N-doped semiconductive polymer using methoxy-modified JLBI for optimized miscibility and Coulomb interactions towards high thermoelectric performance

Abstract

Molecular doping of n-type polymers generates coulombically bound charge carriers or counterion pairs via a charge-transfer process. Synergistically optimizing the miscibility and the release of free charge carriers within a specific polymer:n-dopant pair poses a formidable challenge for achieving high-performance n-type organic thermoelectrics. In this work, the density of free charge carriers and thermoelectric properties were significantly enhanced in the n-doped benchmark semiconductive polymer N2200 by harnessing a novel n-dopant, 9-(1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-8-methoxy-julolidine (MeO-JLBI), which was derived from our previously reported JLBI molecule by employing the lipophilicity of julolidine group and the electron-donating characteristic of MeO group. In contrast to N2200 films doped with the common benzimidazoline n-dopants N-DMBI and JLBI, which showed similar low performance, the MeO-JLBI-doped N2200 exhibited ∼4 times higher free charge carrier density and ∼10 times higher conductivity (1.02 × 10⁻² S cm⁻¹), leading to a ∼5 times higher room-temperature power factor of 3.54 × 10⁻² μW m⁻¹ K⁻² at optimized conditions, which represented one of the highest power factors observed for doped N2200 films to date. Experimental findings demonstrated that the MeO group enhanced the driving force for charge transfer, reduced the Coulomb interactions between counterion pairs in the doped state, and consequently facilitated the generation of free charges more efficiently, while maintaining the same enhanced miscibility with N2200 as observed with JLBI. This study offers an approach that synergistically utilizes electron-donating and lipophilic effects to tackle the key requirements of high miscibility, charge transfer driving force, and reduced Coulomb interactions in n-doped polymers for thermoelectric applications.