PEDOT:PSS

PEDOT:PSS (or Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) is a transparent conductive polymer consisting of a mixture of the ionomers poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate. Due to its unique combination of conductivity, transparency, ductility, and ease of processing, PEDOT:PSS has become a benchmark material in thin-film electronic fabrication.

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate can be used as an interfacial layer for hole transport in organic light emitting diodes, organic photovoltaics, and perovskite photovoltaics. It can also be used as a replacement for transparent conductors such as ITO or FTO and is used commonly in applications where the underlying substrate is flexible.

Ossila supplies a range of PEDOT:PSS and PEDOT dispersions that are suitable for a range of different applications.

The properties of PEDOT:PSS vary from dispersion to dispersion, hence its versatility. The key properties of PEDOT:PSS are the extent to which it is conductive and the size of its work function. Since PEDOT is conductive and PSS insulating, the conductivity of the resulting polymer depends on ratio between these two ionomers and the microstructure of the film. Similarly, a higher presence of PSS at the surface will result in a deeper work function.

PEDOT:PSS typically comes as a water based emulsion which is created via the polymerization of PEDOT monomers in a polystyrene sulfonic acid solution. We supply all our PEDOT:PSS solutions in light resistant bottles with temperature indicators.

Applications

PEDOT:PSS is the subject of a considerable amount of research, and is used for a range of applications within thin-film electronic fabrication. This includes perovskite photovoltaics, organic photovoltaics, organic light emitting diodes, transparent conductors, organic electrochemical transistors, flexible electronics, thermoelectric generators, supercapacitors, and energy storage

Substrate surfaces should be prepared before the deposition of PEDOT:PSS to ensure that they are clean. This can be done using DI water, Hellmanex III, isopropyl alcohol and a UV Ozone Cleaner. Once the surface is ready, a spin coater can be used to create a PEDOT:PSS thin film.

Frequently Asked Questions

What Makes PEDOT:PSS Conductive?

The electrical conductivity of oxidized polythiophenes has been known about for nearly 40 years. The origins of this conductivity are due to the presence of radical states, which are formed due to the oxidation of the thiophene units. These reduced states are delocalized across the polymer chain, and in the presence of the oxidizer, these radicalized states can be stabilized.

In PEDOT:PSS, the PEDOT is oxidized during the polymerisation reaction by the polystyrene sulfonate. This produces an emulsion where the PSS present stabilizes the radical states on the PEDOT.

Although PEDOT itself is conductive, the PSS present within the blend is insulating. The quantity of PSS and the microstructure of the film therefore 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. During deposition this structure is retained forming 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.

Can the Conductivity of PEDOT:PSS be Improved?

Advanced formulations offer increased conductivity of PEDOT:PSS thin films. This is achieved by the addition of secondary solvents (sometimes referred to as secondary dopants) to the solution, or by exposing thin films to these secondary solvents.

It was originally believed that these solvents acted by further doping the PEDOT, hence the name secondary dopant. More recent work has revealed that their presence changes the core-shell structure that is normally seen. When exposed to these solvents the hydrophilic/hydrophobic nature of the PSS/PEDOT components no longer determine the structure. The solvents allow the diffusion and intermixing of the PEDOT and PSS chains, creating a more homogenous film on the microscale. This homogenisation increases the path length of charges along the PEDOT chain, thereby reducing the distance that charges must travel across PSS rich areas.

What Determines the Work Function of PEDOT?

As work function is a surface property of a material, the work function of the PEDOT:PSS blend is determined by the percentage of each component at the surface of the film. PSS has a significantly deeper work function than PEDOT, therefore a higher presence of PSS at the surface will result in a deeper work function. This means that for formulations with higher percentages of PSS, the work function will be higher than those with lower percentages.

In addition, the processing of the PEDOT:PSS film can result in changes to the work function. When PEDOT:PSS forms a core-shell structure, PEDOT is surrounded by PSS. In this case the work function will be dominated by PSS. If the film has been treated such that the components are more homogenously dispersed, however, the work function will become more shallow as the surface becomes richer with PEDOT.

How Should PEDOT Materials be Stored?

The recommended storage temperature of PEDOT:PSS and PEDOT dispersions is between 5 and 10 degrees Celsius. It is therefore recommended that you store the dispersion at the front of a refrigerator. The product is not usable if frozen, so should be kept away from the back of the refrigerator (where it is coldest).

The dispersions can tolerate a few days to a week outside of the refrigerator (e.g. during shipping) without negative consequences on the PEDOT:PSS performance. Over time at elevated temperatures, the PEDOT:PSS can phase separate, aggregate and form a solid that drops to the bottom of the bottle, reducing performance. Brief heating to a max. of 50°C however has no adverse effect on product properties.

What is the Shelf Life of PEDOT Materials?

When stored at 5 - 10 degrees Celsius, the PEDOT:PSS dispersion will give you consistent performance over 12 months if used regularly. The official recommended shelf life from the manufacturer is 9 months after production, if the dispersion is never used or disturbed. After this time, the dispersion will gradually separate/sediment out at a very slow rate, resulting in lower concentrations and ultimately thinner films.

This degradation is mitigated by frequent use of the dispersion due to the gentle agitation caused when decanting. Through constant use, it is possible to use a single bottle of PEDOT:PSS for many years without harming device performance.

Getting Started with PEDOT:PSS and PEDOT Dispersions

Surface Preparation

To ensure a uniform coating, it is important to make sure that the surface of your substrate is as clean as possible before you begin the deposition process. To prepare a substrate for PEDOT:PSS deposition:

1. Choose a substrate (FTO Glass, ITO Glass, Glass, or SiO2 substrate) and place it in a substrate rack
2. Sonicate your chosen substrate for 5 minutes in hot (70°C) DI water with 1% Hellmanex III
3. Dump-rinse twice in boiling DI water
4. Sonicate your chosen substrate for 5 minutes in Isopropyl alcohol
5. Dump-rinse twice in boiling DI water
6. Dry your chosen substrate using filtered compressed gas
7. Place the chosen substrate into a UV Ozone Cleaner and leave for 10 minutes

UV Ozone Cleaner

• Large Cleaning Area
• Powerful UV Lamp
• No Sample Damage

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Thin-Film Deposition of PEDOT:PSS

For the deposition of thin films of PEDOT:PSS on a freshly prepared surface, we recommend using a vacuum-free spin coater and following this five step process:

1. Filter your PEDOT:PSS solution through a 0.45 µm PES filter (or hydrophobic PTFE filter for HTL Solar 3) into an amber vial
2. Preheat a hot plate to 120 °C ready for use in step 5
3. Place a freshly prepared substrate into an Ossila Spin Coater and set to the desired spin speed
4. The substrates should be spun until the films are dry, for PEDOT:PSS films this is typically >30 s
5. Once the spin coating has finished place the samples on a hotplate for 15 minutes to fully dry

For speeds below 1000 rpm we recommend static spin coating. For higher speeds, dynamic spin coating can be used.

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Obtaining a Uniform Coating

The coating quality of the PEDOT:PSS will be dependent upon several factors. These include the PEDOT formulation you are using, the deposition technique, the surface you are depositing onto and the cleanliness of the surface. Ideally, the film should be highly uniform across the entire surface with possible variations at the ends of your sample due to edge effects.

Sometimes due to the wetting conditions of the PEDOT formulation on the surface, the coating may not be uniform. If this occurs there are several things that can be done. The first is to ensure that the surface of your sample is clean, if possible use a combination of solvent cleaning steps and UV ozone or oxygen plasma treatments (as described above) to ensure a completely clean surface. If this does not improve the quality of the surface, secondary solvents can be added. For AI 4083 and PH 1000 the addition of approximately 10% isopropanol can improve the wetting on surfaces.

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