(153g) One-Step Metal-Assisted Chemical Etching with Silver and Copper Nanoparticles for Nanopore-Type Black Silicon Fabrication

In the solar cell industry, anti-reflection (AR) coating is a popular method to suppress the reflectivity of the solar cell surface by forming destructive interference of incident light. However, AR coating only reduces the reflection for a narrow range of light wavelength and incident angle because of its functionality based on a quarter-wavelength coating. A potential replacement for the AR coating, called black silicon, has become attractive in the fields of Si solar cells and other photon devices. A graded-density surface of black silicon allows this novel Si material to possess low reflectivity and a correspondingly high absorption of visible light. Black silicon has been successfully fabricated by several different methods including reactive ion etching, femtosecond pulsed laser technique, and pulsed electrochemical etching. However, these techniques need either expensive instruments with high energy consumption or complicated fabricating processes, which make them unfavorable for industrial applications.

An economical approach for black silicon manufacturing called metal-assisted chemical etching (MacEtching) method has been developed to decrease the fabrication cost. MacEtching usually includes two steps which are metal deposition and electroless chemical etching. In the metal deposition step, novel metal nanoparticles (NPs) which can attract electrons from Si substrate are firstly deposited on a Si substrate to oxidize the underneath Si to SiO₂. Next, in the electroless chemical etching step, the as-formed SiO₂ is etched away and pits are produced under the NPs. The NPs continuously attract electrons from the Si surface to form SiO₂ and make the pits deeper and connect with each other to form black Si with Si nanowire (SiNW)-type as long as the etching is prolonged.

Based on the two-steps MacEtching technique, we have developed one-step MacEtching technique with Ag and Cu NPs to further simplify the fabrication process of black Si. In the one-step MacEtching, the metal deposition and the electroless chemical etching occur simultaneously on the Si wafer which can save time and cost of cleaning and reloading of Si wafer between the two steps. In our one-step Ag MacEtching, a mixture of AgNO₃/H₂O₂/HF aqueous solution is utilized as a Si etchant under sonication. AgNO₃ works as an Ag NP precursor to provide Ag⁺ ions for Ag NP deposition on Si wafers. H₂O₂ is a reducing agent and able to effectively reduce Ag ions to Ag NPs which oxidize Si to SiO₂. HF removes the as-formed SiO₂ underneath the Ag NPs to increase the nanopore length. We find the Ag⁺ ion concentration and etching time significantly effects on the topology and reflectivity of the black silicon surface. The maximum diameter and length of obtained circular nanopore is 550 nm and 6.5 μm, respectively.

We have also studied the one-step MacEtching with Cu NPs to fabricate black silicon because the Cu precursor, Cu(NO₃)₂, is cheaper than Ag precursors, AgNO₃. Herein, H₃PO₃ is used as reducing agent instead of H­₂O₂ since the reduction potential of H­₂O₂ is not high enough to reduce Cu²⁺ ions to Cu NPs. Interestingly, our experimental results show that the Cu²⁺ concentrations in the etchant can not only affect the nanopore size but also the nanopore shape on the Si surface. With higher Cu²⁺ ion concentrations, the shape of the nanopores is rectangle instead of circle.

Compared to other methods, our one-step Ag and Cu-MacEtching have effectively simplified the black Si fabrication process and may cost down the cost of the facilities and energy expenditures, which is beneficial for industrial applications. On the other hand, unlike SiNW arrays, the nanopore-type black silicon possess no high-aspect-ratio needle-liked structure and are not as fragile as SiNW arrays and can better endure the stress during the solar cell assembly.