Recent Advances in the Synthesis of Polyfluorenes as Organic Semiconductors
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Recent Advances in the Synthesis of Polyfluorenes as Organic Semiconductors David J. Brennan, Yu Chen, Shaoguang Feng, James P. Godschalx, Gary E. Spilman, Paul H. Townsend, Scott R. Kisting, Mitchell G. Dibbs, Jeff M. Shaw, Dean M. Welsh, Jessica L. Miklovich, and Debra Stutts The Dow Chemical Company, Midland, Michigan, USA, 48674 ABSTRACT New poly(fluorene-thiophene) alternating copolymers are described in which either the dioctylfluorene or bithiophene units in poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) are replaced by other fluorene or thiophene-based groups, respectively. Improvements in solubility are realized when the bithiophene unit of F8T2 is replaced by dihexylterthiophene or dihexylpentathiophene units. Melting temperatures are also lowered by 50 – 100 ºC in these polymers when compared to F8T2. Replacement of the bithiophene unit of F8T2 with a dihexylpentathiophene unit also results in a significant improvement in hysteresis (< 2 V vs. 3.5 – 5 V for F8T2). Initial results are also reported on the thermal cleavage of the C8 side groups of F8T2, which yields an insoluble polymeric semiconductor film that continues to exhibit transistor switching characteristics as part of a bottom gate device. INTRODUCTION Polyfluorenes are a class of macromolecules that are characterized by a backbone structure that contains fluorene units, optionally in combination with other aromatic or conjugated groups. The most useful materials have been found to be 9,9-dialkylfluorenes in alternating sequence in the backbone with other aromatic or conjugated units [1-3]. In addition to being some of the world’s best light emitting polymers, polyfluorenes have also been designed and evaluated as the organic semiconducting layer in polymeric field effect transistors (PFETs) [4,5]. One such polymer is poly(9,9-dioctylfluorene-alt-bithiophene) copolymer, otherwise known as F8T2 polyfluorene (1). F8T2 has been shown to have charge mobilities as high as 0.02 cm2/V-s with current on/off ratios of up to 106 [6,7]. This initial work also indicated that F8T2 is more resistant to doping by atmospheric oxygen than other polymeric semiconductors such as poly(3hexylthiophene) [7]. In 2003, a number of publications appeared which described the synthesis, characterization, and electrical properties of F8T2. These included preparation of F8T2 with various molecular weights [8,9], early-stage ink formulation [8], and evaluation of bias stress effects [10-12]. The effects that molecular weight [9], morphology [13,14], and different dielectric materials [15,16] have on the electrical performance of F8T2 has also been described. A number of other papers have also been published on a variety of F8T2-related subjects [1721]. S
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While F8T2 polyfluorene has emerged as one of the leading polymeric semiconductors, improvements to polyfluorene-based semiconductors are desirable. These include (a) improving solubility in organic solvents, (b) reducing melting temperatures, (c) changing the solubility characteristics of spun-
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