Pawan's insights into thermoelectric transport in hybrid thermoelectrics

Hybrid (organic-inorganic) materials have emerged as a promising class of thermoelectric materials, achieving power factors (S2σ) exceeding those of either constituent. The mechanism of this enhancement is still under debate, and pinpointing the underlying physics has proven difficult. In this work, we combine transport measurements with theoretical simulations and first principles calculations on a prototypical PEDOT:PSS-Te(Cux) nanowire hybrid material system to understand the effect of templating and charge redistribution on the thermoelectric performance. Further, we apply the recently developed Kang-Snyder charge transport model to show that scattering of holes in the hybrid system, defined by the energy-dependent scattering parameter, remains the same as in the host polymer matrix; performance is instead dictated by polymer morphology manifested in an energy-independent transport coefficient. We build upon this language to explain thermoelectric behavior in a variety of PEDOT and P3HT based hybrids acting as a guide for future work in multiphase materials.

Editor's Pick in RSI: Pawan and Maheswar's shadow mask technique to measure thin film Seebe

Organics, hybrid (inorganic-organic), PLD, MBE, sputtering, evaporation - you name it! No matter how you make your films, we can measure the transport properties including Seebeck, four-probe electrical conductivity and Hall to characterize your film! Pawan and Maheswar calibrated the technique with PEDOT:PSS as well as Nickel and we are now routinely using this in our lab for measurements on organic, hybrid, semiconducting films with <1% error bars! Anas and Wu Jing are now working on the AC version of this technique - should be greatly useful for smaller size samples... Congratulations folks!

Anomalously low electronic thermal conductivity in metallic vanadium dioxide (S. Lee et. al. Science

Decoupling charge and heat transport

In metals, electrons carry both charge and heat. As a consequence, electrical conductivity and the electronic contribution to the thermal conductivity are typically proportional to each other. Lee et al. found a large violation of this so-called Wiedemann-Franz law near the insulator-metal transition in vanadium dioxide nanobeams. In the metallic phase, the electronic contribution to thermal conductivity was much smaller than what would be expected from the Wiedemann-Franz law. The results can be explained in terms of independent propagation of charge and heat in a strongly correlated system.

Thermal conductivity of VO2

Phonon thermal conductivity

Breakdown of the WF law

In electrically conductive solids, the Wiedemann-Franz law requires the electronic contribution to thermal conductivity to be proportional to electrical conductivity. Violations of the Wiedemann-Franz law are typically an indication of unconventional quasiparticle dynamics, such as inelastic scattering, or hydrodynamic collective motion of charge carriers, typically pronounced only at cryogenic temperatures. We report an order-of-magnitude breakdown of the Wiedemann-Franz law at high temperatures ranging from 240 to 340 kelvin in metallic vanadium dioxide in the vicinity of its metal-insulator transition. Different from previously established mechanisms, the unusually low electronic thermal conductivity is a signature of the absence of quasiparticles in a strongly correlated electron fluid where heat and charge diffuse independently.