Classification of solar photovoltaic cells

With the advancement of science and technology, market demand and guidance from industrial policies in different countries around the world, photovoltaic power generation has developed rapidly in recent years and has become the most promising dominant energy source and alternative energy source in the field of new and renewable energy. The most basic device for photovoltaic power generation is the photovoltaic cell, which is a photoelectric component that directly converts solar energy into electrical energy using photovoltaic technology. Currently, the most commonly used types of photovoltaic cells in the world are as follows:

Monocrystalline silicon photovoltaic cells

Monocrystalline silicon photovoltaic cells are an early development with the highest conversion rate and largest output among photovoltaic cells. Currently, the conversion efficiency of monocrystalline silicon photovoltaic cells in China has averaged 16.5%, with laboratory records showing a maximum conversion efficiency of over 24.7%. This type of photovoltaic cell generally uses high-purity monocrystalline silicon rods as raw materials, with a purity requirement of 99.9999%. In order to reduce production costs, photovoltaic cells used on the ground now use solar-grade monocrystalline silicon rods, and the material performance indicators have been relaxed. Some can also use head and tail materials and waste monocrystalline silicon materials processed by semiconductor devices, which are then drawn into special monocrystalline silicon rods for photovoltaic cells. The monocrystalline silicon rod is cut into silicon wafers, with a thickness generally around 180-220um. After testing, cleaning, and fluffing, the surface is doped and diffused with trace elements such as boron, phosphorus, and antimony to form a PN junction, which has the basic characteristics of a cell. In order to prevent a large number of photons from being reflected off the smooth surface of the silicon wafer, a layer of silicon nitride anti-reflection film is deposited on the surface of the silicon wafer using Pevcd method to reduce reflection and also play a protective role. Then, after removing the phosphorus-silicon glass and plasma etching, silver paste prepared in advance is printed onto the silicon wafer by silk-screen printing to make grid lines, and a back electrode is also made. After sintering, the monocrystalline silicon photovoltaic cell is produced.

Polycrystalline silicon photovoltaic cells

Polycrystalline silicon photovoltaic cells are photovoltaic cells based on polycrystalline silicon materials. Since polycrystalline silicon materials are often cast instead of drawn like monocrystalline silicon, the production time is shortened, and manufacturing costs are greatly reduced. In addition, since monocrystalline silicon rods are cylindrical and photovoltaic cells made using them are also circular, the planar utilization rate of photovoltaic modules is relatively low. Compared with monocrystalline silicon photovoltaic cells, polycrystalline silicon photovoltaic cells have certain competitive advantages. However, during the growth process of polycrystalline silicon materials, a large number of dislocations will be generated in the grains due to thermal stress. In addition, impurities such as metal impurities and oxygen and carbon aggregate on the dislocations, resulting in recombination centers that make the electrical performance uneven, greatly reducing the lifetime of minority carriers and affecting the conversion efficiency of photovoltaic cells. The manufacturing process for polycrystalline silicon photovoltaic cells is not much different from that of monocrystalline silicon photovoltaic cells, and the equipment used is basically the same. The main difference is that when producing polycrystalline silicon photovoltaic cells, efforts must be made to minimize the recombination loss of photo-generated carriers at grain boundaries. In recent years, research and development of polycrystalline silicon cells has made rapid progress. Through measures such as phosphorus and aluminum doping, hydrogen passivation, and establishing interface fields, the conversion efficiency of photovoltaic cells has been greatly improved. Currently, the conversion efficiency of industrial polycrystalline silicon cells has reached 12%-15%.

III. Amorphous Silicon Photovoltaic Cells

Amorphous silicon photovoltaic cells are a new type of thin-film solar cell made from amorphous silicon as a raw material. Amorphous silicon is a semiconductor with an amorphous crystal structure. The photovoltaic cells made from it have a thickness of only 1 micron, which is equivalent to 1/300 of the thickness of a monocrystalline silicon solar cell. Its manufacturing process is much simpler and consumes less silicon material compared to monocrystalline and polycrystalline silicon. In addition, it has the advantage of weak light power generation, making it widely used in electronic calculators, electronic watches, copiers, and other fields. The production of amorphous silicon cells generally uses the PECVD method, with main equipment including glass cleaning, gas-phase deposition, laser scribing, and magnetron sputtering. To solve the performance shortcomings of amorphous silicon photovoltaic cells, researchers have begun studying a stacked photovoltaic cell. Stacked photovoltaic cells deposit one or more PIN sub-cells on top of a prepared PIN layer single-junction photovoltaic cell. Combinations of different bandgap width materials are used to expand the spectral response range, reduce attenuation, and increase conversion efficiency. Currently, the highest conversion efficiency of a single-junction photovoltaic cell produced by a US company is 9.3%, and that of a triple-junction cell is 13%. Due to its simple manufacturing process, low silicon material consumption and cost, lightweight, weak light power generation, and strong adaptability, amorphous silicon photovoltaic cells will become the most promising photovoltaic power generation material.

IV. Copper Indium Gallium Selenide (CIGS) Photovoltaic Cells

Copper indium gallium selenide photovoltaic cells are semiconductor thin films made by depositing copper, indium, and selenium ternary compounds on glass or other inexpensive substrates. Due to the good light absorption performance of copper indium gallium selenide cells, their thickness is only about 1/100 of that of monocrystalline silicon photovoltaic cells. The preparation of copper indium gallium selenide thin-film cells generally adopts processes such as vacuum deposition, selenization, and chemical vapor deposition. Among them, vacuum deposition uses separate evaporation sources for copper, indium, and selenium; the gas-phase selenization method first generates copper/indium layer stacks using vacuum deposition or sputtering at a low temperature of 200-300 degrees, and then heats up to 400-550 degrees for thermal treatment in hydrogen selenide gas or selenium vapor to produce copper indium gallium selenide thin films. Copper indium gallium selenide thin-film cells have the characteristics of low material consumption, low cost, stable performance, and no light-induced degradation. Its photoelectric conversion efficiency has developed from an initial 8% in the 1980s to 15% today, and it is expected to reach 20% in the coming years. Due to its own advantages, especially its photoelectric conversion efficiency, which currently ranks first among various photovoltaic cells, copper indium gallium selenide thin-film cells are called the future of low-cost photovoltaic cells internationally, attracting many institutions and experts to research and develop them. However, indium and selenium are relatively rare elements, and the production of these cells will encounter bottleneck factors due to material constraints, which investors must fully consider.

V. Gallium Arsenide (GaAs) Photovoltaic Cells

Gallium arsenide photovoltaic cells are a type of III-V compound semiconductor solar cell. Compared with silicon solar cells, GaAs photovoltaic cells have higher photoelectric conversion efficiency, with the theoretical efficiency of silicon solar cells being 23%, while the single-junction GaAs photovoltaic cell has achieved a conversion efficiency of up to 27%. GaAs photovoltaic cells can be made into thin-film and ultra-thin solar cells, absorbing 95% of sunlight. GaAs photovoltaic cells only need a thickness of 5-10 μm, while silicon photovoltaic cells require more than 150 μm. GaAs photovoltaic cells also have good high-temperature resistance, with efficiency still at about 10% at 200℃, while silicon photovoltaic cells cannot operate at this temperature. In addition, GaAs photovoltaic cells can be made into more efficient multi-junction stacked photovoltaic cells. The theoretical limit efficiency of double-junction GaAs cells is 30%, triple-junction GaAs cells is 38%, and quadruple-junction GaAs cells is 41%. Most GaAs photovoltaic cells are currently produced using liquid phase epitaxy or metal-organic chemical vapor deposition (MOCVD), which is costly and production is limited. In addition, the price of GaAs material is expensive, which greatly limits the popularity and development of GaAs photovoltaic cells. However, GaAs photovoltaic cells are ideal for space applications due to their high conversion efficiency and high-temperature resistance, and are also suitable for concentrated tracking power generation systems, expanding their use on the ground.

VI. Cadmium Telluride (CdTe) Photovoltaic Cells

Cadmium telluride is a compound semiconductor that has an ideal bandgap for photovoltaic energy conversion. Photovoltaic cells made from this semiconductor have high theoretical conversion efficiency, with the highest actual conversion efficiency reaching 16.5%. CdTe photovoltaic cells are typically manufactured on glass substrates, with the first layer being a transparent electrode and subsequent thin layers consisting of cadmium sulfide, cadmium telluride, and a back electrode, which can be either carbon or a metal thin film. There are many deposition techniques for CdTe, such as electrochemical deposition, close-spaced sublimation, close-proximity vapor transport, physical vapor deposition, screen printing, and spray coating. The thickness of the CdTe layer is usually 1.5-3 μm, which is sufficient for light absorption with a thickness of 1.5 μm. CdTe photovoltaic cells have a simple structure and can be easily deposited as large-area thin films at a high rate, making their manufacturing cost lower. They are a promising new type of photovoltaic cell and are the main research and development focus of the United States, Germany, Japan, Italy, and other countries. However, the toxic element cadmium poses a significant environmental pollution risk and hazard to operators' health, which cannot be ignored. Currently, experts are actively researching solutions, and it is believed that this issue will be resolved in the near future, making CdTe photovoltaic cells one of the new energies in future society.

VII. Polymer Photovoltaic Cells

Polymer photovoltaic cells use different redox potentials of different oxidizable-reducible polymers to form a unidirectional conductive device similar to an inorganic P-N junction by multilayer composite deposition onto a conductive material surface. Common materials for polymer photovoltaic cells include polyethylene, polyacetylene, and poly-p-phenylene vinylene. Pure conjugated polymers are not conductive, and they must be ion-injected through physical doping processes to exhibit semiconductor characteristics and form P-type and N-type structures. Polymer solar cells generally have a sandwich structure composed of a conductive glass (positive electrode), a polymer photoactive layer, and Al (negative electrode). When light shines on the photoactive layer from one side, a photovoltaic effect is generated to produce an electric current. Compared with traditional semiconductor photovoltaic cells with complex structures, high costs, and large fluctuations in photovoltage due to changes in light intensity, polymer photovoltaic cells have the advantages of self-design and synthesis of molecular structures, great flexibility in material selection, easy processing, low toxicity, and low cost, making them important for providing cheap electricity for large-scale solar energy utilization. As research on polymer photovoltaic cells has just begun, their lifespan and efficiency cannot yet compare with inorganic materials, especially silicon cells. Whether they can be developed into practical products still requires further research and exploration.