JP2737705B2 - Solar cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell.

[0002]

2. Description of the Related Art Solar cells are attracting attention as a clean energy source. However, due to the high cost of manufacturing solar cells,
Power generation costs are higher than existing commercial power sources, which is a major obstacle to practical application. Therefore, development for increasing the conversion efficiency to reduce the power generation cost has been performed.

Up to now, solar cells made of a single material have been improved in efficiency, and practical use has been attempted. From the viewpoint of increasing the efficiency, a GaAs-based compound semiconductor is very excellent as a material constituting a solar cell, and a conversion efficiency of about 25% is obtained.

However, when a GaAs-based compound semiconductor is used as a constituent material of a solar cell and GaAs is used as a substrate, the production cost becomes extremely high, and the power generation cost becomes high. In order to reduce the power generation cost, it is necessary to improve the conversion efficiency and reduce the production cost. In addition, in order to obtain high efficiency, a structure in which a plurality of solar cells having different band gap energies are stacked is required.

That is, a solar cell made of a semiconductor material having a large band gap energy is arranged on the incident side (upper part) of the solar cell, and a solar cell made of a semiconductor material having a small band gap energy is arranged below the solar cell.
In the upper part, light in the short wavelength region of the solar spectrum is absorbed,
The light is photoelectrically converted. At the lower part, the sunlight spectrum of the remaining long wavelength region which is absorbed and transmitted by the upper part is photoelectrically converted. By dividing and using the solar spectrum in this way, solar energy can be effectively converted to electric energy.

According to the theoretical calculation of Henry et al.
It has been shown that a conversion efficiency of 50% can be achieved with a layered solar cell (CH. Henry, “Limiting Efficiency of I
dealSingle and Multiple Energy Gap Terrestrial S
olar Cells "J. Appl. Phys. 51 4494 (1980)). For example,
A two-stage stacked solar cell in which a GaAs solar cell and an AlGaAs solar cell are connected by using a tunnel junction intermediate layer has been proposed.

[0007] In order to reduce the manufacturing cost, it is necessary to use an inexpensive substrate and to save solar cell constituent materials. For the substrate, the currently used GaAs is replaced by Si.
It is proposed to change to Further, in order to save solar cell constituent materials, it is desirable to make the solar cell thinner.

FIG. 3 is a sectional view of a conventional solar cell.

Each layer is grown by metal organic chemical vapor deposition (MO).
VPE method). Trimethyl gallium (TMG), trimethyl aluminum (TMA) and arsine (AsH ₃ ) were used as the raw materials for Ga, Al and As, respectively. n
Disilane (Si)
H ₆ ) and diethyl zinc (DEZn) were used. The growth temperature is 725 ° C. (10) tilted 2 ° (10
0) Plane thickness 350 μm, carrier concentration 5.0 × 10 ¹⁷
The thickness 4.0μm on the n-type GaAs substrate 1 cm ^-3, an n-type GaAs layer 2 having a carrier concentration 5.0 × 10 ¹⁷ cm ^-3 is grown, the thickness of 0.5μm thereon, a carrier concentration of 1 .0
A p-type GaAs layer 3 of × 10 ¹⁸ cm ⁻³ is grown to form a pn junction, and a p-type Al _0.85 Ga _0.15 As with a thickness of 0.05 μm and a carrier concentration of 1.0 × 10 ¹⁸ cm ⁻³ is formed thereon. Layer 4, a p-type GaAs layer 5 having a carrier concentration of 1.0 × 10 ¹⁹ cm ^{−3 and} a thickness of 0.2 μm were sequentially grown. The grown film thickness is 4.7
5 μm. Further, the thickness of the n-type GaAs layer 2 is set to 2.0
Crystals having a size of μm were also grown. p-type Al _0.85 Ga _0.15 A
The s layer 4 is a window layer that suppresses surface recombination. The p-type GaAs layer 5 is a layer for reducing the contact resistance with the electrode. Electrodes 6 and 7 are formed on the front and back surfaces of the crystal.

[0010]

However, the above-mentioned S
A high efficiency GaAs solar cell using an i-substrate requires a thickness of 3 μm to 4 μm in order to obtain high efficiency. In order to further increase the efficiency, the solar cells must be stacked, but the film thickness increases.

However, since the lattice constant and the coefficient of thermal expansion of the substrate Si and the GaAs crystal constituting the solar cell are different, cracks occur when the GaAs crystal is grown to a thickness of 3 μm or more on Si. However, a large number of crystal defects are generated. This crystal defect works as a center of recombination for extinguishing the charge generated by the photoelectric conversion, resulting in a significant decrease in efficiency.

Accordingly, an object of the present invention is to solve the above problems and to provide a solar cell capable of obtaining a sufficient photoelectric conversion rate in a thin film state.

[0013]

In order to achieve the above object, the present invention provides a solar cell in which a semiconductor comprising a pn junction is provided on a substrate, wherein a light reflecting semiconductor film is provided between the pn junction and the substrate. It is a thing.

The light reflecting semiconductor film is a single layer or a plurality of layers, and the following formula: 150 / n <d <1240 / (2 · Eg · n) where n is the refractive index of the light reflecting semiconductor film, and d is It is preferable that the film thickness (nm) and Eg of the semiconductor film for light reflection satisfy the band gap energy (eV) of the solar cell.

Further, according to the present invention, in a solar cell having two layers of semiconductors having different band gap energies each composed of a pn junction on a substrate, a light reflecting semiconductor film is provided between each pn junction and the substrate. The semiconductor film for reflection has the following formula: 150 / n <d ₁ <1240 / (2 · Eg ₁ · n) <d ₂ <124
0 / (2 · Eg ₂ · n) where d ₁ is the film thickness (nm) of the upper light reflecting semiconductor film, d
₂ is the film thickness (nm) of the lower light reflecting semiconductor film, Eg ₁ is the band gap energy (eV) of the upper solar cell, Eg
₂ is the band gap energy (eV) of the lower solar cell
Is satisfied.

Further, the present invention provides a solar cell comprising three layers of semiconductors having different band gap energies comprising pn junctions on a substrate, wherein a light reflecting semiconductor film is provided between each pn junction and the substrate. The semiconductor film for reflection has the following formula: 150 / n <d ₁ <1240 / (2 · Eg ₁ · n) <d ₂ <124
0 / (2 · Eg ₂ · n) <d ₃ <1240 / (2 · Eg ₃ ·
n) where d ₁ is the thickness (nm) of the upper light reflecting semiconductor film, d
₂ is the thickness (nm) of the middle light reflecting semiconductor film, d ₃ is the thickness of the lower light reflecting semiconductor film (nm), Eg ₁ is the band gap energy (eV) of the upper solar cell, and Eg ₂ is the middle solar light. Battery band gap energy (eV), Eg ₃
Satisfy the band gap energy (eV) of the lower solar cell.

[0017]

According to the above arrangement, the light incident on the solar cell is p
The remaining light that is incident on the n-junction and is photoelectrically converted and is not absorbed by the pn-junction but is transmitted is reflected by the light-reflecting semiconductor film and is re-emitted.
Since the photoelectric conversion is returned to the n-junction, a sufficient photoelectric conversion rate can be obtained even when the film thickness is thin.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a sectional view of one embodiment of the solar cell of the present invention.

Basically, the same method and substrate as in the conventional solar cell shown in FIG. 3 were used, and common members were denoted by common reference numerals.

First, 0.075 μm on the GaAs substrate 1
m, 5.0 × 10 ¹⁷ cm ^-3 n-type Al _0.2 Ga _0.8 As
Layer and n-type Al of 0.075 μm, 5.0 × 10 ¹⁷ cm ^-3
After forming a layer 8 in which a _0.9 Ga _0.1 As layer was alternately grown three times, that is, six times in total, 5.0 × with a thickness of 1.0 μm was formed.
A 10 ¹⁷ cm ⁻³ n-type GaAs layer 2 is grown, and a 0.5 μm, 1.0 × 10 ¹³ cm ⁻³ p-type GaAs layer 3 is formed thereon.
To form a pn junction, 0.05 μm, 1.0 μm
× 10 ¹⁸ cm ^-3 p-type Al _0.85 Ga _0.15 As layer 4, 0.
A p-type GaAs layer 5 of 2 μm and 1.0 × 10 ¹⁹ cm ⁻³ was sequentially grown. The grown film thickness is 2.20 μm. Then, electrodes 6 and 7 were formed on the front and back surfaces of the crystal. Layer 8
Is a light-reflecting semiconductor film for reflecting light having a wavelength shorter than the absorption edge wavelength of GaAs, and the thickness of this layer 8 is 0.075 μm.
Was determined from Equation 1.

[0022]

150 / n <d <1240 / (2 · Eg · n) where n is the refractive index of the light-reflecting semiconductor film, d is the film thickness (nm) of the light-reflecting semiconductor film, and Eg is the solar cell. Are respectively shown.

The growth film thickness of 2.20 μm in this embodiment is 54% smaller than the growth film thickness of 4.75 μm according to the prior art. An antireflection film is provided on the surface of the solar cell to suppress reflection on the crystal surface, but is not provided here because it is not necessary to compare the prior art with the present invention.

Next, the operation of the embodiment will be described.

When light 9 is incident on the solar cell, p-type Al _0.85
After passing through the Ga _0.15 As layer 4 and the p-type GaAs layer 3, pn
Light that reaches the bonding surface Cpn and is in a short wavelength region of the sunlight spectrum is photoelectrically converted. The light of the remaining long wavelength region of the sunlight spectrum which is not absorbed and transmitted by the n-type GaAs layer 2 is applied to the layer 8.
At the pn junction surface Cpn from the back side (the lower side in the figure), is photoelectrically converted, and can be extracted from the electrodes 6 and 7 as a current.
For this reason, the solar spectrum is divided and used, so that solar energy is effectively converted into electric energy, and a sufficient photoelectric conversion rate can be obtained even in a thin film state.

When the intrinsic conversion efficiencies of the three solar cells were measured, the conventional solar cell was 16.8% when the n-type GaAs layer 2 was 4.0 μm, and 1% when the n-type GaAs layer 2 was 2.0 μm.
It was 4.5%. In contrast, the solar cell of this example had an intrinsic conversion efficiency of 16.5%.

In the case of a conventional solar cell, the n-type GaAs layer 2
Is reduced from 4.0 μm to 2.0 μm,
The conversion efficiency decreased by 2.3% (14% in ratio). On the other hand, in the case of the solar cell of this example, the performance was substantially the same as that of the conventional solar cell having the 4.0 μm GaAs layer.

FIG. 2 is a spectral sensitivity curve for comparing the solar cell shown in FIG. 1 with the conventional solar cell shown in FIG. 3. The horizontal axis indicates the wavelength of light, and the vertical axis indicates the quantum efficiency. Each is shown. Curves 10 and 11 are characteristic curves of the conventional solar cell, and curve 12 is a characteristic curve of the solar cell of this embodiment. Curve 10 shows that the thickness of the n-type GaAs layer 2 is 4.0 μm.
m, curve 11 indicates that the thickness of the n-type GaAs layer 2 is 2.0 μm,
Curve 12 shows the case where the thickness of the n-type GaAs layer 2 is 1.0 μm.

As shown in FIG.
It can be seen that when the thickness of the layer 2 is reduced, the sensitivity is reduced in the wavelength region from 650 μm to 870 μm. On the other hand, in the solar cell of this embodiment, the thickness of the n-type GaAs layer 2 is 1.0 μm.
m, but the thickness of the conventional n-type GaAs layer 2 is 4.0.
It can be seen that the sensitivity is substantially the same as that of a solar cell of μm. This is due to the fact that 650 μm once transmitted through the n-type GaAs layer 2.
This indicates that light in the wavelength range from m to 870 μm is reflected by the layer 8 and returned to the n-type GaAs layer 2 for effective use.

In this embodiment, the case where the solar cell has one layer (one pn junction surface Cpn) has been described. However, the present invention is not limited to this, and two or three layers may be used.

In the case of two layers, the thickness of the light reflecting semiconductor film is set so as to satisfy Equation 2.

[0032]

[Equation 2] 150 / n <d ₁ <1240 / (2 · Eg ₁ · n) <
d ₂ <1240 / (2 · Eg ₂ · n) where d ₁ is the film thickness (nm) of the upper light reflecting semiconductor film, d
₂ is the film thickness (nm) of the lower light reflecting semiconductor film, Eg ₁ is the band gap energy (eV) of the upper solar cell, Eg
₂ is the band gap energy (eV) of the lower solar cell
Are respectively shown.

In the case of three layers, the thickness of the light reflecting semiconductor film is set so as to satisfy Equation 3.

[0034]

[Equation 3] 150 / n <d ₁ <1240 / (2 · Eg ₁ · n) <
d ₂ <1240 / (2 · Eg ₂ · n) <d ₃ <1240 / (2 ·
Eg ₃ · n) where d ₁ is the film thickness (nm) of the upper light reflecting semiconductor film, d
₂ is the thickness (nm) of the middle light reflecting semiconductor film, d ₃ is the thickness of the lower light reflecting semiconductor film (nm), Eg ₁ is the band gap energy (eV) of the upper solar cell, and Eg ₂ is the middle solar light. Battery band gap energy (eV), Eg ₃
Indicates the band gap energy (eV) of the lower solar cell.

As described above, according to this embodiment, the light incident on the solar cell is incident on the pn junction and is photoelectrically converted.
The remaining light that is not absorbed and transmitted by the junction is reflected by the light-reflecting semiconductor film, again enters the pn junction, and is photoelectrically converted. Therefore, a sufficient photoelectric conversion rate can be obtained even when the film thickness is thin. .

[0036]

In summary, according to the present invention, the following excellent effects are exhibited.

(1) A sufficient photoelectric conversion rate can be obtained in a thin film state.

[Brief description of the drawings]

FIG. 1 is a sectional view of one embodiment of a solar cell of the present invention.

FIG. 2 is a spectral sensitivity curve for comparing the solar cell shown in FIG. 1 with the conventional solar cell shown in FIG. 3;

FIG. 3 is a cross-sectional view of a conventional solar cell.

[Explanation of symbols]

 1 substrate 2, 4 semiconductor 8 light reflecting semiconductor film (layer)

Claims (2)

(57) [Claims] 1. A solar cell comprising two layers of semiconductors comprising pn junctions and having different band gap energies laminated on a substrate, wherein a light reflecting semiconductor film is provided between each of the pn junctions and the substrate. The reflecting semiconductor film has the following formula: 150 / n <d ₁ <1240 / (2 · Eg ₁ · n) <d ₂ <1240 / (2 · Eg ₂ · n) where d ₁ is the upper light reflecting semiconductor film. Film thickness (nm), d
₂ is the film thickness (nm) of the lower light reflecting semiconductor film, Eg ₁ is the band gap energy (eV) of the upper solar cell, Eg
₂ is the band gap energy (eV) of the lower solar cell
A solar cell characterized by satisfying the following. 2. A solar cell comprising three layers of semiconductors comprising pn junctions and having different band gap energies laminated on a substrate, wherein a light reflecting semiconductor film is provided between each of the pn junctions and the substrate. The semiconductor film for reflection has the following formula: 150 / n <d ₁ <1240 / (2 · Eg ₁ · n) <d ₂ <1240 / (2 · Eg ₂ · n) <d ₃ <1240 / (2 · Eg ₃ ・ N) where d ₁ is the film thickness (nm) of the upper light reflecting semiconductor film, d
₂ is the film thickness (nm) of the middle light reflecting semiconductor film, d ₃ is the film thickness (nm) of the lower light reflecting semiconductor film, Eg ₁ is the band gap energy (eV) of the upper solar cell, and Eg ₂ is the middle solar light. Battery band gap energy (eV), Eg ₃
Is a solar cell satisfying the band gap energy (eV) of the lower layer solar cell.