TCO (transparentconductingoxide) glass, that is, transparent conductive oxide coated glass, is to evenly coat a layer of transparent conductive oxide film on the surface of flat glass by physical or chemical coating methods, mainly including oxides of in, Sn, Zn and CD and their composite multi-component oxide film materials.
TCO glass was first used in flat panel displays, and now ITO type conductive glass is still the mainstream glass electrode product in the flat panel display industry. In recent years, the rising price of crystalline silicon has greatly promoted the development of thin-film solar cells. At present, thin-film solar cells account for more than 10% of the world photovoltaic market. TCO glass is used as a necessary component of the front electrode of photovoltaic cells, and the market demand is growing rapidly, becoming a hot high-tech coated glass product.
1, Characteristics and types of TCO coated glass
In solar cells, the electrodes of crystalline silicon cells are wires welded on the surface of silicon, and the front cover glass only needs to achieve high light transmittance. Thin film solar cells are coated with p-i-n semiconductor film on the conductive film on the glass surface, and then plated with back electrode.
There are many coating materials and processes of transparent conductive oxide, which are continuously screened through scientific research. At present, there are mainly the following three kinds of TCO glass that match the performance requirements of photovoltaic cells.
Ito coated glass is a very mature product, which has the characteristics of high transmittance, firm film, good conductivity and so on. It was used in the front electrode of photovoltaic cells in the early stage. However, with the improvement of light absorption performance requirements, TCO glass must have the ability to improve light scattering, which is difficult for ITO coating, and the laser etching performance is also poor. Indium is a rare element, which has low storage in nature and high price. Ito is not stable in plasma when applied to solar cells, so ITO coating is not the mainstream electrode glass of photovoltaic cells at present.
SnO2 coating, also referred to as FTO, is currently mainly used to produce Low-E glass for construction. Its conductivity is slightly worse than that of ITO, but it has the advantages of relatively low cost, easy laser etching, and suitable optical properties. Through upgrading and improving the production technology of ordinary Low-E, products with better conductivity and haze than ordinary Low-E are manufactured. TCO glass produced by this technology has become the mainstream product of thin-film photovoltaic cells.
The research progress of zinc oxide based thin films is rapid, and the material properties have been comparable with ITO. The structure is hexagonal wurtzite. Among them, aluminum doped zinc oxide films are widely studied. Its outstanding advantages are easy availability of raw materials, low manufacturing cost, non-toxic, easy doping, and good stability in plasma. It is expected to become a new type of photovoltaic TCO product soon. At present, the main problem is the technical problem of industrialized large-area coating.
2, Performance requirements of photovoltaic cells for TCO coated glass
1. Spectral transmittance
In order to make full use of sunlight, TCO coated glass must maintain a relatively high transmittance. At present, the largest output of thin-film batteries is double junction amorphous silicon batteries, and has begun to transform to amorphous / microcrystalline composite batteries. Therefore, amorphous / microcrystalline composite laminates can absorb and utilize more sunlight and improve conversion efficiency, and will soon become the mainstream products of thin-film batteries.
2. Conductivity
The conductive principle of TCO conductive film is to add trace other elements into the intrinsic semiconductor with weak conductivity, which will significantly change the conductivity of the semiconductor. These trace elements are called impurities, and the doped semiconductors are called impurity semiconductors. Indium tin oxide (ITO) transparent conductive glass is to add tin element into indium oxide to improve conductivity. Its conductivity is the best at present, with the lowest resistivity of 10-5 Ω cm.
3. Haze
In order to increase the ability of the semiconductor layer of thin-film cells to absorb light, TCO glass for photovoltaic needs to improve the scattering ability of transmitted light, which is expressed by haze. Haze refers to the appearance of cloud or turbidity caused by light diffusion on the interior or surface of transparent or translucent materials. Expressed as a percentage of the ratio of diffuse luminous flux to the luminous flux of the transmitting material.
In general, ordinary coated glass requires that the smoother the film surface is, the better, and the smaller the haze is, the better. However, TCO glass for photovoltaic needs to have a certain light scattering ability. At present, the commercial TCO glass with good haze control is AFG pv-tco glass, and the haze value is generally 11% - 15%. It does not include the direct transmittance curve when scattering.
4. Laser etching performance
In the manufacturing process of thin-film batteries, the surface needs to be divided into several long strip-shaped battery packs, which are connected in series to improve the output energy efficiency. Therefore, before plating semiconductor film on TCO glass, the conductive film on the surface must be etched, and the etched part must be completely removed from the oxide conductive film to maintain insulation. At present, there are two kinds of etching methods: chemical etching and laser etching. However, due to the requirements of fine etching lines, which are generally tens of microns wide, laser etching has the characteristics of uniform grooves, clean removal and fast production efficiency.
5. Weather resistance and durability
TCO coating generally uses the "hard" plating process, and the film has good wear resistance, acid and alkali resistance. After the installation of photovoltaic cells, especially when the photovoltaic integrated building is installed on the roof and curtain wall, it is not suitable for regular maintenance and replacement, which requires that photovoltaic cells have good durability. At present, the general warranty period in the industry is more than 20 years. Therefore, the shelf life of TCO glass must also reach more than 20 years.
3, Development prospect of TCO glass for photovoltaic
When the photovoltaic industry is developing at a rate of nearly 40% for five consecutive years, there is an unprecedented shortage of polysilicon, the upstream raw material of crystalline silicon solar cells, the mainstream product. The price of polysilicon has risen from $24 / kg in 2003 to the current spot price of more than $300 / kg. Compared with crystalline silicon solar cells, thin-film solar cells have the characteristics of strong weak light application and strong shape plasticity. The rise in the price of crystalline silicon has virtually promoted the development of thin-film solar cells. At present, thin-film solar cells account for more than 10% of the world photovoltaic market. With the rising price of polysilicon, the development of thin-film solar cells is expected to further accelerate.
In 2006, the statistical production of thin-film solar cells in the world was about 190 megawatts. In 2007, the production capacity of thin-film solar cells expanded rapidly, reaching 350 megawatts under the condition of insufficient production caused by the rising price of polysilicon raw materials. The thin-film batteries in China are mainly amorphous silicon thin-film batteries, with an output of about 29 MW in 2007.
Due to the different conversion efficiency of thin-film batteries, we take the mainstream amorphous silicon as the calculation basis, and the nominal power of the battery per square meter is 50 watts, that is, every 50 watts of battery will use 1 square meter of TCO glass. Based on this, it can be calculated that the demand for TCO glass in the photovoltaic industry in the world in 2007 was 7million square meters. This figure seems small, but due to the high-end production technology of TCO glass, there are only two major suppliers in the world at present. With the annual growth rate of more than 50% in the solar cell industry, products have begun to fall short of demand. The price per square meter has reached more than $20.
According to authoritative forecasts, thin-film solar cells will continue to maintain rapid growth. At present, many manufacturers and research institutions are in the process of industrialization of TCO glass, the output will continue to expand, and new high-performance products will continue to be introduced.
It can be seen that due to the rapid development of thin-film solar cells, the current TCO glass market for photovoltaic has become very tight. In the field of conductive glass, photovoltaic TCO glass has unique performance requirements, which are closely related to the output characteristics of solar cells. It can be said that the development of thin-film solar cells will depend on the improvement of photovoltaic TCO to a certain extent. The shortage of energy has promoted the development of photovoltaic industry, and will also promote photovoltaic TCO glass products to become a bright new star in the glass industry.
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