JP3304666B2 - Solar cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell, and more particularly to a solar cell for space which requires high efficiency, high radiation resistance and light weight.

[0002]

2. Description of the Related Art When a solar cell is used in space as a power source for a satellite, it is exposed to high-energy electron beams and proton beams in outer space. The life time of the minority carrier decreases according to the amount, and the output characteristics gradually deteriorate. For solar cells for space use, in order to improve radiation resistance, (1) shallower the PN junction, (2) thinner the cell thickness, and (3) select a resistivity suitable for the structure of the solar cell. And other measures are taken.

In order to improve the output characteristics of a solar cell, irregularities such as a regular pyramid, an inverted pyramid, and a V-groove are formed on the surface to reduce the reflection loss of light on the surface and to reduce the reflection loss of incident light. It has been known that it is effective to improve the reflectivity of the light-emitting device to form a so-called light trapping structure. In order to improve radiation resistance, it is known that it is effective to reduce the thickness of the cell and to improve the collection efficiency of minority carriers by PN junction after irradiation.

FIG. 11 shows prototypes of 100 μm and 70 μm.
FIG. 3 shows the output deterioration curve due to radiation of a so-called BSFR type silicon solar cell having a BSF (back surface electric field) layer and a reflective layer having a thickness of 50 μm and 50 μm. In FIG. 11, 50
Solid line A showing characteristics of a BSFR type solar cell having a thickness of μm.
Even after being exposed to high-energy radiation,
Compared with the solid lines B and C showing the output characteristics of the 70 μm and 100 μm thick BSFR solar cells, it can be seen that a thinner thickness is more advantageous. Therefore, in order to realize a solar cell with high efficiency and excellent radiation resistance, it is necessary to realize a cell as thin as possible having an optical trapping structure.

[0005]

However, in the actual solar cell manufacturing process, manufacturing a thin cell having a thickness of 50 μm or less increases cell cracking during the process and significantly lowers the yield as shown in FIG. However, there is a problem that the cost of the cell increases. The cause of cracks during the process of thin cells is usually the first step of the process, in order to reduce the silicon wafer to the final cell thickness, the subsequent diffusion process, electrode processing, etc. This is because minute chips are generated around the wafer, which leads to cracks.

[0006] Such chipping is more likely to occur as the thickness of the wafer becomes thinner, and it has been difficult to make the cell thickness smaller than the 50 µm cell in the conventional structure and further improve the radiation resistance. Therefore, it has been a problem to realize a structure capable of improving output characteristics and radiation resistance without reducing the cell thickness.

In the case of a cell having a conventional structure, there are the following problems. FIGS. 13A and 13B are schematic cross-sectional views of a flat cell and an inverted pyramid cell having a conventional structure, respectively.

In FIG. 13A, for example, an N-type layer 4 is formed on the surface of a P-type silicon substrate 1, and a BSF is formed on the back surface.
A P ⁺ type layer 3 serving as a layer is formed. A comb-shaped N electrode 5 is formed on the surface of the N-type layer 4, and a P electrode 6 is formed on the surface of the P ⁺ -type layer 3. The entire light receiving surface is covered with an anti-reflection film 7.

In FIG. 13B, the silicon substrate 1
Formed a large number of inverted pyramids on the surface of
A mold layer 4 is formed, and a P electrode 6 is formed on the silicon oxide film 2.
And is connected to the P ⁺ layer 3 through the. The silicon oxide film 2 serves as a reflection film, which suppresses the recombination loss of minority carriers on the back surface and increases the conversion efficiency.

FIG. 13C is a perspective view of an example of an inverted pyramid type cell. A large number of inverted pyramid concave portions are provided on the surface of the P-type silicon substrate 1, and an N-type layer 4 is formed on the surface, and photoelectric conversion is performed by the PN junction. The illustration of the surface reflection film 7 is omitted. FIG. 13 (b)
In the figure, the quadrilateral at the bottom of the inverted pyramid is sharply displayed, but actually has a slight width as shown in FIG.

In a solar cell as shown in FIGS. 13A and 13B, light having a short wavelength has a large absorption coefficient, so that an electron-hole pair is generated at a relatively shallow position. On the other hand, the long-wavelength light travels deeper inside the silicon substrate and generates electron-hole pairs 11 at a position near the back surface away from the PN junction. In the P region, electrons (minority carriers 12) reach the PN junction and can be extracted as electric power. In the solar cell of the conventional structure, the minority carrier has a long diffusion length before irradiation, so that it can reach the PN junction and be extracted as an output. However, when irradiated with radiation, crystal defects occur in the silicon substrate, which act as recombination centers for minority carriers, whereby the diffusion length of minority carriers gradually decreases, and minority carriers generated at positions far from the PN junction. Will not reach.

FIG. 14 is a graph for explaining this, and shows the deterioration of the spectral sensitivity due to the irradiation of the silicon solar cell with radiation. As the irradiation dose increases, the sensitivity at longer wavelengths gradually deteriorates, and light of longer wavelength, that is, light of longer wavelength, This indicates that minority carriers generated at a deep position away from the PN junction become gradually less likely to be captured by the PN junction due to radiation irradiation.

[0013]

In the solar cell of the present invention, for example, a large number of fine concave portions having a gradually decreasing cross-sectional area are formed on a surface of a P-type semiconductor substrate as a first conductive type. For example, an inverted pyramid-shaped recess is formed, a hole is formed at the top of the inverted pyramid, and penetrates the back surface of the semiconductor substrate, and the side surface of the inverted pyramid and part of the back surface of the semiconductor substrate are covered through the hole. Forming an N-type layer of a second conductivity type, forming a P ⁺ -type layer on the back surface of the semiconductor substrate at a position away from the N-type layer, and forming an N-type layer and a P ⁺ -type layer on the back surface of the semiconductor substrate; , An N electrode and a P electrode were respectively taken out. In the N-type layer on the back,
A fine P ⁺ -type layer that reaches the inside but does not
I can.

The following structure can be added to the solar cell having such a structure. A frame-shaped flat portion is provided around the semiconductor substrate.

The ratio of the area of the N-type layer on the back surface to the area of the P ⁺ -type layer is appropriately selected. A passivation layer of, for example, an oxide film is provided on most of the back surface.

[0016]

The bottom of the inverted pyramid on the front surface corresponding to the interconnector of the P electrode on the back surface is flattened.

[0018]

In the solar cell of the present invention, since the entire cross section of the cell has an inverted pyramid structure, the long wavelength light incident on the semiconductor substrate travels as shown in FIG.
Electron-hole pairs are generated at the same distance as in (a) and (b). As is clear from the comparison between FIGS. 13A and 13B and FIG. 1, not only the flat cell of FIG. 13A but also the conventional inverted pyramid cell of FIG.
In the solar cell of the present invention, the distance between the position where the minority carrier is generated by long-wavelength light and the PN junction is significantly shorter than that of the solar cell having the conventional structure. Therefore, even if the minority carrier diffusion length is shortened by irradiation, deterioration of long wavelength sensitivity is reduced, and radiation resistance is improved. That is, the solar cell of the present invention has an inverted pyramid structure having an N-type layer reaching the back surface of the semiconductor substrate, thereby improving output by reducing surface reflection loss of light and improving radiation resistance by reducing effective cell thickness. The effect of improving the properties can be obtained. Further, the solar cell of the present invention has a through-hole at the bottom of the inverted pyramid, so that the N-type layer on the front surface can be guided to the rear surface, and as a result, an N electrode can be provided on the rear surface. There is no light shielding by the N electrode on the surface as in a solar cell having a conventional structure, and the output can be improved accordingly. Usually, the area of the N electrode on the surface occupies 4 to 5% of the whole.

Since the periphery of the cell is flat, the periphery of the cell is reinforced by this frame-shaped flat portion, and even when the thickness of the cell is reduced, cell cracking due to chipping around the cell is reduced.

By providing a P ⁺ -type layer on the majority of the back surface, a BSF layer is formed, and the output of a thin cell according to the present invention can be improved.

By increasing the area of the N-type layer on the back surface and providing a PN junction on most of the back surface, minority carriers and PN
The effective distance of the junction can be further reduced, and the radiation resistance is further improved.

By covering most of the back surface of the semiconductor silicon substrate with a metal film having a high reflectivity such as a silicon oxide film and forming an N-type layer and a P ⁺ -type layer on a part of the back surface, the BS is formed.
The F effect can be obtained, the light reflectance on the back surface can be improved, the light trapping effect can be improved in combination with the inverted pyramid structure on the front surface, and the output characteristics can be improved.

The fine P in the N-type layer on the back side⁺Forming mold layer
And a structure that does not contact the N electrode with an oxide film, etc.
N-type layer and P⁺High impurity concentration junction (high concentration
NP ⁺The effect of bonding) causes shadows on the solar cell, etc.
Function to protect against reverse bias voltage
be able to. Such a function can be achieved with a cell with a conventional structure.
In order to have it, P⁺Form a mold layer
Need a photo-etching process,
In the solar cell according to the invention, P serving as the BSF layer on the back surface⁺Type
Since it can be formed simultaneously with the layer, the photo on the front side
There is an advantage that an etching step is not required.

By flattening a part of the bottom of the inverted pyramid on the front surface corresponding to the interconnector of the P electrode on the back surface, the strength of the connection portion of the interconnector on the P electrode is increased, so that the cell breaks during the assembly of the solar cell. Can be prevented.

[0025]

FIG. 1 is a schematic sectional view of an embodiment of the present invention. For example, a concave portion having a gradually decreasing cross-sectional area extending from the front surface to the rear surface of the P-type silicon substrate 1, for example, a number of inverted pyramid-shaped concave portions 13, 13... Is provided, and through holes 9, 9,. The through-holes need not necessarily be provided at all the tops of the many inverted pyramid-shaped concave portions 13, and may be provided at appropriate intervals within a range that does not deteriorate the fill factor of the solar cell due to an increase in the series resistance of the N-type layer 4. Good.
An N-type layer 4 is formed over a surface of the concave portion 13 of the silicon substrate 1 surrounded by the inverted pyramid and a part of the back surface of the silicon substrate 1 through the through hole 9. Silicon substrate 1
A P ⁺ -type layer 3 is formed on the back surface of the substrate at a position away from the N-type layer 4. An antireflection film 7 is provided on the surface, that is, the surface of the N-type layer 4 on the light receiving surface side. N-type layer 4 on the back
The P electrode 6 is provided on the electrode 5 and the P ⁺ type layer 3. When viewed from the back, a large number of through holes 9, 9.
The electrodes 6 are scattered. By interposing an insulating film such as polyimide between the N electrodes and the P electrodes scattered in this manner, the P electrodes are also formed on the N electrodes,
It can be provided continuously on the back surface, and can be an integrated P electrode.

FIG. 2 is a cross-sectional view of the solar cell shown in FIG. 1 in which a frame-shaped flat portion 8 is provided around the surface of the silicon substrate 1. This flat portion can prevent chipping around the cell. In this embodiment, the back surface of the silicon substrate 1 except for a connection portion between each electrode and the P ⁺ -type layer 3 and the N-type layer 4,
It is covered with a silicon oxide film 2. The illustration of the antireflection film is omitted. Although the boundaries of the inverted pyramid are shown sharp in the figure, they actually have some width.

FIG. 3 is a diagram showing the P on the back surface of the silicon substrate 1 shown in FIG.
FIG. 4 is a cross-sectional view when the area of the ⁺ type layer 3 is increased. Thereby, the back surface field (BSF) effect can be increased and the output of the cell can be improved.

FIG. 4 is a cross-sectional view when the area of the N-type layer 4 on the back surface of FIG. 1 is increased. Since the area of the PN junction on the back surface is increased, the effective distance between the minority carrier and the PN junction is further reduced, and radiation resistance is improved.

FIG. 5 is a cross-sectional view of the silicon substrate 1 of FIG. 1 in which the back surface is covered with a silicon oxide film 2 except for the connection portions of electrodes and through holes. The through-hole may be closed by a silicon oxide film. Any insulating film having a high reflectance other than the silicon oxide film can be used. The BSF effect can be obtained by the P ⁺ type layer on the back surface, the light reflectance on the back surface can be improved by the reflection film, the light trapping effect can be improved together with the inverted pyramid structure on the front surface, and the output characteristics can be improved.

FIG. 6 shows the case where an insulating film such as a silicon oxide film 2 is interposed between the N-type layer 4 on the back side and the N-electrode as shown in FIGS. 4, a fine P ⁺ type layer 10 reaching the silicon substrate 1 is formed.
Since the silicon oxide film 2 or the like does not contact the N electrode 5, the effect of the junction of the N-type layer 4 and the fine P ⁺ -type layer 10 with a high impurity concentration causes shadows or the like on the solar cell. A function of protecting against a reverse bias voltage can be added. In addition, even when an insulating film is not interposed on the back surface as shown in FIGS. 3 and 4, a fine P ⁺ -type layer 10 reaching the silicon substrate 1 is formed in the N-type layer 4 so as not to contact the N electrode 5. By doing so, a similar function can be added. In a cell having a conventional structure, in order to have such a function, it is necessary to form a P ⁺ -type layer on an N-type layer on the surface.
In the solar cell according to the present invention, the P ⁺
Since it can be formed simultaneously with the mold layer, there is an advantage that a photoetching step on the front surface side is not required.

FIG. 7 shows a solar cell as described above.
The bottom of the inverted pyramid on the surface of the silicon substrate 1 corresponding to the interconnector 6-1 welded to connect each P electrode 6 is flattened. As a result, the strength of the connection portion of the interconnector of the P electrode is increased, so that it is possible to prevent the cell from being cracked during the assembly of the solar cell. The N electrode 5 on the back surface and the interconnector 6-1 are insulated.
When the inverted pyramids are regularly arranged, FIG.
As can be inferred from (c), the bottom of the inverted pyramid on the surface of the silicon substrate 1 has a flat lattice shape.

Each of these solar cells can be manufactured by almost the same steps. An example of the manufacturing process will be described below with reference to FIGS.
(E) and FIGS. 9A to 9D will be described.

First, as shown in FIG. 8A, for example, the P-type silicon substrate 1 is thinned to about 50 μm by etching using a strong alkaline solution.

Next, as shown in FIG. 8B, a silicon oxide film 2 is formed on the surface of the silicon substrate 1 by a thermal oxidation method or the like.

Next, as shown in FIG. 8C, in order to form an inverted pyramid structure on the surface, the silicon oxide film 2 on the surface is removed in a lattice shape by photolithography.

Next, as shown in FIG. 8D, the silicon substrate is subjected to anisotropic etching in a thin alkaline solution to form recesses 13, 13,... Having an inverted pyramid structure. .. Are formed at the apexes of the concave portions of the inverted pyramid on the back surface of the silicon substrate 1.

In the case of this etching, the size of the through hole 9 can be easily obtained by the following equation.

[0038]

(Equation 1)

Next, as shown in FIG. 8E, the silicon oxide film 2 partially remaining on the silicon substrate is removed.
Thereafter, a silicon oxide film 2 is formed on the entire surface of the silicon substrate. The bottom of the inverted pyramid actually has a certain width, but is shown sharp in the figure.

Next, as shown in FIG. 9A, a part of the silicon oxide film 2 on the back surface of the silicon substrate 1 is removed, and a P ⁺ -type layer 3 serving as a BSF layer and a bypass function are added by P-type diffusion. A fine P ⁺ -type layer 10 is formed.

Next, as shown in FIG. 9B, the silicon oxide film on the surface of the concave portion and the silicon oxide film on the back surface of the portion where the N-type layer 4 is to be formed are removed by photolithography, and the N-type layer is diffused by N-type. Then, an oxide film, BSG (boron glass) and PSG (phosphorus glass) remaining on a part of the silicon substrate are removed.

Next, as shown in FIG.
A silicon oxide film 2 of about 300 nm is formed by D.

Next, as shown in FIG. 9 (d), a thickness 1 not shown to prevent recombination loss of carriers on the surface.
After a passivation film having a thickness of about 0 to 30 nm is formed by thermal oxidation or the like, the P ⁺ -type layer 3 on the back surface where an electrode is formed is formed.
And the silicon oxide film 2 on the N-type layer 4 is removed by photolithography, an electrode metal is formed by vacuum evaporation or the like, and unnecessary metal is removed by lift-off to form a P electrode 6 and an N electrode 5, T on the surface of the N-type layer 4
An anti-reflection film 7 of iO ₂ —Al ₂ O ₃ or the like is formed by vacuum evaporation or the like. Finally, the silicon substrate is cut with a diamond saw or the like to obtain a solar cell having required dimensions.

[0044]

The solar cell has a smaller average cell thickness and a smaller PN junction as compared with a conventional cell having the same cell thickness as shown in FIGS. 13 (a) and 13 (b). Since it is also formed on the back surface, the position where the minority carrier is generated and the position of the PN junction are relatively significantly shortened, and the output deterioration due to the decrease in the carrier diffusion length due to irradiation is significantly reduced. On the surface of the cell, the reflectance is reduced by multiple incidence of light due to the inverted pyramid structure, and an antireflection film is provided on the surface, and an oxide film is provided on the diffusion layer on the back surface, increasing the reflectance on the back surface, so-called It is possible to improve the optical confinement effect, reduce the recombination loss of carriers on the front surface and the rear surface, and improve the output. In addition, a function of preventing the cell from being damaged due to the reverse bias voltage can be provided by the effect of the P ⁺ N junction by the fine P ⁺ type layer on the back surface.

In the conventional through-hole cell, it is necessary to mechanically form the through-hole using a laser or the like, which is troublesome.
There is an advantage that the through hole can be easily formed by chemical etching when forming the inverted pyramid. In the manufacture of the solar cell according to the present invention, as shown in FIG. 10, when taking out, for example, two unit cells 21 and 21 surrounded by a flat portion 8 having a large number of inverted pyramids from a silicon wafer 20,
It is possible to flow the process while the parts that do not become solar cells around them can be flown flat, and cell cracking in the process due to chipping around the wafer is different from the conventional solar cell of the same thickness Absent. Therefore, a thin solar cell can be effectively realized without lowering the yield, and as a result, the radiation resistance can be improved.

By flattening the cell periphery and the lower part of the electrode of the solar cell of the present invention, cell cracking can be significantly reduced.

By providing the P ⁺ -type layer in the solar cell of the present invention and incorporating the BSF effect, the output after irradiation can be significantly improved.

In the solar cell according to the present invention, by providing a PN junction also on most of the back surface, the collection efficiency of carriers is improved, and the radiation resistance can be further improved.

The solar cell of the present invention can improve the light reflectance by providing an oxide film between the silicon substrate portion on the back surface and the electrode, and can improve the output by improving the light trapping effect.

The solar cell of the present invention can easily add a bypass function to a reverse bias voltage,
The solar cell can be protected in a reverse bias state due to the occurrence of a shadow or the like, and the reliability can be significantly improved.

Further, in the solar cell according to the present invention, when the interconnector is welded to the back electrode, a part of the bottom of the inverted pyramid on the surface of the silicon substrate corresponding to the interconnector at the weld is flattened. It is possible to increase the strength of the part and improve the reliability of the interconnector welded part.

[Brief description of the drawings]

FIG. 1 is a sectional view of one embodiment of the present invention.

FIG. 2 is a sectional view of another embodiment of the present invention.

FIG. 3 is a sectional view of another embodiment of the present invention.

FIG. 4 is a sectional view of another embodiment of the present invention.

FIG. 5 is a sectional view of another embodiment of the present invention.

FIG. 6 is a sectional view of another embodiment of the present invention.

FIG. 7 is a cross-sectional view of a portion of the solar cell according to the present invention in which an interconnector of a surface electrode is provided.

FIGS. 8A to 8E are schematic cross-sectional views of each step of an embodiment of the present invention.

FIGS. 9A to 9D are schematic cross-sectional views of respective steps following FIG.

FIG. 10 is a plan view of a silicon wafer including a cell according to the present invention.

FIG. 11 is a graph of radiation resistance of BSFR solar cells having different thicknesses.

FIG. 12 is a graph showing the relationship between cell thickness and yield.

FIGS. 13A and 13B are cross-sectional views of a solar cell having a flat surface and a solar cell having an inverted pyramid concave portion provided on the surface, respectively, and FIG. 13C is a perspective view of an example of the latter. FIG.

FIG. 14 is a graph of deterioration of spectral sensitivity due to radiation of a solar cell.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Silicon oxide film 3 P ⁺ type layer 4 N type layer 5 N electrode 6 P electrode 7 Antireflection film 8 Flat part 9 Through hole 10 Fine P ⁺ type layer 11 Electron-hole pair 12 Minority carrier

Claims (6)

(57) [Claims] 1. A plurality of recesses formed on a semiconductor substrate of a first conductivity type and formed in the semiconductor substrate so as to have a sequentially decreasing cross-sectional area extending from the front surface to the back surface, and a part of a tip of the recess. Or a through hole reaching the back surface of the semiconductor substrate, a second conductivity type layer formed over a part of the back surface of the semiconductor substrate from the side surface of the recess through the through hole, and provided on the back surface side of the semiconductor substrate. , The second conductivity type
A second conductivity type electrode connected to the second conductivity type layer and a first conductivity type electrode having a higher concentration than the impurity concentration of the semiconductor substrate and formed at a position on the back surface of the semiconductor substrate apart from the second conductivity type layer. A layer and an electrode of the second conductivity type on the back side of the semiconductor substrate
And is connected to the layer of the first conductivity type.
An electrode of the first conductivity type, and the second conductivity type provided on the back side of the semiconductor substrate.
Formed in a layer of
A high concentration fine first conductivity type layer, and one end of the fine first conductivity type layer is
Plate, but the other end has the second conductivity type
Solar cells that are kept from reaching the poles . 2. The solar cell according to claim 1, wherein a flat portion is provided around a surface of the semiconductor substrate. 3. The method according to claim 1, wherein an area of the first conductivity type layer provided on the back surface of the semiconductor substrate of the first conductivity type is larger than an area of the second conductivity type layer provided on the back surface of the semiconductor substrate. The solar cell according to claim 1, wherein: 4. The method according to claim 1, wherein the area of the second conductivity type layer provided on the back surface of the semiconductor substrate of the first conductivity type is larger than the area of the first conductivity type layer provided on the back surface of the semiconductor substrate. The solar cell according to claim 1, wherein: 5. A first conductivity type semiconductor substrate, a back surface of the first conductivity type semiconductor substrate is covered with an oxide film, and a first conductivity type layer and a second conductivity type layer on the back surface of the semiconductor substrate are penetrated therethrough. 2. The solar cell according to claim 1, further comprising: 6. A claim was a flat portion of the bottom portion of the concave portion of the surface of the semiconductor substrate corresponding to the interconnector connecting portion of the electrode provided on the back surface of the semiconductor substrate 1, 2, 3, 4 or 5
The solar cell according to 1.