JP3777668B2 - Method for manufacturing thin film solar cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a thin film solar cell.
[0002]
[Prior art]
Various materials have been studied as materials for solar cells, but silicon Si, which has abundant resources and does not have to worry about pollution, is the center, and over 90% of solar cell production in the world is Si solar cells. . By the way, the subject of a solar cell is low cost, high photoelectric conversion efficiency, high reliability, and short energy recovery years. Single crystal Si is most suitable for the requirements of high conversion efficiency and high reliability. However, this single crystal Si has a problem in cost reduction. Therefore, research and development of solar cells using thin polycrystalline Si and solar cells using thin-film amorphous Si have been actively conducted for solar cells, particularly high-area solar cells.
[0003]
A thin polycrystalline Si solar cell is made by purifying Si by purification technology from metal grade Si using plasma, etc., producing an ingot by a casting method, and a wafer, ie thin polycrystalline Si, is produced by a high-speed slicing technology such as multi-wire. Produced. However, such high-quality slicing techniques such as boron and phosphorus removal from metal grade Si, production of high-quality crystal ingots by casting method, increase of wafer area, multi-wire, etc. require extremely advanced techniques. For this reason, it has not been possible to produce thin polycrystalline Si that is sufficiently inexpensive and of good quality. Moreover, the thickness of the thin polycrystalline Si produced in this way is about 200 μm and does not have flexibility.
[0004]
On the other hand, since amorphous Si can be formed on the surface of a resin substrate by a CVD (chemical vapor deposition) method, it can be formed as a flexible thin-film amorphous Si. However, the conversion efficiency is lower than that of polycrystalline Si or single crystal Si, and there is a problem in the deterioration of the conversion efficiency during use.
[0005]
Single crystal Si can be expected to have high conversion efficiency and high reliability. Thin-film single-crystal Si can be manufactured by SOI (Silicon On Insulator) technology, which is a manufacturing technology for integrated circuits, etc., but the productivity is low, the manufacturing cost is considerably high, and there are problems in application to solar cells. . Further, in the production of single crystal Si, since the process temperature is relatively high, it is difficult to form it on a plastic substrate or glass substrate having low heat resistance. Thus, since it is difficult to form single crystal Si on a plastic substrate, it is difficult to manufacture a flexible thin film single crystal Si.
[0006]
However, in the case of a solar cell, when a solar cell with a solar cell disposed on the surface of the window glass or a solar car having a solar cell disposed on a roof or the like is configured, it is possible to use a flexible solar cell. This is desirable from the viewpoints of simplification of the configuration and easy arrangement of a rational arrangement with a large light receiving area. However, the semiconductor Si that can constitute such a flexible solar cell is only amorphous Si.
[0007]
Further, when the active layer of the solar cell is a thin film single crystal, light is easily transmitted and the lifetime of electrons generated thereby is long. Furthermore, the optical-electrical conversion efficiency (hereinafter simply referred to as efficiency) can be further increased by having an optical confinement effect.
[0008]
[Problems to be solved by the invention]
According to the present invention, a solar cell is composed of a thin film semiconductor, for example, thin film single crystal Si, so that it is highly efficient and can be configured as a flexible solar cell, and more effectively confines light. Therefore, the present invention provides a thin-film solar cell that can easily and surely produce a thin-film solar cell with high efficiency and thus can reduce the energy recovery years.
[0009]
[Means for Solving the Problems]
The method for producing a thin-film solar cell according to the present invention includes a step of changing the surface of a semiconductor substrate into a porous layer, a step of forming a semiconductor film constituting at least an active layer of the solar cell on the porous layer, and the semiconductor A step of peeling the film from the semiconductor substrate in the porous layer, and an etching step of removing the remaining porous film of the porous layer remaining on the peeling surface of the semiconductor film, The step of changing to a porous layer includes a step of forming a surface layer having a porosity of 30% or less and a porosity layer having a porosity higher than the porosity of the surface layer on a lower layer side than the surface layer. A target solar cell is obtained.
[0010]
According to the above-described manufacturing method of the present invention, the semiconductor substrate surface itself is changed to form a porous layer, and a semiconductor film is formed on the porous layer. The semiconductor film is separated from the semiconductor substrate by breakage in the porous layer. The target solar cell is peeled to form a solar cell with a semiconductor film that is excellent in crystallinity and sufficiently thin. Therefore, it is possible to configure a solar cell that is highly efficient and flexible as necessary.
[0011]
Further, according to the method of the present invention, the semiconductor film is peeled off from the semiconductor substrate by a porous layer, and the porous layer has its separation strength by selecting the size of the strain and the porosity. Since it can be selected, it can be reliably and easily peeled off, and a solar cell can be manufactured with high yield.
[0012]
In particular, in the present invention, the remaining porous film of the porous layer on the back surface of the semiconductor film is removed by performing an etching process on the peeled surface, that is, the back surface of the semiconductor film peeled from the semiconductor substrate. Therefore, it is possible to avoid a decrease in electromotive force of the solar cell due to the presence of the porous film. That is, when such a porous film is present on the back surface of the solar cell, a large number of circular vacancies are present in the porous film, so that it passes through the semiconductor film and arrives here. In the present invention, the light is easily scattered in the porous film, and when the porous film has a high impurity concentration, the absorption of the electromotive force increases and the efficiency of the solar cell decreases. Since the porous film remaining on the peeled surface of the semiconductor film is removed by etching, a decrease in electromotive force due to the remaining porous film can be avoided.
[0013]
Moreover, since the unevenness of the back surface of the semiconductor film can be promoted by this etching treatment, so-called texture can be formed on the back surface, and incident light can be effectively reflected and scattered in the semiconductor film, Also, by forming irregularities on the surface side of the semiconductor film by selecting the deposition conditions of the semiconductor film, these irregularities can more effectively confine light in the semiconductor film and generate carriers. Since the efficiency can be increased, higher efficiency as a solar cell can be achieved.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described.
In the present invention, the surface of the semiconductor substrate is changed by, for example, anodization to form a porous layer. This porous layer is a porous layer composed of two or more layers having different porosities. And the semiconductor film which comprises at least an active part of a solar cell is epitaxially grown on the surface of this porous layer.
[0015]
The surface of the semiconductor film is etched, for example, anisotropically etched, so that the unevenness formed on the surface is made more uneven, that is, the so-called texture is formed.
[0016]
The semiconductor film formed on the semiconductor substrate is peeled off from the semiconductor substrate in the porous layer, and the remaining porous film of the porous layer remaining on the peeled surface of the semiconductor is removed by etching and present on this surface. The so-called texture is formed by making the unevenness to be made more uneven, that is, promoted, and a metal electrode layer is deposited on the peeled surface to produce a target thin film solar cell.
[0017]
The semiconductor substrate can be composed of a silicon Si single crystal body, or a SiGe, GaAs, or GaN single crystal semiconductor material substrate. However, when a Si thin film solar cell is formed, it is preferable to use a Si single crystal substrate.
[0018]
Various shapes can be adopted as the shape of the semiconductor substrate. For example, various shapes such as a wafer shape, that is, a disk shape, or a cylindrical ingot formed by pulling a single crystal having a curved substrate surface can be used.
[0019]
The semiconductor substrate can be constituted by a semiconductor substrate doped with n-type or p-type impurities, or a semiconductor substrate not containing impurities. However, in the case of anodizing, a low resistivity semiconductor substrate so-called p-type doped with a high concentration of p-type impurities. ⁺ It is preferable to use a Si substrate. As this semiconductor substrate, p ⁺ When using a type Si substrate, p-type impurities such as boron B are about 10 ¹⁹ atoms / cm ^Three It is desirable to use a Si substrate that is doped to the extent that the resistance is about 0.01 to 0.02 Ωcm. And this p ⁺ When the type Si substrate is anodized, fine pores elongated in a direction substantially perpendicular to the substrate surface are formed, and the pores are maintained while maintaining the crystallinity, so that a desirable porous layer is formed.
[0020]
As described above, the porous layer can be formed by anodizing the surface of the semiconductor substrate. This anodization can be performed using an electrolytic solution containing hydrogen fluoride and ethanol, or an electrolytic solution containing hydrogen fluoride and methanol.
[0021]
This anodization is carried out by a known method such as the surface technology Vol. 46, no. 5, pp. 8-13, 1995 [Anodic conversion of porous Si]. That is, for example, it can be performed by the double cell method whose schematic configuration is shown in FIG. In this method, an electrolytic solution tank 1 having a two-tank structure having first and second tanks 1A and 1B is used. Then, a semiconductor substrate 11 on which a porous layer is to be formed is disposed between both tanks 1A and 1B, and each one of a pair of platinum electrodes 3A and 3B connected to a DC power source 2 is disposed in both tanks 1A and 1B. Is done. In the first and second tanks 1A and 1B of the electrolytic solution tank 1, for example, hydrogen fluoride HF and ethanol C are respectively provided. ₂ H _Five Electrolytic solution containing OH, or hydrogen fluoride HF and methanol CH _Three An electrolytic solution containing OH is accommodated, the first and second tanks 1A and 1B are arranged so that both surfaces of the semiconductor substrate 11 are in contact with the electrolytic solution, and both electrodes 3A and 3B are immersed in the electrolytic solution. Is done. Then, the DC power source 2 is connected to the electrode 3A side immersed in the electrolytic solution in the tank 1A on the surface side where the porous layer of the semiconductor substrate 11 is to be formed, and a current is passed between the electrodes 3A and 3B. Is made. In this way, power is supplied with the semiconductor substrate 11 side as the anode side and the electrode 3A as the cathode side, whereby the surface of the semiconductor substrate facing the electrode 3A side is eroded and becomes porous.
[0022]
When this two-cell method is used, it is not necessary to deposit the ohmic electrode on the semiconductor substrate, and the introduction of impurities from the ohmic electrode into the semiconductor substrate is avoided.
[0023]
The structure of the porous layer to be formed changes depending on the selection of conditions in this anodization, and this changes the crystallinity and peelability of the semiconductor film epitaxially grown thereon.
[0024]
The porous layer forms a porous layer composed of two or more layers having different porosities. The outermost porous layer is formed as a relatively small and dense porous layer having a porosity of 30% or less so that an epitaxial semiconductor film can be satisfactorily grown on the porous layer. On the inner side of the surface layer, that is, the lower layer side, a porous layer having a relatively high porosity is formed along the surface of the substrate to reduce the mechanical strength due to its own high porosity, or this porous layer and other layers The layer is weakened by strain based on the difference in lattice constant, and the semiconductor film can be easily peeled off, that is, separated in this layer. For example, it is possible to form a porous layer that is weak enough to be separated by application of ultrasonic waves.
[0025]
The layer formed on the inner side of the surface of the porous layer with the increased porosity becomes easier as the porosity increases. However, if the porosity is too high, the above-described epitaxial semiconductor film may be peeled off. In addition, peeling may occur or the porous layer may be damaged. Therefore, the porosity of the layer having a large porosity is set to 40% or more and 70% or less.
[0026]
In addition, when a layer having a high porosity is formed in the porous layer, the strain increases as the porosity increases, and the strain on the surface layer of the porous layer increases, and epitaxial growth occurs on the top. Although unevenness can be generated on the surface of the semiconductor film, if the strain on the surface layer is excessively large, cracks are generated in the surface layer or crystal defects are generated in the semiconductor film to be epitaxially grown. Therefore, the porous layer has a higher porosity than the surface layer as a buffer layer to relieve strain between the layer with high porosity and the surface layer with low porosity. Thus, an intermediate porosity layer having an intermediate porosity with a low porosity is formed. By doing so, the porosity of the high-porosity layer is increased to such an extent that the above-described semiconductor film can be reliably peeled off, and an epitaxial semiconductor film having excellent crystallinity can be formed.
[0027]
In order to form a porous layer composed of two or more layers having different porosities, a multi-step anodizing method having two or more steps with different current densities is employed in the anodizing treatment. Specifically, in order to produce a relatively dense porous layer having a low porosity on the surface, that is, a fine pore having a small diameter, first, the first anodization is performed at a low current density. Since the film thickness of the porous layer is proportional to time, the anodization is performed in a time period that achieves a desired film thickness. Thereafter, if the second anodization is performed at a considerably high current density, a porous layer having a high porosity and a high porosity is formed below the porous layer having a low porosity formed first. That is, a porous layer having a low porosity layer having a low porosity and a high porosity layer having a high porosity is formed.
[0028]
In this case, a large strain is generated in the vicinity of the interface between the porous layer having a low porosity and the porous layer having a high porosity due to the difference in lattice constant between the two. When this strain exceeds a certain value, the porous layer separates into two. Therefore, if a porous layer is formed under an anodizing condition in the vicinity of the boundary condition of whether or not separation due to this strain or mechanical strength due to porosity will occur, it will be grown on this porous layer. A semiconductor film such as an epitaxial semiconductor film can be easily separated through this porous layer.
[0029]
In this case, the first anodization with a low current density uses, for example, a p-type silicon single crystal substrate of 0.01 to 0.02 Ωcm, and HF: C ₂ H _Five OH = 1: 1 (HF is 49% solution, C ₂ H _Five When the volume ratio of OH is 95% solution) (hereinafter the same), 0.5 to 10 mA / cm ² It is performed for several minutes to several tens of minutes at a low current density. Further, the second anodization with a high current density is, for example, 40 to 300 mA / cm. ² The current density is about 1 to 10 seconds, preferably about 3 seconds.
[0030]
In the first and second two-stage anodization described above, the strain generated in the highly porous layer inside the porous layer becomes considerably large. Therefore, the influence of this strain extends to the surface of the porous layer. As described above, the generation of cracks and the possibility of generating crystal defects in the epitaxial semiconductor film formed thereon are caused. Therefore, the porous layer has a higher porosity than the surface layer as a buffer layer to relieve the distortion generated between the surface layer having a low porosity and the high porosity layer. An intermediate porosity layer having a lower porosity is formed. Specifically, the first anodization with a low current density is first performed, then the second anodization with a slightly higher current density than the first anodization is performed, and then the third anode with a much higher current density than those. Perform formation. The conditions for the first anodization are not particularly limited. For example, a p-type silicon single crystal substrate of 0.01 to 0.02 Ωcm is used, and HF: C is used as the electrolytic solution. ₂ H _Five When using OH = 1: 1, 0.5 to 3 mA / cm ² The current density of the second anodization is, for example, 3 to 20 mA / cm. ² The current density of the third anodization is, for example, 40 to 300 mA / cm. ² It is preferable to carry out at a degree. For example, 1 mA / cm ² When anodization is performed at a current density of about 16%, the porosity is about 7 mA / cm. ² When anodizing at a current density of about 26%, the porosity is about 200 mA / cm. ² When anodizing is performed at a current density of about 60 to 70%. When epitaxial growth is performed on the anodized porous layer, an epitaxial semiconductor film with good crystallinity can be formed.
[0031]
Further, as described above, when anodizing with three stages of current density is performed, the surface layer with low porosity formed by the first anodizing maintains the low porosity as it is, and the porosity formed by the second anodizing is maintained. A slightly higher intermediate porosity layer, i.e., a buffer layer, is formed inside the surface layer, i.e., at the interface between the surface layer and the non-porous semiconductor substrate. It becomes a two-layer structure with a layer. In addition, the principle of the high porosity layer formed by the above-described third anodization is not known, but the current density is 90 mA / cm. ² If it is more than about, it is formed in the intermediate porosity layer formed by the second anodization, that is, in the intermediate portion in the thickness direction of the intermediate porous layer.
[0032]
Further, in the formation of the intermediate porosity layer, the anodic oxidation for forming the intermediate porosity layer is performed in a multi-stage or gradually under, for example, a condition in which the energization current density is changed, whereby a low porosity surface layer, a high porosity layer, An intermediate porosity layer is formed in which the porosity is increased stepwise or inclined from the surface layer toward the high porosity layer. In this way, the strain between the surface layer and the high porosity layer is further relaxed, and an epitaxial semiconductor film with good crystallinity can be epitaxially grown more reliably.
[0033]
By the way, the separation surface can be formed by applying a large strain due to the difference in lattice constant at the interface between the release layer having a large porosity performed last and the buffer layer having a low porosity performed immediately before. If the device is devised when anodizing is performed, the separation surface is more easily separated. It is the last anodization with a high current density. For example, instead of flowing the time constant for 3 seconds, after an energization for 1 second, the anodization is temporarily stopped, and after leaving the required time, for example, for about 1 minute, The anodization is stopped again by energizing for 1 minute at the same or different high current density, and after leaving the required time, for example, after leaving for about 1 minute, after energizing again for 1 second at the same or different high current density, the anode In this method, current is intermittently passed to stop the formation. When an appropriate anodization condition is selected using this method, a release layer is formed at the interface with the semiconductor substrate, that is, the lowermost surface of the porous layer, and the separation surface is the inside of the intermediate porous layer, that is, the buffer layer as described above. Instead, they are separated at the interface of the porous layer with the semiconductor substrate. The surface of the semiconductor substrate side is electropolished.
[0034]
Therefore, the highly porous layer in which the porous layer is distorted and the surface are separated as much as possible, and the buffer effect of the intermediate porosity layer is maximized, and the epitaxial semiconductor film has good crystallinity. Can be formed. In addition, since the intermediate porous layer is formed only on the surface side in this way, the entire thickness of the porous layer can be reduced, and the consumption thickness of the semiconductor substrate for forming this porous layer is reduced. Therefore, the number of repeated use of the semiconductor substrate can be increased.
[0035]
As described above, by selecting the anodizing conditions, a large strain can be applied to the separation surface, and the influence of this strain can be prevented from being applied to the epitaxial growth surface of the semiconductor film.
[0036]
Further, in order to perform semiconductor epitaxial growth on the porous layer with good crystallinity, it is desired to reduce the micropores that become seeds for crystal growth on the surface layer of the porous layer. As one of means for reducing the micropores in the surface layer as described above, there is a method of increasing the HF concentration in the electrolytic solution in anodizing. That is, in this case, first, in the low current anodization for forming the surface layer, an electrolytic solution having a high HF concentration is used. Next, an intermediate porosity layer serving as a buffer layer is formed. After that, the HF concentration of the electrolytic solution is lowered, and finally anodization with a high current density is performed. By doing so, the fine pores of the surface layer can be miniaturized, so that an epitaxial semiconductor film with good crystallinity can be formed thereon, and in the high porosity layer, Since the porosity can be made sufficiently high, the epitaxial semiconductor film can be peeled off satisfactorily.
[0037]
The change of the electrolytic solution in the anodization of the porous layer is, for example, as an electrolytic solution in the formation of the surface layer, such as HF: C ₂ H _Five Anodization using an electrolytic solution with OH = 2: 1 is performed, and in forming an intermediate porosity layer as a buffer layer, an electrolytic solution having a slightly lower HF concentration, for example, HF: C ₂ H _Five When anodization using an electrolytic solution with OH = 1: 1 is performed and a high porosity layer is formed, the electrolytic solution is further reduced in HF concentration, for example, HF: C ₂ H _Five Anodization with a high current density is performed using an electrolytic solution of OH = 1: 1 to 1: 2.
[0038]
In the formation of the porous layer described above, when the current density is changed from the formation of the surface layer to the formation of the intermediate porosity layer, the anodization is temporarily stopped and then energization for the next anodization is started. It can also be carried out by changing the current density continuously without stopping anodization, that is, without stopping energization.
[0039]
Moreover, when performing anodization, it is preferable to carry out in the dark place which blocked light. This is because when the light is irradiated, the surface of the porous layer becomes uneven, making it difficult to obtain an epitaxial semiconductor film with good crystallinity.
[0040]
By the above steps, a semiconductor substrate having a porous layer formed on the surface (one side or both sides) can be obtained.
[0041]
In this manner, a semiconductor film is formed on the porous layer formed by changing the surface of the semiconductor substrate. Prior to forming the semiconductor film, the porous layer is annealed. This annealing can include heat treatment in a hydrogen gas atmosphere, that is, hydrogen annealing. This hydrogen annealing can re-crystallize the porous layer so that the surface becomes uneven. In addition, when this hydrogen annealing is performed, the natural oxide film formed on the surface of the porous layer can be completely removed and oxygen atoms in the porous layer can be removed as much as possible, so that the surface of the porous layer is smooth. Thus, a semiconductor film having good crystallinity can be formed. At the same time, the pretreatment can further weaken the strength of the interface between the high porosity layer and the intermediate porosity layer, and the semiconductor film can be more easily separated from the substrate. In this case, the hydrogen annealing is performed in a temperature range of about 950 ° C. to 1150 ° C., for example.
[0042]
In addition, if the porous layer is oxidized at a low temperature before hydrogen annealing, the inside of the porous layer is oxidized, so that a large structural change occurs in the porous layer even if thermal annealing is performed in a hydrogen gas atmosphere. Absent. That is, the strain from the peeling layer is hardly transmitted to the surface of the porous layer, and a high-quality crystalline semiconductor film can be formed. In this case, the low-temperature oxidation can be performed in a dry oxidation atmosphere at 400 ° C. for about 1 hour, for example.
[0043]
Then, a semiconductor film is epitaxially grown on the porous layer. Although this porous layer is porous, it is porous while maintaining the crystallinity of the single crystal semiconductor substrate, so that the semiconductor film can be epitaxially grown on the porous layer. The epitaxial growth on the surface of the porous layer can be performed, for example, at a temperature of 700 ° C. to 1100 ° C., for example, by a CVD method.
[0044]
In this case, unevenness is generated on the surface of the epitaxially grown semiconductor film. As described above, the semiconductor film having irregularities on the surface increases the influence of strain on the surface layer in the porous layer to such an extent that no crystal defects are generated. The epitaxial growth conditions can be formed by shortening the hydrogen annealing time and increasing the epitaxial growth time, shortening the hydrogen annealing time when the hydrogen annealing temperature is the same as the epitaxial growth temperature, or increasing the epitaxial growth temperature above the hydrogen annealing temperature. . Specifically, H ₂ During the annealing, the porous layer is recrystallized to form irregularities on the surface of the porous layer under the condition that the surface becomes irregular. After that, if the semiconductor is epitaxially grown at a reduced temperature, the recrystallization in the porous layer progresses slowly, so the strain effect on the epitaxially grown semiconductor film is reduced and a high-quality semiconductor film that does not absorb crystal defects is formed. Can be membrane.
[0045]
H ₂ The annealing temperature and epitaxial growth temperature are the same and H ₂ Annealing time is short or H ₂ Under conditions where the epitaxial growth temperature is higher than the annealing temperature, the porous layer is H ₂ After being recrystallized by annealing, the porous layer is recrystallized during the epitaxial growth to form multiple round holes, so that the surface becomes uneven, but there are defects in the crystal of the deposited semiconductor film. May occur. However, even in such a case, when a solar cell is configured, it can sufficiently withstand use.
[0046]
Then, when the surface of the semiconductor film on which the unevenness is thus formed is etched with a chemical, the unevenness is further promoted. In other words, the hole shape is enlarged and the fine irregularities are enlarged, so that the number of irregularities is increased, and so-called texture is generated. This etching is preferably performed by anisotropic etching. This anisotropic etching is performed by, for example, a mixed solution of hydrofluoric acid and acetic acid, potassium hydroxide KOH, sodium hydroxide NaOH, such as 60% hydrazine N. ₂ H _Four Etc. can be used.
[0047]
In addition, the semiconductor film formed on the porous layer is formed of a single-layer semiconductor film, and a pn junction that forms an active part of the solar cell is formed by impurity ion implantation, diffusion, or the like. In addition, in this semiconductor film epitaxial growth, a multilayer semiconductor film having two or more layers can be formed.
[0048]
As described above, the semiconductor film epitaxially grown on the semiconductor substrate is peeled off from the semiconductor substrate. Prior to this peeling, for example, a support substrate such as a flexible resin sheet is joined on the semiconductor film, and the support substrate and the epitaxial semiconductor film are bonded. Then, the epitaxial semiconductor film can be peeled from the semiconductor substrate together with the supporting substrate through a porous layer formed on the semiconductor substrate.
[0049]
This support substrate can be a surface protection substrate of the solar cell finally obtained. In this case, it is a transparent support substrate. This support substrate is not limited to a flexible sheet, and in the case of constituting a solar cell that does not require flexibility, it can be constituted by a so-called rigid glass substrate, a resin substrate, or, for example, It can also be constituted by a flexible or transparent printed circuit board with required printed wiring.
[0050]
On the peeled surface, that is, the back surface of the semiconductor film that has been peeled off from the semiconductor substrate, a porous film in which at least the surface layer of the porous layer, the intermediate porosity layer, and the like remain is applied. In the present invention, this is removed by etching. Also in this etching, for example, a mixed solution of hydrofluoric acid and acetic acid, potassium hydroxide KOH, sodium hydroxide NaOH, hydrazine N ₂ H _Four Etc. by anisotropic etching. In this way, the porous film remaining on the back surface of the semiconductor film is removed, and similarly, the unevenness generated based on the surface property of the porous layer formed on the back surface is promoted, and the texture is increased. Is generated.
[0051]
In both of the above-described hydrogen annealing and semiconductor epitaxial growth, the semiconductor substrate can be heated to a predetermined substrate temperature by a so-called susceptor heating method, or heated by direct current flowing through the semiconductor substrate itself. It is possible to adopt an electric heating method or the like.
[0052]
Next, examples of the present invention will be described. However, the present invention is not limited to this embodiment.
[0053]
[Example 1]
1 and 2 show process diagrams of this embodiment.
First, a wafer-like (100) plane direction semiconductor substrate 11 made of single crystal Si doped with boron at a high concentration and having a specific resistance of, for example, 0.01 to 0.02 Ωcm was prepared (FIG. 1A).
.
Then, the surface of the semiconductor substrate 11 was anodized to form a porous layer on the surface of the semiconductor substrate 11. In this example, anodization was performed using the two-tank structure anodizing apparatus described in FIG. That is, a semiconductor substrate 11 made of single crystal Si is disposed between the first and second tanks 1A and 1B, and both tanks 1A and 1B are both HF: C. ₂ H _Five OH = 1: 1 was injected. Then, a current was passed by the DC power source 2 between the Pt electrodes 3A and 3B placed soaked in the electrolytic solutions in the electrolytic solution tanks 1A and 1B.
[0054]
First, the current density is 1 mA / cm. ² This was energized for 8 minutes. As a result, a surface layer 12S constituting a porous layer having fine pores with a small diameter, a dense porosity of 16%, and a thickness of 1.7 μm was formed (FIG. 1B). If the micropores on the surface of the porous layer are small, H to be performed later ₂ By annealing, the surface of the porous layer becomes flatter and smoother, and there is an effect that the crystallinity of the Si epitaxial semiconductor film epitaxially grown on the surface is further improved.
Thereafter, the energization is temporarily stopped. Next, the current density is 7 mA / cm. ² As a result, energization was performed for 8 minutes. Thus, an intermediate porosity layer 12M having a porosity of 26% and a thickness of 6.3 μm was formed under the surface layer 12S (FIG. 1C).
Thereafter, the energization is stopped again. Next, the current density is 200 mA / cm. ² And then energized for 3 seconds. In this way, the thickness having a porosity of about 60%, which is higher than the surface layer 12S and the intermediate porosity layer 12M, so as to be sandwiched from above and below by the intermediate porosity layer 12M. A high porosity layer 12H having a thickness of about 0.05 μm is formed (FIG. 2D). In this way, the porous layer 12 is formed by the surface layer 12S, the intermediate porosity layer 12M, and the high porosity layer 12H.
[0055]
The high-porosity layer 12H thus formed is itself fragile because of its high porosity. Further, in the porous layer 12, since the porosity of the intermediate porosity layer 12M and the high porosity layer 12H are greatly different, a large strain is generated at these interfaces and in the vicinity of the interfaces, and the strength in this vicinity becomes extremely weak. However, this strain acts as a buffer due to the presence of the intermediate porosity layer 12M between the high porosity layer 12H and the surface layer 12S, and the surface of the porous layer is greatly affected by this strain. The effect of distortion can be mitigated. Therefore, this strain can effectively avoid the influence of the epitaxial growth performed later on the porous layer on the crystallinity.
[0056]
Thereafter, in a normal pressure Si epitaxial growth apparatus in which epitaxial growth performed later is performed, the semiconductor substrate 11 having the porous layer 12 is made to be H ₂ Heating at 1030 ° C., that is, annealing treatment was performed in the atmosphere. In this annealing, the temperature was raised from room temperature to 1030 ° C. over about 20 minutes, and annealing was performed at this temperature for about 6 minutes. This H ₂ Annealing makes the surface layer of fine holes with a small diameter flat and smooth. At the same time, in the porous layer 12, the separation strength became even weaker in the vicinity of the interface between the intermediate porosity layer 12 </ b> M and the high porosity layer 12 </ b> H. In this case, many irregularities are formed on the surface of the porous layer 12, and the interface between the porous layer 12 and the semiconductor substrate, that is, the interface between the semiconductor substrate portion where the porous layer is not formed. In addition, the fine pores of the porous layer were further recrystallized by thermal energy, resulting in a large number of holes.
[0057]
Then H ₂ In the atmospheric pressure Si epitaxial growth apparatus that was annealed, the temperature was lowered to 100 ° C., and the semiconductor film 13 was epitaxially grown (FIG. 2E). The semiconductor film 13 in this embodiment is p ⁺ -P ^- -N ⁺ A three-layer structure was adopted. SiH _Four Gas and B ₂ H ₆ Epitaxial growth using gas is performed for 10 minutes, ⁺ A first semiconductor layer 131 made of Si is formed, and then B ₂ H ₆ The gas flow rate was changed, Si epitaxial growth was performed for 10 minutes, and a low concentration p doped with boron B at a low concentration ^- A second semiconductor layer 132 made of Si is formed, and B ₂ H ₆ PH instead of gas _Three Gas is supplied and epitaxial growth is performed for 4 minutes. ^- On the epitaxial semiconductor layer 132, n in which phosphorus P is highly doped ⁺ A third semiconductor layer 133 made of Si is formed, and p made of the first to third epitaxial semiconductor layers 131 to 133 is formed. ⁺ -P ^- -N ⁺ A semiconductor film 13 having a structure was formed.
[0058]
Next, SiO 2 is formed on the semiconductor film 13 by surface thermal oxidation. ₂ A film, that is, a transparent insulating film 16 is formed, and pattern etching by photolithography is performed to form an opening 16W that makes contact with an electrode or wiring (FIG. 3F). The openings 16W can be formed in parallel with a stripe shape extending in a direction orthogonal to the paper surface in the drawing while maintaining a required interval. SiO formed in this way ₂ The film can minimize the generation of carriers and recombination at the interface.
[0059]
Then, a metal film is deposited on the entire surface, and pattern etching by photolithography is performed to form electrodes or wirings 17 on the light receiving surface side along the stripe-shaped openings 16W (FIG. 3G). The metal film forming this electrode or wiring 17 is a multilayer formed by, for example, sequentially depositing a Ti film with a thickness of 30 nm, Pd with a thickness of 50 nm, and Ag with a thickness of 100 nm, and further performing Ag plating thereon. It may be constituted by a structural film. Thereafter, annealing was performed at 400 ° C. for 20 to 30 minutes.
[0060]
Next, in this embodiment, a conductive wire 41, a metal wire in this embodiment, is joined to the striped electrodes or wirings 17 along these, respectively, and supported by a transparent adhesive 21 thereon. A substrate, in this example, a transparent substrate 42 made of, for example, a PET (polyethylene terephthalate) sheet having corrosion resistance against etching performed later is bonded (FIG. 4H). Bonding of the conductive member 41 to the electrode or the wiring 17 can be performed by soldering. These conductive wires 41 are led out outward by making one end or the other end longer than the electrode or wiring 17.
[0061]
Thereafter, an external force is applied to the semiconductor substrate 11 and the transparent substrate 42 to separate them from each other. In this way, the semiconductor substrate 11 and the semiconductor film 13 are separated from each other in the porous layer 12, and a thin film semiconductor 23 in which the epitaxial semiconductor film 13 is bonded to the transparent substrate 42 is obtained (FIG. 4I). In this case, the porous film 12 </ b> R due to the remaining porous layer 12 is deposited on the back surface of the thin film semiconductor 23.
[0062]
Next, the semiconductor film 13 supported on the transparent substrate 42 (support substrate) is immersed in a mixed solution of hydrofluoric acid to remove the porous film 12R by etching, so that the back surface of the semiconductor film 13, that is, the back surface of the first semiconductor film 131 is obtained. However, an uneven surface in which the unevenness is promoted by etching is formed (FIG. 5J). At this time, the conductive wire 41 or the like is not eroded on the surface side by the corrosion-resistant transparent substrate 42.
[0063]
Then, a silver paste is applied to the back surface to form the back electrode 24. In this way, the printed circuit board 20 is p. ⁺ -P ^- -N ⁺ A solar cell in which a thin film semiconductor 23 having a structure is formed is formed (FIG. 5F). The back electrode 24 also functions as a protective film on the back surface of the solar cell.
[0064]
As shown in the schematic diagram based on the micrograph in FIG. 7, the semiconductor film 13 thus formed has irregularities in which circular holes are formed on the front surface 13a, and an irregular surface is also formed on the back surface. Has been.
In FIG. 7, the light irradiation to the solar cell is incident from the transparent substrate 42. As a result, the light incident on the semiconductor film 13 is scattered and reflected by the uneven surface on the back surface as shown in FIG. Since the light is effectively returned to the inside of the film 13, the light utilization rate is increased, and carriers are generated effectively, that is, an electromotive force is generated. In particular, when the back electrode 24 is deposited on the uneven surface of the back surface, the reflection of light into the semiconductor film 13, that is, the light confinement can be performed more effectively.
[0065]
[Example 2]
In this embodiment, the unevenness of the surface of the semiconductor film 13 is made more conspicuous so that the optical confinement of the semiconductor film 13 is performed more effectively. In this case, the semiconductor substrate 11 is a normal (100) Si substrate. Also in this embodiment, as described in the first embodiment, the semiconductor film 13 is formed by performing the same steps as in FIGS. 1 and 2, and then anisotropic to the surface of the semiconductor film 13. Etching was performed. In this case, the etching was performed by immersing the semiconductor substrate 11 on which the semiconductor film 13 was formed in a KOH aqueous solution at 70 ° C. In this manner, the convex portion on the surface of the semiconductor film 13 maintains its shape, and the portion where the (100) crystal plane appears is flat and the (111) crystal plane appears. As shown in FIG. 8B, the surface 13 a shown in FIG. 8A was shown before the etching, as shown in FIG. 8, which is a schematic cross-sectional view based on a micrograph of the semiconductor film 13. As described above, the entire surface is formed with irregularities having a V-shaped cross section.
[0066]
Thus, after forming unevenness | corrugation, ie, a texture also in the surface side of the semiconductor film 13, the target solar cell is obtained through the process of FIGS. 3-5 similar to Example 1. FIG.
[0067]
According to this configuration, light can be confined more effectively on both surfaces of the semiconductor film 13.
[0068]
As described above, according to the method of the present invention, a thin-film solar cell that is sufficiently thin and excellent in crystallinity and therefore highly efficient can be configured flexibly, and an optical confinement structure is imparted to the thin-film solar cell. Therefore, higher efficiency can be achieved. Since the texture of the light confinement structure can be formed by simply etching the entire surface, it is not necessary to form irregularities using an etching mask, and the manufacture is simplified.
[0069]
In each of the above-described examples, p is formed when the semiconductor film 13 is formed. ⁺ -P ^- -N ⁺ In the case where the structure is formed, the semiconductor film 13 is formed and then ion implantation and diffusion of impurities are performed to form p. ⁺ -P ^- -N ⁺ A structure can also be formed.
[0070]
In the above-described example, the epitaxial semiconductor film is peeled off from the semiconductor substrate 11 by applying an external force that separates the epitaxial semiconductor film, but in some cases, the epitaxial semiconductor film can be peeled off by ultrasonic vibration.
[0071]
Further, in anodization, when the current density is large, or when the semiconductor, for example, Si is peeled off due to energization for a long time and Si waste is generated and adhered to the inside of the apparatus, for example, an electric field solution tank, the semiconductor substrate After removing 11, unnecessary Si deposits can be dissolved and removed by injecting a hydrofluoric acid solution into the tank instead of the electrolytic solution.
[0072]
Further, the apparatus for anodizing is not limited to the example of FIG. 6, and an apparatus for immersing a semiconductor substrate in a single tank structure can be used.
[0073]
Further, the above-described thin film solar cell is repeatedly manufactured by epitaxially growing a semiconductor having a thickness corresponding to the reduced thickness on the semiconductor substrate whose thickness is reduced by manufacturing the thin film semiconductor and the solar cell. By doing so, since the same semiconductor substrate can be used permanently, a solar cell can be manufactured at lower cost and lower energy.
[0074]
According to the manufacturing method of the present invention described above, the semiconductor substrate is formed with a porous layer on the surface, the semiconductor is epitaxially grown on the semiconductor layer, and the semiconductor substrate is peeled off. However, if the surface of the semiconductor substrate is removed by etching and electrolytic etching after the formation and peeling of the above-described epitaxial semiconductor film, the semiconductor substrate 11 is repeatedly used again to achieve the intended thin film semiconductor. That is, since a plurality of various thin-film semiconductor devices, for example, flexible semiconductor devices, can be manufactured, it can be manufactured at low cost.
[0075]
As described above, when the electrolytic etching is finally performed on the semiconductor substrate 11, the next porous layer 12 forming step can be performed continuously thereafter.
[0076]
Further, although the semiconductor substrate 11 is thinned by forming the porous layer, the thickness of the semiconductor substrate 11 is compensated by epitaxially growing a semiconductor having a thickness corresponding to the thickness reduction. You can also. Further, when the thickness is not compensated when the thickness is not compensated, the semiconductor substrate itself can be used as a thin film semiconductor, and for example, a solar cell can be manufactured. Therefore, the semiconductor substrate can be used almost without waste without finally becoming ineffective, and this can also reduce the cost.
[0077]
Further, according to the above-described manufacturing method, after the support substrate 42 is bonded onto the semiconductor film 13 to integrate the substrate and the epitaxial semiconductor film, the substrate is peeled off from the semiconductor substrate together with the epitaxial semiconductor film. Therefore, there are no restrictions on the type of this substrate, and flexible printed circuit boards, rigid printed circuit boards, metal plates, ceramics, glass, resins, etc., which have never been considered by conventional common sense in semiconductor technology A solar cell can be formed thereon.
[0078]
In addition, when a method of epitaxially growing a semiconductor layer on a porous layer having a single porosity is used, in order to improve the crystallinity of the semiconductor film, the porosity of the porous layer serving as the nucleus of crystal growth Since it is necessary to reduce the rate, it is necessary to reduce the current density and increase the HF mixing ratio of the electrolytic solution after anodization. However, when the porosity is lowered as described above, the porous layer becomes hard and separation of the epitaxial semiconductor film becomes difficult. Therefore, in order to increase the porosity in order to weaken the separation strength, for example, if the current density is increased and the HF mixing ratio of the electrolytic solution is reduced in the anodizing conditions, separation is facilitated in this case. The crystallinity of the epitaxial semiconductor film is extremely deteriorated. However, as described above, by reducing the porosity of the surface portion of the porous layer and forming a porous layer having a two-sided property that the porosity inside the porous layer is large, In addition, the epitaxial semiconductor film can be satisfactorily formed, and the epitaxial semiconductor film can be easily separated. For example, it is possible to form a porous layer that is weak enough to be easily separated by ultrasonic waves.
[0079]
In addition, the higher the porosity layer formed in the porous layer, the easier it is to peel off as the porosity increases. However, the higher the porosity, the greater the distortion, and the influence on the surface layer of the porous layer. For this reason, a crack may arise in a surface layer. In addition, when performing epitaxial growth, it causes a defect in the epitaxial semiconductor film. In contrast, an intermediate porosity that is slightly higher in porosity than the surface layer is used as a buffer layer to relieve the strain generated by these layers between the high porosity layer with extremely high porosity and the surface layer with low porosity. By forming the layer, it is possible to form a high-quality epitaxial semiconductor film that is easy to be peeled off, and to give strain to the surface of the porous layer to such an extent that irregularities can be generated on the surface of the semiconductor film.
[0080]
In addition, in anodization at a high current density, a high porosity layer can be formed in the porous layer at or near the semiconductor substrate side by intermittently passing an electric current, so that it becomes a surface and a release layer. The highly porous layer can be separated as much as possible, so that the buffer layer can be thinned, and the thickness of the porous layer can be reduced accordingly, and the consumption in the direction of decreasing the thickness of the semiconductor substrate can be reduced. Can be further reduced.
[0081]
In addition, when anodizing at a low current density, when the current is gradually increased, the porosity of the buffer layer between the surface layer of the porous layer and the release layer is gradually increased toward the inside. Can further improve the function of the buffer layer.
[0082]
Moreover, a porous layer can be easily formed by performing anodization in the electrolytic solution containing hydrogen fluoride and ethanol, or the liquid mixture of hydrogen fluoride and methanol. In this case, when changing the current density of the anodization, the range of adjusting the porosity is further increased by changing the composition of the electrolytic solution.
[0083]
In addition, by performing anodization in the dark, the occurrence of unevenness on the surface of the porous layer due to light irradiation during anodization becomes significant, and the disadvantage of deteriorating the crystallinity of the epitaxial semiconductor film can be avoided. An epitaxial semiconductor film having good crystallinity can be formed.
[0084]
Further, by forming the porous layer and then heating in a hydrogen gas atmosphere, the surface of the surface layer of the porous layer became smooth, and an epitaxial semiconductor film having good crystallinity could be formed.
In addition, after the porous layer is formed and before the heating step in the hydrogen gas atmosphere, the porous layer is thermally oxidized to oxidize the inside of the porous layer. Even if applied, it is difficult for a large structural change to occur in the porous layer, and strain from the inside is hardly transmitted to the surface of the porous layer, so that an epitaxial semiconductor film with good crystallinity can be formed.
[0085]
【The invention's effect】
According to the above-described manufacturing method of the present invention, a highly efficient solar cell having excellent crystallinity, sufficiently thin, and having a light confinement structure can be manufactured easily, surely and inexpensively, and the energy recovery years can be shortened. Can be measured.
[Brief description of the drawings]
FIG. 1 is a process diagram (part 1) of an embodiment of the method of the present invention; A to C are cross-sectional views of the respective steps.
FIG. 2 is a process diagram (part 2) of an embodiment of the method of the present invention; D and E are sectional views of the respective steps.
FIG. 3 is a process diagram (part 3) of an embodiment of the method of the present invention; F and G are sectional views of the respective steps.
FIG. 4 is a process diagram (part 4) of an embodiment of the method of the present invention; H and I are sectional views of the respective steps.
FIG. 5 is a process diagram (5) of another embodiment of the method of the present invention; J and K are cross-sectional views of the respective steps.
FIG. 6 is a configuration diagram of an example of an anodizing apparatus for carrying out the method of the present invention.
FIG. 7 is a schematic cross-sectional view based on a micrograph of a semiconductor film formed by the method of the present invention.
FIGS. 8A and 8B are schematic cross-sectional views based on a micrograph of a semiconductor film in another example of the method of the present invention. FIGS.
[Explanation of symbols]
11 Semiconductor substrate, 12 Porous layer, 12M Intermediate porosity layer, 12H High porosity layer, 13 Semiconductor film, 131 First semiconductor film, 132 Second semiconductor film, 133 Third semiconductor film, 41 Conductive line, 42 Transparent substrate (support substrate)

Claims (8)

Changing the surface of the semiconductor substrate to a porous layer;
Forming a semiconductor film constituting at least an active layer of a solar cell on the porous layer;
Peeling the semiconductor film from the semiconductor substrate in the porous layer;
Possess the etching step of removing the residual porous layer of the porous layer remaining on the separation surface of the semiconductor film,
The step of changing to the porous layer is a step of forming a surface layer having a porosity of 30% or less and a porosity layer having a porosity higher than the porosity of the surface layer on the lower layer side of the surface layer. A method for producing a thin-film solar cell. An intermediate porosity layer having a porosity higher than the porosity of the surface layer and lower than the porosity layer is formed between the surface layer and the porosity layer on the lower layer side of the surface layer. The manufacturing method of the thin film solar cell of Claim 1.   2. The method of manufacturing a thin film solar cell according to claim 1, wherein the semiconductor film is an epitaxially grown semiconductor film. The method of manufacturing a thin-film solar cell according to claim 1, further comprising an etching step of the surface of the semiconductor film . Performing the etching step by anisotropic etching with a chemical to promote unevenness of the release surface of the semiconductor film; and
2. The method for manufacturing a thin-film solar cell according to claim 1, wherein an electrode is deposited on the uneven surface . The above etching process is carried out using a mixed solution of hydrofluoric acid and acetic acid, KOH, NaOH, N _2. The method for producing a thin-film solar cell according to claim 1, wherein the method is performed using any one of H ₄ . A support substrate is bonded onto the semiconductor film, and then a peeling process between the semiconductor film and the porous layer and an etching process are performed. The manufacturing method of the thin film solar cell of Claim 1 characterized by the above-mentioned. 2. The method of manufacturing a thin-film solar cell according to claim 1, wherein after the etching step, a metal layer is formed on a peeled surface of the semiconductor film from which the porous layer has been removed .

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