High-performance & thermally stable n-type polymer thermoelectrics based on a benzyl viologen radical cation-doped ladder-type conjugated polymer

Abstract

Over the past five years, the understanding of n-doped conjugated polymers has greatly accelerated the science of n-type polymer thermoelectrics. Part of this rapid progress is based on the emergence of new high-performance n-type conjugated polymers. n-Dopants, on the other hand, have not shown a significant breakthrough. Here we show that one of the oldest ladder-type conjugated polymers, poly(benzimidazobenzophenanthrolinedione) (BBL), can have outstanding n-type thermoelectric performance and superior thermal stability (100 °C over 4 days) when n-doped with one of the oldest organic radicals, the benzyl viologen radical cation (BV˙⁺). The BV˙⁺-doped BBL films achieved a maximum conductivity of 1.63 (1.60 ± 0.05) S cm⁻¹ and a power factor of 5.37 (4.85 ± 0.46) μW m⁻¹ K⁻² in large channel devices (channel length = 2.5 mm; channel width = 20 mm), which are among the highest reported in the literature for n-type polymers, making them the best BBL based n-type thermoelectrics. The enhanced thermoelectric properties of the BV˙⁺-doped BBL are a consequence of the electrochemically active BV˙^+/2+ molecules facilitating electron transport between the BBL fibers. Our results suggest that an electrochemically active counter cation is necessary for better electron transport and thus higher conductivity in fiber-forming/(semi)crystalline n-type organic semiconductors. The same is also likely to be true when the counter anion is electrochemically active for fiber-forming/(semi)crystalline p-type organic semiconductors. These findings will have tremendous impact on the chemical design of p- and n-dopants for organic semiconductors in the future.