The key element of photovoltaic technology is the solar photovoltaic cell. The development of solar photovoltaic cells can be roughly divided into three generations. The first generation is silicon solar cells; the second generation is thin-film solar cells; new technologies such as high-power concentrating cells, organic solar cells, flexible solar cells, and dye-sensitized nano-solar cells are collectively referred to as third-generation solar cells. At present, the mainstream is the first generation of silicon-based solar cells, and the market share of thin-film cells is gradually expanding. Except for high-power concentrator cells, most of the third-generation cells are still in the laboratory research and development stage.
Silicon solar cells
Among silicon solar cells, monocrystalline silicon technology is the most mature. The efficiency and cost of such cells are primarily affected by their manufacturing processes. The manufacturing process is mainly divided into several steps such as ingot casting, slicing, diffusion, texturing, screen printing and sintering. The photoelectric conversion efficiency of solar cells produced by this common process is generally 16%-18%.
The conversion efficiency of monocrystalline silicon solar cells is the highest, but the cost is also higher. Polycrystalline silicon solar cells can reduce costs very well. The advantage is that it can directly manufacture large-sized square silicon ingots suitable for large-scale production. The equipment is relatively simple, so the manufacturing process is simple, power saving, and silicon material saving. The material requirements are also relatively low.
In addition to reducing the cost of materials and the cost of solar cells, it is mainly achieved through two aspects: one is to reduce consumables, such as reducing the thickness of silicon wafers; the other is to improve conversion efficiency. The ways to improve the efficiency include the following aspects: The first is to increase the absorption of light, such as surface texturing, preparation of anti-reflection layers, and reducing the width of the front electrode. The second is to reduce the recombination of photogenerated carriers and improve photon utilization, such as emitter passivation technology. The third is to reduce the resistance and increase the absorption of the photocurrent by the electrode, such as partition doping and back electric field technology.
The current record for the photoelectric conversion efficiency of monocrystalline silicon solar cells is 24.7% created by the University of New South Wales' PERL structure solar cells. Its technical features include: the concentration of phosphorus doping on the silicon surface is low to reduce the recombination of the surface and avoid the existence of surface "dead layers"; local high-concentration diffusion is used under the front and rear surface electrodes to reduce the recombination of the electrode area and form a good Ohmic contact; the front surface electrode is narrowed by the photolithography process to increase the light absorption area; the front surface electrode uses a combination of more matching metals such as titanium, palladium, and silver to reduce the contact resistance between the electrode and silicon; the front and rear surfaces of the battery use SiO2 and point contact methods to reduce surface recombination of cells. However, the technology has not yet been industrialized.
In addition to PERL technology, other technologies can also be used to improve conversion efficiency. Such as BP Solar's surface grooved suede cell and back electrode (EWT) through technology. The former mainly reduces the width of the front electrode through the laser grooving process and increases the absorption area of sunlight, and the large-scale production can achieve an efficiency of 18.3%; The back side, thus increasing the light absorption area of the front side, can achieve an efficiency of 21.3%.
Thin film solar cells
Crystalline silicon solar cells are highly efficient and still dominate in large-scale applications and industrial production. However, due to the relatively high price of silicon materials, it is very difficult to greatly reduce its cost. In order to find alternatives to crystalline silicon cells, lower-cost thin-film solar cells have emerged. The mainstream thin film batteries are silicon-based thin film batteries, cadmium telluride (CdTe) thin film batteries, and copper indium gallium selenide (CIGS) thin film batteries.
The thickness of silicon-based thin-film cells is only 2 microns. Compared with crystalline silicon cells with a thickness of about 180 microns, the amount of silicon material is only about 1.5% of that of crystalline silicon cells, and the cost is low. According to the number of PN junctions included, silicon-based thin-film cells are divided into single-junction cells, double-junction cells and multi-junction cells. Different PN junctions can absorb sunlight of different wavelengths. At present, the highest efficiency of single-junction cells can reach 7%, and double-junction cells can reach 10%.
Due to the good light absorption rate of the material, the conversion efficiency of cadmium telluride thin-film cells is higher than that of silicon-based thin-film cells, and the current efficiency can reach 12%. However, the element cadmium has carcinogenic effects and the natural reserves of tellurium are limited, which restricts the long-term development of this battery.
Copper indium gallium selenide thin-film batteries are considered to be the future development direction of high-efficiency thin-film batteries, which can improve the absorption rate of sunlight by adjusting the manufacturing process, thereby improving the conversion efficiency. At present, the conversion efficiency of the laboratory can reach 20.1%, and the product efficiency can reach 13-14%, which is the highest among all thin-film batteries.
Third-generation solar cells
The third-generation cells can theoretically achieve higher conversion efficiencies. At this stage, except for concentrator cells, most of them are still in the laboratory research stage.
Concentrator cells generally use III-V semiconductor materials, mainly because III-V semiconductors have much higher high temperature resistance than silicon, still have high photoelectric conversion efficiency under high illumination, and the multi-junction structure makes their The absorption spectrum and the sunlight spectrum are close to the same, and the theoretical conversion efficiency can reach 68%. At present, three PN junctions are formed by three different semiconductor materials, germanium, gallium arsenide, and gallium indium phosphorus. If large-scale production is carried out, the efficiency can reach more than 40%.
Solar cells are packaged into solar modules, and the application of different solar cells depends on their own characteristics and the development of market demand. In the early days, solar energy was mainly used in communication base stations and artificial satellites, and later gradually entered the civilian field, such as solar roofs. In these scenarios, the installation area is small and the energy density requirement is high, so crystalline silicon modules occupy the main market share. With the development of large-scale solar desert power plants and photovoltaic buildings, comprehensive cost has gradually replaced energy density as an important factor to consider, and the application of thin-film batteries is on the rise. In addition, the application of different technologies is also affected by other factors such as the use environment and climatic conditions.
