Development of self-doped conductive polymers with phosphonic acid moieties

Abstract Conductive polymers are fundamental to modern organic electronics. Among them, self-doped conductive polymers have attracted attention for applications in printable electronics and biosensor materials due to their water solubility and intrinsic ability to achieve charge compensation without external dopants. To date, the charge-compensating moiety in p-type self-doped conductive polymers has primarily been the sulfonic acid group; however, its strong acidity has led to issues such as substrate corrosion, posing challenges. To address this, we have been developing self-doped conductive polymers incorporating phosphonic acid since ∼2012. Our focus on phosphonic acid stems from its several advantages: it is a mild acid, its dibasic nature enables functionalization, and it exhibits a strong binding affinity to metal oxides. This account summarizes the synthesis and properties of self-doped polyanilines and a self-doped poly(3,4-ethylenedioxythiophene) (PEDOT) containing phosphonic acid moieties, with a focus on their electrical conductivity and potential applications. In self-doped polyaniline, the introduction of phosphonic acid groups induced polaron delocalization under basic conditions, a unique feature not observed in other self-doped polyanilines. Furthermore, salt formation with long-chain alkylammonium ions improved solubility in organic solvents. The potential applications of these materials as charge dissipation agents for electron-beam lithography and hole transport materials for organic photovoltaics were also explored. Additionally, the introduction of phosphonic acid moieties via postpolymerization modification was discussed. A key challenge of self-doped polyaniline was its relatively low conductivity (poly(2-methoxyaniline-5-phosphonic acid): up to 1.9 × 10−1 S/cm). This limitation was overcome by developing Phos-PEDOT 2, a phosphonic acid-functionalized PEDOT derivative, which exhibited a conductivity of 1.3 × 102 S/cm—three orders of magnitude higher than that of self-doped polyaniline.