JP2003152205A - Photoelectric conversion element and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a photoelectric conversion element with high photoelectric conversion efficiency by preventing impurities from being redistributed owing to a high-temperature process and preventing characteristics of the photoelectric converting element itself from decreasing. SOLUTION: The manufacturing method for the photoelectric converting element is characterized by forming a 2nd conductivity type impurity diffusion layer on a 1st conductivity type silicon substrate surface by thermally diffusing a 2nd conductivity type dopant, then forming a silicon oxide film on the diffusion layer by using a coating liquid containing a silicon compound, and forming a 1st conductivity type impurity diffusion layer on the reverse surface of the substrate by thermally diffusing a 1st conductivity type dopant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion element and a method of manufacturing the same, and more particularly, to a photoelectric conversion element applicable to the production of a silicon lower photoelectric conversion element of a stacked solar cell and a large area thin crystalline silicon solar cell. The present invention relates to an element and a manufacturing method thereof.

[0002]

2. Description of the Related Art As a photoelectric conversion device for directly converting sunlight into electricity by using an internal photoelectric effect of a semiconductor or the like, single crystal silicon having a single junction, polycrystalline silicon, an amorphous silicon solar cell, etc. are available on the ground. It is used according to various uses.

In the solar cell manufacturing technology, a P-type semiconductor substrate is mainly used. Usually, a PN junction on the light receiving surface is formed by phosphorus diffusion, and a BSF (Back Surfa) on the back surface is formed.
The ce field) and the electrodes are formed by firing using an Al paste. The solar cell thus formed has a photoelectric conversion efficiency of about 16%.

In order to reduce the cost of the photoelectric conversion device, there is a method of using a low-cost substrate, for example, an inexpensive substrate such as a ribbon substrate. However, these substrate qualities are inferior to cast substrates. Further, in order to make the photoelectric conversion device more efficient, there is a method of stacking P-I-N amorphous silicon solar cells to form a photoelectric conversion element.

For example, Japanese Laid-Open Patent Publication No. 59-96777 discloses a laminated solar cell in which an upper amorphous silicon photoelectric conversion element having an N-P structure and a lower crystalline silicon photoelectric conversion element having an N-P structure are directly connected. Is proposed. Further, in JP-A-2-237172, by inserting an ITO film having a thickness of about 250 nm between the upper element and the lower element, a reflectance of about 40% can be obtained near a wavelength of 600 nm. Stacked solar cells have been proposed.

However, in these laminated solar cells, good characteristics cannot be obtained unless the amorphous cell in the upper part is formed into the P-I-N type from the incident side due to the material characteristics. Further, since the photocurrent is rate-controlled by the upper photoelectric conversion element, it is difficult to increase the photoelectric conversion efficiency by these structures.

Further, Japanese Patent Laid-Open No. 4-226084 discloses an upper amorphous silicon photoelectric conversion element having a P-I-N structure and a lower crystalline silicon photoelectric conversion element having a P-N structure (P, microcrystalline silicon: H / N). , Crystalline silicon) is directly connected to a laminated solar cell. Here, in the lower crystalline silicon photoelectric conversion element, an oxide film having a thickness of about 2 nm is inserted between an N-type polycrystalline silicon substrate and a P-type microcrystalline (μc) layer having a thickness of about 10 to 20 nm. This oxide film is formed by immersing the substrate in heated nitric acid. As a result, the level density of the heterojunction interface is reduced by one digit or more.

However, the characteristics of this laminated solar cell are sensitively affected by the level density at the heterojunction interface.
Further, the oxide film formed by heated nitric acid cannot obtain a sufficient interface passivation effect with respect to the polycrystalline silicon substrate. Furthermore, as the oxide film becomes thicker, the fill factor FF of the solar cell is significantly reduced.

[0009]

By the way, in the conventional solar cell manufacturing technique using an N-type substrate, a PN junction is formed by boron diffusion from the front surface of the N-type substrate, and then BSF is formed on the back surface by phosphorus thermal diffusion. To form. Therefore, it is necessary to form a protective film for protecting the surface PN junction layer before the phosphorus thermal diffusion.

Generally, an oxide film formed by a high temperature thermal process such as a dry or wet thermal oxidation method is used as the protective film. Specifically, when forming an oxide film of about 100 nm, a thermal process of about 1000 ° C. is performed.

However, such a thermal process promotes redistribution of boron on the substrate surface, reduces the surface impurity concentration, and deepens the junction depth. FIG. 4 shows the profile of the surface concentration of boron before and after the formation of the wet oxide film and the dry oxide film by the high temperature process. In dry and wet oxidation, it is found that the impurity concentration on the surface is lowered and the junction depth is increased, and the redistribution of impurities occurs.

Such redistribution of impurities causes a problem that the characteristics of the photoelectric conversion element itself are deteriorated. Further, in the case where an upper photoelectric conversion element is stacked on such a photoelectric conversion element via an intermediate layer of an N-type semiconductor layer to form a stacked photoelectric conversion element, boron is re-distributed so that the boron concentration is increased. Below a certain level, the series resistance between heterojunctions due to the electron tunneling effect remarkably increases. as a result,
There is a problem that the fill factor FF of the stacked photoelectric device is reduced (0.7 <FF) and the photoelectric conversion efficiency is reduced. Further, there is a problem that the diffusion length of the substrate minority carriers is reduced by passing through a plurality of high temperature processes, which causes a reduction in photoelectric conversion efficiency of the photoelectric conversion element.

[0013]

According to the present invention, a second conductivity type dopant is thermally diffused on a surface of a first conductivity type silicon substrate to form a second conductivity type impurity diffusion layer, and then, on the diffusion layer. And a method of manufacturing a photoelectric conversion element, comprising forming a silicon oxide film using a coating liquid containing a silicon compound, and thermally diffusing a first conductivity type dopant on the back surface of the substrate to form a first conductivity type impurity diffusion layer. To be done.

Further, according to the present invention, a photoelectric conversion layer having a second conductivity type impurity diffusion layer on the front surface and a first conductivity type impurity diffusion layer on the back surface is provided on the first conductivity type silicon substrate, and the second conversion element is provided. The surface concentration of the conductivity type impurity diffusion layer is 1 × 10 ¹⁹ to 2 ×
Provided is a photoelectric conversion device having a range of 10 ²⁰ cm ⁻³ and a junction depth of 0.1 to 0.5 μm.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for manufacturing a photoelectric conversion element of the present invention, a second conductivity type impurity diffusion layer is formed on the surface of a first conductivity type silicon substrate, and then a silicon compound is contained on the diffusion layer. A silicon oxide film is formed using the coating liquid,
Then, it comprises a step of forming a first conductivity type impurity diffusion layer on the back surface of the substrate.

Here, the first conductivity type means N type, P type, I
It means any of the types, and among them, the N type is preferable. The second conductivity type means any one of N type, P type, and I type, but it is a conductivity type different from the first conductivity type, and P type is preferable.

The silicon substrate may be any substrate which is usually used for photoelectric conversion elements, and for example, a single crystal silicon substrate, a polycrystalline silicon substrate, a microcrystalline silicon substrate,
An amorphous silicon substrate or a silicon substrate partially including the crystal structure thereof can be used. Of these, a polycrystalline silicon substrate is preferable. The silicon substrate may be either a ribbon substrate or a cast substrate. In addition,
The cast substrate is a wafer cut out from an ingot by a cast casting method, and the ribbon substrate is a sheet-shaped wafer drawn from a silicon solution. The size of the silicon substrate is not particularly limited, and silicon wafers of various sizes can be used. Further, it is suitable that the thickness is, for example, about 200 to 400 μm.

As a method of forming the second conductivity type impurity diffusion layer on the silicon substrate, various methods such as ion implantation, plasma doping, vapor phase diffusion, solid phase diffusion and the like can be mentioned. Of these, vapor phase diffusion is preferred. The vapor phase diffusion can be appropriately set depending on the performance of the photoelectric conversion element to be obtained and the like. For example, in a gas atmosphere containing about 10% of a P-type or N-type impurity element, 800 to 1
A method of holding the silicon substrate in the temperature range of about 100 ° C. for about 10 to 60 minutes can be mentioned.

After forming the impurity diffusion layer of the second conductivity type,
A silicon oxide film is formed on this diffusion layer using a coating liquid containing a silicon compound. The coating liquid containing a silicon compound, silicon compound alcohols as a main component _{[R n Si (OH) x} ], ester, those dissolved in an organic solvent alone or a mixture of ketone and the like. The concentration of silicon oxide (SiO ₂ ) in the coating solution is appropriately in the range of 1 to 10 wt%, particularly 1 to 6 wt%. Such a coating liquid is uniformly applied on a silicon substrate and dried / baked. For coating, various methods such as a spin coating method and a doctor blade method can be used. The thickness of the coating solution is 100 to 600 after the thickness of the oxide film after drying / baking.
It is preferable to set it to nm.

For the drying / baking, for example, a silicon substrate coated with the coating solution is introduced into a baking furnace, the ambient temperature is gradually raised, and finally 400 to 500 ° C., preferably 400 to 450 ° C. There is a method of doing. The atmosphere at this time is preferably a nitrogen gas atmosphere, and during the drying / firing, for example, the volume of the heat treatment furnace is about 40.
In the case of about 00 cm ^3, it is suitable to introduce nitrogen gas at 20 liters / minute or less, preferably about 5-10 liters / minute. Further, in the drying / baking, it is suitable to raise the temperature in the furnace at a temperature rising rate of about 10 ° C./minute or less, preferably about 5 ° C./minute. The temperature decreasing rate of the atmosphere after firing is about 30 ° C./minute or less, preferably about 20 ° C./minute or less. Thereby, the denseness of the obtained silicon oxide film can be sufficiently improved, and the occurrence of leakage of the silicon oxide film can be prevented.

Then, a first conductivity type impurity diffusion layer is formed on the back surface of the silicon substrate. When the second conductivity type impurity diffusion layer is formed on the back surface of the silicon substrate in the previous step, the second conductivity type impurity diffusion layer is removed before forming the first conductivity type impurity diffusion layer. It is preferable to expose the first conductivity type silicon substrate. The removal of the second conductivity type impurity diffusion layer can be performed by, for example, dry etching, wet etching, or CMP method. Among them, wet etching using nitric acid and hydrofluoric acid is suitable.

The first conductivity type impurity diffusion layer can be formed by the same method as the second conductivity type impurity diffusion layer. In addition, after the first conductivity type impurity diffusion layer is formed,
The oxide film formed on the conductivity type impurity diffusion layer is preferably removed. Removal can be performed by dry etching,
It can be performed by wet etching or the CMP method. Among them, wet etching using hydrogen fluoride and pure is suitable.

After this step, according to known methods,
A photoelectric conversion element and / or a stacked photoelectric conversion element can be formed by forming a front surface electrode and / or a back surface electrode, stacking another photoelectric conversion element, forming an intermediate layer, and the like.

Further, the photoelectric conversion element of the present invention is mainly constituted by forming a second conductivity type impurity diffusion layer on the front surface of a first conductivity type silicon substrate and a first conductivity type impurity diffusion layer on the back surface thereof. The second conductivity type impurity diffusion layer formed on the surface of the silicon substrate may be formed on only a part of the surface of the silicon substrate, but is preferably formed on the entire surface. The impurity concentration on the surface of this diffusion layer is 1 × 10 ¹⁹ to
Range of 2 × 10 ²⁰ cm ^-3 , junction depth 0.1-0.5μ
It is preferably in the range of m.

The first-conductivity-type impurity diffusion layer formed on the back surface may be formed on only a part of the front surface of the silicon substrate, but is preferably formed on the entire surface. The impurity concentration on the surface of this diffusion layer is 5 × 10 ¹⁹ to 2
× 10 ²⁰ cm ^-3 range, junction depth 0.3-2.0 μm
It is preferably in the range of.

This photoelectric conversion element may be used as a photoelectric conversion device in which electrodes are formed on the front and back surfaces of a silicon substrate, or the electrodes may be provided on either the front surface or the back surface of the silicon substrate, preferably on the back surface. A laminated photoelectric conversion device in which the photoelectric conversion device is formed and another photoelectric conversion element is laminated on the front surface side may be configured. The electrode is a normal conductive film, for example, aluminum,
It can be formed by a single layer film or a laminated film of a metal such as silver or a transparent conductive material such as ITO or SnO ₂ .

The other photoelectric conversion element to be laminated on the photoelectric conversion element of the present invention may be an element having the same structure as that of the present invention, or may be any photoelectric conversion element which is normally used. Good. For example, an element using a polycrystalline, single crystal, amorphous, or microcrystalline semiconductor having a PIN structure as used in the above-mentioned conventional example can be cited. When the photoelectric conversion element has a laminated structure, it is preferable that the photoelectric conversion element is laminated with an intermediate layer interposed. The material, structure, thickness, etc. of the intermediate layer here can be appropriately set according to the characteristics of the photoelectric conversion device to be obtained, but it is preferable that the intermediate layer is transparent, and the photoelectric conversion layers to be laminated are also preferable. A conductive material or a material that can utilize the tunnel effect is preferable so that the elements are connected in series. For example, a selective reflection film having a laminated structure made of two or more kinds of materials having different refractive indexes, a transparent conductive film and the like can be mentioned. Specifically, a single layer film such as a polycrystalline silicon film or a zinc oxide (ZnO) film or a laminated film may be used.

An embodiment of the photoelectric conversion element and the method for manufacturing the same according to the present invention will be described below with reference to the drawings.

The photoelectric conversion device in this embodiment is
As shown in FIG. 1, the upper photoelectric conversion element 11 and the intermediate layer 5
And the lower photoelectric conversion element 12, and the upper photoelectric conversion element 11 and the lower photoelectric conversion element 12 are connected in series in series to form a two-terminal laminated type.

The upper photoelectric conversion element 11 is made of amorphous silicon having a high energy band gap.
It is composed of a P-I-N structure. The intermediate layer 5 is
It consists of a single layer film of zinc oxide (ZnO). The lower photoelectric conversion element 12 is a polycrystalline silicon solar cell.

As shown in FIG. 2, the lower photoelectric conversion element 12 has a crystal surface resistivity of about 1.0 Ω · cm and a thickness of 350 n.
An m-type N-type polycrystalline silicon substrate 3 was used. This substrate 3 is made alkaline (NH ₄ OH: H ₂ O ₂ : H ₂ O ₂ = 1: 1: 5)
And an acidic (HCl: H ₂ O ₂ : H ₂ O = 1: 1: 5) solution at 80 ° C. for 30 minutes and water respectively.

After that, the substrate 3 is introduced into a high temperature boron diffusion furnace, the temperature is gradually raised, and after the temperature is stabilized, 1
Thermal diffusion of boron was performed for 5 minutes. As a result, the substrate 3
Thickness of about 0.3 μm and impurity concentration of about 10 ²⁰ c
A m ⁻³ P-type silicon layer 4 was formed. The substrate 3 was taken out of the diffusion furnace and cooled to room temperature.

Then, a coating liquid in which silicon oxide particles were dispersed in an organic solvent was applied onto the P-type silicon layer 4 on one surface of the substrate 3 by spin coating. At this time, the coating liquid has a silicon oxide particle concentration of 5.4 wt%.
Alcohol solution was used.

As shown in FIG. 3, nitrogen gas was introduced at a rate of 10 liters / minute, the temperature in the furnace was once raised to 90 ° C. at a temperature rising rate of 5 ° C./minute, and the temperature was maintained for 10 minutes to maintain the organic content. The solvent was evaporated well. Then, at a temperature rising rate of 5 ° C./min, 45
The temperature was raised to 0 ° C. and baking was performed for 30 minutes. Further, the temperature inside the furnace was lowered to 100 ° C. at a temperature lowering rate of 20 ° C./minute, and then the substrate 3 was pulled out. As a result, a silicon oxide film 13 having a film thickness of about 500 nm was formed as a protective film.

Next, the silicon oxide film 13 was protected with a resist, and the P-type silicon layer 4 on the back surface of the substrate 3 was removed with mixed acid to expose the surface of the substrate 3. Then, the resist on the substrate 3 was dissolved and removed.

Subsequently, the substrate 3 is transferred to a phosphorus diffusion furnace, and 85
Thermal diffusion of phosphorus was performed using a POCl ₃ source at 0 ° C. to form an N-type silicon layer 2 having a thickness of about 0.32 μm and an impurity concentration of about 10 ²⁰ cm ^{−3 on} the back surface of the substrate 3 as BSF. The silicon oxide film on the front and back surfaces of the substrate 3 is removed, and then the back surface electrode 1 made of Ti / Pd / Ag is vapor-deposited on the back surface of the substrate 3 to a film thickness of about 2 μm and annealed in a nitrogen atmosphere. The lower photoelectric conversion element 12 was completed.

Next, the intermediate layer 5 made of zinc oxide (ZnO) was formed on the lower photoelectric conversion element 12 in an atmosphere of a mixed gas of argon (Ar) and oxygen by an RF sputtering film forming apparatus to have a thickness of 30 nm. Formed by. At this time, the substrate temperature was 200 ° C. and the RF magnetron sputtering power density was 2 W / cm ² . The upper photoelectric conversion element 11 was formed on the intermediate layer 5 by the plasma CVD method. The substrate temperature is kept at about 150 to 180 ° C., the pressure in the film forming chamber is kept at about 0.3 Torr or less, and SiH
₄ (silane), H ₂ (high purity hydrogen), B ₂ H ₆ (diborane), P
An energy of about 30 mW / cm ² or less is supplied to the plasma using H ₃ (phosphine) to form an N-type amorphous silicon semiconductor layer 6 (thickness: about 30 nm) and a hydrogenated I-type amorphous silicon semiconductor layer 7 (thickness: (Approx. 300 nm)
And a P-type amorphous silicon semiconductor layer 8 (thickness: about 12
nm) respectively.

Further, on the P-type amorphous silicon semiconductor layer 7, a transparent conductive film (I
(TO) 9 was formed to a film thickness of 70 nm, and the surface collector electrode 10 made of Ag was formed thereon by electron beam evaporation to complete a tandem photoelectric conversion device.

As described above, when a lower photoelectric element is manufactured by forming a silicon oxide film at a low temperature after boron diffusion and forming BSF by thermal diffusion of phosphorus, conventional conditions under the conditions shown in Table 1 are used. As shown in FIG. 4, the impurity concentration on the surface of the lower photoelectric conversion element can be kept high as compared with the case where the protective film is formed by wet oxidation or dry oxidation.

[0040]

[Table 1]

As a result, the fill factor of the photoelectric conversion element itself is
I was able to improve FF to about 0.80. Further, in the tandem type photoelectric conversion device in which the upper photoelectric element was laminated on this photoelectric conversion element, the fill factor FF could be improved to 0.7 or more. As a result, the conversion efficiency of the stacked solar cell can be significantly improved from the conventional 12% to about 18%.

[0042]

According to the method of the present invention, a silicon oxide film is formed using a specific coating solution as a protective film for the second conductivity type impurity diffusion layer formed on the surface of a silicon substrate. It is possible to form a silicon oxide film at a lower temperature, suppress redistribution of the second conductivity type impurity, and reduce the second conductivity type impurity than a method of forming a silicon oxide film by a commonly used high temperature process. It is possible to obtain a shallow junction in which is highly diffused. Further, the denseness of the silicon oxide film can be sufficiently improved, the intrusion of the first conductivity type impurities in the subsequent diffusion of the first conductivity type impurities can be prevented, and the impurity concentration in the silicon oxide film can be reduced to 0. It can be formed with a high purity of 1 ppm or less, and it becomes possible to prevent leakage of the silicon oxide film.

According to the present invention, the fill factor FF in the photoelectric conversion element can be improved based on the shallow junction in which the impurities of the second conductivity type are diffused at a high concentration, and the photoelectric conversion element with high photoelectric conversion efficiency is provided. can do.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a photoelectric conversion device formed by a method for manufacturing a photoelectric conversion element of the present invention.

FIG. 2 is a process flow for explaining a method for manufacturing a photoelectric conversion element of the present invention.

FIG. 3 is a diagram showing a firing temperature sequence of a silicon oxide film in the method for manufacturing a photoelectric conversion element of the present invention.

FIG. 4 is a boron profile in the photoelectric conversion element of the present invention.

[Explanation of symbols]

1 Back electrode 2 N-type silicon layer 3 N-type polycrystalline silicon substrate 4 P-type silicon layer 5 Middle class 6 N-type amorphous amorphous silicon semiconductor layer 7 I-type amorphous silicon semiconductor layer 8 P-type amorphous silicon semiconductor layer 9 Transparent conductive film 10 Surface collecting electrode 11 Upper photoelectric conversion element 12 Lower photoelectric conversion element 13 Silicon oxide film

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Claims (4)

[Claims] 1. A second-conductivity-type dopant is thermally diffused on the surface of the first-conductivity-type silicon substrate to form a second-conductivity-type impurity diffusion layer, and then a coating liquid containing a silicon compound is used on the diffusion layer. To form a silicon oxide film on the back surface of the substrate.
A method for manufacturing a photoelectric conversion element, which comprises thermally diffusing a conductivity type dopant to form a first conductivity type impurity diffusion layer. 2. The formation of a silicon oxide film by a coating liquid,
In a nitrogen gas atmosphere, the temperature raising and lowering rate is 20 ° C.
Per minute or less, and the drying temperature is 400 to 500 ° C.
The method described in. 3. A photoelectric conversion layer having a second conductivity type impurity diffusion layer on a front surface of a first conductivity type silicon substrate and a first conductivity type impurity diffusion layer formed on a back surface of the first conductivity type silicon substrate. Surface density is 1 × 10 ¹⁹ ~
Range of 2 × 10 ²⁰ cm ^-3 , junction depth 0.1-0.5μ
A photoelectric conversion element having a range of m. 4. The photoelectric conversion layer, wherein the first conductivity type silicon substrate is an N-type silicon substrate and the second conductivity type impurity is boron, and one or more photoelectric conversion elements are further laminated. The photoelectric conversion element described in 1.