Understanding the processing-structure-property relationships of water-dispersible, conductive polyaniline
Polyaniline (PANI), when doped with small-molecule acids, is an attractive candidate for organic and polymer electronics because of its high electrical conductivity. Its utility as functional components in electrical devices, however, has been severely restricted because such PANI has limited processibility stemming from its limited solubility in common solvents. To overcome this barrier, we have developed water dispersible PANI that is template polymerized in the presence of a polymer acid, poly(2-acrylamido-2-methyl-1-propanesulfonic acid), or PAAMPSA. The polymer acid serves two roles: it acts as a dopant to render PANI conductive and excess water soluble pendant groups provide dispersibility of PANI in aqueous media. While the introduction of polymer acids renders the conducting polymer processible, such gain in processibility is often accompanied by a significant reduction in conductivity. As such, PANI that is doped with polymer acids has only seen limited utility in organic electronics. Given the promise of conducting polymers in organic electronics in general, this thesis focuses on the elucidation of processing-structure-property relationships of PANI-PAAMPSA with the aim of ultimately improving the electrical conductivity of polymer acid-doped PANI. By controlling the molecular weight and molecular weight distribution of the polymer acid template, we have improved the conductivity of PANI-PAAMPSA from 0.4 to 2.5 S/cm. The conductivity increases with decreasing molecular weight of PAAMPSA, and it further increases with narrowing the molecular weight distribution of PAAMPSA. Strong correlations between the structure and the conductivity of PANI-PAAMPSA are observed. In particular, the crystallinity of PANI increases with increasing the conductivity of PANI-PAAMPSA. Given that the crystallinity qualifies the molecular order in PANI-PAAMPSA, we observe a linear correlation between molecular order and macroscopic charge transport in PANI-PAAMPSA. PANI-PAAMPSA forms electrostatically stabilized sub-micron particles during polymerization due to strong ionic interactions between the sulfonic acid groups of PAAMPSA and aniline. When cast as films, the connectivity of these particles must play an important role in macroscopic conduction. The size and size distribution of PANI-PAAMPSA particles is strongly influenced by the molecular characteristics of polymer acid template. Templating the synthesis of PANI-PAAMPSA with a higher molecular weight PAAMPSA results in larger particles, and templating with a PAAMPSA having a larger molecular weight distribution results in a large size distribution in the particles. Because conduction in PANI-PAAMPSA films is governed by how these particles pack, the macroscopic conductivity of PANI-PAAMPSA films increases with increasing particle density, that is reducible from the molecular characteristics of PAAMPSA. Moreover, PANI-PAAMPSA particles are structurally and chemically inhomogeneous. The conductive portions of the polymer preferentially segregate to the particle surface. Conduction in these materials is therefore mediated by the particle surface and conductivity thus scales superlinearly with particle surface area per unit film volume. We further have improved the electrical conductivity of PANI-PAAMPSA by more than two orders of magnitude via post-processing solvent annealing with dichloroacetic acid (DCA). Since DCA is a good plasticizer for PAAMPSA and its pKa is lower than that of PAAMPSA (pKas of DCA and PAAMPSA are 1.21 and 2.41, respectively, at room temperature), DCA can effectively moderate the ionic interactions between PANI and PAAMPSA, thereby relaxing the sub-micron particulate structure arrested during polymerization. PANI-PAAMPSA can thus rearrange from a “compact coil” to an “extended chain” conformation upon exposure to DCA. Efficient charge transport is thus enabled through such “extended chain” PANI-PAAMPSA structure. DCA-treated PANI-PAAMPSA exhibits an average conductivity of 48 S/cm. The DCA treatment is not only specific to PANI-PAAMPSA. This treatment can also enhance the conductivity of commercially-available poly(ethylene dioxythiophene) that is doped with poly(styrene sulfonic acid), or PEDOT-PSS. Specifically, DCA-treated PEDOT-PSS exhibits a conductivity of 600 S/cm; this conductivity is the highest among polymer acid-doped conducting polymers reported so far. PANI-PAAMPSA can effectively function as anodes in organic solar cells (OSCs) whose active layer is a blend of poly(3-hexylthiophene), P3HT, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Specifically, the OSCs with PANI-PAAMPSA anodes exhibit an average short circuit current density of 1.95 mA/cm², open circuit voltage of 0.52 V, fill factor of 0.38, and efficiency of 0.39 %. The use of DCA-treated PANI-PAAMPSA as anodes increases device performance (i.e., short circuit current density and thereby efficiency) of OSCs by approximately two and a half fold. The OSCs with DCA-treated PANI-PAAMPSA anodes exhibit short circuit current density and efficiency as high as 4.95 mA/cm² and 0.97 %, respectively. We demonstrated several factors that govern the electrical conductivity of polymer acid-doped conducting polymers. Design rules, such as those illustrated in this study, can enable the development of conducting polymers that is not only easily processible from aqueous dispersions, but also sufficiently conductive for electronic applications, and should bring us closer to the realization of low-cost organic and polymeric electronics.