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With the expansion of technological progress and industrial scale, the cost of photovoltaic power generation has further decreased, and it will become one of the important energy sources for sustainable development in the future.

Key components of photovoltaic technology

The key component of photovoltaic power generation technology is solar photovoltaic cell. The development of solar photovoltaic cells can be roughly divided into three generations. The first generation was 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 the third generation of solar cells. At present, the mainstream is the first generation of silicon solar cells, the market share of thin film batteries is gradually expanding, the third generation of batteries in addition to high-power concentrating cells, most of them are still in the laboratory research and development stage.

Silicon solar cells

In silicon solar cells, monocrystalline silicon technology is the most mature. The efficiency and cost of such cells are mainly affected by their manufacturing process. The manufacturing process is mainly divided into ingot casting, slicing, diffusion, fleecing, screen printing and sintering. The photovoltaic conversion efficiency of solar cells produced by this ordinary process is generally 16%-18%.

The conversion efficiency of monocrystalline silicon solar cells is the highest, but the cost is also higher. Polysilicon solar cells can reduce the cost very well, and its advantage is that it can directly manufacture large-size square silicon ingot suitable for large-scale production, and the equipment is relatively simple, so the manufacturing process is simple, power saving, silicon material saving, and the material requirements are also low.

In addition to reducing the cost of materials, reducing the cost of solar cells is mainly achieved through two aspects, one is to reduce consumables, such as reducing the thickness of silicon wafers; The second is to improve the conversion efficiency. The way to improve the efficiency includes the following aspects: the first is to increase the absorption of light, such as the surface fleece, the preparation of anti-reflection layer, and the reduction of the width of the front electrode. The second is to reduce photogenerated carrier recombination and improve photon utilization, such as emitter passivation technology. The third is to reduce the resistance and increase the absorption of the electrode to the photocurrent, such as zonal doping and back electric field technology.

At present, the highest record of photoelectric conversion efficiency of monocrystalline silicon solar cells is 24.7% created by the PERL structure solar cell of the University of New South Wales. The technical features include: the concentration of phosphorus doping on the silicon surface is low to reduce the surface recombination and avoid the existence of surface “dead layer”; In order to reduce the electrode region recombination and form good ohmic contact, high concentration diffusion is used locally under the front and rear surface electrodes. The front surface electrode is narrowed by photolithography and the absorption area is increased. The front surface electrode adopts more matching metal such as titanium, palladium, silver metal combination 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 the surface compound of the battery. However, the technology has not yet been industrialized.

In addition to PERL techniques, other techniques can be used to improve conversion efficiency. Such as BP Solar’s surface-grooving suede battery and back electrode (EWT) crossing technology. The former is mainly through the laser grooving process to reduce the width of the front electrode, increase the absorption area of sunlight, large-scale production has achieved 18.3% efficiency; The latter, through laser drilling in the battery, leads the electrodes on the front to the back, thereby increasing the absorption area of the front, and can achieve an efficiency of 21.3%.

Thin film solar cell

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 significantly reduce its cost. In order to find an alternative to crystalline silicon cells, lower-cost thin-film solar cells came into being. The mainstream thin film batteries are silicon based thin film batteries, cadmium telluride (CdTe) thin film batteries, copper indium gallium selenium (CIGS) thin film batteries three types.

The thickness of the silicon-based thin film battery is only 2 microns, and compared with the crystalline silicon battery with a thickness of about 180 microns, the amount of silicon material is only about 1.5% of the crystalline silicon battery, and the cost is low. According to the number of PN junctions included, silicon-based thin film batteries are divided into single-junction batteries, double-junction batteries and multi-junction batteries, and different PN junctions can absorb different wavelengths of sunlight. At present, the maximum efficiency of single-junction batteries can reach 7%, and double-junction batteries can reach 10%.

Due to the good absorption rate of the material, the conversion efficiency of the cadmium telluride thin film battery is higher than that of the silicon-based thin film battery, and the current efficiency can reach 12%. However, the element cadmium has carcinogenic effect and the natural reserves of tellurium are limited, which restricts the long-term development of the battery.

Copper indium gallium selenium thin film battery is considered to be the future development direction of high efficiency thin film battery, which can improve the absorption rate of sunlight by adjusting the manufacturing process, so as to improve 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 of all thin film batteries.

Third generation battery

The third generation battery can theoretically achieve higher conversion efficiency. At this stage, in addition to concentrating batteries, most of them are still in the laboratory research stage.

Polymer cells generally use III-V semiconductor materials, mainly because III-V semiconductors have much higher temperature resistance than silicon, still have high photoelectric conversion efficiency under high illumination, and the multi-junction structure makes their absorption spectra and sunlight spectra close to the same, and the theoretical conversion efficiency can reach 68%. At present, the most used is to form three PN junctions by germanium, gallium arsenide, gallium indium phosphorus three different semiconductor materials. If large-scale production, the efficiency can reach more than 40%.