Synergistic Tuning of Doping Efficiency and Charge Transport in n-Type Thermoelectric Polymers via Rational Backbone Design

Despite the growing interest in organic thermoelectrics, n-type polymers generally exhibit lower performance than their p-type counterparts, limiting the development of efficient thermoelectric generators. Herein, we report two new n-type polymers based on cyanothiophene-flanked diketopyrrolopyrrole (CDPP) copolymerized with either diketopyrrolopyrrole (PCDPP-DPP) or dialkoxybithiazole (PCDPP-BTzOR) to investigate the critical yet scarcely explored role of orbital interaction between counits in governing thermoelectric performance. Although PCDPP-BTzOR exhibits a more conformationally locked backbone, higher molecular weight, comparable film crystallinity, and nearly identical LUMO level to PCDPP-DPP, the latter displays markedly superior charge transport and doping response. PCDPP-DPP achieves higher field-effect electron mobility (0.191 vs 0.013 cm² V⁻¹ s⁻¹) and carrier concentration after n-doping (1.4 vs 1.0 × 10²⁰ spins cm⁻³), yielding over an order of magnitude higher electrical conductivity (25.71 vs 1.97 S cm⁻¹). These improvements arise from the deliberate tuning of interunit orbital interactions in PCDPP-DPP, which promotes electronic delocalization and mitigates trapping associated with polarized charge-transfer states. Consequently, PCDPP-DPP attains a high power factor of 23.52 μW m⁻¹ K⁻² and a promising room-temperature figure of merit of ZT = 0.07. Overall, this work establishes orbital interaction engineering as an effective strategy for designing high-performance n-type thermoelectric polymers.