Crystalline silicon solar cell production

What you must know about polysilicon cell texturing technology

admin on June 16, 2022 0 Comments • Tags: #cleaningandtexturing #crystallinesiliconsolarcell

What you must know about polysilicon cell texturing technology

In the process of preparing crystalline silicon cells, in order to effectively improve the absorption of sunlight on the silicon surface and improve the efficiency of solar cells, it is a relatively effective process to increase the anti-reflection effect by forming uneven pits on the surface of silicon wafers. Studies have shown that the spectral absorption effect of silicon wafers in the wavelength range of 400~1000nm is the best, and the reflectivity of ordinary planar silicon wafers for this spectrum is generally 30%~40%. Therefore, in the production process of solar cells, it is necessary to A pit-like microstructure is made on the surface of the silicon wafer to increase the conversion efficiency of the battery, that is, the textured surface.

The suede has the following advantages: ① Multiple reflections of specific wavelengths of light, effectively improving the absorption efficiency of sunlight on the surface of the silicon wafer, and increasing the short-circuit current ISC; ② Extending the optical path of sunlight in the silicon wafer to increase the photogenerated load The number of carriers: ③At the same time, the uneven textured surface can also increase the P-N junction area, thereby increasing the collection rate of photogenerated carriers; ④Experiments show that prolonging the minority carrier lifetime by the textured surface can improve the long-wave spectral response of the solar cell, namely Red light response.

The polysilicon cell texturing technology has the following requirements:
First, the shape of the suede. It is the size and structural parameters of the single structure of the suede. The size of the single microstructure of the suede is relatively small, and when it is relatively narrow, the optical path of the incident light inside the microstructure will be increased, which will increase the light trapping effect of sunlight and improve the reduction. reflection effect. The suede should be uniform, and the suede should be able to completely cover the surface of the polysilicon, because the better the uniformity of the suede, the smoother the P-N junction formed after diffusion, and the higher the open circuit voltage of the battery: the uniformity of the suede It directly affects the subsequent processing technology, such as the coating time of PECVD, and its uniformity also affects the filling performance of the screen printing process paste and the fragmentation rate of the silicon wafer.

Second, the quality of the suede surface. In the process of cutting damage and texturing of polycrystalline silicon ingots, it will bring various effects on the surface of silicon wafers, such as: cracks, lattice distortion and other surface and sub-surface damages. There will be a large number of dangling bonds of silicon atoms at the damage, and it is easy to stretch the few ions for recombination when the “photoelectric effect” occurs: a large number of dangling bonds will also appear in the dislocation position of some lattices, and the distorted position is not conducive to the current carrying. In addition, a large amount of impurities such as metal ions remain between cracks and dislocations, which will seriously affect the ISC and thus the conversion efficiency of solar cells. Therefore, the prepared Suede to ensure the integrity of the suede.

At present, the texturing methods of polysilicon are mainly divided into dry texturing and wet texturing. Dry texturing mainly includes processing technologies such as mechanical grooving, laser processing, and reactive ion etching; wet texturing also includes acid corrosion and alkaline corrosion. In industrial-grade polysilicon texturing, it is mainly acid corrosion in wet texturing technology. Also called isotropic corrosion.

In the industrial line texturing, it is mainly composed of the following parts: the texturing tank, the water tank, the alkali tank (KOH), the water tank cleaning tank (HF), the water tank, and the drying tank. The actual picture of the texturing machine is shown in Figure 1 , In addition, there are automatic feeders and unloaders at the beginning and end of the texturing machine, which realizes the purpose of industrial automation and saves labor costs. The texturing tank is mainly a mixed solution of HF and HNO; the alkali tank (KOH) is mainly to remove the unreacted HF of the previous stage, and can remove part of the porous silicon; the cleaning tank (HF) is mainly neutralized and has no reaction. It has good hydrophobicity, and it is easier to dehydrate the silicon wafer, which is beneficial to the drying of the silicon wafer in the last step. There will be a water tank in the middle of each tank, mainly to wash off the solution of the previous level, so that it will not bring people to the next tank, thus contaminating the solution of the next tank.

Figure 1 - Physical picture of polysilicon texturing machine
Figure 1 – Physical picture of polysilicon texturing machine

Acid etching mainly uses an acid etching solution mixed with HF, HNO3 and water. At a specific reaction temperature (usually 6~9 °C), the Si-H bond is etched and destroyed by acidity, thereby forming uneven pits on the surface of the silicon wafer. Structured suede. As shown in Figure 2, this is a photo of a polycrystalline silicon wafer that has just been textured under a microscope of 500 times. After measurement, the center value of the textured surface has a reflectance of 28% in the wavelength range of 400~1000nm, and its chemical process can be expressed as ( 6.1) to formula (6.3) to express:

Figure 2 - Surface morphology of suede surface magnified 500 times under microscope after suede making
Figure 2 – Surface morphology of suede surface magnified 500 times under microscope after suede making

3Si+4HNO3==3SiO2+4NO↑+2H2O (1)
Si+4HNO3==SiO2+4NO2↑+2H20 (2)
SiO2+6HF—H2SiF6+2H2O (3)

The basic reaction process is: HNO3 reacts with silicon to generate a dense oxide layer on the surface of the silicon wafer, and prevents the subsequent reaction of HNO3, then the oxide layer reacts with HF and is etched away, then HNO3 will continue to interact with silicon again. The tablet reacts, allowing the reaction to continue. In fact, the actual reaction process is quite complicated. Some experimental studies have shown that the NO and NO2 generated in the reaction will dissolve in water and generate very unstable HNO2. HNO2 is hydrolyzed to generate NO+ with strong activity. Nitrogen is positive trivalent, and these trivalent nitrogens It plays a leading role in the oxidation process of the silicon surface.

Read more: What is a polysilicon ingot?

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