# One-Dimensional Nanostructure Engineering of Conducting Polymers for Thermoelectric Applications
## 1 Introduction  
Development of eco-friendly and sustainable approaches for energy generation is a major global challenge faced by the world today. Thermoelectric materials that allow generation of electricity from heat and vice versa to provide a green and economic way for power generation and refrigeration have drawn tremendous interest from both industrial and scientific communities over the centuries [1; 2; 3; 4; 5]. The energy conversion efficiency of a thermoelectric material can be evaluated by the temperature difference and a unitless figure of merit (\(ZT\)), which is defined as \(S^{2}\sigma T/\kappa\), where \(S\) (V K \({}^{-1}\)) is Seebeck coefficient, \(\sigma\) (S m \({}^{-1}\)) is electrical conductivity, \(\kappa\) (W m \({}^{-1}\) K \({}^{-1}\)) is thermal conductivity and \(T\) (K) is absolute temperature. The power factor (\(PF\)) is the product of \(S^{2}\) and \(\sigma\), which is in general used to determine the thermoelectric performance of a thermoelectric material. The traditional high performing thermoelectric materials are mainly inorganic semiconductors or semimetals which possess moderate electrical conductivity and high Seebeck coefficient [6; 7]. However, most of these materials are facing significant challenges in large- scale manufacturing and practical utilization due to their heavy weight, high cost, scarcity, toxicity, poor processability, brittleness and non-flexibility, particularly for applications in wearable sensors and electronic devices. Given the limitations of the inorganic thermoelectric materials, the organic counterparts mainly based on conducting polymers and their nanocomposites, have gained tremendous research interest and clinched considerable achievements over the past two decades [8; 9; 10; 11; 12; 13; 14; 15]. Conducting polymers havebeen considered to be the next promising thermoelectric materials owing to their unique advantages including low cost, high mechanical flexibility, light weight, low or no toxicity, and intrinsically low thermal conductivity. In addition, the electronic structures of these organic semiconductors are fairly tunable via molecular chemistry and doping treatments. Polymer-based thermoelectric materials have intrinsically low thermal conductivity (0.1-0.5 Wm\({}^{-1}\) K\({}^{-1}\)), and therefore, enhancement in Seebeck coefficient and electrical conductivity is more important. Currently, the-state-of-art conducting polymers for thermoelectric applications include poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh) derivatives etc. The thermoelectric performance of these conducting polymers can be further enhanced through incorporation of various inorganic nanofillers (such as Te nanowires, Bi\({}_{2}\)Te\({}_{3}\) nanowires, carbon nanotubes (CNTs), graphene, etc. ), which give rise to the improvement in Seebeck coefficient and/or electrical conductivity while maintaining the low thermal conductivity. It has been acknowledged that polymer's properties are highly related to the morphologies and structures. Nanoscaled conducting polymers, especially those with one-dimensional (1D) nanostructures (such as polymer nanowires, nanotubes, nanofibers, nanorods, etc.) have their own special set of advantages over their corresponding bulk materials due to the highly ordered structure, large surface area and superior carrier transport [9; 16; 17; 18; 19]. Various methods for synthesizing 1D conducting polymers have been developed, including hard or soft template-based methods and template-free methods, such as inverse microemulsion polymerization, electrospinning method, etc. [20]. To date, significant progress has been achieved in the 1D engineering of conducting polymers for a wide variety of applications, such as batteries, supercapacitors, sensors, transistors, electrochromic displays, photovoltaics etc. [21; 22; 23; 24].