How do Solar Cells Work?

Catalog

Ⅰ How do solar cells work?

Solar cells, also known as "solar chips" or "photocells", are photoelectric semiconductor sheets that use sunlight to directly generate electricity. As long as it is exposed to light that meets certain illuminance, it can instantly output voltage and generate current when there is a loop. In physics, it is called photovoltaic or PV for short. Solar cells are devices that directly convert light energy into electrical energy through the photoelectric effect or photochemical effect. Crystalline silicon solar cells that work with the photovoltaic effect are the mainstream, while thin-film cells that work with the photochemical effect are still in their infancy.

Sunlight shines on the semiconductor p-n junction to form a new hole-electron pair. Under the action of the built-in electric field in the p-n junction, the light-generated holes flow to the p-region, and the photo-generated electrons flow to the n-region, and a current is generated after the circuit is turned on. This is the working principle of photovoltaic effect solar cells.

solar cell

There are two methods of solar power generation, one is the light-heat-electric conversion method, and the other is the light-electric direct conversion method.

1 Light-heat-electric conversion

The light-heat-electric conversion method uses the heat generated by solar radiation to generate electricity. Generally, the solar heat collector converts the absorbed heat into the vapor of the working fluid and then drives the steam turbine to generate electricity. The former process is a light-heat conversion process; the latter process is a heat-electric conversion process, which is the same as ordinary thermal power generation. The disadvantage of solar thermal power generation is its low efficiency and high cost. It is estimated that its investment is at least 5-10 times more expensive than ordinary thermal power plants. A 1000MW solar thermal power station requires an investment of 2 to 2.5 billion US dollars, and the average investment of 1 kW is 2000 to 2500 US dollars. Therefore, it can only be applied to special occasions on a small scale, and large-scale utilization is not cost-effective, and it cannot compete with ordinary thermal or nuclear power plants.

2 Light-to-electricity direct conversion

Solar cells generate electricity based on the photoelectric properties of specific materials. Black bodies (such as the sun) radiate electromagnetic waves of different wavelengths (corresponding to different frequencies), such as infrared, ultraviolet, visible light, and so on. When these rays are irradiated on different conductors or semiconductors, electric current will be generated by photons that interact with free electrons in the conductors or semiconductors. The shorter the wavelength and the higher the frequency of the rays, the higher the energy they have. For example, ultraviolet rays have much higher energy than infrared rays. However, not all wavelengths of radiation energy can be converted into electrical energy. It is worth noting that the photovoltaic effect has nothing to do with the intensity of the radiation, and current can only be generated when the frequency reaches or exceeds the threshold that can produce the photovoltaic effect. The maximum wavelength of light that can cause a semiconductor to produce a photovoltaic effect is related to the forbidden bandwidth of the semiconductor. For example, the forbidden bandwidth of crystalline silicon is about 1.155 eV at room temperature. Therefore, light with a wavelength of less than 1100 nm can make crystalline silicon produce a photovoltaic effect.

Solar cell power generation is a renewable and environmentally friendly power generation method. During the power generation process, carbon dioxide and other greenhouse gases will not be generated, and will not cause environmental pollution. According to the production materials, it is divided into silicon-based semiconductor cells, CdTe thin-film cells, CIGS thin-film cells, dye-sensitized thin-film cells, organic materials cells, etc. Among them, silicon cells are further divided into single-crystal cells, polycrystalline cells, and amorphous silicon thin-film cells. The most important parameter for solar cells is conversion efficiency. Among the silicon-based solar cells developed in the laboratory, the efficiency of monocrystalline silicon cells is 25.0%, the efficiency of polycrystalline silicon cells is 20.4%, the efficiency of CIGS thin-film cells is 19.6%, and the efficiency of CdTe thin-film cells is 16.7%, and that of amorphous silicon thin-film cells is 10.1%

A solar cell is a photoelectric element that can convert energy. Its basic structure is made by joining P-type and N-type semiconductors. The most basic material of semiconductors is "silicon", which is non-conductive. But if different impurities are added to the semiconductor, it can be made into P-type and N-type semiconductors. The P-type semiconductor has a hole (P-type The semiconductor is missing a negatively charged electron, which can be regarded as an additional positive charge), and an N-type semiconductor has an additional potential difference of free electrons to generate current. Therefore, when sunlight is irradiated, the electrons in the silicon atom are excited to produce the convection of electrons and holes. These electrons and holes are affected by the built-in potential and are attracted by the N-type and P-type semiconductors, and gather at both ends. If the external electrodes are connected, a loop is formed. This is the principle of solar cell power generation.

To put it simply, the principle of photovoltaic power generation is to use solar cells to absorb sunlight with a wavelength of 0.4 μm to 1.1 μm (for silicon crystals) and directly convert light energy into electrical energy output.

Ⅱ Solar cell components

The composition of solar cell modules and the functions of each part:

1) Tempered glass is used to protect the main body of power generation (such as cells), and the selection of light transmission is required: 1. The light transmission must be high (generally above 91%); 2. Ultra-white tempered treatment.

2) EVA is used to bond and fix the tempered glass and the main body of power generation (such as a battery). The quality of the transparent EVA material directly affects the life of the module. The EVA exposed to the air is easy to age, thus affecting the light transmittance of the module. In addition to the quality of the EVA itself, the lamination process of the module manufacturer is also very important. For example, the EVA glue connection is not up to the standard, and the bonding strength of the EVA and the tempered glass and the backplane is not enough, which will cause the EVA to be early aging.

3) The main function of the cell is to generate electricity. The mainstream in the main power generation market is the crystalline silicon solar cell and the thin-film solar cell. Both have their own advantages and disadvantages. Crystal silicon solar cells have relatively low equipment costs and high photoelectric conversion efficiency. They are more suitable for power generation under outdoor sunlight, but the consumption and cell costs are high; thin-film solar cells have low consumption and cell costs and have very low light effects. Well, it can also generate electricity under ordinary light, but the relative equipment cost is relatively high. The photoelectric conversion efficiency is more than half of that of crystalline silicon cells, such as solar cells on calculators.

4) Backplane: its functions are sealing, insulation, and waterproof (usually TPT, TPE, and other materials must be resistant to aging. Most component manufacturers have a 25-year warranty. Tempered glass and aluminum alloy are generally okay. The key lies in the backplane and silica gel)

5) Aluminum alloy protective laminates play a certain role in sealing and supporting.

6) The junction box protects the entire power generation system and acts as a current transfer station. If the component is short-circuited, the junction box automatically disconnects the short-circuit battery string to prevent burnout of the entire system connection. The most important thing in the junction box is the selection of diodes. The type of cell is different, the corresponding diode is also different.

7) Silicone sealing: it is used to seal the junction between the component and the aluminum alloy frame, and the component and the junction box. Some companies use double-sided tape and foam instead of silica gel. The process is simple, convenient, easy to operate, and the cost is very low.

Ⅲ Basic characteristics

The solar cells have three basic characteristics: the polarity of solar cells, the performance parameters of solar cells, and the volt-ampere characteristics of solar electric environmental protection cells. The specific explanation is as follows:

1. Polarity of solar cells

Silicon solar cells are generally made of P+/N type structure or N+/P type structure. P+ and N+ indicate the conductivity type of the semiconductor material on the front side of the solar cell; N and P indicate the conductivity type of the semiconductor material on the back of the solar cell. The electrical properties of solar cells are related to the characteristics of the semiconductor materials used to make the cells.

2. Parameters of solar cells

The performance parameters of solar cells consist of open-circuit voltage, short circuit current, maximum output power, fill factor, conversion efficiency, etc. These parameters are a measure of the performance of solar cells.

3. The volt-ampere characteristics of solar cells

The P-N junction solar cell includes a shallow P-N junction formed on the surface, a bar-shaped and finger-shaped front ohmic contact, a back ohmic contact covering the entire back surface, and an anti-reflection layer on the front. When the battery is exposed to the solar spectrum, photons with energy less than the bandgap Eg have no contribution to the battery output. Photons with energy greater than the bandgap Eg will contribute energy Eg to the battery output, and energy less than Eg will be consumed in the form of heat. Therefore, in the design and manufacturing process of solar cells, the influence of this part of heat on battery stability and life span must be considered.

Ⅳ Classification 

According to the crystalline state, solar cells can be divided into two categories: crystalline thin-film type and amorphous thin-film type (hereinafter denoted as a-), and the former are divided into single-crystalline and polycrystalline.

According to the material, it can be divided into silicon film shape, compound semiconductor film shape, and organic film shape. The compound semiconductor film shape is divided into amorphous (a-Si:H, a-Si:H:F, a-SixGel-x: H, etc.), IIIV group (GaAs, InP, etc.), IIVI group (Cds series), and zinc phosphide (Zn 3 p 2 ), etc.

According to different materials, solar cells can be divided into silicon solar cells, multi-compound thin-film solar cells, polymer multilayer modified electrode solar cells, nanocrystalline solar cells, organic solar cells, plastic solar cells, among which silicon solar cells are the most mature in development and occupy a dominant position in applications.

1 Silicon solar 

International Space Station Solar Panel

Silicon solar cells are divided into monocrystalline silicon solar cells, polycrystalline silicon thin-film solar cells, and amorphous silicon thin-film solar cells. Monocrystalline silicon solar cells have the highest conversion efficiency and the most mature technology. The highest conversion efficiency in the laboratory is 24.7%, and the efficiency during mass production is 15% (as of 2011, 18%). It still occupies a dominant position in large-scale applications and industrial production. However, due to the high cost of monocrystalline silicon, it is difficult to greatly reduce its cost. In order to save silicon materials, polycrystalline silicon thin films and amorphous silicon thin films have been developed as the alternative products of monocrystalline silicon solar cells.

Compared with monocrystalline silicon, polycrystalline silicon thin-film solar cells have a lower cost and higher efficiency than amorphous silicon thin-film cells. The maximum conversion efficiency of the laboratory is 18%, and the conversion efficiency of industrial-scale production is 10% (as of 2011, 17%). Therefore, polycrystalline silicon thin film batteries will soon dominate the solar cell market.

Amorphous silicon thin-film solar cells have low cost, lightweight, high conversion efficiency, and are convenient for mass production. They have great potential. However, due to the photoelectric efficiency decay effect caused by its materials, the stability is not high, which directly affects its practical application. If it can further solve the stability problem and improve the conversion rate, then amorphous silicon solar cells are undoubtedly one of the main development products of solar cells.

2 Polycrystalline film 

Polycrystalline silicon solar cell

The efficiency of polycrystalline thin-film batteries of cadmium sulfide and cadmium telluride polycrystalline thin-film cells is higher than that of amorphous silicon thin-film solar cells, and the cost is lower than that of monocrystalline silicon cells. It is also easy to mass-produce. However, because cadmium is highly toxic, it causes serious pollution. 

The conversion efficiency of the gallium arsenide (GaAs) III-V compound battery can reach 28%. The GaAs compound material has a very ideal optical band gap and high absorption efficiency, strong anti-radiation ability, is insensitive to heat, and is suitable for manufacturing high-efficiency single-junction batteries. However, the high price of GaAs materials limits the popularity of GaAs batteries to a large extent.

Copper-indium-selenium thin-film batteries (CIS for short) are suitable for photoelectric conversion without the problem of light-induced degradation, and the conversion efficiency is the same as that of polysilicon. With the advantages of low price, good performance, and a simple process, it will become an important direction for the development of solar cells in the future. The only problem is the source of the material. Since indium and selenium are relatively rare elements, the development of this type of battery is bound to be restricted.

3 Organic polymer

Replacing inorganic materials with organic polymers is a research direction that has just begun in solar cell manufacturing. Because organic materials are flexible, easy to manufacture, widely sourced, and low cost, they are of great significance for large-scale use of solar energy and low-cost electricity. However, the research on the preparation of solar cells with organic materials has only just begun, and neither the service life nor battery efficiency can be compared with inorganic materials, especially silicon cells. Whether it can be developed into a product of practical significance remains to be further studied and explored.

4 Nanocrystalline

Nanocrystalline chemical energy solar cells are newly developed, and their advantages lie in their low cost, simple process, and stable performance. The photoelectric efficiency is stable above 10%, and the production cost is only 1/5 to 1/10 of that of silicon solar cells. The life span can reach more than 20 years.

The research and development of this type of battery have just started and will gradually enter the market in the near future.

5 Organic film

Organic thin-film solar cells are solar cells whose core part is composed of organic materials. If you are not familiar with organic solar cells, it is reasonable. Today, more than 95% of mass-produced solar cells are silicon-based, while the remaining less than 5% are also made of other inorganic materials.

6 Dye sensitization

Dye-sensitized solar cells attach a pigment to TiO2 particles and then soak them in an electrolyte. The pigment is irradiated by light to generate free electrons and holes. Free electrons are absorbed by TiO2, flow out from the electrode, enter the external circuit, pass through the electrical appliances, flow into the electrolyte, and finally return to the pigment. The manufacturing cost of dye-sensitized solar cells is very low, which makes it highly competitive. Its energy conversion efficiency is about 12%.

7 Plastic battery

Plastic solar cells use recyclable plastic films as raw materials and can be mass-produced through "roll-to-roll printing" technology, which is low-cost and environmentally friendly. However, plastic solar cells are still immature. It is expected that in the next 5 to 10 years, solar cell manufacturing technologies based on plastics and other organic materials will mature and be put into use on a large scale.