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.