JP2771497B2 - 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 FIG. 2 shows a specific example of a sectional structure and a material structure of a solar cell generally used at present. FIG. 2 shows an example of a solar cell using a GaAs compound semiconductor.
Junction formed in the boundary region between the n-type GaAs emitter layer 5 and the n-type GaAs base layer 6 has a basic structure. Carriers excited in the pn junction region by light allow electrons to flow to the n-side back electrode 9 by the internal electric field. , Holes reach the p-side surface electrode 1 and are extracted therefrom to obtain an output.

[0003] The performance of a solar cell is indicated by its photoelectric conversion efficiency, which at present is about 30% at the maximum, and the rest is lost due to various factors. Excluding the portion that does not essentially contribute to the photoelectric conversion, the following four main types of loss factors can be cited. (1) Reflection loss on the front surface (2) Recombination loss on the front or back surface of the semiconductor (3) Recombination loss inside the semiconductor (4) Series resistance loss In order to increase photoelectric conversion efficiency, these losses must be reduced. It is necessary to reduce it. Various attempts have been made to reduce the loss in the conventional solar cell structure shown in FIG. The antireflection film 2 is for reducing the reflection loss of the above (1) by laminating a material having a minimum surface reflection on the solar cell surface in consideration of the refractive index of the solar cell material. The window layer 3 is a layer in which a material having a larger forbidden band width than the material forming the pn junction is laminated on the surface of the emitter layer 5 to prevent photo-excited carriers from reaching the surface and recombining. The BSF layer 7 is formed by laminating a material having a larger forbidden band width than the base layer 6 or increasing the doping concentration. Play. These are all attempts to reduce the recombination loss of the above (2). In order to reduce the loss of (3), attempts have been made to improve the crystal quality of the semiconductor material itself. In order to reduce the loss in (4), attempts have been made to improve the crystal quality, design the electrode 1, and develop a low-resistance ohmic contact material. Providing a p-type GaAs cap layer 11 between the electrode 1 and the window layer 3 is one of them.

[0004] In particular, focusing attention on the window layer material for the purpose of reducing the recombination loss on the front or back surface of the semiconductor (2), the condition to be satisfied is more than that of the material forming the pn junction. The forbidden band width is large, the light absorption in the window layer is small, the lattice constant and the thermal expansion coefficient are close to those of the semiconductor material forming the solar cell, and a film having good crystal quality can be formed.

[0005] A GaAs compound semiconductor, which is a III-V compound, is expected as a material capable of realizing a very high photoelectric conversion efficiency exceeding 30% when a solar cell is formed. However, the surface of this semiconductor is very active, and its surface recombination velocity (Sf) is 10 ⁶ to 10 ⁷ cm /.
s. Therefore, the window layer of a GaAs-based solar cell has a band gap of about 2 eV and is an indirect transition type GaAs.
As substantial aluminum gallium arsenide having the same lattice constant _{(Al x Ga 1-x As} : x> 0.7) have been used conventionally. Recently, indium gallium phosphide (In _0.5 Ga _0.5 P: composition ratio of In: Ga: 1: 1) having a forbidden band width of about 1.9 eV and also having substantially the same lattice constant as GaAs has been used. I have.

[0006]

As described above, as a window layer of a GaAs-based compound semiconductor solar cell comprising elements of groups III and V of the periodic table, AlGaAs has conventionally been used.
s or In _0.5 Ga _{0 .5} P has been used. Here, AlGaAs has a problem in that aluminum (Al) is very active and a defect occurs due to the incorporation of oxygen into the film. In order to form a high-quality film, the conditions such as the growth temperature must be strictly set. You need to control.

Recently, a material called In _0.5 Ga _0.5 P, which has become mainstream as a substitute for AlGaAs, does not use Al as a raw material, so that In _0.5 Ga _0.5 P / GaAs is used.
Recombination loss at the s interface is greatly reduced. This material is GaAs only when the composition ratio of In and Ga is 1: 1.
And the lattice constant match, but the rate of change of the lattice constant with respect to the composition ratio is much larger than that of AlGaAs, and precise control of the composition ratio is required. Furthermore, since it is a direct-transition type material, there is a problem that loss of light due to light absorption in sunlight having a photon energy of up to about 4 eV cannot be ignored.

Further, In _0.5 Ga _0.5 P is also used as an upper cell of a GaAs cell in a tandem solar cell, and a high photoelectric conversion efficiency of nearly 30% has been reported for a tandem solar cell. Then In _0.5
Aluminum indium phosphide (Al _0.5 In _0.5 P) is used as the window layer of the Ga _0.5 P cell. This material also contains Al, and for the same reason as described above, it is very difficult to form a high-quality film.

An object of the present invention is to provide a III-V compound semiconductor solar cell having an optimum window layer material for reducing the surface maximum coupling loss irrespective of a single junction type or a tandem type. And to propose a structure.

[0010]

According to the present invention, in a solar cell including a pn junction formed of a III-V compound semiconductor material, an emitter layer of the pn junction is composed of an element of Group II and Group VI of the periodic table. Stacking compound semiconductor material layers,
The object is achieved by making the atomic layer in contact with the emitter layer surface of the pn junction a group VI atom.

FIG. 1 shows a typical sectional structure of a solar cell according to the present invention. As a material for the window layer 3 of a solar cell comprising a pn junction formed at the boundary between the emitter layer 5 and the base layer 6 of a III-V compound semiconductor, the group II and VI of the periodic table
The first atomic layer 4 in contact with the surface of the emitter layer 5 of the window layer 3 has a structure in which the first atomic layer 4 is a group VI atom using a compound semiconductor material made of a group III element.

The group II elements of compound semiconductors currently widely used which are composed of group II and group VI elements include Zn and M
g and Cd, and Group VI elements include S, Se and Te. Considering the coincidence with the lattice constant of GaAs, the window layer material may be ZnSe, MgS, a mixed crystal of ZnSe and ZnS,
Alternatively, a mixed crystal of MgSe and MgS is suitable. All of these materials, when laminated on GaAs, have the same zinc-blende-type structure as GaAs, and have a forbidden band width of 2 eV or more, which sufficiently satisfies the conditions as a window layer material.

An atomic layer formed of a compound semiconductor material such as GaAs and in contact with the surface of an emitter layer of a pn junction is represented by S,
By using a group VI atom such as Se or Te, the active surface is terminated, the incorporation of impurities such as O and C into the film surface is reduced, and the surface recombination is reduced. This is due to the surface passivation effect of the group VI atom, and the effect has been confirmed so far for S, Se, and the like. This VI
A solar cell with small surface recombination loss can be obtained due to the surface termination effect of group atoms.

The thickness of the window layer material usually used is
In order to reduce the light absorption there, it is as thin as several tens nm.
Therefore, even when a material such as ZnSe or MgS having a lattice constant slightly different from that of GaAs other than a ternary mixed crystal material having a lattice constant matched with that of GaAs, such as ZnS _0.06 Se _0.94 , satisfactory crystal growth is sufficient. It is possible. Such binary system material By using the, In _{0. 5} Ga _0.5 P which has been conventionally used, the difficulty of composition control in the case of using the Al _0.5 In _0.5 P eliminated.

Further, all of the above materials are direct transition type materials, and recombination loss in the window layer is expected similarly to In _0.5 Ga _0.5 P. However, if an MgS-based material is used, the forbidden band width can be set to about 4 eV. In such a high energy region, there are almost no photons in sunlight and there is no light absorption, so that recombination loss in the window layer can be ignored. Further, in recent solar cell structures, a double hetero structure in which a pn junction region is sandwiched between materials having a smaller refractive index effectively confines incident photons, thereby improving conversion efficiency. InGaP, which has been conventionally used for GaAs solar cells, has a refractive index of 3.45, and AlGaAs has a refractive index of about 3.2. The refractive index of a compound semiconductor material composed of an element belonging to Group II and Group VI of the periodic table is about 2.5, and all have significantly smaller values than conventional materials. Therefore, by forming a double heterostructure with this material, it is possible to increase the light confinement efficiency as compared with the related art and effectively use photons.

According to the present invention, the above-mentioned actions are combined,
A solar cell having higher photoelectric conversion efficiency than before can be manufactured.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific examples of the present invention will be described in detail. In the following figures, in order to facilitate correspondence with the conventional example, the same functional parts as in FIG. 2 which is a conventional example will be described with the same reference numerals as in FIG.

Example 1 A solar cell having a sectional structure shown in FIG. 3 was manufactured. When a simple metal is used as a raw material for the group II and group VI elements constituting the window layer, the crystal growth must be performed at a low temperature because its vapor pressure is extremely high. At this time, as a method for manufacturing the window layer, molecular beam epitaxy (MB
The E) method is suitable. Further, when an organic metal gas source is used as a source of the group II and group VI elements, fabrication by metal organic chemical vapor deposition (MOCVD) is also possible. Since the technique for growing a II-VI compound semiconductor on GaAs by the MBE method has been established by research on fabrication of a blue semiconductor laser, here, the window layer was crystal-grown by the MBE method.

As the semiconductor substrate 8, n-type GaAs (1
00) A substrate was used. The oxide film on the surface of the substrate 8 is
After removal by thermal cleaning at 10 ° C. for 10 minutes, an n-type GaAs base layer 6 doped with silicon at 2 × 10 ¹⁷ cm ⁻³ was grown to 3 μm on the substrate at the same temperature. Subsequently, a p-type GaAs emitter layer 5 doped with beryllium at 2 × 10 ¹⁸ cm ⁻³ was grown to 0.5 μm. Thereafter, the substrate temperature is set to 4
Lower to 00 ° C.

After the temperature is stabilized, the window layer 3 is grown. First S
The shutter for only e is opened, and one to several atomic layers of the Se atomic layer 4 are grown on the surface of the p-type GaAs emitter layer 5.
Thereafter, the Zn shutter is also opened to start growing the ZnSe layer. After the growth of 0.03 μm, the shutters of Se and Zn are closed to terminate the growth of the ZnSe layer. During the growth of the window layer, rf plasma-excited nitrogen is irradiated to perform p-type doping. The doping concentration was 2 × 10 ¹⁸ cm ⁻³ .

On the window layer 3, a p-type GaAs cap layer doped with 4 × 10 ¹⁹ cm ⁻³ of beryllium is formed in a lower region of the surface electrode 1 in order to improve electrical contact between the window layer and the surface electrode 1. 11 was grown 0.3 μm. Thereafter, the substrate is taken out of the growth apparatus into the atmosphere and put into a process step, where electrode deposition, formation of an antireflection film, and the like are performed. An alloy of gold: zinc (Au: Zn) was deposited on the p-type cap layer 11 as the front electrode 1 and an alloy of gold: germanium (Au: Ge) was deposited on the GaAs substrate 8 as the back electrode 9 by an electron beam. Further, as the antireflection film 2, ZnS and MgF ₂ were formed by electron beam evaporation.

In this manner, a solar cell having a small surface recombination loss can be obtained. Although there is a lattice mismatch of 0.27% between ZnSe of the window layer 3 and GaAs of the emitter layer 5, if the thickness of the window layer 3 is 0.03 μm, the mismatch is absorbed as a strain and a good mismatch is obtained. It is possible to perform single crystal growth. FIG. 4 shows a comparison between the quantum efficiency of the solar cell according to the first embodiment and the quantum efficiency of the conventional solar cell shown in FIG. According to the window layer of the present invention, it can be seen that the characteristics of the quantum efficiency especially on the short wavelength side are improved as compared with the conventional example due to the reduction of the surface recombination.

Here, an example in which ZnSe is used as the window layer material has been described, but similar results can be obtained by using MgS instead of ZnSe.

Example 2 A solar cell having the window layer of the present invention and having a high light confinement efficiency by a double heterostructure was manufactured. FIG. 5 shows a cross-sectional structure of the solar cell. In this embodiment, MgS _0.06 Se _0.94 having a refractive index of 2.6, which is smaller than that of GaAs, and a very large forbidden band width of 4 eV is used as a material for forming the double hetero structure. MgS _0.06
Se _0.94 has almost the same lattice constant as GaAs,
The crystal was grown by the method.

As shown in FIG. 5, an n-type semiconductor substrate is used.
GaAs (100) substrate 8 and an oxide film on the substrate surface
Was removed by thermal cleaning at 580 ° C. for 10 minutes.
Then, 2 × 10 2 silicon is deposited on the substrate at the same temperature.¹⁷cm^-3Do
The buffered n-type GaAs layer as the buffer layer 10 is 0.5
μm was grown. Subsequently, the substrate temperature is reduced to 400 ° C.
And make Ga 2 × 10¹⁷cm^-3Doped MgS_0.06Se
_0.94Layer 7 is grown 0.1 μm. Again, the substrate temperature is 58
After the temperature was stabilized to 2 ° C. ¹⁷cm
^-3A doped n-type GaAs base layer 6 is grown to a thickness of 3 μm.
And then add 2 × 10 beryllium¹⁸cm^-3Doped p
Type GaAs emitter layer 5 was grown to 0.5 μm.

Thereafter, the substrate temperature was lowered to 400 ° C., and after the temperature was stabilized, the window layer 3 was grown. First Se and S
Is opened to form a mixed layer 4 of Se and S for one to several atomic layers on the surface. Thereafter, the Mg shutter is also opened to start the growth of the MgS _0.06 Se _0.94 layer. Mg
After growing the S _0.06 Se _0.94 layer to a thickness of 0.03 μm, the shutters for Se, S, and Zn are simultaneously closed to terminate the growth. During the window layer growth, rf plasma excited nitrogen is irradiated,
P-type doping was performed. Doping concentration is 2 × 10 ¹⁸
cm ^-3 .

Further, on the window layer 3, in order to improve the electrical contact between the window layer and the surface electrode 1, p-type Ga doped with 4 × 10 ¹⁹ cm ⁻³ of beryllium is formed in a region below the surface electrode 1.
The As cap layer 11 was grown to 0.3 μm. After this,
In the same manner as in Example 1, the substrate was taken out of the growth apparatus into the air and introduced into a process step, where electrode deposition, formation of an antireflection film, and the like were performed. Gold: zinc alloy (Au: Z) as surface electrode 1
n) was deposited on the cap layer 11, and a gold-germanium alloy (Au: Ge) was deposited on the substrate 8 by electron beam as the back electrode 9. Further, as the anti-reflection film 2, ZnS and MgF
₂ was formed by electron beam evaporation.

FIG. 4 shows the quantum efficiency of the solar cell according to Example 2 in comparison with the conventional example. According to the second embodiment, since the forbidden band width of the window layer material is large and the lattice mismatch is small, the characteristics on the short wavelength side as compared with the first embodiment are improved. In addition, the overall sensitivity was improved due to the light confinement effect of the double heterostructure. Here, an example in which MgSSe, that is, a mixed crystal of MgS and MgSe is used as a material of the window layer has been described, but ZnSe and ZnS
The same effect can be obtained by using a mixed crystal of.

[0029]

According to the present invention, it is possible to effectively absorb photons with a small recombination loss on the surface and to obtain a solar cell with higher efficiency than conventional materials.

[Brief description of the drawings]

FIG. 1 is a sectional structural view of a solar cell.

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

FIG. 3 is an explanatory diagram of the first embodiment of the present invention.

FIG. 4 is a diagram comparing the quantum efficiencies of the present invention and conventional solar cells.

FIG. 5 is an explanatory view of a second embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Surface electrode, 2 ... Antireflection film, 3 ... Window layer, 4 ... Group VI atomic layer, 5 ... Emitter layer of III-V compound semiconductor, 6 ... I
II-V compound semiconductor base layer, 7 BSF layer, 8 semiconductor substrate, 9 back electrode, 10 buffer layer, 11
Cap 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-51-10783 (JP, A) JP-A-63-126281 (JP, A) JP-A-2-218174 (JP, A) JP-A-4-199881 (JP, A A) (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 31/04-31/078

Claims (3)

(57) [Claims] In a solar cell including a pn junction formed of a III-V compound semiconductor material, a compound semiconductor material layer composed of an element of Group II and Group VI of the periodic table is laminated on the emitter layer of the pn junction. A solar cell, wherein the atomic layer in contact with the emitter layer surface of the pn junction is a group VI atom. 2. The solar cell according to claim 1, wherein the compound semiconductor material comprising the Group II and Group VI elements is ZnSe or MgS. 3. A compound semiconductor material comprising a group II element and a group VI element, wherein the compound semiconductor material is a mixed crystal of ZnS and ZnSe or MgS
The solar cell according to claim 1, wherein the solar cell is a mixed crystal of MgSe and MgSe.