Optimization of SiN AR coating for Si solar cells and modules through quantitative assessment of optical and efficiency loss mechanism

Plasma-enhanced chemical vapor deposition (PECVD) SiN films are widely used as antireflection (AR) coating for silicon solar cells and particularly for multi-crystalline solar cells for hydrogen passivation of bulk defects. In this paper, the detailed optical properties of various SiN films and their effect on silicon solar cell efficiency in air and under glass is evaluated by a combination of Monte-Carlo geometrical ray tracing program, Sunrays, and a device modeling program PC1D. Maximum module power under glass and ethylene vinyl acetate (EVA) encapsulation is used as the figure of merit for optimizing the index and thickness of the SiN films. Simulations are categorized by surface morphology (planar or textured) and ambient (air or glass). SiN films with refractive index (n) in the range of 2.03-2.42 are used for this study. It is found that although n = 2.03 is not the optimum index in terms of reflectance under glass (n = 1.5), it produces maximum cell or module efficiency under glass. This is because n = 2.03 film produces much higher cell efficiency (17.9%) in air, therefore, even after a significant optical encapsulation loss of 0.8% in absolute efficiency, the cell efficiency remains highest (17.1%) under glass. In contrast SiN film with an index of 2.4 produces only 0.5% air to glass efficiency loss but due to the low starting efficiency of 17% in air; the final cell efficiency under glass is only 16.5%. In addition, texturing provides a larger window of thickness around the optimum without affecting the optical performance. Similar analysis done for planar cells indicate that optimum index for highest module power is 2.20. This is because reflection is much higher in planar cells, therefore higher index can be tolerated before loss due to absorption in SiN exceeds the gain in reflectance under glass. Copyright (C) 2011 John Wiley & Sons, Ltd.