Revealing unipolar thermoelectric performance in bipolar polymer

Abstract

Conjugated polymers are attracting increased attention as thermoelectric (TE) materials for energy harvesting applications in low-temperature regimes. However, in many doped ambipolar polymers, the simultaneous transport of both holes and electrons under temperature gradients leads to an offset in thermopower (S), which suppresses TE performance and complicates intrinsic understanding of bipolar TE conversion. Herein, we quantitatively investigate the p-n polarity transition in FeCl3-doped bipolar PDPP4T films by measuring the magneto-thermoelectric Nernst effect, combined with Hall and Seebeck effect analyses. Notably, behind the S = 0 point, we observe a significant thermopower offset originating from the balancing contributions of electrons and holes. This countervailing thermopower value is extracted to reach 400 μV K−1, which could ideally produce an estimated maximum unipolar ZT of 0.24 at 175 K, due to rising polaron states and reduced carrier concentration. Our findings reveal the extraordinary hidden unipolar TE performance achievable in doped bipolar polymer towards ultra-low-temperatures thermoelectric.