JPH08204215A - Series connected solar cell - Google Patents

Abstract

PURPOSE: To obtain a structure of series connected solar cell having high conversion efficiency and improved spectral sensitivity characteristics. CONSTITUTION: A tandem solar cell comprising upper and lower cells 4, 2 of mixed crystal of a compound semiconductor connected in series through a tunnel junction layer 3 has a structure where the band gap Eg of a BSF layer 412 is set higher than that of a base layer 42 by equalizing the impurity concentration between the base layer 42 and the BSF layer 412 of the upper cell 4 while differentiating the composition of mixed crystal. Crystal strain is relaxed by providing a strain relax layer 411 having Eg lower than that of the base layer 42 under the BSF layer 412.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a solar cell, which is a semiconductor element for converting solar energy into electric energy, and particularly to a tandem using a compound semiconductor mixed crystal devised to enhance conversion efficiency. The present invention relates to a structure of a series-connected solar cell such as cells.

[0002]

2. Description of the Related Art There are various structures of solar cells. For example, in a pn junction semiconductor solar cell including an n-type emitter layer 433 and a p-type base layer 422 as shown in FIG. With the layer 433 as the light incident side, the photons absorbed in the underlying p-type base layer 422 generate a pair of holes-electrons, of which electrons, which are the minority carriers, move by diffusion to form the pn interface. When it reaches the depletion layer, it flows into the n-type emitter layer 433 due to the large electric field of the depletion layer and becomes a current. However, as shown in FIG. 6B, p
Some of the electrons generated in the mold base layer 422 may enter the back surface electrode layer 8 by diffusion, and they cannot return to the base layer 422 anymore, and eventually combine with holes that are majority carriers and disappear. , Current does not. In order to prevent such a back surface recombination loss, conventionally, a back surface electric field (Back Su) is made so that minority carriers (electrons) generated in the base layer 422 should be kept away from the back surface electrode layer 8 as much as possible.
rface Field; hereinafter referred to as "BSF") Layer 4
A layer called 11 is provided below the base layer 422. As shown in FIG. 6C, the BSF layer acts as a band barrier for the minority carriers generated in the base layer. As the BSF layer, 1) the same material as the base layer is used to increase the impurity density to serve as a barrier against minority carriers, 2) other semiconductor materials have a larger forbidden band width than the base layer material, Also used is a barrier against minority carriers. With such a BSF layer, the conduction band of the base layer sharply rises at the interface with the BSF layer, and the electrons, which are minority carriers, are repelled by this barrier and cannot go further. In order to obtain a sufficient barrier against minority carriers by the method 1) above, it is necessary to make the impurity density of the BSF layer extremely high. This is not a serious problem in the case of a single-layer solar cell, but a high-concentration doping layer is used as the upper cell BSF layer of a stacked solar cell in which solar cells are connected in series (tandem) to obtain a high output voltage. Is used, infrared light absorption due to a high concentration of majority carriers occurs in the layer. This infrared light is supposed to reach the cell in the lower layer of the tandem connection and be absorbed there to become a current, and such infrared light absorption in the BSF layer causes a large energy loss for the solar cell, which is not preferable. On the other hand, the above method 2) does not have a problem of infrared light absorption, but it is generally difficult to find a suitable material.
Further, even if an appropriate material is used, the base layer is grown on the BSF layer by hetero-heteroepitaxial growth, and since the lattice constants of the two differ, crystal defects easily occur at the interface.

As a structure using the BSF layer according to the method 1), a laminated solar cell (cell) as shown in FIG. 5 is known. FIG. 5 shows a so-called tandem cell in which a plurality of single cells are connected in series in order to obtain a high conversion efficiency or a high electromotive voltage. In FIG. 5, a GaAs cell 2 is used as the lower cell (bottom cell),
As an upper cell (top cell), an In _0.5 Ga _0.5 P cell 4 lattice-matched to GaAs is used. Further, a GaAs tunnel junction layer 3 is provided between the lower cell and the upper cell in order to electrically connect them in series. BSF layer 41 of the top cell (upper cell) of FIG.
Has an impurity density higher than that of the base layer 42. In detail, the GaAs bottom cell 2 is Zn
Doped p ⁺ GaAs substrate 11 (p <1 × 10 ¹⁹ c
m ⁻³ ) formed on the p ⁺ GaAs buffer layer 12 having a thickness of 0.3 μm and an impurity density of 7.0 × 10 ¹⁸ cm ⁻³ . And the GaAs cell 2 is p ⁺ Ga
Thickness of 0.1 μm formed on the As buffer layer 12
And p ⁺ InGaP with an impurity density of 3.0 × 10 ¹⁸ cm ^-3
BSF layer 21 of, and a pGaAs base layer 2 provided thereon with a thickness of 3 μm and an impurity density of 2.0 × 10 ¹⁷ cm ⁻³
2. An n ⁺ GaAs emitter layer 23 having a thickness of 0.1 μm and an impurity density of 2.0 × 10 ¹⁸ cm ⁻³ provided on the upper portion thereof,
Furthermore, the thickness of the upper part is 0.1 μm and the impurity density is 2.0 ×
It is composed of 10 ¹⁸ cm ⁻³ of n ⁺ InGaP window layer 24. A pn junction is formed between the emitter layer 23 and the base layer 22. The GaAs tunnel junction layer 3 is n ⁺ InG which is the uppermost layer of the lower cell (GaAs bottom cell) 2.
An n ⁺ GaAs layer 31 having a thickness of 15 nm and an impurity density of 5 × 10 ¹⁸ cm ⁻³ or more formed on the aP window layer 24, and ap ⁺ GaAs having a thickness of 15 nm and an impurity density of 1.0 × 10 ¹⁹
And the layer 32. And on this upper part, p with a thickness of 0.3 to 0.7 μm and an impurity density of 4 × 10 ¹⁷ cm ^−3
+ In _0.5 Ga _0.5 P BSF layer 41; thickness 0.7 to
PIn with 1.5 μm and impurity density of 1.5 × 10 ¹⁷ cm ⁻³
_0.5 Ga _0.5 P base layer 42; n ⁺ In _0.5 Ga _0.5 P emitter layer 43 having a thickness of 50 nm and an impurity density of 3.0 × 10 ¹⁸ cm ⁻³ ; and a thickness of 30 nm, an impurity density of 2 × 10
^The In _0.5 Ga _0.5 P top cell 4 is formed by depositing the ¹⁸ cm ⁻³ n ⁺ AlInP window layer 44 in this order.
An n ⁺ GaAs layer 51 having a thickness of 0.3 μm for ohmic contact is formed on a part of the upper portion of the In _0.5 Ga _0.5 P top cell 4, and an Au—Ge / Ni / Au layer is formed on the n ⁺ GaAs layer 51.
An upper metal electrode layer (surface electrode layer) 7 including a layer 71 and an Au layer 72 thereon is formed. On the back surface of the p ⁺ GaAs substrate 11, an Au layer is formed as a lower metal electrode layer (back surface electrode layer) 8. At the surface of the n ⁺ AlInP window layer 44 of the In _0.5 Ga _0.5 P top cell 4, n ⁺ G
The ZnS layer 61 and the MgF ₂ layer are formed in regions other than the portion where the aAs layer 51 and the upper metal electrode layer 7 are formed.
The antireflection film 6 including the layer 62 is formed.

[0004]

However, in the conventional laminated solar cell as shown in FIG. 5, the BSF layer 41 of the top cell absorbs a large amount of infrared light, and the incident light does not reach the bottom cell sufficiently. However, there is a problem that the wavelength division effect does not work effectively as a tandem cell. Therefore, the conventional stacked solar cell (tandem cell) having a tandem structure has a problem that the conversion efficiency is low, and the open circuit voltage Voc, the short-circuit photocurrent Isc, and the like are also low.

The present invention has been made in view of the above points, and provides a novel laminated solar cell structure in which the infrared light absorption in the top cell is extremely small and the back surface recombination loss can be effectively prevented. Is the purpose.

[0006]

In order to solve the above problems, the present invention is a tandem cell type solar cell in which a top cell 4 and a bottom cell 2 are composed of at least a bonding layer 3 as shown in FIG. The structure of the top cell 4 is characteristic. That is, the top cell 4 includes the first conductivity type first emitter layer 43, the second conductivity type first base layer 42, the second conductivity type first BSF layer 412, and the second conductivity type strain relaxation layer. 411 as a main component and the first BSF layer 41
2. The impurity density of the strain relaxation layer 411 is substantially equal to or smaller than the impurity density of the first base layer, and the forbidden band Eg of the first BSF layer 412 is the first BSF.
It is characterized in that it is larger than the Eg of the layer and the Eg of the strain relaxation layer 411 is smaller than the Eg of the first base layer 42. As shown in FIG. 1, the bottom cell mainly includes the second emitter layer 23, the second base layer 22, the second BSF layer 21, and the like. Note that FIG. 1 is merely an example, and it goes without saying that the present invention may be a series-connected solar cell having three or more cells.

Preferably, the first base layer 42, the first BSF layer 412, and the strain relaxation layer 411 are mixed crystal compound semiconductors composed of the same constituent element, and the composition x of this mixed crystal is preferable. Are the first base layer 42 and the first BSF layer 4
12 and the strain relaxation layer 411 are different from each other.

Preferably, the junction layer 3 is a tunnel junction layer composed of a first conductivity type compound semiconductor 31 and a second conductivity type compound semiconductor 32.

Preferably, the top cell 4, the bonding layer 3 and the bottom cell 2 are formed on the compound semiconductor substrate 11 as described in claim 4, and the lattice constant of the first base layer 42 of the top cell 4 is the compound. It is characterized in that it substantially matches the lattice constant of the semiconductor substrate.

[0010] More preferably, the first base layer 42 as claimed in claim 5, wherein is an In _x Ga _1-x P, a compound semiconductor substrate 11 is GaAs, the composition x is substantially 0.5 There is a feature. Substantially 0.5 means that it may be slightly deviated from 0.5 within a range in which the occurrence of lattice strain is suppressed.

[0011]

According to the features of the present invention, the first BSF layer 41 is
2. Since the impurity density of the strain relaxation layer 411 is equal to or smaller than the value of the base layer 42, there is almost no infrared light absorption by the majority carrier in the BSF layer, which is a problem in the conventional technique. The incident light effectively passes through the top cell and reaches the bottom cell. Therefore, the wavelength division effect as a tandem cell is sufficiently exerted and the conversion efficiency is increased.

In the solar cell of the present invention, as shown in the band diagram of FIG. 4B, the forbidden band width Eg of the first BSF layer 412 is made larger than the Eg of the first BSF layer 42 so that the potential barrier is increased. The backside recombination loss of the minority carriers generated by light absorption is prevented. FIG. 4A shows a band diagram in the conventional technique. In the conventional technique, the first BSF layer 41 and the second base layer 42 are used.
And the forbidden band width Eg of the first BSF layer 41
A potential barrier for minority carriers is formed by increasing the impurity density of. Therefore, in this case, the first
Absorption of infrared light occurs in the BSF layer. In the case shown in FIG. 4B, a concern is that the forbidden band difference ΔEg between the first base layer 42 and the first BSF layer 412 is ΔEg.
The first base layer 42 and the first BSF
This is the occurrence of a difference in lattice constant due to changing the composition ratio x of the mixed crystal of the layer 412. If epitaxial growth is performed in a state where the crystal lattice spacing is different, the crystallinity of the first base layer 42 becomes poor, and a new cause of light absorption is formed.
After all, the spectral sensitivity characteristic cannot be improved. In the present invention, the Eg of the first BSF layer 412 is made larger than the Eg of the first base layer 42, and at the same time, the Eg of the strain relaxation layer 411 formed in contact with the first BSF layer 412.
Is smaller than Eg of the first base layer 42. As a result, the lattice strain is compensated by the first BSF layer 412 and the strain relaxation layer 411 so that the base layer 42 is not affected by the lattice mismatch. That is, the strain relaxation layer 411
The value obtained by subtracting the mixed crystal composition ratio from the lattice spacing of the first base layer 412 to the lattice spacing of the first BSF layer 411 is calculated from the lattice spacing of the first base layer 42 to the lattice spacing of the strain relaxation layer 411. Since it is selected so as to have the same sign with the opposite sign to the value obtained by subtracting, the lattice strain is compensated and the crystallinity of the first base layer becomes good. Therefore, conversion efficiency as a tandem cell, open circuit voltage Vos, short-circuit photocurrent Isc
Is improved.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. The same parts are designated by the same reference numerals in FIG. FIG. 1 shows a GaAs tunnel junction between an upper cell (top cell) 4 and an GaAs lower cell (bottom cell) 2 using In _0.5 Ga _0.5 P lattice-matched with GaAs as an emitter layer and a base layer according to an embodiment of the present invention. It is a tandem cell structure solar cell connected by layer 3. Ga in detail
As lower cell (bottom cell) 2 is Zn-doped p ⁺ Ga.
A thickness of 0.3 μm formed on the As substrate 11 (p <1 × 10 ¹⁹ cm ⁻³ ) and an impurity density of 7.0 × 10 ¹⁸ cm ⁻³
Is formed on the p ⁺ GaAs buffer layer 12.
The GaAs cell 2 has a thickness of 0.1 μm formed on the p ⁺ GaAs buffer layer 12, and has an impurity density of 3.0 ×
10 ¹⁸ cm ⁻³ p ⁺ InGaP BSF layer 21, thickness 3 μm provided on the BSF layer 21, and impurity density 2.0 × 10 ¹⁷
cm ⁻³ pGaAs base layer 22, 0.1 μm thickness provided on the upper part thereof, n ⁺ GaAs emitter layer 23 having an impurity density of 2.0 × 10 ¹⁸ cm ⁻³ , and 0.1 μm thickness on the upper part, and impurity density 2.0 × 10 ¹⁸ cm ^-3 n ⁺ I
and the nGaP window layer 24. Emitter layer 2
A pn junction is formed between 3 and the base layer 22. G
The aAs tunnel junction layer 3 has a thickness of 15 nm and an impurity density of 5 × 10 ¹⁸ cm ⁻³ formed on the n ⁺ InGaP window layer 24 which is the uppermost layer of the lower cell (GaAs bottom cell) 2.
It is composed of the above n ⁺⁺ GaAs layer 31 and the p ⁺⁺ GaAs layer 32 having a thickness of 15 nm and an impurity density of 1.0 × 10 ¹⁹ . The peak current of the tunnel junction layer 3 is about 50 mA /
cm ² . On top of this, pIn _y Ga _{1 -y} P having a thickness of 0.2 μm and an impurity density of 1.5 × 10 ¹⁷ cm ⁻³
(Y> 0.5) A strain relaxation layer 411 is formed, and the strain relaxation layer 411 has a thickness of 0.2 μm and an impurity density of 1.5.
× 10 ¹⁷ cm ^-3 pIn _x Ga _1-x P (x <0.5) B
An SF layer 412 is formed, and a thickness of 0.7 to
PIn with 1.5 μm and impurity density of 1.5 × 10 ¹⁷ cm ⁻³
_0.5 Ga _0.5 P base layer 42; n ⁺ In _0.5 Ga _0.5 P emitter layer 43 having a thickness of 50 nm and an impurity density of 3.0 × 10 ¹⁸ cm ⁻³ ; and a thickness of 30 nm, an impurity density of 2 × 10
^{An 18} cm ⁻³ n ⁺ AlInP window layer 44 is deposited in this order to form an InGaP upper cell (top cell) 4. The In _0.5 Ga _0.5 P top cell 4 has a part of the upper portion thereof with n ⁺ Ga having a thickness of 0.3 μm for ohmic contact.
An As layer 51 is formed, and Au-Ge / Ni is formed on the As layer 51.
The upper metal electrode layer (surface electrode layer) 7 including the / Au layer 71 and the Au layer 72 thereon is formed. p ⁺ Ga
A lower metal electrode layer (rear surface electrode layer) is formed on the rear surface of the As substrate 11.
8 is an Au layer. In _0.5 Ga _0.5 P
On the surface region of the n ⁺ AlInP window layer 44 of the top cell 4, other than the part where the n ⁺ GaAs layer 51 and the upper metal electrode layer 7 thereon are formed, a ZnS layer 6 is formed.
1. The antireflection film 6 made of the MgF ₂ layer 62 is formed. The dopant in FIG. 6 is p ⁺ GaAs buffer layer 12, p ⁺ InGaP-BSF layer 21, pGaAs base layer 22, p ⁺⁺ GaAs layer 32, pIn _y Ga _1-y P.
The strain relaxation layer 411, the pIn _x Ga _1-x P-BSF layer 41
2. Zn is preferably used for the pInGaP base layer 42, and n ⁺ GaAs emitter layer 23, n ⁺ In
_0.5 Ga _0.5 P window layer 24, n ⁺⁺ GaAs layer 31, n ⁺ I
n _0.5 G _0.5 P emitter layer 43, n ⁺ AlInP window layer 4
4, it is desirable to use Si for the n ⁺ GaAs layer 51, but similar effects can be obtained with other dopants.

FIG. 2B shows the impurity density (Nd-Na) profile in the vicinity of the upper cell of the stacked solar cell according to the embodiment of the present invention, and FIG.
The impurity density profile of In the prior art, p ⁺ In rather than pIn _0.5 Ga _0.5 P base layer 42 is used.
_Although the impurity density of the _0.5 Ga _0.5 P-BSF layer is high, the impurity density of the pIn _y Ga _1-y P strain relaxation layer 411 and the pIn _x Ga _1-x P-BSF layer 412 is pIn _0.5 Ga in the present invention. _The impurity density is the same as that of the _0.5 P base layer 42. Therefore, light absorption in the _{^{p + In 0. 5 Ga 0.5 P}} -BSF layer in question in the prior art can be suppressed extremely small. Note that the BSF layer 412,
The impurity density of the strain relaxation layer 411 may be lower than the impurity density of the p base layer 42. In the present invention, FIG.
(B) shows I in the depth direction from the surface in the vicinity of the upper cell.
Although the distribution of the composition (x, y) of n is shown, pIn _x Ga
The composition x of the _1-x P-BSF layer 412 is smaller than 0.5,
The composition y of the pIn _y Ga _1-y P strain relaxation layer 411 is set to be larger than 0.5. (As a reference diagram, the distribution of the composition of the conventional technique is shown in FIG. 3A. In the conventional technique, p ⁺ In _0.5
The composition of the Ga _0.5 P-BSF layer 41 is 0.5 and pIn
It has the same composition as the _0.5 Ga _0.5 P base layer 41. Therefore, in the embodiment of the present invention, the forbidden band width Eg is pIn _x Ga _{1 -x} P-BSF layer 4 as shown in FIG.
12 is larger than Eg of the pIn _0.5 Ga _0.5 P base layer 42 by ΔEg, and E of the pIn _y Ga _1-y P strain relaxation layer 411 is
g is smaller than Eg of the pIn _0.5 Ga _0.5 P base layer 42 by ΔEg.

In FIG. 3 (c), Eg = 1.85 eV of the pIn _0.5 Ga _0.5 P base layer 42 is shown, but this is an example, and strictly speaking, it slightly differs depending on the impurity density, the growth method and the like. It becomes a value. In any case, in the present invention, ΔEg is set to 30 meV to 90 meV to prevent back surface recombination loss, and at the same time, the lattice strain between the BSF layer 412 and the strain relaxation layer 411 is canceled and the BSF layer 41
The influence of the crystal lattice strain caused by increasing Eg of 2 is
It can be set so as not to reach the pIn _0.5 Ga _0.5 P base layer 42. For this purpose, the composition x is 0.025 with respect to 0.5
(Corresponding to ΔEg = 30 meV) to 0.075 (ΔEg
= Corresponding to 90 meV). BSF layer 41
Since the Eg of 2 is larger than the Eg of the p base layer 42,
Even if the impurity density of the F layer 412 is slightly lower than the value of the p base layer 42, back surface recombination loss can be prevented, and therefore, the BS
Light absorption in the F layer 412 can be further reduced. Moreover, due to the effect of the strain relaxation layer 412, the crystallinity of the p base layer 42 becomes good, and the photosensitivity reduction due to the lattice strain hardly occurs.

The tandem cell solar cell shown in FIG. 1 of the present invention can be manufactured by the following manufacturing method. That is, (a) First, metal organic chemical vapor deposition (MOCVD), CB
E (Chemical Beam Epitaxy) method,
MBE (Molecular Beam Epitax)
y) method, MLE (Molecular Layer E)
Pitaxy method or the like, and on the p ⁺ GaAs substrate 11, the p ⁺ GaAs layer 12, the GaAs bottom cell 2, Ga
An As tunnel junction 3, an In _0.5 Ga _0.5 P top cell 4, and an n ⁺ GaAs layer 51 are continuously epitaxially grown. More specifically, the GaAs bottom cell 2 is p ⁺ InG
aP-BSF layer 21, pGaAs base layer 22, n ⁺ G
aAs emitter layer 23, n ⁺ In _0.5 Ga _0.5 P layer 24
In _0.5 Ga _0.5
The P top cell 4 has a pIn _y Ga _1-y P strain relaxation layer 41.
1, pIn _x Ga _1-x P-BSF layer 412, pIn _0.5
The Ga _0.5 P base layer 42, the n ⁺ In _0.5 Ga _0.5 P emitter layer 43, and the n ⁺ AlInP window layer 44 are laminated in this order to form a multilayer epitaxial growth layer. Further, the GaAs tunnel junction is an n ⁺⁺ GaAs layer 31, p ^++. It is a continuous epitaxial growth layer composed of the GaAs layer 32. MOCVD can be performed by either normal pressure MOCVD or low pressure MOCVD.
Desirably, for example, a low pressure MOCVD method in which the pressure is maintained at 6.7 to 10 kPa, and more preferably, a vertical type low pressure MOCV method.
It is better to use method D. As group III source gas, triethylgallium (TEG), trimethylindium (TM)
I), trimethylaluminum (TMA), trimethylamine alane (TMAAl), or the like, and phosphine (PH ₃ ) or arsine (AsH ₃ ) or the like is used as a group V source gas. Alternatively, tertiary butyl phosphine ((C ₄ H ₉ ) PH ₂ ; TBP), tertiary butyl arsine ((C ₄ H ₉ ) AsH ₂ ; TBA) and the like may be used. Examples of n-type dopant gas include monosilane (SiH ₄ ), disilane (Si ₂ H ₆ ), diethyl selenium (DESe), and diethyl tellurium (DE).
Te) or the like may be used, but monosilane is preferable. p
Type dopant gas is diethylzinc (DEZn)
Alternatively, trimethylgallium (TMG) may be used. These raw material gas and dopant gas are introduced into a reaction tube controlled to a reduced pressure of 6.7 kPa to 10 kPa using a mass flow controller or the like. The ratio of the group V source gas to the group III source gas, the so-called V / III ratio, is
For example, about 120 to 170 may be performed. The substrate temperature during growth may be 650 ° C. to 700 ° C., for example, in order to obtain a high Voc value as shown in FIG.
The growth of the aAs bottom cell 2 is preferably 700 ° C.

(B) Next, the wafer having the multilayer structure thus continuously epitaxially grown is taken out from the reaction tube, a photoresist for lift-off is applied, a predetermined pattern is formed by photolithography, and Au-- Vacuum deposition of Ge / Ni / Au. For example, 10
0 nm Au-Ge (12 wt%), 20 nm Ni,
A 70 nm Au film is formed by the EB vapor deposition method. Then, if the photoresist is removed, the upper metal electrode layer 71 having a desired plane pattern shape such as a comb-shaped stripe is formed.
Although a similar pattern can be obtained by etching with an etchant such as a KI / I solution by ordinary photolithography without using the lift-off method, the lift-off method is simpler. Then, the electrode is sintered at 360 to 450 ° C. in an H ₂ atmosphere or an inert gas atmosphere such as N ₂ . Sintering at 360 ° C. for about 2 seconds is preferable.

(C) Next, the surface of the epitaxial growth layer is covered with photoresist or the like, and the back surface of the p ⁺ GaAs substrate 11 is about 6 μm using a brom (Br ₂ ) based etchant.
After etching, the back electrode layer 8 having a thickness of about 1 μm is formed on the surface.
Au plating of. Subsequently, Au-Ge / Ni / Au having a predetermined plane pattern on the surface of the epitaxial growth layer
Only the upper surface portion of the film 71 is plated with an Au plating film 72 of about 1 μm by photolithography to obtain a plane pattern shape of the upper metal electrode layer 7.

(D) Next, for example, 10 mm × 20 mm
The main area of the device consisting of the light receiving surface and the upper electrode layer having a desired planar dimension such as is covered with photoresist, and the surface of the other uncovered epitaxial growth layer is about 30 μm-
Mesa etching is performed to form a mesa with a width of 50 μm and a depth of about 1 μm.

Then, using the plane pattern of the upper metal electrode layer 7 as a mask, the n ⁺ GaAs layer 5 under the upper electrode layer 7 is formed.
The remaining n ⁺ GaAs layer 51 is removed by etching, leaving only 1.

(E) Next, using the lift-off method again, Z
The antireflection film 6 including the nS film 61 and the MgF ₂ film 62 is formed by sputtering or vacuum evaporation.

(F) Next, the width of 30 μm described in (d)
10 by cleavage using a mesa line of ~ 50 μm
A cell of × 20 mm is cut out to complete the solar cell having the shape shown in FIG.

In this embodiment, the two-cell laminated solar cell has been described, but it is clear that the present invention is also effective for a laminated solar cell having three or more cells.

[0024]

According to the present invention, there is almost no infrared light absorption by the majority carriers in the upper cell, the long wavelength side spectral sensitivity characteristic is improved, the incident light effectively reaches the lower cell, and the sufficient wavelength division effect is obtained. Is obtained. Therefore, for example, in a 2-cell stacked solar cell having an electrode occupation rate of 2%, a conversion efficiency of 28% or more can be obtained, which is extremely high efficiency.

Further, according to the present invention, the spectral sensitivity on the long wavelength side is improved, lattice strain and the like in each layer constituting the tandem cell is alleviated, and the carrier lifetime is extended,
Voc is also greatly improved, and a high value of 2.5 V or higher can be obtained. According to the present invention, Isc and fill factor (fill f
(actor; FF) is also improved.

[Brief description of drawings]

FIG. 1 shows In _0.5 Ga _0.5 P / G according to an embodiment of the present invention.
It is sectional drawing of aAs.

2 (b) is a graph showing the impurity density distribution in the depth direction from the surface of the tandem cell according to the embodiment of the present invention.
(A) is a figure which shows the impurity density distribution in a prior art.

FIG. 3 (b) and FIG. 3 (c) respectively show the distribution of the composition (x) in the depth direction from the surface of the tandem cell according to the example of the present invention and the forbidden band width (Eg). It is a figure which shows the change of. FIG. 3A is a diagram showing a composition distribution in the conventional technique for comparison.

FIG. 4 (a) and FIG. 4 (b) are energy band diagrams of a tandem cell according to a conventional technique and an embodiment of the present invention, respectively.

FIG. 5 is a cross-sectional view of a tandem cell in the related art.

FIG. 6 is a schematic diagram for explaining the operation of the solar cell.

[Explanation of symbols]

2 GaAs bottom cell (lower cell) 3 GaAs tunnel junction layer 4 In _0.5 Ga _0.5 P top cell (upper cell) 6 antireflection film 7 upper metal electrode layer (front electrode layer) 8 lower metal electrode layer (back electrode layer) 11 p ⁺ GaAs substrate 12 p ⁺ GaAs buffer layer 21 p ⁺ In _0.5 Ga _0.5 P BSF layer 22 p GaAs base layer 23 n ⁺ GaAs emitter layer 24 n ⁺ InGaP window layer 31 n ⁺⁺ GaAs layer 32 p ⁺⁺ GaAs layer 41 p ⁺ In _0.5 Ga _0.5 P-BSF layer 42 pIn _0.5 Ga _0.5 P base layer 43 n ⁺ InGaP emitter layer 44 n ⁺ AlInP window layer 51 n ⁺ GaAs contact layer 61 ZnS film 62 MgF ₂ film 71 Au-Ge / Ni / Au film 72 Au plated film 411 pIn _y Ga _1-y P strain relaxation layer (y> 0.5) 412 pIn _x Ga _1-x P-BSF layer (x <0.5) 422 Base layer 433 Emitter layer

Claims (5)

[Claims] 1. A first conductivity type first emitter layer, a second conductivity type first base layer in contact with the first emitter layer, and a second conductivity type in contact with the first base layer. Type 1st BS
An F layer, a top cell having at least a second conductivity type strain relaxation layer in contact with the first BSF layer, a first conductivity type second emitter layer, and a second emitter layer contacting the second emitter layer. Formed between the top cell and the bottom cell, and a bottom cell having at least a second base layer of the second conductivity type and a second BSF layer of the second conductivity type in contact with the second base layer. At least a bonding layer that connects the top cell and the bottom cell in series with each other, and the impurity density of the first BSF layer and the strain relaxation layer is the first layer.
Is equal to or smaller than the impurity density of the base layer, the forbidden band width of the first BSF layer is larger than the forbidden band width of the first base layer, and the forbidden band of the strain relaxation layer is A series-connected solar cell having a width smaller than the forbidden band width of the first base layer. 2. The first base layer, the first BSF layer,
The series connection solar cell according to claim 1, wherein the strain relaxation layers are mixed crystal compound semiconductors composed of the same constituent element, and the compositions of the mixed crystals are different from each other. 3. The series connection solar cell according to claim 1, wherein the junction layer is a tunnel junction layer made of a first conductivity type and a second conductivity type compound semiconductor. 4. The top cell, the bonding layer, and the bottom cell are formed on a compound semiconductor substrate, and the first base layer is substantially lattice-matched with the compound semiconductor substrate. The series-connected solar cell according to any of the above. 5. The first base layer is In _x Ga _{1 -x} P
And the compound semiconductor substrate is GaAs, In _x Ga
The series-connected solar cell according to claim 4, wherein the composition x of _1-xP is substantially 0.5.