On The Thermal Conductivity of Conjugated Polymers for Thermoelectrics

X Rodríguez-Martínez and F Saiz and B Dörling and S Marina and JL Guo and K Xu and H Chen and J Martin and I McCulloch and R Rurali and JS Reparaz and M Campoy-Quiles, ADVANCED ENERGY MATERIALS, 14 (2024).

DOI: 10.1002/aenm.202401705

The thermal conductivity (kappa) governs how heat propagates in a material, and thus is a key parameter that constrains the lifetime of optoelectronic devices and the performance of thermoelectrics (TEs). In organic electronics, understanding what determines kappa has been elusive and experimentally challenging. Here, by measuring kappa in 17 pi-conjugated materials over different spatial directions, it is statistically shown how microstructure unlocks two markedly different thermal transport regimes. kappa in long-range ordered polymers follows standard thermal transport theories: improved ordering implies higher kappa and increased anisotropy. kappa increases with stiffer backbones, higher molecular weights and heavier repeat units. Therein, charge and thermal transport go hand-in-hand and can be decoupled solely via the film texture, as supported by molecular dynamics simulations. In largely amorphous polymers, however, kappa correlates negatively with the persistence length and the mass of the repeat unit, and thus an anomalous, albeit useful, behavior is found. Importantly, it is shown that for quasi-amorphous co-polymers (e.g., IDT-BT) kappa decreases with increasing charge mobility, yielding a 10-fold enhancement of the TE figure-of-merit ZT compared to semi-crystalline counterparts (under comparable electrical conductivities). Finally, specific material design rules for high and low kappa in organic semiconductors are provided. A consistent dataset of thermal conductivity (kappa) measurements performed in 17 pi-conjugated materials is presented. A correlation analysis with intrinsic material descriptors reveals distinct regimes for kappa depending on the thin film microstructure. This allows certain amorphous pi-conjugated polymers to reach superior thermoelectric performance than semi-crystalline counterparts when both are benchmarked under similar electrical conductivities. image

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