Role of ionization energy on mixed conduction in polythiophene-derived polyelectrolyte complexes

Conjugated polyelectrolyte complexes formed by the electrostatic compatibilization between a conjugated and an insulating polyelectrolyte are a versatile design platform for highly processable, high-performing polymeric mixed ion−electron conductors. While electrostatic mediation in complexes allows for structure and property control, a fundamental understanding of how the properties of the constituent conjugated polyelectrolyte (CPE) translate to the resulting complex performance is necessary for future designs. To investigate the role of CPE architecture on the overall charge transport properties of the resulting complex properties, here we compare a water-soluble cationic poly(alkoxythiophene) derivative based on poly(3-alkoxy-4-methylthiophene) with an imidazolium pendant unit and bromide counterion to an analogous complex with poly(sodium 4-styrenesulfonate). Through spectroscopic, morphological, electrochemical, and charge transport characterization, we find that poly(alkoxythiophene)-based complexes exhibit high mixed conductivity, enhanced electrochemical stability, improved doping efficiency, and lower oxidation potential, relative to previously reported poly(3-alkylthiophene)-based complexes, making them more suitable candidates for electrochemical applications. Importantly, both CPE and complex films based on the poly(3-alkoxy-4-methylthiophene) chemistry display electronic conductivities on the order of 10−2−10−3 S/cm and impressive ionic conductivities up to the order of 10−4 S/cm, despite the ordered morphology of the 3-alkoxy-4-methylthiophene backbone. We make a key observation that the enhancement of the electronic conductivity of the CPE from an alkyl to alkoxythiophene backbone does not necessarily improve the electronic conduction of the resulting complex as observed in previous reports, thereby underscoring the role of complexation thermodynamics, dielectric strength of the electrostatic complex, and complex morphology on mixed conduction. This study provides fundamental insights governing future design rules of mixed-conducting polyelectrolyte complexes for next-generation energy applications.

由共轭聚电解质与绝缘聚电解质通过静电相容作用形成的共轭聚电解质复合物（Conjugated polyelectrolyte complexes），是一类可加工性优异、性能卓越的聚合物混合离子-电子导体的多功能设计平台。尽管复合物中的静电介导作用可实现结构与性能调控，但要实现未来的设计优化，仍需从基础层面理解构成组分的共轭聚电解质（conjugated polyelectrolyte, CPE）的特性如何转化为最终复合物的性能。为探究CPE的分子结构对最终复合物整体电荷传输性能的影响，本文对比了两种体系：一种是以咪唑鎓为悬挂单元、溴离子为抗衡离子的、基于聚(3-烷氧基-4-甲基噻吩)的水溶性阳离子型聚(烷氧基噻吩)衍生物，另一种则是与聚(4-苯乙烯磺酸钠)形成的类似复合物。通过光谱学、形貌学、电化学及电荷传输表征手段，本文发现相较于此前报道的基于聚(3-烷基噻吩)的复合物，基于聚(烷氧基噻吩)的复合物展现出更高的混合导电率、更优异的电化学稳定性、更高的掺杂效率以及更低的氧化电位，因此更适用于电化学应用场景。重要的是，无论基于聚(3-烷氧基-4-甲基噻吩)化学结构的CPE还是其复合物薄膜，尽管其3-烷氧基-4-甲基噻吩主链呈现有序形貌，但其电子电导率仍可达10⁻²~10⁻³ S/cm，离子电导率更是高达10⁻⁴ S/cm量级，性能优异。本文的关键发现在于：将CPE的主链从烷基噻吩替换为烷氧基噻吩以提升其电子电导率，并不一定会如此前研究所观察到的那样，改善最终复合物的电子传导性能。这一结果凸显了复合热力学、静电复合物的介电强度以及复合物形貌对混合导电行为的调控作用。本研究为下一代能源应用所需的混合导电聚电解质复合物的未来设计准则提供了基础理论依据。