CHEMICAL SCIENCE

Yunnan University’s polymer-based flexible “opto-mechatronic” nano-power generation film has made a new breakthrough


On August 16, 2022, Beijing time, the team of Bao Lixia and Wang Jiliang of the Department of Polymers of the School of Chemical Sciences and Engineering of Yunnan University published a research result entitled “Photomechaelectric Nanogenerator” in the journalMate.

Through molecular design, the result introduces UV-sensitive azobenzene photosensitive units (AZO) and ionic liquid units (ILO) with weak ion-to-ion interactions into the macromolecular backbone in the form of covalent bonds. Through the “layer-by-layer” cis- and transverse isomerization of the AZO unit under the action of ultraviolet-visible light, the anions in the ILO unit are significantly separated, and a dynamic molecular dipole is formed, and the internal potential difference is obtained. Different from the photoelectric effect of conductors and the photovoltaic effect of inorganic and conjugated conductive polymer semiconductors, the “photomechanical” effect described in this study provides a possibility for photoelectric conversion of non-electronically conductive flexible polymers, and also provides a new strategy for the development of flexible nanogenerators, self-powered photoelectric sensors and detectors.

The corresponding authors of the paper are Bao Lixia and Wang Jiliang, and the first authors are Zhao Jin and Zhang Yinghao.

Relatively speaking, solar energy is an inexhaustible and inexhaustible green clean energy. However, solar energy is indirect and cannot meet human requirements for all-weather use of energy. Therefore, the conversion and storage of solar energy has important theoretical and practical significance. Among them, phase change energy storage materials and photovoltaic materials have received widespread attention in the past few decades. The reason for this is that these two types of materials can efficiently store or convert solar energy into electrical energy so that people can easily use it whenever they need it.

The broad “photoelectric effect” includes the internal photoelectric effect (also known as the photovoltaic effect) discovered by the French scientist Becqurel in 1839 and the external photoelectric effect discovered by the German physicist Hertz in 1887, the “photoelectric effect” is the theoretical basis for the conversion of solar energy into electrical energy by metal and inorganic or organically conjugated conductive polymer semiconductors. From Einstein’s external photoelectric effect, it can be seen that the reason why metals can convert solar energy into electrical energy is mainly because the frequency of incident light exceeds the escape frequency of free electrons on the metal surface, in other words, when the energy of the incident photon exceeds the energy required for the free electrons on the metal surface to overflow (that is, escape work), the metal surface will produce “optoelectrons” and obtain direct current. For inorganic or organic conjugated conductive polymer semiconductors, under the action of light, the interior of the uneven p-N semiconductor will produce a redistribution of electrons and holes, which will form a potential difference, which is the photovoltaic effect. From the above mechanism, it can be seen that when light acts on the electron insulating polymer, due to the absence of free electrons in the polymer or the redistribution of electrons and holes, therefore, the electron insulating polymer has no external photoelectric effect or photovoltaic effect, and the light energy cannot be converted into electrical energy.

Figure 1: Schematic diagram of the molecular structure of an electron insulating polymer with photoelectromechanical effects.

In the team a large number of pre-solar energy storage materials (Chem. Eng. J., 2020, 389, 124483; Chem. Eng. J., 2022, 428, 131088; Chem. Eng. J., 2022, 436, 135226; Chem. Eng. J., 2022, 450, 138144) and Molecular Design, Synthesis and Application of Functional Ionic Liquid Crystals or Ionic Plastic Crystals (Electrochimica Acta, 2019, 294, 249-259; J. Power Sources, 2019, 444, 227305; Nano Energy, 2020, 67, 104220; Polymer, 2021, 215, 123388; Carbohydr. Polym, 2021, 255, 117363; J. Energy Chem., 2022, 66, 647-656; J. Energy Chem., 2022, 73, 360-369), this study introduced macromolecular backbones in the form of covalent bonds by molecular design (as shown in Figure 1) by simultaneously introducing ultraviolet light-sensitive azobenzene photosensitive units (AZO) and ionic liquid units with weak ion-to-ion interactions (ILO) in the form of covalent bonds.

Figure 2: Schematic diagram of the generator theory of the opto-electromechanical nano-generator.

In order to achieve the “layer-by-layer” cis- and transverse isomerization of the AZO unit under the action of ultraviolet-visible light, the anions in the ILO unit that are also covalently bonded to the macromolecular chain are effectively separated at the same time as the AZO unit undergoes heterogeneous changes, and then a dynamic molecular dipole is formed, thereby obtaining the internal potential difference, and finally realizing the purpose that the electron insulating polymer can also convert light energy into electrical energy. Different from the mechanism of traditional conductors and semiconductors converting light energy into direct current through photoelectric effect, this study induces the effective separation of an anion ions in the corresponding ILO unit by inverting the layer-by-layer spatial configuration of the AZO unit under the action of ultraviolet light (weak “mechanical deformation”), resulting in dynamic molecular dipoles, forming an internal potential difference, and finally inducing the periodic directional migration of electrons inside the deposition electrode to obtain alternating current. For simplicity, the mechanism described in this study is called “Photomechaelectric effect”, and the related working mechanism is shown in Figure 2.

The relevant research results show that the electronically insulated ZAO-ILO copolymer film (2 cm × 2 cm × 100 μm) prepared by the institute can generate more than 5V of ALTERNa when irradiated by only 3 mW cm-2, and after rectification circuit rectification, it can continuously light up commercial LED diodes many times, and the voltage across the 2200 μF bulk commercial capacitor can be raised to more than 4.5V (as shown in Figure 3) within 10 minutes. Copolymers have shown great application value in the fields of flexible nanogenerators, self-powered photodetectors and photoelectric sensors.

Figure 3: Energy-time dependence curve of photomechanical nano-generators.

The work has been approved by the National Natural Science Foundation of China (21961044, 22169024), the “Major Science and Technology Project” of the Science and Technology Department of Yunnan Province, the “Thousand Talents” Program of Yunnan Province, the “Ten Thousand People” Program of Yunnan Province, the Basic Research Program of Yunnan Province (202105AC160072, 202101BC070001-019, 202101AT070280, 202102AB080017) and Yunnan University Graduate Innovative Research Program (2021Y394) strongly supported. (Source: Science Network)

Related paper information:https://doi.org/10.1016/j.matt.2022.07.027



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