What are Organic Electrochemical Transistors (OECTs)?

Organic electrochemical transistors (OECTs) regulate charge flow through an organic semiconductor channel via ion injection from an electrolyte. The most common organic semiconductor used is PEDOT:PSS. In OECTs, ionic and electronic charges can couple throughout the entire volume of the channel, resulting in higher transconductance compared to Field-Effect Transistors (FETs). However, due to ion diffusion, OECTs have slower response times.

OECT Components

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Gate electrode | Electrolyte | Organic semiconductor film | Source electrode + Drain electrode | Substrate

Gate electrode: in contact with the electrolyte and controls the flow of ions and charge within the device.

Electrolyte: source of ions which flow through the device causing bulk modulation.

Organic semiconductor film: the channel through which the current flows between the source and drain electrodes. Ion transport and electrochemical reactions can take place here.

Source and Drain electrodes: they control the flow of current which enters from the source, through the organic semiconductors to the drain. It is the voltage difference of the two that drives the flow of charge carriers.

How Does a OECT Work?

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The gate voltage (V_G) controls the injection of ions from the electrolyte into the semiconductor channel. This changes the doping state of the organic semiconductor film. It does this by regulating the interaction between the electrolyte (ions) and the semiconductor. For example positive cations from the electrolyte may travel into the semiconductor channel when a positive voltage is applied. The cations will then interact with any negative charge within the organic semiconductor channel, inducing a doping effect.

The drain voltage (V_D) induces a current (drain current, I_D) which is proportional to the quantity of mobile holes or electrons in the organic semiconductor channel. This determines the the doping state of the organic film.

V_G and V_D are both referenced with respect to the source electrode.

OECT Switch

OECTs can operate like a switch: the gate voltage (input) controls the drain current (output). When V_G is applied (switched on) ions are injected into the semiconductor channel which impacts its conductivity and charge flow.

OECT Amplifier

OCETS can operate as an amplifier: a small voltage input can lead to significant changes in conductivity of the channel which results in a much larger modulation of the drain current. This is a result of bulk modulation across entire semiconductor channel.

Depletion Mode

In depletion-mode OECTs contain p-type conducting polymers like PEDOT:PSS as the channel.

• A hole current flows in the channel even without a gate voltage (on state) as holes can hop via PEDOT.
• When a positive gate bias is applied, cations from the electrolyte are injected into the channel, balancing the anions of PSS.
• The number of holes in the channel is reduced, causing the film to become dedoped.
• The holes extracted at the drain are not replenished at the source
• The drain current drops, turning the device off.

Accumulation Mode

For accumulation mode, OECTs made from semiconducting polymers are typically in the OFF state due to the low number of mobile holes in the channel.

• When a negative gate voltage is applied, anions from the electrolyte are injected into the channel.
• This leads to an accumulation of holes (electrochemical doping) in the semiconducting channel.
• The conductivity of the channel increases
• A drain current can flow, turning the device on.

Materials used in OECTs

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Organic Semiconducting Layer

The material most commonly used as the organic semiconducting layer in OECTs is PEDOT:PSS. It is classed as being p-type doped or oxidized as mobile holes can hop between PEDOT chains within the polymer blend. The positive charge within PEDOT chains is balanced by PSS anions. A hole current is produced when a drain voltage is applied.

Other materials have been explored and they fall into a variety of categories:

• Polyelectrolyte-doped (like PEDOT:PSS)
• Small-molecule-doped (PEDOT:TOS)
• Semiconducting (PTHS + Tetrabutylammonium)
• Conducting (PEDOT-S)
• p-type (p(g2T-TT))
• n-type (p(gNDI-g2T))
• polymer blends (PEDOT:PSS + PVA)

In order to determine the feasibility of new organic semiconducting materials the standard components are:

• Au source and drain electrodes
• Glass substrate
• Ag/AgCl gate electrode
• Electrolyte of aqueous NaCl or KCl 

Gate electrode materials

Different gate electrodes have been used within OECT research including:

• Pt
• Ag/AgCl
• PEDOT:PSS
• Graphite
• Gold
• Activated carbon

Electrolytes

The choice of electrolyte affects the OECT’s ion mobility, biocompatibility, response time, and device performance, depending on the specific use case. They generally fall into three categories:

• Aqueous Electrolytes (NaCl, KCl)
• Ionic Liquids (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI))
• Solid or Gel Electrolytes (ionogel, PEG)

Advantages of OECTs

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Organic electrochemical transistors offer several advantages, particularly in the field of bioelectronics and sensing, including:

• The organic materials used have high tunability
• Large flexibility in device architecture
• Integration with a variety of substrates
• Easily deposited
• Range of fabrication processes available
• Biocompatible – operate effectively in aqueous environments
• High sensitivity
• Low operating voltage
• Long-term stability

OECT vs OFET

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Organic field-effect transistors rely on field-effect doping. In other words, the number of mobile electrons (in n-type) or holes (in p-type) within the semiconductor component is regulated by an applied voltage to a metallic electrode. The electrode is separated from the semiconductor by a thin insulating layer called the gate dielectric. Compared to OECTs the degree of current modulation is much lower therefore for every unit of voltage applied by the gate there isn't a big change in the output current (low transconductance). This means OFETs can not be used as amplifiers in the way that OECTs can. However OFETs are not restricted by slow ion diffusion and therefore have much faster response times.

Feature	OECT	OFET
Gating Mechanism	Electrochemical (ion-based)	Electric Field (field-effect)
Operating Environment	Liquid / Electrolyte	Dry
Response Time	Slower (ion movement)	Faster (electric field)
Current Modulation	Higher due to bulk electrochemical doping	Lower due to surface charge effects
Applications	Bioelectronics, circuits, memory devices	Displays, circuits, flexible electronics

PEDOT:PSS and PEDOT Based Polymers

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Further Reading

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• Rivnay, J. et al. Organic electrochemical transistors. Nat Rev Mater. (2018)