Anion-Dependent Molecular Doping and Charge Transport in Ferric Salt-Doped P3HT for Thermoelectric Application

Abstract

The effect of different iron (III) dopants on the doping process and charge transport properties based on a poly(3hexylthiophene) (P3HT) film was investigated. It is found that the doping level is dependent on not only the driving force for charge transfer but also the miscibility between a polymer and a dopant, while the mobile carrier transport is significantly controlled by the microstructure upon doping. A high electrical conductivity (128 S cm −1 ) is obtained for a FeCl 3 -doped P3HT film among three different doped P3HT combinations, although a low doping level is observed in this film. In contrast, a highest doping level but a low electrical conductivity (65 S cm −1 ) is achieved for Fe(OTf) 3 -doped P3HT. Another ferric salt with a larger size anion and strong oxidation ability, Fe(Tos) 3 , endows both much low doping level and low electrical conductivity (9 S cm −1 ). Grazing-incidence wide-angle X-ray scattering (GIWAXS) shows that a much stronger π−π stacking of P3HT and larger crystalline domains may exist in Fe(OTf) 3 -doped P3HT compared with those of FeCl 3 -doped P3HT. However, Hall-effect measurements show that the high electrical conductivity of FeCl 3 -doped P3HT is mainly attributed to higher carrier mobility. Temperature-dependent conductivity experiments demonstrate that smaller activation energy for carrier transport is needed for a FeCl 3 -doped P3HT film. These results indicate that smooth and continuous transport paths are formed in a FeCl 3doped film, contributing to high carrier mobility while discrete domains in Fe(OTf) 3 -doped film hamper the carrier transport. A prototype device with a five-leg FeCl 3 -doped P3HT film connected with a silver paste was fabricated. The measured maximum output power is about 4.64 nW at the temperature difference of 23.3 K. Our results suggest that the interaction between dopant anions and polymer chains is crucial for high electrical conductivity by improving morphologies to achieve ionized carriers' transfer into much mobile carriers.

Iron(III) Dopant Counterions Affect the Charge-Transport Properties of Poly(Thiophene) and Poly(Dialkoxythiophene) Derivatives

“…36 In contrast, when weakly or noncoordinating anion ligands are present, some solvents may substitute nearly all of the ligands around the iron center to yield a solvent-separated ion pair, leaving only X − species as the charge-balancing anions. [31][32][33]37 Some studies have examined the effects of counterion species on the resulting transport properties, 26,29,34,38 but the prevailing structure−property relationships are not clear as doping thermodynamics, kinetics, and spatial incorporation convolute these relationships.…”

Dopant-Stabilized Assembly of Poly(3-hexylthiophene)

“…This is consistent with P3HT oxidatively doped in thin films with FeTs 3 . 36 Furthermore, J-type nanostructures appear to exhibit slightly wider d-spacings, suggesting that they are more heavily doped. 54 Many of the oxidized nanostructures, particularly the J-type nanoparticles, exhibit scattering features consistent with Fe(II) salts (Figure S11k,l).…”

“…Next, iron (III) p -toluenesulfonate (FeTs 3 ) is added to the hot polymer solution at 5–20 mol % (with respect to the polymer repeat unit). FeTs 3 was selected as the dopant because it is soluble in organic solvents and has been previously studied as an oxidative dopant for a range of polymer semiconductors. , The hot polymer-dopant mixture is then allowed to cool over 1 h to 20 °C, forming a violet-purple colloidal suspension (Figure S2).…”

“…The d -spacing of neutral P3HT is 16.6 Å, whereas oxidized nanostructures at every T a and [FeTs 3 ] possess d -spacings greater than 20 Å when calculated from the (100) (Table S3) and greater than 18 Å when calculated from the (200), indicating expansion in the lamellar spacing and therefore an ICT doping mechanism (Table S3). This is consistent with P3HT oxidatively doped in thin films with FeTs 3 . Furthermore, J-type nanostructures appear to exhibit slightly wider d -spacings, suggesting that they are more heavily doped …”

A Dichlorinated Dithienylethene-Diketopyrrolopyrrole-Based Copolymer with Pronounced P–N Crossover: Evidence for Anionic Seebeck Contribution

“…Recently, ferric chloride (FeCl 3 ) has been used as a dopant for high performance p-type organic thermoelctrics. − In one reported study, the S of n-type organic thermoelectric system became positive when doped with a high concentration of n-dopant . Similarly, p-type polymers doped with a high concentration of p-dopants also can exhibit negative S .…”