Remarkably High Conductivity and Power Factor in D–D′-type Thermoelectric Polymers Based on Indacenodithiophene

Abstract

Three p-type thermoelectric (TE) polymers based on the indacenodithiophene moiety substituted with bis(alkylsulfanyl)methylene side-chains (IDTS) are synthesized. The TE characteristics of IDTS-based donor–donor’ (D–D′) type PIDTSDTTT and donor–acceptor (D–A) type polymers, PIDTSBT and PIDTS2FBT are investigated. Remarkably higher electrical conductivity (σ = ≈1000 S cm−1) and power factor (PF = ≈120 μW m−1 K−2) by doping with AuCl3 are measured for PIDTSDTTT compared to D–A type polymers. The higher σ of PIDTSDTTT originates from its higher carrier concentration compared to those of PIDTSBT and PIDTS2FBT. Moreover, the facile polaron-to-bipolaron transition is measured, and the charge carriers are calculated to be more stable with extended delocalization in PIDTSDTTT compared to D–A polymers. The significantly higher doping stability in PIDTSDTTT can be explained in terms of the higher conduction band of bipolarons than the valence band of O2 and H2O, which blocks the facile reduction of bipolarons in air. The energetic structures of doped polaron and bipolaron states, as well as pristine TE polymers, must be carefully considered to realize efficient and stable p-type thermoelectric polymers, where a D–D′ type structure with further enhanced carrier mobility can be considered as a potential molecular framework.