Chunlin Xu

Thermopower is a key parameter for assessing the thermoelectric performance of conducting polymers and probing their charge transport mechanisms. Although pronounced thermopower anisotropy has been observed in aligned polymer films, its microscopic origin remains incompletely understood. Here, combining tight-binding modeling, first-principles calculations, and Boltzmann transport theory, we identify doping-induced electronic dimensional crossover as an intrinsic mechanism underlying this anisotropy. As hole doping increases, the Fermi surface evolves from closed three-dimensional pockets to configurations with quasi-two-dimensional and ultimately quasi-one-dimensional character, without requiring structural changes. These changes in Fermi-surface topology reshape the energy dependence of the directional transport distribution functions, producing pronounced thermopower anisotropy and direction-dependent sign reversals. We further show that this anisotropy can contribute substantially to the Nernst response of aligned conducting polymers. These findings establish a microscopic connection between band filling, Fermi-surface topology, and anisotropic thermoelectric transport.

Charge transport and thermoelectric (TE) properties of conjugated polymers are largely determined by their solid-state energetic disorders. Previous studies have focused on the energetic disorders or traps that result from dihedral angle rotations of the polymer backbone. However, this fails to explain the largely different charge transport properties in many conjugated polymers. Here, we investigate the charge transport and TE properties of two isomeric polymers with highly rigid backbones and low energetic disorders along the chain. Both polymers have similar ultralow charge transport activation energies (<35 meV), but exhibit vastly different charge transport properties. We found that P(PzDPP-Pz) with a linear backbone exhibited a high n-type TE power factor, over 2 orders of magnitude higher than its isomer P(PzDPP-Dz). By utilizing multiple characterizations and molecular simulations, we, for the first time, rule out the well-known dihedral-angle rotation disorders and unveil that the disorders and traps originate from the nanoscale polymer packing structures. Our work provides insight into the origins of the various types of disorders and traps across multilevel nanoscale structures in polymer semiconductors.

Polymer thermoelectrics offer an inherently soft, cost-effective, and lightweight solution to convert ubiquitous heat sources into sustainable electricity. However, their realistic applications are hindered by insufficient performance and the scaling complexity. We introduce irregular hierarchical-porous thermoelectric polymers, featuring irregularly shaped and distributed pores with diameters that range from less than 10 nanometers to micrometers. This porous structure not only enhances multiple phonon-like scattering, achieving a 72% reduction in lattice thermal conductivity, but also unexpectedly improves charge transport through nanoconfinement-enhanced crystallization. The optimized film yields a benchmark figure-of-merit zT of 1.64 at 343 kelvin. Moreover, this method is compatible with easy-to-process spray-coating techniques.

Thermal management in two-dimensional (2D) materials is pivotal, given their extensive technological utility in miniaturized devices. However, an expedient yet precise method for predicting the thermal transport properties of 2D materials remains to be established. In this study, we derive analytical formulas for calculating the scattering rate of three-phonon Umklapp processes, specifically tailored to 2D materials, based on the continuous elasticity assumption, quasi-harmonic approximation, and central force approximation. These formulas enable the construction of a refined Debye–Callaway model that incorporates the effects of both three-phonon Umklapp scattering and boundary scattering, eliminating the need for any fitting parameters. Notably, we explicitly disentangle the contributions of optical phonons, treating them within a framework analogous to the Einstein model. This improved model facilitates efficient and accurate predictions of the anisotropic lattice thermal conductivities in 2D materials. The results generated by our model for 12 2D crystals show excellent agreement with those from fully first-principles calculations reported in the literature. In particular, this study clarifies how the relaxation time fitting parameter adopted in previous work depends on the crystal structure and dimensional characteristics of the materials. It also provides an understanding of the logarithmic divergence of lattice thermal conductivity with respect to the size of 2D crystals within a framework involving only three-phonon scattering processes and Debye approximation. Owing to its low computational cost and high prediction precision, this model can serve as a valuable tool for high-throughput screening and machine learning applications in the identification of 2D materials with tailored thermal conductivity.

The vibrational thermal conductivity of polymer semiconductors is critical to the performance of organic electronic devices, yet its underlying mechanisms remain elusive. This work presents a two-channel model to elucidate the vibrational heat transport in semiconducting polymers with extended side chains. We reveal a striking splitting of vibrational modes along the polymer chain, driven by the substantial difference in force constants between the backbone and side chains. This gives rise to two distinct dispersive longitudinal branches: a backbone branch, characterized by phonon-like propagons that govern thermal conductivity, and a side chain branch, consisting of non-propagating diffusons that make minimal contributions to heat transport. The reduced maximum thermal conductivity of these polymer semiconductors, compared to their side chain-free counterparts, is attributed to phonon scattering by the low-frequency optical modes of the side chains. Our findings establish a foundation for strategically modifying side chains as a potent approach to fine-tuning thermal conductivity.

The vibrational thermal conductivity of polymer semiconductors is crucial for the performance of organic electronic devices, yet its underlying mechanisms remain elusive. This work presents a novel two-channel model to elucidate the vibrational heat transport in semiconducting polymers with extended side chains. We reveal a striking splitting of vibrational modes along the polymer chain, which is driven by the significant difference in force constants between the covalent bonds of the backbone and the van der Waals forces between the side chains. This leads to the emergence of two distinct longitudinal acoustic-like branches: a dominant backbone branch, featuring phonon-like propagons that predominantly dictate thermal conductivity, and a side chain branch, primarily composed of non-propagating diffusons, contributing minimally to heat transport. The reduced thermal conductivity of these polymer semiconductors relative to their side chain-free counterparts is attributed to phonon scattering with the low-frequency optical modes of the side chains, as validated by both first-principles calculations and inelastic neutron scattering experiments. Our findings highlight that the strategic modification of side chains offers a potent approach for fine-tuning thermal conductivity, as demonstrated by substituting side chain atoms with heavier elements. This finding opens new pathways for improving the thermoelectric performance of polymer semiconductors.

Donor-acceptor (D-A) copolymers doped with n-type dopants are widely sought after for their potential in organic thermoelectric devices. However, the existing structural disorder significantly hampers their charge transport and thermoelectric performance. In this Letter, we propose a mechanism to mitigate this disorder through side chain engineering. Utilizing molecular dynamics simulations, we demonstrate that strong Coulomb interactions between counterions and charged polymer backbones induce a transition in the stacking arrangement of the polymer backbones from a slipped to a vertical configuration. However, the presence of side chain steric hindrance impedes the formation of closely packed and ordered vertical stacking arrangements, resulting in greater distances between adjacent backbones and a higher level of structural disorder in the doped films. Therefore, we propose minimizing side chain steric hindrance to enhance the structural order in doped films. Our findings provide essential insights for advancing high-performance thermoelectric polymers.