Michael Lesley Sorensen

Tetrathienoacene Copolymers As High Mobility, Soluble Organic Semiconductors

Increasing the rigidity of the thiophene monomer through the use of an alkyl-substituted core that consists of four fused thiophene rings is shown to be a promising route toward high-performance organic semiconductors. We report on a dialkylated tetrathienoacene copolymer that can be deposited from solution to yield ordered films with a short pi-pi distance of 3.76 A and with a field-effect hole mobility that exceeds 0.3 cm2/V.s. This polymer enables simple transistor fabrication at relatively low temperatures, which is particularly important for the realization of large-area, mechanically flexible electronics.

Alkylsubstituted Thienothiophene Semiconducting Materials: Structure−Property Relationships

A family of conjugated polymers with fused structures consisting of three to five thiophene rings and with the same alkyl side chains has been synthesized as a means to understand structure-property relationships. All three polymers showed well-extended conjugation through the polymer backbone. Ionization potentials (IP) ranged from 5.15 to 5.21 eV; these large values are indicative of their excellent oxidative stability. X-ray diffraction and AFM studies suggest that the polymer with the even number of fused thiophene rings forms a tight crystalline structure due to its tilted side chain arrangement. On the other hand, the polymers with the odd number of fused thiophene rings packed more loosely. Characterization in a field-effect transistor configuration showed that the mobility of the polymer with the even number of rings is 1 order of magnitude higher than its odd-numbered counterparts. Through this structure-property study, we demonstrate that proper design of the molecules and properly arranged side chain positions on the polymer backbone can greatly enhance polymer electronic properties.

Electrophoretic Displays Fabricated on Ultra-Slim Flexible Glass Substrates

Ultra-slim flexible glass substrates enable high performance displays and electronic devices through their inherent benefits of a high-quality surface, process compatibility, thermal and dimensional stability, optical transmission, and barrier properties. This study demonstrates use of flexible glass as a backplane substrate for both active matrix displays with organic thin-film transistors (TFTs) as well as segmented displays. The demonstrated 4.7-inch active matrix VGA displays have a resolution of 170 dpi (640×480 pixels), pixel size of 150 μm × 150 μm, and aperture ratio of 40%. The dimensional stability benefit of flexible glass was clearly observed when compared to the registration of polymer substrate devices. This study demonstrates the capability and benefits of flexible glass substrates in devices fabricated with solution-based processing as a step toward roll-to-roll flexible electronic fabrication.

Thermal reorganization of alkyl-substituted thienothiophene semiconductors

Controlling the structure of polymer thin films under thermal annealing is vital to realize reproducible transport properties and acceptable device lifetimes needed to fabricate electronic circuits. We investigated the behavior of two conjugated polymers with different length alkyl side-chains upon thermal annealing. The longer side-chain polymer showed greater stability with no significant reorganization upon annealing. The shorter side-chain polymer underwent conformation changes upon thermal annealing, which cannot be explained by residual solvent effects, a liquid crystal phase transition, or side-chain melting. Using an in situ study of thermal annealing we can attribute the film reorganization to changes in the side-chain tilt, which result in a positive linear expansion coefficient on the order of 8 × 10−4 K−1. Neither polymer features phase transitions above room temperature nor exhibited significant or abrupt changes during thermal processing, which is a mandatory property for the manufacture of flexible electronics.

Structure vs. property relationships in high mobility fused thiophene polymers

A family of conjugated polymers with fused structures consisting of three to five thiophene rings and with the same alkyl side chains has been synthesized as a means to understand structure - property relationships. All three polymers showed well extended conjugation through the polymer backbone. X-ray diffraction study of the polymer thin films suggests that the polymer with the even number of fused thiophene rings forms a tight crystalline structure due to its tilted side chain arrangement. On the other hand, the polymers with the odd number of fused thiophene rings packed more loosely. Characterization in a field-effect transistor configuration showed that the mobility of the polymer with the even number of rings is one order of magnitude higher than its oddnumbered counterparts. Through this structure - property study, we demonstrate that proper design of the molecules and properly arranged side chain positions on the polymer backbone can greatly enhance polymer electronic properties