The phase behaviour of poly(9,9-dioctylfluorene-co-bithiophene) semiconducting polymer, (F8T2) in top gate thin film transistor device structures fabricated using inkjet printing is investigated. The source, drain and gate electrodes are patterned by inkjet printing from a solution of a conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT) doped with poly(styrene sulfonic acid) (H. C. Starck), and a polymer layer is used as the dielectric.
At room temperature, the as-spun semiconductor films exhibit an isotropic, amorphous phase. Field effect mobilities of more than 4 x 10-3 cm2 / Vs, and on / off current ratios greater than 105 are observed. Upon annealing at elevated
temperatures, crystalline, and liquid crystalline phases are exhibited. The crystalline domains are identified by polarised
optical and atomic force microscopy. We investigate the crystallinity as a function of the annealing temperature. The order in the material is found to correlate well to the field effect mobility in the TFT device structure. The results of TFTs fabricated using inkjet printing to deposit the semiconductor film are also shown.
All-polymer thin film transistors and circuits have been fabricated by inkjet printing. Source, drain and gate electrodes were printed with a solution of conducting conjugated polymer, poly-ethylenedioxythiophene (PEDOT), and semiconductor and gate dielectric were spin-coated from solutions of conjugated polymer and insulator polymer, respectively. The transistors printed in air show comparable performances to the reference samples with gold electrodes. In order to overcome the resolution limit of inkjet printing, water-based PEDOT solution has been deposited onto a pre-patterned substrate which defines a channel by wettability contrast between hydrophilic and hydrophobic surface regions. Polymer transistors with a channel length of 5 microns have been achieved by this approach. In order to improve carrier mobility, main chains of the polymer semiconductor were self-aligned along the channel direction, and a mobility of 0.02 cm2/V+s has been achieved in the printed transistor. We demonstrate simple printed circuits (inverters) with via-holes and load resistors formed by inkjet technology.
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