JP2006332540A - Solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent an unwanted level from being formed that increases a carrier recombination loss at a quantum well to obtain a solar cell having an excellent photoelectric conversion efficiency. <P>SOLUTION: A solar cell has: a first conductive first semiconductor layer (5) using carbon as its material; a second conductive second semiconductor layer (7) using carbon as its material and having a polarity opposite to the first conduction type; and a quantum well (6) formed between the first and second semiconductor layers that uses carbon as its material. The quantum well (6) is made up of: a wall layer (11) composed of a carbon semiconductor thin film; multiple quantum dots (10) imbedded in the wall layer; and an sp<SP>2</SP>combination prevention layer (12) provided around the quantum dots. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a solar cell using carbon as a material, and particularly to a solar cell provided with a quantum well portion in order to improve long wavelength side sensitivity.

For example, a photoelectric conversion element such as a solar cell that generates an electromotive force from light such as visible light is expected as a next-generation clean energy source. Moreover, as a next-generation low-cost solar cell material, a semiconductor thin film mainly composed of carbon called amorphous carbon or diamond-like carbon has been proposed (see Patent Document 1). The carbon material has two stable crystal structures: graphite, which is a conductor and made of sp ² bonds, and diamond, which is an insulator, and made of sp ³ bonds. A semiconductor thin film can be formed by mixing the sp ² and sp ³ bonds. Furthermore, by adjusting the sp ² and sp ³ bonds and the concentration of added hydrogen, it is possible to freely form semiconductor thin films and fine particles (quantum dots) having light absorption characteristics and electrical characteristics suitable for solar cells.

  For this reason, in a solar cell using a carbon material, light absorption characteristics suitable for a wide wavelength range of sunlight can be obtained by adjusting the component ratio in a single material. Therefore, it has an excellent feature that the manufacturing cost can be reduced as compared with a solar cell in which GaAs, Si, Ge, etc., which are conventional crystalline materials, are stacked.

  In addition, as a new method for improving the conversion efficiency of a solar cell, a method of improving the long wavelength sensitivity by forming a quantum well layer in the solar cell element (see Patent Document 2), light having a wide wavelength range by forming quantum dots. A method (Patent Document 3) has been proposed in which the amount of carriers (charges) generated is increased by absorbing the carrier. In order to form a quantum well structure, in general, a thin film having a small band gap of about 1 to 20 nm or a fine dot having the same size diameter is periodically formed on a crystalline semiconductor material GaAs, Si, Ge or the like. It is necessary to provide in. A configuration in which a GaInAs quantum well layer or an InAs quantum dot is provided in GaAs has been proposed, and an improvement in light sensitivity in a long wavelength region has been confirmed.

  In order to provide the quantum well layer, it is necessary to supply a gas containing Ga, In, P, As, or the like using a CVD (chemical vapor deposition) method or an MBE (molecular beam epitaxy) method. Since these materials and manufacturing apparatuses are expensive, they are a great obstacle to providing low-cost solar cells. On the other hand, thin-film solar cells using amorphous silicon (a-Si), CuInSe2-based, and CdS-based materials have been developed as low-cost solar cell materials. In order to form a quantum well structure in these materials, for example, a structure in which an amorphous SiGe layer is provided as a quantum well layer in a-Si has been proposed. However, characteristics comparable to crystalline materials have not been obtained, which is an obstacle to the production of highly efficient solar cells.

  On the other hand, light receiving elements and light emitting elements using cylindrical molecules such as carbon nanotubes mainly composed of carbon elements or spherical molecules such as fullerenes as materials for forming quantum dots have been proposed (Patent Documents 4 and 5). , 6). Among them, application to solar cells is also suggested, but it has not yet been proposed to have a specific structure or superiority over conventional quantum dots.

  Cylindrical molecules such as carbon nanotubes can be produced by chemical vapor deposition from a carbon source such as ethylene or propane and an inexpensive gas such as hydrogen gas. These cylindrical and spherical molecules also contain carbon as a main component, and semiconductor physical properties can be controlled by adjusting the diameter, length, and bonding state. In addition, by adjusting the synthesis conditions, controllability is superior to quantum dots produced using self-shrinkage characteristics due to surface tension, and the possibility of further improving the performance of the solar cell, that is, the photoelectric conversion efficiency. Have.

JP 2003-51603 A JP-A-6-302840 JP 2002-141531 A Japanese Patent Laid-Open No. 2003-243692 JP 2003-285299 A JP 2000-31462 A

However, cylindrical or spherical molecules such as carbon nanotubes and fullerenes constituting the quantum dots provided in the quantum well portion have light absorption characteristics and electrical characteristics determined according to the shape. Therefore, when a bond that contributes to conductive properties such as sp ² bond occurs between the carbon nanotube and the carbon thin film that becomes the wall layer, a new (extra) energy level, for example, a charge is captured in the quantum dot portion. An intermediate level to be coupled is generated, and the energy level at the interface portion between the wall layer and the dot is reduced, resulting in a defect such as a gap therebetween.

Therefore, in manufacturing the quantum well portion, as shown in FIG. 1, when the quantum dots 102 formed using carbon nanotubes or the like are embedded in the carbon thin film that becomes the wall layer 104, the quantum dots 102 in the quantum well portion 100 are formed. As a result, sp ² bonding easily occurs between the wall layer 104 and the quantum dot portion, thereby generating an unnecessary energy level and reducing the energy level.

FIG. 2 is a diagram showing an energy band structure of the quantum well portion 100 shown in FIG. As shown in the figure, when sp ² bonds are formed around the quantum dots 102, the energy level in the vicinity of the interface of the quantum dots 102 decreases, and a gap 106 is generated between the energy levels of the wall layers 104. The existence of this gap increases the recombination loss of electrons and reduces the effect of confining electrons, so that the photoelectric conversion efficiency on the long wavelength side by the quantum well portion is lowered, and the output of the solar cell is hindered.

Therefore, the present invention provides an excellent photoelectric conversion in a solar cell mainly composed of carbon and provided with a quantum well portion by suppressing the occurrence of sp ² bonds between the wall layer constituting the quantum well portion and the quantum dots. It is an object to provide a solar cell that has characteristics and can be manufactured at low cost.

In order to solve the above-described problems, a solar cell of the present invention includes a first semiconductor layer of a first conductivity type made of carbon, a first semiconductor layer made of carbon and having a polarity opposite to that of the first conductivity type. A two-conductivity type second semiconductor layer and a quantum well portion made of carbon formed between the first and second semiconductor layers, wherein the quantum well portion is composed of a carbon semiconductor thin film It includes a wall layer, a plurality of quantum dots embedded in the wall layer, and an sp ² bond prevention layer provided in the peripheral part of the quantum dots.

The quantum dot includes a cylindrical or spherical molecule of carbon. The sp ² bond prevention layer is constituted by bonding a functional group such as an alkyl group to the carbon atom on the surface of the quantum dot. Alternatively, it is configured by bonding hydrogen or a halogen element to a carbon atom.

By actively bonding a functional group, hydrogen, or halogen element to the carbon atom on the surface of the quantum dot, the generation of sp ² bonds that occur when the quantum dot comes into contact with the wall layer is suppressed. Unnecessary energy levels that increase the coupling loss are prevented from being formed in the quantum dot portion. This improves the photoelectric conversion efficiency of the solar cell and increases the output.

When bonding a functional group to the carbon atom, the number of the functional group to be bonded is in the range of 5 to 50% of the number of carbon atoms on the surface of the cylindrical or spherical molecule of carbon constituting the quantum dot. When the number of functional groups to be bonded is large, the energy level of the quantum dots changes and the electron confinement effect of the quantum dots is impaired. Therefore, by making the number of functional groups to be combined in the above range, it is possible to achieve both the effect of suppressing sp ² bond formation and the maintenance of the quantum effect.

The sp ² bond prevention layer may be constituted by bonding hydrogen or halogen element to the carbon atom on the surface of the quantum dot. The addition of the halogen element prevents the wall layer and quantum dots from contacting and bonding. Moreover, since these elements do not easily change the energy level of the quantum dot, the quantum effect of the quantum dot is not impaired.

When a halogen element is used for the sp ² bond prevention layer, the number of halogen elements is in the range of 10 to 90% of the number of carbon atoms on the surface of the cylindrical or spherical molecule of carbon constituting the quantum dot. Thus, it is possible to achieve both the maintenance of suppression effect and quantum effect of sp ² bond formation.

In addition, the sp ² bond prevention layer may be configured by bonding any one of a functional group, hydrogen, and a halogen element to a carbon atom in a portion of the wall layer adjacent to the quantum dot. By adding a functional group or the like to the wall layer side, it is possible to reduce changes in physical properties such as band gap increase and decrease of the quantum dots.

Further, the sp ² bond prevention layer is formed by bonding one of a functional group, hydrogen, and a halogen element to a carbon atom of the wall layer adjacent to the quantum dot, and a carbon atom on the surface of the quantum dot. You may comprise by what combined with any of a functional group, hydrogen, and a halogen element.

By adding a functional group or the like to both the wall layer side and the quantum dot side, the effect of suppressing sp ² bond generation can be further increased.

As described above, in the solar cell of the present invention, contact and bonding between the carbon atoms of the wall layer constituting the quantum well portion and the carbon atoms constituting the quantum dot are suppressed by the presence of the sp ² bond prevention layer. It is difficult to form an intermediate level that captures and recombines carriers at the wall layer side interface of the quantum dot. Therefore, since the quantum well portion exhibits the original quantum effect and improves the light receiving sensitivity on the long wavelength side, it is possible to provide an inexpensive solar cell having excellent photoelectric conversion efficiency.

FIG. 3 is a cross-sectional view showing a schematic configuration of a solar cell according to one embodiment of the present invention. Reference numeral 1 denotes a multilayer light control thin film that operates as an antireflection film for reducing light reflection loss or an optical filter for transmitting unnecessary light and reflecting unnecessary light for the semiconductor material to generate electric charges. . Reference numeral 2 denotes a light-transmitting substrate, which is made of glass, plastic or the like that transmits sunlight. 3 is a transparent electrode composed of an ITO film or SnO ₂ or the like, 4 is a semiconductor layer constituting the photoelectric conversion part of the solar cell, and a thin film mainly composed of carbon in which sp ² bonds and sp ³ bonds are mixed An element that contains hydrogen or a halogen element mixed in the manufacturing process and further controls conductivity is added.

The semiconductor layer 4 basically has a p layer and an n layer constituting a photoelectric conversion part, and a quantum well part for improving the long wavelength sensitivity provided between the p layer and the n layer. In the present embodiment, the semiconductor layer 4 includes an n ⁺ layer 5, a quantum well portion 6, a p layer 7, and a p ⁺ layer 8, and the quantum well portion 6 has a lower carrier concentration than the layers 5 and 7. It is formed by a semiconductor, for example, an n ⁻ layer, i layer or p ⁻ layer. A back electrode 9 is connected to the semiconductor layer 4 and collects electrons or holes.

In this solar cell, electrons and holes generated in the semiconductor layer 4 by absorbing light are transferred to the n ⁺ layer 5 and holes are transferred to the p ⁺ layer 8 by a pn junction formed by the p layer and the n layer. And extracted as a photovoltaic force through the electrodes 1 and 9. In the solar cell shown in FIG. 1, the n ⁺ layer 5 is provided on the light receiving side, but the same effect can be obtained as a solar cell even if the p + layer 8 is provided on the light receiving side.

FIG. 4 is a diagram for explaining the configuration of the quantum well portion 6 and shows a part of the AA cross section of FIG. 1. As shown in the figure, the quantum well portion 6 includes a plurality of quantum dots 10 for generating a quantum effect and a wall layer 11 around the quantum dots 10. Similar to the n ⁺ layer 5, the p layer 7, and the p ⁺ layer 8, the wall layer 11 is made of a material mainly composed of carbon in which sp ² bonds and sp ³ bonds are mixed. Lower than these layers. The quantum dots 10 are composed of carbon cylindrical molecules (for example, carbon nanotubes) or spherical molecules (for example, fullerene). The quantum dot 10 includes one or several of these carbon molecules, and has a diameter of about 3 nm and a length of about 20 nm. The energy band gap of the quantum dots is about 50 to 95% of the wall layer 11.

FIG. 5 shows the structure of a carbon nanotube as an example of a cylindrical molecule, and FIG. 6 shows the structure of fullerene (C ₆₀ ) as an example of a spherical molecule. The carbon nanotubes, fullerenes and the like constituting the quantum dot 10 have light absorption characteristics and electrical characteristics determined according to their shapes, but their energy band gap is smaller than that of the wall layer 11, for example, 50 to 95 of the wall layer 11. %.

  FIG. 7 shows an energy band structure of cross section A in the solar cell shown in FIG. 7 corresponds to each component shown in FIG. 3, and indicates the energy band gap of that portion. The quantum well portion 6 is composed of a wall layer 11 having a large energy band gap and quantum dots 10 having an energy band gap smaller than that of the wall layer 11, thereby causing a quantum effect to appear and increasing the light receiving sensitivity on the long wavelength side. It is improving. Since the semiconductor layer 4 has a light receiving sensitivity mainly for visible light, and therefore, by providing the quantum well portion 6 having a light receiving sensitivity on the longer wavelength side than visible light, photoelectric conversion is efficiently performed in a wider wavelength region. The output of the solar cell is improved.

However, with reference to FIGS. 1 and 2, as described in the section (Problems to be Solved by the Invention), when sp ² bonds contributing to conductivity are formed between the quantum dots 10 and the wall layer 11. In the energy band structure of the quantum well portion, an energy level gap is generated between the wall layer 11 and the quantum dot 10, thereby degrading the electron confinement characteristics.

Therefore, in the solar cell of the present invention, as shown in FIG. 8, in the quantum well portion 6, the sp ² bond prevention layer 12 is provided between the quantum dots 10 and the wall layer 11. The sp ² bond prevention layer 12 is configured by adding a functional group, hydrogen, or a halogen element to carbon atoms constituting the quantum dot 10 or the wall layer 11 or both. As the functional group, an alkyl group containing a methyl group, an ethyl group, a propyl group, or the like can be used. As the halogen element, fluorine, chlorine, bromine, or the like can be used.

9 and 10 show the energy band structure of the quantum well portion 6 in the case of the structure shown in FIG. These energy band structures are shown for the cross section C in FIG. In the quantum well portion 6 having the structure shown in FIG. 8, since the sp ² bond prevention structure 12 is provided between the quantum dot 10 and the wall layer 11, the sp ² bond is suppressed at the interface. As shown in FIG. 9, the energy level of the quantum dot 10 is flat. Alternatively, when conductivity is suppressed in the peripheral portion of the quantum dot 10 as compared with the central portion, a level that increases the energy level may be formed in the peripheral portion of the quantum dot 10 as shown in FIG.

Thus, by providing the sp ² bond prevention structure 12 between the quantum dot 10 and the wall layer 11, an energy level gap that increases electron recombination loss is not generated between the quantum dot 10 and the wall layer 11. Therefore, the original effect of the quantum well portion 6 is exhibited, and a solar cell having high light receiving sensitivity on the long wavelength side can be obtained.

  Below, the specific structure of each part is shown about the solar cell of the structure shown to FIG. C represents the carrier concentration.

Multilayer light control thin film 1
_{Material: MgF 2, ZnS, SiO 2} , TiO 2 , etc.
Light transmissive substrate 2
Material: Quartz, glass, plastic, etc. Thickness: about 0.2-5mm Size: about 1x1cm-100x100cm
Transparent electrode 3
Material: ITO, SnO ₂ etc. Thickness: 0.01-2 μm
Semiconductor layer 4
Material: Mainly composed of carbon.
Additional element: 0.01% (atomic%, the same applies hereinafter) to 30% of hydrogen or halogen element. Si and Ge are added as necessary.
n ⁺ layer 5 : thickness 0.02-1 μm, C = 1 × 10 ^{17 −1} × 10 ²⁰ cm ⁻³ , band gap 1.4 eV (about 0.2-2.0 eV)
Quantum well 6
(A) Wall layer 11 : Crystalline n ^- layer (C = 1 × 10 ¹² to 1 × 10 ¹⁵ cm ⁻³ ), sp ² / sp ³ bond ratio 0.1 to 2.0, band gap 1.4 eV ( 0.2 ~ 2.0eV)
(B) Quantum dot 10 : carbon nanotube, diameter 3 nm, length about 20 nm, band gap 1.1 eV (about 0.15 to 1.8 eV) (about 50 to 95% of the wall layer), quantum dot 10 and wall layer 11, an sp ² bond prevention layer is formed.
p layer 7 : thickness 1-30 μm, C = 1 × 10 ^{14 −1} × 10 ¹⁸ cm ⁻³ , band gap 1.7 eV (about 0.4 to 2.4 eV)
p ⁺ layer 8 : thickness 0.02-1 μm, C = 1 × 10 ¹⁷ to 1 × 10 ²⁰ cm ⁻³ , band gap 1.7 eV (about 0.4 to 2.4 eV)
Back electrode 9
Material: Al, Ag, Ti, Cu, Ni, Cr, etc. Thickness: 0.2-5 μm

Note that the combination of the p layer and the n layer in the above structure is not limited to this, and a p ⁺ layer may be provided on the light receiving surface side.

As described above, in the solar cell having the above-described structure, the sp ² bond prevention layer 12 includes 1) a functional group or the like bonded to the carbon atom of the quantum dot 10 and 2) a wall layer adjacent to the quantum dot. There are cases where a carbon atom is combined with a functional group or the like, and 3) a case where it is provided on both the quantum dot side and the wall layer side. In addition, as a substance to be bonded, a functional group, hydrogen, or a halogen element can be used. Specific examples and effects in each case will be described below.

When the sp ² bond prevention layer 12 is provided on the quantum dot side, hydrogen is bonded to the surface of the carbon nanotube or the like, but is easily detached from the carbon nanotube when the quantum dot 10 is formed. When hydrogen is desorbed, the remaining carbon atoms are sp ² bonded to the carbon atoms on the wall layer side. In order to prevent this, hydrogen on the surface of the carbon nanotube is substituted with an alkyl group or a halogen element to suppress sp ² bond formation. Alternatively, hydrogen is positively added during the generation of the quantum dots 10 to replenish the desorbed hydrogen and suppress sp ² bond generation. With this structure, the effect of reducing sp ² bonds can be easily obtained.

The sp ² bond preventing layer 12 on the carbon atoms of the portion in contact with the quantum dots 10 in the case wall layer 11 side provided on the wall layer side, a functional group, hydrogen, by coupling one of the halogen element sp ² bond preventing layer 12 Configure. In this case, there is an advantage that changes in physical properties such as band gap increase / decrease of the quantum dots 10 can be reduced.

When the sp ² bond prevention layer 12 is provided on both the quantum dot side and the wall layer side , one of a functional group, hydrogen, and a halogen element is applied to the carbon atom on the wall layer 11 side and the carbon atom on the surface of the quantum dot 10. The sp ² bond can be further reduced by forming the sp ² bond prevention layer 12 by bonding the.

Example 1
(A) Wall layer 11: Crystalline n ⁻ layer (C = 1 × 10 ¹² to 1 × 10 ¹⁵ cm ⁻³ ), sp ² / sp ³ bond ratio 0.1 to 2.0, band gap 1.4 eV ( 0.2 to 2.0 eV), a functional group is added to the interface side in contact with the quantum dots.
(B) Quantum dot: carbon nanotube, diameter 3 nm, length about 20 nm, band gap 1.1 eV (about 0.15 to 1.8 eV) (about 50 to 95% of the wall layer), hydrogen or halogen around the carbon nanotube Add elements.

When an alkyl group is used as a functional group for constituting the sp ² bond prevention layer 12, the alkyl group is composed of sp ³ bonds, and has an effect of preventing the wall layer and quantum dots from contacting and bonding. As a result, sp ² bond generation between the wall layer and the quantum dot interface is suppressed, and the quantum well portion exhibits its original effect and improves the photoelectric conversion efficiency of the solar cell.

Example 2
(A) Wall layer 11: Crystalline n ⁻ layer (C = 1 × 10 ¹² to 1 × 10 ¹⁵ cm ⁻³ ), sp ² / sp ³ bond ratio 0.1 to 2.0, band gap 1.4 eV ( 0.2 to 2.0 eV).
(B) Quantum dot: carbon nanotube, diameter 3 nm, length about 20 nm, band gap 1.1 eV (about 0.15-1.8 eV) (about 50-95% of the wall layer), methyl group around carbon nanotube ( adding -CH _3).

When hydrogen or a halogen element is used to form the sp ² bond prevention layer 12, hydrogen and a halogen element are added to the carbon atoms contained in the wall layer side or the quantum dot surface, or both, so that the wall layer and the quantum element are added. This has the effect of preventing the dots from coming into contact with each other. Further, hydrogen and halogen elements hardly change the energy level of the quantum dots, and do not function as elements that increase defects even with respect to the carbon thin film constituting the wall layer.

Example 3
(A) Wall layer 11: Crystalline n ⁻ layer (C = 1 × 10 ¹² to 1 × 10 ¹⁵ cm ⁻³ ), sp ² / sp ³ bond ratio 0.1 to 2.0, band gap 1.4 eV ( 0.2 to 2.0 eV).
(B) Quantum dot: carbon nanotube, diameter 3 nm, length about 20 nm, band gap 1.1 eV (about 0.15 to 1.8 eV) (about 50 to 95% of the wall layer), fluorine around the carbon nanotube (- F) is added.

When an alkyl group having the number of functional groups added to form the sp ² bond prevention layer 12 is added to the surface of the carbon nanotube, sp ² bond generation can be suppressed. However, if the number of functional groups added to the number of atoms on the carbon nanotube surface is increased, the electron confinement effect as a quantum dot is impaired. Therefore, in order to suppress sp ² bond generation and maintain the quantum effect, the number of functional groups to be added is set in the range of 5 to 50% of the number of atoms on the surface of the carbon molecule constituting the quantum dot. For example, the number of functional groups to be added is reduced to 30% to 50% for small functional groups such as methyl groups and 25% to 15% as ethyl groups and propyl groups increase. Thereby, it is possible to achieve both the sp2 bond generation suppression effect and the maintenance of the quantum effect.

Example 4
(B) Quantum dot: carbon nanotube, diameter 3 nm, length about 20 nm, band gap 1.1 eV (about 0.15-1.8 eV) (about 50-95% of the wall layer), methyl group around carbon nanotube ( the -CH _3), added to 35% of the outer carbon elements constituting the carbon nanotubes.

The number of bonds of hydrogen or halogen element added to form the sp ² bond prevention layer 12 When hydrogen or halogen element is added to the surface of the carbon nanotube, sp ² bond generation can be suppressed. However, when the number of halogen elements added to the number of atoms on the carbon nanotube surface is increased, the electron confinement effect as quantum dots is impaired. Therefore, in order to suppress sp ² bond generation and maintain the quantum effect, the halogen element to be added is set to a range of 10 to 90% of the number of atoms on the surface of the carbon molecule constituting the quantum dot. For example, the number of elements to be added is reduced to 50 to 90% for light elements such as fluorine and 40% to 30% as chlorine and bromine increase. Thereby, it is possible to achieve both the sp2 bond generation suppression effect and the maintenance of the quantum effect.

Example 5
(B) Quantum dot: carbon nanotube, diameter 3 nm, length about 20 nm, band gap 1.1 eV (about 0.15 to 1.8 eV) (about 50 to 95% of the wall layer), fluorine around the carbon nanotube (- F) is added to 70% of the peripheral carbon elements constituting the carbon nanotube.

The figure which shows the structure of the quantum well part which does not apply this invention. The figure which shows the energy band structure of the quantum well part shown in FIG. Sectional drawing which shows the structure of the solar cell concerning one Embodiment of this invention. Sectional drawing which shows the structure of the quantum well part of the solar cell shown in FIG. The figure which shows the structure of a carbon nanotube typically. The figure which shows the structure of fullerene typically. The figure which shows typically the energy band structure of the solar cell shown in FIG. FIG. 5 is a partially enlarged view of the quantum well portion shown in FIG. 4. The figure which shows an example of the energy band structure of the quantum well part shown in FIG. The figure which shows the other example of the energy band structure of the quantum well part shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Multilayer light control thin film 2 Optically transparent substrate 3 Transparent electrode 4 Semiconductor layer 5 n ⁺ layer 6 Quantum well part 7 p layer 8 p ⁺ layer 9 Back surface electrode 10 Quantum dot 11 Wall layer 12 sp ² Bond formation prevention layer

Claims (10)

A first conductivity type first semiconductor layer made of carbon; and
A second semiconductor layer of a second conductivity type made of carbon and having a polarity opposite to that of the first conductivity type;
In a solar cell including a quantum well portion made of carbon formed between the first and second semiconductor layers,
The quantum well portion includes a wall layer formed of a carbon semiconductor thin film, a plurality of quantum dots embedded in the wall layer, and an sp ² bond prevention layer provided in the peripheral portion of the quantum dot. A solar cell.   2. The solar cell according to claim 1, wherein the quantum dots include carbon cylindrical molecules or spherical molecules. 3. 3. The solar cell according to claim 1, wherein the sp ² bond prevention layer is configured by bonding a functional group to a carbon atom on the surface of the quantum dot. 4.   4. The solar cell according to claim 3, wherein the functional group is an alkyl group.   5. The solar cell according to claim 3, wherein the number of functional groups is in the range of 5 to 50% of the number of carbon atoms on the surface of a cylindrical or spherical molecule of carbon constituting the quantum dot. Solar cell. 3. The solar cell according to claim 1, wherein the sp ² bond prevention layer is configured by bonding hydrogen or a halogen element to a carbon atom on the surface of the quantum dot. 7. The solar cell according to claim 6, wherein when a halogen element is used in the sp ² bond prevention layer, the number of halogen elements is 10 of the number of carbon atoms on the surface of a cylindrical or spherical molecule of carbon constituting the quantum dot. A solar cell characterized by being in a range of ˜90%. 3. The solar cell according to claim 1, wherein the sp ² bond prevention layer is configured by bonding any one of a functional group, hydrogen, and a halogen element to a carbon atom of a portion of the wall layer adjacent to the quantum dot. A solar cell characterized by the above. 3. The solar cell according to claim 1, wherein the sp ² bond prevention layer is obtained by bonding any one of a functional group, hydrogen, and a halogen element to a carbon atom of a portion of the wall layer adjacent to the quantum dot. And a solar cell comprising a carbon atom on the surface of the quantum dot bonded to any one of a functional group, hydrogen, and a halogen element.   The solar cell according to claim 8 or 9, wherein the functional group is an alkyl group.

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