Influences Of Treatment With Salts Or Alkalies-based Reagents On The Thermoelectric Properties Of PEDOT:PSS Films

Energy is indispensible for industrial production and daily life.Recent years,due to the world’s demand for energy and the environmental impact of global climate change caused by the combustion of fossil fuels,developing environmentally friendly energy sources has become one of the most important channels for the sustainable development of the economy and society.Thermoelectric（TE）material,as a kind of green and functional material which can directly realize the conversion of thermal energy and electricity energy without pollution,can successfully make an access to the recycling of the waste heat and the harvest of the energy,and also can be applied in power generation and refrigeration.Compared with conventional inorganic TE materials,conducting polymers have been considered as great potentials as TE materials because of their intrinsic advantages such as rich sources,low density,low cost,excellent flexibility,and easy processing into versatile forms,especially the inherent low thermal conductivity.Poly（3,4-ethylenedioxythiophene）:poly（styrenesulfonate）（PEDOT:PSS）will be a promising candidate for organic TE materials because of its easy-turning electrical conductivity,low thermal conductivity,facile processing and good environmental stability.Nevertheless,the ZT value of PEDOT:PSS is still much lower than that of inorganic thermoelectric materials,mainly due to its poor Seebeck coefficient and electrical conductivity of its original state.Various attempts have been made to enhance the ZT value of PEDOT:PSS.Doping and dedoping processes strongly affect the carrier mobility and oxidation level of PEDOT chains by Seebeck coefficient and electrical conductivity,playing an important role in TE performance of PEDOT:PSS.Selection of effective doping or dedoping reagents is believed to optimize the electrical conductivity and Seebeck coefficient hence the TE performance.In this work,PEDOT:PSS were pretreated or post treated by different reagents based on salts or alkalies and the TE performance of the polymer films were systematically investigated.The major contents and conclusions are as follows:1.PEDOT:PSS solution was pretreated by different lithium salts and the TE performance of the polymer films has been investigated.It is found that,treatments with different lithium salts show different influences on the TE properties of PEDOT:PSS films.The maximum electrical conductivity was obtained in the LiClO₄-treated films,reaching 522 S cm^-1,which is approximately 2610 times higher than that of pristine film,while the corresponding Seebeck coefficient exhibits a decreasing tendency.Li₂SO₄ and LiNO₃ can also significantly improve the electrical conductivity,on the contrary,LiCl and LiB（C₂O₄）₂ show negligible effect.LiClO₄treatment gave rise to the highest power factor of 1.53μW m^-1 K^-2.Our study has proved that the remarkable improvement of electrical conductivity was mainly due to the doping effect,namely the conformational changes of PEDOT chains.2.PEDOT:PSS solution was pretreated by binary secondary doping with DMSO and LiClO₄ and the TE performance of the polymer films has been investigated.It is found that,doping with only DMSO improved the electrical conductivity of PEDOT:PSS film from 0.3 S cm^-1 to 437 S cm^-1,which then was further increased up to 950 S cm^-1 when LiClO₄ was added as the second dopant,indicating the synergistic effect of both dopants.The obtained electrical conductivity was almost 3000 times that of pristine film,while the Seebeck coefficient was negatively affected.Via binary secondary doping,PEDOT:PSS films possessed a maximum power factor of 8.25μW m^-1 K^-2.Our study has proved that the remarkable improvement of electrical conductivity was mainly due to the induced physical changes,namely the weakening of the electrostatic attraction between PEDOT and PSS and the conformational changes of PEDOT chains.3.Deep eutectic solvent（DES=ChCl/EG）was introduced as the additive or the surface treatment reagent for the doping of PEDOT:PSS and the TE performance of the polymer films has been investigated.It is found that,DES addition simultaneously enhanced the electrical conductivity and Seebeck coefficient of PEDOT:PSS films,reaching the optimized value of 70 S cm^-1 and 29.1μV K^-1,respectively.DES post-treatment was more effective than direct addition in improving the electrical conductivity,giving rise to a maximum value of 620.6 S cm^-1,with the Seebeck coefficient still being higher than that of pristine film.The highest power factor of the polymer film was calculated to be 24.08μW m^-1 K^-2 by DES post-treatment.Our study has proved that DES induced doping and dedoping effect,which leads to the both improved electrical conductivity and Seebeck coefficient.4.Deep eutectic solvent（DES=ChCl/Urea）and its mixture with DMSO were introduced as the surface treatment reagents for the thermal treatment of PEDOT:PSS films and the TE performance of the polymer films has been investigated.It is found that,DES thermal treatment simultaneously enhanced the electrical conductivity and Seebeck coefficient of PEDOT:PSS films,functioning better when temperature is higher.The optimal conductivity and Seebeck coefficient were found to be 85.6cm^-1and 30.1μV K^-1.DES/DMSO mixture could further improve the electrical conductivity and Seebeck coefficient,by changing the ratio between them,reaching an optimal power factor of 25.26μW m^-1 K^-2,with the corresponding electrical conductivity and Seebck coefficient being 424.2 cm^-1and 24.4μV K^-1.Our study has proved that DES presence induced doping and dedoping effect,which was the key reason for the TE improvement.5.Mixture of DMSO/NaOH（EtOH）solution was introduced as the surface treatment reagent for the thermal treatment of PEDOT:PSS films and the TE performance of the polymer films has been investigated.It is found that,relative to pristine film,the thermal treatment by the mixture could simultaneously benefit the electrical conductivity and Seebeck coefficient.By changing the ratios between DMSO and NaOH ethanol solution,the electrical conductivity ranged from 837.2 to0.04 S cm^-1 and the Seebeck coefficient varied from 12.6 to 54.8μV K^-1;At a certain ratio of 10:12,we obtained a maximum power factor of 33.04μW m^-1 K^-2.Our study has proved that DMSO/NaOH ethanol solution induced doping and dedoping effect,which has resulted in the significantly optimized power factor.