Controlling Self‐Doping in Conjugated Polyelectrolyte Copolymers to Vary the Threshold Voltage in OECTs

ABSTRACT

Self‐doped conjugated polyelectrolytes (S‐CPEs) with high electrical conductivity and stable self‐doping, without the use of external dopants, are reported. Two types of soluble copolymers with different levels of self‐doping were synthesized. The degree of self‐doping was precisely controlled by varying the amount of self‐dopable monomer units, 3HOTS ⁻ [TMA ⁺ ] or 3HOT‐[SO ₃ H], over a wide compositional range. These units were copolymerized with a non‐self‐dopable monomer, 3MEEET. Compared to S‐CPEs bearing tetramethylammonium sulfonate side chains, the corresponding S‐CPEs containing free sulfonic acid groups (SO ₃ ⁻ H ⁺ ) show additional self‐acid doping, which results in higher self‐doping levels. As the fraction of self‐dopable repeating units increases, both S‐CPE series show stronger polaron absorption, as evidenced by ultraviolet–visible–near‐infrared (UV–vis‐NIR) spectroscopy, alongside a decrease in barrier to hole injection. Spectroscopic data enabled us to propose the underlying self‐doping mechanism. The degree of self‐doping is found to strongly influence organic electrochemical transistor (OECT) performance, especially the threshold voltage ( V _T ), which can be precisely tuned from −0.6 to +0.1 V. Additional studies examine morphology, crystal orientation, swelling in electrolytes under applied bias, volumetric capacitance, and cytotoxicity. Overall, this copolymer approach enables precise control of charge carrier concentration and conductivity of soluble semiconductor polymers for different applications.