Saturday, May 7, 2011

【 Weak current College 】 solar principle】

Solar energy is virtually inexhaustible, renewable energy. Is clean energy, produces no pollution of the environment. In the effective use of solar energy; solar energy PV utilization is the fastest developing in recent years, the most dynamic areas of research, is one of the most watched one of the project.

Making solar cells primarily for the semiconductor material as a basis for its working principle is use of photoelectric materials absorbed light photo for conversion occurred after the response, depending on the material, the solar cells can be divided into: silicon-based solar cells and thin film cell, where the main stresses of silicon-based solar cells.

I. silicon solar cells

1. silicon solar cells working principle and structure

Solar power generation principle mainly semiconductor PV, General Semiconductor main structure is as follows:

Figure, positive charge that silicon atoms, negative charge represents around the silicon atom next to four electrons.

When the silicon crystals doped with other impurities such as boron, phosphorus, etc, when mixed with boron, silicon crystals will exist in a hole, its formation could refer to the following figure:

Figure, positive charge that silicon atoms, negative charge represents around the silicon atom next to four electrons. And yellow indicates mixed boron atoms, because boron atoms only three electronic around, so you'll have to figure in the blue hole, the hole because no electronic and become very unstable, easy to absorb, in electronic form, and P-type semiconductor (positive).

Similarly, mixed with phosphorus Atom, since phosphorus atoms have five electrons, so there will be an electronic became very active, the formation of N-type semiconductor (negative). Yellow to red phosphorus nuclei, as extra electrons. The following figure.

P-type semiconductor has more holes, and N-type semiconductor contains more e-mail, so that when P type andN type semiconductors together, in the contacts form a potential difference, this is a pn junction.
When a P-type and N type semiconductors together, the two semiconductor interface areas will form a special thin layer), the interface of P-type side of negatively charged, N-type side with the positive electricity. This is because many p-type semiconductor N-type semiconductors holes, more free-electron density difference occurred. N district of electronics spreads to P, P zone in hole will spread to N district, once a diffusion are formed by the N point to the "P" in the field, thus preventing proliferation. Achieve balance, it formed a special thin layer formation potential difference, this is a pn junction.

When the chip is light, PN, N-type semiconductor P-type of hole to move, and the District of P-type region of e-mail to N-type region moving, forming from the N-type region into the current P-type region. And then, in pn junction formation potential difference in, which formed the power supply. (As shown in the figure below)

Because Semiconductor is not a good conductor of electricity, electronics, after the adoption of the p-n junction diode if in flow, resistance is very large, the loss is very large. But if all the coloured metal top, the Sun is not passed, the current will not be able to produce, and therefore generally used metal grid overlay p-n junction (shown in Figure comb electrode), to increase the area of incident light.

In addition very bright on Silicon surfaces, reflection off a lot of sunlight, cannot be the battery use. To do this, the scientists to it on a layer of reflection coefficient of very small screen protector (shown in figure), the actual industrial production base with CVD deposits a layer of silicon nitride film, the thickness of around 1000 El. To reduce the reflection loss to 5 per cent or even smaller. A battery can deliver current and voltage limited, so people will be a lot of battery (usually 36) touse parallel or series, the formation of solar panels.

2. silicon solar cell manufacturing process

The usual crystal silicon solar cells is in thickness 350 ~ 450 μ m high quality silicon chip, the silicon wafers from lifting or casting of Silicon ingot saw mowing.

The above method of actual consumption of silicon materials and more. In order to save material, polycrystalline silicon thin film battery use chemical vapor deposition, including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) technology. In addition, the liquid phase Epitaxial method (LPPE) and sputtering method can also be used in polycrystalline silicon thin film batteries.

Chemical vapor deposition mainly to SiH2Cl2, SiHCl3, SiH4, as SiCl4 or reaction gases, in protective atmosphere generated by reaction and deposition of silicon atoms in the heating of the substrate, the substrate material for general use Si, SiO2, Si3N4, etc. But the study found that in non-silicon substrate is very difficult to form a larger grain and easily between the grain. To solve this problem is to use LPCVD deposition in the substrate layer of relatively thin amorphous silicon layer, and then will this layer of amorphous silicon layer annealing, get a larger grain size, and then in the layer seed on thick poly-SI films, therefore, recrystallization technology is a very important aspect of the current technology mainly solid phase crystallization and melting and recrystallization in the area. Polycrystalline silicon thin film batteries in addition to using recrystallization process, also used almost all the preparation technology of monocrystalline silicon solar cells, which are of solar cell conversion efficiency has improved significantly.

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