**4. Wet-chemistry processes for solar cell fabrication**

Wet-chemistry-based treatment is an important step in solar cell processing for saw damage removal (SDR) for the as-cut wafers, texturing of the surface to increase the absorption of incoming solar radiation and edge isolation after the diffusion process. As discussed in the previous section, there are mainly mono-crystalline and multi-crystalline silicon wafers used for fabrication of solar cells. The wet-chemistrybased processing for the respective types of wafers will be discussed ahead.

#### **4.1 Texturing of mono-crystalline silicon wafers**

As indicated in Section 2, the development of solar cells started primarily with mono-crystalline wafers and hence employed well-established methods from the domain of microelectronics. Alkaline anisotropic etching based on KOH/NaOH

is used for pyramidal texturing of mono-crystalline wafers. An as-cut monocrystalline wafer has a weighted average reflectance of >30% (over wavelength of 300–1,200 nm) which reduces to 11–12% after the texturing process. Typical morphology of an alkaline textured surface is shown in **Figure 5**. The anisotropic etching solution etches the (100) surface of the wafers to expose the (111) faces which have a higher density of silicon atoms and hence a slower etch rate compared to the (100) faces. This results in formation of random pyramid structures which form an angle of 54.7° with respect to the wafer surface.

Typical parameters for the alkaline texturing process are shown in **Table 1**. It should be noted that the values of various parameters are indicative and are not to be taken as absolute as there are a variety of additive manufacturers in the market. Isopropyl alcohol (IPA) was initially used as an additive in the texturing solution, which is not involved in the etching reaction, but acts as a wetting agent to improve the homogeneity of texturing process by preventing the H2 bubbles (generated during the reaction) adhering to the silicon surface [12]. However by 2010, IPA was gradually replaced with alternative additives due to drawbacks like unstable concentration as the bath temperature is close to the boiling point of IPA (82.4°C), high costs, high consumption, health hazards and explosiveness [12]. Many groups have published development work to replace IPA with alternate additives to overcome the disadvantages of IPA, increase the process window and reduce the surface reflectance [12–16]. Additives also reduce the processing time to <10 minutes and increases the bath life to >100 runs.

The texturing process of the mono-crystalline wafers is typically performed in a 'batch' which implies that the wafers are loaded in a carrier with slots to hold the wafers (100 slots in a carrier) and then the batch is processed sequentially in baths for texturing, cleaning, treatment steps to remove the organic residue and metal contamination and drying the processed wafers. The carriers are typically coated with PVDF which has very good resistance to various chemicals, abrasion and mechanical wear and tear. Typical carrier for mono-crystalline wafers handling is shown in **Figure 6**. The batch texturing tool has dedicated baths for each step with dosing tanks for chemicals used in the bath. The tool processes many carriers simultaneously and can reach a throughput of >6,000 wafers/h with processing of four carriers at the same time.

**Figure 5.** *Typical surface morphology of an alkaline textured mono-crystalline wafer.*
