31x Power Factor Boost in PEDOT:PSS Thermoelectric Composites with COOH and OH SWCNT

Organic thermoelectric materials offer a path to flexible, lightweight, low-cost energy harvesting from body heat, IoT-device waste heat, and any low-grade temperature differential where rigid bismuth-telluride modules are inappropriate. PEDOT:PSS, the most studied conducting polymer for organic thermoelectrics, has a moderate Seebeck coefficient but limited intrinsic conductivity. The standard strategy — blend SWCNT into PEDOT:PSS — increases conductivity but typically lowers Seebeck (the universal Wiedemann-Franz-style trade-off in TE materials). A 2020 study from the Department of Chemistry at the University of Massachusetts Amherst by Tonga, Wei, and Lahti, published in the International Journal of Energy Research, broke that trade-off using Cheap Tubes carboxyl (COOH) and hydroxyl (OH) functionalized single-walled carbon nanotubes. The functionalized SWCNT/PEDOT:PSS blends showed simultaneous increases in both electrical conductivity and Seebeck coefficient, raising the thermoelectric power factor to 22 µW/m·K² — an ~31× improvement over the same composite with pristine SWNT (0.7 µW/m·K²). Funded by the U.S. Defense Threat Reduction Agency.

The Research Question

The figure of merit for a thermoelectric material is ZT = S²σT/κ, where S is the Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature. The numerator S²σ is the "power factor" (PF), reported in units of W/m·K². Higher PF means more electrical power produced per unit temperature differential. The hard problem in PF engineering is that S and σ usually move in opposite directions — the same doping that raises carrier concentration (and σ) raises Fermi level and lowers S, and vice versa. The Tonga group set out to ask whether functionalized SWCNT — with COOH or OH groups on the nanotube wall — could break that inverse relationship in a PEDOT:PSS composite by simultaneously improving charge transport and increasing Seebeck via interface energy-filtering effects.

Materials and Methods

SWCNT — functionalized variants from Cheap Tubes

From the paper's Experimental Section (verbatim): "Functionalized SWNTs were purchased from CheapTubes. SWNT-COOH: >90% purity with outer diameter 1-4 nm and 5-30 μm length. SWNT-OH: >90% purity with outer diameter 1-2 nm and 5-30 μm length. SWNT was purchased from Sigma-Aldrich with ≥50-70% purity and diameters of 1.2-1.5 nm."

• SWNT-COOH: >90% purity, 1-4 nm OD, 5-30 μm length — Cheap Tubes.
• SWNT-OH: >90% purity, 1-2 nm OD, 5-30 μm length — Cheap Tubes.
• Pristine SWNT control: Sigma-Aldrich, 50-70% purity — used as the comparison baseline.

The contrast is informative: the Cheap Tubes functionalized material was used because the authors needed both surface-chemistry control (COOH or OH for hydrogen bonding to PEDOT:PSS) and high purity (>90%) to keep amorphous carbon and metal residue from dominating the electrical response. The Sigma pristine material at 50-70% purity served only as the baseline; functionalized SWCNT at >90% purity is what produced the result.

PEDOT:PSS host polymer

• CLEVIOS™ PVP Al 4083, the standard hole-transport-layer formulation.
• ~1.5 wt% solids in water, PEDOT:PSS weight ratio 1:6.

Blend preparation and film casting

• Solvent: water only — the COOH and OH functionalization on the SWCNT walls enables surfactant-free dispersion in aqueous PEDOT:PSS.
• SWCNT loading: swept from 10 wt% to 65 wt% to identify the optimal composition.
• Film casting: drop-cast or spin-cast on substrate; samples sized for in-plane four-probe conductivity and thermoelectric measurement.

Characterization

• Four-probe in-plane electrical conductivity (σ).
• Seebeck coefficient (S) by temperature gradient + thermocouple measurement.
• Power factor PF = S²σ calculated from measured S and σ.
• FTIR and UV-Vis to verify hydrogen-bonding interactions between the SWCNT surface groups and PEDOT chains.

Key Results

Atypical thermoelectric behavior — both σ and S increase together

The headline finding is the breaking of the usual S vs σ trade-off. With COOH-functionalized SWCNT loading swept from 10% to 65%, electrical conductivity climbed monotonically to ~115 S/cm (from 8 S/cm in pristine PEDOT:PSS — a 14× gain). What's unusual is that the Seebeck coefficient went up from 18 µV/K to 42 µV/K over the same composition range, rather than down. The product S²σ therefore got the benefit of both terms, producing the 22 µW/m·K² power factor — a ~31× gain over the pristine-SWNT control composite (0.7 µW/m·K²) and a ~2,400× gain over PEDOT:PSS alone (0.009 µW/m·K²).

COOH outperforms OH

At matched SWCNT loading, the COOH-functionalized blend produced PF = 22 µW/m·K² while the OH-functionalized blend produced PF = 16 µW/m·K². Both are dramatic improvements over the pristine SWNT and PEDOT:PSS-only baselines, but COOH is the stronger surface chemistry for this application — consistent with the carboxylate's stronger hydrogen-bond donor character vs hydroxyl.

Why — the energy-filtering mechanism

The authors attribute the atypical S+σ co-increase to two effects acting in concert:

• Conformational extension of PEDOT chains. Hydrogen bonds between the COOH or OH groups on the SWCNT wall and the polythiophene backbone induce PEDOT chains to adopt extended, linear conformations rather than coiled ones. Extended chains improve charge mobility (raises σ).
• Energy-filtering at SWCNT/PEDOT junctions. The energy barriers at the nanotube/polymer interface block low-energy carriers from contributing to transport, leaving only the higher-energy carriers — which carry more energy per charge and therefore raise the Seebeck coefficient.

Both mechanisms require the surface functionalization. Pristine SWCNT has no anchor for H-bonding with PEDOT (no chain straightening) and no engineered junction barrier (no energy filtering). The COOH and OH variants from Cheap Tubes deliver both.

Why Cheap Tubes Functionalized SWCNT Works Here

• >90% purity matters for thermoelectrics. Amorphous carbon and metal-catalyst residue carry their own electronic states and degrade clean PF measurements. The Sigma-Aldrich pristine baseline at 50-70% purity produced PF = 0.7 µW/m·K²; the Cheap Tubes functionalized material at >90% purity produced PF = 22 µW/m·K² — the purity gap is part of the story.
• Surface-chemistry choice matters. COOH and OH variants both produce dramatic improvements but COOH is stronger. Cheap Tubes supplies both as standard catalog items, letting researchers compare surface chemistries directly without one-off custom synthesis.
• Aqueous processing. The COOH and OH functionalization makes the SWCNT water-dispersible without surfactants — compatible with PEDOT:PSS (CLEVIOS™) which is itself an aqueous formulation. The whole composite is processed in water.

Application Areas

• Wearable and on-skin energy harvesting — body-heat-powered IoT sensors, smartwatch self-charge augmentation, medical monitoring patches. The flexible PEDOT:PSS/SWCNT film is the right form factor.
• Industrial waste-heat recovery (low-grade) — pipe surfaces, exhaust ducts, server-rack air outlets where the temperature differential is <100 °C and rigid bismuth-telluride is impractical.
• Self-powered sensors — environmental monitors, agricultural sensors, structural health monitors in remote installations where battery replacement is costly.
• Soft / printed thermoelectric devices — inkjet- or screen-printed thermoelectric modules for distributed sensing networks.
• Defense and security wearables — the paper's DTRA funding hints at the use case: soldier-mounted sensors and field IoT powered by body heat without battery dependency.

Order the Cheap Tubes COOH and OH SWCNT Used in This Study

The carboxyl- and hydroxyl-functionalized SWCNT used by the Tonga group at UMass Amherst are catalog items from Cheap Tubes:

• SWNT-COOH: >90% purity, 1-4 nm outer diameter, 5-30 μm length. The headline-result variant of the Tonga study (PF = 22 µW/m·K²). Available pristine or with COOH content tailored to the application.
• SWNT-OH: >90% purity, 1-2 nm outer diameter, 5-30 μm length. The second variant of the Tonga study (PF = 16 µW/m·K²). Currently special-order at Cheap Tubes.
• SWNT-NH₂: available as the amine-functionalized counterpart for epoxy crosslinking and biomolecule conjugation.

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