PEDOT:PSS

Plastics are normally considered insulators: they don’t conduct electricity, which is why plastic is used to coat cables in the first place. PEDOT:PSS is an exception. It is a plastic that conducts electricity, and one that is also nearly transparent when applied in an extremely thin layer. The name consists of two parts because the material is a mixture of two very long molecular chains. One chain, PEDOT, transports the electric current. The other, PSS, ensures that the whole thing can be dispersed in water and processed like ink.

Why a conductive plastic is meant to replace indium

Every phone display and every solar cell needs a layer that can do two things at once: conduct electricity and let light through. Until now, this has mostly been handled by indium tin oxide, or ITO for short. That’s a brittle metal oxide, and indium is a rare, expensive raw material. Whoever finds an alternative makes screens and solar modules cheaper and less dependent on a handful of supplying countries.

The second reason is mechanical in nature. ITO cracks when bent, because it is hard and glass-like. PEDOT:PSS is a polymer and therefore flexible. That’s exactly why it shows up wherever electronics need to be foldable, rollable, or stretchable. A sensor that sticks to the skin can practically not be made from ITO.

Then there’s processing. ITO is vapor-deposited in a vacuum, which requires elaborate equipment. PEDOT:PSS comes as an aqueous liquid and can be printed or spin-coated. This opens up the possibility of producing electronics from a roll, similar to how a newspaper is printed.

Two chains, one division of labor

PEDOT alone would be useless. The chains tangle into a hard clump that won’t dissolve in any liquid. That’s why PSS is added. PSS is a long, strongly water-friendly chain that attaches to PEDOT. It keeps the PEDOT pieces suspended individually, much like dish soap disperses fat droplets in water. The result is a dark blue dispersion that can be handled like ink.

The conductivity itself arises through doping. PEDOT is missing electrons in many places, meaning it has positive vacancies. These holes can travel along the molecular chain, and that’s exactly what constitutes the flow of current. PSS supplies the matching negative counter-charges and keeps the system stable. Without this charge balance, the material would immediately fall apart.

A common misconception is that PEDOT:PSS is as conductive as copper. That’s not even close to true. In its raw state, it lies far below metals, and also far below ITO. Only additives such as certain solvents rearrange the chains and boost conductivity by several orders of magnitude. This brings it close to ITO, without matching it everywhere.

From solar cells to skin sensors

PEDOT:PSS is most commonly found in organic solar cells and OLED displays, meaning screens whose pixels light up on their own. There it sits as an extremely thin intermediate layer between the electrode and the active material, ensuring that charges transfer cleanly. You don’t see it, but without this layer many components perform noticeably worse.

A growing field is bioelectronics. Because the material is soft and works in a moist environment, it is suitable for electrodes that sit on or even inside the body. Research groups are using it to build sensors for heart and brain signals that conform to the skin. Metal electrodes are often too rigid here and press uncomfortably.

In the news, PEDOT:PSS mostly comes up in reports about printed electronics, flexible displays, or neurotechnology. Its best-known weakness is also frequently mentioned there: the material is acidic and attracts water, which can attack neighboring layers over the years. Intensive research is therefore underway on more durable variants.