PEDOT:PSS Properties: How to Enhance Them

PEDOT:PSS has a unique combination of electrical, optical, mechanical, and thermoelectric properties. It is one of the most widely used intrinsically conducting polymer (ICP) blends. As a result of it’s desirable properties it has received significant attention in the field of organic optoelectronics, particularly as a p-type semiconductor. This versatile material exhibits a range properties that make it suitable for various applications.

Electrical Properties of PEDOT:PSS

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One of the most notable properties of PEDOT:PSS is its electrical conductivity.  PEDOT provides the polymer blend with its conductive properties. The π-conjugated structure of PEDOT allows for delocalization of electrons along the polymer backbone, enabling efficient charge transport. PSS provides charge balance and stabilizing properties. High electrical conductivity is crucial for applications such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), sensors, and conductive coatings.

Improving PEDOT:PSS Electrical Conductivity

The electrical conductivity of PEDOT:PSS can be modulated in several ways, including:

• PEDOT:PSS ratio - The more PSS there is in the blend the lower the conductivity of the material.
• Doping level - Doping agents such as organic acids or solvents enhance the conductivity by introducing charge carriers into the polymer matrix.
• Film thickness - Electrical conductivity can be greatly improved with multi-layers of PEDOT:PSS
• Processing conditions - Electrical conductivity can be enhanced through pre-treatment of secondary doping with polar solvents, additives to form nanocomposites
• Post-treatment techniques - Post-treatment with solvents and in particular the post acid treatment can result in significant conductivity improvement.

PEDOT:PSS possesses largely tuneable conductivity ranging from 0.1 S cm⁻¹ for undoped PEDOT:PSS films up to 8797 S cm⁻¹ for single-crystal PEDOT nanowires. This wide range of electric conductivity gives rise to the versatile applications of PEDOT:PSS in OLEDs, OPVs, thermos-electronics, supercapacitors and energy storage, sensors and bioelectronics, antistatic packing, flexible and wearable electronics.

Optical Properties of PEDOT:PSS

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PEDOT:PSS is typically a blue dispersion, and its films exhibit minimal to no absorption in the ultraviolet (UV) and visible (Vis) regions of the spectrum. Absorption only increases in the near-infrared (NIR) region.

Optical Transparency

PEDOT:PSS exhibits good optical transparency in the visible spectrum, making it suitable for optoelectronic applications where light transmission is desired. The optical transparency, as well as the conductivity of PEDOT:PSS, makes it highly attractive for various applications in optoelectronics and transparent electronics. One of the primary applications of PEDOT:PSS is as a transparent electrode in devices like organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and flexible electronics. By depositing PEDOT:PSS as a thin, transparent conductive layer, it can replace traditional opaque electrodes (e.g., indium tin oxide, ITO) while maintaining good electrical conductivity and optical transparency. Transparent conductive films based on PEDOT:PSS are used in touch screens and display technologies. These films enable touch sensitivity and act as transparent electrodes for driving OLEDs or liquid crystal displays (LCDs).

The transparency of PEDOT:PSS films depends on factors such as:

• Film thickness - The transparency of PEDOT:PSS films can be tailored by optimizing the film thickness during deposition processes like spin coating or inkjet printing.
• Substrate type - Thin films of PEDOT:PSS on transparent substrates such as glass or flexible plastics can achieve high transmittance while maintaining conductivity.
• Processing conditions - PEDOT:PSS can be doped with various dopants to modify its optical properties, including absorption spectra and refractive index, enabling tailored designs for specific device requirements.

Mechanical Properties of PEDOT:PSS

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PEDOT:PSS films are flexible and exhibit good mechanical properties compared to traditional inorganic conductive materials like indium tin oxide. This flexibility stems from the polymer nature of PEDOT:PSS and the interactions between its constituents, allowing it to conform to curved or flexible surfaces without cracking or delamination. When subjected to tensile forces, PEDOT:PSS can undergo significant elongation before failure. Flexible and stretchable electronics benefit from PEDOT:PSS's mechanical flexibility, enabling applications in wearable devices, flexible displays, and biomedical sensors.

Improving PEDOT:PSS Flexibility

Pure PEDOT:PSS films are relatively brittle and can exhibit poor adhesion to certain substrates. However, the mechanical properties of PEDOT:PSS films can be further enhanced through optimization of film thickness, substrate choice, pre- and post-treatment methods. Strategies such as blending with other polymers, carbon nanomaterials, cross-linking treatments, and surface modifications can improve the mechanical flexibility and adhesion of PEDOT:PSS films. In flexible electronics, PEDOT:PSS is often incorporated into composite materials or layered structures to enhance mechanical robustness while maintaining its desirable electrical conductivity.

There are three main strategies to improve the flexibility of the coated PEDOT:PSS:

1. PEDOT:PSS can be geometrically coated or patterned onto the supporting elastomeric substrate, forming wrinkled structure that can stand afford repeated stretching–releasing process. Nanocomposites of PEDOT:PSS with functionalized reduced graphene oxide (rGO) nanosheets deposited onto a pre-stretched polydimethylsiloxane (PDMS) substrate showed a good stability in stretching–releasing tests at 15% strain.
2. PEDOT:PSS can be made into an insulating elastomeric matrix. The blends of PEDOT:PSS with waterborne polyurethane (WPU) can have a conductivity of ∼ 80 S cm⁻¹ and an elongation at break of > 30%.
3. Stretchable PEDOT:PSS films can be realized by the incorporation of plasticizers such as Zonyl and Triton.  By adding 5 wt% DMSO and 1 wt% Zonyl FS-300 into PEDOT:PSS (PH1000), a stretchable and conductive four-layer film coated on a prestrained PDMS substrate can withstand over 5000 stretching cycles of 0 to 10% strain with no change in resistance. The addition of a compound with two or more hydroxyl groups, like glycerol or malic acid, can significantly enhanced the mechanical stretchability of PEDOT:PSS films from < 10% of pristine PEDOT:PSS films to > 50%. This effectively lowers the interchain interaction with PSS and thus increases the overall mechanical stretchability of PEDOT:PSS. Simultaneously, these additive compounds can induce secondary doping to enhance the conductivity of PEDOT:PSS films.

Thermoelectric Properties of PEDOT:PSS

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Thermoelectric polymers like PEDOT:PSS have advantages including low intrinsic thermal conductivity, low cost, no or low toxicity, and high mechanical flexibility. Especially when compared with the conventional thermoelectric materials that are inorganic semimetals or semiconductors.

PEDOT: PSS stands out as one of the most extensively studied conducting polymers for thermoelectric applications, due to its high electric conductivity when doped with a secondary dopant and its low thermal conductivity. Both high electrical conductivity and Seebeck coefficient are required for employment in thermoelectric devices. Dopants such as small molecules (e.g., organic molecules like dimethyl sulfoxide, DMSO), or nanoparticles (e.g., carbon nanotubes, graphene oxide) can improve charge carrier mobility and increase the Seebeck coefficient.

Improving PEDOT:PSS Thermoelectric Properties

Techniques for improving PEDOT:PSS thermoelectric properties include:

• Solvent Treatment - Dipping PEDOT:PSS into DMSO or mixtures of methanol ethanol
• Composites - Blended with additives like single-walled carbon nanotubes (SWCNT)
• Post-treatment - Soaking in mixtures like trifluoromethanesulfonic acid in methanol
• Solution-shearing deposition – helps with the removal of PSS to increase electrical conductivity.
• Reducing agents - hydrazine, sodium sulfite (Na₂SO₃), sodium borohydride (NaBH₄), or hydroiodic acid (HI).

Chemical stability

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The chemical stability of PEDOT:PSS refers to its resistance to degradation when exposed to various environmental factors such as:

moisture - The PSS chains bearing both acidity and hygroscopicity, could lead to substrate corrosion, device degradation, and ultimate conductivity, transparency and mechanical flexibility lost. Water molecules can hydrolyse the ester bonds in PSS, leading to the detachment of PSS chains from PEDOT and affecting film adhesion and stability.

• Oxygen - can react with PEDOT chains, leading to the formation of oxidative species and the loss of conjugation, resulting in decreased conductivity.
• Heat - causes deformation of the polymer blend
• Light - PEDOT:PSS is subject to UV degradation, UV radiation can induce photochemical reactions in PEDOT:PSS, leading to chain scission, cross-linking, and changes in molecular structure.
• Chemical substances - Exposure to ions, especially in electrolyte solutions, can disrupt the charge balance in PEDOT:PSS films, affecting conductivity and stability.

Improving PEDOT:PSS Chemical Stability

Stability is a critical aspect for the performance and reliability of PEDOT:PSS-based devices, as degradation can lead to a decrease in conductivity, changes in optical properties, and mechanical failure. There have been many techniques used to increase the stability of PEDOT:PSS including:

• Neutralize PEDOT:PSS acidity - mild bases such as imidazole have been used a additives to tune the pH of PEDOT:PSS. Urea has also been used to help neutralize and it also directs phase separation of PEDOT and PSS segments, leading to enhanced conductivity.
• Post-treatment - Perfluorooctane sulfonic acid (PFOSA) can not only improve the film conductivity but also significantly improve its chemical and long term stability. Doping with DMF followed by camphor sulphonic acid (CSA) post-treatment significantly enhances device stability in terms of conductivity and optical transmittance.
• Composites - the addition of graphene into PEDOT:PSS to form graphene/PEDOT:PSS composite can also improve charge extraction hence up to 10 times its conductivity while promote great stability in the perovskite active layer.

PEDOT:PSS and PEDOT Based Polymers

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