Defect-tolerant electron and defect-sensitive phonon transport in quasi-2D conjugated coordination polymers

TL;DR

This study reveals that quasi-2D conjugated coordination polymers can simultaneously exhibit high electrical conductivity and low thermal conductivity, making them promising for thermoelectric applications due to their defect-tolerant charge transport and defect-sensitive heat propagation.

Contribution

The paper demonstrates that these polymers uniquely combine defect-tolerant electron transport with defect-sensitive phonon transport, a property not previously observed in similar materials.

Findings

Electrical conductivity > 2000 S cm-1 with metallic temperature dependence.

Low lattice thermal conductivity (< 0.38 W m-1 K-1) due to lattice disorder.

Identification of a two-carrier ambipolar metallic transport regime.

Abstract

Thermoelectric materials, enabling direct waste-heat to electricity conversion, need to be highly electrically conducting while simultaneously thermally insulating. This is fundamentally challenging since electrical and thermal conduction are usually coupled. Here, we discover that quasi-2D conjugated coordination polymer films exhibit this ideal mix of antithetical properties due to coexistence of defect-tolerant charge transport and defect-sensitive heat propagation. The former is highlighted by the highest conductivities > 2000 S cm-1 with metallic temperature dependence observed in disordered films with paracrystallinity > 10%, while the latter manifests in low, temperature-activated lattice thermal conductivities (< 0.38 W m-1 K-1) originating from small-amplitude, quasi-harmonic lattice dynamics with disorder-limited lifetimes and vibrational scattering length on the order of interatomic spacing. Based on temperature-dependent thermoelectric and magnetotransport experiments we identify a two-carrier (hole-electron), ambipolar metallic transport regime as the origin of relatively small Seebeck coefficients in these materials. Our findings identify conjugated coordination polymers as attractive materials for applications in thermoelectric energy harvesting, (bio)electronics and energy storage.