A recent paper appeared in Nature Materials has revealed new trends in thermoelectricity existing in organic semiconductors which significantly differs from the behaviour widely observed in inorganic semiconductors. The increase of the carrier concentration n in inorganic semiconductors leads to the increase of electrical conductivity σ, but decreases the Seebeck coefficient S. Since the performance of thermoelectric materials is determined by the figure of merit Z=σS^2/κ, where κ is the thermal conductivity, a compromise between all these three parameters needs to be reached. For this reason highly doped semiconductors are the materials that better achieve this trade-off as observed in the classical figure below.
However, Kim et al. at the University of Michigan have proved that this is not the case in organic semiconductors, where a reduction of the dopant concentration leads to an improvement in Z due to the simultaneous increase of S and σ. They attribute this feature to the major influence of the mobility μ rather than the n in the electrical conductivity which is proportional to the product of both (σ=qnμ). In other words, the mobility enhancement (that the authors attribute to a decrease in the tunnelling distances) overwhelms the reduction in carrier concentration. The opposite occurs in inorganic semiconductors where the changes in μ are small relative to changes in n, and S and σ have opposite dependences on n.
This provides a new strategy to optimise the thermoelectric properties of polymer/organic thermoelectrics. Applying this to PEDOT:PSS, a outstanding value of ZT=0.45 at room temperature has been achieved by dedoping the polymer, which represents another significant enhancement in the efficiency of organic thermoelectrics in a short time.