Optical properties of silicon light trapping structures for photovoltaics

Light trapping structures in photovoltaics are essential to suppress reflection losses and increase conversion efficiency. For wafer silicon (Si) solar cells, this is commonly achieved by chemical texturing and the application of an antireflection coating. Such surfaces still show significant reflection losses that are ∼10%. Hence, for further reduction in reflection, new methods for light trapping need to be explored, which are effective for a broad solar spectral and angular range. In this paper, we explore an ultrafast laser texturing method that successfully reduces the reflection below 5% over a broad spectral and angular range and more importantly, is applicable to crystalline, multi-crystalline, thin film silicon and other materials. The optical properties of ultrafast laser textured silicon surfaces produced in a sulfur hexafluoride (SF6) gas ambient are evaluated by total reflection including scattering as a function of wavelength and angle of incidence. The optical results are further compared with other texturing schemes. This study also investigates the silicon bandgap modification induced by ultrafast texturing method. Finally, a comparison is made for the photovoltaic parameters of solar cells made of ultrafast laser textured surfaces, chemically textured surfaces, porous silicon surfaces, and etched silicon surfaces that result in nanowires for light trapping to understand impact of surface texturing on photovoltaic device performance.