简介概要

Preparation of pyramid-SiNWs binary structure with Ag nanoparticles-assisted chemical etching

来源期刊：Rare Metals2019年第4期

论文作者：Zi-Long Zhang Bo Wang Yu Chen Yun-Hui Tang Xue-Mei Song Qing-Liu Li Hui Yan

文章页码：312 - 315

摘要：It is an important way to improve the efficiency of solar cells by using the special microstructures of surface. In this work, a pyramid-silicon nanowires（pyramidSiNWs） binary structure was prepared on the silicon surfaces with the metal-assisted chemical etching（MACE）method. Scanning electron microscope（SEM） was used to observe the micromorphology of the pyramid-SiNWs binary structure. The formation mechanism of the binary structure was discussed. The role of Ag nanoparticles in MACE is considered to be the template and the catalyzer.The optical reflectivity of the silicon surfaces was studied with ultraviolet-visible（UV-Vis） spectrophotometer.Compared with the flat silicon surface and the simple pyramidal structure, the silicon surfaces with the pyramidSiNWs binary structure achieve a much lower reflectance in a wide range of wavelength. The effect of etching time as a parameter on the reflectivity was also discussed.

稀有金属(英文版) 2019,38(04),312-315

Preparation of pyramid-SiNWs binary structure with Ag nanoparticles-assisted chemical etching

Zi-Long Zhang Bo Wang Yu Chen Yun-Hui Tang Xue-Mei Song Qing-Liu Li Hui Yan

Department of Materials Science and Engineering, Beijing University of Technology

作者简介：*Bo Wang,e-mail:wangbo@bjut.edu.cn;

收稿日期：23 December 2014

基金：financially supported by the National Natural Science Foundation of China (Nos. 51032002 and 11274028);

Preparation of pyramid-SiNWs binary structure with Ag nanoparticles-assisted chemical etching

Zi-Long Zhang Bo Wang Yu Chen Yun-Hui Tang Xue-Mei Song Qing-Liu Li Hui Yan

Department of Materials Science and Engineering, Beijing University of Technology

Abstract：

It is an important way to improve the efficiency of solar cells by using the special microstructures of surface. In this work, a pyramid-silicon nanowires(pyramidSiNWs) binary structure was prepared on the silicon surfaces with the metal-assisted chemical etching(MACE)method. Scanning electron microscope(SEM) was used to observe the micromorphology of the pyramid-SiNWs binary structure. The formation mechanism of the binary structure was discussed. The role of Ag nanoparticles in MACE is considered to be the template and the catalyzer.The optical reflectivity of the silicon surfaces was studied with ultraviolet-visible(UV-Vis) spectrophotometer.Compared with the flat silicon surface and the simple pyramidal structure, the silicon surfaces with the pyramidSiNWs binary structure achieve a much lower reflectance in a wide range of wavelength. The effect of etching time as a parameter on the reflectivity was also discussed.

Keyword：

Ag nanoparticle; Pyramid structure; Silicon nanowires; Binary structure; Reflectance;

Received： 23 December 2014

1 Introduction

The solar cell has become a hot spot of investigation into the energy consumption recent years.Among the investigations,the surface antireflection is considered as one of the most important influencing factors to improve the performance of the optical and optoelectronic devices [ 1, 2, 3] .As we know,the reflectivity of a flat silicon surface is high,which can reach35%-40%with a strong spectral dependence.This will seriously reduce the efficiency of light conversion for the silicon-based solar cells.Many coatings,such as SiO_x,TiO_x or Si_xN_y with the intermediate or gradient refractive indices,are studied to improve the antireflection of the solar cells.All the antireflection coatings of single layer or multiple layers were deposited by different techniques such as the plasma-enhanced chemical vapor deposition,sputtering and sol-gel.However,only the coatings within a limited spectral range and in a special incident angle can reach a good antireflection performance.Besides,these coatings on silicon wafer surfaces have many other problems with the application,such as the thermal mismatch,stability and adhesiveness.For these issues,other approaches,such as the fabrication of surface microstructures,are also proposed to develop the high efficiency solar cells [ 3, 4, 5, 6] .

In recent years,both the pyramidal structure and the silicon nanowires (SiNWs) structure have drawn much attention due to their potential applications in solar cells.The morphology of the surface covered with SiNWs makes it possible to reduce the reflectance of solar cells [ 7, 8, 9, 10, 11, 12] .Metal-assisted chemical etching (MACE) becomes the main method for preparing the SiNWs [ 7, 13, 14] .In this paper,a binary structure via the combination of the pyramids and SiNWs was achieved.Ag particles were used as the metal assistant of MACE.The formation mechanism of the binary structure was discussed.And the reflectivity of the binary structure was measured by ultraviolet-visible (UV-Vis) spectrophotometer.

2 Experimental

The substrates used in experiments were unpolished monocrystalline silicon wafers of N-type (100) with the resistivity of 5-10Ω·cm and the thickness of (200±20)μm.Firstly,the silicon substrates were cut into 20 mm×20 mm pieces and then orderly cleaned by acetone,ethanol and deionized water to remove the absorbed dust and surface contamination on the silicon.Secondly,the silicon pieces were immersed in the diluted hydrofluoric acid (5 wt%) to remove the native oxide layer and rinsed in the deionized water.

The etching process consisted of the following three steps.Firstly,a cleaned silicon substrate was placed in the NaOH (2.5 vol%) solution with isopropyl alcohol (IPA)at 80°C for 30 min to form the pyramidal structure on the surface.Secondly,a thin Ag nanoparticle layer was grown on the surface by immerging into the solution of AgNO₃ and HF for 30 s.Finally,the silicon wafer with Ag nanoparticle film was placed into the solution of HF and H₂O₂ for a certain time at room temperature to fabricate SiNWs on pyramidal structure.During the etching process,the concentrations of HF and H₂O₂ were always fixed at 4.8 and 0.2 mol·L^-1,respectively.After the etching process,the silver on the silicon was removed by the mixture solution of HNO₃ and deionized water in the ratio of 1:1.

The surface reflectivity was measured by the ultravioletvisible (UV-Vis,UV-3101PC) spectrophotometer with the wavelength range of 300-1100 nm.And the morphology of the structure was characterized by scanning electron microscope (SEM,S-4800).

Fig.1 SEM image of surface morphology of silicon wafer treated in NaOH and IPA solution at 85℃for 30 min

3 Results and discussion

SEM image of pyramidal structure on the silicon wafer is shown in Fig.1.Owing to the anisotropic etching of monocrystalline silicon in NaOH solution,the whole wafer surface is covered with pyramidal structure.The size of pyramids on the silicon surface is in the range of about5-20μm.

Before the sequent MACE process on the pyramid surfaces,an experiment of MACE on the flat surface was performed to confirm the parameters of MACE.At the first step,an Ag nanoparticle layer was grown on the silicon surface (Fig.2a).Then,etching step starts.The morphology of the flat silicon surface after the MACE is shown in Fig.2b.SiNWs are successfully obtained after etching in the solution of HF and H₂O₂ for 6 min.The length of SiNWs is 4.8μm,and the diameter of SiNWs is about100 nm (Fig.2c).

Then,the silicon surfaces with pyramidal structure were treated with AgNO₃-HF solution for 30 s to obtain Ag nanoparticle layer.After the deposition of Ag nanoparticle layer,samples were immersed in the etching solution of HF and H₂O₂ for different time of 2,4 and 6 min,respectively.The morphologies of the binary structure of pyramidSiNWs with different etching time are shown in Fig.3a-f.Figure 3a-c is the planform images of the surfaces,and Fig.3d-f is the images corresponding to cross section.For the etching time of 2 min,the oriented SiNWs could be fabricated on every face of the pyramidal structure(Fig.3a).It reveals that the density of SiNWs on top and ridges of a pyramid is lower than that on the planes of the pyramid.The length of SiNWs on the pyramid surfaces is about 1.3μm,and the etching is not thorough.Some SiNWs have a thick diameter (Fig.3d).With the etching time increasing,the distribution of SiNWs on the top and ridges of the pyramid changes obviously.When the etching time increases to 4 min and more seriously to 6 min,the SiNWs on the top of the pyramid begin to tilt and even be wisped together.This change might derive from the increase in the aspect ratio of SiNWs on top of pyramids with the etching time (Fig.3e,f).

Fig.2 SEM images of a layer of Ag nanoparticles on flat surface grown for 30 s,b planform of SiNWs on flat surface and c cross section of SiNWs

Fig.3 SEM images of binary structures with silicon in etching solution of HF and H₂O₂ for a,d 2 min,b,e 4 min,and c,f 6 min

The formation mechanism of pyramid-SiNWs binary structure could be realized as a combination of the etching for pyramidal structure and the MACE process (Fig.4).After the etching step for pyramidal structure,an Ag nanoparticle layer as the template and the catalyzer was deposited on the surface of the pyramidal structure in the AgNO₃-HF solution.The essential of MACE method was explained as a reaction of localized primary cells [ 7, 15] .With the promotion of Ag nanoparticles,the surfaces of pyramids were etched to the nanowires.At the last step,Ag particles were removed with the nitric acid.

Fig.4 Schematic diagram of fabrication for pyramid-SiNWs binary structure

During the etching steps of MACE,the changes of the surface reflection could be easily observed by eyes.The wafer surface becomes black soon after immersing into the etching solution,indicating an excellent antireflection property.Figure 5a shows the reflection spectra of an unpolished silicon wafer,textured silicon wafer with pyramids and the pyramid-SiNWs binary structure with2-min etching time,respectively.Obviously,the surface reflectance of a pyramidal structure reaches a low value of about 10%,which is lower than that of the unpolished surface of Si.For the surface with pyramid-SiNWs binary structure,the reflectance decreases to a much lower value(<5%) compared with the pyramidal structure in a wide range from UV light to infrared.However,an excessive etching will increase the reflectance.Figure 5b shows the reflection spectra of binary structure obtained with different etching time.It reveals that the reflectance slightly increases with etching time.The reason may be for the tilting of the SiNWs with excessive length.

4 Conclusion

A pyramid-SiNWs binary structure was achieved by etching processes with the assistance of Ag nanoparticles.The formation mechanism of pyramid-SiNWs is considered as a combination of the etching for pyramidal structure and the MACE process in which the Ag particle layer works as the template and the catalyzer.The measurements of UV-Vis spectra prove that the pyramid-SiNWs binary structure exhibits a very low reflectance(<5%) in a wide range of wavelength,which may have an advantage in the application for solar cells.

Fig.5 Reflection spectra of a unpolished silicon wafer,pyramid structure and pyramid-SiNWs binary structure etching for 2 min and b binary structure obtained with different etching time