Photograph by Chen Yongsheng and his scientific research team Wu Junhui

The flexible organic solar cell device prepared by Professor Chen Yongsheng's team. Photo by Liu Qian / Bright Picture

Schematic diagram of the flexible characteristics of organic solar cells and the main results of this research work

From the use of ancient natural fires to the drilling of wood for fire, to the use of coal and oil, the development of human civilization is essentially the development of energy utilization capabilities. So far, contemporary human civilization and economic development are largely based on the development and utilization of fossil energy. In the 21st century, due to concerns about the non-renewable fossil energy reserves on the planet, and the increasingly severe environmental pollution derived from the extraction and use of fossil energy, people have focused their exploration on green and sustainable energy fields, such as solar energy. , Wind, water ...

"Only solving the scientific problems of efficient use of solar energy is the road to sustainable human development." Professor Chen Yongsheng of the School of Chemistry, Nankai University asserted, "The sun is the mother of all things and the 'source' of energy. If the light energy can be used by two ten thousandths, it can meet all the energy needs of the current human society. "Because of this, Professor Chen Yongsheng and his team condensed their scientific research mission into one sentence-" Ask for energy from the sun " "!

1. Organic solar cells are expected to be commercialized

Among the various technologies for human use of solar energy, solar cells, that is, devices that use the "photovoltaic effect" to directly convert light energy into electrical energy, are currently widely used and also one of the most promising technologies.

For a long time, people have made solar cells based on inorganic materials such as crystalline silicon. However, such battery production has drawbacks such as complicated process, high cost, large energy consumption, and heavy pollution. Whether it is possible to find a new type of organic material with low cost, high efficiency, strong flexibility, and environmental friendliness to develop a new type of solar cell is now becoming the goal of scientists all over the world.

"Using the most abundant carbon materials on the earth as basic raw materials, obtaining high-efficiency and low-cost green energy through technological means is of great significance for solving the major energy problems facing humanity." Chen Yongsheng introduced, starting in the 1970s The research of organic electronics and organic (polymer) functional materials provides opportunities for the realization of this goal.

Compared with inorganic semiconductor materials represented by silicon, organic semiconductors have many advantages such as low cost, material diversity, adjustable functions, and flexible printing preparation. At present, organic light-emitting diode (OLED) -based displays have been commercialized and are widely used in mobile phones and TV displays.

Organic solar cells based on organic polymer materials as the photosensitive active layer have the advantages of material structure diversity, large-area low-cost printing preparation, flexibility, translucency and even full transparency. They have many advantages that inorganic solar cell technology does not have. characteristic. In addition to being a normal power generation device, it also has huge application potential in other fields such as energy-saving building integration and wearable devices, which has aroused great interest in academia and industry.

"Especially in recent years, the research on organic solar cells has achieved rapid development, and the photoelectric conversion efficiency has been constantly refreshed. At present, the scientific community generally believes that organic solar cells have reached the commercialization's" eve of dawn ".

2. Break through the bottleneck: strive to improve the photoelectric conversion efficiency

The bottleneck restricting the development of organic solar cells is the low photoelectric conversion efficiency. Improving the photoelectric conversion efficiency is the primary goal of organic solar cell research and the key to its industrialization. Therefore, the preparation of solution-processable active materials with high efficiency, low cost, and good reproducibility is the basis for improving the photoelectric conversion efficiency.

Chen Yongsheng introduced that early organic solar cell research mainly focused on the design and synthesis of polymer donor materials. The active layer is based on the bulk heterostructure of the fullerene derivative receptor. With the continuous advancement of related research and the higher requirements of the device technology for materials, solution-processable oligomeric small molecule materials with a defined chemical structure have begun to attract strong attention.

"This type of material has the advantages of simple structure, easy purification, and good reproducibility of photovoltaic device results." Chen Yongsheng said that in the early days, most small molecule solutions were not good for film formation, so the main method was to prepare the device by vapor deposition Its application prospects are greatly restricted. How to design a photovoltaic active layer material with good synthetic performance and a definite molecular structure is a key problem recognized by scientists.

With his keen insight and careful analysis of this research field, Chen Yongsheng decisively selected new solution-processable organic small molecule and oligomer active materials that had significant risks and challenges at that time as a breakthrough point in solar power research. From the design of molecular materials to the optimization of the preparation of photovoltaic devices, Chen Yongsheng led the scientific research team to carry out scientific research day and night. After 10 years of unremitting efforts, he finally constructed an oligomeric small molecule organic solar material system with distinctive characteristics.

From the efficiency of 5% to more than 10%, to 17.3%, they are constantly refreshing the world record of photoelectric conversion efficiency in the field of organic solar cells. The design concepts and methods they put forward are widely used by the scientific community. For more than ten years, they have published nearly 300 academic papers in internationally renowned magazines and applied for more than 50 invention patents.

3. A small step in conversion efficiency, a big step in the energy industry

Chen Yongsheng has been thinking about: how high the efficiency of organic solar cells can reach, can it eventually be comparable to silicon-based solar cells? Where are the "pain points" in the industrial application of organic solar cells and how to solve them?

In the past few years, despite the rapid development of organic solar cell technology, the photoelectric conversion efficiency has exceeded 14%, but compared with solar cells made of inorganic and perovskite materials, the efficiency is still low. Although the application of photovoltaic technology needs to consider multiple indicators such as efficiency, cost and life, efficiency is always the first priority. How to give full play to the advantages of organic materials, by optimizing material design and improving battery structure and preparation process, so as to obtain higher photoelectric conversion efficiency?

Starting in 2015, Chen Yongsheng's team began to conduct research on organic stacked solar cells. He believes that to achieve or even exceed the technical performance goals of solar cells based on inorganic materials, designing stacked solar cells is a highly potential solution-organic stacked solar cells can make full use of and play a role in organic / polymer materials. It has the advantages of structural diversity, solar absorption and adjustable energy level, and obtains sub-cell active layer materials with good solar absorption complementation, thereby achieving higher photovoltaic efficiency.

Based on the above ideas, they used the series of oligomeric small molecules designed and synthesized by the team to obtain 12.7% organic stacked solar cells, which refreshed the efficiency of the organic solar cell field at that time. The research results were published in the top journal "Nature · Photonics" in the field. This study was selected as one of the "Top Ten Progress in Chinese Optics in 2017".

How much room for improvement of the photoelectric conversion efficiency of organic solar cells? Chen Yongsheng and his team systematically analyzed and analyzed thousands of literatures and experimental data on materials and devices in the field of organic solar energy, combined with their own research accumulation and experimental results, and predicted the highest achievable organic solar cells including multilayer devices. Photoelectric conversion efficiency, and the parameter requirements for the ideal active layer material. Based on this model, they selected the active layer materials of the front and rear batteries with good complementary absorption in the visible and near infrared regions, and obtained a photoelectric conversion efficiency of 17.3%, which is the organic / polymer reported in the current literature. The world's highest photoelectric conversion efficiency of solar cells has pushed the research of organic solar cells to a new height.

"According to China's 2016 energy demand of 4.36 billion tons of standard coal equivalent, if the photoelectric conversion efficiency of organic solar cells is increased by one percentage point, the corresponding energy demand is generated by solar cells, which means that carbon dioxide emissions can be reduced by about 160 million tons per year. "Chen Yongsheng said.

Some people say that silicon is the most important basic material in the information age, and its importance is self-evident. However, in the view of Chen Yongsheng, silicon materials also have their shortcomings: "Not to mention that silicon materials need to pay huge energy and environmental costs in the preparation process, and their hard and brittle characteristics are also difficult to meet the future flexibility of humans for" wearable "devices. Requirements. Therefore, technology products based on flexible carbon materials with good foldability will be the foreseeable development direction of the new materials discipline. "

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