Improving the thermoelectric power factor of PEDOT:PSS with 4,4'-bipyridine and LiBF 4.

Abstract

Background: Thermoelectric (TE) materials can directly convert heat into electricity, which is beneficial for energy sustainability. Organic conducting polymers are TE materials that have drawn significant attention owing to different favorable properties, such as good processability, availability, flexibility, and intrinsically low thermal conductivity. Among the organic TEs, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is the most extensively investigated material because of its stability and high electrical conductivity. The power factor ( PF) of PEDOT:PSS can be increased using different strategies, such as secondary doping, dedoping, energy filtering, and sequential post-treatments. All these strategies involve the contact of the polymer with different compounds.

Methods: Herein, we have analyzed the impact on the PF of the treatment of PEDOT:PSS with two different systems: (i) a 0.1M solution of 4,4'-bipyridine in 3-methoxypropionitrile and (ii) a 0.1M solution of LiBF ₄ in the same solvent. Impedance, Raman, and ultraviolet-visible-near infrared spectroscopies were employed to understand the variations observed.

Results: The results show that after the treatments, the Seebeck coefficient increased from ca. 12 to ca. 21 μV/K in both cases, and the electrical resistance of the film increased by 46.78% for 4,4'-bipyridine, and only 4.38% in the case of LiBF ₄, reaching at least 2.08 and 3.53 times PF improvements, respectively, with respect to the initial PF value (6.32 μWK ^-2m ^-1). The impedance spectroscopy analysis revealed that only an ohmic behavior existed in all cases. In addition, Raman and UV-vis-NIR analyses identified a dedoping mechanism, which explains the Seebeck coefficient variations identified in both treatments and the increase in electrical resistance for 4,4'-bypiridine. The remarkable lack of resistance increase for LiBF ₄ points to a different phenomenon that could be related to morphological effects.

Conclusion: These two new treatments demonstrate their capability to reach PF values close to the state of the art and expand the catalogue of treatments available for PEDOT:PSS.