An overview of the development of crystalline silicon solar cells

Reports of silicon photovoltaic cells first appeared in 1941. Bell Labs developed monocrystalline silicon solar cells with a photoelectric conversion efficiency of 6% in 1954, and the era of modern silicon solar cells began. Silicon solar cells were first used on spacecraft in 1958. In the following 10 years, the space application of silicon solar cells has been continuously expanded, the process has been continuously improved, and the cell design has been gradually finalized. This is the first period of silicon solar cell development.

The second period began in the early 1970s, during which back surface field, fine gate metallization, shallow junction surface diffusion and surface structuring techniques began to gain traction in the fabrication of solar cells. 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 production of ground-based solar cells had exceeded that of space-based solar cells, and the cost of solar cells continued to decrease. Visit the website to learn more about battery cost and performance.

In the early 1980s, silicon solar cells entered the third period of rapid development. The main features of this period are the introduction of surface passivation technology, reducing contact recombination effect, using post-processing technology to improve carrier lifetime, and improving light trapping effect technology into the manufacturing process of solar cells. The photoelectric conversion efficiency of solar cells has been greatly improved, the commercial production cost has been further reduced, and the application has been continuously expanded.

In the whole development process of solar cells, cells with different structures have appeared successively, such as Schottky (Ms) cells, MIS cells, MINP cells, heterojunction cells [such as ITO(n)/Si(p), a-Si/c-Si, Ge/Si], etc., among which the homogenous PN junction cell structure dominates from beginning to end, and other structures also have an important impact on the development of solar cells.

Solar cells are distinguished by materials, including crystalline silicon cells, amorphous silicon thin film cells, copper indium selenide (CulnSez) cells, cadmium telluride (CdTe) cells, and gallium arsenide cells. At present, the market is dominated by crystalline silicon cells. Since silicon is the second largest element in reserves on the earth, as a semiconductor material, it is the most studied and the technology is the most mature, and the performance of crystalline silicon is stable and non-toxic, so it has become a solar cell. Main material in research and development, production and application.

Amorphous silicon, polycrystalline silicon, monocrystalline silicon
Amorphous silicon, polycrystalline silicon, monocrystalline silicon

Since the commercialization of terrestrial solar cells in the mid-1970s, crystalline silicon has dominated as the basic cell material, and it is believed that this situation will not fundamentally change in the next 20 years. Solar cells prepared from 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 solar cells have high photoelectric conversion efficiency and good stability, but the cost is high; amorphous silicon solar cells have high production efficiency and low cost, but the photoelectric conversion efficiency is relatively low, and the efficiency decays more severely; cast polycrystalline silicon Solar cells have stable photoelectric conversion efficiency and the highest performance-price 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 photoelectric conversion efficiency of cast polycrystalline silicon solar cells is slightly lower than that of Czochralski monocrystalline silicon solar cells. The existence of various defects (such as grain boundaries, dislocations, micro-defects, impurity carbon and oxygen) and contamination of transition group metals in the process of casting polysilicon materials are considered to be the key reasons for the low conversion efficiency of cells. The study of defects and impurities in polysilicon, and the use of appropriate gettering and passivation processes in the process are the keys to further improve cast polysilicon cells. In addition, finding a wet chemical etching method suitable for structuring the surface of cast polysilicon is also an important process for low-cost fabrication of high-efficiency solar cells.

In terms of solid-state physics, silicon materials are not the most ideal photovoltaic cell materials, 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 and copper indium selenide are considered to be two very promising photovoltaic cell materials. Current research has made certain progress, but there is still a lot of work to be done before large-scale production and competing with crystalline silicon solar cells. to do it.
Thanks to technological advances, including silicon wafer thickness, dicing technology, wafer size, and wafer price, the cost per watt of electricity from crystalline silicon solar cells has dropped by about 50 times since 1960. The current price is about 2.5~3 USD/W. According to the National Renewable Energy Laboratory of the United States, the manufacturing cost of thin-film solar cells has also dropped significantly 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 conversion 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 conversion efficiency of commercial products has also improved significantly 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 heavily influenced by production scale, and solar cells are no exception. Comparing silicon wafer type and thin film solar cells, in general, the current capacity of the former is about 10 times that of the latter, so fixed costs can be largely shared. In terms of capacity utilization, due to the substantial growth of the market in recent years, the average capacity utilization rate of silicon wafer solar cells is currently about 80%, while the average capacity utilization rate of thin film solar cells is only 40%. This makes silicon wafer-based solar cells more cost-competitive in production and become the dominant product in the market.

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