Disorder-Tolerant Efficient Doping Strategy of Conjugated Polymers for High-Performance Organic Thermoelectrics

Molecular doping has been a paramount technique to modulate the electronic properties of conjugated polymers (CPs) used in various organic device applications. However, conventional redox doping requires a sufficient energy level offset between the host and dopant; otherwise, doping occurs through the formation of a charge transfer complex, which strongly reduces doping efficiency. Moreover, especially in organic thermoelectrics (TEs), excessive dopants introduced to form highly doped CPs generally cause structural distortion and broadening of the density of states (DOS) in CPs (i.e., dopant-induced disorder), thereby deteriorating their charge transport properties. Herein, we present an efficient doping strategy to attain highly doped CPs with dramatically suppressed structural and energetic disorder using a promising Lewis acid dopant, tris(pentafluorophenyl)borane (BCF), which has recently shown potential for doping CPs regardless of their ionization energy and enabling efficient integer charge transfer by forming a BCF–water complex. Using a non-polar aliphatic solvent, hexane, which can effectively swell the CP films due to its good affinity with the side chains of CPs, we successfully infiltrate BCF into the lamellar spacing of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-<i>b</i>]thiophene] (PBTTT) films via sequential doping, producing a remarkable electrical conductivity of 230 S cm⁻¹ and a TE power factor of 140 μW m⁻¹ K⁻². Concomitantly, the resulting sequentially doped films show significantly high Hall carrier mobility and facilitated bipolaron formation within their high solid-state ordering compared to films doped by solution-mixing. In addition, the sequentially doped films exhibit highly delocalized transport characteristics through highly ordered domains with a narrow DOS, substantiated by charge transport modeling and quantitative investigation of crystalline ordering, charge carrier localization, and energetic disorder. This work would provide an approach for disorder-tolerant doping strategies with a comprehensive understanding of processing–structure–property relationships in highly doped CPs.