PEDOT Coating and Solution Processing

Methods for depositing PEDOT:PSS, are crucial for fabricating various organic electronic devices. These methods play a significant role in determining the film thickness and morphology, electrical properties, and performance of PEDOT-based materials. Commercially available PEDOT based dispersions or EDOT monomer solutions can be readily deposited onto different substrates using different solution processing techniques such as:

• Doctor blading
• Dip coating
• Slot-die coating
• Spin coating
• Spray coating
• Inkjet printing

Detailed in the following, we will explore four major deposition techniques used for PEDOT and PEDOT:PSS films, namely spin coating, dip coating, spray coating, and inkjet printing.

Spin Coating PEDOT

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Spin coating is by far one of the most widely used methods for depositing thin films of PEDOT and PEDOT:PSS. It involves the deposition of a solution onto a substrate followed by spinning at high speeds to achieve a uniform film.

Commercially available PEDOT dispersion or EDOT monomer solution (with other reagents) can be directly dispensed onto the substrate. Typically, a glass or silicon wafer. For glass substrates, ozone cleaning treatment is necessary to increase its wettability before the coating process.

• The substrate is then spun at high speeds (hundreds-thousands rpm) within the spin coater for a specified time.
• Centrifugal force spreads the solution uniformly across the substrate, leading to the formation of a thin film.
• The solvent evaporates during spinning, leaving behind a solid film of PEDOT or PEDOT:PSS.

Post deposition annealing may be performed to improve the electrical conductivity and structural integrity of the film. Annealing conditions such as temperature and duration can be optimized based on the specific requirements of the application. Solvent treatment of the coated films can also be carried out by spin coating the coated film with the pure solvent or non-solvent.

The film thickness can be controlled by:

• Concentration of the dispersion (more concentrated = thicker film)
• Spin speed (lower speed = thicker film)
• Spin time (shorter time = thicker film)

Drying conditions such as humidity can also impact film formation.

Spin coating offers several advantages, including simplicity, scalability, and the ability to achieve uniform films over large areas. Comparing with other coating methods, spin coating also suffers from large material waste with a great deal of solutions spun out of the substrate.

Dip Coating PEDOT

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Dip coating works by immersing substrates into solutions of PEDOT and PEDOT:PSS polymers. The lack of sophisticated expensive equipment means it is a simple and cost-effective method for depositing thin films. Dip coating is a particularly convenient way to coat:

• Textiles
• Yarns
• Fabric
• Cloth

Spin coating of these materials has proven difficult and unpractical.

PEDOT:PSS is popular in smart textile technology due to its relative water dispersibility, ease of manufacturing, environmental stability, commercial availability and high electrical conductivity.

Dip coating provides easy and fast deposition of PEDOT films onto or into any substrate in large areas with the great potential of large-scale production. The coating thickness can be increased by repeating the process several times or by dip coating a large amount of PEDOT:PSS. It is one of the most adaptable conductive polymers to flexible substrates in thin films. Especially compared to conductive metals, such as copper and silver nanoparticles.

Dip coating a small amount of PEDOT:PSS on fabrics and yarns can be achieved by immersing them into either commercially available PEDOT:PSS solutions or a solution of PEDOT:PSS in a solvent of choice with or without additives. The wet fabrics or yarns are then dried at elevated temperature.

For cottons treated with 0.2 wt% of PEDOT:PSS, a remarkable decrease (many orders of magnitude) in sheet resistance by can be observed. Increases in conductivity of the PEDOT coating on fabrics is linked to the chemical bond structure, porosity, and hydrophilicity of the fabric. The addition of polar solvents to PEDOT:PSS can also improve the electrical conductivity of coated fabrics by several orders of magnitude.

Dip coating has been used in the fabrication of sensors, light-emitting diodes, and photovoltaic solar cell devices. It is suitable for coating nonplanar three-dimensional substrates and can be adapted for batch processing. Controlling film thickness and achieving uniformity may require optimization of dipping parameters and solution viscosity. While dip coating is a simple deposition method, it can cause unbalanced coverage or coating build-up as the materials drip dry.

Spray Coating PEDOT

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Spray coating, also known as aerosol deposition, involves spraying a fine mist or aerosol of PEDOT based polymer solution onto the substrate surface. Spray coating serves an alternative to spin coating especially with considering irregular, fragile and oversized substrates and material consumption. A spray system comprising an ultrasonic nozzle or atomizer, solvent delivery system, and substrate holder is used for the deposition process:

• The PEDOT based solution is atomized into droplets or fine particles using the spray system.
• Spray is directed towards the substrate.
• The substrate may be kept stationary or undergo controlled motion to ensure uniform coverage.
• The solvent evaporates, leaving behind a thin film of PEDOT or PEDOT:PSS.

Additional curing or annealing steps may be employed to enhance film properties such as adhesion, conductivity, and stability. Spray coating offers advantages such as scalability, versatility in coating complex geometries, and minimal material waste. It is suitable for coating large area substrates and can be combined with masking techniques for selective deposition.

For uniform coating, parameters are optimized such as:

• Spray pressure
• Nozzle size
• Spray distance from the substrate

Improvements to the thermoelectric properties of PEDOT:PSS films can be achieved by spray coating followed by a two-step sequential post-treatment process. Treatment with ethylene glycol (EG), followed by methylammonium iodide (MAI), leads to changes in the conformation of polymer chains and the preferential orientation of PEDOT crystallites. The non-conductive PSS separates from the conductive PEDOT. The EG treatment of the PEDOT:PSS film saw the great improvement of electrical conductivity  up to 1752.1 S cm⁻¹. Further optimization with 0.05 M MAI in DMSO and deionised water gives a high-power factor of 122.3 μW m⁻¹ K⁻², along with an increased conductivity of 2226.8 S cm⁻¹ and Seebeck coefficient of 22.8 μV K⁻¹.

Inkjet Printing PEDOT

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Inkjet printing involves dispensing droplets of a PEDOT or PEDOT:PSS ink onto a substrate to form desired patterns or layers. Inkjet-printable PEDOT based inks are either commercially available or prepared by dispersing the polymer into a solvent or solvent mixture suitable for inkjet printing. The ink formulation may also include additives and polymer binders to improve stability, viscosity, and printability.

The quality of the printed film is strongly dependent on:

• viscosity of the ink
• surface tension of the ink
• nature of the substrate

There are two main types of inkjet printing technologies:

Thermal inkjet printing: a tiny heating element is used to vaporize a small volume of ink, creating a bubble that forces the ink droplet onto the substrate.

Piezoelectric inkjet printing: uses a piezoelectric crystal that changes shape when an electric field is applied, generating pressure pulses that eject ink droplets from the print head.

Both technologies are capable of high-resolution printing and are suitable for depositing PEDOT:PSS, particularly for flexible and printed electronics applications.

The ink is loaded into an inkjet printer equipped with printheads that eject droplets onto the substrate in a controlled manner. Various printing techniques such as drop-on-demand (DOD) or continuous inkjet (CIJ) can be used. After printing, the deposited ink undergoes drying and possibly annealing processes to remove solvents and enhance film properties. Thermal annealing or exposure to specific atmospheric conditions may be used depending on the ink composition and substrate.

Inkjet printing is a great tool to print flexible, transparent and conductive patterns of PEDOT-PSS composites. Inkjet-printed PEDOT:PSS films find applications in various electronic devices, including OPV, OLED and displays, sensors, antistatic coatings, and electronic textiles.

Inkjet printing offers advantages such as high precision, digital patterning capabilities, and compatibility with flexible substrates. Inkjet printing is a programmable, non-contacted, and scalable process which allows for large area production of substrate coating with desired pattern and precision.

Multi-layered coating

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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.

PEDOT:PSS and PEDOT Based Polymers

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