Application of Solar Cell PL Detection and Photoluminescence

　　1、 The Concept and Application of Photoluminescence

　　Photoluminescence (PL) is a luminescent phenomenon in semiconductor materials. It refers to the process in which electrons in a material absorb external photons and are excited to achieve luminescence. Electrons in excited states are unstable and can transition to lower energy levels, releasing energy in the form of light.

　　The frequency, phase, amplitude, direction, polarization state, etc. of luminescence carry a large amount of basic information of materials, making it a very important method for detecting the electronic structure of materials. In addition, it has no contact with materials, does not damage them, and has high sensitivity and accurate information. It is widely used in various fields such as band gap detection of materials, impurity level and defect analysis, research on material composite mechanisms, and material quality identification.

　　2、 The application of PL detection in the photovoltaic industry is an inevitable trend

　　In recent years, the photovoltaic industry has developed rapidly, especially in Chinese Mainland, where the investment in solar cells is extremely huge. There are thousands of photovoltaic enterprises with obvious overcapacity. In addition, the economic crisis in Europe and the United States has also greatly affected the sales and development of enterprises. Merger and elimination are the inevitable trend of this industry. Therefore, in order to remain invincible during this critical period, improving product efficiency and reducing production costs has become the goal of the entire industry.

　　In the production process of solar cells, there are many serious problems, such as impurities, fragments, cracks, black core surface pollution, uneven diffusion, poor electrodes, etc. It is these defects that limit the photoelectric conversion efficiency and service life of the battery. Meanwhile, due to the lack of comprehensive industry standards, the quality of raw materials for silicon wafers is also uneven, and the presence of some defective wafers directly affects the stability of modules and even photovoltaic systems. Therefore, the solar energy industry needs fast, effective, and accurate positioning and inspection methods to detect potential problems in the production process.

　　Based on the above domestic and international market conditions and the demands of production lines, the application of battery panel PL testing in the photovoltaic industry is crucial, which is determined by the high detection speed of PL testing for the entire production line. PL detection can detect and analyze various processes such as silicon wafers, textured surfaces, diffusion, etching, PECVD, electrode printing, sintering, and battery cells online and offline; It is an essential key equipment for analyzing process issues, improving yield, and reducing costs, providing complete and reliable guarantees for product quality and company quality.

　　3、 Basic principles of photovoltaic cell PL detection

　　PL photoluminescence provides an irreplaceable solution for defect detection in photovoltaic cells. Since it is so important, it is necessary to understand its principles.

　　Usually, a specific wavelength of laser is used as the excitation light source to provide photons with a certain amount of energy. The electrons in the ground state of the battery absorb these photons and enter the excited state. The electrons in the excited state belong to metastable state, and will return to the ground state in a short time, emitting fluorescence with a peak of about 1150 nm (as in the case of silicon wafer batteries) infrared light. Use a highly sensitive and high-resolution camera for photosensitivity, and then analyze the image through software.

　　The intensity of luminescence is proportional to the concentration of non equilibrium minority carriers at this location, and defects will be the strong recombination centers of minority carriers. Therefore, a decrease in the concentration of minority carriers in this area will weaken the fluorescence effect, appearing as dark dots, lines, or certain areas on the image, while areas with less recombination in the sample will appear as brighter areas. Therefore, by observing photoluminescence imaging, it is possible to determine whether the sample has defects, impurities, and other factors that ultimately affect battery efficiency.

　　4、 Analysis results of photovoltaic cell PL tester

　　1. EL detection analysis

　　The EL module in the PL photoluminescence system can clearly detect various defects in the battery cell, including cracks, and can completely replace specialized EL detection equipment.

　　2. Complete online detection and analysis.

　　The PL photoluminescence detection system can detect various processes in the battery production process and accurately analyze problems. It is a unique system that can be detected as a whole, and can truly achieve online detection. The detection speed is very fast, and the detection time for each piece is less than 1 second. Through PL system analysis, problems with diffusion can be accurately identified, such as uneven diffusion thickness and damaged rollers; The subsequent process and even the final battery cell results indicate that the problem of uneven thickness was optimized in the subsequent stages of the crystal, and ultimately did not significantly reduce the efficiency of the battery cell. However, the damage to the roller affects all subsequent stages and ultimately affects the efficiency of the battery cell.

　　3. Analysis of minority carrier lifetime

　　The PL system can analyze the minority carrier lifetime of the entire surface and specified internal positions, and analyze the reasons for different lifetimes.

　　4. Comparison of PL system detection results with other devices

　　The PL detector of Yaohua Laser can not only accurately calculate minority carriers, but also analyze the reasons for the decrease in minority carrier lifetime through various results. This is something that other devices cannot achieve.

　　5、 Summary

　　The PL detector of Yaohua Laser can be applied to almost all solar cells used, such as monocrystalline silicon wafers and batteries, polycrystalline silicon wafers and batteries, thin film batteries, compound batteries, etc. Capable of detecting a complete range of parameters such as minority carrier lifetime, series resistance, silicon wafer classification, and detecting various defects. The detection speed is fast, the time is less than 1 second, and the accuracy is high. It can find every defect and cause that leads to efficiency reduction in real-time, truly achieving accurate and fast online detection. It is an essential testing equipment to reduce production costs, improve battery quality, and revolutionize product competitiveness.