Chemical Texturing Used to Reduce The Reflectance of the Multi-Crystalline Silicon Wafer For Improving Optical Properties and Solar Cell Efficiency


 The aim of this work is to improve the optical properties in the multi-crystalline silicon (mc-Si) by acid texturization. Generally, HF and HNO3 are using the mc-Si wafer acid texturization process and it is toxic chemical acids. In this work, H2O2 is used instead of HNO3 because of H2O2 less toxic chemical compared to HNO3. In this work, we have used the different types of chemical acids in different ratios for etching. Here, we have used HF: H2O2: CH3COOH=3:2:2, HF: H2O2: KMnO4 =3:2:0.2 M and HF: H2O2: HNO3: KMnO4 =3:2:2:0.2M for etching with the etching time of 60 sec. The HF: H2O2: KMnO4 =3:2:0.2M gives the better results as obtained from optical microscope, UV- Visible reflectance studies and X-ray diffraction (XRD) studies. The etched mc-Silicon wafer surface was analyzed by the optical microscope and Scanning Electron Microscope (SEM). The FTIR results indicate the reduction of oxidation in the etched samples. Moreover, the HF: H2O2: KMnO4 =3:2:0.2M textured wafers have the advantages of lower reflectance and increased etching of the mc-Silicon wafer.


Introduction
Surface re ections are considered as a key process to achieve higher e ciencies in solar cells. Mc-Silicon has occupied larger percent of the silicon solar cell market due to its low cost. However, the effective surface texturing in mc-Silicon is a challenging task for solar cell application. The different type of chemical textures was used for the etching process. Acid mediated texture on etching is a key technology to fabricate mc-Silicon for solar cell application. Because, it is a cost effective technique to get high e ciency. The different types of etching process were used to reduce the light re ectance. Reactive Ion Etching (RIE) [1], Metal-catalyzed chemical etching (MACE) [2], Plasma texturing, plasma-less atmospheric pressure dry [3] and acidic texturing [4] were used to reduce the re ectance in mc-Silicon wafer. RIE and MACE methods give less than 5% re ectance. But, these are inadmissible in the photovoltaic eld due to low throughput and process complexity [4]. These are also expensive etching methods. The chemical etching is most widely used to remove unbounded molecule and impurities. In general, alkaline solution based anisotropic etching was adopted for the single crystalline -silicon and in the case of mc-silicon the acid solution based isotropic etching has been performed. Usually, random sized pyramid structures are obtained in c-silicon and "worm like" structures are obtained in mc-Silicon.
More speci cally, the texturization of the (100) oriented C-Silicon wafers have already been commercialized based on alkali or isopropyl alcohol etching process [5].
Mc-Silicon is different from c-Silicon. Randomly oriented crystalline grains make the grain boundaries and it is unavoidable in the mc-Silicon. Mechanical etching and laser etching textures gave regular microstructure surface. But, it reduces photoelectric conversion. Further, it is highly expensive and not suitable for mass production [8]. Acidic etching is a key technology on Texturing the mc-Silicon wafer to reducing the re ectance of the wafer. In addition, it gives higher e ciency, increasing the optical path length and it is least expensive method. Sheng et al., [9] have investigated the crystal orientation and crystalline peak intensity of the etched silicon wafers. The X-ray diffractogram result indicates that the crystalline peak intensity of the etched silicon wafer is slightly shifted to lower 2θ compared to without etched wafer. Shuai Z et al., [10] have optimized chemical etching (MCCE) of the mc-Silicon wafers with different orientation with both alkali and metal-catalyzed chemical etching.
Gangopadhyay et al., [11] have analyzed the different ratios of sodium hydroxide-sodium hypochlorite (NaOH-NaOCl) texturing solution with mc-Silicon. They have showed the formation of Si-Cl bonds through the FTIR imaging and the bonds have improved the quality of the diffused junction.
In this paper, the reaction behavior of the mc-Silicon in HF -H 2 O 2 -CH 3 COOH/ KMnO 4 / HNO 3 -KMnO 4 etching mixtures is studied and the appropriate composition of the texturing reaction mixture was found.
It is obtaining the effective isotropic surface texture of mc-Silicon solar cells in a short time of 60 secs and it is helpful in improving the optical properties. The weight loss, reducing the re ection, surface morphology and crystal structure of the mc-Silicon wafer were determined by using the digital micrometre, UV-Vis-NIR Spectrophotometer, optical microscope and X-Ray diffractometer (XRD), respectively. The etching mechanism is studied for different grains of mc-Silicon wafers. The reduction of the oxidation is also studied by using FTIR.

Experimental Section
The random small size mc-Silicon wafers are cut from the commercial P-type multi-crystalline silicon wafers with 180-micron thickness. Before processing, the wafers were cleaned in RCA1 and the  Table.1. It is indicating that the texturization of mc-Silicon wafer is leading to a signi cant reduction in re ectivity and gives high values of short circuit current [12]. The surface morphology of the mc-Silicon wafers was studied using Scanning Electron Microscopy (SEM) QUANTA 200 3D Dual Beam (FEI) and CMM-23 COSLAB Microscope. The crystal structure and orientation were studied using Analytical Empyrean X-ray diffractometer (XRD) with CuK α radiation ((λ = 1.5406 Å). UV-Vis-NIR spectrophotometer (Lamda 35, PerkinElmer) was used to measure the surface re ectivity. Mc-Silicon wafers with lower re ectance and lower recombination defects enhances the solar cell conversion e ciency. This can be obtained by chemical etching process with low cost. The wet chemical etching process is used for the removal of metal impurities from texturing of the silicon wafer and silicon materials. Acid texturing in a mc-Silicon wafer has been performed using a Hydro uoric (HF) and Hydrogen peroxide (HNO 3 ). The general etching mechanism is of two steps: (1) the SiO 2 is formed by nitric acid on the surface of the mc-Silicon wafer and then (2) SiO 2 is removed from the wafer surface via water-soluble complexes formed by HF [13].
The HNO produces an SiO 2 layer It is dissolved by HF The overall reaction is: The described mechanism of silicon dissolution is an electrochemical process on the silicon surface. In this process, holes (h+) were injected in the valence band of the semiconductor by HNO 3 at local cathodic sites, followed by the formation of a SiO 2 layer which is dissolved by HF at local anodic sites. The formation of SiO 2 layers was con rmed by the X-ray Photoelectron Spectroscopy studies [14]. The electrochemical process works just like conventional operations. The mc-Silicon wafers were cleaned in RCA-1 process at 10 minutes and the purpose of the cleaning was to remove the organic residue on the silicon wafer surfaces. It is placed in deionized water for a few seconds. Three set of chemicals with different volume and molar ratios were selected for etching process at room temperature and etching duration was 60 sec. The rst set of etching chemical solution is HF-H 2 O 2 -CH 3 COOH with volume ratio of The acid etching is su cient to create isotropic surface texture of the wafer and to avoid the saw damages. The re ectance of the mc-Silicon textured surfaces are investigated from wavelength between 300-1000 nm and shown in Figure.1. Three set of etched samples and reference silicon sample re ectance results were compared and shown in Figure.1. It is clearly seen that Set-2 (HF-H 2 O 2 -KMnO 4 ) chemical ratio reduces the re ectivity and the reduction up to 12.5 %. The Set-2 chemical ratios in etching process strongly in uences the effective re ectance. It has higher light absorbance compared to other chemical textured wafers since, the higher light absorbance leads to increase the solar cell e ciency. Kulesza et al., have optimized [12] the time e cient texturization of mc-Silicon in the HF/HNO 3 solution and its effect on the optoelectonic parameters of the solar cells. The optimal chemical etching ratio has increased the solar cell e ciency up to 13.9 %.
3.2 Crystalline structure of the grain orientation

Surface morphology of the mc-Silicon wafers:
The surface structures of the etched mc-Silicon wafers were investigated by using an optical microscope and SEM. Figure.  wafer.
An optimization of the three different textured solutions were helpful to achieving lower re ectance. The SEM images of silicon wafer surface are shown in Figure.4 Figure.6. It reduces more weight and it is shown in Table.2. The HF/H 2 O 2 / KMnO 4 solution is showing the optimal oval pit structure with the lowest re ectance. Wet etching of the mc-Silicon wafers is a critical step in the solar cell making process. Optimized wet etching process improves the optical properties [19,20]. The recombination properties of mc-Silicon solar cells are more complex owing to the structural defects, point defects and interaction among defects. Oxygen segregation in structural defects affects electrical activity and further complicates it. Pizzini et al., [21] con rmed an oxygen segregation in the structural defects and they showed its in uence on dislocation electrical activity. IR absorption spectroscopy is an important tool to study the nature of HF etched silicon. Reduction of oxidation is con rmed in three different textured solutions and it is shown in the IR spectrum of Figure.7. The peak position of Si-O-Si bonds at 1107 cm − 1 and its re ectance are decreased in Figure.7