JPH07283428A - Solar cell - Google Patents

Abstract

PURPOSE:To obtain a sufficient photoelectric conversion efficiency while p and n layers are made thinner by inserting a thin-film layer with a small refractive index than that of p-type and n-type semiconductor layers into the p-type and n-type semiconductor layers. CONSTITUTION:Light entering p layer is reflected by a thin-film layer 4 in n layer, the reflected light is reflected by a thin-film layer 7 in the p layer, and the reflection is repeated, thus confining light between two thin-film layers 4 and 7. Light can be effectively absorbed into a pn junction region sandwiched by the thin-film layers 4 and 7 and a high absorption coefficient can be obtained even if the p and n layers are made thinner. By setting a distance (d) between the thin-film layers 4 and 7 inserted into the p and n layers to values so that an expression I (n indicates the refractive index of p and n-type semiconductor layer constitution materials, Eg (ev) indicates a band gap energy, and d indicates the distance between the thin-film layers and is expressed by nm) can be met, light can be absorbed effectively by the pn junction region and a sufficient photoelectric conversion efficiency can be obtained while the p and n layers are made thinner.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a solar cell, and more particularly to a thin and highly efficient solar cell.

[0002]

2. Description of the Related Art Solar cells have been attracting attention as a clean energy source, but their production costs are high, so that their power generation costs are higher than existing commercial power sources, which is a major obstacle to their practical use. Therefore, research and development are being conducted to improve the conversion efficiency. Up to now, the efficiency of single-material solar cells has been promoted, and attempts for practical use have begun. From the viewpoint of high efficiency, GaAs-based compound semiconductors are extremely excellent as materials for solar cells,
Currently, a conversion efficiency of over 25% is obtained. But,
GaAs compound semiconductor is used as the constituent material of the solar cell,
When a solar cell is manufactured by the current manufacturing method using GaAs also for the substrate, the manufacturing cost becomes very high and the power generation cost becomes high. Therefore, in order to achieve low power generation cost, it is necessary to further improve efficiency and reduce manufacturing cost.

In order to obtain high efficiency, the solar cell must have a structure in which a plurality of solar cells having different band gaps are laminated. Specifically, the incident side of sunlight (upper part)
A solar cell made of a semiconductor material having a large bandgap is arranged in the lower part, and a solar cell made of a semiconductor material having a small bandgap is arranged below it. In the upper solar cell, light in the short wavelength region of the sunlight spectrum is absorbed and photoelectrically converted.
In the lower solar cell, the remaining long-wavelength light that is not absorbed and transmitted in the upper part is absorbed and photoelectrically converted. By dividing and utilizing the spectrum of sunlight, sunlight energy can be effectively converted into electric energy. Henry et al.'S theoretical calculations show that a two-layer structure can achieve an efficiency of 35% (CHHenry, "Limiting Efficiency
of IdealSingle and Multiple Energy Gap Terrestrial
Solar Cells "J. Appl. Phys. 514494 (1980)). For example, a two-stage stacked solar cell in which a GaAs solar cell and an AlGaAs solar cell are connected using a tunnel junction intermediate layer has been proposed.

In order to reduce the manufacturing cost, it is necessary to use inexpensive substrates and save solar cell constituent materials. Regarding the substrate, it has been proposed to replace the currently used GaAs with Si. Further, in order to save the solar cell constituent materials, it is desirable to thin the solar cell.

[0005]

Problems to be Solved by the Invention As described above, Si
Development of high-efficiency GaAs solar cells using substrates has been promoted, but there are the following problems.

In order to obtain high photoelectric conversion efficiency, there are two main factors: sufficient light absorption and effective use of excited carriers. Among these, from the viewpoint of light absorption, it is preferable that the p-type semiconductor layer (p layer) and the n-type semiconductor layer (n layer), which are epi layers, are thick. However, since the lattice constant and the coefficient of thermal expansion are different between the substrate Si and the GaAs-based crystal that constitutes the solar cell, when a GaAs-based crystal is grown on Si with a film thickness of 3 μm or more, cracks occur and crystal defects occur. A large amount will be generated.

On the other hand, from the viewpoint of effectively extracting the excited carriers, the thickness of the p layer and the n layer may be reduced or the diffusion length of the excited carriers may be increased. Research on increasing the diffusion length of excited carriers has been promoted, but no significant progress has been made. Therefore, it is considered to reduce the thickness of the p layer and the n layer. For this purpose, a structure capable of increasing the light absorption coefficient with a thin layer is required.

The present invention was created under such circumstances, and an object thereof is to provide a novel solar cell capable of obtaining a sufficient photoelectric conversion efficiency in a state where the p layer and the n layer are thin. To provide.

[0009]

In order to achieve the above object, the present invention provides a solar cell in which a p layer and an n layer are joined to each other, and the p layer and the n layer have a smaller refractive index than these. It is characterized in that a thin film layer is inserted.

In the solar cell of the present invention, it is desirable that the distance between the thin film layer inserted in the p layer and the thin film layer inserted in the n layer is set so as to satisfy the expression (1).

[0011]

[Equation 1]

GaAs is used as the material for the p-layer and the n-layer.
It is desirable to use AlGaAs as the material of the thin film layer.

[0013]

According to the solar cell of the present invention constructed as described above, by inserting thin film layers each having a smaller refractive index than these into the p layer and the n layer constituting the pn junction of the solar cell, The light incident on the p layer is reflected by the thin film layer in the n layer, the reflected light is reflected by the thin film layer in the p layer, and the light is confined between the two thin film layers by repeating this reflection. . As a result, the pn sandwiched between the thin film layers
Light is effectively absorbed in the junction region, and the p layer and n
It is possible to obtain a high light absorption coefficient even if the layer is thin. If a direct transition type III-V group compound semiconductor, which itself has a high absorption efficiency, is used as the material for the p-layer and the n-layer, light absorption by the thin layer can be more effectively achieved. Also, if the distance d between the thin film layer inserted in the p layer and the thin film layer inserted in the n layer satisfies the above formula (1), the reflection of light between the thin film layers is prevented. This is repeated for light having a wavelength advantageous for photoelectric conversion, and this light is effectively absorbed in the pn junction region.

A method of deriving the equation (1) will be shown below.

The condition (resonance reflection condition) in which light is repeatedly reflected between the two thin film layers is expressed by the following equation (2).

D = 1/2 × λ / n (2) where d is the distance between the thin film layers (see FIG. 5), n is the refractive index of the p layer and the n layer, and λ is the absorption of the pn layer. It is the wavelength of the light you want to do.

By the way, absorption of light occurs with respect to light having a single wavelength longer than the wavelength (emission wavelength) λ _Eg corresponding to the band gap Eg of the semiconductor. The degree of light absorption (light absorption coefficient) increases as the wavelength of light decreases. From the opposite viewpoint, the closer the wavelength of light is to λ _Eg , the easier it is for light to pass.

Therefore, in the present invention, conditions are set so that the above-mentioned reflection is repeated for light having a wavelength of λ _Eg / 2 to λ _Eg . That is, the range of the wavelength λ of the light that the pn layer wants to absorb most is set to the range of the following expression (3). λ _Eg × 0.5 <λ <λ _Eg × 1.0 ・ ・ ・ (3) Now, the emission wavelength λ _Eg is expressed by λ _Eg = 1240 / Eg ・ ・ ・ (4), and this formula (4) is expressed by formula (3). Substituting into 1240 / Eg × 0.5 <λ <1240 / Eg × 1.0 (5). Further, by transforming the equation (2), λ = 2 · n · d (2 ′), and substituting this into the equation (5), 1240 / Eg × 0.5 <2 · n · d <1240 / Eg × 1.0 (6) If this equation (6) is divided by 2 · n, the above equation (1)
Is obtained. In the case of GaAs, the band gap Eg is 1.4
Since 24 eV and the refractive index n are 3.68 (see FIG. 3) for light of wavelength λ = 800 nm, the distance between the two thin film layers is 59 nm <d <118 nm according to equation (1). It will be.

[0019]

EXAMPLES Next, examples of the present invention will be described.

In order to show the effectiveness of the solar cell of the present invention,
A solar cell according to the related art and a solar cell according to the present invention were actually manufactured, and their characteristics were compared.

FIG. 1 is a sectional view of a solar cell showing an embodiment of the present invention. Metal organic vapor phase epitaxy (M
OVPE method) was used. Trimethylgallium (TMG), trimethylaluminum (TMA), and arsine (AsH ₃ ) were used as the raw materials of Ga, Al, and As at that time. Disilane (Si ₂ H ₆ ) and diethyl zinc (DEZn) were used as the n-type and p-type dopants, respectively.
The growth temperature is 725 ° C. Inclined by 2 ° in the (110) direction (10
0) surface as substrate surface, thickness 350 μm, carrier concentration 5.
0.2μ thickness on 0 × 10 ¹⁷ cm ^-3 n-type GaAs substrate 1
m, carrier concentration 5.0 × 10 ¹⁷ cm ^-3 n-type Al0.2 Ga0.
8 As layer 2, n-type GaA with carrier concentration 5.0 × 10 ¹⁷ cm ^-3
After growing the s layer 3, the thickness is 0.01 μm, the carrier concentration is
5.0 × 10 ¹⁷ cm ^-3 n-type Al0.5 Ga0.5 As layer (n-type thin film layer) 4, carrier concentration 5.0 × 10 ¹⁷ cm ^-3 n-type GaAs
Layer 5, p-type GaAs layer 6 having a carrier concentration 1.0 × 10 ¹⁸ cm ^-3, thickness of 0.01 [mu] m, p-type Al0.5 Ga0.5 As layer having a carrier concentration 1.0 × 10 ¹⁸ cm ^-3 (p-type thin film layer) 7, p-type GaAs layer 8 with carrier concentration 1.0 × 10 ¹⁸ cm ^-3 , thickness 0.05 μm,
Carrier concentration 1.0 × 10 ¹⁸ cm ^-3 p-type Al0.85Ga0.15A
s layer 9 and thickness 0.2 μm, carrier concentration 1.0 × 10 ¹⁹ cm ^-3
The p-type GaAs layer 10 was grown. And p-type GaA
After the s layer 10 was etched and shaped as shown in the figure, electrodes 11 and 12 were formed on the front surface and the back surface of the substrate 1. Here, it is the pn layer composed of the p-type semiconductor layer 13 and the n-type semiconductor layer 14 that functions as a solar cell. n-type Al0.2 Ga
The 0.8 As layer 2 is a barrier layer for preventing electrons generated in the n-type GaAs substrate 1 from contributing to the efficiency of the solar cell,
That is, it is a layer provided to clarify how the structure of the pn layer affects the efficiency of this solar cell. The p-type Al0.85Ga0.15As layer 9 is a window layer for suppressing surface recombination, and the p-type GaAs layer 10 is a layer for reducing the contact resistance with the electrode.

The n-type thin film layer 4 and the p-type thin film layer 7 are made of GaAs.
Is a layer for resonance-reflecting light having a wavelength shorter than the absorption edge wavelength. The distance between these two thin film layers 4 and 7 is 40 nm and 60n.
Solar cells were manufactured by changing m, 80 nm, 100 nm, 115 nm and 140 nm respectively. At that time, the interface between the n-type GaAs layer 5 and the p-type GaAs layer 6 (p
n interface). Also, the sum of the thickness of the n-type GaAs layer 3 and the n-type GaAs layer 5 is 1.2 μm, and the p-type G
The sum of the thickness of the aAs layer 6 and the p-type GaAs layer 8 is 1.2 μm.
And Therefore, in the solar cell of the present invention, the thickness of the pn layer that substantially acts as a solar cell is 1.72 μm including the n-type thin film layer 4 and the p-type thin film layer 7.

FIG. 2 is a sectional view of a conventional solar cell. In this case, n-type Al0. With a thickness of 0.2 μm and a carrier concentration of 5.0 × 10 ¹⁷ cm ^-3 was formed on the same substrate 1 as in FIG. 1 by the same method as above.
2 Ga0.8 As layer 2, thickness 4.0 μm, carrier concentration 5.0
× 10 ¹⁷ cm ^-3 n-type GaAs layer 15, thickness 0.5 μm, carrier concentration 1.0 × 10 ¹⁸ cm ^-3 p-type GaAs layer 16, carrier concentration 1.0 × 10 ¹⁸ cm ^-3 p-type Al 0.85 Ga 0. 15As layer 9
And a p-type GaAs layer 10 having a thickness of 0.2 μm and a carrier concentration of 1.0 × 10 ¹⁹ cm ⁻³ was grown. Then, electrodes 11 and 12 were formed on the front surface of the p-type GaAs layer 10 and the back surface of the substrate 1. The pn layer consisting of the n-type GaAs layer 15 and the p-type GaAs layer 16 acts as a solar cell, and the thickness of the pn layer in this case is 4.5 μm.

The refractive index of the material forming each layer of the solar cell shown in FIGS. 1 and 2 is as shown in FIG.

As a result of measuring the intrinsic conversion efficiency of these solar cells, it was 16.8% for the solar cell of the conventional structure shown in FIG.

FIG. 4 shows the measurement results of the solar cell of the present invention, which shows the space dependence of the photoelectric conversion efficiency for the light of wavelength 800 nm between the n-type thin film layer 4 and the p-type thin film layer 7. . From the figure, it can be seen that when the distance between the two thin film layers 4 and 7 is between 60 nm and 115 nm, higher efficiency than before can be exhibited.

From the above, the solar cell of the present invention is only 1.72μ.
4.5 μ in the p-type and n-type GaAs layers 13 and 14 with a thickness of m
It was confirmed that the performance of the solar cell of the related art having a GaAs layer having a thickness of m is better than that of the conventional solar cell.

An antireflection film is usually provided on the surface of the solar cell in order to suppress reflection on the surface of the epitaxial layer, but it is not necessary for comparing the present invention with the prior art, so this is intentionally omitted in this embodiment. Was omitted.

[0029]

In summary, according to the present invention, thin film layers each having a smaller refractive index than these are inserted into the p-type semiconductor layer and the n-type semiconductor layer which form the pn junction of the solar cell, respectively. A thin pn layer between two thin film layers can sufficiently absorb light, and as a result, a highly efficient thin solar cell can be realized.

[Brief description of drawings]

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

FIG. 2 is a cross-sectional view showing a conventional solar cell.

FIG. 3 is a diagram showing wavelength dependence of a refractive index of a material forming each layer of the solar cells of FIGS. 1 and 2.

FIG. 4 shows the conversion efficiency of an n-type thin film layer and p of the solar cell of the present invention.
It is a figure which shows the space dependence with a type | mold thin film layer.

FIG. 5 is a schematic diagram for explaining a process of deriving a mathematical formula that defines a distance between an n-type thin film layer and a p-type thin film layer.

[Explanation of symbols]

 1 n-type GaAs substrate 2 n-type Al0.2 Ga0.8 As layer (barrier layer) 3 n-type GaAs layer 4 n-type Al0.5 Ga0.5 As layer (n-type thin film layer) 5 n-type GaAs layer 6 p-type GaAs layer 7 p-type Al0.5 Ga0.5 As layer (p-type thin film layer) 8 p-type GaAs layer 9 p-type Al0.85 Ga0.15 As layer (window layer) 10 p-type GaAs layer (low resistance layer) 11 electrode 12 electrode 13 p-type semiconductor layer 14 n-type semiconductor layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ken Takahashi 3550 Kidayomachi, Tsuchiura City, Ibaraki Prefecture Hitachi Cable Ltd. Advanced Research Center

Claims (3)

[Claims] 1. A solar cell in which a p-type semiconductor layer and an n-type semiconductor layer are joined to each other, characterized in that thin film layers each having a smaller refractive index than the p-type and n-type semiconductor layers are inserted into the solar cell. And solar cells. 2. The solar cell according to claim 1, wherein the distance between the thin film layer inserted in the p-type semiconductor layer and the thin film layer inserted in the n-type semiconductor layer satisfies the formula (1). The characteristic solar cell. [Equation 1] 3. The solar cell according to claim 2, wherein GaAs is used as a material of the p-type and n-type semiconductor layers, and AlGaAs is used as a material of the thin film layer.