In-depth explanation of crystalline silicon solar cell array

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 about 20 ~ 25 mA/cm2. Generally, it cannot be used as a photovoltaic power source alone. After the solar cells are encapsulated in series and in parallel, a solar cell module is formed, and its power is generally several watts to tens of watts, which is the smallest unit that can be used alone as a photovoltaic power source. The solar cell modules are then installed on the bracket in series and parallel combination to form a solar cell array, which can meet the output power required by the load of the solar photovoltaic power generation system, as shown in Figure 1.

Figure 1 - Solar cells, modules and arrays
Figure 1 – Solar cells, modules and arrays

The commonly used solar cells are mainly crystalline silicon solar cells. The crystalline silicon solar cell consists of a crystalline silicon wafer, the upper surface of the crystalline silicon wafer is closely arranged with metal grid lines, and the lower surface is a metal layer. 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 the solar cell, a current will flow through the load, and the solar cell will generate current; the more photons absorbed by the solar cell, the greater the current will be generated. The energy of the photon is determined by the wavelength. Photons with energy lower than the base energy cannot generate free electrons, and a photon with energy higher than the base energy only generates a free electron. Conversion efficiency drops.

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, since the single-crystal silicon material used has the same quality as that used in the semiconductor industry, the use of single-crystal silicon is relatively expensive. 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 photoelectric conversion efficiency of polycrystalline silicon solar cells is generally lower than that of monocrystalline silicon solar cells. I was fortunate to read an article about battery performance and conversion efficiency before. If you are also interested, please click here to open.

Because polycrystalline silicon solar cells are produced by casting, 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 cells are connected, they can be packaged. In the past, the structure of solar cell modules was mostly: the front was covered with glass with high light transmittance, the front and back of the solar cell were bonded with transparent silicon rubber, the back was backed by aluminum plate glass, and the surrounding area was made of aluminum or stainless steel as a frame , lead out the positive and negative poles to form the component. 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 in China and abroad have adopted a new structure: tempered glass with high light transmittance on the front, a polyethylene vapor film on the back, and EVA or PVB glue on both sides of the solar cell. Lightweight aluminum profile frame with electrodes drawn from the junction box.

After the solar cell module is encapsulated, the photoelectric conversion efficiency of the module is generally 5%~10% lower than that of the solar cell due to the influence of the cover glass and sealant on the light transmission and the performance mismatch between the individual cells. . However, if the thickness and refractive index of the glass glue are better matched, the efficiency will be improved after encapsulation.

Solar cell modules are often exposed to sunlight and are directly affected by the local natural environment, which includes meteorological and mechanical factors of the environment. 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 (such as humidity, temperature cycle, thermal shock, high temperature and high humidity aging, salt spray, low humidity aging, weather resistance, etc.) before they are officially put into production. If it is used in special occasions, some special tests should be carried out.

The physical unit that connects and seals multiple solar cells through wires is called a solar cell module. It 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 It 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.

Factory-produced general-purpose solar modules have generally been specially designed to take into account the charging voltage required by the battery, blocking diode and line voltage drops, and temperature variations. On a solar cell module, the standard number of solar cells is 36 pieces (10cm × 10 cm), which means that a solar cell module can generate about 17V, just enough to effectively charge a set of batteries with a rated voltage of 12V .

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 resist corrosion, wind, hail, rain, etc. The potential quality problems are the sealing degree of the edge and the performance of the junction box on the back of the module.

Read more: Prospects for the development of international solar photovoltaic power generation technology