Impact of Counterion Size on the Thermoelectric Properties and Seebeck Coefficient in Conjugated Polymers

Organic semiconductors (OSCs) are promising materials for the design of next-generation bioelectronic devices, neuromorphic devices, biosensors, electrochromic displays, and thermoelectric devices. Doping (i.e., introducing charge carriers) of OSCs is key to controlling their electronic and optical properties for device applications. To effectively leverage the use of doping in OSCs, it is important to understand how the ions interact with the electronic charge carriers on the OSC. One of the parameters that helps determine the optical, electronic, and thermoelectric properties of doped OSCs is the counterion size, and understanding counterion size effects is important for tuning these properties. The Seebeck effect, which refers to the formation of an electrical potential resulting from a temperature differential across a material, is influenced by doping concentration, charge-carrier mobility, and counterion size. Optimizing the power factor (S²σ) of thermoelectric materials is challenging due to the inverse relationship between the Seebeck coefficient (S) and electrical conductivity (σ), however, by properly selecting the size of the dopant we can effectively adjust this tradeoff. In highly doped polymers, inversion of the sign of the Seebeck coefficient can occur at high doping levels for many π-conjugated polymer-dopant combinations. This Seebeck coefficient inversion is interesting from a fundamental charge transport perspective and could enable the creation of thermoelectric modules using the same polymer-dopant system for both p-type and n-type legs. While the impact of the counterion size and structure is being increasingly investigated, it has not yet been investigated how the counterion size impacts whether Seebeck coefficient inversion occurs. This work examines how counterion size affects Seebeck coefficient inversion in OSCs, and also demonstrates the impact of counterion structure on the thermoelectric performance of electrochemically doped organic semiconductors. It is found that often larger counterions result in higher power factors, but smaller counterions are more likely to result in Seebeck coefficient inversion.