Thermoelectric properties of conducting polymers: The case of poly(3-hexylthiophene)

Conducting polymers have recently been suggested as thermoelectric materials for use in large-area thermogenerators. To help assessing the feasibility of this the electrical conductivity and Seebeck coefficient of a series of heavily doped regioregular poly(3-hexylthiophene) films are measured between 220 and 370 K. $p$-type chemical doping of up to 34% is accompanied by the introduction of negatively charged counterions, ${\text{PF}}_6^{-}$. The counterions produce a disordered environment within which the $p$-type electronic carriers move. This disorder diminishes with increasing doping as the effect of the counterions is smoothed out. Concomitantly the thermally activated electrical conductivity rises strongly while its activation energy decreases. On the other hand, the Seebeck coefficient is found to be weakly dependent on temperature and it decreases with increasing doping. When combined, these results indicate that the thermoelectric power factor reaches a broad maximum between 20% and 31% doping. These results are discussed in terms of the thermally activated hopping-type mobility of bipolarons, deduced from the absence of electron spin resonance signal in the heavily doped materials.

Figure 1

Chemical sketches of the pristine, singled-charged (polaron), and doubly charged (bipolaron) poly (3-hexylthiophene) with their counterions. The chemical reaction of the doping reaction is: $\text{P}3\text{HT}+{\text{NOPF}}_6\to \text{NO}+\text{P}3{\text{HT}}^{+}{\left({\text{PF}}_6\right)}^{-}$.

Figure 2

(Color online) Evolution of the optical-absorption spectrum of P3HT as a function of doping level. Inset is a graph zoom in from 400 to 800 nm.

Figure 3

(Color online) $2\times 2\mu {\text{m}}^2$ height AFM images of P3HT films (A) undoped, (B) doped with a 0.5 mg/mL ${\text{NOPF}}_6$ solution in acetonitrile. (C) Distribution of the fibril width for the undoped (green downward diagonal) and doped (red upward diagonal) deposits.

Figure 4

(Color online) Temperature dependence of electrical conductivity (filled symbols) and Seebeck coefficient (open symbols) for various doping level.

Figure 5

(Color online) Evolution of the activation energy $E_a$ (solid squares) and the pre-exponential factor ${\sigma }_0$ (open triangles) versus doping level.

Figure 6

(Color online) Evolution of the electrical conductivity (at room temperature) with the doping level and a comparison with theoretical trends using Eq. (8). The discrepancy between the rate of change of the conductivity between the open square and the experimental data (dots) is attributed by the variation in electronic coupling between the bipolarons.

Figure 7

(Color online) Schematic of the density of states in pristine and doped P3HT films. CB and VB denote the conduction and valence bands of the pristine P3HT.

Figure 8

(Color online) The evolution of the Seebeck coefficient’s slope $\partial S/\partial \left(1/T\right)$ ($V$, the blue open circles) and its $1/T=0$ intercept ($S_o$, the black solid squares) with doping concentration.

Figure 9

(Color online) The comparison of the theoretical trend (open squares) of evolution of Seebeck coefficient vs doping level and the experimental data (filled circles) at room temperature.

Figure 10

(Color online) Log (conductivity) vs Seebeck coefficient for P3HT films at various doping levels at room temperature.

Figure 11

(Color online) Doping dependence of the room-temperature Seebeck coefficient $S$ (open triangles), electrical conductivity $\sigma$ (filled squares), and thermoelectric power factor $P_T$ (open circles).