(405a) Water-Dispersible, Conductive Polyaniline Makes Better Electrical Contacts to P-Type Organic Semiconductors in Otfts

We present the first detailed report that directly correlates the reduced contact resistance in organic thin-film transistors with fundamental structural and morphological characterization at the organic semiconductor-conducting polymer interface.

Using stamp-and-spin-cast, a patterning technique for depositing water-dispersible, conductive polyaniline, we have successfully fabricated bottom-contact thin-film transistors with pentacene (p-type) as the organic semiconductors. To compare the performance of polyaniline electrodes with gold electrodes, we also fabricated reference organic thin-film transistors with analogous dimensions and geometry that use gold electrodes. Structural and electrical studies on these two batches of thin-film transistors reveal dramatic morphological differences at the channel-electrode interface that influence the linear regime current-voltage characteristics of the devices. Specifically, in bottom-contact thin-film transistors with polyaniline electrodes, the pentacene grains are similar in size and are continuous across the channel-electrode interface. On a molecular level, the fused rings of pentacene are oriented perpendicular to the surface both in the channel and on polyaniline electrodes. Accordingly, the current varies linearly with the source-drain voltage, an indication that the contact resistance is small in such devices. In thin-film transistors with gold electrodes, however, the pentacene grains are different in size and are discontinuous across the channel-electrode interface. Further, the fused rings of pentacene are oriented perpendicular to the channel surface and parallel to the gold surface. Such differences across the channel-electrode interface leads to structural and electronic disorder, which in turn results in current-voltage characteristics that deviate from linearity. Surface scanning potential measurements suggests that this deviation occurs at the channel-electrode interface and constitutes large contact resistance in devices with gold electrodes.