JP2640389B2 - Solar cell manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a solar cell, and more particularly, to a method for manufacturing a solar cell having good energy conversion efficiency.

[Conventional technology]

In various devices, a solar cell is used as a driving energy source.

A solar cell uses a pn junction in a functional part, and silicon is generally used as a semiconductor forming the pn junction. From the viewpoint of the efficiency of converting light energy into electromotive force, it is preferable to use single crystal silicon, but from the viewpoint of increasing the area and reducing the cost, amorphous silicon is advantageous.

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 the conventionally proposed method, a thick polycrystal was sliced into a plate-like body, which was used to reduce the thickness to 0.3 m.
It is difficult to reduce the thickness to less than m, so that the thickness becomes more than necessary to sufficiently absorb the amount of light, and in this regard, the effective use of the material is not sufficient. That is, it is necessary to reduce the thickness sufficiently in order to reduce the cost.

Therefore, attempts have been made to form a polycrystalline silicon thin film using a thin film forming technique such as a chemical vapor deposition (CVD) method. Energy conversion efficiency is lower than that of the crystalline silicon slice method.

Attempts have also been made to increase the crystal grain size by irradiating the polycrystalline silicon thin film formed by the above-described CVD method with laser light and melting and recrystallizing it, but cost reduction is not sufficient, and stable production is also not possible. Have difficulty.

Such a situation is a common problem not only in silicon but also in compound semiconductors.

On the other hand, the method disclosed in U.S. Pat.No. 4,816,420, that is, a sheet-like crystal is formed by selective epitaxial growth and a lateral growth method on a crystal substrate via a mask material and then separated from the substrate. It has been shown that a thin crystalline solar cell can be obtained by the method for manufacturing a solar cell characterized by the above.

[Problems to be solved by the invention]

In the above-described method, the opening provided in the mask material has a line shape. To separate a sheet-like crystal grown by selective epitaxial growth and lateral growth from the line seed, the cleavage of the crystal is used. If the shape of the line seed is larger than a certain size due to mechanical peeling, the ground contact area with the substrate increases, so that the sheet crystal is damaged during the peeling. In particular, when increasing the area of solar cells, no matter how narrow the line width (actually around 1 μm), if the line length becomes several mm to several cm or more, it is a fatal problem. Become.

In the above method, SiO ₂ was used as a mask material.
Although examples of growing a silicon thin film is shown by the selective epitaxial growth and lateral growth at a substrate temperature of 1000 ° C., the silicon layer / SiO ₂ by the reaction between the silicon film and the SiO ₂ to grow in this high temperature It is generally well known that considerable stacking faults (plane defects) are introduced on the silicon layer side near the interface. Such defects have a great adverse effect on the characteristics of the solar cell.
Conversely, it has been reported that the lower the growth temperature is, the lower the defect density is [H. Kitajima, A. Ishitani, N. Endo
and K. Tanno, Jpn. J. Appl. Phys. 22 , L783 (1983)], the growth rate drops rapidly, and the film thickness required for solar cell formation cannot be obtained within a practical time.

[Object of the invention]

The method of the present invention eliminates the above-mentioned disadvantages of the prior art and provides a method of manufacturing a high-quality thin silicon solar cell.

An object of the present invention is to provide an inexpensive solar cell by transferring a thin semiconductor layer formed on a silicon wafer to a substrate such as a glass substrate.

Another object of the present invention is to provide a high-quality solar cell by reducing defects in a grown crystal near an interface between a grown crystal and an insulating layer in selective crystal growth via an insulating layer.

[Configuration of the invention]

The present invention solves the above-mentioned problems in the prior art and has been completed as a result of intensive studies by the present inventors in order to achieve the above object. It is concerned. That is, in the method for manufacturing a solar cell according to the present invention, a step of forming an insulating layer on a silicon wafer, forming a polycrystalline semiconductor layer having voids exposing the surface of the insulating layer in a lattice-like manner on the insulating layer Step, the insulating layer is removed by etching using the gap,
Separating the semiconductor layer from the silicon wafer,
And a step of providing an electrode on the separated semiconductor layer.

The main technique of the present invention is the selective epitaxial growth using the non-nucleation surface formed on the silicon wafer and the region of the silicon seed portion (seed portion) formed on the silicon wafer shown in FIG. The purpose of the present invention is to form a polycrystalline silicon thin film, which is a set of single crystals having the same crystal orientation and size (grain size) by growth.

Here, the general principle of the selective epitaxial growth method will be briefly described. The selective epitaxial growth method refers to a method of performing epitaxial growth using a vapor phase growth method, as shown in FIGS. 2A and 2B, on an insulating layer such as an oxide film formed on a silicon wafer. Is a selective crystal growth method in which an exposed silicon surface in an opening provided in an insulating layer is used as a seed crystal under the condition that nucleation does not occur, and epitaxial growth is performed only in this opening. When the epitaxial layer filling the opening continues to grow, the crystal layer grows in the horizontal direction along the surface of the insulating layer while continuing to grow in the vertical direction. This is called the lateral growth method (Epiatxial Lateral Overgrowth),
As the growth continues in the lateral direction, silicon crystals finally grown from different openings come into contact with each other to form a continuous film.
The appearance of the vertical to horizontal growth ratio or facet at this time generally depends on the growth conditions, the plane orientation of the substrate, the thickness of the insulating layer, and the shape of the opening provided on the insulating layer.

The present inventors have conducted a number of experiments to reduce the size of the opening to a small area of several μm or less,
Regardless of the thickness of the insulating layer, the crystal grows three-dimensionally on the insulating layer with a growth ratio in the vertical direction to the horizontal direction of about 1, and a clear facet appears to obtain a mountain-shaped single crystal. Found that they contacted to form a continuous film (second
FIG. 3).

At this time, it has been found that the insulating film can be almost completely removed by etching through the gap before the completely continuous film is formed, that is, in a state where a gap remains between adjacent mountain-shaped single crystals. It has been found that a polycrystalline thin film which is an aggregate of single crystal bodies can be easily separated from a silicon wafer.

In addition, the present inventors have conducted further experiments and found that when the insulating layer on the silicon wafer was SiO ₂ as shown in FIG. The stacking faults contained in the same figure were controlled by controlling the growth temperature during crystal growth and lowering the temperature in the early stage of growth and then increasing the temperature, or by changing the insulating layer from SiO ₂ to Si ₃ N ₄ , respectively. As shown in (B) and (C), it has been found that it can be greatly reduced. In particular, in the case of (B), a continuous thin film having few defects at the interface can be obtained without significantly lowering the effective growth rate on SiO ₂ .

The present inventors have further repeated experiments to obtain the fourth
At the initial stage of crystal growth, impurities are intentionally introduced at the initial stage of the crystal growth as shown in FIG. It has been found that a good semiconductor layer can be obtained by growing a few high-quality crystal layers and using this as an active region.
Thus, the present invention has been completed. Hereinafter, an experiment performed by the present inventors will be described in detail.

Experiment 1 Selective crystal growth As shown in FIG. 2A, a thermal oxide film as an insulating layer 202 was formed on the surface of a (100) silicon wafer 201 having a thickness of 500 μm.
Å Formed, etched using photolithography, and provided with square openings each having a side a at an interval of b = 50 μm in an arrangement as shown in FIG. 3A. Here, three types of openings of 1.2 μm, 2 μm, and 4 μm were provided as the value of a. Next, selective crystal growth was performed by a normal low pressure CVD apparatus (LPCVD apparatus) shown in FIG. HCl to using SiH ₂ Cl ₂ as a raw material gas, and controls the generation of nuclei on the oxide film of an insulating layer 202 with H ₂ as a carrier gas
Was added. Table 1 shows the growth conditions at this time.

After the growth was completed, the state of the wafer surface was observed with an optical microscope, and as shown in FIG. 2 (C) or FIG. 3 (B), a peak having a particle size of about 20 μm was obtained for any value of a. Single crystal bodies 203 (303) having facets are regularly arranged on lattice points at intervals of 50 μm, and selective crystal growth is performed according to the pattern of opening 301 determined in FIG. 3 (A). I was assured. At this time, the ratio of the crystal grown to the opening is 10% for any value of a.
It was 0%. In the grown single crystal, the ratio of the facet on the surface that clearly appears without being distorted is a
And as shown in Table 2, the smaller the value of a, the smaller the ratio of deviation.

All the obtained single crystal bodies had the same orientation, and it was found that the crystal orientation was accurately inherited from the silicon wafer as the substrate.

Next, under the conditions shown in Table 1, the growth time was further extended to 90 minutes, and growth was performed. After the growth was completed, the wafer surface was observed with an optical microscope in the same manner as above.
As shown in FIG. (C), for any value of a, a single crystal
In 203 (303), adjacent objects are completely in contact with each other,
It was confirmed that a polycrystalline thin film composed of a collection of single crystal bodies arranged neatly in a mask shape when viewed from above the substrate was obtained. At this time, the height of the single crystal body is about 40
μm. The polycrystal was cleaved along the line BB 'shown in FIG. 3 (C), and after the cross section was polished, defects were revealed by Secco etching and the vicinity of the grain boundary was observed with a scanning electron microscope. No clear interface such as that seen in polycrystalline silicon having a non-uniform crystal orientation was observed. However, near the interface with the oxide film, it was observed that the crystal contained many defects as shown in FIG. 4 (A). The surface defect density at this time was about 7 × 10 ⁴ cm ⁻¹ .

In the obtained polycrystal, as shown in FIG. 3 (C), a gap 304 of about 8 μm square remains between adjacent single crystal bodies 303, and an oxide film 302 as an insulating layer is partially exposed. Was seen. It was confirmed by another experiment that the voids 304 disappeared completely by further increasing the growth time.

Next, an attempt was made to remove the insulating layer below the polycrystalline thin film by using the above-mentioned gap.

Experiment 2 Removal of Insulating Layer The silicon wafer on which the polycrystalline thin film with voids obtained in Experiment 1 was grown was immersed in a 49% HF aqueous solution for 24 hours. Thereafter, the wafer was washed with running water and dried, and then the wafer surface was observed with an optical microscope and a scanning electron microscope. As a result, no oxide film was found in the voids on the polycrystalline thin film. Therefore, the epoxy resin was applied to the surface of the polycrystalline thin film, the glass plate was attached as a support, and after the epoxy was cured, the glass plate was separated from the wafer. Was transferred to the glass plate as it was. Observation of the rear surface of the divided polycrystalline thin film with an optical microscope and a scanning electron microscope revealed that although a part of the oxide film remained, most of the oxide film was etched, and the silicon crystal was not etched. It was exposed. Next, defect control was attempted by controlling the growth temperature during crystal growth.

Defect density in SiO ₂ interface vicinity entering the crystal grown on the SiO ₂ as control described above of the defect density due to Experiment 3 growth temperature depends on the temperature during crystal growth, as the growth temperature is low, the defect density is low (H. Kitajima, A. Ishitani, N. Endo and K. Tan)
no, Jpn. J. Appl. Phys. 22 , L783 (1983)]. However, if the growth temperature is lowered, good quality crystals can be obtained, but on the contrary the growth rate drops sharply, making it difficult to grow and contact the mountain-shaped polycrystal to form a polycrystalline film.

In consideration of the configuration of a solar cell, the growth temperature is changed in two stages, and the crystal region near the SiO ₂ interface, which serves as a photocurrent path, does not substantially decrease the growth rate. An experiment was performed to determine whether a crystal having a defect density could be obtained.

Using the same substrate as in Experiment 1, the growth temperature was set to 2 under the conditions shown in Table 3.
When the crystal growth was carried out by continuously changing the stages, a continuous polycrystalline film as shown in FIG. 2 (D) or FIG. 3 (C) was obtained. The polycrystal was cleaved along the line BB 'shown in FIG. 3 (C), and after the cross section was polished, defects were revealed by Secco etching and observed with a scanning electron microscope. In the vicinity of the interface, as shown in FIG.
× 10 ³ cm ^-1 . It has been found that a continuous polycrystalline silicon film with few defects near the SiO ₂ interface can be obtained by changing the growth temperature in two stages and performing continuous crystal growth.

Experiment 4 Defect Density on Si ₃ N _{4 Since} the reaction between SiO ₂ and Si crystal is considered to be one of the causes of crystal defects, an experiment was performed using Si ₃ N ₄ instead of SiO ₂ as the insulating layer. Si ₃ N ₄ is converted to SiH ₂ Cl using a normal LPCVD apparatus.
₂ and NH ₃ are deposited on a silicon wafer by thermal decomposition, and the film thickness is
1000 mm. Crystal growth was performed under the conditions shown in Table 1 in the same manner as in Experiment 1, and the defect density in the crystal near the interface with the Si ₃ N ₄ layer was examined in the same manner as in Experiment 3. FIG. 4 (C) As shown in the figure, it is smaller than that of SiO ₂
^{It was 4} cm ^-1 .

Thus, it was found that the defect density was improved by replacing the insulating layer with Si ₃ N ₄ .

Experiment 5 Reduction of defects due to growth temperature change + Si ₃ N ₄ Further, based on the results of Experiments 3 and 4 above, Si ₃ N
Crystal growth was carried out in the same manner as in Experiment 3 using No. ₄ under the forming conditions shown in Table 3. When the defect density in the crystal near the interface with the Si ₃ N ₄ layer was examined for the grown continuous polycrystalline film in the same manner as in Experiments 3 and 4, the density decreased as shown in FIG. 4 (B). And a value as low as about 7 × 10 ² cm ⁻¹ was obtained.
Next, an inactive region was formed by adding impurities at an early stage of crystal growth, and an attempt was made to use a high-quality crystal layer with few defects grown thereon as the active region.

Experiment 6 Reduction of the influence of defects due to impurity addition In order to minimize the influence of defects and obtain good device characteristics, impurities were added at the initial stage of crystal growth to reduce the area near the interface with the oxide film with many defects. After inactivation, the introduction of the impurity was stopped, and a crystal was continuously grown on the region containing the impurity to provide a region with few defects, which was used as an active layer.

Sb-doped (100) silicon wafer (ρ = 0.02Ω · c
A substrate was produced in the same manner as in Experiment 1 using m). Crystal growth is performed by dividing the growth conditions into two stages under the conditions shown in Table 4,
A continuous polycrystalline film as shown in FIG. 2 (D) or FIG. 3 (C) was obtained. The sectional structure of the crystal grown at this time is the fourth structure.
This is as shown in FIG. Au was vacuum-deposited to a thickness of 200 ° on the surface of the obtained polycrystalline film to form a Schottky barrier, and the reverse saturation current was measured. The recombination current due to defects is reduced as compared with the case where a polycrystalline film is grown under the conditions shown in Table 1 and a Schottky barrier is similarly formed, and the current value is about 10 ² to 10 ³ times lower at all values of a. Was.

It was found that the influence of defects near the SiO ₂ interface can be reduced by introducing impurities into the initial stage of crystal growth by dividing the growth conditions into two stages.

Experiment 7 Reduction of the influence of defects due to impurity addition + Si ₃ N ₄ Further, based on the results of Experiments 4 and 6 above, using Si ₃ N ₄ for the insulating layer, and in the same manner as in Experiment 6 under the formation conditions in Table 4 Crystal growth was performed. A Schottky barrier was formed on the grown continuous polycrystalline film in the same manner as in Experiment 6 and the reverse saturation current was measured. The current obtained was 5 to 10 times lower than the characteristics obtained in Experiment 6. Value.

Next, the solar cell characteristics were evaluated using the obtained low defect crystal thin film.

Experiment 8 Formation of solar cell In the same manner as in Experiment 3, using a Sb-doped (100) silicon wafer (ρ = 0.02 Ω · cm), a polycrystalline silicon having a gap composed of a collection of single crystal silicon was grown, and the surface thereof was grown. B was implanted by ion implantation under the conditions of 20 KeV and 1 × 10 ¹⁵ cm ⁻² and annealed at 800 ° C. for 30 minutes to form ap ⁺ layer on the surface of the polycrystal. The solar cell with p ⁺ / polycrystalline silicon / SiO ₂ / n ⁺ (100) Si structure fabricated in this way
1.5 (Measurements of the I-V characteristic under 100 mW / cm ² irradiation, the open-circuit voltage 0.42V in cell area 0.16 cm ^2, short-circuit photocurrent
22mA / cm ² , fill factor 0.67, conversion efficiency at a = 4μm
6.2% was obtained. It has been shown that a good solar cell can be formed using the polycrystalline silicon thin film formed on the insulating layer.

As described above, the present invention, which has been completed based on the experimental results described above, opens a part of the insulating layer formed on the silicon wear and grows the mountain type single crystal silicon by the selective epitaxial growth method. The present invention relates to a method of manufacturing a solar cell by exfoliating a polycrystalline thin film composed of the aggregate.

FIG. 1 shows the configuration of a solar cell manufactured by the method of the present invention.

A p ⁺ layer 103 is formed on the upper surface of polycrystalline silicon composed of a collection of single-crystal silicon 102, and a space between adjacent single-crystal silicon 102 is filled with an epoxy resin (not shown). I have. A transparent electrode 104 also serving as an anti-reflection film and a current collecting electrode 107 are provided on the p ⁺ layer 103, and a transparent support 106 such as glass is further interposed between the transparent electrode 104 and an epoxy resin 105 thereon. Has been fixed. A back surface electrode 101 is formed on the lower surface of the polycrystalline silicon.

Next, a method for manufacturing a solar cell according to the present invention will be described in accordance with the process shown in FIG. Using Sb-doped (100) silicon wafer (ρ = 0.02 Ω · cm), polycrystalline silicon 503 composed of a collection of single-crystal silicon as shown in FIG. I do. FIG. 2 (D) is a cross-sectional view taken along the line BB 'in FIG. 3 (C), but a cross-sectional view along the line AA' is shown in FIG. A) to (H). FIG. 5 (A) shows a cross section in a direction in which the gap 504 is visible in FIG. 2 (D). (B) Next, after forming ap ⁺ layer on the surface by ion implantation in the same manner as in Experiment 8, H
The oxide film 502, which is an insulating layer, is removed by etching with an F aqueous solution, and (C) an epoxy resin 505 is applied to the polycrystalline surface to fill the void 504. (D) After the resin is hardened, only the resin on the polycrystalline surface is removed by plasma etching, and the resin is left in the voids. (E) A transparent electrode 506 and a collecting electrode 507 are sequentially deposited on the exposed polycrystalline surface.
(F) Further, an epoxy resin 511 is applied again from above, and the polycrystalline surface is fixed by a support 508 such as a glass plate. (G) After the resin is cured, the polycrystalline silicon layer 503 is separated from the silicon wafer 501, and the oxide film remaining on the back surface of the polycrystalline silicon layer 503 is completely removed again with an HF aqueous solution. An electrode 509 is formed on the back surface of the layer 503 by vapor deposition to complete the process.

As the insulating layer used in the solar cell of the present invention, a material whose nucleation density on the surface thereof is considerably smaller than that of silicon is used from the viewpoint of suppressing nucleation during selective crystal growth. For example, SiO ₂ , Si ₃ N _{4 and the} like are typically used.

The source gas for selective crystal growth used in the present invention is SiH ₂ Cl ₂ , SiCl ₄ , SiHCl ₃ , SiH ₄ , Si ₂ H ₆ , SiH ₂ F ₂ , S
Representative examples include silanes such as i ₂ F ₆ and halogenated silanes.

Further, H ₂ is added as a carrier gas or in order to obtain a reducing atmosphere for promoting crystal growth, in addition to the above-mentioned source gas. The ratio of the amount of the source gas and the amount of hydrogen is appropriately determined as desired depending on the formation method and the type of the source gas and the material of the insulating layer, and further, the formation conditions, preferably 1:
The ratio is suitably 10 or more and 1: 1000 or less (introduction flow ratio), and more preferably, 1:20 or more and 1: 800 or less.

In the present invention, HCl is used for the purpose of suppressing the generation of nuclei on the insulating layer. The amount of HCl added to the source gas is appropriately determined depending on the forming method, the type of the source gas, the material of the insulating layer, and the forming conditions. Is determined according to
1: 0.1 or more and 1: 100 or less are appropriate, and more preferably 1: 0.
It is desirable that the ratio be 2 or more and 1:80 or less.

The temperature and pressure are selectively grown performed in the present invention, forming method and the type of raw material gas to be used, varies depending forming conditions of the flow rate ratio and the like of the source gas and H ₂ and HCl, the temperature is, for example normal In the LPCVD method described above, the temperature is suitably 600 ° C. or higher and 1250 ° C. or lower, and more preferably, is controlled to 650 ° C. or higher and 1200 ° C. or lower. In particular, when SiO ₂ is used for the insulating layer, the temperature is preferably set to 850 ° C. or more and 950 ° C. or less in order to suppress defects at the initial stage of crystal growth and to maintain a growth rate of a certain level or more.

Similarly, the pressure is suitably in the range of about 10 ^{-2 to} 760 Torr, and more preferably in the range of 10 ^{-1 to} 760 Torr.

Further, as a support for fixing the polycrystalline thin film used in the solar cell of the present invention, a support having a light transmission property as a light incident surface is used, for example, glass, SiO ₂ , Si ₃ N ₄ , ceramics or the like. A film or sheet of a synthetic resin such as polyimide, polycarbonate, and polyamide is used.

In the present invention, the resin used for filling the voids between the single crystal bodies may be selected and used in terms of heat resistance, electrical insulation, adhesiveness, mechanical strength, dimensional stability, corrosion resistance, weather resistance. preferable. Among such resins, the resin used in the present invention is in a liquid state, and is preferably a curable resin that causes heat curing, ultraviolet curing, or electron beam curing. Specific examples of such a resin include an epoxy resin, a diglycol dialkyl carbonate resin, a polyimide resin, a melamine resin, a phenol resin, and a urea resin.

Hereinafter, an example of resin injection using an epoxy resin will be described, but the resin used in the present invention is not limited to the epoxy resin.

Novolak-type epoxy resin Epicoat 154 (trade name, manufactured by Yuka Shell Epoxy Co., Ltd.) was used as an epoxy resin, and a polyfunctional solid acid anhydride YH-308H [trade name: as a curing agent and a reactive diluent] was used. Yuka Shell Epoxy Co., Ltd.] and alkyl monoglycidyl ether BGE [trade name: Yuka Shell Epoxy Co., Ltd.]
The resin is cast under reduced pressure. The apparatus shown in FIG. 8 is an apparatus in which a resin injection nozzle 806 is provided in a processing container that can be evacuated and evacuated so that resin can be cast under reduced pressure. A resin injection nozzle 806 having a structure similar to that of a generally used injection molding machine can move on a rail 803 together with a resin tank 804. Also, nozzle 8
A constant temperature heater 805 is provided in parallel with the resin tank 06 and the resin tank 804 so that the temperature of the resin can be kept constant.
Further, the apparatus shown in the figure is provided with a squeegee moving means 812 for moving a squeegee (blade) 811 in order to spread the resin applied on the crystal uniformly.

In order to cast a resin using the apparatus, first, the substrate 809 on which the single crystal 807 has been grown first is put into a reduced-pressure vessel 801,
This is set on the support base 808, and the heater 802 arranged in the support base 808 is set at 100 ° C. Next, set the constant temperature heater 805 to 100 ° C and epicoat epoxy resin.
154, curing agent YH-308, reactive diluent BGE in a weight ratio of 100: 7
The resin is stored in the resin tank 804 at a ratio of 0:20, and then the inside of the vacuum container 801 is evacuated and reduced to a pressure of 10 ⁻³ Torr using a vacuum pump 813. When the resin in the resin tank 804 becomes uniform, the resin is discharged from the resin nozzle 806 onto the crystal 807, and the resin on the crystal 807 is distributed using the squeegee moving means 812 such that the resin is distributed in a thickness of about 1 μm. Spread. When the resin has spread, the pressure regulating valve 810 is gradually opened to bring the inside of the pressure reducing vessel 801 to atmospheric pressure, and the heater 802 is opened.
After keeping the temperature of 100 ° C. for 5 hours, the temperature of the heater 802 is raised to 200 ° C. and allowed to stand for 6 hours.
Here, when the substrate 809 is taken out of the decompression container 801 and touches the resin on the single crystal, it is found that the resin is strong.

The single crystal constituting the polycrystalline thin film formed by the method of the present invention can be doped with an impurity element during or after crystal growth to form a junction. As the impurity element to be used, as a p-type impurity, an element belonging to Group IIIA of the periodic table, for example, particularly, B, Al, Ga, In, or the like is particularly preferable. Elements belonging to Group A of the group V of the table, for example, P, As, Sb, Bi and the like are preferable, but B, Ga, P, Sb and the like are particularly preferable. The amount of the impurity to be doped is appropriately determined according to desired electric characteristics. As a substance containing such an impurity element as a component (impurity introduction substance), it is preferable to select a compound which is in a gaseous state at normal temperature and normal pressure or a compound which can be easily vaporized by an appropriate vaporizer. Such compounds include PH ₃ , P ₂ H ₄ , PF ₃ , PF ₅ , PCl ₃ , AsH ₃ , AsF
_{3, AsF 5, AsCl 3,} SbH 3, SbF 5, BF 3, BCl 3, BBr 3, B
It may be mentioned _{2 H 6, B 4 H 10} , B 5 H 9, B 5 H 11, B 6 H 10, B 6 H 12, AlCl 3 or the like. The compound containing an impurity element may be used alone or in combination of two or more.

The shape of the opening provided in the insulating layer when performing the selective crystal growth method used in the method of manufacturing the solar cell of the present invention is not particularly limited, but a square, a circle, and the like are typical examples. Regarding the size of the opening, the facet of the growing mountain-shaped single crystal body collapses as the opening increases as shown in Experiment 1, that is, the crystallinity tends to deteriorate, and the collapse of the facet is suppressed. Therefore, it is desirable that the thickness be several μm or less in order to facilitate peeling. Actually, since it depends on the pattern accuracy of photolithography, when the shape is a square, a is appropriately 1 μm or more and 5 μm or less. In addition, the interval b at which the openings are provided may be 2 irregularities on the surface of the polycrystalline silicon in consideration of the reflectance of light in the above-mentioned mountain facet and the ease of electrode formation.
It is appropriate that the thickness be 10 μm or more and 200 μm or less so as to be in the range of μm or more and 40 μm or less.

The layer configuration of the solar cell according to the method of the present invention is not limited, and can be applied to any structure such as a Schottky type, an MIS type, a pn junction type, a pin junction type, a hetero type, and a dandem type.

〔Example〕

Hereinafter, the formation of a desired solar cell by the method of the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Example 1 As described above, in the same manner as in Experiment 3, a pn-type solar cell composed of a collection of single-crystal silicon having a uniform crystal orientation as shown in FIG. 1 or FIG. 5 (H) was produced. Fifth
FIGS. 7A to 7H show the manufacturing process. The substrate is a P-doped (100) silicon wafer (ρ = 0.05Ωcm)
Was used. PSG (2000 °) is deposited in place of the thermal oxide film and used as an insulating layer 502, the opening is made square, three sides of a = 1.2 μm, 2 μm, and 4 μm are provided at intervals of b = 50 μm. Selective crystal growth is carried out by the LPCVD apparatus under the conditions shown in Table 3, and a polycrystalline silicon thin film composed of a collection of single-crystal silicon 503 having the same crystal orientation as shown in FIG. 3 (C) is formed on a silicon wafer. [FIG. 5 (A)].

At this time, P is simultaneously thermally diffused from the PSG to form an n ⁺ layer on the back surface side of the polycrystalline silicon (not shown).

In the same manner as in Experiment 8, B was implanted into the polycrystalline silicon surface by ion implantation under the conditions of 20 KeV and 1 × 10 ¹⁵ cm ⁻² ,
Annealing was performed at 00 ° C. for 301 minutes to form ap ⁺ layer 510 on the surface of the polycrystal. When performing ion implantation, implantation was performed at an angle several degrees off from the vertical direction with respect to the substrate, so that ions did not reach the peripheral portion of the crystal in contact with the insulating layer in the gap formed between the single crystals. In this way, when forming the back electrode, leakage between the back electrode and the junction can be avoided.

Next, the wafer is immersed in a 49% strength HF aqueous solution for 24 hours, the PSG 502 is removed by etching through a gap 504 of about 8 μm square, the wafer is washed with running water, and dried. Was applied to fill the void 504. Novolac epoxy resin epicoat as epoxy resin
154 (trade name: Yuka Shell Epoxy Co., Ltd.)
A multifunctional solid acid anhydride YH-308H [trade name: Yuka Shell Epoxy Co., Ltd.] is used as a curing agent and a reactive diluent.
Manufactured), alkyl monoglycidyl ether BGE [trade name:
Yuka Shell Epoxy Co., Ltd.] was mixed at a weight ratio of 100: 70: 20, and the resin was cast under reduced pressure. The resin is cast using an apparatus as shown in FIG. 8, and the resin is discharged from the resin injection nozzle 806 onto the polycrystalline surface 807 to form a squeegee 81.
1 spreads the resin evenly. After curing the resin at 100 ° C for 5 hours, O ₂ plasma etching (O ₂ flow rate: 500 sccm / mi
n, pressure: 1 Torr, high-frequency discharge power: 200 W), gap 5
The polycrystalline surface was completely exposed while leaving the resin only at the portion 04, and thereafter the resin was cured at a temperature of 250 ° C. for 3 hours (FIGS. 5B to 5D). Electron beam vapor deposition of a transparent conductive film ITO506 was performed on the entire surface of the polycrystalline surface, and Cr was vacuum-deposited thereon as a current collecting electrode 507 at a thickness of 1 μm (FIG. 5E). An epoxy resin 511 is again applied to the electrode surface in the same manner as described above, and the entire polycrystalline silicon film is fixed by a support 508, which is a glass plate, and after the resin is cured, a single crystal body 503 is collected from the silicon wafer 501. The polycrystalline silicon layer, which is the body, is separated, and some PSG remaining on the
Etching was performed with a HF aqueous solution having a concentration of 10%. Finally, a back electrode 509 was formed on the rear surface of the separated polycrystalline silicon layer by applying Cr at 3000 ° by vacuum evaporation [FIGS. 5 (F) to 5 (H)].

The IV characteristics of the obtained pn-type solar cell were measured under AM1.5 light irradiation. The cell area at this time is 0.16 cm ²
Met. Table 5 shows the results of comparing the characteristics with those of a solar cell similarly manufactured using a polycrystalline silicon film obtained when the growth was performed under the conditions of Table 1 with a growth time of 90 minutes.

In any case, the polycrystalline solar cell formed by the two-stage growth of the present invention has higher characteristics than the polycrystalline solar cell formed at a single substrate temperature of 1030 ° C. The reason that the characteristics tend to deteriorate as a becomes larger is considered to be due to the collapse of facets during crystal growth and, consequently, to an increase in defect levels resulting from a decrease in crystallinity.

Example 2 An amorphous silicon carbide / polycrystalline silicon hetero-type solar cell was produced in the same manner as in Example 1. The substrate is a P-doped (100) silicon wafer (ρ = 0.05Ω · c
m), a PSG film was deposited at a thickness of 3000 mm by a normal pressure CVD method. The opening has a size of only a = 1.2 μm and b = 30 μm
m. Selective crystal growth was performed by the ordinary LPCVD method under the conditions shown in Table 6 to form a polycrystalline silicon thin film composed of a set of polycrystalline silicon having a uniform crystal orientation. At this time, the size of the voids remaining between the single crystals was about 6 μm square. 6 (A) to 6 (H) show the processes of the fabricated hetero solar cell. Although it is almost the same as the case of FIG. 5 shown in the first embodiment, a p-type amorphous silicon carbide 610 is formed on the polycrystalline silicon instead of the p ⁺ layer 510 in FIG.

The p-type amorphous silicon carbide layer 610 was deposited on the surface of the polycrystalline silicon at a thickness of 100 ° by a usual plasma CVD apparatus under the conditions shown in Table 7. At this time, the dark conductivity of the amorphous silicon carbide film was 1010 ⁻² S · cm ⁻¹ , and the composition ratio of C and Si in the film was 2: 3.

Further, as the transparent conductive film 606, ITO was formed by electron beam evaporation of about 1000 °, and as the current collecting electrode 607, Cr was formed by vacuum evaporation of 1 μm.

When the IV characteristics of the thus obtained amorphous silicon carbide / polycrystalline silicon hetero solar cell under AM1.5 light irradiation were measured (cell area 0.16
cm ² ), open circuit voltage 0.52 V, short-circuit photocurrent 20 mA / cm ² , fill factor
The conversion efficiency was 0.56, which was a high value of 5.8%.
This is a result comparable to a conventional amorphous silicon carbide / single crystal silicon hetero solar cell.

Example 3 In the same manner as in Examples 1 and 2, a pn-type solar cell in which an impurity was added to a region near the interface as shown in FIG. FIGS. 9A to 9H show the manufacturing process. The substrate is a P-doped (100) silicon wafer (ρ =
0.05 Ω · cm). A thermal oxide film (1000
Using Å), the openings were a = 1.2 μm and b = 50 μm. Selective crystal growth is performed by the ordinary LPCVD apparatus shown in FIG. 7 under the conditions shown in Table 8, and a polycrystal consisting of a collection of single-crystal silicon 903 having the same crystal orientation as shown in FIG. A silicon thin film was formed on a silicon wafer [FIG. 9 (A)]. Hereinafter, a peelable solar cell having a pn junction was manufactured by the same process as in Example 1.

For the I-V characteristic under AM1.5 illumination of the resulting pn-type solar cell was subjected to measurement, open-circuit voltage 0.47V in cell area 0.16 cm ^2, short-circuit photoelectric current 24mA / cm ^2, a fill factor 0.71, The conversion efficiency was 8%, and the characteristics were further improved as compared with the case where no impurity was added in the initial stage of crystal growth (Example 1).

Embodiment 4 In the same manner as in Embodiments 1, 2, and 3, pi as shown in FIG.
An n-type polycrystalline solar cell was fabricated. The solar cell in Fig. 10 is obtained by adding a small amount of impurities to the source gas during selective crystal growth.
A pin junction is made in a single crystal. The important point here is that the end face of the p / i junction is on the insulating layer 1002 in the area where a gap is formed, so if the insulating layer is removed by etching to remove the obtained polycrystalline body from the wafer, The end face is exposed, and the subsequent steps (such as electrical insulation / separation accompanying the formation of the back electrode) become difficult. Therefore, as shown in FIG. 10 (A), the insulating layer is previously formed into a two-layer structure such as the substrate 1001 / SiO ₂ 1002 / Si ₃ N ₄ 1002 ′, and then the fine layer is formed by the reactive ion etching method. An opening was provided, and a polycrystalline film was obtained by a selective crystal growth method.

SiO ₂ and Si ₃ N ₄ were deposited by the normal pressure CVD method and the LPCVD method, respectively, and the film thicknesses were 1000 ° and 3000 °, respectively. The size of the openings was a = 1.2 μm, and the spacing was b = 50 μm. During the growth of the single crystal body by the selective crystal growth method, the type and the amount of impurities were changed, and the junction was formed by sequentially changing the conductivity type from nip from the substrate side. Table 9 shows the growth conditions.

The switching of the impurities requires that the growing single crystal
until it becomes m is an n layer was formed by introducing PH _3, then ceases PH _3, voids in contact with the single crystal bodies is about 6
At about μm, B ₂ H ₆ was introduced in the future to form a p-layer on the crystal surface by about 2000 °.

A solar cell having a pin junction was produced from the polycrystalline thin film thus obtained by a process as shown in FIGS. 10 (A) to 10 (H). As described above, the insulating layer has a two-layer structure. First, Si ₃ N ₄ 1002 ′ is etched through the void 1004 by reactive ion etching,
The SiO ₂ layer 1002 was removed by immersion in a 49% HF aqueous solution.
In this manner, the end surface of the pin junction is protected by the Si ₃ N ₄ film, and the legs are protruded from the n-layer 1003 ′ even after peeling, so that the connection with the back surface electrode can be easily performed. All other processes were performed in the same manner as in Examples 1 and 2.

Examination of the I-V characteristic at AM1.5 illumination of a pin junction type polycrystalline solar cell produced through the process described above, the open-circuit voltage 0.51V in cell area 0.16 cm ^2, short-circuit photoelectric current 25mA / cm ^2,
The fill factor was 0.7, and a high conversion efficiency of 8.9% was obtained.

As described above, according to the method for manufacturing a polycrystalline solar cell of the present invention, defects in the grown crystal near the interface between the grown crystal and the insulating layer in selective crystal growth are reduced, and a high-quality thin solar cell can be manufactured. It became so.

Further, it is possible to peel off from the wafer and fix it to a support such as glass by utilizing the voids remaining between the single crystal bodies that are the components of the polycrystalline silicon thin film, and it is possible to reuse the wafer many times. This indicated that an inexpensive solar cell with mass productivity was manufactured.

[Effects of the Invention] As described above, according to the present invention, a polycrystalline thin-film solar cell having high conversion efficiency formed on a wafer can be transferred to a support such as glass. This has made it possible to provide low-cost, high-quality thin solar cells with mass productivity to the market.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a pn-type solar cell manufactured by the method of the present invention, and FIG. 5 is a view for explaining the manufacturing process. FIG. 2 is a diagram illustrating a selective crystal growth method.
FIG. 3 is a view for explaining a process in which a mountain-shaped crystal obtained by the method of the present invention grows three-dimensionally. FIG. 4 is a view for explaining how defects entering a crystal are reduced by the method of the present invention. FIG. 6 shows a process of manufacturing a hetero-type solar cell manufactured by the method of the present invention. FIG. 7 is a diagram schematically showing a general LPCVD apparatus. FIG. 8 is a view showing an apparatus for performing resin casting. FIG. 9 and FIG. 10 are diagrams showing the steps of manufacturing pn-type and pin-type solar cells manufactured by the method of the present invention, respectively. In the figure, 201,501,601,809,901,1001 ... silicon wafer, 202,302,
502,602,902,1002,1002 '… insulating layer, 102,203,303,503,6
03,807,903,1003… Single-crystal silicon 204,301… Opening, 304,504,604,904,1004… Void, 305…
Grain boundaries, 903 ', 1003' ... n-type silicon layers, 103, 510, 910, 10
10 ... p-type silicon layer, 105,505,511,605,611,905,911,10
05,1011: epoxy resin, 104,506,606,906,1006 ... transparent conductive layer, 107,507,607,907,1007 ... collecting electrode, 106,508,60
8,908,1008: support, 610: p-type amorphous silicon carbide layer, 101,509,609,909,1009: back electrode, 701
... gas supply system, 702 ... heater, 703 ... quartz reaction tube, 704
... substrate, 705 ... susceptor, 801 ... pressure reducing vessel, 802 ... substrate heater, 803 ... rail, 804 ... resin tank, 805 ... constant temperature heater, 806 ... resin injection nozzle, 808 ... support base, 810
... pressure adjusting valve, 811 ... squeegee, 812 ... squeegee moving means, 813 ... exhaust device.

Claims (10)

(57) [Claims] A step of forming an insulating layer on a silicon wafer, a step of forming a polycrystalline semiconductor layer having voids exposing the surface of the insulating layer in a lattice-like manner, and forming the insulating layer on the insulating layer. A method of manufacturing a solar cell, comprising: a step of separating the semiconductor layer from the silicon wafer by performing etching removal using the gap; and a step of providing an electrode in the separated semiconductor layer. 2. The method according to claim 1, wherein the step of forming the semiconductor layer includes the steps of: providing a minute opening in the insulating layer; and performing selective epitaxial growth and lateral growth on the silicon surface exposed from the opening. The method for manufacturing a solar cell according to (1). 3. The method according to claim 2, wherein the selective epitaxial growth and the lateral growth are performed until the grown adjacent crystals collide with each other. 4. The method for manufacturing a solar cell according to claim 1, further comprising a step of filling said void with a resin. 5. After forming an electrode on the surface of the semiconductor layer,
The method for producing a solar cell according to any one of claims 1 to 4, further comprising a step of applying a resin on the electrode surface and fixing the resin to a substrate. 6. The crystal of the semiconductor layer has a planar defect density of 1 × 10 ⁴ cm ⁻¹ or less in the vicinity of an interface with an insulating layer.
A method for manufacturing the solar cell according to the above. 7. The method according to claim 2, wherein the selective epitaxial growth and the lateral growth are performed by a CVD method in which the substrate temperature is changed stepwise. 8. The method for manufacturing a solar cell according to claim 1, wherein said insulating layer is made of Si ₃ N ₄ or SiO ₂ . 9. The polycrystal comprises a set of angle-shaped single crystals,
The method according to any one of claims 1 to 8, further comprising a step of adding an impurity to the single crystal at an initial stage of growth. 10. The method for manufacturing a solar cell according to claim 1, wherein said semiconductor layer has irregularities on its surface.