Solar cell

A method for generating electric power with use of a solar cell includes steps of: (a) preparing the solar cell including a condensing lens and a solar cell element, wherein the following inequation set (I) is satisfied: d2<d1, d3<d1, 1 nanometer≦d2≦4 nanometers, 1 nanometer≦d3≦4 nanometers, 100 nanometers≦w2, and 100 nanometers≦w3 . . . (I); and (b) irradiating a region S which is included in the surface of the p-type window layer through the condensing lens with light in such a manner that the following inequation (II) is satisfied so as to generate a potential difference between the n-side electrode (110) and the p-side electrode (109): w4≦w1 . . . (II).

TECHNICAL FIELD

The present invention relates to a solar cell.

BACKGROUND ART

FIG. 7shows a solar cell disclosed in Patent Literature 1. The conventional solar cell comprises a plurality of photoelectric conversion layers13, the photoelectric conversion layer13having a solar cell element11and a lens L. The solar cell element11comprises a p-type GaAs buffer layer13a, a p-type InGaP-BSF layer13b, a p-type GaAs base layer13c, an n-type GaAs emitter layer13d, an n-type InGaP window layer13e, and an antireflection layer15. These layers13ato15are stacked on a semiconductor substrate12in this order. The solar cell element11further comprises a separation grid16which separates the photoelectric conversion layer13, a contact layer14around the detector side of the photoelectric conversion layer13, a recoupling prevention layer17around the outer circumference of the contact layer14, a detector side electrode18and a back side electrode19.

Sunlight penetrates the lens L and the antireflection layer15, and the n-type InGaP window layer13eis irradiated with the sunlight. This irradiation of the sunlight generates electric power.

CITATION LIST

Patent Literature

Non Patent Literature

[Non Patent Literature 1]Jenny Nelson, “The Physics of Solar Cells”, World Scientific Pub Co Inc.

SUMMARY OF THE INVENTION

Technical Problem

The conventional solar cell has a conversion efficiency of approximately 20%.

The purpose of the present invention is to provide a solar cell with higher conversion efficiency.

Solution to Problems

The present disclosure provides a method for generating electric power with use of a solar cell, the method comprising steps of:

(a) preparing the solar cell comprising a condensing lens and a solar cell element, wherein

the solar cell element comprises an n-type GaAs layer, a p-type GaAs layer, a p-type window layer, an n-side electrode, and a p-side electrode;

a Z-direction denotes the direction of the normal line of the p-type GaAs layer;

an X-direction denotes a direction orthogonal to the Z-direction,

the n-type GaAs layer, the p-type GaAs layer, and the p-type window layer are stacked along the Z-direction;

the p-type GaAs layer is interposed between the n-type GaAs layer and the p-type window layer along the Z-direction;

the p-side electrode is electrically connected with the p-type GaAs layer;

the n-side electrode is electrically connected with the n-type GaAs layer;

the n-type GaAs layer is divided into a center part, a first peripheral part, and a second peripheral part;

the center part is interposed between the first peripheral part and the second peripheral part along the X-direction;

the first peripheral part and the second peripheral part have a shape of a layer,

wherein

d1represents a thickness of the center part along the Z-direction;

d2represents a thickness of the first peripheral part along the Z-direction;

d3represents a thickness of the second peripheral part along the Z-direction;

w2represents a width of the first peripheral part along the X-direction; and

w3represents a width of the second peripheral part along the X-direction; and

(b) irradiating a region S which is included in the surface of the p-type window layer through the condensing lens with light in such a manner that the following inequation (II) is satisfied so as to generate a potential difference between the n-side electrode and the p-side electrode:
w4≦w1  (II);

wherein

w1represents a width of the center part along the X-direction;

w4represents a width of the region S along the X-direction in the cross-sectional view which includes the Z-direction; and

the first center part overlaps the region S when seen from the Z-direction.

Advantageous Effect of the Invention

The present invention provides a solar cell with higher conversion efficiency.

DESCRIPTION OF EMBODIMENTS

In the step (a), a solar cell is prepared.

FIG. 1Ashows a cross-sectional view of the solar cell according to the embodiment 1. As shown inFIG. 1A, the solar cell comprises a condensing lens101and a solar cell element102.

As shown inFIG. 1B, the solar cell element102comprises an n-type GaAs layer104, a p-type GaAs layer103, a p-type window layer105, an n-side electrode110, and a p-side electrode109. The n-type GaAs layer104, the p-type GaAs layer103, and the p-type window layer105are stacked. A Z-direction denotes a stacking direction. Along the Z-direction, the p-type GaAs layer103is interposed between the n-type GaAs layer104and the p-type window layer105.

The p-side electrode109is electrically connected with the p-type GaAs layer103. The n-side electrode110is electrically connected with the n-type GaAs layer104.

It is preferable that an n-type barrier layer106and an n-type contact layer108are interposed between the n-type GaAs layer104and the n-side electrode110along the Z-direction. Along the Z-direction, the n-type barrier layer106is interposed between the n-type GaAs layer104and the n-type contact layer108. Along the Z-direction, the n-type contact layer108is interposed between the n-type barrier layer106and the n-side electrode110.

Along the Z-direction, it is preferable that a p-type contact layer107is interposed between the p-type window layer105and the p-side electrode109. The p-side electrode109, the p-type contact layer107, the p-type window layer105, the p-type GaAs layer103, the n-type GaAs layer104, the n-type barrier layer106, the n-type contact layer108, and the n-side electrode110are electrically connected in this order.

As shown inFIG. 1B, the n-type GaAs layer104is divided into a center part104a, a first peripheral part104b, and a second peripheral part104c. The center part104ais interposed between the first peripheral part104band the second peripheral part104calong an X-direction. The X-direction is orthogonal to the Z-direction.

As shown inFIG. 2, the thickness d1of the center part104ais greater than the thickness d2of the first peripheral part104band than the thickness d3of the second peripheral part104c. When the thickness d1is the same as the thickness d2and the thickness d3, the higher conversion efficiency is not achieved (see the comparative examples 1 and 3, which are described later).

In the embodiment 1, the thickness d2is not less than 1 nanometer and not more than 4 nanometers. When the thickness d2is less than 1 nanometer, the higher conversion efficiency is not achieved (see the comparative example 10, which is described later). When the thickness d2is more than 4 nanometers, the higher conversion efficiency is not achieved (see the comparative examples 7 to 9, which are described later). Similarly, the thickness d3is also not less than 1 nanometer and not more than 4 nanometers.

The first peripheral part104bhas a shape of a layer. As shown inFIG. 6AandFIG. 6B, the first peripheral part104bmust not have a shape of a taper. This is because the higher conversion efficiency is not achieved (see the comparative examples 4 and 5, which are described later). Similarly, the second peripheral part104calso has a shape of a layer.

As shown inFIG. 2, the center part104ahas a width of w1. The first peripheral part104bhas a width of w2. The second peripheral part104chas a width of w3.

The value of w2is 0.1 micrometer or more. When the value of w2is less than 0.1 micrometer, the conversion efficiency is decreased. For the same reason, the value of w3is 0.1 micrometer or more. See the examples 4 and 5 and the comparative example 10, which are described later.

As described above, the value of d1represents a thickness of the center part104aalong the Z-direction.

The value of d2represents a thickness of the first peripheral part104balong the Z-direction.

The value of d3represents a thickness of the second peripheral part104calong the Z-direction.

The value of w2represents a width of the first peripheral part104balong the X-direction.

The value of w3represents a width of the second peripheral part104calong the X-direction.

The obverse surface of the condensing lens101is irradiated with light. This is described in more detail in the step (b), which is described later. Sunlight is preferred.

The reverse surface of the condensing lens101is preferably in contact with the solar cell element102. The light is focused onto the p-type window layer105by the condensing lens101.

It is preferable that the condensing lens101has a diameter of 2 millimeters to 10 millimeters, a thickness of 1 millimeter to 5 millimeters, and a refractive index of 1.1 to 2.0.

The material of the condensing lens101is not limited. An example of the material of the condensing lens101is glass or resin.

The p-type window layer105is made of a p-type compound semiconductor having a lattice constant close to that of GaAs and having a wider bandgap than GaAs. An example of the material of the p-type window layer105is p-type InGaP or p-type AlGaAs.

The n-type barrier layer106is made of an n-type compound semiconductor having a lattice constant close to that of GaAs and having a wider bandgap than GaAs. An example of the material of the n-type barrier layer106is n-type InGaP or n-type AlGaAs.

The material of the p-type contact layer107is not limited, as long as ohmic contacts are formed in the interface with the p-type window layer105and in the interface with the p-side electrode109. An example of the material of the p-type contact layer107is p-type GaAs.

The material of the n-type contact layer108is not limited, as long as ohmic contacts are formed in the interface with the n-type barrier layer106and in the interface with the n-side electrode110. An example of the material of the n-type contact layer108is n-type GaAs.

As shown inFIG. 1B, the sides of the layers103to108are preferably covered with an insulating film111. An example of the material of the insulating film111is non-doped InGaP, silicon dioxide, or silicon nitride.

When the insulating film111is used, as shown inFIG. 4, the insulating film111is covered with a metal film118. The metal film118improves the heat radiation property of the solar cell element102.

It is preferred that the metal film118is electrically connected with the p-side electrode109and that the metal film118and the n-side electrode110are exposed on one surface (inFIG. 4, the bottom surface).

(Method for Fabricating Solar Cell Element102)

A method for fabricating a solar cell element102is described below with reference toFIGS. 3A to 3G.

First, as shown inFIG. 3A, a sacrificial layer114, the p-type contact layer107, the p-type window layer105, the p-type GaAs layer103, the n-type GaAs layer104, the n-type barrier layer106, and the n-type contact layer108are formed in this order on the surface of a GaAs substrate113by a known semiconductor growth method such as a molecular beam epitaxy method or a metal organic chemical vapor deposition method (hereinafter, referred to as an “MOCVD method”). The sacrificial layer114has a lattice constant close to that of GaAs. The sacrificial layer114is a layer for being etched selectively against GaAs. An example of the material of the sacrificial layer114is AlAs or InGaP.

Next, as shown inFIG. 3B, a first mask115is formed on the n-type contact layer108. The n-type contact layer108, the n-type barrier layer106, the n-type GaAs layer104, the p-type GaAs layer103, the p-type window layer105, and the p-type contact layer107are etched by dry-etching with use of the first mask115. The width of the first mask115is equal to the sum of (w1+w2+w3) shown inFIG. 2. In the dry-etching, a mixed gas of BCl3and SF6may be used.

As shown inFIG. 3C, a second mask116is formed on the n-type contact layer108. The second mask116has a smaller width than the first mask115. This width of the second mask116is the same as the width of w1shown inFIG. 2. With use of the second mask116, the n-type contact layer108and the n-type barrier layer106are etched. Furthermore, the upper portion of a peripheral part of the n-type GaAs layer104is etched. The etching depth of the n-type GaAs layer104is equal to the thickness d1-d3shown inFIG. 2.

As shown inFIG. 3D, the second mask116is removed. The n-side electrode110and the insulating film111are formed. An example of forming the n-side electrode110is a sputtering method or an electron beam deposition technique. An example of forming the insulating film111is a chemical vapor deposition method.

As shown inFIG. 3E, a base substrate117is fixed to the n-side electrode110. The GaAs substrate113and the sacrificial layer114are removed by etching. An example of the base substrate117is a silicon substrate or a glass substrate. A wax or an adhesive sheet may be interposed between the n-side electrode110and the base substrate117optionally.

As shown inFIG. 3F, the p-side electrode109is formed on the p-type contact layer107. Furthermore, a part of the p-type contact layer107which is not in contact with the p-side electrode109is removed by etching. An example of forming the p-side electrode109is a sputtering method or an electron beam deposition technique.

Finally, as shown inFIG. 3G, the base substrate117is removed. Thus, the solar cell element102is obtained. As shown inFIG. 1A, the obtained solar cell element102is fixed to the condensing lens101. Thus, the solar cell is obtained.

In the step (b), the p-type window layer105is irradiated with the light through the condensing lens101to generate a potential difference between the n-side electrode110and the p-side electrode109. As shown inFIG. 2, a region S of the p-type window layer105is irradiated with the light.

The present inventors discovered that the following inequation set (II) is required to be satisfied in the step (b).
w4≦w1  (II)

As described above, the value of w1represents the width of the center part104aalong the X-direction.

The value of w4represents a width of the region S along the X-direction.

When seen along the Z-direction, the center part104aoverlaps with the region S.

In the case where the inequation set (II) is not satisfied, the higher conversion efficiency is not achieved (see the comparative example 4).

As shown inFIG. 2, when the n-type GaAs layer104has the same width as the p-type window layer105, the width of w1is equal to or greater than the width of w4. Specifically, if the following equation: (w1+w2+w3)=(w4+w5+w6) is satisfied, the width of w5is equal to or greater than the width of w2, and the width of w6is equal to or greater than the width of w3. Both of w5and w6correspond to the part which is not irradiated with the light.

EXAMPLES

The present invention is described in more detail by the following examples.

In the example 1, the solar cell element102shown inFIG. 2was fabricated by the method shown inFIGS. 3A to 3G.

Table 1 shows the composition and the thickness of each layer in the solar cell element102according to the example 1.

The condensing lens101was 4 millimeters square and had a thickness of 3 mm. The condensing lens101had a focus spot of 80 micrometers square.

The solar cell according to the example 1 was fabricated as below.

First, as shown inFIG. 3A, the layers104to114shown in Table 1 were grown on the GaAs substrate113by an MOCVD method.

Next, as shown inFIG. 3B, a square resist film115having 100 micrometers square was formed on the n-type contact layer108by photolithography. Using this resist film115as a first mask, the n-type contact layer108, the n-type barrier layer106, the n-type GaAs layer104, the p-type GaAs layer103, the p-type window layer105, and the p-type contact layer107were removed by ICP plasma etching with use of a mixed gas of BCl3and SF6. Thus, a pattern having 100 micrometers square was formed.

After etching, the first mask was removed with a resist stripper liquid. After removed, a square resist film116having 90 micrometers square was formed on the n-type contact layer108. The center of the resist film116corresponded with the center of the resist film115.

Using this resist film116as a second mask, the n-type contact layer108and the n-type barrier layer106were etched. Furthermore, as shown inFIG. 3C, almost all of the peripheral part of the n-type GaAs layer104was etched in such a manner that the peripheral part of the n-type GaAs layer was left slightly. A mixed solution of phosphoric acid and hydrogen peroxide was used to etch the n-type contact layer108and the n-type GaAs layer104. Hydrochloric acid was used to etch the n-type barrier layer106.

After etching, the thickness of the remaining peripheral part of the n-type GaAs layer104was measured with a transmission electron microscope. The thickness was 4 nanometers.

The second mask was removed with a detachment liquid. After removed, as shown inFIG. 3D, a titanium film with a thickness of 50 nanometers and a gold film with a thickness of 250 nanometers were stacked on the n-type contact layer108to form the n-side electrode110with use of an electron beam deposition device.

Next, as shown inFIG. 3D, an insulating film111made of SiN with a thickness of 400 nanometers was formed with use of a plasma chemical vapor deposition device.

Next, wax was applied with a spin coater to the surface where the n-side electrode110was formed. After the wax was dried, as shown in FIG.3E, the n-side electrode110was fixed to the base substrate117made of glass.

After fixed, the GaAs substrate113was removed with use of a mixture of citric acid and hydrogen peroxide. Subsequently, the sacrificial layer114was removed with use of buffered hydrofluoric acid to expose the p-type contact layer107. Thus, the structure shown inFIG. 3Ewas obtained.

As shown inFIG. 3F, a titanium film having a thickness of 50 nanometers, a platinum film having a thickness of 150 nanometers, and a gold film having a thickness of 250 nanometers were formed in this order on the p-type contact layer107to form the p-side electrode109with use of an electron beam deposition device.

After the p-side electrode109was formed, the wax was dissolved with isopropanol to remove the base substrate117. Thus, the solar cell element102shown inFIG. 3Gwas obtained.

The obtained solar cell element102was attached to the condensing lens101in such a manner that the center of the focus position of the condensing lens101corresponded with the center of the solar cell element102. In this manner, the solar cell according to the example 1 was obtained.

The solar cell according to the example 1 was irradiated with sunlight under the condition that w4=90 micrometers and w5=w6=5 micrometers. The volt-ampere characteristics of the solar cell according to the example 1 were measured, and the conversion efficiency was calculated. Table 2 shows them with the data of the examples 2 to 8 and the comparative examples 1 to 14.

The conversion efficiency was calculated according to the following equation (I):
(Conversion efficiency)=(Maximum output value from the solar cell)/(Energy of the sunlight)  (Equation I)

The maximum output value described in the above-mentioned equation (I) denotes the maximum value of the output value defined by the following equation (II):
(Output value)=(Current density obtained from the solar cell)·(Bias voltage obtained from the solar cell)

For more detail, see the pages 11 to 13 disclosed in Non-Patent Literature 1, such as Jenny Nelson, “The Physics of Solar Cells”, World Scientific Pub. Co. Inc.

The experiment identical to that of the example 1 was performed except that d2=2 nanometers.

The experiment identical to that of the example 1 was performed except that d2=1 nanometer.

The experiment identical to that of the example 1 was performed except that w1=99.8 micrometers and w2=w3=0.1 micrometer.

The experiment identical to that of the example 1 was performed except that w1=99 micrometers and w2=w3=0.5 micrometers.

The experiment identical to that of the example 1 was performed except that w4=86 micrometers and w5=w6=7 micrometers.

The experiment identical to that of the example 1 was performed except that w1=80 micrometers, w2=w3=10 micrometers, w4=80 micrometers, and w5=w6=10 micrometers.

The experiment identical to that of the example 1 was performed except that w1=80 micrometers, w2=w3=10 micrometers, w4=76 micrometers, and w5=w6=12 micrometers.

Comparative Example 1

The experiment identical to that of the example 1 was performed except that d2=d3=2.5 micrometers and w4=100 micrometers.

Comparative Example 2

The experiment identical to that of the example 1 was performed except that d2=d3=2.5 micrometers.

Comparative Example 3

The experiment identical to that of the example 1 was performed except that w4=100 micrometers.

Comparative Example 4

The experiment identical to that of the example 1 was performed except that the p-type GaAs layer103was formed by a wet-etching technique, instead of the ICP plasma etching, which is a dry etching, so as to obtain the solar cell shown inFIG. 6A.

Comparative Example 5

The experiment identical to that of the example 1 was performed except that the p-type GaAs layer103and the n-type GaAs layer104were formed by a wet-etching technique to obtain the solar cell shown inFIG. 6B.

Comparative Example 6

The experiment identical to that of the example 1 was performed except that d2=d3=0.1 micrometers.

Comparative Example 7

The experiment identical to that of the example 1 was performed except that d2=d3=0.01 micrometers.

Comparative Example 8

The experiment identical to that of the example 1 was performed except that d2=d3=0.005 micrometers.

Comparative Example 9

The experiment identical to that of the example 1 was performed except that d2=d3=0 micrometers.

Comparative Example 10

The experiment identical to that of the example 1 was performed except that w1=99.9 micrometers and w2=w3=0.05 micrometers.

Comparative Example 11

The experiment identical to that of the example 1 was performed except that w4=98 micrometers and w5=w6=1 micrometer.

Comparative Example 12

The experiment identical to that of the example 1 was performed except that w4=94 micrometers and w5=w6=3 micrometers.

Comparative Example 13

The experiment identical to that of the example 1 was performed except that w1=80 micrometers, w2=w3=10 micrometers, w4=88 micrometers, and w5=w6=6 micrometers.

Comparative Example 14

The experiment identical to that of the example 1 was performed except that w1=80 micrometers, w2=w3=10 micrometers, w4=84 micrometers, and w5=w6=8 micrometers.

The examples 1 to 8 and the comparative examples 1 and 2 show that it is necessary that the following inequation set: d2<d1and d3<d1is satisfied.

The examples 1 to 3 and the comparative examples 6 to 9 show that it is necessary that the following inequation set: 1 nanometer≦d2≦4 nanometers and 1 nanometer≦d3≦4 nanometers is satisfied.

The examples 4 and 5 and the comparative example 10 show that it is necessary that the following inequation set: 100 nanometers≦w2and 100 nannometers≦w3is satisfied.

The examples 1, 6 to 8 and the comparative examples 11 to 14 show that it is necessary that the following inequation: w4≦w1is satisfied.

INDUSTRIAL APPLICABILITY

The present invention provides a solar cell with higher conversion efficiency.