Technical Field of the Invention

The invention relates to a method of texturing the monocrystalline silicon wafer surface developed for silicon solar cells by etching at room temperature.

State of the Art of the Invention (Prior Art)

Surface texturing of silicon wafers is one of the most critical processes in terms of increasing the efficiency of solar cells. There are many techniques and methods applied in this regard. Among the methods used, the method, which is generally prepared around 70 - 80°C and where basic chemicals such as potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) are used, is widely used. Pyramids are formed on the surface of monocrystalline silicon wafers with the etching due to orientation with the said basic chemicals in this method. The biggest feature of basic chemicals is that they depend on the crystal orientation during etching. Therefore, the orientation of the silicon wafers used is important and silicon wafers are generally (100) used in solar cell applications.

Meanwhile, solution price performance is very important in these methods, whose main purpose is to minimize surface reflectance. The spectral reflectance value is reduced to below 11% with pyramid formation with basic chemicals. The average height of the pyramids formed on the surface of silicon wafers is 4 pm and the base is 4 pm.

Metal assisted etching methods are also used to texture the surface of silicon wafers in addition to basic chemicals. Metal salts or metals and oxidants and reducing chemicals are generally used in metal assisted etching (MAE) methods. Basically used metals: silver, gold, platinum, copper, palladium and their salts. If direct metals are used, the desired metal is grown by thermal evaporation or sputtering techniques on the silicon surface. Then, surface shaping is performed by photolithography and surface etching is performed in the solution containing oxidizing and reducing agents. The structures formed can vary from nanometer size to micrometer size. Generally used oxidants are: hydrogen peroxide (H2O2), nitric acid (HNO3), AgNCh, Fe(NO)3 and Ni(NO)s, and the most commonly used chemical as reducing agents is hydrofluoric acid (HF).

Various structures were produced by copper assisted chemical etching method for texturing monocrystalline silicon solar cells including nanoporous, inverted pyramid, V-groove, steep pyramid, and hybrid structures in the study conducted by Xiaolong Du et al. It is mentioned that the surface of monocrystalline silicon wafers is formed on the silicon surface with the copper assisted chemical etching method in the experimental part of the study. The etching medium contains different amounts of Cu(NO3)2, HF and H2O2 in the single-step copper assisted etching method. The copper assisted chemical etching method takes place at a temperature close to room temperature (30°C). Copper nanoparticles are formed on the silicon surface during the etching period.

The nano-texturing of a silicon surface was obtained by one-step copper assisted chemical etching (CACE), which offers a simple approach for the large-scale production of inverted pyramid textured silicon surfaces in the study conducted by Altyeb Ali Abaker Omer et al. The effects of H2O2 concentration, etching time and reaction temperature on the inverted pyramid-like structure and anti-reflection ability were systematically investigated. The results show that the lowest average reflectivity (4.3%) in the wavelength range of 300-1000 nm was obtained under optimum conditions of 0.06 mol/L copper nitrate, 3 mol/L H2O2 concentration and 2 mol/L hydrofluoric acid (HF) for 5 minutes at 60°C.

The invention relates to a low cost method for preparing a black silicon structure and belongs to the field of photoelectric technology in patent document CN106229386A. The invention uses Ag and Cu bimetal to aid chemical corrosion of the multi-crystalline silicon surface. The invention reduces AgNCL consumption by tens of times, the process is simple and the production cost of black silicon is reduced compared to the existing Ag auxiliary chemical corrosion. The method has great application potential in the preparation of black silicon solar cells with high conversion efficiency. The chemical etching process is carried out at room temperature in the medium of 0.01 mM - 2.0 mM AgNCF, + 1 mM - 100 mM Cu (NCL + 0.1-10 M HF + 0.1-1 M H ₂ O ₂ . Methods of surface texturing of silicon wafers with low cost and high performance suitable for mass production are needed since surface texturing is an extremely critical stage in increasing the efficiency value of silicon cells and reducing optical losses.

Brief Description and Objects of the Invention

The present invention relates to the method of etching the surface of monocrystalline silicon wafers at room temperature which meets the aforementioned needs, eliminates all the disadvantages and provides some additional advantages.

The primary object of the invention is to support etching by forming nanoparticles on the surface of silicon wafers, thereby enabling solar cells to trap more light.

The spectral surface reflection of the silicon wafers was reduced to about 10% by supporting the etching on the surface of the silicon wafers with the invention method carried out at room temperature (16-24°C). The surface reflection obtained after the pyramid created using basic chemicals is approximately 12% and this reflection value has been reduced to lower values with the invention. It is ensured that the solar cell traps more light for this reason.

The invention process can be used at least 10 times with the same efficiency unlike the processes performed with basic chemicals. Due to the fact that the invention takes place at room temperature, the low amount of HNO3 chemical vapor flying through the solution because the temperature is the element that increases the repeatability of the invention.

The chemicals used in the invention method are copper nitrate trihydrate (Cu(NO3)2) (or copper sulfate, copper chloride), nitric acid and hydrofluoric acid. The Cu(NO3)2 used supports etching by forming nanoparticles on the surface of the silicon wafers. The formed silicon oxide layer is etched with the help of hydrofluoric acid while the silicon surface is oxidized with the help of nitric acid. Cu(NO3)2 was preferred for orientational etching on the surface, and HF and HNO3 were preferred for oxidation and etching at room temperature. The invention can form pyramids on the silicon surface at room temperature with the copper assisted chemical etching method. It is very important to reduce both temperature and reflection for mass production lines.

H2O2 is used as an oxidizing chemical in the previous part. HNO3 is used instead of H2O2 in the invention. An effective and efficient oxidation reaction occurs at room temperature and supports orientational etching thanks to the HNO3 used. The cathode reactions, oxidation amounts and shapes created by these two oxidizing chemicals are completely different from each other. The cathode reactions for the two chemicals are shown below.

Cathode reaction with HNO3:

Cathode reaction with H2O2:

The oxidation mechanism on the surface of silicon wafers completely changes due to the complex structure of HNO3 chemical in water. The effective oxidation amount of the silicon surface is completely different for the two solutions mentioned. Both the morphology and reflectance characteristics of the structures formed on the silicon surface are different from each other due to this difference. The oxidizing chemicals used directly affect the etching temperature, etching speed and direction and surface morphology of the process. All structural features of the resulting product change for this reason.

Definitions of Figures Describing the Invention

The figures and related descriptions required to better understand the invention are as follows. Figure 1: SEM image of the surface of the silicon wafer after copper assisted etching at room temperature.

Figure 2: Detailed SEM image of the surface of the textured silicon wafer surface with copper etching at room temperature.

Figure 3: Reflectance graph after etching at room temperature.

Figure 4: Graph of reflectance amounts of standard pyramid and room temperature etching (invention) methods.

Detailed Description of the Invention

Etching of surfaces of silicon wafers and forming pyramids by the copper assisted chemical etching method at room temperature is described only for clarifying the subject matter better and without any limiting effect in this detailed description.

The invention is a method of texturing the monocrystalline silicon wafer surface by etching and it comprises the following steps:

(i) Adding and dissolving 1 - 20 mM inorganic copper salt in 0.5 - 15 M hydrofluoric acid (HF), 0.2 - 10 M nitric acid (HNO3) and deionized water (DI) at room temperature of 16-24°C,

(ii) Immersing the silicon wafer in the solution obtained and keeping it for 1 to 20 minutes,

(iii) Rinsing from its chemicals in deionized water,

(iv) Cleaning of copper nanoparticles formed on silicon wafers with chemicals containing HNO ₃ . In step (i), preferably 3-7 M HF, 1-3 M HNO3 and 3-8 mM copper salt are used, and the reflectance values range from 10% to 20%. More preferably, 5 M HF, 1.5 M HNO3 and 5.8 mM copper salt are used and 10% reflectance value is reached as a result of 10 minutes of etching.

Inorganic copper salt can be copper nitrate trihydrate (Cu(NO3)), copper sulfate or copper chloride.

Firstly, the solution to be used in chemical etching was prepared in the invention. The solution is formed by dissolving 5.4 mM copper nitrate trihydrate (Cu(NO3)) in 90 mL hydrofluoric acid (HF), 35 mL nitric acid (HNO3) and 375 mL deionized water (DI). Mechanical shaking methods such as mixing, foaming etc. should be used in order to mix CU(NO3) salt in the solution. After the solution is prepared, silicon wafers are immersed in the solution and kept for 1 to 20 minutes. The waiting time is sufficient for pyramid formation on the surface. Afterward, the silicon wafers are removed from the solution and rinsed with deionized water. Copper nanoparticles are formed on the silicon surface during the etching period. These nanoparticles dissolve in the solution and again adhere to the surface, allowing the etching to continue depending on the crystal orientation. The half-cell reactions of etching are as follows:

Cathode reactions;

Anode reaction;

HNO3 is reduced in metal and copper ions are reduced by taking electrons from the silicon surface while giving h ⁺ to the system in cathode reactions. The silicon surface is oxidized and dissolved, so that etching occurs spontaneously and continuously in the anode reaction. It accelerates the etching by supporting it in the area where the metal is located in other metal assisted etching methods. Metal dissolution and growth by nucleation reactions in the solution are not successive continuous reactions. But copper nanoparticles continuously grow by nucleation and dissolved on the silicon surface in copper assisted etching. Different intensities of growth occur in different orientations depending on the surface energy of the silicon in each successive growth reaction. Pyramid-like structures are formed on the surface thanks to this orientational growth reaction.

A general SEM image of the surface of the silicon wafer after copper assisted etching is shown in Figure 1. The peaks and bases of the pyramids formed randomly on the surface can be seen from this figure.

Figure 2 shows a closer SEM view of the surface of the silicon wafer. One of the biggest differences between the standard pyramid surface and the invention pyramid surface is that the density of sharp pyramid peaks on the surface is fairly low as can be seen from this image. Therefore, electrically reduced recombination losses and cells with higher performance are obtained in solar cell application. The invention provides superiority over the solutions (TMAH, KOH) used for other pyramid formation with this feature. The bright parts that appear on the SEM image in Figure 2 are copper nanoparticles growing on the surface. HNO3:H ₂ O (between 1:1 and 1:10) solution is used to clean these particles from the surface after etching.

The most important measurement method after the surface texturing of silicon wafers is the spectral reflectance profile at different wavelengths. Figure 3 shows the reflection graph of the silicon wafer whose surface was etched with solution at room temperature. The result is very close to each other compared to the pyramid created by basic chemicals. The amount of reflection is below 11% on average.

The most important aspect that differs the invention from other methods and techniques is that the silicon surface is textured with metal-assisted etching at room temperature. The orientational growth of copper nanoparticles and orientational etching on the surface were achieved thanks to the HNO3 chemical used in the solution. The common method used in mass production lines is performed by using KOH or TMAH chemicals around 70 - 80°C. The invention has realized the usability of the same solution at least 10 times with the same efficiency again while switching this temperature to the room temperature. A significant decrease in the volatility of the HNO3 chemical at room temperature made reproducible processes possible. Figure 4 shows the reflectance amounts of silicon wafers whose surface is textured with the standard pyramid at room temperature. The invention is particularly superior in terms of low reflectance values in the standard surface texturing below the wavelength of 550 nm.