Understanding the thermoelectric transport properties of organic semiconductors through the perspective of polarons

Optimism remains high in the field of organic thermoelectrics, and the possibility of achieving so-called figures of merit (ZT) approaching or exceeding one remains a selling point for this field. Higher the figure of merit, ZT, higher is the efficiency of conversion from waste heat into reusable energy. ZT = 1 is normally considered the benchmark value for a thermoelectric material to be commercially useful.

we understand the efficiency of waste heat to electricity conversion in organic semiconductors from the viewpoint of the fundamental physics of charge and heat transport. We first look at a few well-known models that attempt to understand the thermoelectric transport physics in organic semiconductors and compare them with a model based on so-called polaronic charge conduction. Rather than just summarize the existing literature, as is customary in review articles, the approach we’ve used is to link the many reported experimental observations with each other. We do this as an attempt to build a unified picture of thermoelectric transport physics in organic semiconductors.

Our modelling attempt is univariate and deliberately simple. While it is difficult to justify a microscopic interpretation for the lumped variable we use, the model’s primary aim is to scope out new organic materials for thermoelectric power generation applications purely by looking at two measurable quantities, namely, the electrical conductivity and the Seebeck coefficient. A similar mathematical model was previously used to identify polaronic transport in disordered conductive oxide-based materials. We’ve shown that it may also be applicable to disordered organic semiconductors, seeing as the transport physics of disordered materials is expected to be similar.

We’ve looked back at the scientific highlights from the last 15 years of research on the thermoelectrics of organic semiconductors. We’ve discussed both the strengths and limitations of using organic thermoelectrics in power generation and showcase new applications such as thermoelectric radiation sensing that go well beyond power generation. The intrinsic limitations of organic thermoelectrics in power generation do not apply to newer promising fronts of thermoelectric development. In conclusion, we hope our viewpoint inspires further development in the field of organic thermoelectrics including new modelling attempts that construct a unified understanding of the physics of transport in organic semiconductors.