Nanfang University professor returns from overseas to focus on global challenges and promote the new generation of solar cells to production lines

China Energy Storage Network: Solar energy is the most abundant renewable energy source that humans have, and it is also the cleanest and most reliable future energy source. Solar cells have attracted the attention of scientists from all over the world because they can directly convert solar radiation into electrical energy through the photovoltaic effect. Among them, the research on perovskite solar cells, the third generation of high-efficiency solar cells, has become an international frontier.

Recently, Professor Tan Hairen's research group from the School of Modern Engineering and Applied Sciences at Nanjing University and scholars from the University of Oxford in the UK have cooperated to use coating printing, vacuum deposition and other technologies to achieve the world's first large-area all-perovskite laminated photovoltaic module preparation. Certified by an internationally authoritative third-party testing organization, the module has a stable photoelectric conversion efficiency of up to 21.7%, which is the world's highest efficiency of perovskite photovoltaic modules known to date. The relevant results were recently published in the internationally authoritative academic journal Science.

In recent years, Tan Hairen's research group has made breakthroughs in the research of large-area all-perovskite tandem solar cells. In 2019, the research group first proposed a new tunneling structure, breaking through the difficulty of preparing all-perovskite tandems; and at the beginning of this year, they developed a new method to enhance the passivation of surface defects of perovskite grains, creating a world record of 26.4% photoelectric conversion efficiency of all-perovskite tandem cells. The relevant results were published in the international authoritative academic journal Nature.

"Upgradable" perovskite brings unlimited potential to solar cells

In less than half a year, the research results have been successively noticed by top international journals. Why is the research progress of all-perovskite tandem solar cells so concerned?

"Higher efficiency and lower cost - the outstanding advantages of all-perovskite tandem solar cells make it considered as one of the most promising photovoltaic technologies, which is expected to break through the bottleneck of further development and utilization of solar energy." Tan Hairen told reporters.

Perovskite solar cells are solar cells that use perovskite-type organic metal halides as light-absorbing materials. Because the perovskite formula is adjustable, it is "upgradable". Different perovskite layers can be designed to be superimposed or superimposed with other photovoltaic materials for incident light of different wavelengths, thereby capturing as many photons as possible and achieving high-efficiency conversion. It has higher energy conversion efficiency than existing crystalline silicon solar cells on the market.

Another advantage of perovskite solar cells is that they are relatively simple and cheap to prepare. "Crystalline silicon must be heated to thousands of degrees Celsius to produce materials with suitable purity and crystal structure to generate electricity, while perovskites can be made by mixing chemicals in a liquid solvent and coating the surface with the solution, and the required temperature is no more than 100 degrees Celsius," said Tan Hairen.

When Tan Hairen obtained his doctorate in the Netherlands in 2015, his research was mainly on crystalline silicon solar cells. While doing postdoctoral work in Canada, Tan Hairen began to explore all-perovskite tandem cells. "Four years ago, before I returned to China, top international institutions had already conducted relevant research, but most people were working on single-layer perovskites, which could not break through the physical limits in principle." Tan Hairen told reporters that the energy conversion efficiency limit of single-layer perovskite solar cells is around 33%, while the theoretical conversion efficiency of tandem perovskite cells can reach 45%. If the advantages of perovskite materials can be fully exploited and their strengths can be used to make up for their weaknesses, the photoelectric conversion efficiency of the entire solar cell will be improved.

From "fingernails" to "window glass", opening up a new path for industrialization

The so-called stacked cells are like building Lego, building one layer after another. "Each floor attracts light of different wavelengths. In theory, the more layers you build, the less light energy is lost." Tan Hairen introduced that the research team spent a lot of time on the preparation and growth of perovskite materials. "Perovskite stacked cells mean that each layer has a different band gap. Having multiple layers means that high-energy photons excite electrons in layers with wider band gaps, while low-energy photons excite electrons in layers with narrower band gaps. In this way, more of the total solar energy is converted into electrical energy."

In the preparation of perovskite materials, researchers can "tune" their power generation capabilities by controlling the types of molecules produced during the manufacturing process, allowing the material to produce an ideal "band gap," the energy required to push an electron to a higher energy level so that it can carry a charge on a circuit. In order to obtain the optimal formula for all-perovskite stacked cells and achieve the ideal efficiency, Tan Hairen led the research team to explore breakthroughs in the structural principle design and material preparation of perovskite cells step by step.

"Although small-area perovskite cells in the laboratory have achieved high conversion efficiency, the commercialization process of large-area perovskite photovoltaic cells still faces many challenges." Tan Hairen told reporters that in the previously published research report, the perovskite stacked cells prepared by the research team were only the size of a "fingernail". "To solve practical application problems, it is necessary to realize the preparation of perovskite stacked cells on a larger area."

In the latest achievement published in Science, the research team proposed for the first time a mass-producible preparation method for all-perovskite tandem cells. To obtain a mass-producible preparation method, it is necessary to continuously try and verify the process design. "Every day, I see new results and feel very excited when there is a slight breakthrough." Tan Hairen led the team to use coating printing, vacuum deposition and other preparation technologies to replace the spin coating film forming process commonly used in the laboratory to prepare a 20 square centimeter all-perovskite tandem cell. "Although it is only the size of a palm, we have proved the feasibility of this technology to the world."

The long-term stability of the interconnection structure in the battery block is also a key bottleneck for industrialization. "In a tandem perovskite photovoltaic module, there is a complex interconnection structure in the connection area between each two sub-cells. Due to the direct contact between the perovskite light-absorbing layer and the back metal electrode in the interconnection area, the metal material will be corroded and the electrical properties of the perovskite material will be reduced." Tan Hairen said that in order to overcome this problem, the team used the atomic layer deposition method to prepare a tin dioxide electron transport layer between the perovskite light-absorbing layer and the back metal electrode to ensure that the different materials in the interconnection area are conductive and coexist harmoniously.

"I hope that after several years of hard work, we can increase the efficiency to 26%-27% on a large scale for industrialization, and witness the transition of perovskite solar cells from the laboratory to the production line as soon as possible." Tan Hairen said that this was one of the reasons why he chose Nanjing University when he returned to China. It was precisely because of Jiangsu's developed photovoltaic industry chain that the team will further promote the industrialization of all-perovskite tandem solar cell technology. At present, a research achievement transformation center has been established at the Suzhou campus of Nanjing University. It will jointly explore the preparation of larger-area all-perovskite tandem solar cells with relevant industry companies, and open up a new path for the mass production and commercialization of large-area perovskite tandem cells. "We will expand the solar panel to the size of 1 square meter of window glass. In the future, it is expected to be used in the roof glass of new energy vehicles, industrial plant roofs, building photovoltaics, ground power stations, etc. In terms of the process and technical conditions for the preparation of perovskites, we will continue to innovate and verify."