Novel texturing of tri-crystalline silicon surfaces

Tri-crystalline silicon (tri-Si) is a promising candidate material for solar cell fabrication because it can be made quickly and at low cost. Tri-Si has a higher mechanical strength and thus requires a thinner wafer than either polycrystalline or single-crystal Si, which decreases the unit price per wafer. Furthermore, tri-Si ingots grow more quickly than single-Si ingots, enabling higher production capacity.

One of the key parameters for improving the efficiency of tri-Si is reflectance, which can be lowered by etching. Accordingly, we have developed a novel, two-step texturing process for tri-Si surfaces. We start with HF:HNO₃:deionized (DI) water (2.5:2.5:5) etching, which is followed by exposing the tri-Si wafer to acidic vapors to generate fine holes. The etching depth can be up to 2.5μm. Through this process we have achieved a surface reflectance of 12.3%, which is about 10% lower than that achieved using normal acidic texturing. An antireflection coating of SiN_x is used to optimize the reflectance of the nanoporous structures. In addition, we have reached a fill factor of 0.78 with an efficiency of 16.2% on a 12.5 × 12.5cm wafer. This high efficiency is mainly due to the increased short-circuit current density of 34mA/cm².

Figure 1 shows a scanning electron microscopy (SEM) image of the surface morphology of the tri-Si surface after the first texturing step, here called acidic texturing. Tri-Si wafers were immersed in an acidic solution of HF:HNO₃:DI water (0.25:0.25: 0.5) for 65s. The etching depth was 4μm per side. Porous structures formed on both sides of the wafer. Both the micropores and elongation structures are evident in Figure 1. The porous structures are uniformly and randomly distributed on the surface. The length and width of the elongation structures are approximately 10 and 2μm, respectively.

Figure 1. Scanning electron microscopy (SEM) image of acidic texturing of a tri-Si surface. The magnification is ×30K.

Figure 2 presents SEM images of the tri-Si surface morphology achieved in the second step, here called vapor texturing. After acidic texturing, tri-Si wafers were exposed to the acidic vapor produced by reaction of the waste Si with a solution of 70% HF and 30% HNO₃ by volume. The etching time was 8min, and the etching depth was 2.5μm on each side. The vapor-texturized surface features pits and bumps, which are around 10μm long and 4μm wide. The size distribution of the nanoporous structure varies from 5 to 10nm and seems to be uniform.

Figure 2. SEM images of the second, vapor-texturing step of a tri-Si surface. (left) Magnification ×5K. (right) Magnification ×30K.

The chemical processes in the vapor-texturing step can be expressed as:¹

These two reactions are related to the dissolution of Si atoms and the formation of porous structures. The H₂SiF₆compound, which is observed on the surface of the Si substrate, may occur in the form of droplets. The proposed mechanism for the formation of (NH₄)₂SiF₆ is:²

In fact, with an acid mixture that is poor in HNO₃, a small amount of (NH₄)₂SiF₆ is produced during reaction (1), due to the presence of NO₂. Moreover, drops of H₂SiF₆ from reaction (2) partially cover the etched surface, preventing the small quantity of nitric acid vapors from continuing the etching. However, for an acid mixture rich in HNO₃, a significant amount of NO₂ reacts with Si in the presence of HF, enabling the formation of (NH₄)₂SiF₆ as a major phase. The (NH₄)₂SiF₆ powder-like phase is highly porous,³ allowing the acidic vapors to diffuse into the nanoporous matrix and continue the etching process. During this process, the formation of nanosized pores is accompanied by the production of (SiF₆)²⁻ ions, due to hydrolysis of the primary etching product.

The Si wafers in solar cells are becoming thinner in an attempt to reduce manufacturing costs. Because of this, the wafer breakage rate is increasing. In addition to its positive effects on efficiency and reflectance, the texturing process described here can reduce the wafer breakage rate. The nanostructured surface of the wafers after vapor texturing provides additional strength when compared to typical acidic texturing processes. Future work will incorporate tri-Si wafers that have undergone this novel texturing process into solar cells. We expect these cells to be low cost, high yield, and high efficiency.