The two key issues in the development of solar cells are: improving conversion efficiency and reducing costs. Because of its low cost and easy mass production, amorphous silicon thin-film solar cells are generally valued and developed rapidly. As early as the early 1970s, Carlson and others had already started the research and development of amorphous silicon cells, and its research and development work has developed rapidly in recent years. At present, many companies in the world are producing amorphous silicon solar cell products. The manufacturing method of amorphous silicon solar cells is completely different from that of monocrystalline silicon and polycrystalline silicon solar cells. The silicon material consumes very little and the power consumption is lower.
Although amorphous silicon is a good battery material as a solar material, its optical band gap is 1.7eV, making the material itself insensitive to the long-wave region of the solar radiation spectrum. This limits the conversion efficiency of amorphous silicon solar cells. In addition, its photoelectric efficiency will decay with the continuation of the illumination time, the so-called light-induced degradation effect, which makes the performance of amorphous silicon solar cells unstable. The way to solve these problems is to prepare tandem solar cells, which are made by depositing one or more PiN layer cells on the prepared PIN layer single junction solar cell. The tandem solar cell improves the conversion efficiency. The key problems to solve the instability of single junction battery are:
(1) It combines materials with different band gaps to improve the response range of the spectrum.
(2) The first layer of the top cell is thin, and the electric field intensity generated by the light does not change much, ensuring that the photogenerated carriers in the first layer are extracted.
(3) The carrier produced by the bottom cell is about half of that of a single cell, and the light-induced degradation effect is reduced.
(4) The sub-cells of the tandem solar cell are connected in series.
Because amorphous silicon has very unique physical properties and processing advantages in manufacturing technology, it has become one of the research focuses and cores of large-area and high-efficiency solar cells. Amorphous silicon has a high absorption coefficient for sunlight and produces the best photoconductivity value. It is a good light conductor, it is easy to achieve high concentration doping, obtain excellent PN junction, and can control its band gap change in a wide range of composition.
Because the atomic arrangement in amorphous silicon lacks the regularity in crystalline silicon, there are many defects. Therefore, in the pure amorphous silicon PN junction, the tunnel current often dominates, making it show the characteristics of tunnel current without rectifying characteristics. In order to obtain good diode rectification characteristics, a thicker intrinsic layer i must be added between the P layer and the N layer to control the tunnel current. Therefore, amorphous silicon solar cells generally have a PiN structure. In order to improve efficiency and improve stability, sometimes it is also made into a multilayer PiN structure laminated battery, or insert some transition layers.
Amorphous silicon solar cells are the most complete thin-film solar cells developed, and their structure is usually PiN (or NiP) type. The P and N layers are mainly used to establish an internal electric field, and the i layer is composed of amorphous silicon. Because of the high light absorption capacity of amorphous silicon, the thickness of the i-layer is usually only 0.2 to 0.5 μm. Its band gap is about 1.1~1.7eV, which is different from 1.1eV of wafer silicon. Amorphous materials are different from crystalline materials, and the structure uniformity is low. Therefore, electrons and holes are conducted inside the material. If the distance is too long, the probability of the two overlapping is extremely high. To avoid this phenomenon, the i-layer should not be too thick, but if it is too thin, it will easily cause insufficient light absorption. In order to overcome this problem, this type of solar cell is designed in a multi-layer structure stack to take into account both light absorption and photoelectric conversion efficiency.
There are various structures of amorphous silicon solar cells. Among them, there is a better structure called PiN battery. It first deposits a layer of phosphorus-doped N-type amorphous silicon on the substrate, then deposits a layer of undoped i-layer, and then deposits a layer of boron-doped P-type amorphous silicon, and finally evaporates a reflective film with an electron beam, and vaporizes the silver electrode. For this kind of manufacturing process, a series of deposition chambers can be used to form a continuous process in production to realize mass production.
At the same time, amorphous silicon solar cells are very thin and can be made into a stacked type or manufactured by an integrated circuit method. On a plane, using an appropriate mask process, multiple series cells can be fabricated at a time to obtain a higher voltage. Because ordinary crystalline silicon solar cells have a voltage of only about 0.5V, the amorphous silicon tandem solar cells produced in Japan can reach 2.4V. At present, the problem of amorphous silicon solar cells is that the photoelectric conversion efficiency is low, the international advanced level is about 10%, and it is not stable enough, and the conversion efficiency often declines. Therefore, it has not been widely used in large-scale solar power supplies, but mostly used in low-light power supplies, such as pocket electronic calculators, electronic clocks and copiers. In addition, due to its low stability, it directly affects its practical application. If stability and conversion efficiency can be further resolved, then amorphous silicon solar cells are undoubtedly one of the main development products of solar cells, which will promote the development of solar energy utilization because of its low cost, light weight and more convenient application. It can be combined with the roof of the house to form an independent power source for the building.
There are many methods for preparing amorphous silicon thin-film solar cells, including reactive sputtering, PECVD, LPCVD, etc. The reaction raw material gas is SiH4 diluted with H2, and the substrate is mainly glass and stainless steel. The prepared amorphous silicon film undergoes different battery processes to produce single junction cells and stacked solar cells respectively.
Amorphous silicon solar cells are generally formed by decomposing and depositing silane (SiH4) gas by high-frequency glow discharge and other methods. Due to the low decomposition and deposition temperature (about 200°C), the energy consumption is low during production and the cost is relatively low. This method is suitable for large-scale production. The area of monolithic solar cells can be large (for example, 0.5m×1.0m), neat and beautiful. At present, two major advances have been made in the research of amorphous silicon solar cells:
(1) The conversion efficiency of amorphous silicon solar cells with a three-stack structure reaches 13%.
(2) The annual production capacity of tri-stack solar cells reaches 5MW.
Due to the large absorption coefficient of amorphous silicon to sunlight, amorphous silicon solar cells can be made very thin. Usually the thickness of the silicon film is only 1~2μm, which is 1/500th of the thickness of monocrystalline silicon or polycrystalline silicon cells (about 0.5mm), so the production of amorphous silicon cells consumes less resources.
Amorphous silicon has a light fatigue effect due to the instability of its internal structure and a large number of hydrogen atoms, which is aimed at the long-term stability of amorphous silicon solar cells. In the past 10 years, after hard research, although some improvements have been made, the problem has not been completely solved, so it has not been widely promoted and applied.
At present, the research of amorphous silicon solar cells mainly focuses on improving the properties of amorphous silicon film itself to reduce defect density, accurately designing the cell structure and controlling the thickness of each layer, and improving the interface state between the layers in order to achieve high efficiency and high stability. At present, the highest efficiency of amorphous silicon single junction cells can reach about 14.6%, the industrial production can reach 8%~10%, and the highest efficiency of laminated amorphous silicon solar cells can reach 21.0%.
Due to the limitation of material characteristics, the room for further improvement of the efficiency of crystalline silicon solar cells is limited. At present, the more potential for growth should be multi-junction tandem solar cells. The market share of silicon-based solar cells is 96%, of which monocrystalline silicon is 39%, polycrystalline silicon is 44%, and amorphous silicon is 13%.