Regulated low cost pre-treatment step for surface texturization of large area

for cell Khan Haryana, Article history: Received 30 April 2009 Received in revised form Accepted 8 February 2010 Available online 15 March 2010 Keywords: Conventional pre-treatment process for saw damage removal before texturization of monocrystalline silicon wafers is by higher concentration (6–10%) caustic etch at 50–60 1C. In this paper a novel low cost in removing the mechanically damaged surface and other surface their process often create other productivity problems like higher ARTICLE IN PRESS Contents lists available at ScienceDirect Solar Energy Materials Solar Energy Materials fax: +91 129 2201040. structure on the silicon surface [5,6]. Pre-texturization damage removal process of the silicon helps contaminants from the wafer surface and this process generates better textured surface. However, long and additional steps in repeated reflections [3,4]. In order to enhance the pyramid nucleation, the interfacial energy of silicon/electrolyte should be reduced, so that sufficient wettability for the silicon surface can be achieved. Isopropyl alcohol (IPA) is generally mixed in the solution in order to achieve good uniformity of pyramidal damage etching, defect centers and contaminants on wafer surface contribute to low open circuit voltage (V oc ) by enhancing the cell leakage current. A modified process has been developed by Gangopadhyay et al. [8] to remove these organic and inorganic Reduction of optical losses in single crystal silicon (C-Si) solar cells by surface texturing is one of the important issues of modern silicon photovoltaics. Many researchers [1,2] have shown the selective etching of /100S silicon by using sodium or potassium hydroxide solution. For monocrystalline silicon solar cells, these anisotropic etches are used to form pyramidal structure that can collect the reflected light and trap the light inside the cells by or KOH solution often results in non-uniform or incomplete silicon etching. Ultimately it results in bad texturing, like non- uniform pyramid heights or even no pyramid formation on some wafer areas [3]. Presently most of the C-Si PV industries use thin silicon wafers (160–200mm thickness) and high silicon etching rate of NaOH/KOH pre-treatment solution [7] poses difficulty in monitoring the time duration of damage removal etching. During higher duration of silicon etching, the breakage rate of silicon wafers increases during texturization. For less or incomplete Single crystalline silicon solar cell Industrial low cost process Texturization pre-treatment process NaOH–NaOCl SEM, DIV, LIV analysis 1. Introduction (NaOH) and sodium hypochlorite (NaOCl) solution is reported for industrial large area, high efficiency, single crystalline silicon solar cells. The moderate silicon etching rate of hot NaOH–NaOCl solution generates a better control on removal of damaged surface. This new damage etching process also helps in the formation of optimized pyramidal structure on silicon wafer during texturization. This process is highly suitable for thin starting raw wafers with thicknesses in 160–200mm range used by most of cell manufacturing industries. Substantial reduction of yield loss due to breakage of wafers is achieved by using this modified process. Optimized recipe of this surface texturization process is ascertained by the Scanning Electron Microscopic (SEM) study of front textured surface on non-metallized and metallized areas. Also reflectivity, cell dark and illuminated voltage–current characteristic measurements validate the superiority of this process to the existing one, which finally leads to low cost, improved quality solar cells for any monocrystalline PV industry. this OCl C0 breaks and reacts with silicon to form silicon dioxide (SiO 2 ) and chlorine ion [12]. This way NaOCl oxidizes other surface contaminants present on the silicon wafer. Simultaneously highly reactive chlorine ions (Cl C0 ) react with hazardous metallic impurities and dissolve them in the solution as soluble chlorides. Finally, a higher concentration (20%) of NaOH at 80–82 1C is used in our pretreatment solution instead of 6–8% NaOH solution at 50–60 1C. Concentrated NaOH solution is highly reactive to silicon at more than 80 1C. However, protective SiO 2 layer formed on silicon wafers by the presence of Fig. 5. The SEM micrograph of silver grid finger on emitter surface of the cell fabricated using modified pre-treatment process. solar cell fabricated using the modified pre-treatment process under NaOCl in the etching solution drastically reduces the silicon etching rate by NaOH. So NaOH etches silicon uniformly with less vigorousity to yield a clean silicon surface just before the final texturization step. Also the evolution of chlorine during the process helps as a neutralization agent for sodium ions. The etching temperature range of 80–82 1C is also critical. Below 80 1C etch rate by NaOH becomes very slow and so NaOH becomes unable to etch through the SiO 2 layer formed by NaOCl. Above 82 1C NaOCl decomposes into oxygen and sodium chloride [9] and thus NaOCl fails to produce SiO 2 layer which is responsible in protecting silicon from fast etching by NaOH. This finally leads to undesirable non-uniform surface etching and thickness reduction of textured wafers. The SEM micrographs of the textured silicon surfaces using the modified pre-treatment process are shown in Fig. 3(a–c), respectively, at 1000X, 2000X and 5000X magnifications. Uniform pyramid formation is clearly visible in Fig. 3(a). Adequate and optimized surface silicon etching just before the NaOH texturization step results in much cleaner silicon surface as Wavelength (nm) 0 4 8 12 16 Reflectivity (%) Textured wafer with modified pre-treatment process Textured wafer with old pre-treatment process 200 400 600 800 1000 1200 Fig. 6. The variation of reflectivity over useable wavelength range of emitter surface of the cell fabricated using modified and old pre-treatment process. ARTICLE IN PRESS shown in Fig. 3(b). This silicon etching process facilitates the formation of large number of small pyramids on silicon. Absolutely clean silicon surface along pyramidal edges and the corner of the pyramids after texturization is also observed in higher magnification SEM micrograph in Fig. 3(c). For commercial screen printed solar cell fabrication, lower pyramidal height (o8mm) is preferred because of better metal finger lines coverage over the textured surface. The SEM micrographs are obtained on the front silver grid fingers of the cell fabricated using modified pre-treatment recipe under magni- fications of 2000X and 5000X and are shown in Fig. 4(a) and (b), respectively. The small black spots indicate the silicon portion just beneath the silver metal. The presence of uniform silver on grid finger is evident from Fig. 4(b) and shows a high metal density of the textured sample. The cleanliness of the pre-treatment process is also verified from the clean metal surface in Fig. 4(b). All these SEM studies, thus, clearly indicate excellent surface quality of the textured and final cell surfaces, and thus confirm the suitability of the modified regulated pre-treatment of silicon by the NaOH– NaOCl solution for damage removal prior to the NaOH texturization step. The uniformity of front silver grid coverage over the textured surface is observed in the SEM micrograph shown in Fig. 5. This uniformity contributes to a better fill factor (FF) for the solar cell after proper co-firing of screen printed ohmic contacts. For the textured wafers formed by the old and modified pre- treatment processes, reflectivity measurements are carried out over the usable range of solar spectrum. From the reflectance graph shown in Fig. 6, average reflectivity is found to be reduced from 8.29% in the old to 8.08% in the modified process in the whole wavelength range. This observed marginal decrease in surface reflectance in the wafer textured using the modified pre- treatment process generates a marginal increase in short circuit current of the cell. The illuminated V–I (LIV) characteristics of the cells fabricated using textured wafers with both the old and modified pre- treatment processes are shown in Fig. 7. All the electrical performance parameters corresponding to LIV characteristics are listed in Table 1. This high FF of 0.757 in the modified pre-treated cell supports the theory of the formation of a good front metal coverage on the grid finger lines, thereby ensuring a good metal contact. These values are quite common in our fabricated cells in mass production level. The dark V–I characteristics of the cells are shown in Fig. 8. The lower value of the leakage current of cell fabricated using the modified pre-treatment process as compared to the old process, is responsible for the better V oc here. The high value of shunt resistance also has its own contribution for better FF in this cell. This can offer better electrical performance of solar modules in outdoor applications by generating more ampere–hour of charging of battery. The higher value of V oc establishes better texturization results in our proposed modified recipe. 3 4 5 Maximum delivered 2.25 2.19 0.4 0.6 Cell fabricated using modified pre-treatment process Cell fabricated using old pre-treatment process P.K. Basu et al. / Solar Energy Materials & Solar Cells 94 (2010) 1049–1054 1053 Voltage (mV) 0 1 2 Current (A) Cell fabricated using modified pre-treatment process Cell fabricated using old pre-treatment process 200 4000 600 Fig. 7. The illuminated voltage–current (LIV) characteristics of solar cells fabricated using the modified and old pre-treatment process measured at AM1.5 Global spectrum and 1 SUN intensity. Table 1 Electrical parameters of solar cell fabricated using the modified pre-treatment process solution. Damage removal process Open circuit voltage V oc (V) Short circuit current I sc (A) Fill factor FF Modified pre-treatment process 609 4.87 0.757 Old pre-treatment process 605 4.83 0.750 using NaOH–NaOCl solution and old pre-treatment process using only NaOH power P max (W) Efficiency Z (%) Shunt resistance R sh (O) Series resistance R s (mO) 15.11 29 20.6 14.75 28 22.7 0.4-0.4 -1.2 Voltage (V) -0.2 0 0.2 Current (A) Fig. 8. The dark voltage–current (DIV) characteristics of solar cells fabricated using the modified and old pre-treatment processes. ARTICLE IN PRESS 4. Conclusion The pre-treatment process of texturization of monocrystalline silicon wafers is optimized by the modified process reported here. It helps in controlling surface etching or polishing to generate appropriate pyramidal surface. This process is extremely suitable to thin wafers used in industry for volume production without compromising with cell electrical qualities. Also SEM micrographs and reflectivity studies confirm improved textured surface qualities. This texture uniformity leads to better V oc , FF and finally better cell efficiency to consistently achieve more than 15% efficiency large area industrial solar cell. There is also a drastic reduction in diode leakage current which leads to better module performance in outdoor applications. Acknowledgement Authors would like to thank Prof. (Dr.) V.K. Agarwal, Director and Management of Echelon Institute of Technology, Faridabad, India, for continuous stimulation and support for the present research work. References [1] E.D. Palik, O.J. Glembocki, I. Heard Jr., P.S. Burno, L. Tenerz, Etching roughness for (1 0 0) silicon surfaces in aqueous KOH, J. Appl. Phys. 70 (1991) 3291–3300. [2] Q.B. Vu, D.A. Stricker, P.M. Zavracky, Surface characteristics of (1 0 0) silicon anisotropically etched in aqueous KOH, J. Electrochem. Soc. 143 (1996) 1372–1375. [3] F. Restrepo, C.E. 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