Water-dispersible, conductive polyaniline for organic thin-film electronics

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2007-12

Authors

Lee, Kwang Seok, 1973-

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Abstract

Water-dispersible, conductive polyaniline is an attractive candidate for organic thin-film electronics due to its solution-processability which facilitates low-cost processing. The successful incorporation of water-dispersible, conductive polyaniline into organic electronic applications relies on the development of proper processing techniques for material deposition and the elucidation of how processing conditions affect structural development and macroscopic properties of the material. This thesis focuses on understanding the processing-structure-property relationships of a waterdispersible, conductive polyaniline that is doped with poly(2-acryl-amido-2-methyl-1- propanesulfonic acid), or PANI-PAAMPSA. Such understanding has facilitated the incorporation of PANI-PAAMPSA into functional organic thin-film transistors (TFTs). We have developed simple, direct patterning techniques by exploiting the wetting and adsorption characteristics of PANI-PAAMPSA. Conductive PANI-PAAMPSA features can be selectively patterned in the hydrophilic regions on a molecular template. Conductive PANI-PAAMPSA features, which are directly patterned on insulating substrates, can be used as functional electrical components immediately after patterning. PANI-PAAMPSA features as small as 5μm can be routinely created with average electrical conductivity of 0.2S/cm. The patterned PANI-PAAMPSA features effectively function as source and drain electrodes in pentacene thin-film transistors (TFTs). Specifically, bottom-contact pentacene TFTs with PANI-PAAMPSA electrodes exhibit an average mobility of 0.2cm2 /V-s and on/off current ratios of 104 , which are on par with the requirements for backplane circuits for driving display applications. In bottom-contact devices, PANIPAAMPSA makes efficient electrical contact to pentacene by promoting growth of continuous pentacene grains across the channel/electrode interface. Pentacene at such an interface adopts upright orientation, i.e., the fused rings of pentacene are oriented perpendicular to the surface, which leads to more efficient charge injection and extraction at the pentacene/PANI-PAAMPSA interface compared to a pentacene/gold interface. Despite the fact that PANI-PAAMPSA has enhanced charge injection and extraction at the channel/electrode interface in pentacene TFTs, the bulk resististivity of this material remains high (≅5Ωcm). We have successfully reduced the bulk resistivity of PANI-PAAMPSA by more than two orders of magnitude through a simple dichloroacetic acid (DCA) treatment. Our characterization reveals that DCA induces drastic structural changes of PANI-PAAMPSA. DCA moderates the ionic interactions between PANI and PAAMPSA. PANI-PAAMPSA chains can thus rearrange from the “compact-coil” to the “extended chain” conformation. Efficient charge transport is thus enabled through such “extended chain” PANI-PAAMPSA conformation. The use of DCA-treated PANIPAAMPSA as functional electrodes increases device performance (i.e., mobilities and on/off currnet ratios) of TFTs utilizing functionalized acenes by more than an order of magnitude. Specifically, bottom-contact triisopropylsilyl pentacene TFTs with DCAtreated PANI-PAAMPSA electrodes exhibit mobilities and on/off current ratios as high as 0.12 cm2 /V-s and 105 , respectively. Lastly, we show that the molecular structure of vacuum-deposited organic semiconductor molecules, i.e., pentacene and dihexylthiophene anthracene (DHT-ANT) can be drastically different depending on the nature of the surface chemistry of the substrates. Specifically, pentacene and DHT-ANT grow two-dimensional (2D) grains when the molecule-substrate interactions are weak. In these 2D grains, the fused rings of the molecules are generally oriented upright on the substrate surface. On the other hand, if the molecule-substrate interactions are strong, the molecules tend to grow onedimensional (1D) grains. The fused rings of the molecules are generally parallel to the substrate surface. The details of the molecular orientation in turn significantly influence the electronic band structures of the organic semiconductors. Specifically, molecules with fused rings lying flat on substrate surfaces exhibit higher work functions compared to molecules with fused rings oriented upright. We demonstrated several examples showing how the processing-structureproperty relationships of PANI-PAAMPSA facilitate its incorporation in organic TFTs. Such relationships are beneficial for organic electronics as the field moves towards real applications on a commercial scale.

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