Novel strategy for boosting thermoelectric performance of organic materials with low electrical conductivity

To realize a wearable power generator, organic thermoelectric materials are desired to exhibit a dimensionless figure-of-merit [zT = α2σT/(κe + κl)] of more than unity, where T, α, and σ denote the temperature, thermopower, and electrical conductivity, whereas κe and κl are the thermal conductivities mediated by carriers and phonons, respectively. Although conventional materials exhibit a maximum power factor (PFmax = α2σ) in the high-σ region of more than 10 Scm− 1, zT is not necessarily maximized at the σ giving PFmax because of a marked increase in κe in the high-σ region, which has been a barrier for boosting the zT of organic materials up to a practical level. Here, we report that an exceedingly large PFmax of 1.1 × 10− 3 Wm−1K− 2 was realized for the composite film consisting of fullerene molecules and molybdenum trioxide nanoclusters by keeping the giant Seebeck effect at a low σ of less than 10− 2 Scm− 1. Since κe is negligibly small in the low-σ region, zT of 0.81 is expected to be achieved at room temperature, which is the highest value among organic materials to the best of our knowledge.