(1) Single crystal solar cell Single crystal solar panel Among silicon-based solar cells, single crystal solar cells have the highest conversion efficiency (16% ~ 20%) and the most mature technology. At present, the electrical grounding technology of monocrystalline silicon is close to maturity. In battery manufacturing, technologies such as surface texturing, emitter passivation, and partition doping are used. Be widely adopted. The developed batteries mainly include planar monocrystalline silicon batteries and trench buried gate electrode monocrystalline silicon batteries.
The improvement of conversion efficiency is mainly due to the surface microstructure treatment and zoning doping process of monocrystalline silicon. In this regard, the German Flawn Hof Solar System Research Institute has always maintained a world-leading level. In this study, the surface of the battery is textured by lithography and photography to make an inverted pyramid structure. The oxide passivation layer with the thickness of 13nm is combined with two anti-reflection coatings on the surface, and the aspect ratio of the gate is improved by improved electroplating process. The conversion efficiency of large-area (225cm2) single crystal solar cells prepared by Kyocera Corporation is 19.44%, and China Beijing Solar Research Institute is also actively engaged in the research and development of high-efficiency crystalline silicon solar cells. The conversion efficiency of planar high efficiency monocrystalline silicon battery (2cm × 2cm) is 19.79%, and that of trench buried gate electrode crystalline silicon battery (5cm ×5cm) is 19.79%. The conversion efficiency of monocrystalline silicon solar cell is undoubtedly the highest, and it still occupies a dominant position in large-scale application and industrial production. However, due to the influence of the price of monocrystalline silicon material and the corresponding complicated battery technology, the cost of monocrystalline silicon remains high.
(2) Polycrystalline silicon solar cells
Polycrystalline silicon solar cells have low cost, high conversion efficiency (14% ~ 16%) and mature production technology, occupying the main photovoltaic market, and are the leading products of solar cells at present. Polysilicon solar cells have become the mainstream technology with the highest share of solar cells in the world. However, the efficiency of polysilicon solar cells is lower than that of monocrystalline silicon cells. Compared with the power generation efficiency per unit cost, the two are close.
(3) Amorphous silicon solar cells
The advantage of amorphous silicon lies in its strong absorption ability to visible spectrum (500 times stronger than crystalline silicon), so only a thin layer is needed to effectively absorb photon energy. Moreover, the production technology of this amorphous silicon thin film is very mature, which can not only save a lot of material costs, but also make it possible to make large-area solar cells. The main disadvantages are low conversion rate (5%-7%) and photo-induced degradation (so-called S-W effect, that is, the photoelectric conversion efficiency will decrease with the continuation of illumination time, making the battery performance unstable). Therefore, it is not competitive in the solar power generation market, and it is mostly used in the small-size electronic product market with low power. Such as electronic calculators and toys.
In the1980s, amorphous silicon was the only commercialized thin-film solar cell material. When the amorphous silicon solar cell appeared, it caused a lot of investment. From 1985 to the beginning of 1990, the proportion of amorphous silicon solar cells once reached one-third of the total solar cells in the world, but due to poor stability, it could not be effectively improved, resulting in a decline in output. According to different kinds of materials, thin-film batteries can be divided into: thin-film crystalline silicon solar cells (C-Si); Thin film amorphous silicon solar cells (a-Si for short), II-VI compound solar cells (CdTe, indium copper selenide) and III-V compound solar cells, such as gallium arsenide (GaAs), indium phosphide (InP) and indium gallium phosphide (InGaP). Except that the conversion efficiency of III-V compound solar cells can be higher than 30% by adopting multilayer thin film structure, the efficiency of other concentrating thin film solar cells is generally below 10%.
At present, there are three industrialized materials for thin film photovoltaic cells: amorphous silicon (a-Si), copper indium selenium (CIS, CIGS) and cadmium telluride (CdTe), among which amorphous silicon thin film cells have the largest production ratio. In 2007, it accounted for 5.2% of the global output.
(1) Ⅲ-ⅴ compound solar cells
The typical III-V compound solar cells are gallium arsenide (GaAs) cells, and the conversion rate is over 30%. This is because ⅲ-ⅴ family is a semiconductor material with direct energy gap. Under the radiation condition of AM 1, it can absorb about 97% of light only with a thickness of 2um. Thin film solar cells made of GaAs thin films grown on monocrystalline silicon substrates by chemical vapor deposition are used in space because of their high efficiency. The new generation of GaAS multi-junction solar cells has the highest conversion efficiency at present. Because of its wide absorption spectrum range, the conversion efficiency can reach over 39%. But also has stable performance and long service life. However, this kind of battery is expensive, and the average price per watt can be dozens of times higher than that of polysilicon solar cells, so it is not the mainstream of civil use.
Because of its direct energy gap, high absorption coefficient, good anti-reflection damage and insensitivity to temperature changes, it is suitable for application in three main fields, such as thermoelectric TRV, concentrating system and space.
Since August, 2007, gallium arsenide batteries have been transformed from the use on satellites to the large-scale application of concentrated solar power plants. Gallium arsenide high-efficiency concentrating cells have been proved to be an effective way to build solar power plants at low cost abroad.
(2) Ⅱ-ⅵ compound solar cells
II-VI compound solar cells include cadmium telluride thin film cells and copper indium gallium selenide thin film cells.
The direct energy gap of cadmium telluride battery is 1.45eV, which is just within the energy gap range of ideal solar cells. In addition, it has a high absorption coefficient. It has become one of the ideal solar cell materials with high efficiency. In addition, it can be made by various rapid film forming technologies. Because of the easy modular production and good commercial performance in recent years, CdTe/ glass has been widely used in roof materials. However, cadmium pollution is a hidden danger in the development of this thin film battery. The United States and Germany have implemented the recycling mechanism of CdTe solar cells, which has injected positive energy into the market. Because the battery manufacturing process only takes a few minutes, it is easy to mass-produce quickly, so the United States is quite optimistic about the market prospects. It is considered that the number of amorphous silicon solar cells may exceed in the future.
Copper, indium, gallium and selenium have wide light absorption range and good stability in outdoor environment. Because of its high conversion efficiency and low material manufacturing cost, it is considered to be one of the most promising thin film batteries in the future. In terms of conversion efficiency, with the help of the condenser, the current conversion efficiency can reach about 30%, and the highest level under standard environmental test is 19.5%, which is comparable to monocrystalline silicon solar cells. Cu(InGa)Se2 solar cell is not only suitable for large-area surface use, but also has the ability to resist radiation damage, so it also has the potential to be applied in space field. After 30 years of development, the penetration rate of CIGS batteries is still not high. Small-scale mass production stage did not clearly see its cost advantage expected by the world. Therefore, how to make the mass production technology of solar cells mature and greatly reduce the manufacturing cost is the subject of future efforts. Another development direction is to develop CIGS technology with wide energy gap (greater than 1.5eV) and no loss of efficiency. Developing a low-temperature manufacturing process that can produce high-quality CIGS thin films is also the key point to reduce manufacturing costs. Attracted by the market potential of low material cost and high component efficiency, in recent years, in addition to Shell Solar, Wrth Solar, Showa Shell, ZSW, even Honda has followed suit. The hidden danger of CIGS solar cell development is that the reserves of In and Ga are limited, and they may face the same problem of insufficient silicon materials under the competition of other semiconductor and photoelectric industries. At the same time, the manufacturing process is complex and the investment cost is high, which restricts the market growth; CdS has the disadvantage of potential toxicity, which limits the market development. The University of Toledo is in a leading position in the field of flexible substrate amorphous silicon solar cells. The initial efficiency of the single-junction amorphous silicon germanium battery laboratory has reached 65,438+03%. Their technical team participated in the establishment of MWOE and Xunlight companies, and actively planned greater production capacity.
Japan is also a world leader in the research of flexible substrate solar cells. In Japan, Sharp Company, Sanyo Company, TDK Company and Fuji Company have invested a lot of manpower and material resources in the development of flexible substrate amorphous silicon solar cells, and built several megawatt polyester film flexible battery production lines.
Sanyo Company used flexible amorphous solar cells as energy source for the first time on unmanned solar aircraft, and completed the flight across the American continent, showing the great potential of flexible amorphous thin-film solar cells as aircraft energy source. The amorphous silicon solar cells prepared by Sharp and TDK on polyester film have been able to produce modules with an area of 286cm2, and the efficiency has reached 8.65438 0%. The efficiency of small-area battery reaches 1 1. 1%. The stable efficiency of a-Si/a-SiGe laminated battery of Fuji Company reached 9%, and a factory was built in Kumamoto, Japan. In 2006, the output of amorphous silicon battery with plastic substrate reached 15MW.
On the other hand, the European Union has joined some research institutions of its member countries, including Neuchatel University, VHF Technology Company, Roth &; Rau company and others have carried out joint research on flexible battery with polyester film substrate, and now a small batch production line has been realized. On June 65438+10/October 65438 +0, 2005, the European Union launched the "FLEXCELLENCE" project, which lasted for three years. The goal is to develop efficient equipment and technology for roll-to-roll production of thin-film battery components, build a flexible battery production line of more than 50 MW, and hope to control the production cost at 0.5 Euro per watt. According to the report in 2007, at present, the laboratory efficiency of amorphous silicon laminated battery with polyester film substrate in Neuchatel University reaches 10.8%, and the annual production capacity of VHF-technologies Company is 25MW.
The research progress of flexible substrate thin film battery in China is slow. In the mid-1990s, chrona Company in Harbin developed a single-junction amorphous silicon thin film battery with flexible polyimide substrate, with an initial efficiency of 4.63% and a power-to-weight ratio of 23 1.5w/kg, but little progress has been made since then. In recent years, Nankai University has made some progress in the research of flexible substrate amorphous silicon thin film batteries. They obtained 0. 1 15cm2 single-junction thin film battery on polyimide substrate, with an initial efficiency of 4.84% and a power-to-weight ratio of 34 1W/kg.
As for the industrialization of flexible substrate batteries, Tianjin Jinneng Battery Co., Ltd. is currently building a 6MW amorphous silicon flexible battery production line, and the 30MW production line has started project demonstration. Xinjiang Tianfu Photovoltaic Optical Display Co., Ltd. is building a 1MW amorphous silicon flexible battery production line, and plans to build 8MW in the future. It is expected that the battery cost of both companies will be high, because their equipment and technology are imported from abroad. Generally speaking, China has the technical foundation for developing amorphous silicon thin film batteries, but the research on flexible substrates is still in its infancy, which is far from foreign countries.