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Principle Of Power Generation Of Solar Cells
- Jul 31, 2018 -

Photovoltaic power generation uses solar cells to convert solar energy directly into electrical energy based on the principle of photovoltaic volts. Whether it is independent use or grid-connected power generation, photovoltaic power generation system is mainly composed of solar panels (components), controllers and inverter photovoltaic power generation. Photovoltaic power uses the photovoltaic effect of the semiconductor interface to convert light energy directly into electrical energy. The key component of technology is solar cells. The solar cells are packaged and protected in series to form a large-area solar cell module, which is combined with a power controller to form a photovoltaic power generation device.

Solar cells: Solar cells are mainly based on semiconductor materials. Their working principle is to use photoelectric materials to absorb light energy and then generate photoelectric conversion reactions. Depending on the materials used, solar cells can be divided into:

1. Silicon solar cells;

2. A battery made of a multi-component compound such as an inorganic salt such as gallium arsenide III-V compound, cadmium sulfide or copper indium selenide;

3. A solar cell prepared by functional polymer materials;

4. Nanocrystalline solar cells, etc.

Silicon solar cell

1. Silicon solar cell working principle and structure

The principle of solar cell power generation is mainly the photoelectric effect of semiconductor. The main structure of general semiconductor is as follows:

图片1.png 

In the figure, a positive charge represents a silicon atom, and a negative charge represents four electrons surrounding a silicon atom. When other impurities such as boron, phosphorus, etc. are incorporated into the silicon crystal, when boron is doped, a hole exists in the silicon crystal, and its formation can be referred to the following figure:

图片2.png 

In the figure, a positive charge represents a silicon atom, and a negative charge represents four electrons surrounding a silicon atom. The yellow color indicates the boron atom incorporated. Since there are only three electrons around the boron atom, the blue hole shown in the figure is generated. This hole becomes unstable due to the absence of electrons, and it is easy to absorb electrons. Neutralization forms a P(posiTIve) type semiconductor. Similarly, after the phosphorus atom is incorporated, since the phosphorus atom has five electrons, an electron becomes very active to form an N (negaTIve) type semiconductor. Yellow is the phosphorus nucleus, and red is the extra electron. As shown below.

图片3.png 

The N-type semiconductor contains a large number of holes, and the P-type semiconductor contains a large amount of electrons, so that when the P-type and N-type semiconductors are combined, a potential difference is formed at the contact surface, which is a PN junction.

When the P-type and N-type semiconductors are combined, a special thin layer is formed in the interface region between the two semiconductors, and the P-type side of the interface is negatively charged, and the N-type side is positively charged. This is because the P-type semiconductor has multiple holes, and the N-type semiconductor has many free electrons, and a concentration difference occurs. The electrons in the N region diffuse into the P region, and the holes in the P region diffuse into the N region. Once diffused, an "internal electric field" from N to P is formed, thereby preventing diffusion. After reaching equilibrium, such a special thin layer formation potential difference is formed, which is the PN junction.

图片4.png 

When the wafer is exposed to light, in the PN junction, the holes of the N-type semiconductor move toward the P-type region, and the electrons in the P-type region move toward the N-type region, thereby forming a current from the N-type region to the P-type region. A potential difference is then formed in the PN junction, which forms a power supply. (As shown below)

图片5.png 

Since the semiconductor is not a good conductor of electricity, if the electron flows in the semiconductor after passing through the p-n junction, the resistance is very large and the loss is very large. However, if all the metal is applied to the upper layer, the sunlight cannot pass, and the current cannot be generated. Therefore, the p-n junction (as shown in the comb electrode) is generally covered with a metal mesh to increase the area of the incident light. In addition, the surface of the silicon is very bright, reflecting a lot of sunlight and cannot be used by the battery. To this end, the scientists applied a protective film with a very small reflection coefficient (pictured) to reduce the reflection loss to 5% or less. The current and voltage that a battery can provide are limited, so many batteries (usually 36) are used in parallel or in series to form a solar photovoltaic panel.

2. Production Process of Silicon Solar Cell

 A typical crystalline silicon solar cell is fabricated on a high quality silicon wafer having a thickness of 350 to 450 μm, which is sawed from a lifted or cast silicon ingot.

图片6.png 

The above method actually consumes more silicon material. In order to save materials, polycrystalline silicon thin film batteries are currently used in chemical vapor deposition, including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) processes. In addition, liquid phase epitaxy (LPPE) and sputter deposition methods can also be used to prepare polycrystalline silicon thin film batteries. The chemical vapor deposition is mainly carried out by using SiH2Cl2, SiHCl3, SiCl4 or SiH4 as a reaction gas to form silicon atoms under a certain protective atmosphere and deposited on a heated substrate. The substrate material is generally selected from Si, SiO2, Si3N4 and the like. However, it has been found that it is difficult to form large crystal grains on a non-silicon substrate, and it is easy to form voids between the crystal grains. To solve this problem, a thin layer of amorphous silicon is deposited on the substrate by LPCVD, and then the amorphous silicon layer is annealed to obtain larger crystal grains, which are then deposited on the seed crystal. Thick polycrystalline silicon film, therefore, recrystallization technology is undoubtedly a very important link, the current technology mainly includes solid phase crystallization and medium melting recrystallization. In addition to the recrystallization process, the polycrystalline silicon thin film battery employs almost all technologies for preparing single crystal silicon solar cells, and the solar cell conversion efficiency thus obtained is remarkably improved. Third, nanocrystalline chemical solar cells Silicon solar cells in solar cells are undoubtedly the most mature development, but because of the high cost, far from meeting the requirements of large-scale promotion and application. To this end, people continue to explore processes, new materials, thin film and other aspects, and the newly developed nano TIO2 crystal chemical solar cells are valued by scientists at home and abroad. For example, dye-sensitized nanocrystalline solar cells (DSSCs) mainly include a glass substrate coated with a transparent conductive film, a dye-sensitized semiconductor material, a counter electrode, and an electrolyte.

图片7.png 

Anode: dye-sensitized semiconductor film (TIO2 film)

Cathode: Platinum-plated conductive glass

Electrolyte: I3-/I- As shown, the white spheres represent TiO 2 and the red spheres represent dye molecules. The dye molecules absorb the solar energy energy to transition to the excited state, the excited state is unstable, and the electrons are rapidly injected into the adjacent TiO 2 conduction band. The electrons lost in the dye are quickly compensated from the electrolyte, and the electricity entering the TiO 2 conduction band is Eventually it enters the conductive film and then produces a photocurrent through the outer loop.

图片8.png 

The advantages of nanocrystalline TiO 2 solar cells are their low cost and simple process and stable performance. Its photoelectric efficiency is stable at more than 10%, and the production cost is only 1/5 to 1/10 of the silicon solar cell. The lifetime can reach more than 20 years. However, due to the research and development of such batteries, it is estimated that they will gradually enter the market in the near future.


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