The evolution of organic thermoelectric material based on conducting poly(3,4-ethylenedioxythiophene)

Abstract

Thermoelectric (TE) material, as one of new energy materials, is regarded as one of the most important energy-saving materials, which can directly achieve the interconversion between heat and electricity. Presently, inorganic semi-conductors are considered to be the best thermoelectric materials. However, the development of new thermoelectric material has attracted great attention owing to the scarce resource and limited performance of inorganic thermoelectric materials. As the discovery and wide application of conducting polymers (CPs), organic thermoelectric materials have come into the sights of people over the past 30 years. The development of CPs as a promising thermoelectric material has gained great attention over the past decades. Various CPs have been developed and investigated on thermoelectric performance such as polyacetylene (PA), polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh) and their derivatives since 1989. Among numerous CPs, poly(3,4-ethylenedioxythiophene) (PEDOT) shows many superior properties compared to others due to its excellent environmental stability, water solubility, easy processibility, and high electrical conductivity, which brings new strategy for studies of high performance organic thermoelectric materials. The conductive PEDOT combined with an insulating poly(styrenesulfonate) (PSS) can form a stable aqueous PEDOT:PSS with a good film-forming property and has been regarded as one of the most potential thermoelectric material. In 2008, the thermoelectric performance of PEDOT:PSS pellets were reported systematically for the first time. Although its thermoelectric figure-of-merit ( ZT ) was as low as 10 - 3, it was one of the highest values for CPs at that time. Soon afterwards, the thermoelectric performance of the free-standing PEDOT:PSS film achieved a ZT value as high as 10 - 2 in 2010. During these years, great attention focused on the derivatives of PTh, polyselenophene (PSh), PANi, and polycarbazole (PCz). Since 2010, PEDOT:PSS has come into sight of researchers in the world. The past few years witnessed great development of thermoelectric performance of conducting PEDOT:PSS ( ZT ~10 - 1). In recent ten years, the thermoelectric figure-of-merit ( ZT ) of PEDOT has been enhanced by three orders of magnitude from 10 - 4 to 10 - 1 as one of the most promising organic thermoelectric materials. Presently, the enhanced thermoelectric performance for PEDOT concern in the increased electrical conductivity via an easy method, especially for PEDOT:PSS. A large electrical conductivity of PEDOT:PSS thin film more than 3000 S cm - 1 has been achieved by adding an organic solvent or post-treatment with organic solvents, acid, and ionic liquids. Compared to inorganic thermoelectric materials, PEDOT:PSS can achieve a high electrical conductivity without an obviously decreasing Seebeck coefficient. A further improvement of thermoelectric performance for PEDOT:PSS has been devoted to the optimization of Seebeck coefficient via the pH value adjustment, the reduction of the oxidized level of conductive PEDOT, or composite with inorganic thermoelectric materials. A large thermoelectric power factor has become a reality. Although there are large gaps from actual industrial application for thermoelectric PEDOT ( ZT ˃1), yet most efforts focus on the high performance PEDOT. A large number of new techniques and methods have been developed to improve the thermoelectric performance of PEDOT. This review pays the attention to the advantages and characteristics, development history, and performance improvement of PEDOT as a promising organic thermoelectric CP from discovery to development. In order to achieve a high performance thermoelectric PEDOT, more attention should be paid to the development of low dimensional PEDOT crystal, control of oxidized level, extension of conjugated chain length of PEDOT, new preparation method and techniques, and the effects of crystal structure on electron transport properties as well as the flexible PEDOT thermoelectric devices in the future. Additionally, it is necessary to keep up with the development of n-type organic thermoelectric materials.