Photovoltaic cells in series can harness more solar energy

Multilayer structure of tandem solar cells

In the process of developing renewable energy, photovoltaic cells that use solar energy to generate electricity are the focus of many researchers, and the most popular one is to find materials or structures with high photoelectric conversion rates. Nowadays, the application of organic polymers that have conductive ability and can increase light absorption and conversion in photovoltaic cells has shown great potential. Researchers believe that organic polymer materials, with their low cost and high output, are expected to make photovoltaic cells cheap and light.

Photoelectric conversion efficiency increased from 8.62% to 10.6%

In the past few years, people have carried out a lot of meticulous and arduous research work to improve the photoelectric conversion efficiency, including developing new materials, improving photovoltaic cell structure and developing new processing technology.

Among the numerous studies, the achievements of the Henry Samory School of Engineering and Applied Sciences at the University of California, Los Angeles and the California NanoSystems Institute are particularly noteworthy. The researchers have greatly improved the performance of polymer solar cells by introducing a new series structure into photovoltaic cells. In July 2011, the photoelectric conversion rate of the researchers' photovoltaic cells was confirmed to be 8.62%, creating the world's best level.

After using a new infrared absorbing polymer material provided by Sumitomo Chemical in photovoltaic cells, researchers recently announced that they have increased the photoelectric conversion rate of tandem photovoltaic cells to 10.6%. This result has been recognized by the National Renewable Energy Laboratory of the U.S. Department of Energy and has become a new world record. The researchers are confident that the tandem structure they developed has a wide range of application value.

[page] Before the breakthrough by the University of California researchers, the performance of tandem photovoltaic cells was always lower than that of single-layer photovoltaic cells, mainly because people had not found suitable polymer materials. The engineering researchers at the University of California, Los Angeles showed people the high-efficiency single-layer tandem polymer photovoltaic cells. Yang Yang (transliteration), a professor of materials science and engineering at the university, used a double-decker bus to vividly explain the new structure photovoltaic cells. He said that an ordinary bus can accommodate a considerable number of passengers. If one more layer is added to it, more passengers can be carried in the same space, and this is why tandem polymer photovoltaic cells achieve high efficiency.

Tandem structure can absorb more spectrum of sunlight

It is reported that the tandem structure consists of two cells with different absorption spectral bands. Its uniqueness lies in that it has a low-frequency bandgap conjugated polymer material specially designed to form a tandem structure. The bandgap determines the band in which the polymer absorbs the solar spectrum. With the help of a dual-cell structure with different spectral absorption bands, tandem photovoltaic cells provide an effective way to harvest a wider spectrum of solar radiation. However, simply combining two cells does not naturally improve efficiency. Researchers said that in order to effectively obtain light energy, the photoelectric conversion materials used in tandem photovoltaic cells must match each other.

In actual research, in order to achieve the goal of more efficient use of solar radiation, the research team led by Yang Yang combined multiple photosensitive layers with complementary spectral absorption to form a series of polymer photovoltaic cells. Their series structure includes a front cell with a higher spectral bandwidth and a rear cell with a lower spectral bandwidth, and the two cells are connected by a special interlayer.

Compared to single-layer photovoltaic cells, tandem photovoltaic cells are more efficient in harnessing solar energy, especially because they minimize the loss of other energy. By using more sunlight absorbing layers (each layer absorbs a different band of the spectrum), tandem photovoltaic cells can maintain current and increase voltage. Researchers say these factors lead to improved efficiency of photovoltaic cells.

"Compared with single-junction photovoltaic cell structures, we spent much less time on tandem photovoltaic cells," said Li Gang, an engineering researcher at the University of California, Los Angeles. "The fact that we have achieved such a great result in improving photovoltaic cell efficiency in such a short period of time fully demonstrates the great potential of tandem photovoltaic cell technology."

Yang Yang said that compared with traditional photovoltaic cell production, the new tandem photovoltaic cell only needs to add a very low-cost wet coating process to be manufactured. He believes that since the added process is compatible with existing production equipment, the technology can be commercialized in the near future.

Some experts believe that the research team's work provides a new direction for polymer chemists to design new materials for tandem polymer photovoltaic cells. At the same time, the research results also represent an important progress in the commercialization of polymer photovoltaic cells. Yang Yang and his team hope to increase the photoelectric conversion rate to 15% in the next few years.

The research team's work was supported by the National Science Foundation, the Air Force Office of Scientific Research, the Office of Naval Research, and the Department of Energy, with the National Renewable Energy Laboratory participating in the research. The head of the Sumitomo Chemical research group said that the spectrum of sunlight is very wide, including visible light as well as invisible infrared and ultraviolet light. The company is pleased to see that Sumitomo Chemical's low-bandwidth polymer has contributed to the researchers' creation of a new world record for photovoltaic conversion efficiency.