PEDOT:PSS applications beyond hole transport layer

PEDOT:PSS is still the most widely used conductive polymer, both in research and in real-world applications. This is due to its unique properties, which include relatively high conductivity, transparency, solid environmental and chemical resistance, film formation properties, while presenting low toxicity, environmental impact, and cost.

In the MADRAS project, PEDOT:PSS is mainly used as a Hole Transport Layer (HTL) for the development of an optical photosensor, which is used for the measurement of transmitted/reflected light.

This sensor, in conjunction with a thin-film transistor (TFT) backplane, then allows the creation of a 500-pixel density (PPI) fingerprint sensor. In order to achieve this, all layers including PEDOT:PSS must exhibit high homogeneity and quality.

Within the optical photosensor, PEDOT:PSS serves as an HTL and also partially blocks electron transport. PEDOT:PSS can also be used in other areas of sensor applications, where, for example, layers with a certain complex ratio show very good characteristics in relative humidity sensing.

PEDOT:PSS can be used for capacitive sensing, as a flextrode for medical applications or even as a temperature sensor due to the temperature dependence of the electrical resistance of PEDOT:PSS.

PEDOT:PSS enables many other interesting applications thanks to its unique features. One of the most common is its use as a Transparent Conductive Layer (TCL), where the conductivity of the layer prepared from dispersion can reach over 1000 S/cm.

If the layers are treated with sulfuric acid or ethylene glycol, for instance, such layers can reach up to 3000 S/cm and a transparency of more than 85%. Such layers can be used as TCLs in solar cells, where they fully replace Indium Tin Oxide (ITO). PEDOT:PSS also serves as a TCL in organic light-emitting diodes (OLEDs), where a light source showing a brightness of over 600 cd/m² at 6V can be prepared by printing.

As a transparent conductive layer, PEDOT:PSS is often used also in electroluminescent panels of LEC type – a Light-Emitting Capacitor whose emissive layer is based on doped zinc sulfide, most often elements such as copper, silver, manganese, etc.

These displays can be completely screen printed including the PEDOT:PSS layer. For the most commonly used colours, like blue and blue-green emission, the high transparency of PEDOT:PSS in the blue region is also advantageous and, on the contrary, the higher own-self PEDOT:PSS absorption in the red part of the visible spectrum does not matter.

This LEC display has been used in the past as a light source for backlighting monochrome liquid crystal displays (LCD) of Motorola phones, where PEDOT:PSS was used as a TCL.

PEDOT:PSS can also be used in electrochromic displays, either as a TCL or as an electrochemically active layer itself. With suitable electrolyte types, the latter also shows high stability with the possibility of switching on/off more than 10.000 times.

These types of displays are capable of operating at low voltages of around 1V, so they can be prepared by printing, where the conductive paths, switches, display, and battery are printed. From the given electrochromic elements, matrix displays or simple display elements can be prepared.

These displays could serve as simple informative interfaces for smart integrated functionalities. Electrochromic displays have been considered for several years in large-area applications such as electronic windows for their shading capabilities.

The electrochemical properties of PEDOT:PSS can also be used in organic electrochemical transistors (OECTs), where PEDOT:PSS forms a so-called channel between the source and drain electrodes.

This channel is locally oxidised/reduced through gate and electrolyte, thus changing the electrical resistance of the channel between source and drain by 3-4 orders of magnitude. This allows the creation of modulatable switches based on PEDOT:PSS to turn on/off displays, OLED/LEDs, etc.

The electrochemical properties of PEDOT:PSS can also be exploited in the field of electrical energy storage where, for example, different oxidation states of the PEDOT polymer can be used to store energy.

PEDOT:PSS could be used in supercapacitors, where it can form one or even both electrodes. However, the relatively low energy density per weight allows more applications in the field of low-power sources for printed electronics.

Of greater importance is the use of PEDOT:PSS as a conductive binder for cathode or anode materials for Li-ion batteries. It provides a binder function that holds the electrode material particles together, while its high conductivity ensures very good charge transport and thus low internal resistance of battery cells. This enables fast charging and discharging of Li-ion batteries.

It is clear from above-mentioned applications, as well as from many others described in the literature, that the applications of the PEDOT:PSS conducting polymer are very broad and therefore it is not surprising that the work on conducting polymers, including PEDOT:PSS, was awarded the Nobel Prize in Chemistry in 2000 to Alan MacDiarmid, Alan Heeger, and Hideki Shirakawa.