Nano holes on micro pyramids; broadband optical light trapping in thin wafer based Si (<100 μm) solar cells

Standard lithographic free wet etching process applied to thin (100 μm) p-type 156^∗156mm² Czochralski (CZ) silicon to generate random micro pyramids with variable heights from 2 to 5 μm, which leads to reduce the surface reflection upto 10% by scattering. The present study mainly focus on to optimize etching recipe for maskless damage free neutral beam etching (NBE) to drill low aspect ratio nano holes on micro pyramids. This nano holes offers a new way to manipulate light absorption and scattering processes, to reduce the surface reflection down to 4% in broad wavelength length range, is well suits for high performance thin wafer based Si photovoltaics for less consumption of silicon in industrial usage. NBE perform at room temperature with a mixture of sulfur hexafluoride (SF₆) and oxygen (O₂) with different gas ratios and bias powers. Optimized etching recipe for damage free NBE is as fallows, O₂ (65%) is added to a defined SF₆ (35%) and when lower bias (Vpp=50 volts) is applied to accelerate the ions in the plasma toward the neutralization plate, which forms low aspect ratio nano structure on micro pyramids with optimal etch depth by neutral particles. Surface morphology of etched profiles and its surface reflections have been investigated by field emission scanning electron microscope (FE-SEM) and spectrophotometer respectively. Nano structures with optimal etch depth of 70-100 nm forms on micro pyramids and reduces its surface reflection in visible and near-IR region (upto 3.5%) compare to their micro pyramid (10%) counterpart. This structures are well suits for next generation high efficiency thin (<100 μm) wafer based Si solar cells.