Effect of Chemical Post-Treatment on Laser Texturing of Polycrystalline Silicon Wafer Surface

The laser etching process involves the interaction of a laser with the material, resulting in material removal. As shown in Figure 15, using the Gaussian laser processing principle as an example, the silicon wafer undergoes localized energy deposition during etching. This causes a small portion of the surface material to vaporize and splatter, while the majority of the material melts and is ejected. As a result, the etched silicon wafer surface is covered with solidified droplets and molten residue, with an underlying damage layer formed by high etching temperatures and impact. To maintain the silicon wafer's electrical properties, chemical post-treatment is necessary to remove the cladding and damage layers. The effects of chemical post-treatment are illustrated in Figure 16. Chemical post-treatment effectively eliminates or mitigates surface defects, improving the overall texture quality. Post-treatment solutions can be acidic, alkaline, or a combination thereof, each producing distinct etching effects depending on the corrosion conditions.

Figure 15 Principle of pulsed laser irradiation of polysilicon

Figure 16 Texturing surface microstructure: (a) after laser ablation (b) after chemical etching (Inset: related cross-sectional scanning electron micrograph)

Zielke et al. employed a frequency-doubled Nd:YVO4 laser to process silicon wafers, producing a microstructure on the wafer surface with a diameter of approximately 7 μm and a depth of around 4 μm. After laser texturing, the wafers underwent a two-step wet chemical etching process. In the first step, surface burrs were removed by immersing the wafer in a 25% NaOH solution at 40°C for 3 minutes. In the second step, the wafer was etched using a mixture of 40% HF and 100% HNO3 in a 1:30 volume ratio to remove laser-induced damage. The optimal etching effect occurred when the surface microstructure had a depth of 3.2 μm.

Ding et al. studied the defect densities of boron-doped DWS polycrystalline silicon wafers by classifying them into four groups (A, B, C, and D) based on photoluminescence imaging. They employed an industrial laser with a 532 nm wavelength, using a pulse repetition frequency of 500 kHz and laser power settings of 100% and 50% for texturing. To examine the effect of solution corrosion on surface reflectivity, they applied an acidic solution composed of 49% HF, 69% HNO3, 98%H2SO4, and H2O in a 1:7.2:3.5:8.3 volume ratio, etching for 140 seconds at 8°C. In the subsequent step, they applied an acidic solution containing 49% HF, 69% HNO3, and H2O in a 1:3.1:1.5 volume ratio, etching for 70 seconds at 10°C to further investigate the impact of corrosion on surface reflectivity.

Choi et al. immersed the sample in a 25% NaOH solution at 40°C for 4 minutes after UV laser texturing to remove surface residue. This was followed by chemical etching at room temperature for 4.5 minutes using a mixture of NaOH, CH3COOH, and Han F solutions in a 30:10:4 volume ratio, which formed a V-shaped microstructure. After slag removal and chemical etching, the V-shaped cross-sectional microstructure formed a regular honeycomb pattern on the surface. The results showed that UV laser texturing reduced surface reflectivity by 3.3% compared to polycrystalline silicon solar cells textured with isotropic acid etching.

Laser texturing of polycrystalline silicon wafer solar cells typically involves three main steps, as shown in Figure 17. First, laser texturing creates the textured surface; second, anisotropic alkaline cleaning removes surface slag produced during texturing; and third, isotropic etching is performed using a strong acid solution (e.g., HF). Alkaline etching is performed before acid etching because it is more effective at removing surface slag and the damage layer. Acid etching, being isotropic, preserves the morphology of the textured microstructure but is less effective at removing slag and the damage layer.

Figure 17 (a) Polycrystalline silicon laser textured surface (b) KOH alkaline washing (c) HF/HNO3 acid washing

Acid etching is highly toxic and entails significant costs. As a result, researchers have investigated the use of the alkaline etching (anisotropic etching) process at different stages following laser texturing. Despite being generally less effective than acid etching, alkaline etching offers certain advantages. Acid etching is more effective at reducing surface reflectivity than alkaline etching. However, once an anti-reflection coating is applied and the cells are encapsulated, the reflectivity difference between acid and alkaline etching becomes negligible. Furthermore, studies have shown that the performance of polycrystalline silicon solar cells etched with either alkaline or acid methods differs only marginally. Therefore, provided that alkaline etching offers electrical properties comparable to acid etching, it is a viable option for laser texturing.

Ha et al. used a pulsed fiber laser with an output power of 15 W and a wavelength of 1064 nm to texture the surface of polycrystalline silicon wafers. Their experiment involved etching the wafers with a mixed solution containing 2.0% KOH and 5.9% isopropyl alcohol (IPA), or a 20% KOH solution, at the same temperature and duration. Kim et al. used an Nd:YAG picosecond laser with an output power of 11 W and a wavelength of 1064 nm for wafer texturing. Following laser texturing, the polycrystalline silicon surfaces were etched with either a 25% tetramethylammonium hydroxide (TMAH) solution or a mixture of 25% TMAH and 99% CH3COONa in a 5:10:85 mass ratio, under identical temperature and time conditions.

The results indicated that, with increasing cleaning time, etching with both solutions led to a smoother textured surface. Additionally, in Ha et al.'s study, reflectivity decreased more gradually with the KOH/IPA mixed solution than with the KOH solution, with better preservation of the surface texture. In Ha's experiment, etching polycrystalline silicon wafers with a KOH/IPA mixed solution for 5 minutes at 80°C resulted in a significant increase in solar cell efficiency, from 4.6% to 12.6%. In Kim's experiment, the optimal etching time was 5 minutes at 70°C, resulting in an increase in solar cell efficiency from 7.5% to 15.2%. (Solar cell efficiency is the ratio of incident light power converted into maximum electrical output.)

In the laser texturing of polycrystalline silicon wafer surfaces, laser parameters, processing environment, and methods significantly affect light trapping efficiency (i.e., reflectivity) and surface morphology. The effect on texture quality, defined as the reduction of defects such as cladding and damage layers created during laser processing, is limited, resulting in overall improved texture quality. As shown in Table 4, post-treatment chemical etching following laser texturing of polycrystalline silicon wafers significantly enhances the microstructure quality, facilitates subsequent processing steps in solar cell production, and improves the photoelectric conversion efficiency of the final solar cells.

Table 4 Effect of various chemical post-treatment etching methods on surface texturing and efficiency of polysilicon solar cells after laser texturing

Conclusion

Laser texturing generates microstructures on the surface of multicrystalline silicon wafers through laser etching, enhancing their photoelectric conversion efficiency. However, this process also introduces cladding and damage layers that may adversely impact cell performance. Chemical post-treatment, including both acidic and alkaline etching, effectively removes these layers, refining surface properties. Research indicates that acidic etching significantly reduces reflectivity but is costly and environmentally hazardous. While alkaline etching is slightly less effective, it offers comparable electrical properties to acidic etching at a lower cost. In conclusion, chemical post-treatment is essential for improving the quality of laser texturing and enhancing the efficiency of solar cells.