A heavily doped D-D′-type polymer with metal-like carrier transport hybrid doping

A new donor-donor (D-D′)-type polymer (PIDTSCDTS) based on electron-rich indacenodithiophene (IDT) and cyclopentadithiophene (CDT) moieties is synthesized with extended π-conjugation and pronounced chain planarity through substitution with sp 2 -hybridized alkenyl side chains. Its thermoelectric (TE) properties are studied in detail depending on the doping methods: sequential doping (SqD), solution-mixed doping (MxD), and hybrid doping (HyD) with AuCl 3 and FeCl 3 as p-dopants. The carrier generation and transport depend strongly on the doping method. A higher conductivity ( σ ) of up to ∼500 S cm −1 is achieved using HyD (compared with other methods) owing to the significantly higher (bi)polaron generation (of up to ∼10 21 cm −3 ) by doping both the amorphous and crystalline regions. Analyzing the relationship between the Seebeck coefficient ( S ) and the conductivity ( σ ) based on the Kang-Snyder model, metal-like carrier transport is observed for polymers obtained via SqD and HyD, which has been barely reported in organic TE polymers. The higher transport coefficient ( σ E 0 ) in the HyD films reduces the S - σ trade-off relation, leading to a larger maximum power factor, PF max , of ∼40 μW m −1 K −2 with simultaneously enhanced σ and S values compared with SqD films (16 μW m −1 K −2 ). To further optimize the TE properties, the degenerate doping and band-like carrier transport with maximizing the σ E 0 need to be carefully considered for the molecular design and doping process. Combining a planar electron-rich D-D′-type molecular design and optimal hybrid doping successfully demonstrates metal-like carrier transport whilst reducing the Seebeck coefficient-conductivity trade-off relation under degenerately doped conditions.