Is there a constant Lorentz number for organic thermoelectric materials?

Organic thermoelectric (TE) materials have attracted great interest due to their diverse advantages of excellent flexibility, light weight, good processability and highly adjustable molecular structure. Although significant advances have been achieved in enhancement of TE performance, the present trend of excessive pursuing high electrical conductivity (σ) and power factor is misleading, especially for those heavily-doped conjugated polymers and highly electrically-conductive materials. Herein, we wish to highlight a question: Is there a constant Lorentz number for organic thermoelectric materials? Based on Wiedemann-Franz law and an empirical relation between σ and Seebeck coefficient, the optimal σ range is discussed. Then, an estimated Lorentz number range is provided based on the widely studied PEDOT-based TE materials. Finally, a modified equation of figure of merit is tentatively proposed, which covers the electronic contribution to thermal conductivity.