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Monocrystalline silicon wafers and polycrystalline silicon wafers have significant differences in crystal quality, electrical properties, and mechanical properties. The difference between single crystal and polycrystalline is mainly in the preparation of raw materials. Single crystal is the Czochralski lifting method, polycrystalline is the ingot casting method, and there are only some minor differences in the back-end manufacturing process.

The development history of crystalline silicon cells

In 1839, French scientist Becquerel discovered the photovoltaic effect of liquids.

In 1917, Polish scientist Czkowski invented CZ technology, which was later improved and developed into the main preparation method of monocrystalline silicon for solar energy.

In 1941, Orr discovered the photovoltaic effect on silicon materials.

In 1954, American scientists Chapin and Pearson made the first practical single-crystal silicon solar cell at Bell Labs in the United States.

From 1955 to 1975, due to the high cost of single crystal cells, the industry continued to focus on reducing the cost of crystal manufacturing, and proposed the ingot single crystal process.

In 1976, the single crystal ingot technology failed, and the German Wacker Company took the lead in using ingot polycrystalline for solar cell production, sacrificing crystal quality to reduce power generation costs.

From 2005 to 2010, polycrystalline cell technology rapidly expanded its share based on relatively cheap cost.

In 2013, the conversion efficiency of Panasonic's HIT single crystal cell reached 25.6%, breaking through the highest theoretical efficiency limit in the photovoltaic industry. People re-evaluated the performance and cost ranges of various technologies.

From 2013 to 2015, the introduction of continuous and rapid crystal pulling technology and diamond wire slicing technology reduced the cost gap between monocrystalline modules and polycrystalline modules to less than 3%, and the unit investment cost of power plants using monocrystalline modules and polycrystalline modules was the same.

It is expected that by 2016, with the application of high-efficiency technologies such as PERC, the cost of monocrystalline modules and polycrystalline modules will reach the same level.

Conversion efficiency comparison

The three main parameters that affect the conversion efficiency are: Voc (open circuit voltage), Isc (short circuit current), FF (fill factor), the formula is: Eta=Voc×Isc×FF

From the decomposition of photoelectric conversion efficiency parameters, the parameters of monocrystalline cells are ahead of those of polycrystalline cells, as shown in Table 1. Generally speaking, the mass production efficiency of domestic monocrystalline cells under the current process is about 19.55%, and if done well, it can reach 19.8%-19.9%, depending on whether it is a three-bar or four-bar; the mass production efficiency of polycrystalline cells is generally 18.12% or so.

Process difference

In terms of process, single crystal is more environmentally friendly and lower cost than polycrystalline. The process technology of the battery includes texturing, diffusion, etching, coating, printing, sintering, etc. The main difference between the preparation process of single crystal battery and polycrystalline battery is in the texturing link, and the rest of the links are only differences in control standards.

Single crystal texturing adopts alkaline solution corrosion, and silicate and hydrogen by-products are generated during the corrosion process. By replacing or partially replacing isopropyl alcohol (IPA) with texturing auxiliary liquid, lower BOD and COD sewage discharge can be achieved, and The single crystal texturing system has very low requirements on equipment hardware, and it is easier to achieve environmental protection and process control.

Polycrystalline is corroded by acid solution, which requires the use of high concentrations of nitric acid and hydrofluoric acid. The main by-products are fluorosilicic acid and NOx, and NOx is an air pollutant that is difficult to be thoroughly treated. Considering these factors, it is necessary to use strict Closed automation equipment. The equipment acquisition and maintenance cost of polycrystalline texturing is much higher than that of monocrystalline.

Temperature Coefficient Comparison

Monocrystalline materials have no grain boundaries, high material purity, small internal resistance, and small temperature rise; The temperature increase of the polycrystalline is more pronounced. Under the highest light intensity, the working temperature of single crystal is about 5~6℃ lower than that of polycrystal, and the working temperature of polycrystal in some areas can be more than 10℃ higher than that of single crystal, so the power loss of polycrystal is larger, and the power of single crystal The loss is small.

From the point of view of the temperature coefficient itself, the temperature coefficient of single crystal is slightly lower than that of polycrystalline, so the power loss of single crystal is also less than that of polycrystalline when the temperature is increased by 1 °C.

Brief description of PERC battery technology

A few years ago, the photovoltaic industry focused on the selective emitter cell technology for high-efficiency cells? Now the industry no longer does selective emitter cells and pays more attention to PERC cells, because selective emitter cells mainly improve the short-wave The absorption capacity is reflected in the components. Since EVA itself absorbs the short wavelength band of ultraviolet light, it does not show obvious advantages in components, and the selective emitter technology is eliminated. PERC cells are mainly reflected in the absorption of near-infrared and infrared bands, while EVA does not absorb solar energy in the infrared band, so PERC technology can better reflect the improvement of cell efficiency to the improvement of module efficiency.

PERC batteries have the following characteristics:

① The absolute value of cell efficiency can be increased by 1% on single crystal and 0.5% on polycrystalline, so the advantage of using PERC technology on single crystal is greater.

② PERC technology has high compatibility with existing production lines, is easy to upgrade production lines, and can reduce the cost per watt of cells.

③ PERC batteries have become the mainstream technology in the industry and gradually replace conventional batteries.

④ Through process optimization, the mass production efficiency can be gradually increased to 21% in the past 1-2 years. The efficiency of SolarWorld's recent P-type monocrystalline silicon PERC cell in the laboratory has reached 21.7%.

The above-mentioned P-type PERC cell technology, the next-generation N-type PERC technology, can not only solve the problem of LID, but also further improve the conversion efficiency of mass production to 22%.

In the second half of 2015, LEO Solar will mass-produce high-efficiency PERC monocrystalline cell modules at its Hefei base, and will add 2GW of PERC cell production capacity in Taizhou, Jiangsu.

Brief introduction of IBC battery technology

IBC cells are also produced using N-type monocrystalline silicon wafers. At present, the highest efficiency in the laboratory can reach 25%, and the average efficiency in mass production is 23%. As can be seen from Figure 8, there is no grid line on the front of the IBC cell, and all grid lines are concentrated at the back. Its biggest feature is that the manufacturing process is relatively complicated. There are currently sixteen or seven manufacturing processes, and the cost is relatively high, which limits the development of this technology. The industry is currently focusing on developing low-cost IBC technology.

Panasonic has combined IBC and HIT technologies to set a new world record for conversion efficiency, as high as 25.6%. Its open circuit voltage reaches 740mV, Jsc is 41.8mA/cm2, FF is 82.7%, and the silicon wafer thickness is 150μm.

Application example of IBC cells: Sunny Power 2 uses high-efficiency N-type IBC monocrystalline cells to cover the wings, with a conversion efficiency of 23%, and completely relies on solar power to complete the round-the-world flight.

Prediction of battery technology development trend

① In the future, the market share of monocrystalline will gradually surpass that of polycrystalline.

② The market share of N-type high-efficiency batteries will gradually increase, depending on the speed of N-type battery cost reduction.

③ The market share of PERC batteries will surpass the current conventional batteries after 2018, and the share will gradually expand.

④ PERC batteries will have a long life cycle and will coexist with N-type batteries in the market for a long time.

Comparison of investment income of single and polycrystalline power plants

At present, the high-power modules packaged in 60 pieces have a single crystal mass production power of 275W and a polycrystalline mass production power of 260W. Since the number of monocrystalline modules used in each square array is small, brackets, fixtures, combiner boxes, photovoltaic cables, basic engineering, installation engineering, etc. are effectively saved. Therefore, in terms of total investment cost, monocrystalline systems and polycrystalline The system is basically the same. The specific analysis data are shown in Table 3 below.

In terms of power station operation, monocrystalline can save 5% of land rent and 6% of operation and maintenance costs than polycrystalline energy, and monocrystalline energy generation per watt is at least 3% higher than that of polycrystalline energy, then at 25% capital ratio, 15 years Under the financing structure of loan term, the IRR of capital investment in monocrystalline power plants in central my country will be at least 2.78% higher than that in polycrystalline power plants.

Comparison of power generation and long-term reliability

So far, the vast majority of power stations that have undergone long-term operation tests use monocrystalline components. Typical cases include:

① In 1982, the first grid-connected photovoltaic system in Europe was built in Switzerland, using monocrystalline modules with an installed capacity of 10KW and an average annual attenuation of 0.4%.

② In 1984, the 1MW photovoltaic power station in California used monocrystalline modules, which are still in good operation.

③ In 1984, the earliest photovoltaic power station in Lanzhou used monocrystalline modules, with an average annual attenuation of 0.37%.

④ In 1994, the earliest single crystal power station in Ningbo, Zhejiang, had a total power reduction of 13.1% in 21 years.

⑤ The MW-level rooftop power station in Germany has been operating for 18 years. The annual attenuation of Siemens monocrystalline modules is about 0.4%, and there is no quality problem so far.

⑥ "Finding the Most Beautiful Old Modules" is the first stop in Yunnan. The rooftop single crystal system has been in operation for nearly 30 years, and the corrected light decay in the last 15 years does not exceed 8%.

At present, there are a large number of power station operation examples in China, which prove that in the same region, under the same construction conditions and BOS conditions, the power generation per watt of monocrystalline is significantly higher than that of polycrystalline. Typical cases include:

① Qingdao Longsheng Photovoltaic Carport, the power generation per watt of monocrystalline is 6.6% higher than that of polycrystalline.

② Sun Yat-Sen University "Comparison of Actual Power Generation Performance of Six Solar Cell Photovoltaic Arrays" (2008.1-2008.7) confirmed that the power generation per watt of single crystal is 5.7% higher than that of polycrystalline.

③ Test data of Zhejiang University Key Laboratory of Silicon Materials (2013.7-2014.6): The power generation of single crystal with the same nominal capacity is more than 7% higher than that of polycrystalline.

④ Comparison of single and polycrystalline array data of a photovoltaic project in Hohhot: The annual power generation of a single-crystalline array with the same capacity of the same project is 7% higher than that of polycrystalline.

⑤ Compared with the 30MW power plants of Zhongwei and Tongxin in Ningxia, the single crystal power generation is 6.52% higher.

⑥ Comparison of 10MW single crystal system/10MW polycrystalline system of Golmud Sunshine Energy: the power generation of single crystal is 5.12% higher than that of polycrystalline.

Summarize

① Monocrystalline silicon wafers have higher mechanical strength and lower fragmentation rate than polycrystalline silicon wafers.

② Monocrystalline silicon cells have higher conversion efficiency and greater room for efficiency improvement than polycrystalline silicon cells.

③ During the 25-year life cycle, the actual power generation of monocrystalline silicon power plants is more than that of polycrystalline silicon power plants (about 6%).

④ In terms of long-term reliability, the decay of monocrystalline silicon power plants is about 3% lower than that of polycrystalline silicon. Loye Solar guarantees that the monocrystalline silicon cell modules produced will not decay by more than 16.2% within the 25-year guarantee period.

⑤ The price of monocrystalline modules is 0.1-0.15 yuan per watt higher than that of polycrystalline silicon, the cost of monocrystalline silicon systems is basically the same, and the return on investment IRR of monocrystalline silicon power plants is at least 2.78% higher than that of polycrystalline power plants.

⑥ Monocrystalline has the advantage of being highly intensive and maximizing roof resources. On the same roof area, the installation volume of monocrystalline system is 7.8% higher than that of polycrystalline system, which is more suitable for distributed power station applications.

⑦ Benefiting from the growing number of rooftop photovoltaic installations and the increasing demand for more efficient products, monocrystalline silicon cells and module products will rapidly occupy a higher market share in the next few years, becoming a new growth point in the photovoltaic industry.