Numerical calculation of the reflectance of sub-wavelength structures on silicon nitride for solar cell application

In this study, we calculate the spectral reflectivity of pyramid-shaped silicon nitride (Si₃N₄) sub-wavelength structures (SWS). A multilayer rigorous coupled-wave approach is advanced to investigate the reflection properties of Si₃N₄ SWS. We examine the simulation results for single layer antireflection (SLAR) and double layer antireflection (DLAR) coatings with SWS on Si₃N₄ surface, taking into account effective reflectivity over a range of wavelengths and solar efficiency. The results of our study show that a lowest effective reflectivity of 1.77% can be obtained for the examined Si₃N₄ SWS with the height of etched part of Si₃N₄ and the thickness of non-etched layer of 150 and 70 nm, respectively, which is less than the results of an optimized 80 nm Si₃N₄ SLAR (∼5.41%) and of an optimized DLAR with 80 nm Si₃N₄ and 100 nm magnesium fluoride (∼5.39%). 1% cell efficiency increase is observed for the optimized Si solar cell with Si₃N₄ SWS, compared with the cell with single layer Si₃N₄ antireflection coatings (ARCs); furthermore, compared with DLAR coated solar cell, the increase is about 0.71%. The improvement on the cell efficiency is mainly due to lower reflectance of Si₃N₄ SWS over a wavelength region from 400 to 600 nm that leads to lower short circuit current.