Black silicon (b-Si), a type of porous Si, has attracted attention for use in Si-based solar cells as a replacement of the conventional anti-reflection (AR) coating.1 Surface morphology of b-Si possesses a graded refractive index between the Si surface and the air, and brings a low reflectivity and a correspondingly high absorption of incident light.2 A metal-assisted chemical etching method, which typically includes two steps, has been studied to economically fabricate b-Si. In the first step, metal deposition, a noble metal, such as gold (Au) and silver (Ag), is deposited on the Si surface as nanoparticles (NPs) to withdraw electrons from the Si surface for SiO2 formation. In the second step, electroless chemical etching, the as-formed SiO2 is etched by HF to produce pits under NPs. As etching continues, the pits become deeper and ultimately connect with others, and the remaining Si substrate would form b-Si which consists of Si nanowires or nanopores.
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