In 1839, the French Becqueral observed the photovoltaic effect in a chemical cell for the first time. In 1876, the photovoltaic effect was also observed in solid selenium (Se), and Se/Cuo photovoltaic cells were subsequently developed. Reports about silicon photovoltaic cells appeared in 1941. Bell Labs Chapin and others developed monocrystalline silicon solar cells with an efficiency of 6% in 1954, and the era of modern silicon solar cells began. Silicon solar cells were first used in spacecraft in 1958. In the following 10 years, the application of silicon solar cells in space continued to expand, the process continued to improve, and the design of silicon solar cells was gradually finalized. This is the first period in the development of silicon solar cells. Tai’s second period began in the early 1970s. During this period, back surface field, fine grid metallization, shallow junction surface diffusion and surface structuring technology began to be introduced into the manufacturing process of silicon solar cells, and the photoelectric conversion efficiency of solar cells has been greatly improved. At the same time, silicon solar cells began to be used on the ground and continued to expand. By the end of the 1970s, the output of ground-use solar cells had exceeded the output of space-use solar cells, and the cost of solar cells continued to decrease.
Read more: Photovoltaic effect of solar cells
In the early 1980s, silicon solar cells entered the third period of rapid development. The main characteristics of this period are the introduction of surface passivation technology, reducing contact recombination effects, post-processing to increase carrier life, and improving light trapping effect technology into the manufacturing process of solar cells. The efficiency of solar cells has been greatly improved, the cost of commercial production has been further reduced, and the application has continued to expand.
In the entire development process of solar cells, various solar cells with different structures have appeared successively, such as Schottky (Ms) cells, M1S cells, MINP cells, heterojunction cells[such as ITO(n)/Si(p), a-Si/c-Si, Ge/Si], etc. Among them, the homogenous PN junction cell structure dominates from beginning to end, and other structures also have an important influence on the development of solar cells.
Solar cells are distinguished by materials, including crystalline silicon cells, amorphous silicon thin-film cells, copper indium selenium (CIS) cells, cadmium telluride (CdTe) cells, arsenide cells, etc. At present, the market is dominated by crystalline silicon cells. Since silicon is the second most abundant element on the earth, as a semiconductor material, people have studied it the most, the technology is the most mature, and the performance of crystalline silicon is stable and non-toxic, so it has become the main material in the research, development, production and application of solar cells.
Since the commercialization of ground-based solar cells in the mid-1970s, crystalline silicon has occupied a dominant position as a basic solar cell material, and it is believed that this situation will not undergo a fundamental change in the next 20 years. Solar cells made of crystalline silicon materials mainly include: monocrystalline silicon solar cells, cast polycrystalline silicon solar cells, amorphous silicon solar cells and thin-film crystalline silicon cells.
Monocrystalline silicon battery has high conversion efficiency and good stability, but the cost is high; amorphous silicon solar battery has high production efficiency and low cost, but the conversion efficiency is relatively low, and the efficiency decays relatively severely; cast polycrystalline silicon solar cells have stable conversion efficiency and the highest cost-performance ratio; thin-film crystalline silicon solar cells are still in the research and development stage. At present, cast polycrystalline silicon solar cells have replaced Czochralski monocrystalline silicon as the most important photovoltaic cell material. However, the conversion efficiency of cast polycrystalline silicon solar cells is slightly lower than that of Czochralski monocrystalline silicon solar cells. There are various defects in cast polysilicon materials, such as grain boundaries, dislocations, micro defects, impurity carbon and oxygen, and transition metals contaminated during the process, which are considered to be the key reasons for the low conversion efficiency of cast polysilicon solar cells. Therefore, research on the rules of defects and impurities in cast polysilicon, and the use of appropriate gettering and passivation processes in the process are the key to further improving cast polysilicon cells. In addition, finding a wet chemical corrosion method suitable for surface structuring of cast polysilicon is also an important process for low-cost preparation of high-efficiency solar cells.
From the perspective of solid-state physics, silicon is not the most ideal solar photovoltaic cell material. This is mainly because silicon is an indirect energy band semiconductor material with a low light absorption coefficient, so it has become a trend to study other photovoltaic cell materials. Among them, cadmium telluride (CdTe) and copper indium selenium (CulnSe2) are recognized as two very promising solar photovoltaic cell materials. The current research has made some progress, but there is still a lot of work to be done before large-scale production and competition with crystalline silicon solar cells. Due to advances in technology, including wafer thickness, cutting technology, wafer size, and wafer prices, there have been substantial improvements. Since 1960, the unit wattage cost of power generation with crystalline silicon solar cells has dropped by about 50 times, and the current price is about US$2.5 to US$3/w. According to reports from the National Renewable Energy Laboratory of the United States, the manufacturing cost of thin-film solar cells has also fallen sharply in the past 10 years, and the trend is faster than that of silicon wafers, but its price is still about 50% higher than that of silicon wafers.
At present, the photoelectric efficiency of a single silicon wafer cell in the laboratory has reached 25%, which is very close to the theoretical value of 29%. The photoelectric efficiency of commercial products has also made great progress since 1970, reaching about 12% in recent years. This technological achievement, relatively speaking, is beyond the reach of most thin-film technologies.
Production costs are often affected by the scale of production, and solar cells are no exception. Comparing silicon wafer type and thin film type solar cells, the current capacity scale of the former is about 10 times that of the latter, so the fixed cost can be shared by a large margin. Secondly, in terms of capacity utilization rate, at present, due to the substantial market growth of silicon wafer manufacturers in recent years, the average capacity utilization rate is about 80%, while thin film manufacturers are only about 40%. This makes silicon wafer solar cells more cost-competitive in production and becomes the dominant product in the market.