Mitchell G. Dibbs

Polyfluorenes as organic semiconductors for polymeric field effect transistors

Well-characterized F8T2 polyfluorene (Dow Chemical) has been prepared with weight average molecular weights (Mw) ranging from about 20,000 to 120,000. This semiconducting polymer has been used by Plastic Logic to fabricate arrays of 4,800 thin film transistors (TFTs) with 50 dpi, to be used as backplanes for active matrix displays. In this paper, the effects that molecular weight and thermal treatment have on the electrical characteristics of F8T2-based TFTs are reported. First, transistor performance improves with increasing molecular weight, with maximum values of TFT mobility approaching 1x 10-2 cm2 /V-s. Consistently higher mobilities are obtained when the F8T2 semiconductor makes contact with PEDOT/PSS versus gold electrodes. Alignment of F8T2 on a rubbed polyimide substrate is maintained after quenching, as determined by measurement of the dichroic ratios. Early-stage results on the development of inks based on F8T2 polyfluorene are also reported.

Fluorene arylene copolymers for organic photovoltaic devices

Fluorene arylene copolymers are a class of aromatic macromolecules that have an alternating backbone structure consisting of a 9,9-dialkylfluorene together with one (or more) additional aromatic group(s). Fluorene when combined with chromophoric and/or charge transporting aromatic monomers to form polyfluorenes have received a great deal of attention over the last several years as the emissive layer in polymeric light emitting diodes. The emission of green, red, or blue light can be controlled by the choice of the aryl backbone segments and alkyl side groups in the polymer. More recently, polyfluorenes have been designed and evaluated as the organic semiconducting layer in polymeric field effect transistors (pFETs). This work has led to a class of polymeric semiconductors with an excellent combination of charge mobility, environmental stability, and processability. These polymers have also been shown to have optoelectronic properties. The high optical density, high charge carrier mobility, and the potential to tune the absorption spectra makes this class of materials an ideal candidate for further study in the area of organic photovoltaics. This paper reviews the synthesis and characterization of polyfluorenes, focusing on the optimization of electronic properties for the conversion of light into electric current.

Recent Advances in the Synthesis of Polyfluorenes as Organic Semiconductors

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°C 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.