CN111933750A - Preparation method of thermal oxidation alkali polishing SE-PERC solar cell - Google Patents

Abstract

The invention relates to the field of SE-PERC solar cell production. A method for preparing a SE-PERC solar cell by thermal oxidation alkali polishing. A textured surface is formed on both sides of a silicon wafer; phosphorus is diffused on the surface of the silicon wafer, and the square resistance of the silicon wafer after the phosphorus is diffused is 100-170Ω/sq; Laser heavy doping, after completion, the square resistance difference between the heavily doped area of the silicon wafer and the undoped area is 30-70Ω/sq; a thermal oxide silicon dioxide protective layer is formed on the heavily doped area on the front of the silicon wafer, two The thickness of the silicon oxide protective layer is 1.0-4.0nm; the phosphorous silicate glass on the back of the silicon wafer is removed; the backside of the silicon wafer is alkali-polished; the silicon wafer is oxidized; The backside of the silicon wafer is subjected to laser film opening; the back electrode silver paste and back electric field aluminum paste are printed on the backside of the silicon wafer, and the positive electrode silver paste is printed on the front side; high-temperature sintering is performed to form a silicon-based battery; the silicon-based battery after sintering is completed Perform electrical injection.

Description

A kind of preparation method of thermal oxidation alkali polishing SE-PERC solar cell

technical field

The invention relates to the field of SE-PERC solar cell production.

Background technique

SE-PERC solar cells are one of the most popular high-efficiency cells on the market today, which combine front-side laser heavy doping (SE) with local contact back passivation (PERC) to greatly improve solar cell efficiency. SE-PERC solar cell includes, from bottom to top, back electrode, back electric field, SiNx/SiNxOx stack, P-type silicon, N++ layer, N+ layer, silicon oxide, silicon nitride and positive electrode, the N++ layer is propelled by front-side laser phosphorous silicon Phosphorus in glass (PSG) implementation. The preparation method of the existing SE-PERC battery is mainly an acid etching preparation method, and the preparation process is: texturing-diffusion-front laser-acid etching and polishing+PSG removal-annealing-back deposition passivation film-deposition reduction. Reverse film - back laser opening - back electrode, back electric field and positive electrode printing - high temperature sintering. In the preparation process of SE-PERC cells, although the process of acid polishing is simple, it has low reflectivity, large light projection loss, and low conversion efficiency. Therefore, it is necessary to improve the preparation method of SE-PERC cells.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is: how to prevent the increase of square resistance caused by the loose surface phosphorus in the traditional alkali polishing process and the problem of poor silver-silicon contact after electrode printing, so as to further improve the conversion efficiency of the battery.

The technical scheme adopted in the present invention is: a preparation method of thermal oxidation alkali polishing SE-PERC solar cell, which is carried out according to the following steps:

(1) The suede is formed on both sides of the silicon wafer;

Wet etching technology is used to form a textured surface on both sides of the silicon wafer; wet etching technology is used to etch to form a textured surface with an inverted pyramid structure. Preferably, after texturing, the weight of the silicon wafer is reduced by 0.2-0.8g, preferably 0.5-0.6g; after texturing, the reflectivity of the silicon wafer is 8-14%; preferably 10-13%, more preferably 11% -12%; Controlling the reflectivity of the silicon wafer after texturing is beneficial to control the reflectivity of solar cells to sunlight in the later stage, effectively increase the absorption rate of solar cells to sunlight, and improve the conversion efficiency of solar cells.

(2) Phosphorus diffusion on the surface of the silicon wafer;

Phosphorus is diffused on the surface of the silicon wafer through low-pressure diffusion technology; preferably, the square resistance of the silicon wafer after diffusion is 100-170Ω/sq, more preferably 120-140Ω/sq; increasing the surface square resistance of the silicon wafer can reduce the The surface doping concentration can not only improve the short-wave effect of the battery and increase the short-circuit current, but also reduce the dark saturation current and increase the open-circuit voltage caused by surface recombination, and optimize the battery performance.

(3) Laser heavy doping is performed on the front side of the silicon wafer;

The front-side doping is performed using the Maiwei laser; the laser doping increases the doping concentration of the electrode region; reduces the ohmic contact between the silver paste and the silicon wafer, thereby improving the fill factor; and improving the performance of the solar cell. Preferably, after heavy doping, the difference between the square resistance of the heavily doped area of the silicon wafer and the square resistance of the doped area is 30-70Ω/sq, preferably 45-55Ω/sq; the square resistance of the heavily doped electrode area is here The conversion efficiency of solar cells can be improved within the range.

(4) A thermal oxide silicon dioxide protective layer is formed on the heavily doped area on the front side of the silicon wafer;

Through high-temperature thermal oxidation technology, a thermal oxide silicon dioxide protective layer is formed on the heavily doped area on the front side of the silicon wafer, which effectively prevents the increase of square resistance caused by surface phosphorus loosening during the traditional alkali polishing process, and prevents the silver silicon after electrode printing. Poor contact reduces the problem of battery conversion efficiency. Preferably, the thermal oxide silicon dioxide protective layer is formed with a thickness of 1.0-4.0 nm, more preferably 2.5-3.5 nm.

It should be noted that, it is generally believed that the laser heavy doping is to use the phosphorous silicate glass produced in the phosphorus diffusion process as the doping source, and the phosphorous in the phosphorous silicate glass layer is pushed into the depth of the silicon wafer by laser irradiation, thereby forming heavy doping. Impurity region; that is, laser heavy doping is performed directly after phosphorus doping, which can ensure a high concentration of doping source, thereby reducing the square resistance of the heavily doped region to the greatest extent, improving the ohmic contact between the electrode and the substrate, and improving the Solar cell conversion efficiency. In the present invention, after the front-side laser heavy doping is performed, a thermal oxide silicon dioxide protective layer process is added; according to the traditional idea, part of the front-side phosphorous silicate glass will be removed in the process of removing the back-side phosphorous silicate glass, so as to reduce the phosphorus in the laser heavy-doping process. source, which is not conducive to improving the conversion efficiency of solar cells. However, by increasing the thermal oxide silicon dioxide protective layer process in the present invention, a large number of studies have proved that not only does not reduce the square resistance of the heavily doped area (the difference between the square resistance of the doped area and the square resistance of the doped area is 40-60Ω) /sq); at the same time, it also effectively overcomes the problem of loss of phosphorus in the heavily doped area due to alkali polishing in the traditional alkali etching process and improves the square resistance; the efficiency of SE-PERC solar cells is increased by nearly 1%, and an unexpected technology has been achieved. Effect.

Preferably, after the thermal oxide silicon dioxide protective layer is added in the present invention, the square resistance of the laser heavily doped area on the front side of the silicon wafer is 60-80Ω/sq; the thermal oxide silicon dioxide protective layer is added in the present invention before alkali polishing, which is effective The problem of washing off the loose phosphorus on the surface of the laser heavily doped area in the traditional alkali polishing process is prevented; the low square resistance of the heavily doped area is ensured; thus the ohmic contact is improved; and the conversion efficiency of the solar cell is improved.

(5) Remove the phosphosilicate glass on the back of the silicon wafer;

Wherein, HF solution is used to remove the phosphosilicate glass from the back of the silicon wafer; preferably, the HF solution is used to remove the phosphosilicate glass from the back of the silicon wafer; the volume concentration of the HF solution is 3%-9%. The HF solution can quickly remove the phosphosilicate glass on the back of the silicon wafer; it can prevent the silicon wafer from being damaged due to too long reaction time.

(6) Alkali polishing the back of the silicon wafer;

The KOH solution containing additives is used to polish the backside of the silicon wafer; preferably, the backside of the silicon wafer is polished in a polishing tank filled with a KOH (containing additive) solution; further preferably, the volume concentration of the KOH solution is 5%- 10%, more preferably 6-8%. KOH solution is alkaline, and the polishing effect is more obvious.

Preferably, the weight reduction of the silicon wafer during polishing is 0.1-0.3 g, more preferably 0.12-0.20 g; the reflectivity of the back surface of the silicon wafer after polishing is 35-45%, more preferably 40-55%. Alkali polishing can effectively improve the reflectivity of the backside of the silicon wafer, so that the long-wavelength transmittance is significantly reduced, thereby reducing the transmission loss of light, increasing the current density Jsc, and improving the conversion efficiency of SE-PERC solar cells.

It should be noted that in the process of preparing SE-PERC solar cells by the traditional acid etching method, the mixed solution of HF and HNO3 is used to polish the back of the silicon wafer; however, the back reflectivity after polishing is low, generally lower than 30% ; Reduce the absorption of sunlight, reduce the current density, and reduce the conversion efficiency of solar cells. In the present invention, the KOH solution containing additives is used for polishing, which can effectively improve the reflectivity of the back surface of the solar cell.

(7) Oxidize the silicon wafer;

The oxidation treatment is carried out by using thermal oxygen; preferably, the temperature after oxidation is controlled at 500-800°C; more preferably, it is 650-750°C. The high-temperature oxygen supply process can effectively oxidize the silicon wafer and play a passivation role, thereby reducing the junction recombination, increasing the open circuit voltage, and improving the product yield.

(8) Deposit a silicon nitride film on the front side of the silicon wafer;

Wherein, the anti-reflection film is a silicon nitride film; the anti-reflection film can be deposited by PECAD method; preferably, the deposition thickness is 60-90 nm; the front refractive index of the silicon wafer after the anti-reflection film is deposited is 1.5-3.0; The anti-reflection film can effectively improve the absorption rate of solar energy and improve the conversion efficiency of solar cells.

(9) Deposit a passivation film on the back of the silicon wafer;

Wherein, the passivation film is a SiNxOx/SiNx film; the passivation film can be deposited by PECAD; preferably, the deposition thickness is 80-160 nm. The passivation film can effectively reduce the recombination on the back of the silicon wafer, increase the open circuit voltage, and improve the conversion efficiency of solar cells.

(10) Laser film opening on the back of the silicon wafer;

Among them, the backside passivation film is opened by Dier laser to form ohmic contact between aluminum and silicon, the laser power is 20-40W, and the diameter of the opening spot is 20-40µm. (11) Print the back electrode silver paste and the back electric field aluminum paste on the back of the silicon wafer, and print the positive electrode silver paste on the front;

(12) Sintering at high temperature to form a silicon-based battery;

Among them, the firing temperature is 300-900°C, preferably 400-900°C, and preferably 400-850°C.

(13) Electric injection is performed on the silicon-based battery after sintering.

The beneficial effects of the present invention are as follows: the present invention effectively improves the backside reflectivity of the solar cell by using the backside alkali polishing, so that the long-wavelength transmittance is significantly reduced, thereby reducing the transmission loss of light, increasing the current density Jsc, and further improving the SE- Conversion efficiency of PERC solar cells. By adding a thermal oxidation process, the present invention forms a thermally oxidized silicon dioxide protective layer on the heavily doped area on the front side of the silicon wafer, which effectively prevents the increase in square resistance caused by surface phosphorus loosening during the traditional alkali polishing process, and prevents the electrode after printing. Poor silver-silicon contact reduces cell conversion efficiency. Through the preparation process of the present invention, the conversion efficiency of the solar cell can be improved to ≥22.40%.

Detailed ways

Example 1

The preparation method of the high-efficiency SE-PERC solar cell in this embodiment is as follows:

Texturing: 1200pcs P-type silicon wafer is used as the base material, wet etching technology is used to form a textured surface on the surface of the silicon wafer, the weight loss is controlled to 0.46g, and the reflectivity is 12.3%;

Diffusion: Using low-voltage diffusion technology to form a PN junction, the sheet resistance of the silicon wafer after diffusion is 122Ω/sq;

Laser heavy doping is performed on the front side of the silicon wafer, and the square resistance drop of the silicon wafer is controlled to 48Ω/sq;

A thermal oxide silicon dioxide protective layer is formed on the heavily doped area on the front side of the silicon wafer with a thickness of 1.1 nm;

Remove the phosphosilicate glass on the back of the silicon wafer, and remove the PSG layer on the back with an HF solution with a volume concentration of 6%;

Alkaline polishing was performed on the back of the silicon wafer, and KOH/additive solution with a volume concentration of 6.1% was used for back polishing, the weight loss was 0.12g, and the reflectivity was controlled at 39%;

The silicon wafer is oxidized, and the temperature is controlled at 700℃;

A silicon nitride film is deposited on the front side of the silicon wafer, the film thickness is controlled at 72nm, and the refractive index is 2.12;

A passivation film is deposited on the back of the silicon wafer, and the passivation film is SiNxOx/SiNx film with a thickness of 121nm;

The backside of the silicon wafer is laser-opened with Dier laser, the laser power is 30W, and the aperture diameter is 29µm;

The back electrode silver paste and the back electric field aluminum paste are printed on the back of the silicon wafer, and the positive electrode silver paste is printed on the front;

High-temperature sintering is performed to form a silicon-based battery, and the sintering temperature is 820 °C;

The silicon-based cell after sintering is completed for electrical injection.

Example 2

The preparation method of the high-efficiency SE-PERC solar cell in this embodiment is as follows:

Texturing: 1200pcs P-type silicon wafer is selected as the base material, wet etching technology is used to form a textured surface on the surface of the silicon wafer, the weight loss is controlled to 0.53g, and the reflectivity is 12.5%;

Diffusion: Use low-voltage diffusion technology to form a PN junction, and the sheet resistance of the silicon wafer after diffusion is 131Ω/sq;

The laser is heavily doped on the front side of the silicon wafer, and the square resistance drop of the silicon wafer is controlled to 52Ω/sq;

A thermal oxide silicon dioxide protective layer is formed on the heavily doped area on the front side of the silicon wafer with a thickness of 2.2nm;

Remove the phosphosilicate glass on the back of the silicon wafer, and remove the PSG layer on the back with an HF solution with a volume concentration of 6%;

Alkaline polishing was performed on the back of the silicon wafer, and KOH/additive solution with a volume concentration of 6.1% was used for back polishing, the weight loss was 0.15g, and the reflectivity was controlled at 40%;

The silicon wafer is oxidized, and the temperature is controlled at 700℃;

A silicon nitride film is deposited on the front side of the silicon wafer, the film thickness is controlled at 72nm, and the refractive index is 2.12;

A passivation film is deposited on the back of the silicon wafer, and the passivation film is SiNxOx/SiNx film with a thickness of 123nm;

The backside of the silicon wafer is laser-opened with Dier laser, the laser power is 30W, and the aperture spot diameter is 28µm;

The back electrode silver paste and the back electric field aluminum paste are printed on the back of the silicon wafer, and the positive electrode silver paste is printed on the front;

High-temperature sintering is performed to form a silicon-based battery, and the sintering temperature is 820 °C;

The silicon-based cell after sintering is completed for electrical injection.

Example 3

The preparation method of the high-efficiency SE-PERC solar cell in this embodiment is as follows:

Texturing: 1200pcs P-type silicon wafer is selected as the base material, wet etching technology is used to form a textured surface on the surface of the silicon wafer, the weight reduction is controlled to 0.55g, and the reflectivity is 12.3%;

Diffusion: Use low-voltage diffusion technology to form a PN junction, and the sheet resistance of the silicon wafer after diffusion is 157Ω/sq;

Laser heavy doping is performed on the front side of the silicon wafer, and the square resistance drop of the silicon wafer is controlled to 58Ω/sq;

A thermal oxide silicon dioxide protective layer is formed on the heavily doped area on the front side of the silicon wafer with a thickness of 3.8nm;

Remove the phosphosilicate glass on the back of the silicon wafer, and remove the PSG layer on the back with an HF solution with a volume concentration of 6%;

Alkaline polishing was performed on the back of the silicon wafer, and KOH/additive solution with a volume concentration of 6.1% was used for back polishing, the weight loss was 0.19g, and the reflectivity was controlled at 41%;

The silicon wafer is oxidized, and the temperature is controlled at 700℃;

A silicon nitride film is deposited on the front side of the silicon wafer, the film thickness is controlled at 71nm, and the refractive index is 2.13;

A passivation film is deposited on the back of the silicon wafer, and the passivation film is SiNxOx/SiNx film with a thickness of 125nm;

The backside of the silicon wafer is laser-opened with Dier laser, the laser power is 31W, and the aperture diameter is 30µm;

The back electrode silver paste and the back electric field aluminum paste are printed on the back of the silicon wafer, and the positive electrode silver paste is printed on the front;

High-temperature sintering is performed to form a silicon-based battery, and the sintering temperature is 820 °C;

The silicon-based cell after sintering is completed for electrical injection.

The performance of the SE-PERC solar cells in Examples 1-3 was measured, and the results are shown in Table 1.

Table 1

It can be seen from the table that the efficiency of the SE-PERC solar cell in the present invention is above 22.30%. Compared with traditional SE-PERC solar cells, the efficiency has been significantly improved.

Claims (4)

1. A preparation method of a thermal oxidation alkali polishing SE-PERC solar cell is characterized by comprising the following steps: the method comprises the following steps

Firstly, forming a suede on two sides of a silicon wafer;

secondly, performing phosphorus diffusion on the surface of the silicon wafer, wherein the sheet resistance of the silicon wafer after the phosphorus diffusion is 100-170 omega/sq;

thirdly, performing laser heavy doping on the front surface of the silicon wafer, wherein the difference value of the sheet resistance of a heavily doped region and the sheet resistance of an undoped region of the silicon wafer is 30-70 omega/sq;

fourthly, forming a thermal oxidation silicon dioxide protective layer in the heavily doped region on the front surface of the silicon wafer, wherein the thickness of the silicon dioxide protective layer is 1.0-4.0 nm;

fifthly, removing phosphorosilicate glass on the back surface of the silicon wafer;

sixthly, performing alkali polishing on the back of the silicon wafer;

step seven, carrying out oxidation treatment on the silicon wafer;

step eight, depositing a silicon nitride film on the front surface of the silicon wafer;

step nine, depositing a passivation film on the back of the silicon wafer;

step ten, performing laser film opening on the back of the silicon wafer;

printing back electrode silver paste and back electric field aluminum paste on the back of the silicon wafer, and printing positive electrode silver paste on the front of the silicon wafer;

step twelve, performing high-temperature sintering to form the silicon-based battery;

and step thirteen, performing electric injection on the silicon-based battery after sintering.

2. The method for fabricating a thermal alkaline oxide polished SE-PERC solar cell as claimed in claim 1, wherein: in the second step, the sheet resistance of the silicon wafer after phosphorus diffusion is 120-140 omega/sq.

3. The method for fabricating a thermal alkaline oxide polished SE-PERC solar cell as claimed in claim 1, wherein: and in the third step, after the step is finished, the difference value of the sheet resistance of the heavily doped region and the sheet resistance of the undoped region of the silicon wafer is 45-55 omega/sq.

4. The method for fabricating a thermal alkaline oxide polished SE-PERC solar cell as claimed in claim 1, wherein: in the fourth step, the thickness of the silicon dioxide protective layer is 2.5-3.5nm, and the sheet resistance of the laser heavily doped region on the front surface of the silicon wafer is 60-80 omega/sq.