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From the use of ancient natural fire, to the use of drilling wood for fire, to the use of coal and oil, the development of human civilization is essentially the development of energy utilization ability. So far, human civilization and economic development are largely based on the development and utilization of fossil energy. In the 21st century, due to the concern about the non-renewable fossil energy reserves on the earth, as well as the increasingly severe environmental pollution derived from the exploitation and use of fossil energy, people will explore the green sustainable energy field, such as solar energy, wind energy, water energy...
"Only solving the scientific problem of efficient use of solar energy is the way to sustainable development of mankind." Professor Chen Yongsheng, School of Chemistry, Nankai University, asserted, "The sun is the mother of all things and the 'source' of energy. If the solar energy that reaches the earth at any time could be harnessed to two parts per 10,000, the entire energy demand of human society could be met. Because of this, Professor Chen Yongsheng and his team condensed their scientific research mission into one sentence - "to the sun for energy"!
1. Organic solar cells are expected to be commercialized
In the human use of solar energy technologies, solar cells, that is, the use of "photovoltaic effect" to directly convert light energy into electrical energy devices, is currently widely used, but also one of the most promising technologies.
For a long time, people have been more based on inorganic materials such as crystalline silicon to prepare solar cells. However, the production of this kind of battery has drawbacks such as complicated process, high cost, high energy consumption and heavy pollution. Whether to find a new 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 material on earth as the basic raw material, obtaining efficient and low-cost green energy through technical means is of great significance for solving the major energy problems facing mankind at present." Chen Yongsheng introduced that the research of organic electronics and organic (polymer) functional materials, which started in the 1970s, has provided opportunities for the realization of this goal.
Compared with inorganic semiconductor materials represented by silicon, organic semiconductor has many advantages such as low cost, material diversity, adjustable function and flexible printing. At present, displays based on organic light-emitting diodes (OLeds) have been commercially produced and are widely used in mobile phone and TV displays.
The organic solar cell based on organic polymer material as the photosensitive active layer has the advantages of material structure diversity, large area low-cost printing preparation, flexibility, translucent and even full transparency, and has many excellent characteristics that inorganic solar cell technology does not have. In addition to being a normal power generation device, it also has great 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 of organic solar cells has achieved rapid development, and the photoelectric conversion efficiency is constantly refreshed." At present, the scientific community generally believes that organic solar cells have reached the 'dawn' of commercialization." Chen Yongsheng said.
2. Break through the bottleneck: strive to improve the efficiency of photoelectric conversion
The bottleneck restricting the development of organic solar cells is that the photoelectric conversion efficiency is low. 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, and the active layer was based on the bulk heterostructure of fullerene derivative receptors. With the continuous advancement of related research and the higher requirements of materials in device technology, solubilizable oligomolecular materials with determinable chemical structure have attracted intense attention.
"These materials have the advantages of simple structure, easy purification, and good reproducibility of photovoltaic device results." Chen Yongsheng said that in the early stage, most small molecule solutions were not good at forming films, so evaporation was mainly used to prepare devices, which greatly limited their application prospects. How to design and synthesize photovoltaic active layer materials with good performance and determined molecular structure is a key problem recognized by scientists.
With his keen insight and careful analysis of the research field, Chen Yongsheng decisively selected the new organic small molecules and oligomers active materials that could be processed with solution, which had major risks and challenges at that time, as the breakthrough point of solar power generation 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, and after 10 years of unremitting efforts, finally constructed a unique oligomer small molecule organic solar material system.
From efficiency of 5% to more than 10%, and then to 17.3%, they continue to break the world record in the field of photovoltaic conversion efficiency of organic solar cells. Their design concepts and methods have been widely used by the scientific community. Over the past decade, they have published nearly 300 academic papers in internationally renowned magazines and applied for more than 50 invention patents.
3. One small step for efficiency, one giant leap for energy
Chen Yongsheng has been thinking about how high the efficiency of organic solar cells can be achieved, and whether they can finally compete with silicon-based solar cells? Where is the "pain point" of the industrial application of organic solar cells and how to crack it?
In the past few years, although organic solar cell technology has developed rapidly, the photoelectric conversion efficiency has exceeded 14%, but compared with inorganic and perovskite materials made of solar cells, the efficiency is still low. Although the application of photovoltaic technology should consider a number of indicators such as efficiency, cost and life, efficiency is always the first. How to make use of the advantages of organic materials, optimize the material design and improve the battery structure and preparation process, so as to obtain higher photoelectric conversion efficiency?
Since 2015, Chen Yongsheng's team has begun to conduct research on organic laminated solar cells. He believes that in order to reach or even exceed the goal of the technical performance of solar cells based on inorganic materials, the design of laminated solar cells is a very potential solution - organic laminated solar cells can make full use of and play to the advantages of organic/polymer materials, such as structural diversity, sunlight absorption and energy level adjustment. A sub-cell active layer material with good complementary sunlight absorption is obtained, thus achieving higher photovoltaic efficiency.
Based on the above ideas, they used a series of oligomeric small molecules designed and synthesized by the team to prepare 12.7% organic laminated solar cells, refreshing the efficiency of the organic solar cell field at that time, the research results were published in the field of the top journal "Nature Photonics", and the study was selected as "Top Ten Advances in Chinese Optics in 2017".
How much room to improve the photoelectric conversion efficiency of organic solar cells? Chen Yongsheng and his team systematically analyzed thousands of literatures and experimental data on materials and devices in the field of organic solar energy, and combined with their own research accumulation and experimental results, predicted the actual maximum photoelectric conversion efficiency of organic solar cells including multi-layer devices, as well as the parameter requirements for ideal active layer materials. Based on this model, they selected the active layer materials of the front cell and the rear cell with good complementary absorption capacity in the visible and near infrared regions, and obtained a verified photoelectric conversion efficiency of 17.3%, which is the world's highest photoelectric conversion efficiency reported in the current literature of organic/polymer solar cells, pushing the research of organic solar cells to a new height.
"According to China's energy demand of 4.36 billion tons of standard coal equivalent in 2016, 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 Chen Yongsheng's view, silicon materials also have their disadvantages: "Not to mention the huge energy and environmental costs that silicon materials need to pay in the preparation process, its hard and brittle characteristics are difficult to meet the flexible requirements of future human 'wearable' devices." Therefore, technical products based on flexible carbon materials with good folding will be the foreseeable development direction of the new materials discipline."
