Design and fabrication of sub-wavelength structure on silicon nitride for solar cells

In this study, we explore the spectral reflectivity of pyramid-shaped silicon nitride (Si₃N₄) sub-wavelength structures (SWS). First, a multilayer rigorous coupled-wave approach is advanced to investigate the reflection properties of Si₃N₄ SWS. We first optimize Si₃N₄ SWS structure for the best effective reflectance properties. 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 nm and 70 nm. Then, we develop a simple and scalable approach for fabricating SWS on silicon nitride by means of self-assembled nickel nano particle masks and inductively coupled plasma ion etching. Silicon nitride SWS surfaces with diameter of 120 nm - 180 nm and a height of 150 nm - 160 nm were obtained for lowest average reflectivity for wavelength range from 190 nm to 1000 nm. A low reflectivity below 1% was observed over wavelength from 600 to 800 nm. The measured reflectivity is closely matched with the simulated results. Using the measured reflectivity data, the solar cell characteristics has further been compared for single layer anti-reflection coatings and SWS structures and a 1.31% improvement in efficiency was observed.