JP2002280582A - Polycrystalline solar cell and polycrystalline film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polycrystalline silicon thin film solar cell good in characteristics and a solar cell which suppresses the recombination of charges in a grain boundary and has high conversion efficiency. SOLUTION: The polycrystalline solar cell having an active layer 12 formed on a polycrystalline silicon substrate 11 is characterized by that the active layer is composed of a polycrystal and a film thickness in a grain boundary 16 of the polycrystal forming the active layer 12 ranges from 0.20 to 0.85 in film thickness at a polycrystal grain center. The active layer is grown in a liquid phase and the polycrystalline silicon substrate 11 is also formed on a metal silicon wafer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell. More specifically, the present invention relates to a solar cell in which thin-film polycrystalline silicon is stacked on a low-cost substrate.

[0002]

2. Description of the Related Art Solar cells have been widely studied as power sources that are system-connected to driving energy sources of various devices and commercial power.

[0003] For a solar cell, it is desired that an element can be formed on a low-cost substrate such as a metal due to cost requirements. On the other hand, silicon is generally used as a semiconductor constituting a solar cell. Among them, single crystal silicon is the most excellent from the viewpoint of efficiency of converting light energy into electromotive force, that is, photoelectric conversion efficiency. However, amorphous silicon is considered to be advantageous from the viewpoint of increasing the area and reducing the cost. In recent years, the use of polycrystalline silicon has been studied for the purpose of obtaining low cost comparable to amorphous silicon and high energy conversion efficiency comparable to single crystal silicon. However, in a method conventionally proposed for such a single crystal or polycrystalline silicon, a lump-shaped crystal is sliced into a plate-shaped substrate.
It was difficult to: Therefore, the substrate has a thickness greater than necessary to sufficiently absorb the amount of light, and the material is not effectively used. That is, in order to reduce the cost, it was necessary to further reduce the thickness.

[0004] As a method of manufacturing a polycrystalline silicon substrate aiming at low cost, there has been proposed a method of forming a silicon wafer by a spin method in which molten Si droplets are poured into a crucible. 0.1 to 0.2 mm, which is sufficient for light absorption as crystalline Si (20 to
(50 μm), the thickness was still not sufficiently reduced. In addition, such thinning makes it difficult for the silicon wafer itself to have the strength as a substrate, which inevitably requires another inexpensive substrate for supporting the silicon wafer. Was.

Therefore, there has been reported an attempt to form a solar cell by forming a substrate made of metal-grade silicon and then forming a silicon layer having a film thickness necessary and sufficient for light absorption on the substrate by a liquid phase growth method ( TFCiszek, THWang, X.Wu, RWBurr
ows, J. Alleman, CRSchwerdtfeger and T. Bekkedahl,
“Si thin layer growth from metal solution on sin
gle-crystal and cast metallurgical-grade multicrys
talline Si substrates ", 23rd IEEE Photovoltaic spe
cialists Conference, (1993) p.65).

However, in the above-described method, a silicon layer is formed on a low-purity silicon substrate made of metal-grade silicon using copper, aluminum or tin as a metal solvent. In order to perform an etch-back for removal, a metal as a solvent is mainly left in crystal grain boundaries, and sufficient characteristics as a solar cell have not been obtained. In order to solve this problem, there has been disclosed a method in which copper and an aluminum alloy are used as a solvent and etching back is not performed (THWang, TFCi
szek, CRSchwerdtfeger, H. Moutinho, R. Matson. “G
rowth of silicon thin layers on cast MG-Si from me
tal solutions for solar cells ". Solar Energy Mater
ials and Solar Cells 41/42 (1996) p.19), and mass production has problems such as complicated control of alloy composition.

[0007] The applicant of the present invention has disclosed in
No. 8205, a method for manufacturing a solar cell including a step of forming a silicon layer on a crystalline silicon substrate by a liquid phase growth method using a metal solvent, wherein the total impurity concentration on the surface of the crystalline silicon substrate is 10 ppm or more. And
The present invention proposes a method for manufacturing a solar cell, wherein the metal solvent is indium.

[0008] G. Ballhorn, KJ Weber, S. Armand, MJ
Stocks, AWBlakers. “High-efficiency multicrystal
line silicon solar cells ”. Solar Energy Material
and Cells 52 (1998) p.61-p.68, when a silicon layer is formed on a polycrystalline silicon substrate by a liquid phase growth method, flat growth is achieved by repeating growth and meltback. It is stated that it can be obtained.

However, in general, in the case of a polycrystalline solar cell, there are many defects at grain boundaries, and charge recombination there is large. In order to improve the conversion efficiency of a polycrystalline solar cell, it is necessary to suppress the recombination of charges at grain boundaries.
Conventionally, for this purpose, a thin film of amorphous silicon or microcrystalline silicon is formed on polycrystalline silicon, and dangling bonds at grain boundaries are terminated by H atoms contained therein to inactivate the grain boundaries, and the grain boundaries are deactivated. Recombination was suppressed.

However, this method imposes great restrictions on the subsequent process conditions in order to prevent the termination of the terminated H atoms. The effect was not enough.

Japanese Patent Application Laid-Open No. 2000-195792 discloses that a buffer layer (silicon oxide film) and an amorphous semiconductor layer (amorphous silicon film) are formed in this order on a substrate, and then the amorphous semiconductor layer is formed. There is disclosed a technique in which a layer is melted by applying energy and then crystallized. The amorphous semiconductor layer is melted by applying energy, and then expands in volume during crystallization, so that remarkable irregularities are formed on the polycrystalline surface. The publication also discloses a technique for accurately flattening the unevenness. That is, the carrier mobility is improved by flattening the irregularities formed in the crystallization step in a subsequent surface treatment (polishing) step.

However, this method requires a step of flattening the polycrystalline surface, which not only causes an increase in manufacturing cost, but also causes cracks or chips in the polycrystalline semiconductor layer in the polishing step. Defects may be formed.

[0013]

SUMMARY OF THE INVENTION An object of the present invention is to provide a thin film polycrystalline silicon solar cell having good characteristics. That is, an object of the present invention is to provide a solar cell having high conversion efficiency by suppressing recombination of charges at grain boundaries.

Another object of the present invention is to provide an inexpensive solar cell.

An object of the present invention is to provide a polycrystalline film capable of achieving high conversion efficiency when used in, for example, a solar cell.

[0016]

According to the present invention, there is provided a polycrystalline solar cell comprising a polycrystalline silicon substrate having an active layer formed on a polycrystalline silicon substrate, wherein the active layer is made of polycrystalline, and Is characterized in that the film thickness at the grain boundary portion of the polycrystal forming the film is in the range of 0.20 to 0.85 with respect to the film thickness at the central portion of the polycrystal.

The polycrystalline solar cell according to the present invention is characterized in that, in the above polycrystalline solar cell, the active layer is formed by liquid phase growth.

In the polycrystalline solar cell according to the present invention, in the above-mentioned polycrystalline solar cell, the thickness of the polycrystalline grain forming the active layer at the grain boundary portion is 0.25 or more and 0 at the central portion of the polycrystalline grain. .75 or less.

The polycrystalline solar cell according to the present invention is characterized in that, in the above polycrystalline solar cell, the polycrystalline silicon substrate is metal-grade silicon containing 10 ppm or more of impurities.

In the polycrystalline film of the present invention, the thickness at the grain boundary is 0.20 or more and 0.85 at the center of the polycrystalline grain.
It is characterized in the following range.

The polycrystalline film according to the present invention is characterized in that the polycrystalline film is formed by liquid phase growth.

In the polycrystalline film of the present invention, the thickness at the grain boundary is 0.25 to 0.75 with respect to the thickness at the center of the polycrystalline grain.
It is characterized in the following range.

The polycrystalline film according to the present invention is characterized in that the polycrystalline film is formed on polycrystalline silicon.

The polycrystalline film of the present invention is characterized in that, in the above polycrystalline film, the polycrystalline silicon substrate is metal-grade silicon containing 10 ppm or more of impurities.

[0025]

The inventor of the present invention has solved the above-mentioned problems in the prior art, and has conducted intensive studies focusing on the structure and charge collection efficiency of the polycrystalline semiconductor layer in order to achieve the above object. To complete the present invention.

One of the major features of the polycrystalline semiconductor layer is the difference in the structure and properties of the central part and the grain boundary part of the crystal grains constituting the polycrystal. That is, when the polycrystalline layer as the active layer is irradiated with light, charges generated at the center of the crystal grain can be taken out to an external circuit as a photocurrent. However, the charge generated in the vicinity of the crystal grain boundary has a large proportion reaching the grain boundary. In general, the crystal grain boundary has many recombination centers of charges having many defects, so that many charges recombine and disappear at this grain boundary portion, and the ratio of being able to be taken out to an external circuit as a photocurrent is reduced. .

On the other hand, the present inventors have found that the thickness of the central part of the crystal grain and the thickness of the grain boundary part can be adjusted to a desired ratio by controlling the growth condition of the polycrystal. . The present inventors have found that by optimizing the balance (that is, the structure of the polycrystalline layer), it is possible to efficiently reduce the charges that recombine at the crystal grain boundary portion, and have reached the present invention.

The details of the method for adjusting the thickness of the central portion of the crystal grains and the thickness of the grain boundary portion to a desired ratio are as follows. Generally, when growth is performed while maintaining a thermal equilibrium state in which deposition and meltback coexist, a flat film tends to be formed. Conversely, the more the thermal equilibrium deviates from the thermal equilibrium, the more likely the difference between the thickness at the center of the crystal grain and the thickness at the grain boundary is. Therefore, the difference tends to increase as the temperature drop rate during liquid phase growth increases. Also, the higher the degree of supersaturation before liquid phase growth, the more likely the difference will be. In addition, for example, by circulating a metal solvent and always bringing a solvent having a high degree of supersaturation into contact with the substrate, meltback can be suppressed, and a difference between the thickness of the central portion of the crystal grain and the thickness of the grain boundary portion can be provided. If the difference is too large, the thickness of the central part of the crystal grain and the thickness of the grain boundary part can be adjusted to a desired ratio by introducing a step of melt back.

By setting the ratio of t1 / t2 to 0.20 or more and 0.85 or less, more preferably 0.25 or more and 0.75 or less, the traveling distance of the electric charges in the vicinity of the grain boundary is shortened, and the traveling time of the electric charges is reduced. Becomes shorter. As a result, recombination of charges in the solar cell is reduced, the current taken out to an external circuit is increased, and the conversion efficiency is improved.

If it is less than 0.20, the thickness of the crystal grain boundary becomes significantly smaller than the thickness of the crystal grain, and the leakage current at the crystal grain boundary increases. In particular, when the thickness of the crystal grain boundary portion is extremely small, the leakage current increases so that insulation between the upper high-concentration semiconductor layer and the current collecting electrode and the lower high-concentration semiconductor layer cannot be maintained. Conversely, when the thickness of the crystal grain portion is extremely large, recombination due to defects in the crystal grain portion increases in the course of the charge running in the crystal grain portion. If it exceeds 0.85, the traveling distance of the charges generated in the vicinity of the crystal grain boundary becomes long, and the effect of the present invention is not sufficiently exhibited because recombination due to defects in the crystal grain boundary increases.

The present invention provides a structural advantage of polycrystal, that is, a difference between a thickness of a film near a grain boundary and a thickness of a central portion of a crystal grain (= unevenness).
This is completely different from the conventional technology that requires a flattening process.

According to the second feature, the amount of silicon of the solar cell grade to be used can be greatly reduced as compared with a normal polycrystalline wafer solar cell.

In addition, by performing liquid phase growth by bringing indium into contact with the surface of the metal-grade silicon substrate, it is possible to form a silicon layer suitable for a solar cell with less contamination of impurities.

Before liquid phase growth with indium, the substrate surface layer is dissolved with a metal solvent and then reprecipitated, so that most of the impurities can be removed from the surface layer by the segregation effect. Therefore, a higher quality silicon layer can be formed by liquid phase growth on this. At this time,
Boron B is likely to remain in the re-deposited layer among the impurities contained in the substrate, so that the re-deposited silicon layer becomes p-type (p ⁺ ). This p-type (p ⁺ ) layer can be used as a BSF (Back Surface Field) layer when manufacturing a solar cell.

In the present invention, the thickness of the film near the grain boundary (t)
The method (1) for measuring the thickness (t2) of the central portion of the crystal grain is not particularly limited, but for example, a stylus type film thickness measuring device is simple and preferable. These include Alpha Step (product name), Tari Step (product name)
And so on. The grain portion and the grain boundary portion are each 1 mm with reference to the substrate portion on which no crystal is grown.
Measure over a length and average each part.
At that time, measure each of multiple locations (for example, average 10 points)
Is preferred. The thickness (t2) of the central portion of the crystal grain of the present invention is the thickness of the substrate and the central portion of the crystal grain (portion parallel to the substrate) as shown in FIG. Not the thickness.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted a number of experiments to improve the conversion efficiency of a polycrystalline silicon solar cell. As a result, it was found that most of the charges generated by light irradiation were recombined at the crystal grain boundaries and could not be taken out to an external circuit. In order to improve this, it has been found that a remarkable improvement can be obtained by making the film thickness near the grain boundary thinner than the central part of the crystal grain. In particular, an improvement in the conversion efficiency is observed when the thickness of the polycrystalline grain boundary is in the range of 0.20 to 0.85 with respect to the thickness of the polycrystalline grain.
Significant improvements were obtained in the following ranges.

The ratio between the thickness of the polycrystalline grain boundary and the thickness of the polycrystalline grain could be set to a desired value by controlling the growth conditions in the liquid phase growth method.

In addition, metal-grade silicon (impurity 10 ppm)
It has been found that a thin-film crystal solar cell having good characteristics can be formed by depositing a silicon layer on the surface of a low-purity silicon substrate as described above by a liquid phase growth method using indium. It has also been found that a thin-film crystalline solar cell with good characteristics can be formed by depositing a silicon layer on the surface of a plate-like metal-grade silicon melted / solidified in a crucible by a liquid phase growth method using indium.

Further, after dissolving / reprecipitating the surface layer of the plate-shaped metal-grade silicon melted / solidified in the crucible with a metal solvent, depositing a silicon layer thereon by a liquid phase growth method using indium. And found that a better solar cell could be formed, and completed the present invention.

An embodiment of the present invention will be described with reference to FIGS.

FIG. 1 shows one structural example of a solar cell according to an embodiment of the present invention.

In FIG. 1, reference numeral 11 denotes a metal-grade polycrystalline silicon wafer. A solar cell grade silicon polycrystalline layer 12 is formed on the metal-grade polycrystalline silicon wafer 11 by reflecting its crystal structure as it is.

Reference numeral 16 denotes a crystal grain boundary of the 11th metal-grade polycrystalline silicon wafer, which is also inherited by 12 solar cell grade silicon polycrystalline layers. Eleven metal-grade polycrystalline silicon wafers are generally strong p-type because they contain a large amount of B as an impurity. The resistivity is 0.1
A typical value is about −0.001 Ω · cm.

Depending on the impurities contained, it may show n-type.

In some cases, the conductivity type is not clear. In this case, it is preferable to use a solar cell having the structure shown in FIG.

Therefore, in order to prevent inadvertent bonding between the metal-grade polycrystalline silicon wafer 11 and the solar cell grade silicon polycrystalline layer 12, when the metal-grade polycrystalline silicon wafer 11 is p-type, The polycrystalline silicon layer 12 of the battery grade (for example, less than 10 ppm of impurities) is also preferably p-type. For this reason, it is preferable that a group III element is slightly dissolved together with silicon in the metal solvent when the silicon polycrystalline layer 12 of the solar cell grade silicon polycrystalline layer 12 is formed.

When the metal-grade polycrystalline silicon wafer 11 is of the n-type, it is necessary that the twelve solar cell grade silicon polycrystalline layers are also of the n-type. Therefore, it is necessary to dissolve a group V element together with silicon in the metal solvent at the time of preparing the silicon polycrystalline layer.
The thickness of the solar cell grade silicon polycrystalline layer 12 is preferably from 10 to 200 μm, more preferably from 20 to 200 μm.
200 μm.

The crystal growth method used in the method of manufacturing the 12 solar cell grade silicon polycrystalline layers includes a liquid phase growth method, an LPCVD method, a normal pressure CVD method, a plasma CVD method, a photo CVD method, and a sputtering method. A liquid phase growth method is preferably used from the viewpoint of growth rate and crystallinity.

Reference numeral 13 denotes a semiconductor layer for forming a pn junction with 12, which is a layer of a type opposite to 12.

When 12 is p-type, 13 is n-type and 12
When is n-type, it is preferable that 13 be p-type.

The method for forming the semiconductor layer 13 is as follows.
In this method, impurities having a polarity opposite to that of 12 are introduced into the surface of the layer by ion implantation or thermal diffusion. As impurities, P, As, Sb, etc. for n-type, and B, P-type for p-type.
Al, Ga, etc. are each suitably selected.

To form a semiconductor junction, a semiconductor layer 13 having a conductivity type different from that of polycrystalline Si may be deposited on the surface of polycrystalline Si by a liquid phase growth method, various CVD methods, or the like. In this case, the crystal structure of the semiconductor layer 13 may or may not inherit the crystal structure of the twelve polycrystalline Si layers. If not succeeded, a film of microcrystalline silicon, amorphous silicon carbide or the like can be used.

The junction depth or the thickness of the semiconductor layer depends on the amount of impurities to be introduced.
It is preferably in the range of 1 μm, more preferably 0.02 to 0.5 μm.

Reference numeral 14 denotes a grid-like current collecting electrode. The collecting electrode is formed by vacuum evaporation or sputtering,
It is formed in a grid shape by patterning as required into a desired shape.

The current collecting electrode 14 may be made of a material having high conductivity, for example, a metal, and a material which can make an ohmic contact with a large-diameter Si crystal thin film which is preferably polycrystalline Si can be preferably used. The current collecting electrode 14 is 1
It may have a layered structure, or may have a multilayered structure by combining a plurality of materials (for example, metals).

The layer thickness of the current collecting electrode is not particularly limited as long as each layer has a required function, and may be appropriately designed within a range where necessary power can be taken out.

Reference numeral 15 denotes an antireflection film, which is made of TiO ₂ , Sn
A film of O ₂ , In ₂ O ₃ , ZnO or the like is formed by a sputtering method or the like.

The film thickness is determined in consideration of the refractive index of silicon and the antireflection film at a wavelength at which the photocurrent is maximized so that the antireflection effect is maximized.

FIG. 2 is a structural example according to another embodiment of the present invention. 2 correspond to 11 to 16 in FIG. 27 is a highly doped layer introduced between 21 metal-grade polycrystalline silicon wafers and 22 solar cell grade silicon polycrystalline layers. The polarity is the same as 22, but the resistivity is low. Usually, it is 0.1 Ω · cm or less.

This structure is effective for making ohmic contact with the solar cell grade silicon polycrystalline layer 22 when the polarity of the metal grade polycrystalline silicon wafer 21 is not clear.

Hereinafter, a manufacturing process of the solar cell having the structure shown in FIG. 1 will be described.

[0062] Granular metal-grade silicon was charged into a quartz crucible. The metal-grade silicon was melted by placing the crucible in a vacuum induction heating furnace and maintaining the temperature above the melting point of silicon (〜1415 ° C.) for a certain period of time. Next, an ingot of metal-grade silicon was produced by lowering the temperature and solidifying. The obtained metal-grade silicon ingot is 350 m thick.
m to produce a metal-grade silicon wafer. After cleaning the metal-grade silicon wafer with a detergent and an organic solvent, the wafer was etched with a 1.5% HE aqueous solution for 1 minute to remove an oxide film on the surface. The obtained metal-grade silicon wafer was placed in a boat made of carbon graphite or the like, and a silicon layer was deposited on the substrate by a liquid phase growth method using indium in a hydrogen atmosphere. A bond was formed on the surface of the silicon layer. A current collecting electrode pattern was formed thereon. An anti-reflection film was formed thereon, and a solar cell having the structure shown in FIG. 1 was manufactured.

A method of manufacturing a solar cell having the structure shown in FIG. 2 will be described. Granular metal-grade silicon was charged into a quartz crucible. The metal-grade silicon was melted by placing the crucible in a vacuum induction heating furnace and maintaining the temperature above the melting point of silicon (〜1415 ° C.) for a certain period of time. Next, an ingot of metal-grade silicon was produced by lowering the temperature and solidifying. The obtained metal-grade silicon ingot is 350 m thick.
m to produce a metal-grade silicon wafer. After cleaning the metal-grade silicon wafer with a detergent and an organic solvent, the wafer was etched with a 1.5% HE aqueous solution for 1 minute to remove an oxide film on the surface. The obtained metal-grade silicon wafer is placed in a boat made of carbon graphite or the like, and a metal solvent such as indium is brought into contact with the metal-grade silicon substrate and placed in an electric furnace, and the metal-grade silicon substrate is placed in the electric furnace. The surface layer was dissolved in the solvent. Then, the temperature was lowered to make the concentration of silicon in the solvent saturated or supersaturated, and silicon was deposited again on the surface of the metal-grade silicon substrate. At this time, the deposited silicon became p-type (p ⁺ ). The obtained metal-grade silicon substrate was placed in a boat made of carbon graphite or the like, and a silicon layer was deposited on the substrate by a liquid phase growth method using indium in a hydrogen atmosphere. A bond was formed on the surface of the silicon layer. A current collecting electrode pattern was formed thereon. An antireflection film was formed thereon, and a solar cell having the structure shown in FIG. 2 was manufactured.

Hereinafter, an example of a method for manufacturing a solar cell according to the present invention will be described in detail.

The liquid phase growth method is preferably performed in a hydrogen atmosphere for the purpose of removing a natural oxide film present on the silicon substrate surface. The growth temperature is preferably in the range of 500 to 1100C, more preferably in the range of 700 to 1050C.

In the liquid phase growth method, it is preferable to use indium as the metal solvent. 9 for indium
A high-purity 9.9% to 99.9999% is used.

As the liquid phase growth method, a slow cooling method and a temperature difference method are generally used.
JP-A-191987) can also be used.

As the metal-grade silicon used for the solar cell substrate of the present invention, a low-purity metal, specifically, one containing an impurity element of 0.1% to 2% is inexpensive and easily used.
It is also possible to reduce the amount of impurities by performing a treatment with an acid such as hydrochloric acid in advance as needed before granulation or powdering and before melting.

As the material of the crucible used in the liquid phase growth method, quartz or carbon graphite is used from the viewpoint of easiness of processing and cost, but a material capable of releasing the molten / solidified silicon can be applied and Any material having a melting point higher than that of silicon may be used, and silicon carbide, silicon nitride, boron nitride, or the like may be used. In addition, the crucible is formed asymmetrically or by attaching a heat sink to control the heat flow during solidification, thereby segregating impurities contained in metal-grade silicon to one side of the sheet surface and increasing the crystal grain size. It is also possible to make it.

As the release agent applied to the crucible, a release agent which does not react with molten silicon and has a large contact angle is selected. Specifically, a material containing Si ₃ N ₄ as a main component is used, and SiO ₂ or the like is added as necessary. As a method of coating the release agent into the crucible, an organic solution or a silanol solution in which powdered Si ₃ N ₄ is dispersed is sprayed into the crucible, and a heat treatment at 400 ° C. or more is performed to form a coating.

The metal soot used for dissolving / reprecipitating the surface of the metal-grade silicon substrate used in the present invention is selected to have a relatively low melting point and sufficiently dissolve the surface layer of the silicon substrate. As such a material, for example, indium, gallium, tin and the like are preferable.

As a boat for liquid phase growth using indium and a boat for dissolving / reprecipitating the surface of a metal-grade silicon wafer with a metal solvent used in the liquid phase growth method, carbon graphite is mainly used. , Silicon carbide, silicon nitride and the like can also be used. As a method for bringing the metal solvent into contact with the surface of the silicon wafer, a slide method or a tipping method is mainly used.

As a furnace used for manufacturing a metal-grade silicon ingot, a vacuum induction heating furnace is preferable from the viewpoint of controllability, and a furnace that can stably maintain a temperature equal to or higher than the melting point of silicon is used. From the viewpoint of maintaining the crystallinity of the ingot, it is preferable that the temperature can be lowered at a rate of about -30 ° C / min or less. The furnace used for dissolving / reprecipitating the surface of a metal-grade silicon wafer substrate with a metal solvent is an ordinary electric furnace, and the specifications thereof conform to those of the above vacuum induction heating furnace. Also in this case, an induction heating furnace may be used.

FIG. 3 is a sectional view of only the solar cell grade silicon layer. In the present invention, the ratio between the thickness t1 of the crystal grain portion and the thickness t2 of the crystal grain boundary portion is defined.

Hereinafter, an experiment performed by the present inventor to obtain the above-described solar cell will be described in detail.

(Experiment 1) In this experiment, a metal-grade polycrystalline silicon substrate was placed in a boat made of carbon graphite, and a supercooling degree of 30 ° C. and a film formation starting temperature of 980 ° C. were set in a hydrogen atmosphere in a metal solvent. Was changed, and the film thickness of the crystal grain portion and the crystal grain boundary portion of the silicon layer grown in the liquid phase was measured. The descending speed is 1 ℃ / min, 2 ℃
/ Min, 3 ° C / min, 4 ° C / min, 5 ° C / mi
n, 6 ° C./min, 7 ° C./min. These samples are referred to as samples 1-9, respectively. As a result, the film thickness of the crystal grain boundary portion and the crystal grain portion of each sample was measured by a stylus type film thickness meter. The ratio (t1 / t2) was as shown in Table 1.

[0077]

[Table 1]

(Experiment 2) Since all the samples obtained in Experiment 1 were found to be p-type by pn judgment by the thermoelectromotive force method, POCl ₃ was used as a diffusion source on the surface of the deposited silicon layer at 900 ° C. P diffusion was performed at a temperature to form an n ⁺ layer, and a junction depth of about 0.5 μm was obtained. Formed n ⁺
The dead layer on the layer surface is removed by etching,
A junction depth with an appropriate surface concentration of μm was obtained.

[0079] Further about as a transparent electrode on the n ⁺ layer 0.
1 μm thick ITO was formed by electron beam evaporation.

On the transparent electrode, a current collecting electrode (Cr (0.0
2 μm) / Ag (1 μm) / Cr (0.004 μm))
Was formed by vacuum evaporation. Al was deposited on the back surface of the metal-grade silicon wafer as a substrate to form a back electrode.

The thin-film crystal solar cell manufactured as described above is
AM1.5 (100mW / cm ^Two) I under light irradiation
-V characteristics were measured. As a result, the release as shown in Table 2
Voltage, short-circuit photocurrent, fill factor and conversion efficiency
Was.

[0082]

[Table 2]

From the results of Table 2, Experiments 1 and 2, the relationship between the conversion efficiency and the ratio of the film thickness between the crystal grain boundary and the crystal grain is as shown in FIG.

FIG. 4 shows that the conversion efficiency exceeds the practical minimum level of 10% when the thickness ratio between the crystal grain boundary portion and the crystal grain portion is between 0.2 and 0.85. 0.2
It can be seen that it is significantly higher between 5 and 0.75.

(Experiment 3) In this experiment, a method of forming a p ⁺ silicon layer with less impurities on a metal-grade silicon wafer was examined. The metal-grade silicon wafer is placed in a carbon boat, the metal solvent of indium is brought into contact with the sheet, placed in an electric furnace, and maintained at 1050 ° C. to dissolve the surface layer of the metal-grade silicon wafer in the indium solvent. Was. After maintaining this state for a while and sufficiently saturating, the temperature was lowered by controlling the electric furnace, and silicon in the solvent was deposited again on the surface of the silicon wafer. After the deposition for a certain period of time, the indium solvent brought into contact with the wafer was removed to obtain a desired silicon deposition layer.

Table 3 shows the results of analysis of elements contained in the silicon layer deposited on the surface of the obtained silicon wafer.

[0087]

[Table 3]

From Table 3, it was confirmed that the amount of impurities in the deposited silicon layer was further reduced to 1/100 or less as compared with the metallic grade silicon.

Further, when the pn was determined by the thermoelectromotive force method, it was found that the deposited silicon layer was p-type (p ⁺ ).

From the results of the above-described experiments, a high quality silicon layer can be obtained by forming a silicon layer on a crystalline silicon substrate having an impurity concentration of 10 ppm or more by a liquid phase growth method using indium as a solvent. Also, a metal wafer is prepared by melting / solidifying granular metal-grade silicon, and the surface thereof is dissolved / reprecipitated with a metal solvent to form a p-type (p +) silicon layer, and then indium is coated with indium as a solvent. It was found that a thin-film crystal solar cell having good characteristics can be formed by forming a silicon layer by the liquid phase growth method used in Example 1.

The thickness of the silicon layer formed in the present invention is preferably 10 μm from the viewpoint of efficient light absorption.
m or more, more preferably 10 μm or more and 100 μm or less,
It is more preferable that the thickness be 10 μm or more and 50 μm or less.

[0092]

EXAMPLES Hereinafter, the solar cell according to the present invention will be described in more detail, but the present invention is not limited to these examples.

(Example 1) In this example, a silicon layer was deposited on a metal-grade silicon wafer sliced from an ingot by a liquid phase growth method using indium, and a thin film solar cell was formed using the silicon layer as an active layer.

An ingot was prepared by casting using metal-grade silicon having a purity of 98% as a raw material, and sliced into a wafer having a thickness of 0.5 mm to prepare a metal-grade silicon wafer substrate. Elemental analysis near the surface of the produced metal-grade silicon wafer substrate was performed, and the results in Table 4 were obtained.

[0095]

[Table 4]

The crystal grain size of the metal-grade silicon wafer substrate is several mm to several cm, and the specific resistance is 0.01 Ω · c.
The m-grade (p-type) metal-grade silicon wafer substrate was placed in a carbon boat, and high-purity silicon was dissolved in an indium solvent at 980 ° C. for 1 hour and 40 minutes before film formation. Thereafter, the wafer is placed in a solvent, and a liquid crystal growth method using indium in which high-purity silicon is dissolved is used as a solvent. In a hydrogen atmosphere, a degree of supercooling is 10 ° C., a growth start temperature is 950 ° C., and a descent rate is 4 ° C./min. To form a silicon layer until the temperature drops to 800 ° C.

When the film thickness of the formed silicon layer was measured, the center of the crystal grain was 51 μm and the grain boundary was 28 μm.
m.

Next, P was thermally diffused on the surface of the silicon layer at a temperature of 900 ° C. using POCl ₃ as a diffusion source to form an n ⁺ layer, and a junction depth of about 0.5 μm was obtained. The dead layer on the surface of the formed n ⁺ layer was removed by etching to obtain a junction depth having an appropriate surface concentration of about 0.2 μm.
Further collector electrode on the n ⁺ layer (Cr (0.02μm) /
Ag (1 μm) / Cr (0.004 μm)) was formed by electron beam evaporation. Furthermore, an anti-reflection film Ti
About 60 nm of O ₂ was formed by a sputtering method, and Al was also deposited on the back of the metal-grade silicon wafer to form a back electrode.

The thin-film crystal solar cell thus manufactured was measured for IV characteristics under AM1.5 (100 mW / cm ² ) light irradiation.
cm ² in open circuit voltage 0.62 V, the short circuit photocurrent 35 mA / c
m ² and fill factor were 0.75, and a conversion efficiency of 16.3% was obtained.

(Comparative Example 1) A silicon film was produced in the same manner as in Example 1, except that the descent rate was 0.5 ° C./min in the liquid phase growth method, and the other conditions were the same. The film thickness at the center of the crystal grain of the obtained silicon film was 51 μm, and the film thickness at the crystal grain boundary was 46 μm. Hereinafter, a solar cell was manufactured in the same steps as in Example 1. When the IV characteristics under AM1.5 (100 mW / cm ² ) light irradiation were measured, the cell area was 2 cm ² , the open voltage was 0.52 V, and the short-circuit photocurrent was 30 mA.
/ Cm ² , fill factor 0.58, conversion efficiency 9.0%
Met.

(Example 2) In Example 1, the liquid phase growth was performed.
After forming a silicon film by the plasma CVD method,
Attached to the_Four0.05 sccm gas, H_TwoGas 8
0 sccm, PH _ThreeGas (1% PH_Three, 99% H_Two) 0.
2 sccm under 60 Pano pressure, 105 W at 50 W
Hz VHF power is applied for 5 minutes, and n
Form a 20-nm microcrystalline silicon layer to form a PN junction
Done. Next, 550 of the ITO film is formed by a sputtering method.
nm. The ITO film consists of a transparent conductive film and an anti-reflection film.
There are both effects. A current collecting electrode (Cr (0.
02 μm) / Ag (1 μm) / Cr (0.004 μm)
m)) was formed by electron beam evaporation.

The thin-film crystal solar cell thus manufactured was measured for IV characteristics under AM1.5 (100 mW / cm ² ) light irradiation.
Open voltage 0.63 V, short-circuit photocurrent 34 mA / c in cm ²
m ² and a fill factor of 0.71, resulting in a conversion efficiency of 15.2%.

(Example 3) A metal-grade silicon wafer was placed in a carbon boat, and a metal solvent of indium was brought into contact with a sheet and placed in an electric furnace. The layer was dissolved in the In solvent. In this state, when several hours have passed and saturation is sufficient, the electric furnace is controlled to lower the temperature at a rate of 0.5 ° C./min, and silicon in the solvent is deposited again on the silicon wafer surface to 700 ° C. Was. After deposition for 2 hours, the boat was slid to remove the indium solvent on the wafer, and a desired silicon deposition layer was obtained.

Table 5 shows the results of analyzing the elements contained in the silicon layer deposited on the surface of the metal-grade silicon wafer.

[0105]

[Table 5]

From Table 5, it was confirmed that the amount of impurities in the deposited silicon layer was significantly reduced as compared with the granular metal-grade silicon as the raw material. Further, the thickness of the obtained reprecipitated silicon layer was about 15 μm from a cross-sectional observation by SBM / EDX.

The pn judgment by the thermoelectromotive force method revealed that the deposited silicon layer was p-type (p ⁺ ).

On the surface of the silicon wafer, that is, on the deposited silicon layer, liquid phase growth was performed using indium other than the indium metal solvent used in the above-mentioned dissolution / reprecipitation step as a solvent. Before film formation, high-purity silicon was dissolved in an indium solvent at 980 ° C. for 1 hour and 40 minutes. Thereafter, the wafer is placed in a solvent, and in a hydrogen atmosphere, the degree of supercooling is 30 ° C., and the growth start temperature is 950 ° C.
The silicon layer was formed at a temperature lowering rate of 4 ° C./min.
The thickness of the obtained silicon layer is 53 μm at the center of the crystal grain.
m, 30 μm at the grain boundary.

The n + layer was formed on the surface of the silicon layer by thermal diffusion of P at a temperature of 900 ° C. using POCl ₃ as a diffusion source to obtain a junction depth of about 0.5 μm. N + formed
The dead layer on the layer surface is removed by etching,
A junction depth with an appropriate surface concentration of μm was obtained.

[0110] On of the produced n ⁺ layer, the collector electrode (Cr
(0.02 μm) / Ag (1 μm) / Cr (0.004
μm)) was formed by electron beam evaporation.

On the current-collecting electrode, Ti was used as an anti-reflection film.
An O ₂ film was formed by a 600 nm sputtering method.

Further, Al was deposited on the back surface of the silicon wafer as the substrate to form a back electrode.

The IV characteristics of the thin-film crystal solar cell fabricated as described above were measured under AM1.5 (100 mW / cm ² ) light irradiation. As a result, the cell area is 2 cm
² , the open-circuit voltage is 0.60 V and the short-circuit photocurrent is 34 mA.
/ Cm ² and a fill factor of 0.75, resulting in a conversion efficiency of 15.3.
%.

Example 4 A metal-grade silicon wafer substrate was placed in a carbon boat. 930 ° C before film formation
Then, high-purity silicon and B were dissolved in the indium solvent for 1 hour and 30 minutes. Thereafter, the wafer is placed in a solvent, and a liquid crystal growth method using indium as a solvent is performed in a hydrogen atmosphere in a degree of supercooling of 10 ° C. and a growth start temperature of 930.
A silicon layer containing B was formed at a temperature of 800 ° C., an end temperature of 800 ° C., and a descent rate of 0.5 ° C./min.

When the film thickness of the silicon layer containing B was measured, the center of the crystal grain was 12 μm and the grain boundary was 11 μm. The pn determination by the thermoelectromotive force method showed that the deposited silicon layer was p-type (p ⁺ ).

On the surface of the silicon wafer, that is, on the deposited silicon layer, liquid phase growth was performed using indium other than the indium metal solvent used in the above-mentioned dissolution / reprecipitation step as a solvent. Before film formation, high-purity silicon was dissolved in an indium solvent at 980 ° C. for 1 hour and 40 minutes. Thereafter, the wafer is placed in a solvent, and in a hydrogen atmosphere, the degree of supercooling is 30 ° C., and the growth start temperature is 950 ° C.
A silicon layer was formed at a temperature lowering rate of 4 ° C./min until the temperature dropped to 680 ° C. The thickness of the obtained silicon layer was 55 μm at the center of the crystal grain and 31 μm at the grain boundary.

Using a POCl 3 diffusion source as a diffusion source, P was thermally diffused at a temperature of 900 ° C. to form an n ⁺ layer on the surface of the silicon layer, and a junction depth of about 0.5 μm was obtained. N + formed
The dead layer on the layer surface is removed by etching,
A junction depth with an appropriate surface concentration of μm was obtained.

A current collecting electrode (Cr) was formed on the n ⁺ layer thus formed.
(0.02 μm) / Ag (1 μm) / Cr (0.004
μm)) was formed by electron beam evaporation.

On the current collecting electrode, Ti was used as an anti-reflection film.
An O ₂ film was formed by a 600 nm sputtering method.

On the back surface of the silicon wafer as the substrate, Al was deposited to form a back electrode.

The IV characteristics of the thin-film crystal solar cell fabricated as described above were measured under AM1.5 (100 mW / cm ² ) light irradiation. As a result, the cell area is 2 cm
² , the open-circuit voltage is 0.61 V and the short-circuit photocurrent is 33 mA.
/ Cm ² and a fill factor of 0.76, resulting in a conversion efficiency of 15.3.
%.

[0122]

As described above, according to the present invention, by making the film thickness of the grain boundary portion smaller than the film thickness of the central portion of the crystal grain at a predetermined ratio, the charge traveling near the grain boundary can be achieved. The distance is shortened and the travel time of the charge is shortened. As a result, recombination of charges in the solar cell is reduced, the current taken out to an external circuit is increased, and the conversion efficiency is improved.

In particular, when the predetermined ratio is 0.25 or more and 0.75 or less, the effect is more remarkable.

By forming polycrystals by liquid phase growth, they can be manufactured at low cost.

Further, the amount of silicon of the solar cell grade to be used can be significantly reduced as compared with the ordinary polycrystalline wafer solar cell.

In addition, a silicon layer suitable for a solar cell with less impurities can be formed by performing liquid phase growth by bringing indium into contact with the surface of the metal-grade silicon substrate.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating a configuration example of a solar cell according to an embodiment of the present invention.

FIG. 2 is a conceptual diagram showing another configuration example of the solar cell according to the embodiment of the present invention.

FIG. 3 is a schematic diagram of a cross section of a polycrystalline silicon layer according to an embodiment of the present invention.

FIG. 4 is a graph showing the relationship between the conversion efficiency and the ratio of the film thickness between the crystal grain boundary and the crystal grain.

[Explanation of symbols]

11,21 Metal-grade polycrystalline silicon wafer 12,22 Solar cell grade polycrystalline silicon layer 13,23 Silicon layers 12 and 22, respectively, and P
Semiconductor layer for forming N junction 14, 25 Current collecting electrode 15, 25 Antireflection film 16, 26 Crystal grain boundary 27 Highly doped layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akishi Nishida 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Hiroshi Sato 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Incorporated (72) Inventor ▼ Taka ▲ Yasuyoshi I F-term (reference) 5F051 AA03 AA16 CB11 CB20 DA03 FA06 GA04 HA03 3-30-2 Shimomaruko, Ota-ku, Tokyo

Claims (11)

[Claims] In a polycrystalline solar cell in which an active layer is formed on a polycrystalline silicon substrate, the active layer is made of polycrystal, and the thickness of the polycrystalline grain forming the active layer at a grain boundary portion is 0.20 or more with respect to the film thickness at the center of the polycrystalline grains.
A polycrystalline solar cell characterized by being in the range of 85 or less. 2. The polycrystalline solar cell according to claim 1, wherein said active layer is a layer formed by liquid phase growth. 3. The method according to claim 1, wherein the thickness of the polycrystal forming the active layer at the grain boundary is in the range of 0.25 to 0.75 with respect to the thickness of the center of the polycrystal. Item 1
Or the polycrystalline solar cell according to 2. 4. The polycrystalline solar cell according to claim 1, wherein the polycrystalline silicon substrate is metal-grade silicon containing impurities of 10 ppm or more. 5. The polycrystalline solar cell according to claim 1, wherein liquid phase growth is performed by bringing indium into contact with the surface of the metal-grade silicon substrate. 6. A polycrystalline film characterized in that the film thickness at the grain boundary part is in the range of 0.20 to 0.85 with respect to the film thickness at the central part of the polycrystalline grain. 7. The polycrystalline film according to claim 6, wherein the polycrystalline film is a liquid phase grown film. 8. The polycrystal according to claim 6, wherein the film thickness at the grain boundary is in the range of 0.25 to 0.75 with respect to the film thickness at the center of the polycrystal grain. film. 9. The polycrystalline film according to claim 6, wherein said polycrystalline film is formed on a polycrystalline silicon substrate. 10. The polycrystalline silicon substrate has a thickness of 10 pp.
10. The polycrystalline film according to claim 9, wherein the polycrystalline film is a metal-grade silicon substrate containing at least m impurities. 11. The polycrystalline film according to claim 6, wherein liquid phase growth is performed by bringing indium into contact with the surface of the metal-grade silicon substrate.