Organic thermoelectric materials: a new leader in future green energy

New molecules "send" great skills

π (pi), pronounced "pi", is a magical Greek letter. We know it from the number pi. For chemists, π refers more to a chemical bond. In a π bond, the electron clouds of different atoms are arranged and overlapped in a "shoulder-to-shoulder" manner, and the electrons originally bound around a certain atom can move between the two atoms that form the chemical bond.

In the eyes of scientists, the big π bond can "dispatch" great powers!

What is even more surprising to chemists is that in some organic molecules, electrons can "run" freely between multiple atoms. Molecules with such π bonds are called π-conjugated molecules.

Today, this type of molecule has become one of the brightest stars in chemical research. How to efficiently design and synthesize π-conjugated molecules and how to precisely control their assembly structure and electronic behavior are the most challenging basic scientific issues in this field.

Since 2014, the Chinese Academy of Sciences has launched the strategic priority research project (category B) "Molecular Engineering of Functional pi(π)-Systems" (hereinafter referred to as the "Prime Research Project").

During the five years of work supported by the pilot project, Zhu Daoben and Wan Lijun, academicians of the Chinese Academy of Sciences, served as chief scientists, and scientific researchers from many units including the Institute of Chemistry, Changchun Institute of Applied Chemistry, Shanghai Institute of Organic Chemistry and Institute of Microelectronics worked together to produce a number of original and world-leading innovative results.

Scientists published 1,183 papers in internationally renowned academic journals and applied for 169 invention patents, 64 of which have been authorized. Special project goals and technical indicators were fully exceeded.

In the international evaluation organized by the Institute of Chemistry for this pilot project, international peer experts participating in the evaluation spoke highly of the project's achievements: "This project has been a huge success, exceeding its initial goals and developing a complete platform for exciting materials and device research."

At the same time, based on the scientific research results of the special project, scientists have also developed flexible device preparation and integration technology with independent intellectual property rights, which has effectively promoted the development of this strategic emerging industry in my country and made scientific and technological preparations for the organic electronics industry to subvert future life.

Starting today, China Science Daily will review the achievements made by scientists in six different fields with the support of this project.

Organic thermoelectric materials: new leaders in future green energy

Thermoelectric effect is a fundamental and ubiquitous energy conversion phenomenon. Based on this effect, electricity generation by temperature difference and refrigeration driven by electric field can be realized.

Organic thermoelectric materials represented by π-conjugated molecular materials have excellent solution processability, flexibility and low thermal conductivity, and exhibit outstanding thermoelectric performance.

Chinese scholars play a leading role in this field. In 1986, researchers from the Institute of Chemistry of the Chinese Academy of Sciences and the Max Planck Institute in Germany worked together to discover that BEDT-TTF single crystals can exhibit completely opposite thermoelectric potentials at different temperatures, which means that organic systems have the potential to become good thermoelectric materials.

More than 20 years later, basic research on organic thermoelectric materials gradually entered a period of prosperous development.

With the support of the pilot project, the scientific research team has developed a series of high-performance material systems and prepared new functional organic thermoelectric devices.

Zhu Daoben, an academician of the Chinese Academy of Sciences, introduced that the outstanding work of scientists in this field includes:

First, a thermoelectric material with a new record in thermoelectric properties was obtained. The researchers used an electrochemical method to prepare a coordination polymer, which overcame the obstacle of structural disorder to improving thermoelectric performance, greatly improved the conductivity of the material, and achieved a thermoelectric figure of merit (ZT value) of 0.3, which is the highest value for n-type molecular thermoelectric materials.

Second, a method for rapidly searching for new thermoelectric materials has been developed. The researchers proposed a new method for field-controlled thermoelectric properties of high-mobility semiconductors to accelerate the search for promising organic thermoelectric materials. This new method lays the foundation for the rational design of organic thermoelectric materials.

Third, a batch of new devices based on thermoelectric materials were prepared.

For example, for the first time, a self-powered temperature-pressure dual-parameter sensor was prepared using microstructured organic thermoelectric materials, and the first thermally suspended organic thermoelectric device was constructed and electro-cooling was achieved... The successful preparation of these devices demonstrated the broad application prospects of organic thermoelectric materials in self-powered health monitoring and ultra-thin solid-state refrigeration.

International assessment experts pointed out that while organic electronics are often limited to "conventional possible applications" such as field-effect transistors, light-emitting diodes and solar cells, the Institute of Chemistry's work in the field of organic thermoelectric materials has broken through these limitations.

"Young researchers in our research group often imagine the future of organic thermoelectric materials together. One interesting scene is very impressive," said Zhu Daoben. "When you go camping, you can bring a kettle made of organic thermoelectric materials, so you don't have to worry about your mobile phone running out of power. The kettle is connected to a charging cable, and the heat energy of boiling water is converted into electrical energy. The hot water also charges the mobile phone."

Researchers believe that due to the wide availability of thermal energy and low cost, thermoelectric materials are expected to play an important role in solving major challenges facing mankind, such as the energy crisis and environmental problems.

Figure 1: Peltier effect temperature distribution of organic thermoelectric materials

Figure 2: Photo of an organic thermoelectric integrated device that can be used for self-powered pressure-temperature dual-parameter sensing.