Fully Organic Nanocomposites with High Thermoelectric Power Factors by using a Dual-Stabilizer Preparation

The thermoelectric properties of fully organic nanocomposites were investigated, for which meso-tetra(4-carboxyphenyl) porphine (TCPP) and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were used as instrinically conductive and semiconducting stabilizers, respectively. The electrical conductivity (σ) of these dual-stabilizer organic composites increased to approximately 9500 S m⁻¹ as the concentrations of both the multiwalled carbon nanotubes (MWNTs) and PEDOT:PSS were increased. The thermopower (or Seebeck coefficient,S) and thermal conductivity, however, remained relatively unaffected by the increase in concentration (≈40 μV K⁻¹ and ≈0.12 W m⁻¹ K⁻¹, respectively). Replacing MWNTs with double-walled carbon nanotubes (DWNTs) increased σ and S to approximately 96 000 S m⁻¹ and 70 μV K⁻¹, respectively, at 40 wt % DWNTs. This study suggests that σ and S can be simultaneously tailored by using multiple stabilizing agents to affect the transport properties of the junctions between nanotubes. Combining semiconducting and intrinsically conductive molecules as CNT-stabilizers has led to a power factor that is among the best for a completely organic, free-standing film (≈500 μW m⁻¹ K⁻²). These flexible, segregated-network nanocomposites now exhibit properties that rival the more conventional inorganic semiconductors, particularly when normalized by the mass.