(318d) Nanopore-Type Black Silicon Anti-Reflection Layers Fabricated By a One-Step Silver-Assisted Chemical Etching

Solar cells need low surface reflectance to maximize the amount of absorbed incident photons for improving the energy conversion efficiency. Anti-reflection (AR) coatings are traditional materials to suppress the reflection of silicon (Si)-based solar cell surface, but can only work for a narrow range of light wavelength and incident angle. To overcome the disadvantages of AR coatings, a novel type of porous Si material called black silicon (b-Si) has been considered as replacement for the AR coatings. B-Si is directly formed from the Si wafer of the solar cell and its surface possesses a graded refractive index resulting in a low reflectivity of visible light. Although b-Si has been successfully fabricated by several different methods, most of the techniques need either expensive instruments and high energy consumption or complicated fabricating processes, causing them unfavorable for industrial applications.

Recently, an economical and efficient approach for industrial b-Si manufacturing called metal-assisted chemical etching method has been developed to avoid the above disadvantages. The metal-assisted chemical etching method includes two steps: metal deposition and electroless chemical etching. In the metal deposition step, a noble metal, such as Au, Ag, and Pt is deposited on the Si surface usually as nanoparticles (NPs). The metal NPs attract electrons from the silicon surface promoting the oxidation to SiO₂ in the presence of an appropriate oxidant. In the electroless chemical etching step, the as-formed SiO₂ is etched away by HF (as H₂SiF₆) and a pit is produced under each NP. As these reactions occur in a continuous process the pits become deeper and ultimately connect with each other, and remaining Si substrate forms b-Si that consists of a silicon nanowire (Si-NW) structure. Unfortunately, this Si-NW structure is very fragile making the incorporation of b-Si into typical fab processes difficult.

The proposed method here, one-step silver (Ag)-assisted chemical etching method, is more facile technique which can synthesize b-Si in 20 minutes under room temperature without any pricy facility. During the process of the one-step method, the deposition of Ag NPs and the chemical etching of Si occurred simultaneously on the Si wafer surface. In our b-Si fabrication process, cleaned Si wafers were etched in a Si etchant consist of HF, H₂O_2, H₂O, and AgNO₃ with various concentrations in sealed plastic ware at room temperature.The effects on the surface morphology and the corresponding surface reflectivity of the concentration of the Ag catalyst precursor, the HF and H₂O₂ concentrations in the Si etchant, and etching time have been systematically investigated. The AgNO₃ was utilized as an Ag NP precursor in a HF:H₂O₂:H₂O solution to compose of a Si etchant; the H₂O₂ not only facilitates the Si etching but also reduces Ag⁺ ions to Ag NPs on the Si wafer. After the Si etching, the Ag remained on the b-Si samples were removed by an Ag etchant consisting of NH₄OH and H₂O₂. Compared to other methods, the one-step Ag-assisted chemical etching simplifies the b-Si fabrication process and may cut down the cost of the facilities and energy expenditures, which is beneficial for industrial applications. In addition, unlike Si-NW arrays, the nanopore-type b-Si possesses no high-aspect-ratio needle-liked structure and is not as fragile as Si-NW arrays and can thus better endure the stress during the solar cell assembly.

According to the results, we found that lower reflectivity is a balance between sufficient Ag catalyst to create large numbers of nanopores on a Si surface and excessive Ag that brings deeply etched channels which would potentially short-circuit a solar cell junction. In addition, a mechanism for describing the formation process of black Si by one-step Ag-assisted chemical etching method is also proposed. With [Ag⁺] of 500 μM, nanopores are hardly observed on the Si wafer surface causing the relative reflectivity of Si wafer surface cannot be effectively improved. The lowest average relative reflectivity, 0.17%, occurs with a [Ag⁺] of 50 μM and the HF:H₂O₂:H₂O ratio of 1:5:2, with nanopore length longer than 6 μm. However, as the [Ag⁺] decreases to 5 μM, a low relative reflectivity (2.60%) and a short nanopore length (< 250 nm) can be obtained by 10 minute etching time and the HF:H₂O₂:H₂O ratio of 5:1:20, indicating that this method can be used as a simple (one-pot), low cost (low [Ag⁺]), energy efficient (room temperature), method for the synthesis of AR layers for Si-based solar cell applications.

We propose that the simplicity of the process and low reaction times, along low consumption of Ag precursor will make such a one-step method scalable for industrial AR applications where process cost and rate are important. Moreover, the nanopore length of b-Si fabricated with 5 μM [Ag⁺] can be controlled to be around 250 nm, which is shorter than a typical p-n junction depth in Si-based solar cells, and would not interfere with the function of the cell.