INDUSTRIAL DEPOSITION OF SILICON NITRIDE ON MC-SI FOR SOLAR CELL PRODUCTION EMPLOYING MICROWAVE PECVD

Abstract

Silicon photovoltaic possesses a great market share of more than 90% in solar industry. Due to the cost and throughput concerns, multi-crystalline silicon solar cell are gaining its dominant position over crystalline solar cells represented by a share of more than 55% and this share is still increasing. However, the major issue existing for multi-crystalline solar cells is the relatively high concentration of defects and impurities in the wafers due to the wafer fabrication methods and grain boundaries for poly-silicon. Processes such as gettering, phosphorous diffusion and back-side field formation via aluminum alloy have been implemented to passivate these defects and impurities however not enough. PECVD SiN coating helps significantly to achieve desirable passivation performance thus becomes a priority in multi-crystalline silicon solar cell production. The deposited SiN layer is functioning not only as a passivation facilitator making use of the H content inside this amorphous layer, but also as an anti-reflection layer to improve transmittance of usable lights into the solar cells. The PECVD system used in REC Cell is characterized by its linear microwave plasma sources showing improvement in surface passivation, better reactant gas utilization and deposition uniformity. The dependence of solar cell performance on the PECVD processing parameters are discussed in the report, via their influences on the functions of providing anti-reflection, surface and bulk passivation. The effect of substrate material on the deposited layer properties is also discussed. Based on this, a hypothesis is provided to analyze the phenomenon of edge discoloration within wafer. Two proposed solutions have been tried to solve this problem and provide support to the hypothesis.