The solar cell is the smallest unit of photoelectric conversion, and its size generally ranges from 4 to 100 cm2. The working voltage of a single 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 battery cells are packaged in series and parallel, they become solar battery modules, and their power is generally a few watts to tens of watts, which is the smallest unit that can be used as a photovoltaic power source alone. The solar cell components are then installed on the bracket through a series-parallel combination to form a solar cell square array, which can meet the output power required by the solar LED lighting system load, as shown in Figure 1.
Commonly used solar cells are mainly silicon solar cells. The crystalline silicon solar cell consists of a crystalline silicon wafer. On the upper surface of the crystalline silicon wafer, metal grid lines are closely arranged, and the lower surface is a metal layer. The silicon wafer itself is P-type silicon, and the surface diffusion layer is an N region. The junction of these two regions is the so-called PN junction. The PN junction forms an electric field. The top of the solar cell is covered by a layer of anti-reflection film to reduce the reflection loss of solar energy.
Connect a load between the upper and lower surfaces of the solar cell. When sunlight shines on the solar cell, current will flow through the load. The more photons the solar cell absorbs, the greater the current generated. The energy of the photon is determined by the wavelength. Photons with energy lower than the base energy cannot produce free electrons. A photon with energy higher than the base energy produces only one free electron. The excess energy will cause the solar cell to generate heat, and the thermal energy loss will reduce the conversion efficiency of the solar cell.
Read more: Efficiency and loss of Photoelectric conversion of solar cells
There are currently three commercially available silicon solar cells in the world: monocrystalline silicon solar cells, polycrystalline silicon solar cells and amorphous silicon solar cells. For monocrystalline silicon solar cells, because the monocrystalline silicon materials used have the same quality as those used in the semiconductor industry, the production cost of monocrystalline silicon cells is relatively high. The crystal orientation of polycrystalline silicon solar cells is irregular, which means that the positive and negative charge pairs cannot be completely separated by the PN junction electric field. Because charge pairs may be lost due to crystal irregularities at the boundary between crystals, the efficiency of polycrystalline silicon solar cells is generally lower than that of monocrystalline silicon solar cells. Multi-product silicon solar cells are produced by casting, so its cost is lower than that of monocrystalline silicon solar cells. Amorphous silicon solar cells are thin-film batteries 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 commercialized monocrystalline silicon solar cells is 13%~15%, the photoelectric conversion efficiency of commercialized polycrystalline silicon solar cells is 11%~13%, and the photoelectric conversion efficiency of commercialized amorphous silicon solar cells is 5%~8%.
The solar cell module contains a certain number of solar cells, which are connected by wires. After the single battery is connected, it can be packaged. The structure of the previous components is mostly: the front is covered with glass with high light transmittance, the front and back of the solar cell are bonded with transparent silicone rubber, the back is supported by aluminum plate glass, and the surrounding is surrounded by aluminum or stainless steel as a frame. The positive and negative electrodes are drawn out to form the solar cell module. The quality of such solar cell modules is not easy to guarantee, and the packaging is labor-intensive. In recent years, most solar cell modules have adopted new structures. The front side is made of high-transmittance toughened glass, and the back side is a layer of polyethylene fluoride film. Both sides of the solar cell are hot-pressed with EVA or PVB glue. The sides are surrounded by lightweight aluminum frame, and the electrodes are led out by the junction box.
After the solar cell module is packaged, due to the influence of the cover glass and sealant on the light transmission and the performance mismatch between the individual cells, the efficiency of the module is generally 5% to 10% lower than that of the solar cell. If the thickness and refractive index of the glass glue are matched well, the efficiency will be improved after packaging.
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, humidity, temperature cycling, thermal shock, high temperature and high humidity aging, salt spray, low humidity aging, weather resistance, outdoor exposure, shock, vibration and other tests before being officially put into production. Some special tests are required if they are used in special occasions.
The general-purpose solar cell modules produced by the factory have generally been specially designed in consideration of factors such as the required charging voltage of the battery, blocking diodes and line voltage drops, and temperature changes. The standard number of single cells that constitute a solar cell module is 36(10cm×10cm), which means that a solar cell module can generate a voltage of about 17V, which can effectively charge a set of batteries with a rated voltage of 12V.
Solar cell modules should have certain anti-corrosion, wind-proof, hail-proof, and rain-proof capabilities. The main function of the alloy sheet on the back of the solar cell module is moisture-proof and anti-fouling. The solar cell is embedded in a layer of polymer. In this solar cell module, the solar cell and the junction box can be directly connected by wires. When the application field requires higher voltage and higher current and a single component cannot meet the requirements, multiple components can be formed into a solar cell square array to obtain the required voltage and current. The reliability of solar cells depends to a large extent on their resistance to corrosion, wind, hail, and rain. The potential quality problems are the sealing of the moving edge and the junction box on the back of the module.