A heavily doped D-D′-type polymer with metal-like carrier transport via 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 pi-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 AuCl3 and FeCl3 as p-dopants. The carrier generation and transport depend strongly on the doping method. A higher conductivity (sigma) of up to similar to 500 S cm(-1) is achieved using HyD (compared with other methods) owing to the significantly higher (bi)polaron generation (of up to similar to 10(21) cm(-3)) by doping both the amorphous and crystalline regions. Analyzing the relationship between the Seebeck coefficient (S) and the conductivity (sigma) 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 (sigma(E0)) in the HyD films reduces the S-sigma trade-off relation, leading to a larger maximum power factor, PFmax, of similar to 40 mu W m(-1) K-2 with simultaneously enhanced sigma and S values compared with SqD films (16 mu W m(-1) K-2). To further optimize the TE properties, the degenerate doping and band-like carrier transport with maximizing the sigma(E0) need to be carefully considered for the molecular design and doping process.

Automated summary

A new donor-donor (D-D')-type polymer (PIDTSCDTS) with extended pi-conjugation and pronounced chain planarity is synthesized by substituting sp(2)-hybridized alkenyl side chains with electron-rich indacenodithiophene (IDT) and cyclopentadithiophene (CDT) moieties. The thermoelectric (TE) properties of the polymer are studied with different doping methods, revealing that hybrid doping (HyD) achieves a higher conductivity (sigma) of up to 500 S cm(-1) due to enhanced (bi)polaron generation. Analysis of the Seebeck coefficient (S) and conductivity (sigma) relationship shows metal-like carrier transport in polymers obtained via sequential doping (SqD) and HyD, resulting in a larger maximum power factor (PFmax) of about 40 mu W m(-1) K-2 compared to SqD films. Careful consideration of degenerate doping and band-like carrier transport is needed for further optimizing the TE properties.