From crystalline silicon cells to thin film cells: an overview of photovoltaic cell processes


The upstream of the photovoltaic industry chain is the collection of crystalline silicon raw materials and the processing and production of silicon rods, ingots, and silicon wafers. The midstream of the industry chain is the production of photovoltaic cells and photovoltaic cell components. At present, crystalline silicon cells are divided into two types: monocrystalline silicon and polycrystalline silicon. The downstream of the industrial chain is the integration and operation of the photovoltaic power station system

From a technical point of view, the technologies involved in the upstream of the photovoltaic industry chain are polycrystalline silicon preparation technology, single crystal silicon pulling rod technology, polycrystalline silicon ingot technology and crystalline silicon slicing technology. The technologies involved in the midstream of the industrial chain include battery technology. The technologies involved in the downstream of the industrial chain include photovoltaic power station grid connection technology, photovoltaic power generation performance evaluation technology and photovoltaic intelligent data platform technology.

The upstream of the photovoltaic industry chain includes crystalline silicon raw materials and silicon wafers. As the most upstream crystalline silicon manufacturing in the industry chain, the technical requirements of this link are very high at present, and it has certain technical barriers and monopoly. At present, the technology for preparing polysilicon mainly includes three types: modified Siemens method, silane method and metallurgical method.

The improved Siemens method is based on the Siemens method with the addition of tail gas recovery and silicon tetrachloride hydrogenation process, which realizes a closed-circuit cycle of the production process, which can not only avoid the direct discharge of highly toxic by-products to pollute the environment, but also realize the recycling of raw materials. Reduced production costs (for low single conversion rates)

The silane method refers to the thermal decomposition of high-purity silane into high-purity silicon in a reactor. The silane method can be divided into two categories. The earlier one is the Silane Siemens method, which uses silane (SiH4) instead of TCS as the raw material of the CVD reduction furnace, and produces high-purity polysilicon by thermal decomposition and vapor deposition of silane. Bar stock, REC Silicon, a subsidiary of REC, has used this method to produce electronic-grade polysilicon;

Later, another type of method appeared – Silane FBR, which uses STC, H2, metallurgical silicon and HCI as raw materials in a fluidized bed (FBR) at high temperature (above 500 °C, not very high) and high pressure ( 20bar or more) hydrogenation to generate TCS, TCS produces silane gas through a series of disproportionation reactions, and the silane gas is passed into a fluidized bed (FBR) reactor with small particles of silicon powder for continuous thermal decomposition reaction to generate granular polycrystalline silicon .

Among the three production processes, the improved Siemens method is the current mainstream method. According to ITRPV’s prediction, due to the potential factor of cost control, the silane fluidized bed method will gradually replace the share of the improved Siemens method in the future. Mainstream preparation methods

Wafer manufacturing is the next step in crystalline silicon manufacturing, and it also belongs to the upstream level of the entire industrial chain. Different from the crystalline silicon manufacturing link, this link is capital-intensive, with low technical content, and the product process is related to the input equipment, which can be divided into monocrystalline silicon wafers and polycrystalline silicon wafers.

In the cost of silicon wafers, the cost of polysilicon accounts for a relatively high proportion, and the price of polysilicon changes frequently. Therefore, in cost accounting, the cost of silicon wafers is divided into silicon cost and non-silicon cost. The cost difference between single crystal and polycrystalline is mainly reflected in the difference between the cost of pulling rods and ingots. The cost advantage of polycrystalline over single crystals has always been based on the higher efficiency of ingots compared to pulling rods.

In the slicing process, the slicing cost of single polycrystalline is roughly similar, which is related to the selected slicing method. The cost of diamond wire slicing is lower than that of mortar slicing, while the cost of single crystal diamond wire slicing is slightly lower than that of polycrystalline diamond wire slicing. At present, diamond wire slicing has basically been popularized for single crystal slicing, and polycrystalline is transitioning from mortar slicing to diamond wire slicing, and the speed is very fast.

At present, in the photovoltaic market, solar cells are mainly crystalline silicon products, and there are also a small number of thin film products. Crystalline silicon cells include two types of monocrystalline silicon and polycrystalline silicon. From the perspective of cell conversion efficiency, the conversion efficiency of conventional polycrystalline mass production is 18.8%, and the efficiency of combined black silicon technology is about 19.2%.

The efficiency of conventional single crystal is 20-20.2%, while the power generation per watt of PERC single crystal modules is 2.5%-3% higher than that of conventional modules. Low and so on, and of course low attenuation. Therefore, in terms of system cost, the cost of cables, supporting structures, inverters, installation and land can be saved, and finally it can bring high benefits to users.

Cost reduction and technological progress have resulted in a significant cost-effectiveness advantage for single crystals. At present, the conversion efficiency of monocrystalline cells in my country is about 2% higher than that of polycrystalline cells on average. Even if the transformation of diamond wire and black silicon is completed, the cost advantage of polysilicon is still not obvious. As the cost of pulling crystals gradually decreases, the cost-effectiveness of single crystals is highlighted.

The general trend of silicon wafer prices is down, and the mismatch between supply and demand in the short term leads to fluctuations, and the long-term trend depends on cost reduction. Since 2016, the price difference between single and polycrystalline has been increasing. On the one hand, it reflects the supply and demand of monocrystalline and polycrystalline in varying degrees, and on the other hand, it is also due to the rapid progress in the efficiency of the downstream cells and modules of the monocrystalline route.

In the future, monocrystalline cells will have more room for efficiency improvement and faster implementation speed than polycrystalline cells, and the efficiency gap will further expand.

Therefore, the single crystal technology route occupies a larger market share in the process of reducing the cost of photovoltaic power generation by improving efficiency

Finally, thin-film batteries are generally made of photosensitive materials several microns thick attached to the surface of glass, stainless steel, etc.

The main advantages are:

(1) The thin-film battery uses less raw materials, simple manufacturing process, low energy consumption, can be continuously produced in a large area, and can use low-cost materials such as glass or stainless steel as the substrate.

(2) It can be made into flexible products that can be bent, and has a wide range of applications.

(3) Good low light performance and power output in the case of low irradiance.

The main disadvantages are:

(1) The photoelectric conversion rate of thin-film cells is low, and the mass production efficiency of copper indium selenium with the highest conversion efficiency can only reach 15%, and the mass production efficiency of silicon-based thin-film cells is below 10%.

(2) The equipment and technology investment of thin-film batteries is several times that of crystalline silicon batteries.

(3) The yield rate of thin-film battery module production is not satisfactory. The yield rate of non/microcrystalline silicon thin film battery modules is currently only around 60%. The mainstream manufacturers of CIGS battery packs are only 65%.



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