JP3368822B2 - 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 an improvement of a tandem type solar cell in which unit solar cells having different band gaps are laminated.

[0002]

2. Description of the Related Art A tandem solar cell is known in which photoelectric conversion efficiency is improved in a wide wavelength range by stacking unit solar cells having different band gaps. Japanese Patent Laying-Open No. 4-226084 also discloses such a tandem solar cell.

FIG. 9 shows a cross-sectional view of such a conventional tandem solar cell. In FIG. 9, the solar cell 1
0 indicates an upper cell 12 which is a unit solar cell on the light incident side (upper side in the figure) and a lower cell 14 which is a unit solar cell on the back surface side.
The tunnel diode 16 is sandwiched between and. An upper electrode 18 is provided on the light incident side and a lower electrode 20 is provided on the back surface side. In such a tandem solar cell, a solar cell having a large bandgap (Eg) is generally used for the upper cell and a solar cell having a small bandgap is used for the lower cell.

FIG. 10 shows the D of the solar cell shown in FIG.
The band structure along the dashed line of is shown. As shown in FIG. 10, since the upper cell 12 and the lower cell 14 are separated from each other in band level, carrier transfer of electrons and holes is hindered at the junction surface between the upper cell 12 and the lower cell 14. . For this reason, the tunnel diode 16 is arranged at this junction to enable the movement of carriers at the junction. As a result, the upper cell 12 and the lower cell 1
4 and 4 are connected in series. As a result, it becomes possible to function as one solar cell as a whole. With the above configuration, the upper cell 12
Moreover, since the wavelength of the light absorbed in the lower cell 14 can be made different, the range of the wavelength of the light that can be absorbed is widened, and the photoelectric conversion efficiency can be improved.

[0005]

However, in the conventional solar cell 10 shown in FIG. 9, since the upper cell 12 and the lower cell 14 are connected in series, the upper cell 12
It is necessary to equalize the currents flowing in the lower cell 14 and the lower cell 14. Therefore, in the solar cell 10 shown in FIG. 9, the thicknesses of the upper cell 12 and the lower cell 14 need to be set such that the amounts of carriers generated in the two cells are equal and the amounts of flowing currents match. . Therefore, there is a problem that the thickness cannot be most suitable for photoelectric conversion efficiency.

Further, although the tunnel diode 16 exists between the upper cell 12 and the lower cell 14, there is a problem that the tunnel diode 16 has a large resistance loss and carrier recombination loss.

The present invention has been made in view of the above conventional problems, and an object thereof is to make the thickness of a solar cell the most advantageous thickness for photoelectric conversion efficiency and to reduce carrier recombination loss. It is to provide a solar cell that can.

[0008]

In order to achieve the above object, the present invention is a tandem type solar cell in which unit solar cells having different band gaps are laminated, and the tandem type solar cell is provided on the light incident surface of the solar cell. , An upper electrode which is one electrode of the unit solar cell on the light incident side, and an n layer and a p layer formed on the back surface of the solar cell and formed on the back surface side of the solar cell, respectively, and are independently connected to each other. The other electrode of the unit solar cell and the back surface electrode which is also used as a pair of electrodes of the unit solar cell on the back surface side.

Further, in the above solar cell, a multiple quantum well layer is formed between unit solar cells having different band gaps.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

Embodiment 1. FIG. 1 shows a cross-sectional view of the structure of the solar cell according to the present invention. In FIG. 1, a solar cell 10 includes an upper cell 12 which is a unit solar cell composed of a semiconductor material having a wide band gap (Eg) and a lower cell 14 which is a unit solar cell composed of a semiconductor material having a narrow band gap. It has a tandem type structure in which is laminated. The upper cell 12 is configured by laminating an n ⁺ layer, a p layer, and a p ⁺ layer, and constitutes the light incident side unit solar cell according to the present invention. Further, an upper electrode 18 is provided so as to be connected to the n ⁺ layer formed on the uppermost part thereof. Furthermore, n
^An insulating film 24 is formed on the ⁺ layer. Insulating film 24
Is made of a transparent material, and the insulating film 24
The light enters the solar cell 10 via.

In the lower cell 14, the n ⁺ layers and the p ⁺ layers are alternately provided on the back surface of the p layer serving as the substrate. A negative electrode 26 is independently connected to each n ⁺ layer, and a positive electrode 28 is independently connected to each p ⁺ layer, and constitutes a back electrode according to the present invention. These negative electrode 26 and positive electrode 28 correspond to the lower cell 1
The upper electrode 18 is one electrode of the upper cell 12 and is also used as the other electrode. The lower cell 14 corresponds to the rear unit solar cell according to the present invention.

For the above-mentioned upper cell 12, for example, AlGaAs can be used as a material. Its bandgap is 1.82 eV. In this case, n
The dopant concentration of the ⁺ layer is 1 × 10 ¹⁹ cm ⁻³ , and the thickness thereof is 0.1 μm. The p-layer has a dopant concentration of 1 × 10 ¹⁶ cm ⁻³ and a thickness of 1.0 μm. Further, the p ⁺ layer has a dopant concentration of 1 × 10 ¹⁹ cm ⁻³
And its thickness is 0.1 μm.

As the material of the lower cell 14, for example, Si can be used. Its bandgap is 1.11 eV. In this case, the n ⁺ layer has a dopant concentration of 1 × 10 ¹⁹ cm ⁻³ and a thickness of 1.0 μm.
m. Further, the dopant concentration of the p layer is 5 × 10 ¹³ c
m ^-3 and its thickness is 100 μm. Furthermore, p ⁺
The dopant concentration of the layer is 1 × 10 ¹⁹ cm ⁻³ and its thickness is 1.0 μm.

The material of the upper cell 12 is InG.
It is also possible to use aP or GaAs. The band gaps thereof are 1.88 eV and 1.42 eV, respectively. GaAs can also be used as the material of the lower cell 14, and its band gap is 1.42 eV.
Is. Further, Ge can be used as the lower cell 14, and its band gap is 0.66 eV.

FIG. 2 shows the band structure of the broken line portion A shown in FIG. Further, FIG. 3 shows a band structure of a broken line portion B shown in FIG. In addition, in FIG.
The band structure of the broken line portion C shown in FIG. 1 is shown. These are all band structures when the solar cell 10 is irradiated with light.

The band structure at the broken line portion A in FIG.
As shown in FIG. 2, when the solar cell 10 is irradiated with sunlight, electrons and holes that are carriers are generated in the lower cell 14, the electrons move to the n ⁺ layer, and Collected in the negative electrode 26 of the electrode, holes move to the p ⁺ layer,
Collected in the positive electrode 28 of the lower electrode. In this case,
Since all the generated carriers move only in the lower cell 14, the upper cell 12 and the lower cell 1 as in the conventional example.
It is not necessary to match the values of the currents flowing in 4 and 4. Further, since the tunnel diode is not necessary, the loss due to carrier recombination does not occur here.

As shown in FIG. 3, the broken line portion B of FIG.
In, the p ⁺ layer, which is the lowermost layer of the upper cell 12, has the highest energy level in the conduction band. Therefore, the solar cell 10 is irradiated with sunlight, and the electrons generated in the upper cell 12 move toward the upper electrode 18. Also, the electrons generated in the lower cell 14 move toward the negative electrode 26 of the lower electrode.

Further, the energy level of the valence band in the broken line portion B becomes low in the n ⁺ layer which is the uppermost layer and the lowermost layer of the solar cell 10 in the broken line portion B. Therefore, holes cannot move along the broken line portion B. Therefore, the holes generated in the upper cell 12 and the lower cell 14 move in the respective cells in the lateral direction of FIG. 1 and move along the broken line portion C.

As shown in FIG. 4, the broken line portion C of FIG.
In, the energy level of the valence band is flat. This is done by appropriately adjusting the dopant concentrations of the p layer and p ⁺ layer of the upper cell 12 and the p layer of the lower cell 14 so that the energy level of the valence band of these layers becomes flat during light irradiation. Because there is. In the portion shown in FIG. 4, that is, the broken line portion C in FIG.
Are n ⁺ layers on the light-receiving surface side of the formation, p ⁺ layer is formed on the back side. With such a configuration, as shown in FIG. 4, holes generated in the upper cell 12 and the lower cell 14 move to the p ⁺ layer on the back surface side and are collected in the positive electrode 28 of the back surface electrodes. . In this case, the holes generated in the upper cell 12 move on the interface between the upper cell 12 and the lower cell 14. However, as mentioned above, there is no need to use a tunnel diode because of the flat energy level in the travel path.

Further, the energy level of the conduction band in the broken-line part C, because the higher becomes the structure by the p ⁺ layer of p layer and p ⁺ layer and the bottom cell 14 of the upper cell 12, the lower cell 1
The electrons generated in the p-layer 4 of FIG. 4 cannot move along the broken line portion C in FIG. Therefore, the electrons generated here move in the lower cell 14 in the lateral direction of FIG. 1 and move along the broken line portion B. The upper cell 12
For electrons generated in the p layer and p ⁺ layer of
2 towards the n ⁺ layer and will be collected at the upper electrode 18.

As described above, the negative electrode 26 and the positive electrode 28 provided on the back surface side of the lower cell 14 function as a pair of electrodes of the lower cell 14, and the positive electrode 28 serves as one electrode provided on the upper cell 12. The upper electrode 18 also functions as the other electrode of the upper cell 12. With this structure, the electrons generated in the lower cell 14 are collected in the negative electrode 26 and the holes are collected in the positive electrode 28. Regarding the electrons generated in the upper cell 12,
It moves to the n ⁺ layer of the upper cell 12 along the broken line portion B and is collected by the upper electrode 18. Further, the holes generated in the upper cell 12 and the lower cell 14 move to the p ⁺ layer of the lower cell 14 along the broken line portion C and are collected in the positive electrode 28. Thus, in the solar cell 10 according to the present embodiment,
Two unit solar cells, upper cell 12 and lower cell 1
4 is laminated, but it is not necessary to match the current values in the upper cell 12 and the lower cell 14 because the structure is not a normal series junction structure. Therefore, the thickness of each cell can be set to the most convenient thickness for the light absorption efficiency, and the photoelectric conversion efficiency can be improved.

In addition, the upper cell 12 and the lower cell 14
Since the n ⁺ layer is provided under the cell, electrons generated in each cell move to the n ⁺ layer of each cell and are collected by the upper electrode 18 on the light incident side and the negative electrode 28 on the back surface side. For this reason,
The distance traveled by the electrons, which are minority carriers, can be shortened, and recombination loss can be reduced.

Further, unlike ordinary tandem type solar cells connected in series, it is not necessary for carriers to pass through the energy barrier, and a tunnel diode for this purpose is also unnecessary. Therefore, it is not necessary to form an extra tunnel diode, and it is possible to prevent resistance loss and loss due to carrier recombination in this tunnel diode.

In this embodiment, the p-type substrate is used as the substrate for forming the solar cell, but it can be an n-type substrate. In this case, the above-described relationship between the conduction band and the valence band and the relationship between the electron and the hole have opposite behaviors, but the same functionally can be realized.

Embodiment 2. FIG. 5 shows a sectional view of Embodiment 2 of the solar cell according to the present invention. In FIG.
The lowermost layer of the upper cell 12 has two different band gaps.
A multiple quantum well layer 22 is formed by laminating thin films of different types. This multiple quantum well layer 22 is used as a means for improving photoelectric conversion efficiency in a solar cell by utilizing the intersubband excitation. However, in the past, this multiple quantum well layer 22 was used in a form formed between pn junctions, but in the present embodiment, the upper cell 12 and the lower cell 1 are used.
It is a shape formed between 4 and. This is because especially minority carriers generated in the solar cell 10 do not have to pass through the multiple quantum well layer 22, and recombination loss of minority carriers in the multiple quantum well layer 22 is prevented. The present embodiment has the same structure as the solar cell of the first embodiment shown in FIG. 1 except that the multiple quantum well layer 22 is provided between the upper cell 12 and the lower cell 14. There is.

For the multiple quantum well layer 22, for example, AlGaAs can be used as a material having a wide band gap. Its band gap (Eg) is 1.8
It is 2 eV. In this case, the dopant concentration is 1 × 1.
It is 0 ¹⁶ cm ^-3 and its thickness is 20 nm. Also,
GaAs can be used as the material having a narrow band gap. Its band gap is 1.42 eV. In this case, the dopant concentration is 1 × 10 ¹⁶ cm ⁻³ and its thickness is 10 nm.

Besides, GaAs can also be used as a material having a wide band gap. Its band gap is 1.42 eV. In this case, the dopant concentration is 1 × 10 ¹⁶ cm ⁻³ and its thickness is 30 nm. InG is used as a material having a narrow band gap.
aAs can also be used. Its bandgap is 1.25 eV. In this case, the dopant concentration is 1 × 1
It is 0 ¹⁶ cm ^-3 and its thickness is 15 nm.

FIG. 6 shows the band structure of the broken line portion A shown in FIG. Further, FIG. 7 shows a band structure of a broken line portion B shown in FIG. In addition, in FIG.
The band structure of the broken line portion C shown in FIG. 5 is shown. These are all band structures when the solar cell 10 is irradiated with light.

In FIG. 6, the band structure of the broken line portion A of FIG. 5 is the same as that of the first embodiment shown in FIG. Therefore, the electrons generated in the lower cell 14 are n ⁺
Transferred to the layer and collected on the negative electrode 26. Also, holes are p
It moves to the ⁺ layer and is collected in the positive electrode 28.

As shown in FIG. 7, the broken line portion B of FIG.
In, the energy level of the conductor is
Of the p-layer and the multiple quantum well layer 22 of FIG. Therefore, the electrons generated in the p layer of the upper cell 12 and the multiple quantum well layer 22 move to the n ⁺ layer of the upper cell 12 and are collected in the upper electrode 18. Further, since the electrons generated in the lower cell 14 have a high energy barrier in the multiple quantum well layer 22, they move to the n ⁺ layer provided on the back surface of the lower cell 14,
It is collected in the negative electrode 26 of the back surface electrode. As described above, also in the present embodiment, the n ⁺ layer is formed on the light incident side surface and the back surface of the solar cell 10, and the electrons generated in the upper cell 12 are generated in the light incident side n ⁺ layer and the lower cell. The electrons generated in 14 move to the n ⁺ layer on the back surface side. Therefore, the electrons, which are minority carriers, do not have to pass through the multiple quantum well layer 22, which has a large carrier recombination loss. Also,
The distance traveled by the electrons can also be shortened. Thereby, the power generation efficiency of the solar cell 10 can be improved.

As shown in FIG. 7, the energy level of the valence band in the broken line portion B becomes low in the n ⁺ layers on both sides of the solar cell 10, and the p layer and the multiple quantum well layer 22 between them are low.
Is getting higher. Therefore, the holes generated in the upper cell 12 and the lower cell 14 cannot move along the broken line portion B in FIG. Therefore, the cells move in the horizontal direction in FIG. 5 and move along the broken line portion C.

As shown in FIG. 8, the broken line portion C of FIG.
, The p layer of the upper cell 12 and the multiple quantum well layer 22
And the energy level of the valence band of the p layer of the lower cell 14 becomes flat. This can also be realized by appropriately adjusting the dopant concentration of each layer as described in FIG.
Therefore, the holes generated in the upper cell 12, the multiple quantum well layer 22, and the lower cell 14 move to the p ⁺ layer formed on the back surface side of the lower cell 14, and are collected in the positive electrode 28. In this case, holes generated in the upper cell 12 are also included in the multiple quantum well layer 2
However, in the present embodiment, the multiple quantum well layer 22 is formed of a p-layer, and recombination loss is small in the case of holes that are majority carriers, so there is no particular problem.

As shown in FIG. 8, the energy level of the conduction band in the broken line portion C is high in the p layer of the upper cell and the multiple quantum well layer 22, and is also high in the p ⁺ layer of the lower cell 14. Become. Therefore, the electrons generated in the lower cell 14 cannot move toward the n ⁺ layer formed in the upper cell 12. Therefore, the electrons generated in the lower cell 14 move laterally in FIG. 5, move to the n ⁺ layer of the lower cell 14 along the broken line portion B, and are collected in the negative electrode 26. The electrons generated in the upper cell 12 move toward the n ⁺ layer provided in the upper cell 12 and are collected by the upper electrode 18.

As described above, also in this embodiment, the electrons, which are minority carriers generated in the upper cell 12 and the lower cell 14, both have a large recombination loss, and thus the multiple quantum well layer 2 is formed.
Since it does not have to pass through 2, it is possible to prevent electrons from being reduced by recombination loss. Thereby, power generation efficiency can be improved. Further, the holes generated in the upper cell 12 pass through the multiple quantum well layer 22 as described above, but the holes are majority carriers in this embodiment, and the energy level difference in the valence band is small, so Coupling loss is small. Thus, the recombination loss of carriers is small, and the photoelectric conversion efficiency can be improved by the effect of the multiple quantum well layer 22, and the solar cell 10 according to the present embodiment can be improved.
The power generation efficiency of can be further improved.

In this embodiment as well, the p-type substrate is used as the substrate for forming the solar cell, but it may be an n-type substrate. In this case, the above-described relationship between the conduction band and the valence band and the relationship between the electron and the hole have opposite behaviors, but the same functionally can be realized.

[0037]

As described above, according to the present invention,
Since the minority carriers generated in the respective unit solar cells having different band gaps move only in the generated unit solar cells, the recombination loss can be significantly reduced. Moreover, since each unit solar cell does not have a structure of a series junction and a carrier can be taken out for each unit solar cell, it is not necessary to match the current value flowing in each unit solar cell. For this reason, each unit solar cell can be formed to have a thickness with the best photoelectric conversion efficiency, and also in this respect, power generation efficiency can be improved.

Further, by forming a multiple quantum well layer between each unit solar cell, the photoelectric conversion efficiency can be further improved.

[Brief description of drawings]

FIG. 1 is a sectional view of Embodiment 1 of a solar cell according to the present invention.

FIG. 2 is a diagram showing a band structure of a broken line portion A shown in FIG.

3 is a diagram showing a band structure of a broken line portion B shown in FIG.

FIG. 4 is a diagram showing a band structure of a broken line portion C shown in FIG.

FIG. 5 is a sectional view of Embodiment 2 of the solar cell according to the present invention.

6 is a diagram showing a band structure of a broken line portion A shown in FIG.

7 is a diagram showing a band structure of a broken line portion B shown in FIG.

8 is a diagram showing a band structure of a broken line portion C shown in FIG.

FIG. 9 is a cross-sectional view showing a structure of a conventional tandem solar cell.

FIG. 10 is a view showing a band structure of the tandem solar cell shown in FIG.

[Explanation of symbols]

10 solar cell, 12 upper cell, 14 lower cell, 1
6 tunnel diode, 18 upper electrode, 20 lower electrode, 22 multiple quantum well layer, 24 insulating film, 26 negative electrode, 28 positive electrode.

Continuation of the front page (56) Reference JP-A-1-225372 (JP, A) JP-A-3-206670 (JP, A) JP-A-60-201670 (JP, A) JP-A-57-1268 (JP , A) JP 51-113481 (JP, A) JP 63-222469 (JP, A) JP 3-181180 (JP, A) JP 8-204214 (JP, A) JP 9-199742 (JP, A) JP-A-4-27169 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 31/04-31/078

Claims (2)

(57) [Claims] 1. A tandem-type solar cell in which unit solar cells having different band gaps are stacked, which is provided on a light incident surface of the solar cell and serves as one electrode of the unit solar cell on the light incident side. The upper electrode is provided on the back surface of the solar cell, and is independently connected to the n layer and the p layer formed on the back surface side of the solar cell, and the other electrode of the unit solar cell on the light incident side and the back surface side A solar cell, comprising: a back electrode that also serves as a pair of electrodes of the unit solar cell. 2. The solar cell according to claim 1, wherein a multiple quantum well layer is formed between unit solar cells having different band gaps.