JP3838911B2 - Method for manufacturing solar cell element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solar cell element and a manufacturing method thereof.
[0002]
[Prior art]
FIG. 5 is a diagram showing a conventional solar cell element. In FIG. 5, 1 is a silicon substrate containing one conductivity type impurity, 1a is a reverse conductivity type impurity diffusion region formed on the surface portion of the silicon substrate 1, 2 is an antireflection film, 3 is a front electrode, and 4 is a back electrode. It is. The back electrode 4 includes an electrode extraction part 4a made of silver or the like and a current collecting part 4b made of aluminum or the like. The electrode extraction part 4a of the front surface electrode 3 and the back surface electrode 4 is covered with a solder layer 11 in order to facilitate connection with other wiring members (not shown).
[0003]
FIG. 6 shows a method for manufacturing this conventional solar cell element. First, as shown in FIG. 6B, the fine irregularities 1a are formed on one main surface side of the plate-like silicon substrate 1 having one conductivity type as shown in FIG. 6A by the reactive ion etching method or the like. Many are formed to have a rough surface. This is because reflection on the surface of the silicon substrate 1 is reduced as much as possible and light is effectively taken into the silicon substrate 1. The polycrystalline silicon substrate 1 is formed by a reactive ion etching method because it cannot be satisfactorily roughened by wet etching using an alkaline aqueous solution.
[0004]
Next, as shown in FIG. 6C, a region 1 a in which a reverse conductivity type semiconductor impurity is diffused is formed on the surface portion of the silicon substrate 1 exhibiting one conductivity type, and a semiconductor junction is formed in the silicon substrate 1. To do. Further, as shown in FIG. 6D, a notch 1d is provided in the peripheral portion on the other main surface side of the silicon substrate 1 to divide the reverse conductivity type impurity diffusion region 1b. An antireflection film 2 made of a silicon nitride film or the like is formed on one main surface side of the semiconductor substrate 1.
[0005]
Thereafter, as shown in FIG. 5, the front surface electrode 3 is formed on one main surface side of the silicon substrate 1 and the back surface electrode 4 is formed on the other main surface side of the silicon substrate 1. The surface electrode 3 is formed by applying a paste obtained by kneading a silver powder and glass frit to a resin on the antireflection film 2 and baking the electrode material 3 so that the electrode material 3 penetrates the antireflection film 2 and is a main component of the silicon substrate 1. It is formed by bonding to the region 1a in which the reverse conductivity type impurity on the surface side is diffused. The back electrode 4 is applied to the other main surface side of the silicon substrate 1 by applying and baking a paste 4a in which silver powder and glass frit are kneaded in resin and paste 4b in which aluminum and glass frit are kneaded in resin. Baking while diffusing conductive semiconductor impurities in high concentration.
[0006]
As described above, in the conventional solar cell element, after one main surface side of the silicon substrate 1 is roughened, the region 1b in which the reverse conductivity type semiconductor impurity is diffused is formed, and the surface electrode 3 has the reverse conductivity side impurity. It was provided so as to be connected to the diffused region 1b.
[0007]
[Problems to be solved by the invention]
However, the reverse conductivity type impurity diffusion region 1b in this conventional solar cell is formed so that the sheet resistance on one main surface side of the silicon substrate 1 is 60 to 300Ω / □ in consideration of the current collection efficiency. Therefore, the contact resistance with the surface electrode 3 is increased, the series resistance of the solar cell element is increased, and the carrier recombination at the surface electrode 3 portion is increased, so that high efficiency of the solar cell cannot be achieved. It was.
[0008]
The present invention has been made in view of such problems of the prior art, and in a solar cell structure having minute unevenness that obtains a high generated current by low reflection, there is little increase in series resistance, and the electrode side. It is an object to provide a highly efficient solar cell element by suppressing the diffusion of minority carriers and reducing carrier recombination in the ohmic contact electrode portion.
[0009]
[Means for solving problems]
In order to achieve the above object, a method of manufacturing a solar cell element according to claim 1 includes a step of forming a high concentration diffusion region having a sheet resistance of 10 to 40Ω / □ on one main surface side of a silicon substrate, A step of protecting a portion of the high-concentration diffusion region that is under the electrode with a mask, a step of removing the high-concentration diffusion region other than the region protected by the mask while forming fine protrusions by a dry etching method, A step of forming a low-concentration diffusion region having a sheet resistance of 60 to 300 Ω / □ in a region where the fine protrusions are formed on one main surface side, and the high-concentration diffusion region from which the mask has been removed. Forming an electrode.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the invention according to the claims will be described with reference to the drawings.
FIG. 1 is a view showing a solar cell element according to claim 1, wherein 1 is a silicon substrate, 2 is an antireflection film, 3 is a front electrode, and 4 is a back electrode.
[0012]
The silicon substrate 1 has one conductivity type such as p-type, for example, and a fine unevenness 1a is formed on the surface portion to prevent reflection. The fine unevenness 1a prevents reflection at the surface portion of the silicon substrate 1 and effectively incorporates it into the silicon substrate 1, so that the height difference is 2.0 μm or less and the aspect ratio (height of the convex portion / It is desirable that the length of the base of the convex portion is 2 or more. The fine irregularities 1a are formed by a dry etching method such as an RIE method. This RIE method is particularly advantageous when a polycrystalline silicon substrate is used as the silicon substrate 1 because a large number of fine irregularities can be formed uniformly without being affected by the crystal plane orientation.
[0013]
On the surface of the silicon substrate 1, a region 1b containing a reverse conductivity type impurity such as phosphorus is formed with a thickness of about 0.5 μm. This region 1b contains a reverse-conductivity-side impurity so that its sheet resistance is 60 to 300Ω / □. When the sheet resistance of this region 1b is 60 Ω / □ or less, the impurity concentration on the light receiving surface becomes high and the surface recombination on the light receiving surface increases, which is unsuitable because it causes deterioration in solar cell characteristics. Moreover, when the sheet resistance of this area | region 1b is 300 ohms / square or more, the fall of FF by the increase in surface resistance is caused and it is unsuitable. The region 1b containing the reverse conductivity type impurity is cut at the peripheral portion 1d on the other main surface side of the silicon substrate 1 in order to separate the positive electrode and the negative electrode.
[0014]
Of the region 1b containing the reverse conductivity type semiconductor impurity, the region 1c forming the surface electrode 3 contains the reverse conductivity type impurity so that the sheet resistance becomes 10 to 40Ω / □. Thus, when the reverse conductivity type impurity is included so that the sheet resistance of the region 1c to which the surface electrode 3 is connected is 10 to 40Ω / □, the leakage current due to the penetration of the junction is small as shown in FIG. Therefore, the open circuit voltage is improved, and the conversion efficiency of the solar cell is improved. In addition, when a reverse conductivity type impurity is included so that the sheet resistance of the region 1c to which the surface electrode 3 is connected is 10 to 40Ω / □, as shown in FIG. The sheet resistance of the reverse conductivity type impurity diffusion region 1a on the main surface side can be increased, the surface concentration is reduced, so that surface recombination is reduced, the open voltage of the solar cell is improved, and the conversion efficiency of the solar cell is improved. To do. At this time, in order to make the sheet resistance below the electrode 10Ω / □ or less without lowering the characteristics of the solar cell element, a long process is required. This leads to an increase in solar cell manufacturing costs and is not suitable. On the other hand, if it is 40Ω / □ or more, an increase in series resistance due to an increase in contact resistance and a penetration of electrodes due to a shallow junction result in deterioration of the characteristics of the solar cell element, which is unsuitable. FIG. 3 shows an example in which the sheet resistance other than the lower part of the electrode on one main surface side is 20Ω / □.
[0015]
The sheet resistance on the one main surface side of the silicon substrate 1 is adjusted to 10 to 40Ω / □ or 60 to 300Ω / □ by appropriately adjusting the temperature, time, impurity concentration, etc. when the semiconductor impurities are thermally diffused. Adjust it.
[0016]
An antireflection film 2 is formed on the light receiving surface side of the silicon substrate 1. The antireflection film 2 is made of, for example, a silicon nitride film having a thickness of about 80 nm, and the surface thereof has a fine concavo-convex shape that inherits the fine concavo-convex shape of the silicon substrate 1 as it is. The antireflection film 2 is desirably formed by, for example, a plasma CVD method. That is, when the antireflection film 2 is formed by the plasma CVD method, hydrogen is injected into the silicon substrate 1 in the process of forming the film, and the interface state of the polycrystalline silicon grains is lowered to increase the efficiency. Can be planned.
[0017]
A surface electrode 3 is formed in the region 1c where the reverse conductivity type impurity is diffused at a high concentration. The surface electrode 3 is composed mainly of silver, for example. A back electrode 4 is formed on the other main surface side of the silicon substrate 2. The back electrode 4 is composed of, for example, an electrode extraction part 4a mainly composed of silver or the like and a current collecting part 4b mainly composed of aluminum or the like. Furthermore, the electrode extraction part 4a of the front surface electrode 3 and the back surface electrode 4 is covered with a solder layer 5 in order to easily connect a plurality of solar cell elements.
[0018]
In order to obtain the sheet resistance of the diffusion layer, there are a method of obtaining the electric resistance directly from the current-voltage characteristics such as the four-probe method and a method of obtaining the sheet resistance from the depth distribution of the impurity concentration. The depth distribution measurement of the impurity concentration is performed by electrical measurement methods such as CV method and spreading resistance method, and physical measurement methods such as SIMS (secondary ion mass spectrometry) and RBS (Rutherford backscattering method). There is. However, since the physical analysis method is expensive and takes a lot of time for measurement, the four-probe method, which is particularly convenient as a method of electrical measurement, is widely used.
[0019]
Next, a method for manufacturing the above-described solar cell will be described. As shown in FIG. 2A, the silicon substrate 1 is formed with a high concentration phosphorus diffusion layer 2 in which phosphorus is diffused at a high concentration. The high concentration phosphorus diffusion layer 2 has a depth of about 1 μm. This high-concentration diffusion region 1c will be a connection portion of the surface electrode 3 later, and is formed so that its sheet resistance is relatively low, 10 to 40Ω / □.
[0020]
Next, as shown in FIG. 2B, a plasma-resistant resist 12 is printed in the region where the surface electrode of the silicon substrate 1 is to be formed, and as shown in FIG. In the state where the high-concentration phosphorus diffusion layer 2 is protected by the plasma-resistant resist 12, etching is performed to form minute irregularities 1a. The purpose of the step of forming the minute unevenness 1a includes not only the formation of the unevenness 1a but also the removal of the high-concentration phosphorus diffusion layer 1c existing in the photoactive region. Therefore, the etching depth when forming the unevenness 1a requires a high-concentration phosphorus diffusion layer 1c or more on the surface where the unevenness 1a is formed.
[0021]
Next, as shown in FIG. 2D, the resist 13 protecting the electrode lower region is removed. Here, in the high-concentration phosphorus diffusion layer 2 which is the lower part of the surface electrode 3, the minute unevenness 1a is not formed. Therefore, the high concentration phosphorus diffusion region 1c can be easily distinguished from the photoactive region. This surface shape brings about a great merit that alignment is easy in the printing of the surface electrode in a later step. That is, when the paste for forming the surface electrode 3 is screen-printed, it is possible to visually align the screen pattern and the position where the surface electrode of the silicon substrate 1 is formed, and the formation of the surface electrode is very easy. become.
[0022]
Subsequently, as shown in FIGS. 2E to 2G, a pn junction 6 is formed on the silicon substrate 1 and pn separation is performed at the peripheral portion 1d on the other main surface side of the silicon substrate 1, as in the conventional process. After performing, the antireflection film 3 is formed by plasma CVD. Further, a paste for forming the front surface electrode 3 and the back surface electrode 4 is screen-printed and fired by a screen printing method, and after firing, the paste is covered with a solder layer 5 to complete (see FIG. 1). Thereby, the solar cell element including the substrate 1 is completed.
[0023]
In the solar cell element as shown in FIG. 1, the minute unevenness (2.0 μm or less) structure for preventing reflection and the shallowly diffused n-type diffusion layer region 1a greatly affect the short-circuit current. On the other hand, since the high concentration phosphorus diffusion region 1c formed under the surface electrode 3 has a sufficient depth, the surface electrode 3 is less likely to break the pn junction. Furthermore, since the high concentration phosphorus diffusion region 1c formed under the surface electrode 3 has a small surface resistance, the generated carriers can be collected efficiently, which contributes to the improvement of the photoelectric conversion efficiency of the solar cell. be able to.
[0024]
Actually, compared with the solar cell element structure of FIG. 5 which is a product using minute unevenness as the prior art, the photoelectric conversion efficiency is improved by about 1.0% in the solar cell element of FIG. 1 according to the present invention. The
[0025]
【Example】
A substrate made of 15 cm square polycrystalline silicon having a thickness of 300 μm and a specific resistance of 1.5 Ωcm is immersed in a 25% aqueous sodium hydroxide solution and etched 10 μm on one side. Things were made. The sheet resistance of the high concentration phosphorus diffusion layer was 20Ω / □. On the substrate subjected to high concentration diffusion, a photoresist was spin-coated, exposed, and developed to form a mask in the surface electrode region. Next, reactive ion etching was performed on all of the highly diffused substrate and the undiffused substrate to form minute irregularities.
[0026]
Next, phosphorus (P) was diffused so that the sheet resistance of the surface portion of the silicon substrate was 40Ω / □, 70Ω / □, 80Ω / □, and 100Ω / □. Further, pn separation was performed, and an SiN film having a refractive index of 1.2 and a film thickness of 80 nm was formed on one main surface side of the silicon substrate by a plasma CVD method to form an antireflection film. Next, a silver paste was screen-printed on the front surface, and an aluminum paste and a silver paste were screen-printed on the back surface, and baked using a belt furnace. Finally, the silicon substrate is immersed in a solder bath, and the light-receiving surface electrode and the silver electrode on the back surface are coated with solder to form a solder layer.
[0027]
The open circuit voltage was measured for each sample. The result is shown in FIG. In a sample having a high-concentration phosphorus diffusion layer under the electrode, the open-circuit voltage is improved as the diffusion sheet resistance increases. On the other hand, in the sample without the high-concentration phosphorus diffusion layer, the open-circuit voltage with respect to the diffusion sheet resistance decreases at a peak of 70Ω / □. As a result, even in a solar cell structure having minute irregularities (2.0 μm or less) using RIE, the sheet resistance of the photoactive region can be increased by having a high-concentration phosphorus diffusion layer below the surface electrode. It has become possible.
[0028]
Furthermore, the phosphorus concentration of the high-concentration phosphorus diffusion layer under the electrode was changed, and the sheet resistance was changed to 20Ω / □, 40Ω / □, 50Ω / □, and 60Ω / □. The device fabrication procedure was the same as described above, and the diffusion sheet resistance of the photoactive layer was 70Ω / □. The FF for each sample is shown in FIG. From this, it was found that the FF decreases when the sheet resistance at the lower part of the electrode is 50Ω / □ or more.
[0029]
【The invention's effect】
As described above, according to the first aspect of the present invention, the step of forming a high concentration diffusion region having a sheet resistance of 10 to 40Ω / □ on one main surface side of the silicon substrate, and the electrode of the high concentration diffusion region A step of protecting the lower portion with a mask, a step of removing a high-concentration diffusion region other than the region protected by the mask while forming fine protrusions by a dry etching method, and the main surface side of the silicon substrate Forming a low-concentration diffusion region having a sheet resistance of 60 to 300 Ω / □ in a region where fine protrusions are formed, and forming the electrode on the high-concentration diffusion region from which the mask has been removed; Therefore, it is possible to provide a solar cell having a structure having a high-concentration phosphorus diffusion layer below the surface electrode of the solar cell in which minute irregularities are formed while suppressing an increase in the manufacturing cost of the solar cell.
[Brief description of the drawings]
1 is a view showing an embodiment of a solar cell element according to claim 1;
FIG. 2 is a diagram showing an embodiment of a method for manufacturing a solar cell element according to claim 2;
FIG. 3 is a diagram showing the relationship between the sheet resistance and the open circuit voltage except for the lower part of the electrode on one main surface side in the solar cell element according to claim 1 and the conventional solar cell element.
FIG. 4 is a diagram showing the relationship between the sheet resistance at the lower part of the electrode on the one main surface side and the open circuit voltage in the solar cell element according to claim 1;
FIG. 5 is a diagram showing a structure of a conventional solar cell element.
FIG. 6 is a diagram showing a conventional method for manufacturing a solar cell element.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1; Silicon substrate, 1a; Low concentration diffusion layer, 1b; Fine unevenness, 1c; High concentration diffusion region, 1d; Separation part, 2; Antireflection film, 3; Front electrode, 4: Back electrode, 4a; Part, 4b; current collecting part, 5; solder layer

Claims (4)

Forming a high concentration diffusion region having a sheet resistance of 10 to 40Ω / □ on one main surface side of the silicon substrate;
A step of protecting a portion corresponding to the lower part of the electrode of the high concentration diffusion region with a mask;
Removing a high concentration diffusion region other than the region protected by this mask while forming fine protrusions by a dry etching method;
Forming a low-concentration diffusion region having a sheet resistance of 60 to 300 Ω / □ in the region where the fine protrusions are formed on one main surface side of the silicon substrate;
Forming the electrode on the high-concentration diffusion region from which the mask has been removed, and a method for manufacturing a solar cell element. The method for manufacturing a solar cell element according to claim 1 , wherein the mask is formed by screen printing a plasma resistant resist. The method for manufacturing a solar cell element according to claim 1 , wherein a metal mask is used as the mask. Method of manufacturing a solar cell device according to any one of claims 1 to 3, wherein the dry etching method is reactive ion etching.

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