KR930010126B1 - Manufacturing method of solar cell - Google Patents

Abstract

The method for mfg. a monocrystalline silicon-diffused solar cell is characterized by (a) cleaning a P-type silicon wafer, (b) forming an oxide film on the back surface of the wafer by the chemical deposition, (c) diffusing a phosphorus on the whole surface by the use of a solid source, (d) forming a three layered electrode on the whole and back surface of the wafer, (e) making the solar cell by the silicon etch, and (f) antireflection coating it with a silicon nitride to form an antireflection film.

Description

Method of manufacturing single crystal silicon diffusion solar cell

1 is a plan view showing a front metal grid pattern according to the present invention.

2 is a process chart for explaining a method for manufacturing a single crystal silicon diffusion solar cell according to the present invention.

* Explanation of symbols for main parts of the drawings

1 solar cell 2 grid finger

3: pad

The present invention relates to a method for manufacturing a diffusion solar cell using a single crystal silicon wafer, and more particularly, to a method for manufacturing a single crystal silicon diffusion solar cell, which simplifies the process and improves efficiency.

In a conventional energy conversion device (Korean Patent Publication No. 81-1712), after cleaning a P-type silicon wafer, boron is diffused on the back surface to form a P ⁺ layer, and impurities before diffusing phosphorus on the front surface of the wafer. In order to prevent penetration, an oxide film is coated on the back surface, phosphorus is diffused on the entire surface of the wafer, heat treatment is performed, and Ti (titanium), Pd (palladium), and Ag (silver) are vacuum deposited. A grid pattern that occupies about 10% is defined, the anti-reflective coating is performed, and then the grid pattern is exposed again to complete the solar cell through the final electroplating on the exposed grid pattern.

Such silicon-matched solar cell is a kind of PN diode, and when the front surface of the solar cell is exposed to sunlight, electron-hole phases are generated by the light energy of sunlight, and they are separated from each other around the potential barrier in the thermal equilibrium state. When electricity is generated and a load is connected to the solar cell, short-circuit current, open voltage, fidelity, etc. can be measured, so that the efficiency of the solar cell can be known.

However, in the manufacturing of the solar cell as described above, since the area of the metal grid occupies about 10% of the area of the solar cell, only about 90% of the solar cell receives light, thereby reducing the short circuit current. . In addition, in the diffusion process of the whole surface, first pre-diffusion and then heat treatment for 2 to 5 hours to deepen the depth of the joint to increase the dead layer of the surface, thereby reducing efficiency . In addition, as a final process, Ti, Pd and Ag are vacuum-deposited, grid patterns are defined, anti-reflective coating is applied, and the grid patterns are exposed again to finally perform electroplating on the exposed grid patterns. As described above, the above-described process was repeatedly performed twice, and thus, the process was complicated.

The present invention has been made to solve the above-mentioned conventional problems, and the present invention will be described in detail below. First, the theoretical background of a solar cell is described as a solar cell is a kind of diode, when sunlight is exposed to the front surface of the cell, electron-hole phase is generated by the light energy, these electrons and holes are mutually centered around the potential barrier It will separate and generate electricity.

At this time, if the load is connected to the solar cell, the relationship between the current and voltage is

Where I _L is the current flowing through the load, I _Ph is the current generated by the light, Io is the saturated dark current, q is the amount of carrier charge, V _L is the voltage drop across the load resistance, and n is the diode constant. (diode quality factor), K is the Boltzmann constant, and T is the absolute temperature.

Is given by.

Ni represents an intrinsic carrier density, Dn represents a diffusivity of electrons, Dp represents a hole diffusion rate, Na represents an acceptor impurity density, and Nd represents a donor impurity density.

Also, the open voltage (V _DC ) is determined from the condition of L _L = 0

The short-circuit current Is is determined from the condition of V _L = 0.

Isc = I _Ph ---------------------------- (4)

Becomes

In addition, two other important factors of the solar cell, the fill factor and the efficiency n are expressed as follows.

Where Im and Vm are the maximum outputs that can be extracted, Pin is the incident intensity, and Jsc is the short-circuit current density.

In general, the focus of solar cell manufacturing is low cost, high efficiency, and high reliability, so that it is necessary to increase Jsc, Voc, and FF in order to obtain high efficiency.

In order to increase Jsc in Equation (6), the surface of the solar cell absorbs a lot of light, while the reflection must be reduced. To increase Voc, recombination occurs in bulk, auger, surface, and metal. Should be reduced.

In addition, FF can be increased by reducing the series resistance of the solar cell and increasing the shunt resisance.

In order to consider the above matters, the following matters should be carefully designed in the manufacture of solar cells.

end. Base material: any type, doping concentration, direction, thickness, price

I. Front and back diffusion: any method, doping concentration, junction depth

All. Front gap: total electrode area, grid width, distance between grid, metal type, price

la. Surface recombination layer

hemp. Anti-reflection coating

bar. Process-induced effects

The present invention has been made in consideration of the above matters, and the present invention will be described in detail with reference to FIG.

In the embodiment of the present invention, the crystal orientation grown by the floating zone (100) is (100), the resistivity is 0.1 to 0.2Ω-cm, and the 3-inch P-type single crystal silicon wafer having a thickness of about 15 mils. Was selected. The reason for choosing P-type is that when the electron mobility is faster than hollow, and the junction is formed by diffusion process on the front side, Phosphorus diffusion can be processed at lower temperature than boron diffusion, so high temperature process can be avoided. Because there is.

In addition, the reason why the crystal orientation is selected as (100) is that the (100) plane has a wider gap in which impurities can enter than the (110) plane or (111) plane, so that the lattice defects due to the impurity injection are less and the surface state is better This is because the directional etching can be performed by using this process as it is for the load.

In addition, the reason why the resistivity of 0.1 to 0.2 Ω-cm is chosen is that the substrate having such a resistivity can achieve the maximum efficiency, and the wafer thickness is 15 mils. 15mil is most desirable.

As such, after the P-type single crystal silicon wafer is selected, the first process must be performed.

Metal tweezers are boiled in DI water for about 15 minutes and washed. Teflon tweezers, boats, beakers, etc. H ₂ O ₂ : H ₂ O: HCl = 5: 2: 1 to boil for about 2 hours to wash the experiment tools first.

The initial cleaning process of the wafer is as follows.

In order to wash the organic matter attached to the wafer surface, it is washed for about 10 minutes while boiling in the order of TCE, acetone, methanol, and then for about 5 minutes with deionized water. Next, in order to remove the oxide layer on the wafer surface, the solution is washed for about 15 seconds in a HF: H ₂ O: 1: 10 solution, and then again for 5 minutes with deionized water. Thereafter, the wafer surface was washed for about 5 minutes while boiling in a solution of NHOH ₄ : H ₂ O ₂ : H ₂ O = 1: 1: 5, and then washed for 5 minutes with deionized water, and finally the metal impurities on the wafer surface. In order to eliminate the H ₂ O: H ₂ O ₂ : HCl = 5: 2: 1 The solution is washed for about 15 minutes while boiling, and then washed again with deionized water for about 5 minutes, then dried (dry).

Cleaning is very important in the semiconductor process, especially before starting the first process, before the high temperature process, and before applying the metal.

This is because, if this process is wrong, a lot of damage is caused when using a high-quality wafer, which degrades the performance of the wafer. If possible, clean the tubes and rods of furnaces used in high processing.

The next process is to coat the silicon oxide film on the back, which serves as a barrier when diffusing phosphorus on the front. The silicon oxide film is coated by a high temperature oxidation method and a chemical vapor deposition method.

In the high temperature oxidation method, when an oxide film is grown, an oxide film is coated on both the front and rear surfaces of the wafer, and thus the oxide film must be removed on the front surface by using photolithography.

This method is time consuming, expensive, and degrades yield. Therefore, in the present invention, a silicon oxide film having a thickness of about 5,000 kPa is formed only on the back side by chemical vapor deposition.

The next step is a washing step performed before the phosphorus diffusion step.

It should be noted that the wafer should be cleaned during the initial cleaning process as described above, and the step of removing the oxide film should be avoided because there is an oxide film.

The following phosphorus diffusion process diffuses for about 20 minutes at 830 ° C using a solid source without using a liquid source, so that the sheet resistance is adjusted to about 100Ω / ㅁ and the junction depth is 0.2 to 0.25 It is formed to about 탆.

In the present invention, by eliminating the heat treatment process performed after the diffusion process and controlling only the above-mentioned phosphorus diffusion process, the resistance of the wafer surface is made uniform and the bonding depth is reduced, thereby maximizing the dead layer existing on the surface layer. Reduced.

This reduction of the dead layer results in an improvement in short circuit current density.

After the phosphorus diffusion process, the oxide film formed on the entire surface of the wafer should be removed and washed well to form the electrode well before forming the electrode. Wash for about 5 minutes while boiling in H ₂ SO ₄ : H ₂ O ₂ = 1: 1, wash for about 10 seconds in HF: H ₂ O = 1: 10, washed for 5 minutes with deionized water and dried After that, a metal film is immediately formed.

Hereinafter, the process of designing the metal grid will be described. The front metal grid of a solar cell is related to the resistance of the sun and the series resistance.

The series resistance includes substrate resistance, horizontal resistance received when electrons move to the grid, contact resistance between silicon and metal, and metal self resistance.

Therefore, when designing the front grid, it is desirable to reduce the total metal area to increase the area of sunlight and reduce the resistance of electrons by reducing the gap between the metal grids.

In the present invention, unlike the conventional grid pattern, as shown in Figure 1, the grid is designed.

That is, the size of the entire solar cell 1 is 1 cm × cm, the width of the grid finger 2 is 10 μm, the size of the pad 3 is 600 μm × 600 μm, and the grid finger 2 is one solar cell. The total area of the electrode was designed to be about 1% of the area of the solar cell, so that the series resistance was kept low, and the short circuit current was improved by receiving a lot of sunlight.

Since the electrode of the solar cell is coated on the silicon, the electrode metal should make an ohmic contact with the silicon, have good adhesion, and be solderable.

A metal having such a property is a three-layer electrode of Ti / Pd / Ag. Ti has good adhesion with silicon, and Ag has excellent electrical conductivity and is a good soldering material.

Pd is used to prevent Ti and Ag from reacting with each other quickly when wet.

In the present invention, the degree of vacuum is maintained at 10 ⁻⁷ torr or less, Ti is deposited to a thickness of about 500 kW using an electron beam evaporator, and Pd is about 500 kW and Ag is used using a thermal evaporator. It is deposited by a thickness of 1,000.

Thereafter, a photolithography process is performed to form a pattern of the front electrode. After the pattern is formed by a photolithography process, the metal is etched in the order of Ag, Pd, and Ti.

At this time, each metal must be completely etched in each etching process, and each time the etching is completed, it must be cleaned with deionized water.

In the present invention, Ag is performed at NH ₄ OH: H ₂ O ₂ = 60: 20, Pd is performed at HCl: NHO ₃ = 30: 10, and Ti is performed at H ₂ O: HCl: NH ₄ F = 150: 60: 30, respectively. .

After the formation of the front and back electrodes, a final metal thickness of about 8 μm was formed by silver electroplating to increase electrical conductivity and reduce series resistance.

At this time, the current density was maintained at 2.5 mA / cm 2 for about 30 minutes. If the current density is too large, the parts other than the electrode may be plated with silver, so care should be taken, and the solution should be fluid so that the silver is plated uniformly.

Since 12 solar cells were made on the 3-inch wafer as shown in FIG. 1, these solar cells should be separated from each other.

In the present invention, a chemical etching method is performed by a simple and effective method different from the conventional method.

That is, after photo etching, silicon is etched about 10 μm in HF: NHO ₃ : CH ₃ COOH = 44: 26: 29 for about 5 to 10 seconds.

This process has the same effect as that of the conventional oxide film, and at the same time avoids the high temperature process of forming the oxide film, there is an advantage that the process can be simplified and simplified.

After the etching process, the remaining photo resist is removed, and then heat treated for about 15 minutes at 400 ° C. and N ₂ atmosphere to enhance ohmic contact and improve adhesion.

Finally, an antireflective coating is applied, which reduces surface reflectance and reduces recombination at the surface, thereby improving the selectivity of the wavelength region.

As a material, silicon nitride is used, which is relatively easy to process and exhibits excellent reflection characteristics. Since silicon nitride has a refractive index of 2.0 and sunlight is distributed in a range of 4,000 to 6,000 mW, the selective wavelength is about 6,000 mW. It can be seen from the destructive interference condition that the thickness is appropriately 750 Å.

In the present invention, silicon nitride of about 750-800 kPa was coated to finally manufacture the solar cell.

As a result of testing the performance of the single crystal silicon diffusion solar cell of the present invention prepared by the manufacturing method as described above was able to obtain the following conclusions.

As described in detail above, it is understood that high efficiency can be easily obtained while maintaining the process simply by forming the front and rear recombination preventing films in the fabrication process and undergoing a process such as directional etching.

Claims (4)

After cleaning the P-type silicon wafer, an oxide film is formed on the back surface of the P-type silicon wafer by chemical vapor deposition, and phosphorus is diffused using a solid source on the front surface, and the grid area is about 1% of the area of the solar cell. Forming three-layer electrodes on the front and back of the wafer so as to be maintained, and then, a solar cell is made by silicon etching, and then an anti-reflection coating is performed using silicon nitride to fabricate a single crystal silicon diffusion solar cell. . The method of claim 1, wherein the washing is performed for about 10 minutes while boiling in TCE, acetone methanol, and then washed with deionized water for about 5 minutes, and washed for about 15 seconds in a HF: H ₂ O = 1: 10 solution. After washing again with deionized water for about 5 minutes, boiling in a solution of NH ₄ OH: H ₂ O ₂ : H ₂ O = 1: 5 and washing for about 5 minutes, then about 5 minutes with deionized water Single crystal silicon diffusion solar cell, characterized in that it is washed for about 15 minutes while boiling and then dehydrated for about 5 minutes with boiling solution of H ₂ O: H ₂ O ₂ : HCl = 5: 2: 1 Manufacturing method. The method of claim 1, wherein the phosphorus diffusion process using a solid source to diffuse for 20 minutes at 830 ℃ to obtain a uniform sheet resistance of 100 Ω / ㅁ and a thin junction of 0.2 ~ 0.25 ㎛ to reduce the dead layer to reduce the short circuit current A method of manufacturing a single crystal silicon diffusion solar cell, characterized by inducing enhancement. The method of manufacturing a single crystal silicon diffusion solar cell according to claim 1, wherein after forming the front and back electrodes, each solar cell is separated by etching silicon using HF, HNO ₃ , and CH ₃ COOH, which is simple.