Non-Wiedemann-Franz behavior of the thermal conductivity of organic semiconductors

Abstract

Organic semiconductors have attracted increasing interest as thermoelectric converters in recent years due to their intrinsically low thermal conductivity compared to inorganic materials. This boom has led to encouraging practical results in which the thermal conductivity has predominantly been treated as an empirical number. However, in an optimized thermoelectric material, the electronic component can dominate the thermal conductivity, in which case the figure of merit ZT becomes a function of thermopower and Lorentz factor only. Hence the design of effective organic thermoelectric materials requires understanding the Lorenz number. Here, analytical modeling and kinetic Monte Carlo simulations are combined to study the effect of energetic disorder and length scales on the correlation of electrical and thermal conductivity in organic semiconductor thermoelectrics. We show that a Lorenz factor up to a factor ∼5 below the Sommerfeld value can be obtained for weakly disordered systems, in contrast with what has been observed for materials with band transport. Although the electronic contribution dominates the thermal conductivity within the application-relevant parameter space, reaching ZT > 1 would require minimization of both the energetic disorder and also the lattice thermal conductivity to values below κ lat < 0.2 W/mK.

Backbone-driven host–dopant miscibility modulates molecular doping in NDI conjugated polymers

“…This suggests that TEG side chains are a promising approach to reduce the thermal conductivity of CPs, a key factor to improve their figure of merit for thermoelectric applications. 44…”

Reduction of the Lattice Thermal Conductivity of Polymer Semiconductors by Molecular Doping

“…For metals and highly doped semiconductors, κ e is proportional to the electrical conductivity following the Wiedemann–Franz law, κ e = σ LT , where L is the Lorenz number and T is the temperature. 11 Although in such materials the electrical and vibrational contributions to ZT are inevitably coupled, it cannot be concluded that this relation also holds for doped organic semiconductors. In fact, several studies for PEDOT:PSS, PEDOT:Tos, and doped polyacetylene suggest that the correlation between σ and κ is not yet fully understood.…”

“… 12 − 15 Furthermore, a recent theoretical work based on Monte Carlo simulations questioned the validity of the Wiedemann–Franz law in conducting polymers. 16 One of the reasons leading to this ambiguity is, perhaps, that there are few reports that simultaneously address electrical and thermal conduction. 13 , 17 …”

Thermoelectric Materials: Current Status and Future Challenges

“…Other interesting routes to explore include the spin Seebeck effect (Sierra et al, 2018) as well as the use of topological states (Xu et al, 2017;Fu et al, 2020) and non-Wiedemann Franz materials (Scheunemann and Kemerink, 2020) that exhibit low electronic thermal conductivity. Compositionally graded materials (Cramer et al, 2018;Ou et al, 2020) can achieve large zTaverage by connecting in series a composition with high-performance at high temperatures with a composition with high-performance at low temperatures.…”