# One-Dimensional Nanostructure Engineering of Conducting Polymers for Thermoelectric Applications
## 2 D-Conducting Polymers and Their Thermoelectric Properties
### Poly (3-alkylthiophene)  
Polythiophene and its derivatives are regarded as the most promising conducting polymers for thermoelectric applications [43; 44; 45]. Coupled on the good environmental stability of the polymer, a significant overlap among the \(\pi\) electron orbitals delocalized throughout the polymer chains and highly orientated semicrystalline structure give rise to a high carrier transport [46]. Among the various polythiophenes, poly(3-alkylthiophene) (P3 AT) is a noteworthy family which is extensively investigated for their electrical transport mechanisms, processing methods and device applications in solar cells, sensors, optoelectronics and so on [44; 47; 48; 49; 50]. A variety of 1D nanostructured materials based on P3 AT have been prepared by different kinds of approaches. It has been reported that continuous long nanofibers from P3 AT can be produced by electrospinning method, [24; 51; 52] while the shorter P3 AT nanofibers are mostly formed via self-assembly in solution with good crystallinity owing to strong interactions along their longitudinal axis [53; 54; 55; 56]. Other approaches, such as vapor-assisted imprinting [57], electrochemical synthesis [58; 59] and solution casting method [60; 61] using sacrificial templates have also been employed to prepare P3 AT nanowires or nanotubes for diverse applications.  
[PERSON] et al. developed high-aspect-ratio P3 AT nanofibers by the Whisker method utilizing anisole as a solvent [46]. The formation and properties of the fibers largely depend on the length of the alkyl chain and molecular weight of the polymer. In a typical experiment, a P3 AT polymer is well dissolved in anisole under heating and stirring. Then, during a quick cooling process to room temperature, nanofibers can grow in the solution. Using the Whisker method, [PERSON] et al. prepared poly(3-hexylthiophene) (P3 HT) nanofibers (50-60 nm in width and several microns in length), which further self-assembled to form networks via drop casting and subsequent doping by silver perchlorate to adjust the level of oxidation (Figure 5a-d) [62]. The nanonet structure exhibits a ZT value (0.0026) that is six times greater than its bulk polymer counterpart, owing to the highly  
Figure 4: (**a**) The effect of the H\({}_{2}\)SO\({}_{4}\) concentration on the gelation process of PEDOT:PSS. Shaded regions present that PEDOT:PSS is able (green) and unable (gray) to gel with various H\({}_{2}\)SO\({}_{4}\) contents. (**b**) Digital photographs of as-fabricated PEDOT:PSS fiber. SEM images of PEDOT:PSS fibers (**c**) before and (**d**) after ethylene glycol post-treatment. (**e**) Schematic diagram for thermoelectric device and (**f**) the output voltage and power density of five couples of legs at various temperature differences ranging from 5 to 60 K. Readpated with permission from American Chemical Society [42].  
regularly spaced, crystalline supramolecular structure, the 1D nanofiber morphology and the close electrical contact between each nanofibers inside the nanonet structure. [PERSON] et al. also prepared P3 HT nanofiber mats with high crystallinity using the similar method (Figure 5e,f) [63]. They found the thermal conductivity of the nanofiber mat was two times lower than the non-fibrous P3 HT polymer despite having comparable electrical conductivity, which is due to presence of a more efficient electron transport route and a large void fraction in the fibrous mats.