At 23:00 Beijing time on May 1, 2021, Chen Jianhua, associate researcher of Yunnan University, Huang Wei, researcher of University of Electronic Science and Technology of China, professor Xie Zhaoqian of Dalian University of Technology, professor Yu Xinge of City University of Hong Kong, Zheng Ding Research Assistant Professor of Northwestern University, Professor Tobin J. Marks, and Professor Antonio Facchetti published an article entitled “Highly” on Nature Materials stretchable organic electrochemical transistors with strain-resistant performance”.
By fusing high-performance electron-ion composite semiconductors with a new device structure of “ordered porous + pre-stretching”, the researchers developed an organicelectrochemicaltransistor (OECT) with effective immune stress effects. Stable performance output is maintained under tensile conditions up to 140%, and real-time sensing amplification of bioelectric signals and synaptic-like functions are realized under complex stress conditions.
The corresponding authors of the paper are Huang Wei, Zheng Ding, Xie Zhaoqian, Yu Xinge, Tobin J. Marks and Antonio Facchetti; the first authors are Chen Jianhua and Huang Wei. The research was also carefully guided by Professor Guo Xugang of southern university of science and technology, professor Yu Junsheng and professor Cheng Yuhua of the University of Electronic Science and Technology of China.
In applications such as wearable electronics, human-machine interfaces, and biomimetics, components with excellent mechanical bendability and highly stable output characteristics are an indispensable part of the next generation of sensors and their systems. Among them, organic organic chemical transistors (OECT) have the advantages of simple structure, low working voltage (< 1V), good biocompatibility, high sensing sensitivity, and excellent current amplification ability, and have shown broad application prospects in the fields of biological signal detection and monitoring, disease diagnosis, and human-computer interaction. Especially in recent years, high-performance OECT with good flexibility and even stretchable characteristics has been successfully developed, further expanding the application of OECT in wearable timely detection and sensorless detection. However, the current research on flexible electronics pays more attention to the stability of the device after mechanical stress, and ignores the change of signal output characteristics during the application of stress. Maintaining stable charge transmission performance under mechanical deformation of flexible stretchable components is a prerequisite for reliable application. Although the current research on the design, properties, and ion-electron transport mechanism of OECT materials has gradually deepened, the preparation of OECT with stable output under tensile conditions is still the focus and difficulty in this field. The reason for this is that changes in the size of the OECT device and the thickness of the active layer during the stretching process will significantly affect the transport efficiency and path of ions and electrons, and further affect their output characteristics.
Recently, Yunnan University, together with university of electronic science and technology of China, Dalian University of Technology, City University of Hong Kong, Northwestern University, etc., jointly reported a new method of stretchable OECT and its sensor that can effectively immunize the effects of complex stresses, and successfully developed an OECT that can be stretched up to 140% by combining the preparation method of ordered honeycomb semiconductor polymers with bidirectional pre-stretching technology, and its output characteristics have maintained a high degree of stability in the stretchable range.
Figure 1: Chemical, redox, optical, electrical properties and membrane morphology of polymers. a) molecular structure of the polymer, b) cyclic voltammetry curve, c) device properties of OECT prepared by spin coating, d) OECT preparation process based on transfer films, e) optical and SEM images of honeycomb polymer films, f) device properties of OECT based on transfer compaction and honeycomb films, g) transient characteristics of OECT based on transfer honeycomb films.
Through material design, the introduction of hydrophilic glycol side chains into the polymer DPP-g2T based on pyrrolidone can not only effectively improve the hydrophilicity, electrochemical stability, and OECT properties of the material, but also improve the mechanical properties of the material. The biphilic molecular design is also conducive to the preparation of well-structured honeycomb-like membranes. Because honeycomb-like polymer films have faster ion migration and exchange rates, honeycomb-based OECT shows higher and more stable device performance.
Figure 2: Finite element simulation, tensile and electrical properties of dense and honeycomb structures. a) Finite element analysis of Free-standing films, b) finite element analysis of films on elastomers, c) the onset strain of microscopic cracks in thin films, d, e) electrical properties of films under different tensile states.
Finite element theory calculations found that honeycomb structure has unique mechanical advantages, and the stress concentration phenomenon is effectively suppressed by extra-surface buckling deformation during tensile process. Compared with densely structured films, honeycomb structures not only have higher tensile properties, but also have significantly reduced peeling between them and the elastic substrate, which is conducive to the stability of OECT’s devices. Through the densification of intrinsic stretching, the morphological characterization of honeycomb-like SEM morphology and the characterization of OECT properties, the experimental results are consistent with theoretical calculations.
Figure 3: a) Preparation process of pre-stretched OECT based on dense, honeycomb-like films, b) SEM images of different strain stretches and repeated stretches after stress release.
Although the honeycomb-based OECT has good tensile ability, due to the influence of the OECT device configuration itself, the change of device size and active layer thickness during the stretching process leads to unstable output current. To solve this problem, combined with the bidirectional pre-stretching strategy, the research team prepared a stretchable OECT and characterized the morphology of the active layer by SEM. The study found that the dense film has a serious accumulation after the release of stress, and its morphology cannot be restored after re-stretching and stress release; while the honeycomb-like film is not completely accumulated after the stress is released, and has good morphological stability, after repeated stretching and stress release, the morphology has not changed significantly, so it is conducive to obtaining a stable current output.
Fig. 4: a.b) Electrical properties of OECT pre-stretched in different directions and under strain based on 100% strain of honeycomb film, c) transversion and output current changes of different tensile times under 30% strain in different directions, d) electrical properties of OECT pre-stretched based on honeycomb film 150% strain in different directions and under strain.
Thanks to the pre-stretching strategy and the mechanical advantages of the honeycomb structure, the pre-stretched OECT based on the honeycomb film shows excellent device stability. Its output current, starting voltage, trans-inversion and other parameters are almost unchanged when the biaxial tensile strain does not reach the pre-tensile strain. After 10,000 stretches under 30% strain, the performance degradation is negligible, showing excellent stability. After exceeding the pre-tensile stress, the device is inactivated due to the rupture of the metal electrode, so this problem can be solved by preparing a stretchable electrode. Compared with honeycomb-like films, OECT based on dense films has significant current attenuation during stretching, mainly due to the peeling between the active layer and the electrode, which proves the necessity of honeycomb structure.
Figure 5: Electrocardiogram (ECG) test results and synapse simulation characteristics of OECT in different tensile states.
The application of OECT based on honeycomb membrane 100% strain pre-stretching to electrocardiogram (ECG) detection and artificial synapse studies showed excellent performance, and when the biaxial tensile strain was within 60%, its output signal was not significantly affected, which will effectively promote the application of OECT and related flexible electrons and sensors in the field of flexible stretchable bioelectronics. (Source: Science Network)
Related paper information:https://doi.org/10.1038/s41563-022-01239-9