# Recent Advances in Black Silicon Surface Modification for Enhanced Light Trapping in Photodetectors
## 2 Wet Methods for Black Silicon Fabrication
### Metal-Assisted Chemical Etching (MACE)
#### 2.1.1 Silver-Assisted Chemical Etching  
As this process can be used to create intricate nanostructures on Si surfaces, [PERSON] et al. utilized Ag-assisted chemical etching to enhance the light-trapping efficiency and the performance of multi-crystalline BSi solar cells [24]. This has been achieved by introducing an additive to create uniform nanostructures on crystalline samples, as shown in Figure 1. The nanostructures exhibited uniform depth and diameter, along with exceptional properties. The Ag-assisted chemical etching method, along with the additive, is a highly effective approach for large-scale, high-efficiency, multi-crystalline BSi solar cell production. It is worth noting that the average efficiency of the solar cells was raised from 0.64% to 19.24% using the MACE process. Furthermore, [PERSON] et al. explored the relationship between the etching time and the absorption enhancement of broadband light in BSi [25]. Ag was thinly applied to Si samples and then heated in a nitrogen (N\({}_{2}\)) environment to create Ag NPs. Subsequently, the etching process was in a mixture solution of hydrofluoric acid (HF), hydrogen peroxide  
Figure 1: Schematic process-flow diagram of the experimental method to enhance light-trapping efficiency through Ag-assisted chemical etching. The diagram illustrates the application of an additive for creating uniform nanostructures, which significantly increased cell efficiency. The schematic details the uniformity of nanostructure depth and diameter, highlighting the method’s effectiveness for large-scale high-efficiency solar cell production [24]. (H\({}_{2}\)O\({}_{2}\)), and liquid water (H\({}_{2}\)O) (HF:H\({}_{2}\)O\({}_{2}\): H\({}_{2}\)O) for unlike intervals at room temperature, leading to the formation of BSi. The BSi nanotextures created a rough surface morphology, which reduced broadband optical reflection. It has been observed that the crystalline Si (c-Si) samples etched for 70 s exhibited the lowest broadband reflection, reaching 3% at a wavelength of 600 nm. The BSi nanotextures in this sample were between 50 and 100 nm in width and 300 and 400 nm in height. [PERSON] et al. have successfully optimized the etching time for enhancing broadband absorption in BSi, with an ultimate goal of increasing the efficiency of solar cells [26]. Their method involved using the one-step electroless Ag-assisted chemical etching method, which produced BSi nanowires with a diameter of approximately 100 nm and heights ranging from 2.0 to 2.9 \(\upmu\)m. Notably, after etching for 80 s, BSi nanowires with a height of 2.9 \(\upmu\)m exhibited 94% absorption at 600 nm. On a related note, [PERSON] et al. investigated the effects of Ag catalyst concentration in MACE on the mechanisms of nanostructure formation on Si and their optical properties [27]. They employed HF along with silver nitrate (AgNO\({}_{3}\)) concentrations of 2 mM, 3 mM, 4 mM, and 8 mM to create p-type Si nanostructures. The experimental results showed that the decrement of catalysis molarity concentration during etching processes led to the creation of nanostructures ranging from 140 to 60 nm, thereby offering valuable insights into their formation. These nanostructures were found to form BSi, causing a reduction in reflectance and demonstrating that MACE holds great promise as a method for antireflection coatings on BSi-based solar photovoltaic (PV) cells. Both studies exemplify significant advancements in nanofabrication techniques aimed at reducing reflectance and enhancing light trapping. In addition, an examination was conducted of the Ag thin films' thickness influence on the morphological and optical characteristics of BSi, created via two-step Ag-assisted chemical etching. They found that a 15 nm thin film of Ag led to surface coverage of Ag NPs at approximately 72.5%, forming dense, spherical shapes with an average nanopore height and diameter of about 420 nm and 200 nm, respectively [28]. This configuration led to an improvement in optical absorption across the wavelength spectrum of 300 to 1100 nm.