JP2006114815A - Solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To increase the amount of formation of carrier which contributes to a photovoltaic power in such a manner that a photodetecting layer in a solar cell can absorb a light of energy smaller than an energy band gap E0, or namely, to improve conversion efficiency. <P>SOLUTION: The solar cell is configured by a pin structure, includes a quantum dot D having a three-dimensional quantum closing action in the i layer 3 of the photodetecting layer so that the energy band structure of the quantum dot D and the barrier layer surrounding it form a type II. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a solar cell that is highly efficient by forming quantum dots in a light detection layer.

  Currently, bulk or quantum wells such as Si and GaAs are used as semiconductor solar cells, and the ideal limit of conversion efficiency is about 30 [%]. In order to obtain the required power, the area must be large. It must be expensive.

  Thus, various efforts have been made to improve the efficiency of solar cells. For example, a structure in which quantum dots are introduced into a light detection layer, that is, an active layer is known.

  For example, Patent Document 1 discloses an invention relating to a structure in which quantum dots are formed in a quantum well in an i layer that is an active layer of a pin solar cell, and wavelength light other than the wavelength light to which the quantum well is sensitive. It comes to be sensitive to.

  However, the quantum dot in the invention of Patent Document 1 is a normal type I, and there is no blocking layer around the quantum dot. Expectation is not expected.

  Patent Document 2 discloses an invention related to a structure in which a plurality of quantum dot layers in the i layer, which is an active layer of a pin solar cell, is provided, and the sensitivity is improved by increasing the total number of quantum dot layers. It can be done.

  However, the quantum dot in the invention of Patent Document 2 is also a normal type I, and there is no blocking layer around the quantum dot. Expectation is not expected.

  Furthermore, various developments have been made on the method of forming quantum dots and the structure of the quantum dots. For example, Patent Document 3 discloses that a plurality of extremely thin islands having a height of about 1 [nm] are stacked to form a height. One quantum dot of about 5 [nm] to 30 [nm] is formed, and the composition in the quantum dot is changed by changing the composition of the thin island itself or by changing the stacking number ratio of different thin islands. An invention to control is disclosed.

According to the invention disclosed in Patent Document 3, the composition of quantum dots, that is, the energy band gap can be controlled freely, and the composition of thin islands and the number ratio of different thin islands can be controlled. The required blocking layer can also be formed by changing.
JP 2002-141531 A JP-A-8-264825 Japanese Patent No. 26969206

  In the present invention, the photodetection layer in the solar cell can absorb light having energy smaller than the energy band gap E0 to increase the generation amount of carriers contributing to the photovoltaic power, that is, conversion efficiency. Try to improve.

  In the solar cell according to the present invention, an energy band structure of a quantum dot and a barrier layer surrounding the quantum dot, including a quantum dot having a pin structure and including a quantum dot having a three-dimensional quantum confinement action in an i layer as a light detection layer. Is basically type II.

  By adopting the above means, the i layer, which is a light detection layer in a solar cell having a pin structure, can absorb light having energy smaller than the energy band gap E0, and contributes to photovoltaic power. The amount of carriers generated increases and the conversion efficiency improves.

  A light detection layer in a pin-structure solar cell, that is, an i layer is provided with a fine structure exhibiting a three-dimensional quantum confinement effect represented by quantum dots.

Figure 1 represents the energy band diagrams for explaining an operation example of the solar cell is in Embodiment 1 of the present invention, D1 is the quantum dot region, E _V is the upper end of the valence band, E _C is conducted the lower end of the band, E0 denotes the energy band gap, E ₁ and E ₂ is the light with smaller energy than the energy band gap E0, respectively. The light having energy E ₁ or E ₂ is light having a wavelength that is not absorbed only by the bulk.

  In the solar cell of the present invention, the quantum dots formed in the light detection layer are type II, and conversion efficiency is improved by using this configuration. Incidentally, in a conventional solar cell having quantum dots, all quantum dots are of type I.

See FIG. 1 (A). Assume that light of energy E ₁ is incident on and absorbed by the solar cell having the illustrated energy band structure.

See FIG. 1B. In this state, the holes move out of the quantum dot, so that the electrons stay in the quantum dot for a long time.

See FIG. 1C. Electrons in the quantum dot absorb light of energy E ₂ and are excited by the conduction band of the barrier layer around the quantum dot, resulting in generation of photovoltaic power.

  In general, when carriers are confined in a simple quantum well, that is, in the case of one-dimensional confinement, it is known that carriers in the conduction band fall into the quantum well, which contributes to a decrease in conversion efficiency. .

  However, in the case of a three-dimensional confinement structure such as a quantum dot, the number of carriers that fall from the barrier layer to the quantum dot is reduced due to a phenomenon inherent to the quantum dot called a phonon bottleneck, so that a decrease in conversion efficiency is suppressed.

  Even in the case of the solar cell of the present invention using type II quantum dots, it is possible to enjoy the effect due to the phonon bottleneck, and there are few carriers that fall into the quantum dots from the barrier layer, and thus the photoexcited The carrier can be used effectively.

  FIG. 2 shows an energy band diagram for explaining an example of the operation of the solar cell according to the second embodiment of the present invention. The same symbols as those used in FIG. It has meaning, (A) shows the case of the present invention, and (B) shows the case of the quantum well mentioned for reference.

  As is clear from FIG. 2 (A), the excited carriers in the conduction band jump over without falling into the type II quantum dot region D1, but the carrier falls in the simple quantum well of FIG. 2 (B). End up.

  By the way, it is preferable to prevent the carriers at the lower end of the conduction band or the upper end of the valence band from being excited and fall into the quantum dot, so that a carrier injection blocking layer is formed in a part of the periphery of the quantum dot. It is effective to keep it.

  FIG. 3 shows an energy band diagram for explaining a solar cell which is a third embodiment of the present invention, and in order to facilitate understanding, a quantum dot having a carrier injection blocking layer viewed in a plane is shown. It is added. The same symbols used in FIG. 1 indicate the same parts or have the same meaning.

In the figure, D is a quantum dot, B ₁ is a carrier (in this case, electron) injection blocking layer, B ₂ is a carrier (in this case, hole) injection blocking layer, and B _c is conduction caused by the carrier injection blocking layer B _1. A barrier at the lower end of the band and B _V indicate a barrier at the upper end of the valence band due to the carrier injection blocking layer B ₂ . The quantum dot D exists in the i layer sandwiched between the p layer and the n layer.

As it will be readily Chitoku from the figure, the carrier injection blocking layers B ₁ and B ₂ may be formed on only one side of the quantum dot D, as the method of forming the same, for example, a GaAs _x Sb _1-x Quantum In the case of the dot D, it is achieved by the method disclosed in Patent Document 3, that is, by laminating while gradually changing the composition.

As an example of the composition in that case,
Dot composition: GaAs _x Sb _1-x (x = 0)
Mother composition: GaAs _x Sb _1-x (x = 0.5)
Stopping layer composition: GaAs _x Sb _1-x (x = 1)
It is.

  FIG. 4 shows an energy band diagram for explaining the case where the solar cell of Example 3 is operated, and the same symbols as those used in FIG. 3 indicate the same parts or have the same meanings. Shall.

The energy band diagram shown represents a state in which an electric field generated by the i layer sandwiched between the p layer and the n layer is applied, so that the energy band is inclined and the quantum dot A state is shown in which carriers that try to enter D are bounced back by the barriers B _c and B _V depending on the carrier injection blocking layer B ₁ or B ₂ .

  FIG. 5 is a cutaway side view showing a specific structure of a solar cell according to the present invention. In the figure, 1 is a substrate, 2 is an n layer, 3 is an i layer (photodetection layer), and D is a quantum. Dots, 4 indicate a p-layer, 5 indicates a p-side electrode, and 6 indicates an n-side electrode.

The main data of each part in the illustrated solar cell is illustrated as follows:
(1) Substrate 1
Material: n-GaAs
Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ]
(2) n layer 2
Material: n-GaAs
Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ]
Thickness: 100 [nm]
(3) i layer (light detection layer) 3
Base material: GaAs
Quantum dot: GaSb
Base material (GaAs barrier layer) thickness: 20 [nm] to 50 [nm]
Number of layers: 50 layers (4) p layer 4
Material: p-GaAs
Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ]
Thickness: 100 [nm]
It is.

  When growing each layer, the MBE (molecular beam epitaxy) method using a solid source is applied, and the growth temperature when growing the i layer 3 which is a quantum dot layer is 450 [° C.] to 550 [° C.], and The growth temperature of the other layers was 550 [° C.] to 650 [° C.].

  The conversion efficiency in the solar cell was calculated for AM1.5. In this case, quantum dots D made of GaSb are grown so as to be non-uniform in size, the variation is 0.94 [μm] to 1.12 [μm], and the base material of the i layer 3 is as described above. It is assumed that the absorption coefficient of the base material and the quantum dots D is sufficiently large.

  The conversion efficiency is 46 [%] when all the carriers generated by the absorption in the quantum dot D go out of the barrier due to the absorption of other infrared light, and the conversion efficiency of the solar cell in the GaAs bulk Is calculated as 36 [%], so that the efficiency is improved by 10 [%].

In each of the embodiments described above, a compound semiconductor is used as a constituent material of the solar cell, but it can be replaced with a Si semiconductor.
(1) Substrate 1
Material: n-Si
Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ]
(2) n layer 2
Material: n-Si
Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ]
Thickness: 100 [nm]
(3) i layer (light detection layer) 3
Quantum dot: SiGe
Si barrier layer thickness: 20 [nm] to 50 [nm]
Number of layers: 50 layers (4) p layer 4
Material: p-Si
Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ]
Thickness: 100 [nm]
Can be.

  In the present invention, the present invention can be implemented in many forms including the above-described embodiment, which will be exemplified below as supplementary notes.

(Appendix 1)
A solar cell comprising a pin structure and including a quantum dot having a three-dimensional quantum confinement action in an i layer as a light detection layer, and an energy band structure of the quantum dot and a barrier layer surrounding the quantum dot forming type II battery.

(Appendix 2)
The i layer, which is a light detection layer,
The energy band structure has no or few discontinuities at the bottom of the conduction band and produces a deep quantum well at the top of the valence band;
The solar cell according to (Appendix 1), which performs three-dimensional quantum confinement only for holes.

(Appendix 3)
The i layer, which is a light detection layer,
The energy band structure is either discontinuous or slightly discontinuous at the top of the valence band and produces a deep quantum well at the bottom of the conduction band;
The solar cell according to (Appendix 1), which performs three-dimensional quantum confinement only for electrons.

(Appendix 4)
The quantum dot material and the barrier layer material in the i layer which is the light detection layer are GaAs _x Sb _{1 -x} / GaAs _y Sb _{1 -y} (x ≠ y)
The solar cell according to (Appendix 2), characterized by comprising a combination of the following.

(Appendix 5)
The quantum dot material and the barrier layer material in the i layer which is the light detection layer are Si _x Ge _1-x / Si _y Ge _1-y (x ≠ y)
The solar cell according to (Appendix 2), characterized by comprising a combination of the following.

(Appendix 6)
The solar cell according to (Appendix 2), wherein a carrier injection blocking layer serving as a barrier for blocking the injection of holes into the quantum dots is formed on the n layer side of the quantum dots.

(Appendix 7)
The solar cell according to (Appendix 3), wherein a carrier injection blocking layer serving as a barrier for blocking injection of electrons into the quantum dots is formed on the p layer side of the quantum dots.

(Appendix 8)
The solar cell according to any one of (Appendix 1) to (Appendix 7), wherein the size of the quantum dots is non-uniform.

It is an energy band diagram for demonstrating the operation example of the solar cell which is Example 1 in this invention. It is an energy band diagram for demonstrating the operation example of the solar cell which is Example 2 in this invention. It is an energy band diagram for demonstrating the solar cell which is Example 3 in this invention. It is an energy band diagram for demonstrating the case where the solar cell which is Example 3 is operated. It is a principal part cutting side view showing the specific structure of the solar cell by this invention.

Explanation of symbols

1 substrate 2 n layer 3 i layer (light detection layer)
D quantum dot 4 p layer 5 p side electrode 6 n side electrode

Claims (5)

A solar cell comprising a pin structure and including a quantum dot having a three-dimensional quantum confinement action in an i layer as a light detection layer, wherein the energy band structure of the quantum dot and the barrier layer surrounding the quantum dot constitutes type II . The i layer, which is a light detection layer,
The energy band structure has no or few discontinuities at the bottom of the conduction band and produces a deep quantum well at the top of the valence band;
The solar cell according to claim 1, wherein the solar cell performs three-dimensional quantum confinement only for holes. The i layer, which is a light detection layer,
The energy band structure is either discontinuous or slightly discontinuous at the top of the valence band and produces a deep quantum well at the bottom of the conduction band;
The solar cell according to claim 1, wherein the solar cell performs three-dimensional quantum confinement only for electrons. 3. The solar cell according to claim 2, wherein a carrier injection blocking layer serving as a barrier for blocking the injection of holes into the quantum dots is formed on the n layer side of the quantum dots. The solar cell according to claim 3, wherein a carrier injection blocking layer serving as a barrier for blocking injection of electrons into the quantum dots is formed on the p-layer side of the quantum dots.

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