JP3027116B2 - Solar cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a solar cell and a method for manufacturing the same.

[0002]

2. Description of the Related Art A solar cell has a pn junction of a semiconductor as a basic structure. Carriers excited in a pn junction region by incident light diffuse through the semiconductor and reach an internal electric field region.
Due to the internal electric field, electrons reach the n-side electrode, and holes reach the p-side electrode, and an output is obtained by being extracted therefrom to the outside.

FIG. 2 is a schematic sectional view showing the structure of a currently manufactured gallium arsenide (GaAs) solar cell. The basic configuration is such that the GaAs emitter n-type layer 4 and the base p
Although it is a pn junction layer formed from the mold layer 5, gallium indium phosphorus (Ga _0.5 In _0.5 P) having the same lattice constant as GaAs and forming a good interface is introduced to reduce the recombination loss of carriers. It has a hetero structure. In the figure, Ga stacked on the emitter layer 4 is shown.
_{The 0.5} In _0.5 P layer 3 is generally called a window layer. The window layer 3 is a layer for preventing photogenerated carriers from reaching the surface to reduce surface recombination loss. Further, the Ga _0.5 In _0.5 P layer 6 laminated under the base layer 5 is called a back surface field (BSF) layer and is a layer for repelling carriers reaching the back surface and reducing recombination loss on the back surface. This layer is indispensable for improving the collection efficiency of generated carriers.

[0004]

As described above, a compound semiconductor solar cell comprising a group III element and a group V element of the periodic table generally has a heterostructure in which different kinds of semiconductor materials are stacked. As a manufacturing method, a growth method such as a metal organic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method is currently mainstream.
In this case, the film growth process proceeds by the group III element reaching and bonding to the surface of the growth substrate covered with the group V element having a high vapor pressure.

Here, considering the film forming sequence in the conventional solar cell manufacturing shown in FIG. 2, for example, from the Ga _0.5 In _0.5 P layer from the BSF layer 6 to the base layer 5, the GaAs
In growing the s layer, it is necessary to switch from phosphorus (P), which is a group V element, to arsenic (As). As described above, P and As, which are Group V elements, have a high vapor pressure. In this case, even if the supply of P is shut off by a method such as a shutter operation, it takes some time before the P is wiped off.

[0006] Therefore, a so-called interruption time, in which no element is supplied when switching the group V element, is usually provided. If the interruption time is short, an interface product in which group V elements such as gallium indium arsenide phosphide (GaInAsP) are mixed in the reaction with As supplied next is formed. On the other hand, when the interruption time is long, re-evaporation of P from the surface of the Ga _0.5 In _0.5 P film, that is, so-called group V penetration occurs remarkably, and GaInAs also reacts with the next supplied As.
P is formed. This layer is made of Ga _0.5 In _0.5 P and GaAs
Since it has a band gap narrower than s, it becomes a recombination region of conduction carriers, and as a result, the efficiency of the solar cell is reduced. The same can be said for the growth of the window layer 3 from the emitter layer 4.

As described above, the group III elements and V in the periodic table
In a solar cell having a heterojunction of a first compound semiconductor composed of a group III element and a second III-V compound semiconductor composed of a different group V element different therefrom,
Reduction of the efficiency of the solar cell due to switching of the group V element during film growth is a major problem. An object of the present invention is to propose a solar cell structure that is optimal for reducing loss at a hetero interface in order to solve the above-mentioned problems of the conventional technology.

[0008]

In order to achieve the above object, the present invention provides a first compound semiconductor comprising a group III element and a group V element of the periodic table, wherein a different group V element is formed on the first compound semiconductor. In a solar cell having a structure in which a second III-V compound semiconductor is laminated, the interface has a forbidden band width larger than at least one of the first and second compound semiconductors and constitutes them. The present invention proposes a solar cell structure in which a third III-V compound semiconductor layer containing both group V elements as constituent elements is introduced. To fabricate a device structure requiring such precise film control, a molecular beam epitaxy (MBE) method or a metal organic chemical vapor deposition (MOCV) method is used.
A growth method such as the D) method is suitable.

The above-mentioned problem of the prior art is caused by the difficulty in switching the atmosphere of the group V element during film growth. It is easy to introduce a compound semiconductor layer containing both elements into the interface. For example, Ga _0.5 as shown in FIG.
In the case of a GaAs solar cell using In _0.5 P as the window layer 3 and the BSF layer 6, gallium arsenide phosphorus (GaAsP) or aluminum gallium arsenide phosphorus (AlGaAsP) is suitable as the third III-V compound semiconductor layer 9. The introduction portions are at three places of the hetero junction between the Ga _0.5 In _0.5 P layer and the GaAs layer, which are shown in FIG.

According to the solar cell structure of the present invention, it is possible to solve the problem of a decrease in efficiency due to switching of a group V element during film growth. The operation of the solar cell having the cross-sectional structure of FIG. 1 will be described with reference to the energy band diagram of FIG. 3 as an example. 3A is an energy band diagram of the window layer 3, the emitter layer 4, the base layer 5, and the BSF layer 6 in an ideal state in the conventional structure shown in FIG.
(B) is an energy band diagram of an actually manufactured device, and (c) is a structure of the present invention in which a GaAsP layer having a large mixed crystal ratio of P atoms is introduced. FIG. 4D is an energy band diagram in the case where the compound semiconductor and the second compound semiconductor are larger than each other. FIG. 4D shows the introduction of a GaAsP layer having a small mixed crystal ratio of P atoms (the band gap of the third compound semiconductor is the first band gap). The energy band diagram in the case of either the compound semiconductor or the second compound semiconductor is shown. However, in the figure, the bending of the band due to the difference in the forbidden band width at the hetero interface is omitted, and only the effective energy difference is shown.

In an ideal state, the state is as shown in FIG. 3A. Electrons generated deep in the element are also repelled by the BSF layer 6 on the back surface and can reach the n-layer side on the front surface. In addition, holes are conducted without loss toward the back surface. Therefore, it can be expected that the carrier collection efficiency is increased by the effect of introducing the heterostructure. However, when actually manufactured, a product such as GaInAsP is formed at the hetero interface.
The energy band at this time is as shown in (b) including a region with a narrow forbidden band width. Since the conducting carriers are trapped in the low energy region before reaching the electrodes, the recombination loss increases and the performance of the solar cell decreases.

On the other hand, in the structure of the present invention, the recombination loss in the conventional structure is reduced by introducing the third compound semiconductor layer 9 and replacing the region where the interface product is formed with a region having a large band gap. . As shown in FIGS. 3C and 3D, there are two types of third compound semiconductor layers to be introduced. First, GaAs having a large mixed crystal ratio of P atoms
When a layer in which the forbidden band width of the third compound semiconductor is larger than that of either the first compound semiconductor or the second semiconductor is introduced as in sP, the state becomes as shown in FIG. Also, P
The bandgap of the third compound semiconductor, such as GaAsP having a small mixed crystal ratio of atoms, is larger than that of the first compound semiconductor or the second compound semiconductor.
(D) when a layer larger than either of the compound semiconductors is introduced. In either case, the recombination region disappears, and the loss in the conventional structure can be reduced. In particular, in (c), the introduced third compound semiconductor layer 9 has a larger BSF for minority carriers.
Show the effect. In the case of introducing an AlGaAsP layer, both (c) and (d) are possible depending on the composition ratio.

At this time, the third compound semiconductor layer is
If a material having a lower refractive index than the first compound semiconductor layer and the second compound semiconductor layer is used, the refractive index difference at the hetero interface can be increased as compared with the conventional structure, and the light confinement efficiency can be improved. By the above operation, in the structure of the present invention, a high carrier collection efficiency close to an ideal element can be realized, and a highly efficient solar cell can be manufactured.

[0014]

Embodiments of the present invention will be described below. In the solar cell of the conventional structure shown in FIG. 2 Embodiment 1, by applying the present invention from the switching of the time and GaAs to GaAs from Ga _0.5 In _0.5 P when switching to Ga _0.5 In _0.5 P A solar cell whose sectional structure is schematically shown in FIG. 1 was manufactured. The growth method is gas source MBE (GS-MB
E) Method. This is a method in which a gaseous raw material is used for supplying a group V element. Here, Arsine (AsH)
₃ ) For P, phosphine (PH ₃ ) was used after thermal cracking. The semiconductor substrate to be manufactured is p-type GaAs (1).
00) Substrate.

In an As atmosphere under the supply of AsH ₃ , G
After removing the oxide film on the surface of the aAs substrate 8 by thermal cleaning at 580 ° C. for 10 minutes, the shutters of Ga and Be are opened, and beryllium (Be) -doped p-type buffer layer 7 is formed on the substrate 8 at the same temperature. A GaAs layer (doping concentration: 7 × 10 ¹⁸ cm ⁻³ ) was grown to 0.3 μm. Thereafter, the substrate temperature is lowered to 500 ° C., and the shutters of Ga, In, and Be are simultaneously opened, and BSF is formed as the BSF layer 6.
An e-doped p-type Ga _0.5 In _0.5 P layer (doping concentration 4.5 × 10 ¹⁸ cm ⁻³ ) was grown to 0.1 μm. BS
After the growth of the F layer 6 is completed, the In shutter is closed first, and Ga
P and As were supplied simultaneously with the shutter open to form a GaAsP layer as the third compound semiconductor layer 9. Thus, the introduction of the third compound semiconductor layer 9 can be realized very easily only by the shutter operation. A sufficient effect can be obtained when the film thickness of the third compound semiconductor layer 9 is about 1 nm.

Next, at a substrate temperature of 580 ° C., a p-type GaAs base layer 5 doped with Be (doping concentration 2 × 10 ¹⁷
cm ⁻³ ) of 3.0 μm, followed by silicon (Si) doped n-type GaAs emitter layer 4 (doping concentration 2 × 10 ¹⁸
cm ⁻³ ) was grown to 0.1 μm. Then, Ga and As
The shutter of P is opened at the same time as the shutter of
As a compound semiconductor layer 9, a GaAsP layer was introduced. Then, the substrate temperature is lowered again to 500 ° C., and after the temperature is stabilized, n-type Ga _0.5 In _0.5 doped with Si is used as the window layer 3.
A P layer (doping concentration: 2 × 10 ¹⁸ cm ⁻³ ) was grown to 0.03 μm. The third compound semiconductor layer 9 was similarly introduced after the growth of the window layer 3. Lastly, the n-type GaAs cap layer 2 doped with Si (doping concentration: 3 × 10 ¹⁸ cm ⁻³ )
Was grown 0.5 μm.

FIG. 4 shows the results of a relative comparison between the quantum efficiency of the solar cell fabricated in this way and the quantum efficiency of the conventional solar cell shown in FIG. As is clear from FIG. 4, the solar cell according to the present invention shows a remarkable improvement in the quantum efficiency on the short wavelength side and the long wavelength side. Further, in a gallium indium arsenide (Ga _0.5 In _0.5 As) solar cell, which has recently attracted attention as a bottom cell of a tandem solar cell, indium phosphorus (InP) is applied to the window layers 3 and B.
The third compound semiconductor layer 9 is used as the SF layer 6 and has an appropriate composition ratio of indium arsenide phosphide (InAsP) or aluminum indium arsenide phosphide (AlInAs).
A similar effect can be obtained by introducing P). [Embodiment 2] A solar cell was manufactured in which the interface loss was reduced and the light confinement efficiency was improved. In order to improve the light confinement efficiency, the first and second compound semiconductors are used as the third compound semiconductor.
It is necessary to select a material having a smaller refractive index than that of the compound semiconductor described above, and the smaller the refractive index is, the higher the confinement efficiency is. Introducing Al is effective for lowering the refractive index. Here, a solar cell similar to that of Embodiment 1 except that an AlGaAsP layer was used as the third compound semiconductor layer instead of the GaAsP layer, and whose sectional structure is schematically shown in FIG. 1 was manufactured.

A BSF layer 6 comprising a Be-doped p-type Ga _0.5 In _0.5 P layer (doping concentration 4.5 × 10 ¹⁸ cm ^-3 ) and a Be-doped p-type GaAs base layer 5 (doping concentration 2 × 10 The introduction of the AlGaAsP layer to the interface with ¹⁷ cm ⁻³ ) is performed by closing the In shutter first after the growth of the BSF layer 6 is completed, and leaving P, As, and A with the Ga shutter open.
1 at the same time. N doped with Si
-Type GaAs emitter layer 4 (doping concentration 2 × 10 ¹⁸ cm
^-3 ) and n-type Ga _0.5 In _0.5 P window layer 3 doped with Si
(Doped concentration 2 × 10 ¹⁸ cm ⁻³ ) at the interface
After the growth of the emitter layer 4, the introduction of the sP layer was performed by simultaneously supplying P and Al with the Ga shutter and the As shutter open. After the growth of the window layer 3, AlGaAsP is similarly formed as the third compound semiconductor layer 9.
Layers were introduced. The thickness of each of the third compound semiconductor layers 9 was about 1 nm.

As shown in FIG. 1, in a double hetero structure in which two sets of three types of semiconductor layers exist with the emitter layer 4 and the base layer 5 interposed therebetween, the light confinement efficiency becomes higher and carrier carriers become higher. Increases collection efficiency. The solar cell fabricated here has AlGa as the third compound semiconductor layer 9.
The use of the AsP layer improved the quantum efficiency as a whole by several percent as compared with the solar cell of the first embodiment using the GaAsP layer. The same effect can be obtained when the AlInAsP layer is used as the third semiconductor layer in the Ga _0.5 In _0.5 As solar cell.

[0020]

According to the present invention, recombination loss due to interface products due to switching of group V elements during growth can be reduced, and a highly efficient solar cell can be manufactured.

[Brief description of the drawings]

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

FIG. 2 is a sectional structural view of a conventional solar cell.

FIG. 3 is an explanatory diagram of an energy band of each element;
(A) is for an ideal device, (b) is for an actual device,
(C) shows the case of introducing a GaAsP layer having a large P atom mixed crystal ratio, and (d) shows the case of introducing a GaAsP layer having a small P atom mixed crystal ratio.

FIG. 4 is a diagram showing the relative quantum efficiency of a solar cell according to the present invention assuming that a conventional example is 1.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Surface electrode, 2 ... Cap layer, 3 ... Window layer, 4 ... Emitter layer, 5 ... Base layer, 6 ... BSF, 7 ... Buffer layer, 8 ... Semiconductor substrate, 9 ... Third compound semiconductor layer

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Katsura Tamura 1-280 Higashi-Koigabo, Kokubunji-shi, Tokyo Inside Hitachi, Ltd. Central Research Laboratory (72) Inventor Junko Minemura 1-280 Higashi-Koigabo, Kokubunji-shi, Tokyo (56) References JP-A-60-218881 (JP, A) JP-A-63-261882 (JP, A) JP-A-2-36536 (JP, A) (58) Fields studied (Int. Cl. ⁷ , DB name) H01L 31/04-31/078

Claims (7)

(57) [Claims] 1. A structure in which a first compound semiconductor layer made of a group III-V element of the periodic table and a second group III-V compound semiconductor layer containing a different group V element as a constituent element are stacked. have a, the second compound and the first compound semiconductor layer
In a solar cell having the same lattice constant of the semiconductor layer ,
At the interface between the first compound semiconductor layer and the second compound semiconductor layer, the first compound semiconductor layer has a forbidden band width larger than at least one of the first compound semiconductor layer and the second compound semiconductor layer. A solar cell comprising a third group III-V compound semiconductor layer containing both of the constituent group V elements as constituent elements. 2. The solar cell according to claim 1, wherein said third compound semiconductor layer has a lower refractive index than said first compound semiconductor layer and said second compound semiconductor layer. 3. The solar cell according to claim 2, wherein two sets of said first, second and third compound semiconductor layers are provided with a pn junction layer interposed therebetween. 4. The semiconductor device according to claim 1, wherein said first compound semiconductor is gallium indium phosphide, said second compound semiconductor is gallium arsenide, and said third compound semiconductor is gallium arsenide phosphorus. 4. The solar cell according to 2 or 3. 5. The semiconductor device according to claim 1, wherein said first compound semiconductor is gallium indium phosphide, said second compound semiconductor is gallium arsenide, and said third compound semiconductor is aluminum gallium arsenide phosphor. 4. The solar cell according to 1, 2, or 3. 6. The semiconductor device according to claim 1, wherein said first compound semiconductor is gallium indium arsenide, said second compound semiconductor is indium phosphide, and said third compound semiconductor is indium arsenide phosphide. 4. The solar cell according to 2 or 3. 7. The semiconductor device according to claim 1, wherein said first compound semiconductor is gallium indium arsenide, said second compound semiconductor is indium phosphide, and said third compound semiconductor is aluminum indium arsenide phosphide. 4. The solar cell according to 1, 2, or 3.