics of a PSC are uniquely dependent on chain length. The molecular weight of a PSC plays a critical role in determining both the final thin film morphology as well as the dominant mechanism of charge transport (i.e. inter- vs. intra- chain transport). Moreover, conformation/orientation changes resulting from flow are anticipated to be sensitive to chain length. We extend our investigation to polymers of variable molecular weight and observe that the extent of alignment, as well as the effect of such an alignment on subsequent charge transport, is highly influenced.