Texturing of large area multi-crystalline silicon wafers through different chemical approaches for solar cell fabrication

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Abstract

Surface texturing of silicon can reduce the reflectance of incident light and hence increase the conversion efficiency of solar cells. Comparatively lesser concentrated (10%) standard alkaline (NaOH/KOH) solution does not give good textured multi-crystalline silicon (mc-Si) surface, which could give satisfactory open-circuit voltage. This is due to grain-boundary delineation with steps formed between successive grains of different orientations. In this work an attempt has been made to obtain a well-textured mc-Si surface through three different approaches. The first two are with two different types of acid solutions and the third with concentrated alkaline NaOH. Solutions of HF–HNO3–CH3COOH/H2O system with different concentrations of HF and HNO3 were used for texturing. The results on the effect of texturing of these three solutions on the surface morphology of very large area (125 mm×125 mm) mc-Si wafer as well as on the performance parameters of solar cell are presented in this paper. Attempts have been made to study extensively the surface morphologies of mc-Si wafers in two effective regions of the isoetch curves of the HF:HNO3:diluent's system. Also we studied the reflectance, uniformity, spectral response, short-circuit current, open-circuit voltage, fill factor and dark current–voltage of the cells fabricated using wafers textured with the three different methods. Short-circuit current of the solar cells fabricated using acid-textured wafers were measured to be in the range of 4.93 A. This value is 0.37 and 0.14 A higher than the short-circuit current values measured in the cells fabricated with isotextured and alkaline-textured wafers, respectively.

Introduction

Multi-crystalline silicon (mc-Si) surface texturing is a key issue in fabricating low cost and high-efficiency solar cells in mass production level. The well-established texturing method for mono-crystalline silicon is the anisotropic etching with aqueous solutions of potassium or sodium hydroxide (KOH/NaOH), which results in the surface covered with pyramids. This is due to the difference in etch rates of the 〈1 0 0〉 and 〈1 1 1〉 orientated planes of silicon [1]. The conventional alkali texturing is not so effective for mc-Si where the grains are randomly oriented. Moreover, mc-Si texturing with standard NaOH/KOH solution at comparatively lower concentration (10%) does not give textured surface, which could give satisfactory open-circuit voltage. This is mainly due to the grain-boundary delineation with steps formed between successive grains of different orientations. Several texturing techniques are under investigation, but none have reached the status of mass production for standard screen-printed solar cells [2], [3], [4], [5], [6]. Texturing with concentrated NaOH solution (40%) at high temperature introduces less effect on mc-Si grain boundaries. Although reactive ion etching (RIE) is one of the well established, one out of many techniques under investigation for texturing mc-Si surface for solar cell fabrication [7], [8], it is a complicated processes and requires expensive instrumentation. Another disadvantage of RIE technique is the essential requirement of an additional etching step in order to achieve better cell performance, which ultimately hamper industrial production throughput. On this background, we made an attempt to texture the mc-Si wafers without exposing the etched steps at the grain boundaries and to obtain a near uniform surface. We also examined the viability of this approach for industrial application.

The wet texturing of silicon surface with solutions containing HF/HNO3 leads to isotropical etching resulting in features with rounded structures on surfaces. We have investigated the effect of texturing the mc-Si wafer for solar cell application using solutions chosen from two different regions of the isoetch [9] curves of HF–HNO3–CH3COOH/H2O system. In some regions of isoetch curves of HF–HNO3–CH3COOH/H2O system, huge amount of heat is evolved during rapid etching, which results in polished surface. Solutions belonging to some other regions of the curve generate porous structures on the silicon surface, which ultimately convert into a uniform surface with significant etch pits to capture appreciable percentage of incident light.

An comparative study of the surface morphology of mc-Si wafers treated with solutions from the two different regions of the isoetch curves for silicon (HF:HNO3:diluent's system) as well as with NaOH has been carried out in this work, along with the study of the performance parameters of the fabricated solar cells.

Section snippets

Texturization

p-Type, mc-Si wafers with resistivity of about 0.5–2 Ω cm were used in this experiment. HF–HNO3–CH3COOH solution with varying volume ratios of HF, HNO3, and CH3COOH was employed in acid texturing process of the silicon surface. The etching time in each process was also varied. Seven sets of etching solutions with different ratios of HF, HNO3, and CH3COOH, which fall in the isoetch region, were used to carry out the etching process (Table 1). Another experiment was carried out with HNO3–HF–H2O

Results and discussion

Most of the experiments related to acid texturing were carried out with a HF–HNO3–CH3COOH mixture in volume ratios and etching time as mentioned in Table 1. The composition of the acids in the mixture was such that the etching rate fell in the lower region of the isoetch curves as shown in the Fig. 1. In this region, the etching contour runs parallel to the lines of constant HF. There is an excess of HNO3 and the etch rate is governed by the ability of the HF to dissolve the SiO2 as it is

Conclusion

We have investigated the texturing of mc-Si solar cells with solutions from the two different regions of isoetch curves of HF–HNO3–CH3COOH/H2O system and with NaOH solution of high concentration. Most of the electrical parameters of the acid-textured (HF:HNO3:CH3COOH=2:15:5) mc-Si solar cell are higher than that of isotextured (HF:HNO3:H2O=14:1:5) and NaOH-textured mc-Si solar cells. Surface flatness helps to improve fill factor and open-circuit voltage due to better metal coverage of the front

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    Citation Excerpt :

    Additionally, due to the random orientation of crystals in the mc-Si wafers, they are preferably produced using anisotropic etching rather than isotropic etching [1–3]. Wet texturing employs a mixture solution of nitric acid (HNO3), hydrofluoric acid (HF) and acetic acid (CH3COOH), which has isotropic etching effects on silicon wafers [4–6], and when isotropic etching is used, a reflectance higher than that of monocrystalline silicon wafers is be obtained [7–9]. Additionally, previous studies have reported that metal-catalyzed etching (MACE), which employs noble metal ions, can effectively reduce the reflectance of mc-Si wafers [10–12].

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