Plasma etched c-Si wafer with proper pyramid-like nanostructures for photovoltaic applications

摘要

Dry and maskless texturing process of mono-crystalline silicon wafers using CF4/O-2 plasma in a reactive ion etching (RIE) system has been developed with the aim of obtaining optimized surface texture characterized by low optical reflection loss for photovoltaic applications. In this study c-Si has been successfully subjected to plasma etching at unusual process conditions, specifically high value of substrate temperature (15 degrees C) coupled with high value of process pressure (40 Pa). Different c-Si surface textures are produced at different etch durations.Specifically, pyramid-like morphology appears at very short etch time and crater-like nanostructures form at longer etch time, whereas the former gives the lowest average reflectance ( 5%). Furthermore, we clearly observe a slowing down of the etching rate starting from 5 min and in correspondence with the appearance of the sponge-like structure coupled with pores formation. Geometric parameters of morphological features, such as RMS and average inclination angle of the surface features, have been correlated with some specific texture functions such as angular distribution of scattered light Ps(theta) obtained by Angular Resolved Scattering (ARS) technique. Shape and slope of the texture features strongly determine the amount of forward scattered light and, as a consequence, the amount of reflectance decrease. Surface texture obtained after only 3 min of etch and characterized both by well distributed small normal pyramids and by high average inclination angle, proves to be more effective in giving reflection reduction respect to other surface textures characterized by inhomogeneously distributed large cavities. Finally, we can well suppose that this small pyramid-like texture on Si-substrates can be used for the preparation of highly efficient heterojunction-based solar cells (Si-HJT).