Polyfluorenes as Organic Semiconductors for Polymeric Field Effect Transistors
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Polyfluorenes as Organic Semiconductors for Polymeric Field Effect Transistors David J. Brennan, Paul H. Townsend, Dean M. Welsh, Mitchell G. Dibbs, Jeff M. Shaw, Jessica L. Miklovich, and Robyn B. Boeke The Dow Chemical Company, Advanced Electronic Materials, Midland, Michigan, USA, 48674. INTRODUCTION Polyfluorenes (1) are a class of polyaromatic macromolecules that are characterized by an alternating backbone structure that consists of a 9,9-dialkylfluorene unit in combination with another aromatic group. The nature of this aromatic unit plays a key role in the electronic properties of the polymers. For example, polyfluorenes which combine chromophoric and charge transporting aromatic units have received a great deal of attention over the last several years as the emissive layer in polymeric light emitting diodes [LUMATION* Light-Emitting Polymers (LEPs)]. The emission of green, red, or blue light can be controlled by the choice of the alkyl side groups (R) and aryl backbone segments (Ar) in the polymer [1].
Ar R
n
R
1 More recently, polyfluorenes have also been designed and evaluated as the organic semiconducting layer in polymeric field effect transistors (PFETs) [2-4]. This effort has led to a class of polymeric semiconductors with an excellent combination of charge mobility, high on/off ratios, environmental stability, and processability. One such polymer is poly(9,9dioctylfluorene-co-bithiophene) alternating copolymer, otherwise known as F8T2 polyfluorene (2). 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 [2]. Early-stage work indicates that F8T2 is more resistant to doping by atmospheric oxygen than other polymeric semiconductors such as poly(3-hexylthiophene) [2]. Inks based on solutions of F8T2 in xylene, mesitylene, and other solvents have also been prepared. This paper will focus on the recent advances in the synthesis, fabrication, and electrical characterization of F8T2 polyfluorene.
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2 *Trademark of The Dow Chemical Company
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DISCUSSION Polyfluorene synthesis and physical properties The synthesis of polyfluorenes is carried out by use of a modified Suzuki-type polymerization process as described previously [1, 5]. In this process, the bisboronate ester of a 9,9-dialkylfluorene reacts with an aromatic dibromide in the presence of a palladium catalyst, yielding the high molecular weight polyfluorene. The synthesis of F8T2 is accomplished using the monomers shown in Scheme 1. After purification, F8T2 is isolated as a yellow-orange fibrous polymer that is soluble to varying degrees in solvents such as THF, chloroform, toluene, xylene, and mesitylene. The synthesis of F8T2 polyfluorene can be scaled up to 20-30 gram quantities of dry polymer (after purification) using conventional laboratory glassware. Scheme 1 O
O B
B
O
O
+
Pd-based catalyst Br
S
Br
S
F8T2 2
F8T2 polyfluorene has been characterized by a combination of molecular weight analysis (inherent viscosity and gel permeation chromatography), dif
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