Study On Thermoelectric Properties Of Doped PEDOT

Thermoelectric materials are important energy conversion materials that directly transform heat to electric energy, being used in thermoelectric power generation or cooling devices due to their obvious advantages including no moving part, long lifetime, non-noise and environment-friendly. Presently, the most widely used thermoelectric materials are (Bi2Te3)-based alloys with a figure of merit(ZT) in the range of 1 to 2.5. However, the high cost and high toxicity in their synthesis procedures limit the large-scale applications of these alloy-kinds materials. Motivated by the desires of sustainable developing strategies, organic materials with extremely low-cost, good flexibility and environmental friendliness are expected to partially release these burdens as thermoelectric materials at room temperature in near future. One of potential organic thermoelectric materials (OTEMs) is the conducting polymers with special geometric structures, high electric conductivity and low thermal conductivity. Poly(3,4-ethylenedioxythiophene) (PEDOT) as one of conducting polymers are applied to thermoelectric fields and obtain the highest ZT of 0.42 under appropriate doping, which promotes the development of organic thermoelectric devices. However, doping effects on the thermoelectric properties of PEDOT has not yet been fully explained in detail.In this study, we focus on the Tos/Tos- doping effects in the pristine PEDOT crystals and study their thermoelectric properties by using the DFT methods. Firstly, we optimize the pristine and doping PEDOT structures with constraining the lattice constants to the experimental values. Second, we studied their electronic, thermal and thermoelectric properties based on the optimizedstructures. Acquired results as follow:(1) The pristine PEDOT crystal structure exhibits semiconductor properties with a small bandgap of 0.452eV. The doping PEDOT crystal structures with Tos- exhibit metallic-like properties without any bandgap near by the fermi level. The remarkable differences may be caused by the structure changes(from aromatic-like to semi-quinoid-like) and the charge transfer between PEDOT layers and Tos- layers.(2) Comparing to pristine PEDOT, To- doping PEDOT structures present higher electronic conductivity, higher Tos- doping concentration higher electronic conductivity.(3) Tos- doping effectively influenced Seebeck coefficient of PEDOT crystal structures and a appropriate doping concentration is very important for optimizing Seebeck coefficient. Ultimately a optimized doping concentration of 12.5% is acquired, higher doping concentration is bad for obtaining the high Seebeck coefficient.(4) Tos molecular doping structures present completely different results from Tos" doping. Tos doping nearly changes the PEDOT chain structures or promotes the charge transfer. Therefore, Tos doping make no contribution to improve the electronic conductivity and Seebeck coefficient of pristine PEDOT crystal.(5) PEDOT crystal structures present anisotropic transfer properties, optimized transfer direction is lattice c direction, next-best is lattice b direction, the poorest is lattice c direction.In addition to the above research, we also study the performance of organic thermoelectric devices(OTEDs) based on the above research results. Results indicate that the performance of OTEDs is closely related to the thermoelectric properties of PEDOT crystals structures, better thermoelectric properties better performance. Furthermore, the thermoelectric effects and thermal resistance effects common influence the performance of OTEDs, an appropriate balance is very important for optimizing performance of organic thermoelectric devices.