Water-dispersible, conductive polyaniline for organic thin-film electronics (2007)

Water-dispersible, conductive polyaniline is an attractive candidate for organic\nthin-film electronics due to its solution-processability which facilitates low-cost\nprocessing. The successful incorporation of water-dispersible, conductive polyaniline\ninto organic electronic applications relies on the development of proper processing\ntechniques for material deposition and the elucidation of how processing conditions\naffect structural development and macroscopic properties of the material. This thesis\nfocuses on understanding the processing-structure-property relationships of a waterdispersible,\nconductive polyaniline that is doped with poly(2-acryl-amido-2-methyl-1-\npropanesulfonic acid), or PANI-PAAMPSA. Such understanding has facilitated the\nincorporation of PANI-PAAMPSA into functional organic thin-film transistors (TFTs).\nWe have developed simple, direct patterning techniques by exploiting the wetting\nand adsorption characteristics of PANI-PAAMPSA. Conductive PANI-PAAMPSA\nfeatures can be selectively patterned in the hydrophilic regions on a molecular template. \nConductive PANI-PAAMPSA features, which are directly patterned on insulating\nsubstrates, can be used as functional electrical components immediately after patterning.\nPANI-PAAMPSA features as small as 5μm can be routinely created with average\nelectrical conductivity of 0.2S/cm.\nThe patterned PANI-PAAMPSA features effectively function as source and drain\nelectrodes in pentacene thin-film transistors (TFTs). Specifically, bottom-contact\npentacene TFTs with PANI-PAAMPSA electrodes exhibit an average mobility of\n0.2cm2\n/V-s and on/off current ratios of 104\n, which are on par with the requirements for\nbackplane circuits for driving display applications. In bottom-contact devices, PANIPAAMPSA\nmakes efficient electrical contact to pentacene by promoting growth of\ncontinuous pentacene grains across the channel/electrode interface. Pentacene at such an\ninterface adopts upright orientation, i.e., the fused rings of pentacene are oriented\nperpendicular to the surface, which leads to more efficient charge injection and extraction\nat the pentacene/PANI-PAAMPSA interface compared to a pentacene/gold interface.\nDespite the fact that PANI-PAAMPSA has enhanced charge injection and\nextraction at the channel/electrode interface in pentacene TFTs, the bulk resististivity of\nthis material remains high (≅5Ωcm). We have successfully reduced the bulk resistivity of\nPANI-PAAMPSA by more than two orders of magnitude through a simple dichloroacetic\nacid (DCA) treatment. Our characterization reveals that DCA induces drastic structural\nchanges of PANI-PAAMPSA. DCA moderates the ionic interactions between PANI and\nPAAMPSA. PANI-PAAMPSA chains can thus rearrange from the “compact-coil” to the\n“extended chain” conformation. Efficient charge transport is thus enabled through such \n“extended chain” PANI-PAAMPSA conformation. The use of DCA-treated PANIPAAMPSA\nas functional electrodes increases device performance (i.e., mobilities and\non/off currnet ratios) of TFTs utilizing functionalized acenes by more than an order of\nmagnitude. Specifically, bottom-contact triisopropylsilyl pentacene TFTs with DCAtreated\nPANI-PAAMPSA electrodes exhibit mobilities and on/off current ratios as high\nas 0.12 cm2\n/V-s and 105\n, respectively.\nLastly, we show that the molecular structure of vacuum-deposited organic\nsemiconductor molecules, i.e., pentacene and dihexylthiophene anthracene (DHT-ANT)\ncan be drastically different depending on the nature of the surface chemistry of the\nsubstrates. Specifically, pentacene and DHT-ANT grow two-dimensional (2D) grains\nwhen the molecule-substrate interactions are weak. In these 2D grains, the fused rings of\nthe molecules are generally oriented upright on the substrate surface. On the other hand,\nif the molecule-substrate interactions are strong, the molecules tend to grow onedimensional\n(1D) grains. The fused rings of the molecules are generally parallel to the\nsubstrate surface. The details of the molecular orientation in turn significantly influence\nthe electronic band structures of the organic semiconductors. Specifically, molecules\nwith fused rings lying flat on substrate surfaces exhibit higher work functions compared\nto molecules with fused rings oriented upright.\nWe demonstrated several examples showing how the processing-structureproperty\nrelationships of PANI-PAAMPSA facilitate its incorporation in organic TFTs.\nSuch relationships are beneficial for organic electronics as the field moves towards real\napplications on a commercial scale.