Modeling of Surface Reflectance of Acid Textured Multicrystalline Silicon Wafer for Solar Cell Application

Transcription

1 International Journal of Electronics and Computer Science Engineering 1065 Available Online at ISSN Modeling of Surface Reflectance of Acid Textured Multicrystalline Silicon Wafer for Solar Cell Application Firoz Khan 1, Abdul Mobin 2 1 Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu, South Korea 2 Physics of Energy Harvesting Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi , India 2 - mobina@nplindia.org Abstract- This paper deals with the theoretical modeling of surface reflectance of acid textured multicrystalline wafer used in the processing of solar cell to minimize the surface reflectivity and there by enhancing the efficiency of the cell without wasting the wafers and time in optimizing the texturization process for the development of high efficiency solar cell. In the present study, texturization of mc-si has been carried out in acidic solution using different compositions of HF, HNO3 and H3PO4 at different temperatures and for varying etching time ( s). The best results were obtained for wet acidic ice cooled solution of HF:HNO3:H3PO4 :: 10:1:5 when textured for 165 s. Attempts have been made to explain the reflectivity behavior of the textured surfaces of the mc-si wafers on the basis of the scanning electron microscopic (SEM) micrographs and theoretical modeling of reflectivity assuming the textured surface as a part of a hemisphere. It was observed that the average experimental reflectivity decreases as the etching time increases from 45 s to 165 s and attains a minimum value of 18% for the etching time of 165 s. The experimental reflectivity curve for the etching time of 165 s matches very well with the calculated reflectivity curve for h/d ratio of 0.50 in the wavelength range of nm. These techniques are seems to be useful for large area industrial solar cell applications and the developed theoretical modeling software package may be suitably used for optimizing the texturization parameters of mc-si wafers for achieving the minimal reflectance values for increasing the efficiency of solar cell. Keywords Multicrystalline silicon, Solar cell, Texturization, Reflectivity, SEM micrographs, Modeling I. INTRODUCTION Multicrystalline silicon (mc-si) substrates being less expensive compared to monocrystalline silicon substrates, efforts are being made globally to achieve the better efficiency and reliability of mc-si solar cells as close to those of c-si solar cells as possible. For silicon (Si) wafers used for the fabrication of solar cells, any texturing method strives to enhance the absorption of band gap radiations in the active part of the solar cell. Pyramid formation on the surface of <100> single crystalline (c-si) wafers with anisotropic alkaline etching solution is an important and effective means to reduce the reflectivity of the front surface of Si solar cells. However, this technique can not reduce the reflectivity sufficiently for multicrystalline silicon (mc-si) because of the different orientations of the grains and the presence of grain boundaries. Texturization of mc-si substrates for improving the optical confinement in the active region of solar cell is not as simple as in the case of c-si. Texturization of mc-si has been done in acidic as well as in alkaline solutions [1-6]. Significant contribution to acidic texturization of mc-si was made by Nishimoto et al 1999 [1], where they mainly used HF:HNO3:H3PO4 ::12:1:3, HF:HNO3:H3PO4 ::12:1:9 and HF:HNO3:H3PO4- ::12:1:12 solutions for short durations of time and used H3PO4 as a catalytic agent. They studied scanning electron microscopic (SEM) micrographs of textured surface and found that the textured surface is a part of a hemisphere. It is clear from our investigation that the acid textured surfaces are not always parts of hemispheres. After a thorough investigation, we have observed that the parabolic structures are also present in the acid textured surface. Theoretical

2 IJECSE, Volume 2, Number 3 F Khan et al calculation shows that reflectivity of the textured surface starts declining when h/d ratio exceeds 0.2 and keeps on declining as h/d increases further, where D is the diameter and h is the depth of the textured hemisphere [1, 3]. Macdonals et al 2004 [2] studied the texturization of mc-si for application in industrial solar cells using wet acid, masked and mask less reactive ion etching (RIE) methods and found masked RIE method to be the best. Xi et al 2004 [3] also observed that the parts of the hemispherical structure in acid texturization and according to their calculations the reflectivity at 500 nm starts declining when h/d exceeds Jung and Young 2004 [4] made an interesting study of texturing mc-si using ultrasonic vibration during alkaline texturization to improve uniformity. Gangopadhyay et al 2005 [5] used mc-si substrates of size 15 cm 15 cm and textured them in a solution of NaOH NaOCl at 800 C to restrict the step height to < 5 µm and obtained % efficiency in an industrial production line with a yield > 95%. Tsujino et al 2006 [6] used acidic texturization using metallic catalyst and attained 16.6% efficiency on mc-si solar cells having the area of 4 cm2. In the present study, texturization of mc-si was carried out at different temperatures from 0-25 C using the different compositions of HF, HNO3 and H3PO4. It was found that the best control of texturization was obtained when the amount of catalytic agent H3PO4 was restricted to less than 6% and the solution was kept ice cooled. The composition HF:HNO3:H3PO4 :: 10:1:5 was found to be the most suitable for texturization and the process was carried out for 75, 105, 135, 165 and 195 s. Reflectivity (R %) of the textured Si wafers were measured in the wavelength (λ) range of nm. The lowest value of R was obtained when the wafers were textured for 165 s. An attempt has been made to explain the variations in R (%) by taking the SEM micrographs of textured surface, and theoretical modeling of reflectance of the textured surface assuming the surface as a part of a hemisphere, as it has been studied by a number of researchers [1,3]. However, these researchers [1, 3] have calculated the reflectivity at a single wavelength using the above mentioned model, whereas we have calculated the reflectivity for the wavelength ranging from nm for different h/d ratios. The experimentally measured reflectivity curves have been compared with the theoretically calculated reflectivity curves in the above wavelength range. II. EXPERIMENTAL The starting material was 100mm x 100mm multicrystalline silicon wafers lapped on both sides and having Ωcm resistivity, type p conductivity (B doped) and having minority carrier lifetime T 10 µs. Circular wafers of size 50 ± 1mm diameter were laser scribed out of the starting wafer. The circular wafers were chemically cleaned by a method developed by Radio Corporation of America (RCA). Both sides of the cleaned wafers were damage free etched in ice cooled solution of HNO3: HF: CH3COOH:: 3:1:3. The wafers were again RCA cleaned and textured in an ice cooled solution of HF: HNO3: H3PO4:: 10:1:5 for time periods 45, 75, 105, 135, 165 and 195 s. The SEM micrographs under 5000X magnification were taken for each of the textured wafers using SEM model LEO 440. Subsequently, the reflectivity of the textured wafers was measured on Shimadzu Spectrophotometer Model UV-3101-PC in the spectral range of nm. III. Result and Discussion The SEM micrographs of acid textured wafers for (a) 45 s, (b) 75 s, (c) 105 s, (d) 135 s, (e) 165 s and (f) 195 s are shown in Fig.1. Fig 1(a) shows initiation of texturing and the surface roughening at the weakest defect site resulting into some reduction in R. It is observed in Fig.1 (b) that the texturization is nearing completion at one point and R decreases further whereas in Fig 1(c) two points are textured which results in lowering the reflectivity than the case of Fig1 (b). It is further observed in Fig.1 (d) that quite a few points are textured though surroundings are irregular, resulting in an even lower value of R. The largest number of textured points and the surroundings tend to form parts of hemispherical morphology leading to the lowest value of R as shown in Fig. 1(e). Fig.1 (f) depicts the over etched region that has lost some points of texture, though the surroundings are close to parts of hemispheres and therefore the reflectivity increases.

3 1067 Modeling of Surface Reflectance of Acid Textured Multicrystalline Silicon Wafer for Solar Cell Application Figure 1. SEM micrographs (5000X) of acid textured mc-si surfaces in ice cold solution of HF:HNO3:H3PO4:: 10:1:5 for (a) 45 s, (b) 75 s, (c)105 s, (d)135 s, (e)165 s and (f) 195 s. The reflectivity values of the textured silicon surface have been calculated for different h/d ratios using the model given by Nishimoto et al 1999 [1], assuming the textured surface as a part of a hemisphere. The plots of average reflectivity (Ra) based on the following the Eq. (1) are shown in Fig 3.

4 IJECSE, Volume 2, Number 3 F Khan et al R a = 1100 λ = R dλ λ dλ λ = 400 The curves for the experimental reflectivity (%) vs. wavelength (nm) are plotted in Fig.3 along with the calculated reflectivity values for h/d ratios of 0.25, 0.30 and The average reflectivity as shown in Fig. 3 indicates that Ra decreases monotonically with the acidic etching duration upto 135 s and then falls down sharply for the etching duration of 165 s, however, Ra (%) goes up sharply for the etching duration of 195 s. It is clear from Fig.1 that the acid textured surfaces are not always parts of hemispheres [1, 3]. Our thorough and minute SEM micrographic studies of the said samples shows the presence of some parabolic structures on the Figure 2. Plot of experimental reflectivities (in symbols) vs. the wavelenth (nm) of acid textured mc-si for different etching time (in sec). The calculated reflectivities (solid line) vs. the wavelenth (nm) for different h/d ratios using the model [1]. surface when it is deeply and sharply etched. However, the surface morphology depends on the composition of the solution, its temperature and etching duration. In industrial solar cells, the junction depth (xj) is higher ( μm) because contacts are made by screen printing technology, whereas in laboratory solar cell xj is smaller ( μm) because contacts are made by vacuum evaporation of metals. Therefore in industrial solar cells, high energy photons ( nm) are absorbed in the front emitter (0-1.5 μm) of the heavily doped region, so called dead layer of the emitter. The resultant spectral response (SR) in this wavelength region is also very small. On the other hand the low energy photons ( nm) are absorbed close to the back contact of the solar cells and the minority carriers generated face considerable difficulties to reach the p-n junction. Moreover,

5 1069 Modeling of Surface Reflectance of Acid Textured Multicrystalline Silicon Wafer for Solar Cell Application in this spectral range SR and solar intensity are low. Therefore the wavelength regions of nm and nm make rather small contributions to the short circuit current of the cell; as a result, if these two regions are excluded, the acidic texturization of mc-si for 165 s as shown in Fig.2, is quite good for optical confinement of terrestrial solar radiation for mc-si solar cell. However, the textured surface as discussed earlier has been modeled as a part of hemisphere only and not the combination of hemisphere and parabola which is required for an advance study. Using the hemisphere model and following the Nishimoto et al 1999 [1] studies, the reflectivity of the textured surface has been calculated for different values of h/d ratios and the theoretical reflectivity curves have been plotted in the wavelength region of nm. The experimental Ra -curves were compared with the simulated Ra-curves for h/d ratios of 0.25, 0.30 and The average experimental reflectivity for etching time varying from s and 195 s lie between the calculated average reflectivity curves for h/d ratio of 0.25 and 0.30 as shown in Fig.2. The curve for the Figure 2. The average experimental reflectivity Ra (in symbol) of acid textured mc-si as a function of etching time (in sec). The average calculated reflectivity (solid line) for different h/d ratios. average experimental reflectivity for etching time of 165 s is matched with the curve for the average calculated reflectivity for h/d ratio of 0.50 in the wavelength range of nm. It is clear from Fig. 3 that the average calculated reflectivity value remains constant and has the maximum value of 35.8% for the h/d ratio ranging from The average calculated reflectivity decreases for the h/d ratio ranging from and attains the minimum value of 18.7%. On the other hand, it is observed that the average experimental reflectivity decreases for the etching time ranging from s and attains a minimum value of 18% for an etching time of 165 s. The experimental reflectivity curve for the etching time of 165 s matched very well with the calculated reflectivity curve for h/d ratio of 0.50 in the wavelength range of

6 IJECSE, Volume 2, Number 3 F Khan et al nm. The lower value of reflectivity for the experimental curve in the wavelength range of nm may be attributed to the formation of porous silicon layer which acts as an antireflection coating. IV.CONCLUSION A simple method of texturization of mc-si has been developed which gives a minimum reflectivity of % in the spectral range of nm, the most useful wavelength range of solar spectrum for industrial solar cells. The result of R (%) as a function of texturing time has been explained on the basis of the textured surface observed through SEM micrographs. A theoretical modeling software package for simulating the reflectivity of acid textured mc-si based on the Nishimoto et al 1999 [1] studies has been developed and used for plotting the reflectivity curves for different h/d ratios for the wavelength ranging from nm, and also for plotting the reflectivity curves for different wavelengths for the h/d ranging from and found to be in agreement with the experimental reflectivity curves. However, using the developed model, the minimum reflectivity can be obtained for h/d ratio of 0.50 for the wavelength ranging from nm. The technique of texturization of mc-si wafer presented in this paper is useful for large area industrial solar cell applications and the developed theoretical modeling software package may be suitably used for optimizing the texturization parameters of mc-si wafers without wasting the costly wafers and time in the texturization process to achieve the minimal reflectance values for increasing the efficiency of solar cell. V. REFERENCE [1]. Y. Nishimoto, T. Ishihara and K. Namba 1999 Journal of Electrochemical Society, [2]. D.H. Macdonald, A. Cuevas, M.J. Kerr, D. Ruby, S. Winderbaum and A. Leo 2004 Solar Energy, [3]. Zhengiang Xi, Deren Yang, Wu Dan, Chen Jun, Xianhang Li and Duanlin Que 2004 Semiconductor Science and Technology, [4]. Jung M. Kim and Young K. Kim 2004 Solar Energy Material and. Solar Cells, [5]. U. Gangopadhyay, S.K. Dhungel, K. Kim, U. Manna, P.K. Basu, H.J. Kim, B. Karunagaran, K.S. Lee, J.S. Yoo and J. Yi 2005 Semiconductor Science and Technology [6]. K. Tsujino, M. Matsumura and Y. Nishimoto 2006 Solar Energy Material and Solar Cells,