JPS6320025B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a solar cell, and particularly to a solar cell in which the structure of the electrode on the light incident side and the method for forming the same are improved.

(Technical Background and Problems of the Invention) As a method for forming electrodes on the light incident side of a solar cell, a metal such as aluminum, silver, titanium, or nickel is deposited on the entire surface, and the front electrode is formed in a fine comb shape, a mesh shape, or a lattice shape. The first method is photo-etching into shapes, the second method is nickel or silver plating, and the third method is printing a paste containing silver or aluminum and then firing it. has been made into The first method is easy to form fine patterns and is effective in reducing series resistance and improving photoelectric conversion efficiency when the diffusion junction is shallow. However, in order to increase the film thickness, the deposition speed is The drawback was that it was slow and took a lot of time. Although the second method has relatively simple equipment and operation, it is necessary to roughen the substrate surface in order to improve the adhesion of the film, and if mechanical polishing is used for this purpose, processing distortion remains, resulting in poor performance. Although anisotropic etching with alkali etc. can form a well-ordered surface with etching pits, extreme care must be taken to avoid damaging the delicate surface during bond formation, electrode formation, resist application, and mounting on the panel. required handling. In the third printing method, electrodes are formed directly through a screen mask, but a high-temperature firing process is required afterwards.
In order to ensure sufficient conductivity, it had to be quite thick, making it difficult to form fine patterns.

In order to further improve the electrical characteristics of solar cells, the inventors conducted a microscopic analysis of the electrode structure and the condition of the P-n junction surface obtained by conventional methods, and while investigating the relationship with the electrical characteristics, they discovered that recent research has We have found that in order to put the advanced high-efficiency solar cells into practical use, special innovations are needed in the materials and properties of the electrodes. That is, in order to obtain high efficiency, it is first necessary to make the diffusion layer shallow in order to improve sensitivity to short wavelength light. In contrast to such shallow junctions, ordinary electrode metals may sometimes penetrate through the junction due to heat treatment or changes over time. For this reason, an intermediate layer is required between the silicon and the main electrode constituent metal to protect the electrode and bond from diffusion and reactions with water and oxygen in the air, and titanium, palladium, titanium, and platinum are required. Two layers are formed by a vapor deposition method or a sputtering method. Attempts have been made to create such a base electrode using nickel using a plating method or a printing method, but it has been found that these are inappropriate for recent high-efficiency solar cells where the junction depth is 0.5 μm or less. In other words, the bonding depth is 0.2μm.
It has been found that the only base electrode that can be practically used for a thickness of 0.3 .mu.m is a multi-layered one made by the above-mentioned vapor deposition or sputtering method. Next, when the diffusion layer is shallow, the electrical resistance of the surface in the lateral direction increases, resulting in a reduction in efficiency due to the influence of the series resistance component. Therefore, as a countermeasure to this problem, first, a so-called fine electrode is used, in which the electrode structure is made up of an extremely large number of fine lattice-like collections. The above can be easily achieved by applying a microfabrication technique using a photoetching process to the base electrode. Furthermore, in order to lower the series resistance, the resistance of the electrode itself must be lowered. For this purpose, it is necessary to increase the thickness of the electrode. However, it is difficult to layer a thicker layer of electrically conductive material on the base electrode using vapor deposition or sputtering methods.
This was difficult in terms of microfabrication technology, took time, and was problematic in terms of production costs.

Therefore, we proposed a method in which a base electrode is formed by vapor deposition or sputtering, and then a good electrical conductor is laminated by plating. In this example, silver was formed by electroplating and copper was formed by electroplating on the base electrode. Subsequently, as a result of further experiments, the junction depth was optimized to further improve the efficiency of solar cells, and the electrodes were made finer than several tens of microns. When a metal layer was formed on the base electrode by electroplating, the internal We found that the electrodes tend to peel off easily due to stress. This becomes especially noticeable when the plating layer is made thicker in order to lower the electrode resistance. On the other hand, it is also possible to form a plated layer by an electroless plating method based on chemical principles that are not based on electrical phenomena. When an electroless plating layer was formed on the base electrode, extremely strong adhesion was obtained due to the so-called anchor effect, which generates unevenness on the surface unique to electroless plating. However, because there is a limit to the thickness that can be formed, a sufficient reduction in resistance was not observed.

As a result of the above basic experiments, it was predicted that the characteristics and reliability could be significantly improved by combining the advantages of the double glazing method and further carefully examining the underlying electrode.

Further investigation focused on the improved adhesion, and the following findings were obtained. That is, the crystal grain size of the electroplated layer is larger than the crystal grain size of the electroless plated layer. This is consistent with the fact that electroless plating has better adhesion to the silicon base, and it is considered that this is caused by the minute irregularities on the surface that produce the anchor effect.

On the other hand, in consideration of electrical conductivity, it is more preferable that the crystal grain size of the electrode material is larger.

Based on the above knowledge, the inventors conducted experimental research and completed the present invention in order to develop a further improved solar cell and method for manufacturing the same.

(Objective of the present invention) That is, an object of the present invention is to provide a solar cell having a fine electrode with high adhesion to a substrate and excellent electrical properties.

(Summary of the Invention) That is, the present invention provides a solar cell having a base electrode made of titanium and platinum or titanium and palladium on the surface of a semiconductor substrate having a junction, and a good electrical conductor formed on the base electrode. A good conductor consists of a first layer made of a metal with a small crystal grain size and formed in contact with the base electrode, and a metal made of a metal with a crystal grain size larger than the crystal grain size of the metal of this first layer formed on the first layer. The solar cell is characterized in that it has a second layer comprising:

(Embodiments of the Invention) Hereinafter, embodiments of the present invention will be described in detail with reference to FIG.

Figure 3 shows orientation (100), thickness 250μm, specific resistance 10Ω・
1 is a partial cross-sectional view of a solar cell formed using a silicon single crystal of cm. First, phosphorus is deposited onto the P-type CZ silicon semiconductor substrate 21 using Pocl ₃ at 900° C. for 10 minutes, and then sintered in nitrogen gas for 15 minutes. At this time, the surface concentration is 2×10 ²⁰ cm ^-2 and the junction depth is
A 0.2 μm N ⁺ layer 22 was formed. Thereafter, the oxide film on the front surface and the diffusion layer on the back surface are removed, and aluminum paste A-3484 (model name: Engelhardt) is formed on the entire back surface by screen printing. A 200 mesh stainless steel screen was used as the printing screen. Then, when it is fired in the air at 850°C for 20 seconds, the silicon surface layer on the back side becomes alloyed, forming the P ⁺ layer 23.
can be formed. Excess sintered components of the aluminum paste that did not contribute to alloying were removed using a mixed etching solution consisting of hydrochloric acid and hydrofluoric acid to form P ⁺ layer 2.
Expose 3. Subsequently, a titanium film 24 with a thickness of 400 Å and a palladium film 25 with a thickness of 200 Å are formed on the P ⁺ layer 23 as a base electrode 26 at a substrate temperature of 250° C. by vacuum evaporation.

Next, plasma is applied to the entire front surface as an anti-reflection film 33.
A silicon nitride film with a thickness of 700 Å is formed using the CVD method.
The wafer was placed in a parallel plate, capacitively coupled device, the substrate temperature was set at 300°C, a reaction gas of ammonia gas and silane gas, and nitrogen gas as a carrier gas were introduced into the Belgear, and a high frequency power of 50 KHz was applied.
Devotion was performed by inputting 500W. As a result, a uniform film with low reflectance was obtained. Next, spin coat OFPR800 (Tokyo Ohka, product name) positive photosensitive resin at 3000 rpm. Thereafter, pre-bake for 30 minutes in a clean oven maintained at 80°C. Next, a fine electrode pattern with a grid width of 10 μm is exposed to light at 10 mJ/cm ² using a contact exposure method using an ultra-high pressure mercury lamp, and developed with a special developer NMD-3 (Tokyo Ohka, product model name). Wash with water and post-bake at 140°C for 30 minutes. As a result, the pattern opening corresponding to the surface electrode shape is
Form into OFPR-800 layer. Then, through this opening, buffered viscous acid (fluoric acid: ammonium fluoride: water = 1:
3:4) to expose the substrate in the shape of an electrode pattern. Subsequently, a titanium film 27 is applied as a conductive film over the entire surface by vacuum evaporation.
400 Å, palladium film 28200 Å, substrate temperature 140 Å
It is formed while being maintained at a temperature of .degree. C., and is used as the surface-side base electrode 29. Thereafter, the resist layer is removed using acetone.

Next, an electroless copper layer of 0.5 μm is plated as a first electroless plating metal layer 30 on the palladium on the front and back surfaces by an electroless plating method. Metsuki's solution consists of copper sulfate, potassium sodium tartrate, and sodium hydroxide, and is formed using Fehring's solution to which a small amount of formalin is added to form a solution of about 0.5 μm. Next, add 200g each of copper sulfate, sulfuric acid, and chloride ions (hydrochloric acid).
Electricity was applied for 20 minutes at a current density of 1 A/dm ² at 25° C. in a plating solution containing 50 g/dm/dm/dm to form a 5 μm thick electroplated metal second layer 31 on the electroless copper. In this way, even with the same copper component, by reducing the crystal grain size of the first layer and increasing the crystal grain size of the second layer, an electrode with good adhesion and good conductivity can be formed. In this case, since silicon nitride has a plating mask effect, the step of forming a plating mask is not necessary. In this example, the same copper as the electroless plating was used for the electroplating, but the same effect can be expected even if the electroless plating or electroplating is made of different metals such as silver. Furthermore, in order to stabilize the electrode surface, nickel sulfate 30g/, sodium citrate 10g/
, Sodium succinate 20g/, Sodium acetate 20g/, Diethylborazane 3ml/, Methanol 50ml/ mixed with a small amount of stabilizer to bring the pH to 6-7.
An electroless nickel plating layer of 0.5 μm is formed as the third electroless plating metal layer 32 using a plating solution prepared at a liquid temperature of 65° C. As a result, an electrode with extremely high conductivity and excellent adhesion was obtained. When a partial cross section is observed with an electron microscope, the electroless plated layer in contact with the base electrode is an aggregate of extremely small crystal grains of about 1.0 to 0.05 μm, which is about 10 times smaller than the crystal grain size of the upper electroplated layer. It was determined that it was one-times smaller and had improved adhesion. With the conventional electroplated layer only, electrode peeling occurred during microfabrication with a line width of 30 μm, resulting in 30-50% defects during the process, but with the structure and method of the present invention, the defective rate was 0.5%. It was below. In addition, when we measured the series resistance value, we found that
Compared to the 0.08Ω of the conventional method using only electroless plating, the present invention has an improvement of 0.009Ω by an order of magnitude, and the adhesion to the base is improved compared to the conventional method using only electroplating. It's almost twice as good. Reducing the series resistance is expected to improve conversion efficiency, and the characteristics were evaluated after connecting the lead wires.

AM1100m by solar simulator
When evaluated by irradiating with simulated sunlight at W/ ^cm2 , the conversion efficiency was 14.9%, an improvement of about 10% compared to conventional electroplated layers or electroless plated layers. .

As described above, according to the present invention, it is possible to manufacture with high yield a highly reliable solar cell having electrodes that have excellent adhesion, conductivity, and microfabrication properties and can withstand shallow bonding for high efficiency. Summer.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a solar cell according to the present invention. 21...Semiconductor substrate, 22...N ⁺ layer, 23...
...P ⁺ layer, 24, 27...Titanium film, 25, 28
... Palladium film, 26, 29 ... Base electrode, 3
0... Electroless plated metal first layer, 31... Electroless plated metal second layer, 32... Electroless plated metal third layer
Layer 33...Antireflection film.

Claims (1)

[Claims] 1. In a solar cell comprising a base electrode made of titanium and platinum or titanium and palladium on the surface of a semiconductor substrate having a junction, and a good electrical conductor formed on the base electrode, the good electrical conductor is A first layer made of a metal with a small crystal grain size and formed in contact with the base electrode, and a second layer made of a metal with a crystal grain size larger than the crystal grain size of the metal of this first layer and formed on the first layer. A solar cell characterized by having two layers. 2. The solar cell according to claim 1, wherein the metal of the first layer is an electroless plated metal, and the metal of the second layer is an electroplated metal. 3. The solar cell according to claim 2, wherein the second layer of metal is further provided with a third layer made of electroless plating metal. 4. The solar cell according to claims 1 and 2, wherein the main component of the metal in the first layer and the second layer is silver or copper. 5. The solar cell according to claim 3, wherein the main component of the metal in the third layer is nickel. 6. The solar cell according to claim 1, wherein an antireflection film is formed on at least a portion of the surface of the semiconductor substrate where the base electrode and the electrically conductive material are not formed. 7. The solar cell according to claim 6, wherein the antireflection film is made of a material that also serves as a plating mask. 8. The solar cell according to claims 6 and 7, wherein the antireflection film is a silicon nitride film.