English
Oxygen is one of the main unavoidable impurities in Czochralski single crystal silicon. Most oxygen atoms exist in silicon in an interstitial state, forming Si—O—Si bonds. Currently, the interstitial oxygen detected through infrared absorption spectroscopy is generally referred to as interstitial oxygen. Due to the different solubility and diffusion coefficients of oxygen in crystal silicon at different temperatures, during the cooling process after crystal growth and in the silicon device processing, oxygen precipitates in suitable forms through different heat treatments, resulting in oxygen donor phenomena and oxygen precipitation, which have a harmful impact on the quality of silicon crystals.
The adjustment of the TOPCon battery SE technology process will intensify the conversion of interstitial oxygen to form more precipitates, leading to the appearance of oxygen precipitation rings, i.e., concentric circles. These precipitation rings affect the minority carrier lifetime of the silicon wafer and subsequently the conversion efficiency of the battery. High oxygen content is one of the key factors causing concentric circles in N-type silicon wafers. Therefore, efficient TOPCon solar cells impose higher quality requirements on single crystal silicon wafers. Currently, the photovoltaic single crystal silicon industry pursues high production capacity and low cost, using larger furnace models and thermal fields for crystal pulling. In large-size thermal fields, to ensure that the temperature of the silicon melt at all positions inside the quartz crucibles is higher than the melting point and does not condense, more heat transfer occurs at the quartz crucible walls. This makes controlling oxygen impurities more difficult, thereby posing higher requirements on the thermal field and flow field.
Concentric circles can be divided into mild and severe concentric circles. Mild concentric circles are growth stripes generated radially during the crystal growth process due to uneven impurity distribution, which does not cause a decrease in battery conversion efficiency. Severe concentric circles result from the combined action of oxygen atoms, self-interstitials, and vacancy defects, leading to a ring distribution that reduces minority carrier lifetime, affecting battery efficiency and yield. The secondary high-temperature process of TOPCon batteries is more sensitive to the oxygen content of the wafer. When the oxygen content is too high, the limit solubility of oxygen increases under high temperatures, while the number of oxygen nuclei is few but the diffusion rate of oxygen is fast, making oxygen precipitation easy to grow, prone to forming severe concentric circles that impact battery conversion efficiency.
We all know that the main component of the quartz crucibles is SiO2, which is the main source of oxygen content in single crystal silicon. We usually influence the forced convection inside the melt through the quartz crucibles rotation speed, thereby changing the volatilization amount of SiO on the melt surface and affecting the oxygen content of the single crystal silicon rod. The lower the quartz crucibles rotation speed, the lower the oxygen content at the crucible-melt interface, which is beneficial for the quality of single crystal silicon; however, in actual crystal pulling production, a low quartz crucibles rotation speed will adversely affect crystal formation.
High-purity quartz crucibles use quartz sand divided into inner and outer layers. The inner layer sand requires higher purity and is still mainly imported. The better quality domestic quartz sand can only be used for the middle and outer layers. Additionally, the inner layers of the quartz crucibles have a protective barium coating that effectively prevent reactions with the silicon solution. However, during the second feeding process, quartz materials are mainly used for the feeder, inevitably causing quartz cone spattering. Since the thermal expansion coefficients of quartz and silicon differ, this can lead to cracking and damage of the quartz cone, with more serious cases resulting in falling into the quartz crucibles. Quartz used for feeders typically has relatively low purity, much lower than that of crucible quartz sand. This portion of quartz can heavily contaminate the silicon material, releasing large amounts of impurities and introducing substantial oxygen into the silicon liquid, affecting the oxygen content of the single crystal silicon rod.
