In general, the power factor (P) of a thermoelectric device is described as a product of the electrical conductivity (σ) and Seebeck coefficient (S) as P=S2σ, whereas these two quantities tend to show opposite behavior upon carrier doping. Thus, it is indispensable to clarify the accurate relationship between σ and S in doped films in order to obtain a guideline to improve P. In semicrystalline polymers such as PBTTT (Fig. 1(a)), an empirical relation of S∝σ-1/4 (P∝σ1/2) has been reported in a wide range of σ by using chemical doping, although no physical background for this relation has been clarified [1]. However, different σ-S relation has also been pointed out recently based on the data analyses of various reports including the same data set in ref. 1 [2]. Such difference seems to arise from the different doping conditions in chemical doping. In this presentation, we report our recent challenge to determine the accurate σ-S relation in various high-mobility polymer materials by using an electrolyte-gated thin-film transistor (TFT) structure, which enables a continuous electrochemical doping on the same device up to high carrier densities.
Fig. 1(b) shows a schematic illustration of the TFT device and measurement setup [3]. We adopted an ionic liquid as the electrolyte, which was drop-casted on the polymer thin film fabricated on the glass substrate. A temperature gradient was given by a couple of Peltier devices and the induced voltage (ΔV) and temperature difference (ΔT) between source (S) and drain (D) electrodes were measured to obtain seebeck coefficient (S=ΔV/ΔT) under various gate voltages (Vg). The electrical conductivity was simultaneously measured at each Vg.
Fig. 1(c) shows an example of σ-S and σ-P relations upon electrochemical doping obtained for two PBTTT devices. We observe highly reproducible data for these devices. In the low conductivity region (σ<100 S/cm), the empirical relation of S∝σ-1/4 is clearly confirmed, whereas the deviation from this relation becomes evident in the high conductivity region (σ>100 S/cm). The maximum of P appears around the boundary of low and high conductivity regions. In the high conductivity region, the σ-S relation seems to approach S∝σ-1, typical to the Mott’s relation expected for metals. This behavior is consistent with the semiconductor-metal transition of PBTTT occuring at highly-doped regions, which is observed from the temperature dependence of the conductivity as well as the electron spin resonance (ESR) measurements.
Similar σ-S relation was commonly observed in a wide range of polymers such as donor-acceptor (DA) type copolymers. The maximum value of P can be enhanced by the surface treatment of the substrate by self-assembled monolayers or by aligning the polymer main chain, presumably due to the increase of the mobility, although the S∝σ-1/4 relation is unchanged in the low conductivity regions. The above σ-S relation insensitive to the mobility is indicative of the contribution of domain boundaries in thermoelectric properties.
[1] A. M. Glaudell et al., Adv. Energy Mater. 5, 1401072 (2015).
[2] S. D. Kang et al., Nat. Mater. 16, 252 (2017).