Hierarchical Etching for Improved Optical Front-side Properties of Monocrystalline Si Solar Cells

Abstract

We investigate a hierarchical surface structuring process to reduce the optical reflectance of monocrystalline Si solar cells to below the reflectance of state-of-the-art alkaline texturing. First, alkaline texturing is performed, yielding random pyramids with a few micrometers in size. Second, plasma etching forms step-shaped structures smaller than the wavelength of visible light. Eventually, the surface morphology is formed by <111> planes with many steps, edges and inverted pyramids in a few hundreds of nanometers scale. We investigate the reflectance on double-side textured samples with SiNx/Al2O3 front-side and SiNx rear-side passivation. These dielectric stacks exhibit a low reflectance for conventional alkaline textured surfaces. The short circuit current density potential of the best hierarchical structured samples is 0.4 mA/cm2 higher when integrating the AM1.5G spectrum from 300nm to 1000 nm. Furthermore, our results suggest that structures with dimensions smaller than the thickness of an antireflection- and passivation (AR) layer cannot be covered conformal by the AR layer when using plasma-enhanced chemical vapor deposition. Thus, there seems to be a lower limit for the structure size, and wet chemical cleaning sequences – performed after plasma etching - have to be adjusted accordingly.