Chlorophyll solar cell schematic drawing by Li Na
Through more than 1 billion years of evolution, natural light-synthesizing organisms on the earth have gradually formed a perfect conversion system from light energy to chemical energy, which can realize the whole process from light energy capture to energy transfer and finally to charge separation.
From this, people can't help but start to imagine, can they imitate nature's creation and make a solar cell with chlorophyll?
Recently, the research team of Wang Xiaofeng, a professor at the School of Physics of Jilin University, has collaborated with the research teams of Ritsumeikan University and Nagahama University of Biological Science and Technology in Japan to develop two types of double-layer or three-layer full-chlorophyll biosolar cells with different structures. Derivatives as photosensitive materials in bio-solar cells achieve a high photoelectric conversion efficiency of 4.2%. Related papers have been published in ACS Energy Letters.
From chlorophyll to solar cell
Chlorophyll molecules are the most abundant and environmentally friendly functional organic semiconductor materials in nature. Using chlorophyll and its derivatives as the main materials to prepare new solar cells, it can not only realize the effective use of cheap renewable natural resources, but also can be imitated The natural system's light energy conversion process realizes potentially high photoelectric conversion efficiency.
"Initially, scientists simply extracted pigment-protein complexes from living organisms and dispersed them on conductive substrates to make bio-batteries." Wang Xiaofeng told the China Science Journal. Although this can often get a weak current, but the photoelectric conversion efficiency is very low. Moreover, the biologically active protein is extremely unstable in vitro, and the working time of the battery is very short, so it has no practical application value.
Previously, scientists first semi-synthesized a series of chlorophyll and its derivatives as dye molecules for dye-sensitized solar cells to obtain higher photoelectric conversion efficiency. Afterwards, chlorophyll derivatives were applied to planar heterojunction and bulk heterojunction organic small molecule solar cells. Subsequently, chlorophyll aggregates were used as additive-free hole-transporting materials in perovskite solar cells, and gradually optimized to obtain higher cell efficiency.
From the experience accumulated by these pioneers, Wang Xiaofeng and others found that although the structural framework of chlorophyll is the same, the difference in structure such as the central metal and the peripheral functional groups will lead to differences in chlorophyll's stability, absorption spectrum, and charge transfer ability.
For example, the direct introduction of carboxyl groups on the chlorophyll macrocycle can be used as a binding site with titanium dioxide, thereby effectively injecting electrons; replacing magnesium with zinc as the central metal can improve the stability of chlorophyll, and can self-assemble into chlorophyll aggregates, which is particularly strong The length of the charge diffusion, effective transfer of photo-generated charge, etc.
On the basis of this understanding, in order to simulate the charge transfer method that can be regarded as the electron donor and acceptor photosystems in nature's Z-type photosynthesis, Wang Xiaofeng and the cooperation team began to explore the most abundant chlorophyll a in nature, transform and assemble into double Bilayer or three-layer full-chlorophyll material bio-solar cell.
In the three-layer structure, the uppermost layer simulates the photosystem II (electron donor) with a bipolar chlorophyll a derivative containing a dicyano group, and the middle layer uses a chlorophyll a aggregate containing a hydroxyl group and a central metal of zinc to simulate For the photosystem I (electron acceptor), the bottom layer uses a chlorophyll a derivative that can bond with titania nanoparticles.
This combination of cascade chlorophyll a derivatives can achieve the most efficient light absorption, charge extraction and transfer.
Light and dark reactions complement each other
Wang Xiaofeng said that in their solar cells, artificial chlorophyll derivatives as raw materials are obtained by simply chemically modifying chlorophyll raw materials widely present in nature.
The preparation of the battery is also relatively simple. After extraction and purification, the chlorophyll derivative is dissolved in an organic solvent and spin-coated on the surface of the conductive glass using a homogenizer, and the thickness of the chlorophyll derivative film is controlled by controlling the rotation speed and spin-coating time. In the same spin coating method, the electron transport layer, the hole transport layer, or other organic active layers are spin-coated on the upper and lower layers of the chlorophyll derivative film, respectively, and finally a metal electrode is deposited on the top layer by a metal evaporation coating machine.
"Because the entire production process does not have strict requirements on the external environment, it is suitable for large-scale production." Wang Xiaofeng admitted that the cost of artificial chlorophyll batteries using conductive glass substrates is estimated to be 100 yuan per square meter, which is higher than organic photovoltaic and calcium that rely on polymer materials. Titanium batteries are cheap."
Photosynthesis includes light reaction and dark reaction stages. The work of Wang Xiaofeng and others mainly focuses on the photoreaction stage. The subsequent dark reaction can be the reduction of carbon dioxide by platinum/TiO2-photocatalytic reaction to produce organic matter.
At the other end of the earth, a cooperative team from Germany and France published a paper in the US Journal of Science on May 8, using microfluidic technology to integrate and encapsulate photosynthetic membranes in droplets of cell size to prepare bionic chloroplasts , And program and control the bionic chloroplast by adjusting the internal composition of the droplet and using light as an external trigger.
"The main innovation of this work lies in the artificial dark reaction process." Wang Xiaofeng said, "But this system does not solve the artificial construction of the light reaction process, and still uses natural chloroplasts." He believes that because the protein framework of natural chloroplasts is in vitro Understability will affect the practical application of this result.
"If this work can be combined with our chlorophyll bio-battery system to simulate the photoreaction process, it may be more practical." Wang Xiaofeng said.
Lucky few
Wang Xiaofeng believes that due to the low material consumption, light weight, low energy consumption, low cost and environmental friendliness of artificial chlorophyll solar cells, it is conducive to modular large-scale production and is expected to replace traditional silicon solar cells and become the mainstream of photovoltaic power generation in the future.
"I think the most interesting application is to combine it with organic agriculture, and use chlorophyll batteries to provide lighting energy for organic farming." Wang Xiaofeng said. Because artificial chlorophyll solar cells have good light transmittance, they can be used in car roofs, windows and building roofs to increase the available surface area for collecting solar energy. Due to the simple preparation method of artificial chlorophyll solar cells, the flexible substrate can also be used to prepare it as a wearable electronic device, adding bricks and tiles to the smart life.
"I often hear about the pollution of cyanobacteria to the waters, but I do not know that cyanobacteria are also good raw materials for the production of chlorophyll batteries. We can turn waste into treasure." Wang Xiaofeng said.
When using their artificial chlorophyll solar cells to produce hydrogen by hydrolysis, they are optimistically estimated, "According to the pilot level, the factory buildings are included, and the future cost is eager to achieve 10~20 yuan/kg. This cost efficiency is higher than the current ordinary photocatalytic system. 3~4 times".
However, compared with other types of photovoltaic cells, whole chlorophyll solar cells are a trail that is rarely visited, and there are relatively few researchers. The importance of R&D needs to be understood and participated by more people.
"We are the minority and the lucky few." Wang Xiaofeng said, "The efficiency of the organic polymer solar film was only 1% when it first came out. After long-term optimization, it can now reach 15% to 16%."
Through further optimization of the spectral range, fill factor, photovoltaic voltage and conductive materials, the full chlorophyll solar cell system does have potential to be tapped. Wang Xiaofeng believes that after more and more researchers pay attention and begin to study artificial chlorophyll solar cells, their commercialization will enter a critical period in the next 5 to 10 years. (â– Chi Han, a trainee reporter of this newspaper)
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