JPH09162432A - Solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To excite the carriers at a well part at high probability by making the band gap energy of a quantum well have energy band structure which increases by stages toward the barriers on both sides. SOLUTION: The band gap energy within a well layer increases by stages Eg2 and Eg1 from a well part W1 to a well part W2, toward the barrier shown by band gap energy Eg. Therefore, even if the energy difference (Eg-Eg) between the well part W1 and the barrier is large, the stage of small energy difference which is relatively easily excited by light or heat is made by the well part W2 at the middle stage, so the carriers at the well part W1 can be excited in the well part W2 relatively easily, that is, at high probability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell composed of a pn junction made of a semiconductor material, and more particularly to a solar cell suitable for manufacturing using a compound semiconductor material.

[0002]

2. Description of the Related Art In comparison with elemental semiconductors such as Si, the bandgap energy of compound semiconductors such as GaAs, for example, is close to the optimum value in view of the spectral distribution of the sun's rays. It is advantageous in obtaining a battery. From this, with the improvement of the crystal growth technique for compound semiconductors, compound semiconductors are being widely used as semiconductor materials for solar cells.

In order to further improve the conversion efficiency of such a solar cell comprising a pn junction of a compound semiconductor, light having a wavelength longer than a predetermined bandgap wavelength determined by the type of semiconductor material used is absorbed, It has been proposed to employ a quantum well structure for converting this into current in a solar cell. In the quantum well structure, a quantum well having a bandgap energy smaller than a predetermined bandgap energy is defined in the energy band structure.
This quantum well can absorb light on the long wavelength side having energy smaller than that of light having a predetermined bandgap wavelength. Therefore, in addition to the carriers excited by the absorption of light having a higher energy than that of light of a predetermined band gap wavelength, the absorption of light on the long wavelength side also causes carriers such as electrons and holes in the quantum well. Is generated.

Therefore, in theory, a solar cell having a quantum well structure should have higher efficiency than one having no quantum well structure, but at present the efficiency is lower than that. . The reason is that carriers such as electrons flowing in the conduction band toward the electrode or holes flowing in the valence band toward the electrode fall into the well of the quantum well structure, and the carriers and quantum It is considered that the majority of carriers generated in the well recombine and disappear in the quantum well without reaching the electrode across the barrier of the quantum well structure, that is, without functioning as a current.

In order to reduce the disappearance of carriers in such a quantum well, for example, J. Appl. Phys. 67 (7).
p3490-3493, 1, April, 1990 KW
It has been proposed by J. Barnham and others that the barrier thickness in the multiple quantum well structure is successively reduced to reduce the quantum well width, thereby increasing the energy level of each well as a whole. It was technically difficult or impossible to do.

[0006]

Therefore, the present invention intends to improve the efficiency of a solar cell having a quantum well structure.

[0007]

Means for Solving the Problems The present invention basically comprises:
In order to increase the probability of carriers in the well of the quantum well structure surmounting the barrier of the quantum well, the concept is to gradually increase the bandgap energy of the quantum well toward the barrier. For the purpose of <construction>, the solar cell of the present invention is composed of a pn junction made of a semiconductor material and has a quantum well structure in the energy band structure. It is characterized by gradually increasing toward the barrier (corresponding to claim 1). As a specific configuration of the solar cell of the present invention, it is desirable that the band gap energy level difference of the adjacent portion in the quantum well of the energy band is 0.1 eV or less (corresponding to claim 2). As a specific configuration of the solar cell of the present invention, it is desirable that the quantum well structure is formed by stacking compound semiconductors having different band gap energies (corresponding to claim 3).

<Operation> FIG. 1 is an explanatory view of an energy band structure for explaining the basic principle of the present invention. In order to explain the operation of the present invention with reference to an explanatory view showing this band structure, p formed by a p-type semiconductor and an n-type semiconductor will be described.
The n-junction forms a transition region T between the p-type region (p) and the n-type region (n). Ev represents a valence band and Ec represents a conduction band. In the transition region T, a well W having a bandgap energy smaller than the bandgap energy Eg of both p-type and n-type semiconductors is formed. Well W has band gap energy E
It comprises a pair of well portions W ₁ having a bandgap energy Eg ₁ smaller than g and both well portions W ₁ have a well portion W ₂ having a bandgap energy Eg ₂ smaller than the bandgap energy Eg ₁ .

Assuming that there is no middle well portion W ₁ when viewed from the deepest well portion W ₂ , the well portion W
_The barrier seen from ₂ has a relatively large value according to the difference between the bandgap energy Eg and the bandgap energy Eg ₂ , but since the well portion W ₁ that is the middle step is provided, the barrier height is high. Is divided into smaller values according to the number of stages in between. Carriers generated in the well portion W ₂ or dropped into the well portion W ₂ are excited in the middle well portion W ₁ by absorption of light, or include carriers excited in the middle well portion W ₁ and the middle well portion W ₁ is also included. the probability of the W ₂ is excited to the conduction band Ec or the valence band Ev is compared to the probability that carriers by absorption of light is excited directly from the well portion W ₂ in the conduction band Ec or the valence band Ev, the excitation Significantly increased due to the low energy required. For this reason, the carriers in the well portion W ₂ are transferred to the well portion W _{1 in the} middle stage.
Thus, the probability that the carrier electrons are excited in the conduction band Ec and the carrier holes are excited in the valence band Ev are increased.

As a result, the number of carriers trapped and lost in the well W including the well portion W ₂ is reduced, and the probability of disappearance due to carrier recombination is reduced, so that the carriers in the well W are effectively converted into a current. You can take it out. The well portion W ₁ and the well portion W ₂ are relatively easily prepared, for example, by appropriately selecting the composition or material of the compound semiconductor according to their band gap energy Eg and sequentially stacking the selected compound semiconductors. Can be formed.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the illustrated embodiments. <Specific Example 1> FIG. 1 is a longitudinal sectional view of a solar cell according to a specific example 1 of the present invention. The solar cell 10 includes a base layer 12 formed on the substrate 11, an emitter layer 13 formed on the base layer 12, and a well layer 14 inserted between the layers 12 and 13.

In the illustrated example, n-type GaAs is used as the substrate 11, and n-type Al _0.2 Ga is used as the base layer 12.
_0.8 As (Carrier density: 2 × 10 ¹⁷ particles / cm ³ , film thickness: 3
μm) and p ⁺ -type Al _0.2 is used as the emitter layer 13.
Ga _0.8 As (carrier density: 2 × 10 ¹⁸ pieces / cm ³ , film thickness:
0.5 μm) is used. Also, in the illustrated example,
N ⁺ -type Al _0.4 Ga is provided between the substrate 11 and the base layer 12.
_0.6 As (Carrier density: 2 × 10 ¹⁸ particles / cm ³ , film thickness:
A well-known BSF (back surface) layer 15 of 0.3 μm) is inserted, and p ⁺ -type Al _0.8 Ga _0.2 As (carrier density: 2 × 1) is formed on the surface of the emitter layer 13.
A well-known window layer 16 of 0 ¹⁸ pieces / cm ³ and a film thickness of 0.02 μm is formed.

In the illustrated example, the well layer 14 inserted between the base layer 12 and the emitter layer 13 is a pair of intrinsic Al layers.
_0.1 Ga _0.9 As layer 17 (undoped, film thickness: 0.1 μm)
And the intrinsic GaAs layer 18 between the intrinsic Al _0.1 Ga _0.9 As layer 17 and
(Undoped, film thickness: 0.3 μm). N-type Al _0.2 Ga forming the base layer 12 and the emitter layer 13
Compared to _0.8 As and p ⁺ -type Al _0.2 Ga _0.8 As bandgap bandgap energy Eg of (1.67eV), the bandgap energy Eg ₁ intrinsic Al _0.1 Ga _0.9 As layer 17 of the well layer 14 (1.55 eV ) Is small and the well layer 14
Band gap energy Eg ₂ of the intrinsic GaAs layer 18 of
Indicates an even smaller value (1.43 eV). As a result, in the energy structure of the pn junction of the solar cell 10 shown in FIG. 2, the intrinsic Al _0.1 Ga _0.9 As layer 17 of the well layer 14 is formed.
Form the well portion W ₁ in the energy band structure shown in FIG. 1, and the intrinsic GaAs layer 18 forms the well portion W ₂ , so that the band gap energy is directed toward the barrier in the transition region T of the pn junction layer. A quantum well W is formed which gradually increases from Eg ₂ to Eg ₁ .

[0014] and the band gap energy Eg ₁ of the middle of the well portion W _1, the difference between the band gap energy Eg ₂ of the bottom of the well part W ₂ is a 0.12eV, is the value of the about 60% 0 0.72 eV is the energy step on the conduction band Ec side, and the remaining about 40%, 0.28 eV, is the energy step on the valence band Ev side. In addition, the band gap energy Eg ₁ of the middle of the well portion W _1, the difference between the forbidden band band gap energy Eg, 0.12
eV, similarly, 0.072, which is a value of about 60% thereof.
eV is the energy level difference on the conduction band Ec side, and the remaining about 40%, 0.048 eV, is the energy level difference on the valence band Ev side. These band gap energy steps differ slightly depending on the electrons and holes, but are about 0.1.
By setting eV or less, it is possible to expect escape from the well W with a high probability of carriers.

BSF layer 15 and base layer 1 on substrate 11
2, the well layer 14, the emitter layer 13, and the window layer 16 are, for example, trimethylgallium “(CH
₃ ) ₃ Ga ”, trimethylaluminum“ (CH ₃ ) ₃
With Al "and arsine" AsH ₃ ", using hydrogen gas as a carrier gas, it is possible by 700 ° C., vacuum MOCVD (metal organic vapor phase epitaxy) carried out under reduced pressure of 100Torr method to form. As dopant of p-type and n-type, "(CH _{3) 2} Zn" and "Si ₂ H _6" was used, respectively. During the crystal growth of each layer, for example, when the intrinsic GaAs layer 18 is formed, Al is unnecessary as a dopant. Therefore, the crystal growth is temporarily stopped while the supply of unnecessary material such as trimethylaluminum is interrupted. Planned.

After forming each of the layers 12 to 16 on the substrate 11,
In the window layer 16, as a contact layer 19 for electrodes,
P ⁺ type GaAs (carrier density: 2 × 10 ¹⁹ pieces / cm ³ , film thickness: 0.3 μm) is formed. This contact layer 19
A front surface electrode 20 is formed on the upper surface by a well-known photolithography method, and a rear surface electrode 21 is formed on the rear surface of the substrate 11. Further, the antireflection film 22 covering the exposed surface of the window layer 16 is formed, whereby the solar cell 10 having the quantum well structure is completed.

In the solar cell 10 of the first embodiment according to the present invention, the sunlight incident through the antireflection film 22 is absorbed by the base layer 12, the well layer 14 and the emitter layer 13. In the n region of the base layer 12 and the p region of the emitter layer 13, electrons and holes are generated according to the absorption of this light as in the conventional solar cell, and the electrons are transmitted to the back electrode 21 by the internal electric field. , And holes are surface electrodes 2
Diffuse toward 0 toward the corresponding electrodes 21 and 20, respectively. As is well known in the art, the BSF layer 15 and the window layer 16 prevent disappearance of carriers diffusing toward the electrodes 20 and 21 by surface recombination on the surface of the emitter layer 13 and the back surface of the base layer 12, respectively. However, this serves to prevent a decrease in conversion efficiency.

On the other hand, the well layer 14 formed in the transition region T
Is smaller than the bandgap energy Eg of the p region (13) and the n region (12), the light on the long wavelength side that cannot be absorbed by the p region (13) and the n region (12) is absorbed by the well layer 14. To be done. Electrons and holes are generated in the well layer 14 due to the absorption of the light on the long wavelength side.

In this well layer 14, carriers generated in the well layer 14 and carriers diffused toward the electrode 20 or 21 may be trapped. However, the band gap energy in the well layer 14 may be trapped. Toward the barrier indicated by the band gap energy Eg, the well portion W ₁
To the well portion W ₂ in a stepwise manner (Eg ₂ , Eg ₁ ). Therefore, even if the energy difference (Eg−Eg ₁ ) between the well portion W ₁ and the barrier is large, the middle well portion W ₂ forms a step with a small energy difference that is relatively easily excited by light or heat. Therefore, the carriers in the well portion W ₁ are excited to the well portion W ₂ relatively easily, that is, with a high probability. Also, the carrier in the well portion W ₂ is likewise relatively easily able to escape from the well portion W _2, herniated carrier electrons, towards the base layer 12, also prolapsed carrier holes emitter layer It is diffused toward 13.

As a result, the frequency of annihilation due to recombination of carriers in a quantum well as in a conventional quantum well structure solar cell can be greatly reduced, and it is generated in the quantum well or falls into this quantum well. Since the carriers can be effectively used as a current, the efficiency of the solar cell 10 is greatly improved as compared with the conventional one, and high photoelectric conversion efficiency can be obtained. Further, in the structure of the solar cell 10, a desired well structure can be relatively easily manufactured by selecting the semiconductor material forming the well layer 14 according to the band gap energy thereof.

The semiconductor material forming the well layer 14 can be appropriately selected, but the band gap energy difference between the well portion W ₁ and the well portion W ₂ which are the adjacent portions in the well layer 14 is 0.1 eV or less. As such, it is desirable to select the material. By setting the bandgap energy difference between the well portions W ₁ and W ₂ to be 0.1 eV or less, it is possible to take out as an effective current with a high probability without erasing the carriers in the well layer 14. Further, when the energy difference between the well and the barrier is large, the well layer 14 may have a multi-stage structure having a multilayer structure of four or more layers in order to increase the number of stages in the well portion as necessary.

Further, an example is shown in which the energy step is formed in a V shape on both sides of the well portion W ₁ which is the central portion of the well layer 14, but the energy step is formed only on one side of the well portion W _1. It is possible to obtain the same effect. Further, although FIG. 2 illustrates the example in which the single well layer 14 is provided, a plurality of wells 14 can be provided.

<Second Embodiment> FIG. 3 is a vertical sectional view of a solar cell 10 showing a second embodiment of the present invention. In the solar cell 10 shown in FIG. 3, three well layers 14 are formed, and the same reference numerals as those in FIG. 2 are attached to the components that perform the same functions as those shown in FIG. In the solar cell 10 according to the second specific example, in the transition region T between the base layer 12 and the emitter layer 13, as a multiple quantum well structure,
Three well layers 14 are formed. The structure of each well layer 14 is the same as in the first specific example shown in FIG.
GaAs layer 18 and a pair of intrinsic Al _0.1 Ga sandwiching the GaAs layer 18
_It consists of _0.9 As layer 17.

An intrinsic Al _0.2 Ga _0.8 As layer is formed between the well layers 14.
A layer 28 (undoped, film thickness: 0.1 μm) is arranged, and the intrinsic Al _0.2 Ga _0.8 As layer 28 has the same bandgap energy E as the base layer 12 and the emitter layer 13.
has g. The intrinsic Al _0.2 Ga _0.8 As layer 28 can be formed by the low pressure MOCVD method similar to that described in the first specific example.

FIG. 4 is an explanatory diagram of the energy band structure of the solar cell 10 according to the second specific example shown in FIG. Each well layer 14 is provided in the transition region T between the n-type region and the p-type region formed by the base layer 12 and the emitter layer 13.
Are formed in series, and each quantum well W has the same structure as that shown in FIG.
The band gap energy is increased stepwise (Eg ₂ , Eg ₁ ) from the well portion W ₁ to the well portion W ₂ toward the barrier indicated by the band gap energy Eg.

Therefore, the carriers in each quantum well W can be escaped to the n-type region or the p-type region with a high probability as in the first specific example, whereby the carriers in the quantum well W are caused to flow into the current. Can be effectively taken out. Further, since the three quantum wells W are provided, it is possible to absorb light on the long wavelength side in each quantum well W, and the total number of carriers excited in the quantum well W increases, It is expected that the conversion efficiency will be improved according to this increase.

In the first and second embodiments, the quantum well W
Although the example in which the is formed in the transition region T of the pn junction has been described, the quantum well W according to the present invention is, instead of being formed in the transition region T, within a range where sunlight passing through the antireflection film 22 is transmitted. , P-type region or n-type region.

<Embodiment 3> FIG. 5 is a drawing similar to FIG. 2 showing a third embodiment of the present invention, showing an example in which the quantum well W is formed in the base layer 12 constituting the n-type region. . In the solar cell 10 according to the third specific example, each layer can be formed by the low pressure MOCVD method similar to that described above. That is, after the BSF layer 15 is formed on the substrate 11, the base layer 12
Lower part 12a of On the lower 12a, a pair of n-type Al _{0. 1} Ga _0.9 As layer 17 and the quantum well W consisting of between the n-type GaAs layer 18 is formed.

[0029] In the first and second specific examples the quantum well W is formed in the transition region T, but Al _{0. 1} Ga _0.9 As layer 17 and the GaAs layer 18 is formed of an intrinsic semiconductor layer of undoped, in a third specific example of the quantum well W is formed on the n-type region, Al _0.1 Ga _{0. 9} as layer 17 and the GaAs layer 18 as described above, it is preferable that the n-type. Therefore, when the quantum well W is formed in the p-type region, the Al _0.1 Ga _0.9 As layer 17 is used.
The p-type is adopted for the and GaAs layers 18, respectively.

After the quantum well W is formed, the upper portion 12b of the base layer 12 is formed, the emitter layer 13 is further formed thereon, and then the window layer 16, the contact layer 19, and the electrodes 20 and 21 are formed. The solar cell 10 is completed.
In the solar cell 10 according to the third specific example, the base layer 12
Since the transition region T is formed at the junction between the upper portion 12b of the above and the emitter layer 13 thereabove, a quantum well structure is not formed in this transition region T, but it is transmitted to the inside of the base layer 12. The generated light generates carriers that effectively act as a current in the quantum well W formed in the base layer 12. Therefore, the solar cell 10 of the third specific example can also obtain excellent conversion efficiency.

When forming a quantum well in a p-type region or an n-type region, a multiple quantum well structure having a large number of quantum wells can be adopted, and a multi-stage structure can be adopted in each quantum well. In addition, for all the multiple quantum well structures, it is possible to form the bandgap energy of some of the quantum wells in stages without forming the bandgap energy of the wells in stages. It is possible to obtain a unique effect due to.

In each of the specific examples, the thickness dimension of each of the layers 11 to 16 is described, but these values are merely examples and can be appropriately selected. Further, as the semiconductor material, Al _0.2 Ga _0.8 As is adopted as the semiconductor material forming the pn junction,
An example in which a combination of GaAs and Al _0.1 Ga _0.9 As is adopted as the well of the quantum well structure has been described, but semiconductor materials having different band gap energies can be appropriately combined and used. As an example of such a combination, InP / InGaAs / InGaAs
P, AlGaAs / GaAs / InGaAsP, InG
aP / GaAs / InGaAsP, AlGaAs / In
There are combinations such as GaAs / GaAs. Also,
The growth method of each crystal layer is not limited to the low pressure MOCVD method, but M
A BE (molecular beam epitaxy) method, a hydride vapor phase epitaxy method, or the like can be appropriately selected.

[0033]

According to the present invention, as described above, the bandgap energy of the quantum well structure is increased stepwise toward the barrier in the quantum well, so that the carriers in the quantum well are in the conduction band. Alternatively, it is possible to effectively excite the valence band, thereby reducing the carriers annihilated by the recombination in the quantum well, and effectively using these carriers as a current, the efficiency of the solar cell is improved. It can be greatly improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an energy band structure showing the principle of the present invention.

FIG. 2 is a vertical cross-sectional view of a solar cell showing a first specific example of the present invention.

FIG. 3 is a drawing similar to FIG. 2 showing a second embodiment of the present invention.

FIG. 4 is an explanatory diagram of an energy band structure according to a second example of the present invention.

FIG. 5 is a view similar to FIG. 2, showing a third embodiment of the present invention.

[Explanation of symbols]

10 solar cell 12 base layer 13 emitter layer 14 well layer W quantum well W ₁ well portion W ₂ well portion

Claims (3)

[Claims] 1. A solar cell comprising a pn junction made of a semiconductor material and having a quantum well structure in the energy band structure, wherein the band gap energy of the quantum well increases stepwise toward the barriers on both sides. A solar cell having a structure. 2. The solar cell according to claim 1, wherein a bandgap energy step difference between adjacent portions of the well is 0.1 eV or less. 3. The solar cell according to claim 1, wherein the quantum well structure is formed by stacking compound semiconductor materials having different bandgap energies.