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

Abstract

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 sp²-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 AuCl₃ and FeCl₃ 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 ∼10²¹ 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_E0) 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_E0 need to be carefully considered for the molecular design and doping process.