Transistor Paint: Environmentally Stable N-alkyldithienopyrrole and Bithiazole-Based Copolymer Thin-Film Transistors Show Reproducible High Mobilities without Annealing By Junying Liu, Rui Zhang, Itaru Osaka, Sarada Mishra, Anna E. Javier, Detlef-M. Smilgies, Tomasz Kowalewski, and Richard D. McCullough* 1. Introduction Field effect transistors (FETs) are fundamental components in electronic devices. Organic and polymeric semiconductors are potential active components in FET devices and have received considerable attention due to the promise of high efficiencies and mechanically flexible fabrication. [1–3] Solution processing tech- niques such as spin coating, inkjet printing, and screen printing allow for large-area fabrication, thus offering a significant advantage over existing inorganic materials in terms of ease of processing and reduced costs. [4–8] More importantly, the electronic and optical properties of conjugated polymers can be readily tuned by controlling the p conjugation and con- nectivity through polymer engineering. [9] To truly take advantage of these proper- ties for FETs, organic semiconductors require high charge carrier mobility, good air stability, and processibility. Unfortu- nately, only a few polymers have mobilities above 0.1 cm 2 V 1 s 1 with on/off currents above 10 5 , qualities required for many applications. [10–17] Thus designing and synthesizing novel semiconducting poly- mers with good FETperformance, as well as understanding their morphology depen- dence on charge carrier mobilities, is still an important area of research. One particular polymer that has shown great potential for use in OFET is N-alkyl- dithieno[3,2-b:2 0 ,3 0 -d]pyrroles (DTP) and thiophene (Th) based polymer (PBThDTP) (Scheme 1). [18] This polymer exhibits excellent solubility in common organic solvents, and has exhibited a field effect mobility as high as 0.21 cm 2 V 1 s 1 and an average mobility of 0.13 cm 2 V 1 s 1 without thermal annealing. In spite of these desirable high charge carrier mobilities, this polymer does not have a very deep highest occupied molecular orbital (HOMO) level, leading to the possibility of partial oxidation in air, and therefore has resulted in devices with reduced stability. An improvement in air stability has been achieved by deepening HOMO level; that is, increasing the ionization potential (IP) of the polymers. Two major strategies have been developed to increase IP without jeopardizing charge carrier mobilities. In one strategy, the effective conjugation length is shortened by: i) localizing charge by introducing fused aromatic rings [11–15,19] and ii) increasing the rotational freedom of the backbone. [10] In the second strategy, comparatively electron deficient units are introduced into the polymer backbone to increase the IP. [20–24] For example, electron-deficient bithiazole (Tz) units have been successfully incorporated into Th polymers and oligomers to increase IP. [22–24] In comparison with bithiophene, the Tz unit is relatively electron deficient and more rigid. Therefore, replacing bithiophene units in PBThDTP with Tz moieties can lead to polymers with higher IP and more rigid, planar polymer structures. FULL PAPER www.afm-journal.de [*] Prof. R. D. McCullough, J. Liu, Dr. R. Zhang, Dr. I. Osaka, Dr. S. Mishra, A. E. Javier, Prof. T. Kowalewski Department of Chemistry Carnegie Mellon University 4400 Fifth Avenue, Pittsburgh, PA 15213 (USA) E-mail: rm5g@andrew.cmu.edu Dr. D.-M. Smilgies Cornell High Energy Synchrotron Source (CHESS) Cornell University, Ithaca, NY 14853 (USA) DOI: 10.1002/adfm.200900926 New solution processable 4-(2-hexyldecan)-4H-bisthieno[2,3-d:3(,2(-b]pyrrole and 4,4(-dialkyl-2,2(-bithiazole-based copolymers (PBTzDTPs) are synthesized with excellent FET performance. These novel copolymers have considerable potential in printable electronics as they have high charge carrier mobilities, excellent air stability, good solution processibility, and no requirement for post-deposition thermal annealing, all requirements for this field of application. The thin film transistors fabricated from PBTzDTPs achieve field effect mobilities as high as 0.14 cm 2 V 1 s 1 with current on/off ratios up to 10 6 without thermal annealing. In addition, the devices exhibit stable performance in air, showing no significant degradation over 60 days. Moreover, the polymers described here provide an excellent example of the systems in which higher mobility performance does not require higher crystalline, long-range ordered structures. Such a system appears to be particularly promising for rapid fabrication techniques, where kinetic conditions usually prevent the development of long-range order. Adv. Funct. Mater. 2009, 19, 3427–3434 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3427