"Doped state regulation" significantly improves the performance of n-type organic electrochemical transistors

An organic electrochemical transistor (OECT) is a three-terminal device based on organic semiconductors (Figure 1a).

Driven by the gate (G) voltage, the ions in the electrolyte enter/move out of the channel, thereby electrochemically oxidizing/reducing doped organic semiconductor layers, drastically changing the current between the source (S) and drain (D) electrodes, thereby realizing the modulation
of the gate voltage signal to the source drain current signal.
Organic electrochemical transistors can work in aqueous solutions, the operating voltage is small (usually less than 1V), and the transconductance (response sensitivity of source leakage current to gate voltage) exceeds that of high mobility materials such as graphene, and has good biodevice interface and biocompatibility
.
Therefore, organic electrochemical transistors have a wide range of applications in the fields of biochemical sensors, neural interface devices, and neuromorphic computing, which have attracted increasing attention (Figure 1b).

As a biochemical sensor, organic electrochemical transistors can detect metabolites (Na+, K⁺, glucose, lactic acid, cortisol hormones, etc.
) in biological fluids such as sweat and tears, so as to monitor human life and health, and recently, have been successfully used in the detection
of new coronavirus.
Due to the good interface of biological devices, organic electrochemical transistors can be used as neural electrodes and brain-computer interfaces for diagnosis and treatment
.
In addition, organic electrochemistry works on a similar principle to synapses and can replace traditional electronic devices for brain-like computational research
.

Fig.
1 a.
Schematic diagram of the structure of organic electrochemical transistor devices; b.
Three organic electrochemical transistor applications: biochemical sensors, neural interface devices, neuromorphic computing

However, the practical application of organic electrochemical transistors is still hindered
by the material level.
Compared with p-type materials, n-type materials are far behind in terms of type and performance, which greatly limits the construction and practical application
of complementary logic circuits based on organic electrochemical transistors.
Influenced by the design of field-effect transistor (OFET) materials, traditional n-type organic electrochemical transistor material designs often reduce the minimum unoccupied molecular orbital (LUMO) energy level
by introducing more electron-deficient groups.
However, most of the materials based on this "low LUMO energy level" design strategy are based on complex structures, and the synthesis steps are long and expensive, and the performance improvement effect is relatively limited
.
Therefore, a simple and efficient design strategy
for high-performance n-type organic electrochemical transistor materials is required.

In response to these challenges, the Lei Ting research group of the School of Materials Science and Engineering of Peking University proposed a new n-type organic electrochemical transistor material design strategy - "doped state regulation" (Figure 2a).

Since the entire semiconductor is highly doped by the electrolyte during the working process of the organic electrochemical transistor, the transport characteristics of the device working state download flow cannot be simply determined by the properties of the molecules in the neutral state, but should be determined
by the properties of the molecules in the doped state.
Through the structural design, the charge is evenly distributed on the polymer skeleton, which can effectively convert the traditional p-type polymer into a high-performance n-type polymer
.
Based on this concept, polymer P(gTDPP2FT) (Figure 2b) exhibits record high n-type organic electrochemical transistor performance (Figure 2c) with characteristic parameters μC* of 54.
8 F cm−1 V⁻¹s?1 and a reduced switching response time of ^1.
75/0.
15 ms
.
Theoretical calculations and controlled experiments show that this transition is mainly caused
by more uniform charge distribution, more stable polarons, and enhanced backbone planarity and conformational stability in the charged state.
This work is the first to propose a method and significance for understanding and regulating the molecular properties of polymers in doped states
.
The work was published in Nature Communications under the title "Switching p-type to high-performance n-typeorganic electrochemical transistors viadoped state engineering
.
"

Figure 2a.
The "doped state regulation" strategy
designed in this work.
b.
Structural formula
of P(gTDPP2FT) polymer material.
c.
Comparing the performance of n-type organic electrochemical transistor materials, P(gTDPP2FT) in this work shows the highest mobility and μC* value

PhD student Li Peiyun and postdoctoral fellow Shi Junwei are the co-first authors of the paper, and Lei Ting is the corresponding author
.

The above research work is supported
by the National Natural Science Foundation of China, the High Performance Computing Platform of Peking University, the Laboratory of Molecular Materials and Nanoprocessing (MMNL) of the School of Chemistry and Molecular Engineering, Peking University, and Shanghai Light Source.