The solar cell is the smallest unit of photoelectric conversion, and its size generally ranges from 4 to 100 cm². The working voltage of the solar cell is about 0.5V, and the working current is 20~25mA/cm². Generally, it cannot be used as a photovoltaic power source alone. After the solar cells are packaged in series and parallel, they become solar cell modules. The power is generally several watts to several tens of watts, which is the smallest unit that can be used alone as a photovoltaic power source. The solar cell modules are then assembled on the support in series and parallel to form a solar cell array, as shown in Figure 1.
The commonly used solar cells are mainly silicon solar cells. The silicon solar cell consists of a crystalline silicon wafer. Metal grid lines are closely arranged on the upper surface of the crystalline silicon wafer, and the lower surface is a metal layer. The crystalline silicon layer is the P region, the surface diffusion layer is the N region, and the junction between these two regions is the so-called PN junction. The top of the solar cell is covered with an anti-reflection film to reduce the reflection loss of solar energy.
When a load is connected between the upper and lower surfaces of a solar cell, current flows through the load, and the solar cell generates current; the more photons absorbed by the solar cell, the greater the current generated. The energy of the photon is determined by the wavelength. Photons with energy lower than the base energy cannot generate free electrons. A photon with energy higher than the base energy will only generate a free electron. The excess energy will make the solar cell heat, and the loss of heat energy will make the solar energy The conversion efficiency of the battery decreases.
There are currently three commercialized silicon solar cells in the world: monocrystalline silicon solar cells, polycrystalline silicon solar cells and amorphous silicon solar cells. For single crystal silicon solar cells, the cost of single crystal silicon solar cells is relatively expensive because the single crystal silicon materials used have the same quality as those used in the semiconductor industry. The crystal orientation of polycrystalline silicon solar cells is irregular, which means that the positive and negative charge pairs cannot all be separated by the PN junction electric field, because the charge pairs may be lost on the boundary between crystals due to crystal irregularities, so The efficiency of polycrystalline silicon solar cells is generally lower than that of monocrystalline silicon solar cells. Polycrystalline silicon solar cells are produced by casting, so their cost is lower than monocrystalline silicon solar cells. Amorphous silicon solar cells are thin-film cells with low cost, but the photoelectric conversion efficiency is relatively low, and the stability is not as good as that of crystalline silicon solar cells. The photoelectric conversion efficiency of general commercial monocrystalline silicon solar cells is 13%~15%, the photoelectric conversion efficiency of commercial polycrystalline silicon solar cells is 11%~13%, and the photoelectric conversion efficiency of commercial amorphous silicon solar cells is 5%~ 8%.
A solar cell module contains a certain number of solar cells, which are connected by wires. After the single cell is connected, it can be packaged. In the past, the structure of solar cell modules was mostly covered with glass with high light transmittance on the front. The front and back of the solar cell were bonded with transparent silicone rubber, and the back was supported by aluminum plate glass. , surrounded by aluminum or stainless steel as a frame, and the positive and negative electrodes are drawn to form a solar cell module. The quality of the solar cell module is not easy to guarantee, and the packaging is labor-intensive. In recent years, most of the solar cell modules in various countries have adopted a new structure; the front side is made of tempered glass with high light transmittance, the back side is a layer of polyethylene fluoride film, the two sides of the solar cell are sealed with EVA or PVB glue by hot pressing, and the surrounding is light weight. Aluminum profile frame, the electrodes are drawn from the junction box.
After the solar cell module is encapsulated, due to the influence of the cover glass and sealant on the light transmission and the performance mismatch between the individual cells, the module efficiency is generally 5%~10% lower than that of the solar cell. The thickness of the glass glue is better matched with the refractive index, etc., and the efficiency is improved after encapsulation.
Solar cell modules are often exposed to sunlight and are directly affected by the local natural environment, which includes environmental meteorological factors and mechanical factors. In order to ensure the reliability of use, the solar cell modules produced by the factory generally undergo a series of performance and environmental tests before they are officially put into production. , Outdoor exposure, shock, vibration and other tests, such as application in special occasions, some special tests should be carried out.
The general-purpose solar cell modules produced in the factory have generally been specially designed taking into account the charging voltage required by the battery, blocking diodes and line voltage drops, and temperature changes. The standard number of solar cells on a solar cell module is 36. (10cm x 10cm), which means that a solar cell module can generate about 17V, just enough to effectively charge a set of batteries rated at 12V.
Multiple solar cells are connected by wires. The sealed solar cell module has certain anti-corrosion, wind-proof, hail-proof, rain-proof and other capabilities. The main function of the alloy sheet on the back of the solar cell module is moisture-proof and anti-fouling. The solar cells are embedded in a layer of polymer. In this solar cell module, the solar cell and the junction box can be directly connected with wires. When the application field requires higher voltage and current and a single component cannot meet the requirements, multiple components can be formed into a solar cell array to obtain the required voltage and current. The reliability of solar cells depends to a large extent on its ability to prevent corrosion, wind, hail, and rain. The potential quality problems are the sealing of the edge and the junction box on the back of the module.