Preparation And Device Design Of PEDOT:PSS/SWCNTs Composite Thermoelectric Films

The rapid development of self-charging technology for flexible microelectronics and multifunctional electronics,represented by wearable or implantable electronic devices,creates great prospects for a new era of medical sensing,human-computer interaction,device connectivity and a range of other applications.Flexible thermoelectric(TE)devices can be attached to heat sources with different degrees of curvature and make good thermal contact with them to achieve efficient thermal energy conversion and power supply for electronic devices.TE devices often consist of p-and n-type TE materials connected alternately in series-parallel conversion circuits to obtain high output power.Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)/single-walled carbon nanotubes(SWCNTs)composite films have the flexibility of PEDOT:PSS and SWCNTs,the high conductivity and Seebeck coefficient of SWCNTs,and the low thermal conductivity of conductive polymers.It is a potential candidate for the development of high-performance flexible TE materials and the construction of micro self-charging TE devices.However,the low TE properties of PEDOT:PSS/SWCNTs composites make it difficult to maintain high efficiency energy harvesting for subsequent applications.Also,the lack of device design and other aspects has hindered the development of PEDOT:PSS/SWCNTs material systems.In this paper,the TE properties of p-type PEDOT:PSS/SWCNTs composite films were optimized using secondary doping and secondary binary de-doping.Secondly,the n-type conversion of the SWCNTs films was also accomplished.Simultaneously,p-n type matrix flexible TE devices with vertical temperature gradients were constructed.Specifically,the TE properties of the PEDOT:PSS/SWCNTs composite films were optimized by a polar solvent dimethyl sulfoxide(DMSO)doping and strong reducer borohydride sodium(NaBH4)de-doping process.To further enhance the TE properties of the PEDOT:PSS/SWCNTs composite films,a binary de-doping agent consisting of sodium bicarbonate(NaHCO3)and the polar solvent ethylene glycol(EG)was used to de-dope the PEDOT:PSS/SWCNTs composite films,avoiding the effect of a single strong reducing agent de-doping on the microstructure of the composite films.Simultaneously,highly stable n-type SWCNTs films were prepared using amine-rich doping of the electron donor polyethyleneimine(PEI).It was matched with the p-type PEDOT:PSS/SWCNTs composite film to construct a p-n matrix-type flexible TE device,which achieved a high output power.The main conclusions of this paper include the following.1.The treatment process of DMSO secondary doping and NaBH4 de-doping adjusted the doping level of PEDOT:PSS/SWCNTs composite films.The method significantly improved the TE properties of p-type composite films and increased the power factor to 411 μW m-1 K-2.DMSO doping is beneficial to the formation of highly ordered stacking structure of PEDOT molecular chains,and forms an interconnected conductive network with SWCNTs,which greatly improves the conductivity of the composite film.The NaBH4 de-doping process reduces the oxidation level of PEDOT:PSS molecules and the carrier concentration of the composite film,which further improves the Seebeck coefficient.However,the de-doping process leads to a larger interlayer distance and even the collapse of the layered structure,resulting in a decrease in conductivity.The obtained high performance p-type PEDOT:PSS/SWCNTs composite films were cut and assembled into 8-legged horizontal TE devices.The maximum open-circuit voltage of the TE devices was 6.34 mV and the maximum output power was 391 nW at a temperature difference of 20 K.2.To minimize the sharp decrease in electrical conductivity caused by microstructural changes during the de-doping of NaBH4,the TE properties of p-type PEDOT:PSS/SWCNTs composite films were optimized by DMSO secondary doping and EG/NaHCO3 binary de-doping,and the power factor was further increased to 500μW m-1 K-2.The binary de-doping agent(composed of EG and NaHCO3)was selected to replace the single component NaBH4 de-doping agent,so that the interlayer distance and layered structure of the PEDOT:PSS/SWCNTs film were maintained during the de-doping process,and the power factor was increased by nearly 20%.The improved conductivity of p-type PEDOT:PSS/SWCNTs composite films is mainly attributed to the effect of DMSO doping.It removes the excess insulating PSS chains and induce phase separation and chain conformational changes in PEDOT molecules,enhancing intra-and inter-grain coupling.In addition,the conductive network structure composed of PEDOT:PSS and SWCNTs facilitates carrier transport and further improves the conductivity of the composite films.The large increase in Seebeck coefficient of the composite films was mainly attributed to the post-treatment process of EG/NaHCO3 which adjusted the oxidation level of PEDOT molecules and reduced the carrier concentration.3.In order to develop TE devices with high output power,it is necessary to find stable n-type TE materials top-type composite films.Therefore,the doping of PEI,an amine-rich electron donor,changed the structure of SWCNTs films and successfully prepared n-type SWCNTs films with high air stability.At 1 wt%PEI doping concentration,the power factor of n-type SWCNTs films reached 185 μW m-1 K-2.The air stability test of TE properties shows that the conductivity and Seebeck coefficient of n-type SWCNTs films decreased slightly with increasing exposure time in air environment during 10 days.The power factor of n-type SWCNTs films only decreased 20.8%after 30 days.A novel matrix-type flexible TE device with vertical temperature gradient was designed and assembled by cutting the p-type PEDOT:PSS/SWCNTs composite film with n-type SWCNTs film.The device has excellent flexibility and can be applied to different curved scenarios,such as the human wrist or other cylindrical heat sources.The maximum open-circuit voltage of the TE device is 23.2 mV and the maximum output power is 2.6 μW at a temperature difference of 48 K.Compared with the horizontal TE device with the same number and length of legs,the p-n matrix TE device has better effect on maintaining temperature difference,and the thermal voltage generated within 30 min is twice that of the horizontal TE device.The design and fabrication of the new structured,high-performance flexible TE device has advanced the development of self-charging technology for wearable and implantable electronic devices.