JPH01132173A - Solar cell - Google Patents

Abstract

PURPOSE:To obtain a high-efficiency solar cell whose photodetecting face has a low- reflectance surface structure without using a single-crystal substrate and without a need for a complicated process such as an etching operation or the like by a method wherein the surface which has been formed on the basis of a specific nucleation face and which is composed of a semiconductor crystal of a mountain-shaped facet is used for a photodetecting face. CONSTITUTION:This solar cell is composed of a semiconductor crystal 7A which has been formed on a non-nucleation face 5, whose nucleation density is sufficiently large, which has been formed on the basis of a nucleatoin face of a microscopic surface area so as to generate only a single nucleus and whose surface is a mountain-like facet; a photodetecting face is formed on the mountain-like facet surface. For example, first electrodes 2a and second electrodes 2b are formed to be comb-shaped on a substrate 4; an insulating film 5 is deposited; after that, contact holes are made only in the first electrodes 2a; n<+> type Si single crystals 7a are grown from the first electrodes 2a; SiO2 films 5b are formed on their surface. Then, contact holes are made in the SiO2 film 5 on the second electrodes 2b; p-type Si single crystals 7b are grown; after that, an oxide film 5a is removed; i-type semiconductor crystals are grown; an Si single-crystal 7A is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a solar cell, and more particularly to a concentrating type solar cell with good energy conversion efficiency.

Regarding the method.

[Prior Art] Various measures have been attempted in the past to improve energy conversion efficiency, which is a major technical issue regarding solar cells.

One of these is an improvement in the crystallinity of semiconductors, which form the functional parts of solar cells. That is, polycrystalline is preferable to amorphous, and single crystal is preferable to polycrystalline. However, using a semiconductor single crystal substrate is disadvantageous in terms of large area and low cost.

On the other hand, reducing the reflectance of the solar cell light-receiving surface is also effective in improving energy conversion efficiency. Therefore, MgF on the surface of the light-receiving surface.
A method of providing an antireflection film consisting of , or Tag is used. Still, since the light-receiving surface is a flat mirror surface, some reflection is unavoidable.

Therefore, textured 5uface or b
A so-called concentrating type solar cell has been proposed, which has a light-receiving surface called a rack surface and having a cross-sectional shape shown in FIG. When light is incident on a light-receiving surface having a surface structure as shown in FIG. 1, the light reflected from one surface is not reflected from an adjacent surface, so that the reflection is reduced as a whole.

However, in the past, concentrating type solar cells have used Sii crystal substrates whose surfaces have been etched. However, in order to perform such surface processing, it is necessary to perform selective etching on, for example, the 5i (100) plane. That is, in conventional concentrating type solar cells, the application of such surface treatment is limited because the etching process is increased. Furthermore, since a Si single crystal substrate must be used as the substrate,
As the substrate temperature rises, the functionality of the solar cell deteriorates. Therefore, it has been desired to develop a solar cell of a concentrating type in which an arbitrary substrate can be selected as the substrate.

In view of the above-mentioned prior art, an object of the present invention is to provide a highly efficient solar cell having the above-mentioned surface structure on the light-receiving surface without using a single crystal substrate.

Another object of the invention is to provide a solar cell that does not require complicated processes such as etching when manufacturing such a solar cell.

[Means for solving the problem] The above problem is caused by the problem that the nucleation density is sufficiently higher than that of the non-nucleation surface and the nucleation density is sufficiently small that only a single nucleus is generated. Consists of a semiconductor crystal with a mountain-shaped facet-like surface formed based on the nucleation surface of the surface area,
The problem is solved by a solar cell characterized in that the chevron-shaped facet-like surface forms a light-receiving surface.

[Operation] One of the major features of the present invention is that it utilizes the principle of selective deposition.

Here, the general principle of the selective deposition method when a plate-like body (ie, a substrate) is used as the base will be explained.

The selective deposition method utilizes the differences between materials in factors that affect nucleation during the thin film formation process, such as surface energy '-1 attachment coefficient, desorption coefficient, and surface diffusion rate. This is a method to form a thin film in a specific manner. Figure 2 (A) and (
B) is an explanatory diagram of the selective deposition method. First, as shown in FIG. 2A, a thin film 2, which becomes a nucleation surface and is made of a material having different factors from that of the substrate 1, is formed on a desired portion of the substrate. When a thin film made of an appropriate material is deposited under appropriate deposition conditions, a phenomenon may occur in which the deposit 3 grows only on the thin film 2 and does not grow on the non-nucleation surface 5 of the substrate 1. can. By utilizing this phenomenon, it is possible to grow a deposit 3 shaped in a self-aligned manner, and it becomes possible to use a conventional resist or to omit the conventional resist.

Examples of materials on which such a selective deposition method can be performed include Sio2 as the substrate 1, Si, GaAs, Si3N4 as the thin n'J2, and St, W, GaAs as the deposit 3.

There are InP etc.

In addition, FIG. 3 shows the surface of 5in2 and Si3N.

3 is a graph showing changes in nucleation density over time, taking the surfaces of .

As the graph shows, 5i0
The nucleation density on 2 is saturated below 103 cm-2, and the nucleation density on 2
The value hardly changes even after 0 minutes. On the other hand, on Si3N4, it is saturated once at about 4X10'cm-2, does not change much for 10 minutes, but increases rapidly after that.

Note that this measurement example shows a case in which SiCu4 gas is diluted with H2 gas and deposited by CVD under conditions of a pressure of 175 Torr and a temperature of 1000°C. Other S
i H4, S i H2CH12゜5iHCJ2. ,
A similar effect can be obtained by using SiF, etc. as a reaction gas and adjusting pressure, temperature, etc. Vacuum deposition is also possible.

In this case, nucleation on the 5in2 film is hardly a problem, but by adding an etching gas such as 60°C gas to the reaction gas, nucleation on the 5i02 film can be further suppressed. The deposition of Si can be completely eliminated.

This phenomenon is largely due to the difference in adsorption coefficient, desorption coefficient, surface diffusion coefficient, etc. for Si on the material surfaces of 5in2 and Si3N4, but Si atoms themselves react with SiO2, causing monoxide with high vapor pressure. As silicon is generated, 5in2 itself is etched,
The fact that such an etching phenomenon does not occur on Si3N4 is also thought to be a cause of selective deposition (T, Yonehara, S, Yoshi
Hara, S, Miyazawa Journal of
Applied Physics 53.6839.
1982).

In this way, the material of the deposition surface is, for example, Sin.

If St, N, and St are selected as the deposition material, a sufficiently large difference in nucleation density can be obtained as shown in the graph. Although 5 in 2 is preferable as the material of the deposition surface here, the material is not limited to this, and a difference in nucleation density can be obtained even with Sin.

Of course, it is not limited to these materials, and as shown in the same graph, the difference in nucleation density is 102
It is sufficient if it is twice or more, and sufficient selective deposition can be performed even with materials such as those exemplified later.

Another method for obtaining this difference in nucleation density is to locally implant St, N, etc. onto the 5in2 to create an excess of Si, N, etc.
You may also form a region having the following. Alternatively, the Si3N4 film may be slightly exposed from the 5i02 thin film.

The above is an explanation of the selective deposition method. By forming a foreign material with a sufficiently higher nucleation density than the material on the deposition surface in a sufficiently fine structure so that only a single nucleus grows, the presence of the fine foreign material The applicant of the present application has proposed a method (selective nucleation method) in which single crystals are selectively formed only at locations where the nucleation occurs.

The selective growth of single crystals is also considered to be influenced by the electronic state of the deposited surface, especially the state of dangling bonds. Also, materials with low nucleation density (e.g. 5i02)
does not need to be a bulk material, and may be formed only on the surface of an arbitrary material or substrate, forming the above-mentioned deposition surface.

FIG. 4 is a conceptual diagram thereof. Figure 4 (A) is similar to Figure 2 (A).
), finely patterned seeds 6 made of a material with a high nucleation density are arranged on a non-nucleation surface 5 made of a material with a low nucleation density. At this time, as before, 5 is SiO, 6 is St, N4
That's fine.

4 is an arbitrary board. The 5eed of 6 is a surface area that is small enough to generate and grow only a single nucleus, that is, the diameter is 1
It is preferable that the size is about ~2 μm, and the area is about 1 to 4 μd. Next, FIG. 4(B) is a diagram of an initial state in which an island-shaped single crystal has grown from a single nucleus on 5eeda. This deposition was performed using a normal epitaxial device, and the source gases were 5iCIL4, 5iHCIt.
,.

As mentioned above, SiH2Cl12, SiH4, etc. are possible.

Incidentally, the HCJZ additive gas has the effect of etching Si, and the nucleation density can be controlled by adjusting the flow rate. When 5eed6 is 2 μm square, the flow rate ratio (1/mi
n) S i H2CIt2 : HCj2 :H2
=1.2:1.6:100, temperature 960℃, pressure 150
Torr was appropriate. When grown under the above conditions, all the island-shaped single crystals 7 become single crystals.

Next, FIG. 4(C) shows an island-shaped single crystal 7 grown selectively. That is, for example, if deposition is performed under the above conditions, no new nuclei will be generated on the non-nucleation surface (S L 02 film) 5 and only the island-shaped single crystals 7 will grow to become large single crystals. This large single crystal is hereinafter referred to as a single crystal.

At this time, as shown in FIG. 4(C), the single crystal St has a unique crystal outer shape (facet).

Therefore, when forming an element on the grown single crystal 7A, the upper part may be flattened by lapping or the like as shown in FIG. 4(D).

However, the present invention attempts to utilize this facet effectively.

When island-shaped single crystals are selectively grown on the deposition surface of an arbitrary substrate by the method described above, the single crystals are surrounded by specific facets. This is thought to result from the anisotropy of surface energy and growth rate. Therefore, the outer shape of one single crystal is mountain-shaped as shown in FIG. Therefore, fine nucleation planes are separated from each other by a sufficient distance and arranged in a two-dimensional matrix on the deposition surface, and the mountain-shaped single crystals grown from these planes are grown until their grain boundaries touch each other. From this, a surface structure as shown in FIG. 2 can be automatically obtained without surface treatment.

[Embodiment Example] (First Embodiment Example) A first embodiment example of the present invention will be described based on FIGS. 6 to 10.

As shown in FIG. 6, metal is sputter-deposited on an arbitrary substrate 4, and then patterned to form an electrode (first electrode) 2a for an n-type semiconductor and an electrode (second electrode) for a p-type semiconductor.
2b is formed into a comb shape. The distance between the electrodes 15a and 15b may be determined as appropriate depending on the desired size of the semiconductor crystal.

Note that the arbitrary substrate may be made of an insulating material such as quartz, alumina, ceramics, or glass, as long as it has a certain degree of heat resistance. Further, as the material of the electrode, a material having a high nucleation density such as Mo, AX, Cu, and W may be used. Further, the first electrode and the second electrode may be formed of the same material, or may be formed of different materials.

Next, as shown in FIG. 7, an insulating film of, for example, 5 in 2 is deposited on the substrate 4 and the electrodes 2a, 2b, and a contact hole is opened only on one electrode (the n-type electrode 2a in this example). exposes the electrode 2a. Electrode 2 exposed at this time
A becomes a nucleation surface with a high nucleation density, and the insulating film 5 becomes a surface with a low nucleation density, that is, a non-nucleation surface.

Next, using a selective nucleation method as shown in FIG.
from the type electrode to the n0 type St which is the first semiconductor crystal of conductivity type.
A single crystal 7a is grown.

When the Si single crystal grows to a certain extent, the crystal formation process is stopped (the crystal that has grown to a certain extent is called an island-shaped single crystal), and each surface of this island-shaped Si single crystal is thermally oxidized, and 5i02 is applied to the surface. @ (This film becomes a non-nucleation surface) 5b is formed.

The size of the n-type wheel crystal Sl is arbitrary, but it is approximately 5 to 6 μm.
is preferred.

Next, as shown in FIG. 9, S i O,
A contact hole is made in the membrane 5. At this time, the surface of the n9-type island-like single crystal Si contains SiO, which has a low nucleation density.
Since the film 5a is present, nucleation occurs only on the p-type electrode.Next, a p-type Si single crystal 7b, which is a second semiconductor crystal, is grown.

Next, as shown in FIG. 10, the oxide film 5a on the surface of the n0 type island-shaped single crystal is etched with a suitable etching solution, for example, an HF solution, to expose the island-shaped Si single crystal surface again. Then, an i-type semiconductor crystal is grown using the nI-type crystal and the p-type crystal as nucleation surfaces, and a Si single crystal 7A (
i-type semiconductor). As the growth continues, grain boundaries are formed where the crystals are adjacent to each other, and facets appear as shown in FIG.

(Second Embodiment) A second embodiment of the present invention will be described with reference to FIGS. 11 to 13.

First, as shown in FIG. 11, a metal material such as AIL, Mo, Cu, W, etc. is sputter-deposited on the entire surface of the alumina substrate 4, and then buttering is performed to form the lower electrode (second electrode) 2b. . For example, 310
．． @5 2 μm square contact holes are provided in a two-dimensional matrix.

Here, the material of the substrate 4 is not limited to alumina, but may be any material as long as it has high heat resistance.

Also, the material of the lower electrode 2b is AJ! , Mo, W, C
Any material having a high nucleation density such as u may be used.

In the configuration shown in FIG. 11, the Si nucleation density is low on the 5in2 film 5, and this becomes a non-nucleation surface (NDs). On the other hand, metal materials such as AIL and Mo are more Si than Si.
Since the nucleation density is large, this becomes the nucleation surface (NDL).

Next, as shown in FIG. 12, the lower electrode 2b is subjected to, for example, a Si crystal formation treatment by, for example, a thermal CVD method.
A Si single crystal nucleus is generated only on the top, and an island-shaped single crystal 7b grows from the nucleus.

Note that at this time, by doping with an appropriate element, a semiconductor crystal of a desired conductivity type can be formed.

Next, in order to form a crystal of a conductivity type opposite to that of the island-shaped single crystal 7b, the dopant is changed to form a single crystal body 7A (see FIG. 13).

Note that a solar cell is formed by depositing a comb-shaped electrode 8 on, for example, the grain boundary portion of the polycrystalline body shown in FIG. 13 and forming an upper electrode (first electrode) 2a.

[Example] (First Example) First, in FIG. 6, on a substrate 4 made of ceramic glass,
AJ2 metal was sputter-deposited and then patterned to form an n-type semiconductor electrode (first electrode) 2a and a p-type semiconductor electrode (second electrode) 2b in a comb shape. Figure 6 (A)
is a side sectional view, and FIG. 6(B) is a perspective view. p indicates a p-type electrode, and n indicates an n-type electrode. In addition, the distance between the electrodes 15a and 15b is 40 μm between centers, and the width of the electrodes is 20 μm.
The m1 thickness was set to 0.6 μm.

Next, as shown in FIG.
I5 was deposited to a thickness of 500 using a CVD device, and a contact hole was made only in the n-type electrode. The contact hole is RI E (Reactive IonEtchi)
ng) device to a size of 2 μm square.

Next, using a selective nucleation method as shown in FIG.
A conductive type first semiconductor crystal, n'1 type S, is placed on the type electrode.
t single crystal 7a was grown. Here, the formation of the n-type Si single crystal uses S i H2CJ22, HC as the source gas.
II, using a mixed gas of H2, 5iHz CJ12:H
Cjl:H2=i, flow rate ratio of 2:1.4:100 (bu7
m1n). PH3 was used as a dopant gas. Further, the growth was performed at a temperature of 900° C. and a pressure of 150 Torr.

When the St single crystal grows to a certain extent, the crystal formation process is stopped (this crystal that has grown to a certain extent is called an island-like single crystal), and each surface of this island-like Si single crystal is thermally oxidized to coat the surface with Sin, A film 5b (this film serves as a non-nucleation surface) was formed.

In this example, the size of the Si single crystal was approximately 5 to 6 μm.

Next, as shown in FIG. 9, the 5i02 film 5 on the p-type electrode is
A 2 μm contact hole was made in the same way as in the case of electrode 2a. Then, a p-type St single crystal 7b, which is a second semiconductor crystal, was grown under the same conditions as those used for growing the n0-type Si single crystal 7a. However, P as a dopant gas
BH3 was used instead of H5. At this time, the surface of the n0-type island-like Si single crystal has a 5i02 film 5a with a low nucleation density.
Therefore, it is not formed in the n-type Si single crystal.

Next, as shown in FIG. 10, the oxide film 5a on the surface of the n0 type island-shaped Si single crystal is removed by etching with an HF solution, and the island-shaped Si single crystal surface is exposed again. Then, an i-type semiconductor crystal is formed using the n1-type St single crystal and p-type Si single crystal as seeds, and the St single crystal 7A (i-type semiconductor)
I got it. The conditions for formation are the gas flow rate ratio S i H2CIL
2: HCl1:H, = 1.2:2.0: 100,
The forming temperature was 920° C. and the pressure was 150 Torr. As the growth continued, grain boundaries were formed where the crystals were adjacent to each other. As a result, facets appeared as shown in FIG. The solar cell formed in this way is AMIRY
Open circuit voltage: 0.62V, short circuit current: 32mA/am during irradiation
, and had electrical properties with a fill factor of 0.8.

Since the solar cell according to this example had an aperture ratio of 100% on the entrance surface, the light incidence efficiency was extremely good.

(Second Embodiment) A second embodiment of the present invention will be described using FIGS. 11 to 13. First, as shown in FIG. 11, M is applied to the entire surface of the alumina substrate 4.
An o film was sputter-deposited to a thickness of 1 μm, and this was used as the lower electrode (second electrode) 2b.

A 5i02 film 5 was deposited on the substrate to a thickness of 1000 mm by CVD, and then 5
Contact holes of 2 μm square were provided in a two-dimensional matrix at 0 μm intervals.

Next, when St was deposited on this substrate by the thermal CVD method, as shown in FIG.
Growth was completed when the thickness reached approximately 6 μm. however,
The crystal growth conditions were as follows.

Gas used: 5i2HCIL2 (source gas) BH3
(Dopant gas) HCJI! (Etching gas) H2 (Carrier gas) Gas flow rate ratio; S 12 HCl2 : HCl2 : H2=1.
2:1. 6: 100 (J2/min)
Substrate temperature: 900°C In this example, since B is doped, 5iJIL
The crystal became p-type.

Next, use PH3 as a dopant gas on top of this,
An n-type Si single crystal 7A serving as the first semiconductor crystal was formed by selective epitaxial growth (FIG. 13). Then, the adjacent si! II- The crystal bodies 7A are in contact with each other and form grain boundaries 11, while facet planes 1 are formed in the upper part.
0 was formed. Then, on the n-type 31 single crystal, we made n0-type Si with high PH and gas concentration for contact with the electrode.
The j$ crystal layer 9 was formed to have a thickness of 1 μm. The crystal growth conditions are
Gas flow rate ratio 5i2H (, Q□ :HCl2:H2=l,
2:1.6: 100 (j2/m1n), formation temperature 9
The temperature was 20° C. and the pressure was 150 Torr.

Finally, a comb-shaped electrode 8 of AfL was formed at the grain boundary portion of the St single crystal by vapor deposition to form an upper electrode Fi (first electrode) 2a. When the energy conversion efficiency of the solar cell thus formed was measured, it was found to be about 16%, a good value. This is a significantly higher conversion efficiency than amorphous silicon solar cells, which are conventional large-area, low-cost solar cells.

[Effects of the Invention] As explained above, according to the present invention, the following numerous effects can be obtained.

(a) Whereas in conventional single-crystal solar cells, the substrate is limited to wafers, in the solar cell according to the present invention, any substrate can be used as long as it has heat resistance higher than the process temperature of the subsequent process. Therefore, a large-area substrate such as glass can be used as a substrate, which is inexpensive.

(b) Since the grown single crystal has a unique facet shape, even light rays incident at the angle of total reflection are absorbed by other crystal planes after reflection, resulting in high light absorption efficiency and , solar cells with high conversion efficiency can be obtained.

(c) In conventional concentrating type solar cells, the functionality of the element deteriorates due to an increase in substrate temperature, but in the solar cell according to the present invention, the base substrate can be selected arbitrarily, so a material with good thermal conductivity can be used. By using a substrate processed into a shape with good heat dissipation, a solar cell of a concentrating type with high conversion efficiency can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram showing a light-receiving surface of a concentrating type solar cell. FIG. 2 is a diagram showing the concept of the selective deposition method. FIG. 3 is a graph showing the time dependence of nucleation density. FIG. 4 is a diagram showing the concept of the selective nucleation method. FIG. 5 is a perspective view showing a crystal with facets. 6 to 10 are manufacturing process diagrams for explaining the first embodiment. 11 to 13 are manufacturing process diagrams for explaining the second embodiment. DESCRIPTION OF SYMBOLS 1... Base body, 2... Thin film, 2a... First electrode, 2
b... Second electrode (lower electrode), 3... Deposit, 4...
・Substrate, 5.5a, 5b-non-nucleation surface, 6 ・-s
e e d (Si, N4), 7... Island-shaped single crystal, 7
A... Single crystal, 7a... First semiconductor crystal, 7b
...Second semiconductor crystal, 11...Crystal grain boundary. Incident light Figure 2 (A) (B) Time (minutes) Figure 4 (D) Figure 8 Figure 9 Figure 12

Claims (2)

[Claims] (1) A surface formed based on a nucleation surface provided on a non-nucleation surface, which has a sufficiently higher nucleation density than the non-nucleation surface and has a surface area so small that only a single nucleus is generated. 1. A solar cell comprising a semiconductor crystal having a chevron-shaped facet, the surface of the chevron-shaped facet forming a light-receiving surface. (2) The solar cell according to claim 1, wherein the upper electrode is provided in the valley of the mountain-shaped facet.