PEDOT:PSS Conductivity

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PEDOT:PSS has conductivities in the range of 10^-4 - 10³ S cm^-1

PEDOT:PSS is conductive because it contains the conjugated intrinsically conductive polymer (ICP) PEDOT. The conjugated nature of PEDOT means there is continuous overlap of π-orbitals along the polymer backbone. This allows for the delocalization of electrons across multiple atoms. This electronic structure enables the easy movement of charge carriers such as electrons and holes.

Although PEDOT is conductive, PSS is insulating. The quantity of PSS and the microstructure of the film have a significant impact on the electronic properties of PEDOT:PSS. In a water-based dispersion, the PEDOT and PSS form a micelle structure in which the hydrophobic PEDOT core is surrounded by a shell of hydrophilic PSS. This structure is retained during deposition and forms localized regions of conductive PEDOT surrounded by insulating regions of PSS. It is this core-shell structure which results in the low conductivity values that can arise for standard formulations of PEDOT:PSS.

Changing the ratio of PEDOT to PSS can drastically affect the properties of your PEDOT material:

Ratio PEDOT:PSS (w/w)	Example	Conductivity	Stability of Suspension	Potential Application
1:20	CH 8000	Low	High	Use in OLEDs to reduce unwanted charge drift.
1:2.5	PH 1000	High	Low	Electrode or transport layer in device

Mechanism of PEDOT:PSS Conductivity

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PEDOT:PSS conductivity is due to both intra- and inter-grain charge transport mechanisms. Areas of crystallized polymer are described as grains. They contain multiple PEDOT polymer chains. The movement of charge carriers within and between these grains results in conductivity.

Intra-grain: Ultra-fast transport within conjugated polymer chains, moderately fast hops between close chains, slow π–π stacking interactions.

Inter-grain: Charge transport between grains is very reliant on morphology. Can be increased by elongating grains or increasing the interconnectivity of PEDOT.

The most limiting factor in conductivity is the slow π–π stacking interactions. A small decrease in stacking distance can dramatically increase interchain conductivity. Combining this with morphological changes where grains can interact more can help increase conductivity.

How to increase PEDOT:PSS conductivity

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The conductivity of PEDOT:PSS can be increased at different points of processing:

• Before thin film formation via additives, dopants and nanocomposites.
• During thin film formation by reducing drying speeds and multi-layered coating of PEDOT:PSS.
• After thin film formation via post treatments.

A PEDOT:PSS conductivity of 5012 S cm^-1 has been recorded following solvent additive doping (15 wt.% EG:DMSO 1:1), plasma post treatment and 1:1:1 water:EG:ethanol solvent post treatment. Slowly dried films showed the best performance compared to films that were annealed immediately.

Removal of insulating PSS is key to increasing conductivity. Most treatments disrupt the coulombic interactions between PEDOT and PSS. This allows for PSS removal. As a result there are changes in PEDOT morphology which increases charge transfer.

PEDOT:PSS Additives and Dopants

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Examples of PEDOT:PSS additives and dopants include:

• Polar and high boiling point solvents
• Ionic liquids
• Anionic surfactants
• Salts

Solvent additives for PEDOT:PSS

Solvents are mixed with PEDOT:PSS dispersions as a percentage of volume (usually ~5 - 15 %). Some of the most conductive PEDOT:PSS examples in literature have dimethyl sulfoxide (DMSO) or ethylene glycol (EG) doping. The polar nature of these solvents allows them to interact with PSS- chains and disrupt their ionic bonds with PEDOT. The high dielectric constant of these polar solvents reduces the electrostatic interactions between the positively charged PEDOT and negatively charged PSS. This alteration affects the morphology of the polymer grains, enabling more efficient packing of PEDOT molecules. Often, PSS is also removed from the system, contributing to higher overall conductivity.

Solvents used as dopants: EG, polyethylene glycol, diethylene glycol, DMSO, dimethyl sulphate, THF, DMF, sorbitol, glycerol, meso-erythritol, xylitol, methoxy ethanol, methanol, ethanol, acetone and water.

Solvent mixtures can also be used as additives, like the one mentioned above (15 wt.% EG:DMSO 1:1). Often water is used with a miscible polar solvent. Water solvates hydrophilic PSS and the organic cosolvent solvates hydrophobic PEDOT. This causes the separation of the polymers and elongates the PEDOT chain morphology resulting in increased conductivity.

Ionic liquid additives in PEDOT:PSS

Ionic liquids (eg. 1-butyl-3-methylimi-dazolium tetrafluoroborate ((BMIm)BF₄)) cause the PSS chains to swell. This changes the overall morphology of the polymer network allowing PEDOT chains to be more interconnected.

Ionic liquids used as dopants: 1-butyl-1-methylpyrrolidium chloride, 1-benzyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium bromide, and 1-butyl-3-methylimidazolium tetrafluoroborate.

Other dopants follow similar mechanisms of disrupting the interactions between PEDOT and PSS, allowing for PSS removal and efficient crystallization of PEDOT.

PEDOT:PSS Nanocomposites

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Nanomaterials can provide additional pathways for charge transport within PEDOT:PSS layers. They improve the connectivity of PEDOT chains and therefore increase conductivity. They help promote the crystallisation of PEDOT, reduce PSS content through displacement.

Nanomaterials used as dopants: metals, metal oxides, polymers and carbon nanomaterials such as reduced graphene oxide (rGO) and carbon nanotubes (CNTs).

For instance, CNTs exhibit high aspect ratios and excellent electrical properties, making them suitable candidates for enhancing the charge transport properties of PEDOT:PSS nanocomposites. Assisted by rGO incorporation, EG doped PEDOT:PSS film conductivity from 3 S cm⁻¹ to nearly 1225 S cm⁻¹ for a 10 wt% rGO composite.

Multi-layered coating of PEDOT:PSS

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PEDOT based film thickness can be increased through layering. Electrical conductivity and thermoelectric properties of spin-coated PEDOT:PSS film can be significantly enhanced by multi-layered coating technique. Regardless of the film thickness, the electrical conductivity (σ) of a single layer coated PEDOT:PSS film is more or less constant. However, electrical conductivity can be greatly improved with multi-layers of PEDOT films on top of each other onto the same substrate. Electrical conductivity up to 2 S cm⁻¹ can be achieved after only 5 layers of PEDOT:PSS film without additional reagents or solvent treatment.

Post-treatment of PEDOT:PSS

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Once you have your thin film of PEDOT:PSS you can treat it further to improve it's conductivity.

O₂ plasma treatment of PEDOT:PSS

Mild O₂ plasma treatments modify the surface electronic structure of PEDOT:PSS. Nanoscale polymer grains become uniformly distributed (around 20 nm). This leads to improved charge carrying capabilities.

O₂ plasma treatments the remove excess PSS (polystyrene sulfonate) from the surface of thin films of PEDOT:PSS, leading to a higher PEDOT to PSS ratio. This facilitates better charge carrier mobility. By etching away some of the material, oxygen plasma can create a rougher surface. This increases the effective surface area and can improve the interface between PEDOT:PSS and other layers or electrodes in a device, enhancing the overall device performance.

Oxygen plasma treatment is commonly used to treat the substrate before PEDOT:PSS deposition to improve wettability.

Acid treatment of PEDOT:PSS

Acid treatments consist of soaking PEDOT:PSS thin films in acid. Concentrated acids release protons that change PSS- into PSSH. Water can easily wash PSSH away. The protons also weaken the attraction between PEDOT+ and PSS-, leading to a phase-segregated structure that improves conductivity. The alignment and stacking of PEDOT's conjugated planes is also improved. An example treatment:

• Drop-cast a 1:5 v/v H₂SO₄ / methanol solution on a preprepared PEDOT:PSS thin film
• Remove the H₂SO₄ by rotating the substrate at a rotation speed of 600 rpm.
• Heat the sample at 150 °C for 10 min
• Repeat the process to grow a film with your desired thickness.

Solvent treatment of PEDOT:PSS

Solvents induce separation of the PEDOT and PSS as discussed in the additives and doping section. Solvents penetrate the polymers and disrupt their coulombic forces. PSS can then be washed away.

Solvent treatment of the coated films can also be carried out by spin coating the coated film with the pure solvent or non-solvent. Example treatment with the solvent dimethyl sulfoxide (DMSO):

• Drop a small amount of DMSO directly on the PEDOT:PSS film at 120 °C and dry for 15 min.
• Rinse film with DI water several times to remove excess PSS
• Dry the DMSO treated PEDOT:PSS film at 120 °C and dry for 15 mins.

PEDOT based polymers

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• Multifunctional
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References

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Hosseini, E. et al. (2020). The key mechanism of conductivity in PEDOT:PSS thin films exposed by anomalous conduction behaviour upon solvent-doping and sulfuric acid post-treatment, AJ. Mater. Chem. C. doi:10.1039/C9TC06311K

Lee, I. et al. (2016). Simultaneously Enhancing the Cohesion and Electrical Conductivity of PEDOT:PSS Conductive Polymer Films using DMSO Additives, ACS Appl. Mater. Interfaces. doi:10.1021/acsami.5b08753

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