JP5509059B2 - Solar cell - Google Patents

Description

  The present invention relates to a solar cell having a superlattice structure.

In recent years, photovoltaic devices have attracted attention as a clean energy source that does not emit CO ₂ , and the spread thereof is progressing. Among these photovoltaic elements, the currently most popular photovoltaic element is a single-junction solar cell using silicon. However, the energy conversion efficiency is approaching the Shockley-Quisser theoretical limit value (hereinafter referred to as the SQ theoretical limit). For this reason, development of third generation solar cells exceeding the SQ theoretical limit has been carried out.

  As this third generation solar cell, intermediate-band solar cells in which intermediate bands or localized levels (which may be called minibands from the viewpoint of quantum structure) are formed in a forbidden band. Has been proposed. In the intermediate band solar cell, the intermediate band is formed in the forbidden band of the base semiconductor, thereby enabling the excitation of electrons from the valence band to the intermediate band and the excitation of electrons from the intermediate band to the conduction band. Light with energy smaller than the forbidden band width of semiconductors can be absorbed. For this reason, the intermediate band solar cell is expected to have high energy conversion efficiency.

For example, in an intermediate band solar cell model, it has been reported that the non-light-collecting energy conversion efficiency is about 46% (see Non-Patent Document 1).
Further, an intermediate band solar cell having a tunnel barrier and having a plurality of quantum dots embedded in an inorganic matrix and an intermediate band solar cell having a quantum dot embedded in an energy enclosure barrier are known (Patent Document 1 and 2).
Non-Patent Document 2 shows a model of an intermediate band solar cell using a plurality of intermediate bands in order to explain the phenomenon of an intermediate band solar cell made of InGaAs.

JP-T 2009-520357 Special table 2010-509772

PHYSICAL REVIEW LETTERS, 97, 247701, 2006 APPLIED PHYSICS LETTERS, 96, 013501, 2010 APPLIED PHYSICS LETTERS 96, 203507, 2010

  However, the energy conversion efficiency of the intermediate band solar cell is not always sufficient. For this reason, a solar cell with higher energy conversion efficiency is desired.

  This invention is made | formed in view of such a situation, and provides the solar cell with higher energy conversion efficiency.

  The present invention includes a p-type semiconductor layer, an n-type semiconductor layer, and a light-absorbing semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer, and the number of the light-absorbing semiconductor layers is at least two. Provided is a solar cell comprising the above energy level in a forbidden band of the light absorbing semiconductor layer.

The inventors of the present invention have proposed that a plurality of energy levels be formed in a forbidden band of the light absorbing semiconductor layer in the light absorbing semiconductor layer sandwiched between the p type semiconductor layer and the n type semiconductor layer. And we repeated examination. As a result, it has been found that a superlattice semiconductor layer having at least two energy levels has higher energy conversion efficiency than a conventional light-absorbing semiconductor layer having a single energy level.
According to this invention, a solar cell with higher energy conversion efficiency is provided.

It is a schematic sectional drawing which shows the structure of the solar cell which concerns on one Embodiment of this invention. It is a band figure in case the superlattice semiconductor layer concerning one Embodiment of this invention has four intermediate | middle bands. In FIG. 2, (1) is a diagram for explaining the positional relationship of the six levels, and (2) is a diagram for explaining the relationship between each level and the carrier generation rate G and the luminescence recombination R therebetween. It is. It is a band figure of the superlattice semiconductor layer concerning a comparative example. 3, (1) is a diagram for explaining the positional relationship between the energy levels of the comparative example, and (2) is a diagram for explaining the relationship between the level, the carrier generation rate G and the luminescence recombination R therebetween. FIG. It is a graph which shows the relationship between the band gap and the maximum energy conversion efficiency in the case of non-condensing obtained by the simulation of Experiment 1. It is a graph which shows the relationship between the band gap at the time of condensing and the maximum energy conversion efficiency obtained by simulation of Experiment 1. FIG. It is a band figure in case the superlattice semiconductor layer concerning one Embodiment of this invention has three intermediate | middle bands. In FIG. 6, (1) is a diagram for explaining the positional relationship of the five levels, and (2) is a diagram for explaining the relationship between each level and the carrier generation speed G and the luminescence recombination R therebetween. It is. It is a graph which shows the relationship between the band gap and the maximum energy conversion efficiency in the case of non-condensing obtained by the simulation of Experiment 2. It is a graph which shows the relationship between the band gap at the time of condensing and the maximum energy conversion efficiency obtained by simulation of Experiment 2. FIG. It is a band figure in case the superlattice semiconductor layer concerning one Embodiment of this invention has two intermediate | middle bands. In FIG. 9, (1) is a diagram for explaining the positional relationship between the four levels, and (2) is a diagram for explaining the relationship between each level and the carrier generation speed G and the luminescence recombination R therebetween. It is. It is a graph which shows the relationship between the band gap and the maximum energy conversion efficiency in the case of non-condensing obtained by the simulation of Experiment 3. It is a graph which shows the relationship between the band gap in the case of condensing and the maximum energy conversion efficiency obtained by simulation of Experiment 3. FIG. It is a figure which shows the band structure calculation result of the 4 level intermediate | middle band solar cell obtained by the simulation of Experiment 4. FIG. In this solar cell, the valence band offset between the quantum dot layer and the barrier layer is zero, and the quantum dot layer and the barrier layer are composed of InAs _0.7 Sb _0.3 and AlSb, respectively. It is a figure which shows the result of having simulated the relationship between a voltage and an electric current when light is irradiated to the solar cell in FIG. It is a figure which shows the band structure calculation result of the 5 level intermediate | middle band solar cell obtained by the simulation of Experiment 4. FIG. In this solar cell, the valence band offset between the quantum dot layer and the barrier layer is zero, and the quantum dot layer and the barrier layer are composed of InAs _0.7 Sb _0.3 and AlSb, respectively. It is a figure which shows the result of having simulated the relationship between a voltage and an electric current when light is irradiated to the solar cell in FIG. It is a figure which shows the band structure calculation result of the 6 level intermediate | middle band solar cell obtained by the simulation of Experiment 4. FIG. In this solar cell, the valence band offset between the quantum dot layer and the barrier layer is zero, and the quantum dot layer and the barrier layer are composed of InAs _0.7 Sb _0.3 and AlSb, respectively. It is a figure which shows the result of having simulated the relationship between a voltage and an electric current when light is irradiated to the solar cell in FIG. It is a figure which shows the band structure calculation result of the 4 level intermediate | middle band solar cell obtained by the simulation of Experiment 4. FIG. In this solar cell, the valence band offset of the quantum dot layer and the barrier layer is not zero, and the quantum dot layer and the barrier layer are composed of InAs and AlSb _0.5 As _0.5 , respectively. It is a figure which shows the result of having simulated the relationship between a voltage and an electric current when light is irradiated to the solar cell in FIG. It is a figure which shows the band structure calculation result of the 5 level intermediate | middle band solar cell obtained by the simulation of Experiment 4. FIG. In this solar cell, the valence band offset of the quantum dot layer and the barrier layer is not zero, and the quantum dot layer and the barrier layer are composed of InAs and AlSb _0.5 As _0.5 , respectively. It is a figure which shows the result of having simulated the relationship between a voltage and an electric current when light is irradiated to the solar cell in FIG. It is a figure which shows the band structure calculation result of the 6 level intermediate | middle band solar cell obtained by the simulation of Experiment 4. FIG. In this solar cell, the valence band offset of the quantum dot layer and the barrier layer is not zero, and the quantum dot layer and the barrier layer are composed of InAs and AlSb _0.5 As _0.5 , respectively. It is a figure which shows the result of having simulated the relationship between a voltage and an electric current when light is irradiated to the solar cell in FIG.

The solar cell of the present invention includes a p-type semiconductor layer, an n-type semiconductor layer, and a light-absorbing semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer, It is characterized in that at least two energy levels are provided in the forbidden band of the light absorbing semiconductor layer.
Here, the light absorbing semiconductor layer refers to a layer formed of a semiconductor that absorbs light, for example, a superlattice semiconductor layer. The energy level may be a quantum level or an intermediate band.

Therefore, in an embodiment of the present invention, the superlattice semiconductor layer includes a p-type semiconductor layer, an n-type semiconductor layer, and a superlattice semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer. May be a solar cell having a superlattice structure in which barrier layers and quantum layers are alternately and repeatedly stacked, and having at least two or more energy levels in the forbidden band of the superlattice semiconductor layer.
In another embodiment of the present invention, the superlattice semiconductor layer includes a p-type semiconductor layer, an n-type semiconductor layer, and a superlattice semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer. Has a superlattice structure in which a barrier layer and a quantum dot layer made of quantum dots are alternately and repeatedly stacked, and the quantum level of the quantum dot is either the conduction band or the valence band of the quantum dot. At least two solar cells may be formed in the forbidden band of the barrier layer.
In another embodiment of the present invention, the superlattice semiconductor layer includes a p-type semiconductor layer, an n-type semiconductor layer, and a superlattice semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer. Has a superlattice structure in which barrier layers and quantum dot layers composed of quantum dots are alternately and repeatedly stacked, and the superlattice structure electronically couples the wave function between the quantum dots, so that at least two The solar cell may be characterized in that the intermediate band described above is formed in either the conduction band or valence band of the quantum dot and the forbidden band of the barrier layer.
Hereinafter, an embodiment in which the intermediate band is formed will be described as an example.

In the embodiment of the present invention, for example, an InAs _x Sb _1-x quantum dot layer or an AlSb barrier layer can be used for the superlattice semiconductor layer of the solar cell. Further, it is possible to use an InAs quantum dot layer / AlSb _y As _1-y barrier layers. The InAs _x Sb _1-x quantum dot layer and the AlSb barrier layer have the lattice constant adjusted to that of AlSb by appropriately changing the element ratio x, or the valence band energy offset (the valence band between the quantum dot layer and the barrier layer). This is preferable in that the (energy difference) can be reduced to zero.

Here, the superlattice structure is a structure in which a barrier layer and a well layer (quantum well layer or quantum dot layer), both of which are made of semiconductors and having different band gaps, are repeatedly stacked, and the electron wave function of the well layer is adjacent. A structure that interacts with the wave function of a well. The well layer is also referred to as a quantum layer.
A quantum dot is a semiconductor fine particle having a particle size of 100 nm or less, and is a fine particle surrounded by a semiconductor having a larger band gap than the semiconductor constituting the quantum dot.
A quantum well is a semiconductor thin film having a thickness of 100 nm or less, and is a thin film surrounded by a semiconductor having a larger band gap than the semiconductor constituting the quantum well.
The quantum dot layer is a layer composed of a plurality of quantum dots and is a well layer having a superlattice structure.
The quantum level refers to a discrete energy level of electrons formed from a barrier layer and quantum dots or quantum wells. The quantum level is also referred to as a quantum energy level.
The barrier layer is made of a semiconductor having a larger band gap than the semiconductor constituting the quantum dots, and constitutes a superlattice structure.
The intermediate band refers to a band connected to one formed in the middle of the forbidden band in the semiconductor constituting the barrier layer.
Note that the electron wave function of the well layer of the superlattice structure interacts with the electron wave function of the adjacent well layer, and a resonant tunneling effect occurs between the quantum levels of the quantum wells, and the quantum levels are connected to one. The formed intermediate band is also called a mini band.

  Further, in the embodiment of the present invention, two intermediate bands may be provided in addition to the configuration of the invention. More specifically, the energy level that forms the bottom of the conduction band of the barrier layer, the energy level that forms the top of the valence band of the barrier layer, and two quantum levels between these levels. The superlattice semiconductor layer may have a level, and the quantum level may form two intermediate bands (hereinafter referred to as a four-level intermediate band solar cell in this specification).

  In this form (four-level intermediate band solar cell), the width of the forbidden band (energy gap) of the barrier layer is preferably 1.0 eV or more, more preferably 1.6 eV or more and 3.5 eV or less. A solar cell having such a forbidden band has higher energy conversion efficiency than a solar cell including a superlattice semiconductor layer having a single intermediate band.

Further, in this embodiment (four-level intermediate band solar cell), the width of the forbidden band of the barrier layer is Eg, and the energy level at the top of the valence band in the barrier layer and the intermediate band has the highest energy level. When the level difference from the low intermediate band is ΔEvi2, the following expression (1) is preferably satisfied.
ΔEvi2 ≧ (Eg / 2) (unit: eV) (1)
More preferably, the following formula (2) is satisfied.
ΔEvi2 ≧ (Eg / 2 + 0.125) (unit: eV) (2)
If the form satisfying such an expression is used to form two intermediate bands using the potential between the conduction band bottom of the quantum dot layer and the conduction band bottom of the barrier layer, the energy level of the lowest intermediate band is formed. Is close to the bottom of the conduction band of the barrier layer, the electron wave function of the quantum dot layer of the superlattice structure greatly interacts with the wave function of the adjacent quantum dot layer. For this reason, it becomes easy to form an intermediate band in which one quantum level is connected, and carrier movement is likely to occur.

Further, in the embodiment of the present invention, in addition to the configuration of the invention, the intermediate band may be at least three or more.
For example, there may be three intermediate bands. More specifically, the energy level that forms the bottom of the conduction band of the barrier layer, the energy level that forms the top of the valence band of the barrier layer, and three quantum levels between these levels. A solar cell in which the superlattice semiconductor layer has a level and the quantum level forms three intermediate bands may be used (hereinafter referred to as a five-level intermediate band solar cell in this specification).

  In this form (five-level intermediate band solar cell), the width of the forbidden band (energy gap) of the barrier layer is preferably 1.1 eV or more, more preferably 1.5 eV or more and 3.8 eV or less. A solar cell having such a forbidden band has higher energy conversion efficiency than a solar cell including a superlattice semiconductor layer having a single intermediate band.

Further, in this embodiment (five-level intermediate band solar cell), the forbidden band width of the barrier layer is Eg, and the energy level at the top of the valence band in the barrier layer and the intermediate band has the highest energy level. When the level difference from the low intermediate band is ΔEvi13, the following expression (3) is preferably satisfied.
ΔEvi13 ≧ (Eg / 2) (unit: eV) (3)
More preferably, the following formula (4) is satisfied.
ΔEvi13 ≧ (Eg / 2 + 0.075) (unit: eV) (4)
In the case of satisfying such a formula, when three intermediate bands are formed using the potential between the conduction band bottom of the quantum dot layer and the conduction band bottom of the barrier layer, the energy level of the lowest intermediate band is formed. Is close to the bottom of the conduction band of the barrier layer, the electron wave function of the quantum dot layer of the superlattice structure greatly interacts with the wave function of the adjacent quantum dot layer. For this reason, it becomes easy to form an intermediate band in which one quantum level is connected, and carrier movement is likely to occur.

Further, in the embodiment of the present invention, in addition to the configuration of the invention, the intermediate band may be at least four or more.
For example, there may be four intermediate bands. More specifically, the energy levels constituting the bottom of the conduction band of the barrier layer, the energy levels constituting the top of the valence band of the barrier layer, and four quantum levels between these levels. A solar cell in which the superlattice semiconductor layer has a level and the quantum level forms four intermediate bands (hereinafter referred to as a six-level intermediate band solar cell in this specification).

  In this form (six-level intermediate band solar cell), the width of the forbidden band (energy gap) of the barrier layer is preferably 1.3 eV or more, more preferably 1.5 eV or more and 3.8 eV or less. A solar cell having such a forbidden band has higher energy conversion efficiency than a solar cell including a superlattice semiconductor layer having a single intermediate band.

Further, in this form (six-level intermediate band solar cell), the width of the forbidden band of the barrier layer is Eg, and the energy level at the top of the valence band in the barrier layer and the intermediate band has the highest energy level. When the level difference from the low intermediate band is ΔEvi24, the following expression (5) is preferably satisfied.
ΔEvi24 ≧ (Eg / 2) (unit: eV) (5)
More preferably, the following expression (6) is satisfied.
ΔEvi24 ≧ (Eg / 2 + 0.05) (unit: eV) (6)
In the case of satisfying such an expression, when the four intermediate bands are formed using the potential between the conduction band bottom of the quantum dot layer and the conduction band bottom of the barrier layer, the energy level of the lowest intermediate band is formed. Is close to the bottom of the conduction band of the barrier layer, the electron wave function of the quantum dot layer of the superlattice structure greatly interacts with the wave function of the adjacent quantum dot layer. For this reason, it becomes easy to form an intermediate band in which one quantum level is connected, and carrier movement is likely to occur.

  In these forms (4 level intermediate band solar cell, 5 level intermediate band solar cell and 6 level intermediate band solar cell), the quantum energy level is the level formed on the conduction band side of the quantum dot, the value Any level of the levels formed on the electron band side may be used, or both levels may be included.

In the embodiment of the present invention, in addition to the configuration of the present invention, the barrier layer may be made of AlSb, and the quantum dot layer may be made of InAs _1-x Sb _x (0 ≦ x ≦ 1). Further, the barrier layer is _{AlSb y As 1-y (0} ≦ y ≦ 1), wherein to the quantum dot layer may be made of InAs, the barrier layer is made of AlAs, the quantum dot layer consists of InAs Also good. The barrier layer may be made of GaN, and the quantum dot layer may be made of In _z Ga _{1 -zN} (0 <z ≦ 1).

  As described above, the intermediate band formed by the structure using the quantum dots has been described as an example. However, these forms are the embodiments of the light absorption semiconductor layer having the energy level described above, and the quantum level is formed. The present invention may be applied to the superlattice semiconductor layer embodiment and the superlattice semiconductor layer embodiment in which the intermediate band is formed.

  Next, this invention is explained in full detail using embodiment shown in drawing. The embodiments and examples described below are merely specific examples of the present invention, and the present invention is not limited thereto.

Embodiment
FIG. 1 is a schematic cross-sectional view showing a configuration of a solar cell according to one embodiment of the present invention. As shown in FIG. 1, a solar cell 20 according to an embodiment of the present invention includes a p-type semiconductor layer 4, an n-type semiconductor layer 12, a superstructure sandwiched between a p-type semiconductor layer 4 and an n-type semiconductor layer 12. The superlattice semiconductor layer 10 has a superlattice structure in which quantum dot layers 6 and barrier layers 8 composed of quantum dots 7 are alternately and repeatedly stacked. Of these configurations, the p-type semiconductor layer 4 is also referred to as a base layer (or p-type base), and the n-type semiconductor layer 12 is also referred to as an emitter layer (or n-type emitter). Therefore, hereinafter, the p-type semiconductor layer 4 is referred to as a base layer (p-type semiconductor layer) 4 and the n-type semiconductor layer 12 is referred to as an emitter-layer (n-type semiconductor layer) 12.

  The solar cell 20 of the present embodiment shown in FIG. 1 further includes a p-type semiconductor substrate 1, a buffer layer 3, a window layer 14, a contact layer 15, a p-type electrode 18, and an n-type electrode 17. Since these are arbitrary constituent elements, they can be appropriately provided according to the usage mode of the solar cell. Therefore, the base layer (p-type semiconductor layer) 4, the emitter (n-type semiconductor layer) 12, and the superlattice semiconductor layer 10 will be described below.

1. p-type semiconductor layer and n-type semiconductor layer (base layer and emitter layer)
The base layer (p-type semiconductor layer) 4 is made of a semiconductor containing a p-type impurity, and can form a pin junction or a pn junction together with the i-type semiconductor layer and the emitter-layer (n-type semiconductor layer) 12.
The emitter layer (n-type semiconductor layer) 12 is made of a semiconductor containing an n-type impurity, and can form a pin junction or a pn junction together with the i-type semiconductor layer and the base layer (p-type semiconductor layer) 4.
When this pin junction or pn junction receives light, an electromotive force is generated. Moreover, the solar cell 20 can output electric power by this.

2. Superlattice Semiconductor Layer The superlattice semiconductor layer 10 is sandwiched between a base layer (p-type semiconductor layer) 4 and an emitter-layer (n-type semiconductor layer) 12, and can constitute a pin junction or a pn junction. The superlattice semiconductor layer 10 has a superlattice structure in which quantum dot layers 6 and barrier layers 8 are alternately and repeatedly stacked. The superlattice semiconductor layer 10 may be an i-type semiconductor, and may be a semiconductor layer containing a p-type impurity or an n-type impurity if an electromotive force is generated by receiving light.

  The superlattice semiconductor layer 10 can have two intermediate bands in the forbidden band of the barrier layer 8 (that is, between the conduction band and the valence band of the barrier layer 8). In the solar cell according to the embodiment of the present invention, the superlattice semiconductor layer 10 may have two or more intermediate bands in the forbidden band of the barrier layer 8, and the position (energy level) where the intermediate band is formed is There is no particular limitation. That is, the position (energy level) may be determined depending on what wavelength light is converted into the solar cell. For example, the position (energy level) in the space solar cell and the ground solar cell is It may be different. For example, the superlattice semiconductor layer 10 may have two intermediate bands on the conduction band side of the quantum dot layer 6 and may have one intermediate band on each of the conduction band side and the valence band side. Two intermediate bands may be provided on the valence band side of the dot layer 6. The superlattice semiconductor layer 10 may have three intermediate bands or four intermediate bands.

  Here, when the energy levels formed by the quantum dots are continuous up to the conduction band or valence band (where continuous means that the energy levels are formed at an energy interval within 25 meV, for example) The beginning of the energy level can be regarded as the lower end of the conduction band and the upper end of the valence band.

The superlattice semiconductor layer 10 having two or more such intermediate bands can be formed, for example, by adjusting the size of the quantum dot layer 6 or the thickness of the quantum dot layer that is a well layer of a superlattice structure. As will be described later in Experiment 4, for example, a barrier layer 8 made of AlSb having a layer thickness of 3.0 nm is formed on a rectangular solid (2.7, 2.7, 9.0) nm (here, parentheses). The numerical value in the figure shows that two intermediate bands are formed in the superlattice semiconductor layer 10 by forming quantum dots having three side lengths of (a, b, c) with InAs _0.7 Sb _0.3. it can. In addition, by forming a rectangular dot of (2.5, 2.5, 8.5) nm in InAs on the barrier layer 8 formed of AlSb _0.5 As _0.5 having a layer thickness of 3.0 nm, Two intermediate bands can be formed in the superlattice semiconductor layer 10.

Further, when the superlattice semiconductor layer 10 has two intermediate bands, the forbidden band width (band gap) of the barrier layer 8 is preferably 1.6 eV to 3.5 eV. With such a forbidden bandwidth, the energy conversion efficiency can be made higher than that of a solar cell having one intermediate band.
Here, the maximum value of the energy conversion efficiency of a solar cell having one intermediate band is 46.7% in the case of non-condensing (the forbidden band width of the barrier layer 8 is 2.4 eV), and in the case of 1000 times condensing And 57.3% (the forbidden band width of the barrier layer 8 is 2.1 eV).

As described in Experiment 4, the superlattice semiconductor layer 10 having such a forbidden band width can be formed by using AlSb as the barrier layer 8 when InAs _0.7 Sb _0.3 is a quantum dot, for example. Further, when InAs is a quantum dot, it can be formed by using AlSb _0.5 As _0.5 as the barrier layer 8. In this way, a superlattice semiconductor layer having a desired forbidden band width can be obtained by selecting a semiconductor material having an appropriate physical property value or adjusting a mixed crystal ratio of semiconductor materials constituting the superlattice semiconductor layer 10. 10 can be formed. The superlattice semiconductor layer 10 having a desired forbidden band width can also be formed by adjusting the size of the quantum dots that are the well layers constituting the superlattice semiconductor layer 10 and the thickness of the barrier layer.

The barrier layer 8 constituting the superlattice semiconductor layer 10 and the material constituting the quantum dot layer 6 are AlSb, InAs _x Sb _1-x , AlSb _x As _1-x , AlAs, GaAs, In _x Ga _1-x As. Can be used. Furthermore, for example, a group IV semiconductor of the periodic table, a compound semiconductor composed of group III and group V, a compound semiconductor composed of group II and group VI, or a mixed crystal material thereof may be used. Further, a chalcopyrite-based material may be used, or a semiconductor other than these may be used. For example, the barrier layer 8 is made of GaNAs, the quantum dot layer 6 is made of InAs, the barrier layer 8 is made of GaP, the quantum dot layer 6 is made of InAs, and the barrier layer 8 is made of GaN. Ga _x In _1-x N for the material, GaSb for the material of the quantum dot layer 6 for the material of the barrier layer 8, AlAs for the material of the barrier layer 8, InAs for the material of the quantum dot layer 6, and the material of the barrier layer 8 Alternatively, CuGaS ₂ and CuInSe ₂ or the like may be used as the material of the quantum dot layer 6.

When the p-type semiconductor substrate 1 is formed of GaAs and the base layer (p-type semiconductor layer) 4 is formed of AlSb _0.5 As _0.5 , as shown in FIG. 1, the superlattice semiconductor layer 10 and the base layer (p-type) are formed. The interface of the semiconductor layer 4 or the base layer (p-type semiconductor layer) 4 is exposed (for example, etching is performed up to the base layer (p-type semiconductor layer)), and the p-type electrode 18 is formed on the exposed surface. Thus, the solar cell 20 can be formed using GaAs for the p-type semiconductor substrate 1 and AlSb _0.5 As _0.5 for the base layer (p-type semiconductor layer) 4.

3. Manufacturing method of solar cell The quantum dot layer or the quantum well layer is formed by a method called electron-lithography or a method called Stranski-Krastanov (SK) growth using a molecular beam epitaxy (MBE) method or a metal organic chemical vapor deposition (MOCVD) method. Quantum dots can be produced by using a technique, a droplet epitaxy method, or the like. The SK growth method can adjust the mixed crystal ratio of the quantum dot or quantum well layer by changing the material composition ratio of the above method, and can change the quantum dot of the quantum dot by changing the raw material, growth temperature, pressure, deposition time, etc. The size or the thickness of the quantum well layer can be adjusted.

  In the production of the solar cell of the present embodiment, for example, a solar cell having a superlattice structure is produced by using a molecular beam epitaxy (MBE) method or a metal organic chemical vapor deposition method (MOCVD) excellent in film thickness control. can do. Here, with reference to FIG. 1, the manufacturing method is demonstrated about one form of the solar cell which has the superlattice structure of FIG. 1 demonstrated above.

For example, the p-GaAs substrate 1 is cleaned with an organic cleaning solution, then etched with a sulfuric acid-based etching solution, washed with running water, and then installed in the MOCVD apparatus. A buffer layer 3 is formed on this substrate. The buffer layer 3 is a layer for improving the crystallinity of the light absorption layer to be formed thereon. Subsequently, a 300 nm AlSb _x As _1-x base layer 4 and an AlSb _x As _1-x layer to be a barrier layer 8 are grown on the buffer layer 3, and then a quantum dot layer made of InAs is formed using a self-organization mechanism. Form.

  The crystal growth of the barrier layer and the quantum dot layer is repeated from the quantum dot layer closest to the p-type semiconductor to the quantum dot layer closest to the n-type semiconductor.

Subsequently, a 250 nm AlSb _x As _1-x layer 12 is crystal-grown to form a pin structure, and then an AlAs layer is formed as the window layer 14.

  Subsequently, a comb-shaped electrode is formed on the contact layer 15 by photolithography and lift-off technology, and the n-type electrode 17 is formed by selectively etching the contact layer 15 using the comb-shaped electrode as a mask. A solar cell can be formed. For example, the p-type electrode 18 can be formed on the base layer 4 by partially etching until reaching the base layer 4.

  Si can be used as the n-type dopant, and Be can be used as the p-type dopant. For example, Au can be used as the electrode material, and the electrode material can be formed by vacuum vapor deposition using a resistance heating vapor deposition method.

  The formed solar cell can confirm whether or not two intermediate bands are formed, for example, by measuring the emission spectrum by PL (photoluminescence) measurement. For example, an Ar laser is used as an excitation light source and a Ge photodetector is used as a detector, and the photoluminescence of the superlattice semiconductor layer 10 is measured at 11K. By obtaining energy (photon energy) corresponding to the emission band of the measured emission spectrum, it is possible to confirm at which level the intermediate band is formed. The forbidden bandwidth of the barrier layer 8 can also be confirmed. Further, the formation of an intermediate band may be confirmed by measuring a light absorption spectrum.

  In addition, the example shown here is an example, each material such as a substrate, a buffer layer, a quantum dot, a dopant, and an electrode used for the solar cell having the superlattice structure of the present embodiment, a cleaning agent used in each process, The substrate processing temperature, manufacturing apparatus, and the like are not limited to the examples shown here.

Simulation experiment [Experiment 1]
A simulation experiment was performed using a detailed equilibrium model to calculate the energy conversion efficiency. In order to explain this calculation method, a band diagram is shown in FIG.

FIG. 2 is a band diagram in the case where the superlattice semiconductor layer according to one embodiment of the present invention has four intermediate bands. That is, this figure shows, as an example, a quasi-intermediate band formed from the lowest (bottom) conduction band, the highest (vertex) valence band, and quantum dots of the barrier layer constituting the superlattice semiconductor layer. A band diagram in the case where the total of the levels is 6 levels (hereinafter referred to as a 6 level semiconductor) is shown. In FIG. 2, (1) is a diagram for explaining the positional relationship of the six levels, and (2) explains the relationship between each level and the carrier generation rate G and the carrier emission recombination rate R between them. FIG.
In addition, the band diagram (energy band diagram) used in this specification is described as conventionally used. That is, the energy level is expressed based on the energy of electrons. Here, the electrons are stable to move to lower energy, and take the energy level of such a state. Furthermore, holes are stable to move to higher energy and take the energy level of such a state.

  In FIG. 2, the energy level at the bottom (bottom) of the conduction band of the semiconductor (barrier layer) serving as the host of the six-level semiconductor is Ec, and the energy level at the top (top) of the valence band is Ev. To do. Further, assume that the intermediate energy level (intermediate band) on the side close to Ec is Ei21, and there are levels of Ei22, Ei23, and Ei24 sequentially from Ei21 toward the Ev side. In this state, let us consider a carrier generation rate for absorbing photons and generating carriers, and luminescence recombination in which electrons and holes recombine to emit light. Here, G represents a carrier generation rate for absorbing photons between these bands to generate carriers, and R represents emission recombination in which electrons and holes recombine to emit light, and a subscript of G or R represents a band where a transition occurs. Represents.

  First, the photon flux included in the energy range from Eini to Efin (Eini <Efin) can be expressed by the following equation (1).

  Here, N represents a photon flux obtained from Planck's radiation law. Further, h represents the Planck constant, c represents the speed of light in vacuum, μ represents the chemical potential of the electron-hole pair, k represents the Boltzmann constant, and T represents the temperature of the substance.

  Next, when this photon flux is used, the carrier generation rate G and the luminescence recombination R between two levels among the six levels (Ec, Ev, Ei21, Ei22, Ei23, and Ei24 levels) are: It can represent with the following formula | equation (2) and Formula (3).

Here, C ₀ is the light collection magnification, H is a geometrically determined constant determined by the distance between the sun and the earth, Ts is the surface temperature of the sun, and T ₀ is the temperature of the solar cell. Eini and Efin represent arbitrary energy satisfying Eini <Efin.

  The current density J taken out from the external electrode connected to the 6-level intermediate band solar cell using these equations can be expressed as the following equation (4). However, since the intermediate band width is very narrow and the energy range in which electrons can transition between the intermediate bands is narrow (the difference between Eini and Efin is small), between the intermediate bands (Ei21, Ei22, Ei23, Ei24, any 2 Electronic transitions between carrier generation and luminescence recombination between two levels are ignored.

  Here, q represents the elementary charge amount. In addition, the subscripts of the carrier generation rate G and the light emission recombination R represent the bands where the transition occurs (two energy levels where the electron transition occurs) as shown in FIG. , The subscript CV represents the electron transition between the energy level Ec and the energy level Ev, the subscript CI1 represents the electron transition between the energy level Ec and the energy level Ei21, and the subscript VI2 Represents a transition of electrons between the energy level Ev and the energy level Ei22. The other subscripts also indicate two energy levels at which electron transitions occur according to similar rules.

  Since no current flows between the intermediate band (the intermediate energy level) and the external electrode, the intermediate band current becomes 0, which can be expressed as the following formulas (5) to (8).

  Here, the subscripts of the carrier generation rate G and the luminescence recombination R indicate two energy levels at which electron transition occurs according to the same rules as in Equation 4.

On the other hand, the solar energy Pin can be expressed as _in the following formula (9).

At this time, when the output voltage is V and the output current is J, the energy conversion efficiency η is expressed by the following equation (10).

  From the above formula, the maximum energy conversion efficiency can be calculated for the 6-level intermediate band solar cell. Although the 6-level intermediate band solar cell has been described, the maximum energy conversion efficiency can be calculated using the same formula for other level intermediate-band solar cells.

  In Experiment 1, the maximum energy conversion efficiency was calculated for the 6-level intermediate band solar cell and the solar cell according to the comparative example by changing the band gap of the barrier layer and the energy level of the intermediate band. FIG. 3 shows a band diagram of a conventional intermediate band solar cell as a comparative example, and FIGS. 4 and 5 show the results of Experiment 1. FIG.

  FIG. 3 is a band diagram of a superlattice semiconductor layer according to a comparative example. That is, this figure shows, as an example, a quasi-intermediate band formed from the lowest (bottom) conduction band, the highest (vertex) valence band, and quantum dots of the barrier layer constituting the superlattice semiconductor layer. 1 shows an example of a band diagram in the case where the total of the levels is 3 levels (the number of intermediate bands is 1) (here, the sun constituted by the superlattice semiconductor layer in which the barrier layer and the quantum dots are at the 3 levels) The battery is hereinafter referred to as a conventional intermediate band solar cell). FIG. 4 is a graph showing the relationship between the band gap and the maximum energy conversion efficiency in the case of non-condensing, obtained by the simulation of Experiment 1. Further, FIG. 5 is a graph showing the relationship between the band gap and the maximum energy conversion efficiency when the light is collected, obtained by simulation of Experiment 1.

Here, in the simulation of Experiment 1, calculation is performed with T _s = 6000K and T ₀ = 300K, and the condensing magnification is calculated for C ₀ in Equations (2) and (9) when C ₀ = 1 and C ₀ = Two patterns in the case of 1000 were used. When C ₀ = 1, “non-condensing” is displayed (FIG. 4), and when C ₀ = 1000 is displayed “1000 times condensing” (FIG. 5).

Referring to FIG. 4, in the case of non-condensing, the 6-level intermediate band solar cell is compared with the conventional intermediate band solar cell when the band gap of the barrier layer (also referred to as a parent semiconductor) is Eg <1.2 eV. It can be seen that there is almost no difference in the energy conversion efficiency.
On the other hand, when the band gap of the barrier layer is Eg ≧ 1.2 eV, the 6-level intermediate band solar cell has a band gap / intermediate band energy level (hereinafter referred to as a combination of these). By optimizing the band lineup, the energy conversion efficiency may be increased (FIG. 4). Here, when the band gap of the barrier layer is Eg = 1.2 eV, the energy level interval between each of the 6 levels and the nearest level can be as narrow as room temperature energy. Considering the ease of controlling the interval, it is more preferable that Eg ≧ 1.3 eV.
Referring to FIG. 4, a 6-level intermediate band solar cell has energy that cannot be obtained by a conventional intermediate band solar cell when the band gap of the barrier layer is in the region of Eg = 1.8 to 3.8 eV. It can be seen that the conversion efficiency is achieved. For example, the maximum energy conversion efficiency of a conventional intermediate band solar cell is about 46.7%, whereas the maximum energy conversion efficiency of a 6-level intermediate band solar cell is that the band gap of the barrier layer is Eg = 2. When it is 6 to 2.7, it is about 56.6%.
Thus, it can be seen from the results of FIG. 4 that by selecting the optimum band lineup, the 6-level intermediate band solar cell achieves energy conversion efficiency that cannot be obtained with the conventional intermediate band solar cell. (The result of FIG. 4 can also be understood by referring to Table 1 described later.)

Referring to FIG. 5, in the case of 1000 times condensing, similarly to the case of non-condensing, the 6-level intermediate band solar cell has a band gap of a barrier layer (also referred to as a parent semiconductor) of Eg <1.1 eV. It can be seen that there is almost no difference in the energy conversion efficiency compared to the conventional intermediate band solar cell.
On the other hand, when the band gap of the barrier layer is Eg ≧ 1.1 eV, by optimizing the band gap and the intermediate band energy level of the 6 level intermediate band solar cell than the conventional intermediate band solar cell, Energy conversion efficiency may be increased (FIG. 5). Here, when the band gap of the barrier layer is 1.1 ≦ Eg ≦ 1.4 eV, the energy level interval between each of the six levels and the closest level can be as narrow as room temperature energy. Considering the ease of controlling the energy level interval, it is more preferable that Eg ≧ 1.5 eV.
Referring to FIG. 5, a 6-level intermediate band solar cell has energy that cannot be obtained by a conventional intermediate band solar cell when the band gap of the barrier layer is in the region of Eg = 1.5 to 3.5 eV. It can be seen that the conversion efficiency is achieved. For example, the maximum energy conversion efficiency of a conventional intermediate band solar cell is about 57.3%, whereas the maximum energy conversion efficiency of a 6 level intermediate band solar cell is that the band gap of the barrier layer is Eg = 2. It is about 67.7% at 4 to 2.5 eV.
In this way, from the result of FIG. 5, by selecting the optimum band lineup even in the case of 1000 times condensing, the energy conversion efficiency that cannot be obtained by the conventional intermediate band solar cell is obtained by the 6 level intermediate band solar cell. Was found to achieve. (The results in FIG. 5 can also be understood by referring to Table 2 described later.)

  The results of Experiment 1 are shown in Tables 1 and 2. Tables 1 and 2 compare the energy conversion efficiency and energy levels of the 6-level intermediate band solar cell and the conventional intermediate band solar cell, and are a table showing a part of the results of Experiment 1. Table 1 shows the case where the condensing condition is “non-condensing”, and Table 2 shows the case where the condensing condition is “1000 times condensing”.

  Here, in Tables 1 and 2, ΔE represents an energy difference between two energy levels (bands). For example, ΔEci21 represents an energy level 70 of Ec and an energy level 61 of Ei21. (See FIG. 2). As described above, the alphanumeric characters described after ΔE indicate two energy levels. Other descriptions such as ΔEii212 and Tables 3 to 6 are also described in the same rule.

Referring to Tables 1 and 2, when compared with Eg of the same size, the optimum band lineup of the 6-level intermediate band solar cell exceeding the conversion efficiency of the conventional intermediate band solar cell is ΔEci21 ≧ 0.05 eV, or ΔEvi24 It can be seen that ≧ 0.05 eV, and that the optimum band lineup is | (ΔEci21−ΔEvi24) | ≧ 0.65 eV. Further, it can be seen that MIN (ΔEii212, ΔEii223, ΔEii234) ≧ 0.10 eV.
Here, MIN (A, B, C) means the smallest numerical value among the numerical values of A, B, and C. Hereinafter, in this specification, MIN (A, B,...) Is used to mean the smallest numerical value in parentheses.

  In addition, referring to Tables 1 and 2, when compared with the maximum conversion efficiency of the conventional intermediate band solar cell, the optimum band that exceeds the maximum conversion efficiency of the conventional intermediate band solar cell for the 6 level intermediate band solar cell. It can be seen that the lineup is ΔEci21 ≧ 0.05 eV or ΔEvi24 ≧ 0.05 eV. It can also be seen that this optimum band lineup satisfies | (ΔEci21−ΔEvi24) | ≧ 0.65 eV. Further, it can be seen that MIN (ΔEii212, ΔEii223, ΔEii234) ≧ 0.125 eV.

For example, when a 6-level intermediate band solar cell is configured by quantum dots and the band offset between the quantum dots and the barrier layer in the valence band is 0, or the quantum level formed in the valence band is one band. When it can be considered (that is, when the four intermediate band levels are produced using the potential due to the conduction band offset), the larger the ΔEvi24 (the smaller the ΔEci21 + ΔEii212 + ΔEii223 + ΔEii234), the lower the intermediate band energy level Ei24 is. Near the bottom of the conduction band of the layer, the electron wave function of the quantum dot layer is likely to interact greatly with the wave function of the adjacent quantum dot layer. Therefore, an intermediate band of Ei21, Ei22, Ei23, and Ei24 is easily formed, and carrier movement is more likely to occur. From this point of view, ΔEvi24 ≧ (Eg / 2) eV is preferable. When Table 1 and Table 2 are examined, the optimum band lineup of the 6-level intermediate band solar cell is ΔEvi24 ≧ (Eg / 2 + 0.05) eV. ing.
For example, in the case of Eg = 2.5 eV in a 6-level intermediate-band solar cell under 1000-fold concentration, the band lineup that provides 67.7% energy conversion efficiency, that is, the combination of ΔEci21 and ΔEvi24 is (ΔEci21, ΔEvi24) = (1.325 eV, 0.575 eV) (0.575 eV, 1.325 eV). These satisfy the above band lineup, and a more preferable combination than the above condition ΔEvi24 ≧ (Eg / 2 + 0.05) eV is (ΔEci21, ΔEvi24) = (0.575 eV, 1.325 eV).

[Experiment 2]
Next, the total of the level of the intermediate band formed from the lowest part (bottom) of the conduction band of the barrier layer constituting the superlattice semiconductor layer, the highest part (vertex) of the valence band, and the quantum dots is five levels. A simulation experiment was performed on the solar cell having the same calculation method as in Experiment 1. In this simulation experiment, several examples of the solar cell having the above five levels were given and the energy conversion efficiency was calculated. The band diagram of the superlattice semiconductor layer of this solar cell is shown in FIG. 6, and the experimental results are shown in FIGS.

  FIG. 6 is a band diagram in the case where the superlattice semiconductor layer according to one embodiment of the present invention has three intermediate bands. That is, this figure shows, as one example, a band diagram in the case where the barrier layer and the quantum dots that constitute the superlattice semiconductor layer have five levels. FIG. 7 is a graph showing the relationship between the band gap and the maximum energy conversion efficiency in the case of non-condensing obtained by the simulation of Experiment 2. Further, FIG. 8 is a graph showing the relationship between the band gap and the maximum energy conversion efficiency when the light is collected, obtained by the simulation of Experiment 2.

In the simulation experiment 2, similarly to the simulation experiment 1, calculated at _{T s = 6000K, T 0 =} 300K, condensing magnification, the C ₀ in the formula (2) (9), the C ₀ = 1 And two patterns of C ₀ = 1000. These indicate “non-condensing” when C ₀ = 1 (FIG. 7) and “1000 times condensing” when C ₀ = 1000 (FIG. 8).

Referring to FIG. 7, in the case of non-condensing, the five-level intermediate band solar cell is compared with the conventional intermediate band solar cell when the band gap of the barrier layer (also referred to as a parent semiconductor) is Eg <1.2 eV. It can be seen that there is almost no difference in the energy conversion efficiency.
On the other hand, when the band gap of the barrier layer is Eg ≧ 1.2 eV, by optimizing the band gap / intermediate band energy level of the 5-level intermediate band solar cell than the conventional intermediate band solar cell, The energy conversion efficiency may be increased (FIG. 7). Here, when the band gap of the barrier layer is Eg = 1.2 eV, the energy level interval between each of the five levels and the closest level can be as narrow as room temperature energy. Considering the ease of controlling the interval, it is more preferable that Eg ≧ 1.3 eV.
Referring to FIG. 7, the five-level intermediate band solar cell has energy that cannot be obtained by a conventional intermediate band solar cell when the band gap of the barrier layer is in the region of Eg = 1.8 to 3.8 eV. It can be seen that the conversion efficiency is achieved. For example, the maximum energy conversion efficiency of the conventional intermediate band solar cell is about 46.7%, whereas the maximum energy conversion efficiency of the five-level intermediate band solar cell is that the band gap of the barrier layer is Eg = 2. When it is 6 to 2.7, it is about 55.4%.
Thus, it can be seen from the results of FIG. 7 that by selecting the optimum band lineup, the five-level intermediate band solar cell achieves energy conversion efficiency that cannot be obtained with the conventional intermediate band solar cell. (The results shown in FIG. 7 can also be understood by referring to Table 3 described later.)

Referring to FIG. 8, in the case of 1000 times condensing, as in the case of non-condensing, the five-level intermediate band solar cell has a band gap of a barrier layer (also referred to as a parent semiconductor) of Eg <1.1 eV. It can be seen that there is almost no difference in the energy conversion efficiency compared to the conventional intermediate band solar cell.
On the other hand, when the band gap of the barrier layer is Eg ≧ 1.1 eV, the 5-level intermediate band solar cell is more optimized than the conventional intermediate band solar cell by optimizing the band gap and the intermediate band energy level. The energy conversion efficiency may increase (FIG. 8).
Referring to FIG. 8, when the band gap of the barrier layer is in the region of Eg = 1.5 to 3.4 eV, the energy of the five level intermediate band solar cell cannot be obtained by the conventional intermediate band solar cell. It can be seen that the conversion efficiency is achieved. For example, the maximum energy conversion efficiency of a conventional intermediate band solar cell is about 57.3%, whereas the maximum energy conversion efficiency of a five-level intermediate band solar cell is that the band gap of the barrier layer is Eg = 2. It is about 66.5% at 3 to 2.4 eV.
Thus, from the result of FIG. 8, even in the case of 1000 times condensing, by selecting the optimum band lineup, the energy conversion efficiency that cannot be obtained by the conventional intermediate band solar cell is obtained by the five-level intermediate band solar. You can see that the battery achieves. (The result of FIG. 8 can also be understood by referring to Table 4 described later.)

  The results of Experiment 2 are shown in Tables 3 and 4. Tables 3 and 4 compare the energy conversion efficiency and energy levels of the five-level intermediate band solar cell and the conventional intermediate band solar cell, and are a table showing a part of the results of Experiment 2. Table 3 shows the case where the condensing condition is “non-condensing”, and Table 4 shows the case where the condensing condition is “1000 times condensing”.

  Referring to Tables 3 and 4, when compared with Eg of the same size, the optimal band lineup of a five-level intermediate band solar cell that exceeds the conversion efficiency of a conventional intermediate band solar cell is ΔEci11 ≧ 0.05 eV, or ΔEvi13 It can be seen that ≧ 0.05 eV. It can also be seen that this optimum band lineup satisfies | (ΔEci11−ΔEvi13) | ≧ 0.625 eV. It can also be seen that MIN (ΔEii112, ΔEi113) ≧ 0.15 eV.

  Also, referring to Tables 3 and 4, when compared with the maximum conversion efficiency of the conventional intermediate band solar cell, the optimum band that exceeds the maximum conversion efficiency of the conventional intermediate band solar cell for the five-level intermediate band solar cell It can be seen that the lineup is ΔEci11 ≧ 0.175 eV or ΔEvi13 ≧ 0.175 eV. It can also be seen that this optimum band lineup satisfies | (ΔEci11−ΔEvi13) | ≧ 0.625 eV. It can also be seen that MIN (ΔEii112, ΔEii113) ≧ 0.175 eV.

For example, when a five-level intermediate band solar cell is configured by quantum dots and the band offset between the quantum dots and the barrier layer in the valence band is zero, or the quantum level formed in the valence band is one band. When it can be considered (that is, when three intermediate band levels are produced using the potential due to the conduction band offset), the larger the ΔEvi13 (the smaller the ΔEci11 + ΔEii112 + ΔEii113), the lower the intermediate band energy level Ei13 is. Near the bottom of the conduction band of the layer, the electron wave function of the quantum dot layer is likely to interact greatly with the wave function of the adjacent quantum dot layer. Therefore, an intermediate band of Ei11, Ei12, and Ei13 is easily formed, and carrier movement is more likely to occur. Examining Table 3 and Table 4 from such a viewpoint, the optimal band lineup of the five-level intermediate band solar cell is preferably ΔEvi13 ≧ (Eg / 2 + 0.075) eV.
For example, in the case of Eg = 2.4 eV in a five-level intermediate band solar cell under 1000 times focusing, the band lineup that provides 63.5% energy conversion efficiency, that is, the combination of ΔEci11 and ΔEvi13 is (ΔEci11, ΔEvi13) = (1.30 eV, 0.575 eV) (0.575 eV, 1.30 eV). This satisfies the band lineup, and a more preferable combination than the above condition ΔEvi13 ≧ (Eg / 2 + 0.075) eV is (ΔEci11, ΔEvi13) = (0.575 eV, 1.30 eV).
[Experiment 3]
Next, the total of the levels of the lowest band (bottom) of the conduction band of the barrier layer constituting the superlattice semiconductor layer, the top (vertex) of the valence band, and the intermediate band formed from the quantum dots is 4 levels. A simulation experiment was performed on the solar cell having the same calculation method as in Experiment 1 and Experiment 2. In this simulation experiment, several examples of the solar cell having the above five levels were given and the energy conversion efficiency was calculated. The band diagram of the superlattice semiconductor layer of this solar cell is shown in FIG. 9, and the experimental results are shown in FIGS.

  FIG. 9 is a band diagram when the superlattice semiconductor layer according to one embodiment of the present invention has two intermediate bands. That is, this figure shows, as an example, a band diagram in the case where the barrier layer and the quantum dot constituting the superlattice semiconductor layer have four levels. FIG. 10 is a graph showing the relationship between the band gap and the maximum energy conversion efficiency in the case of non-condensing, obtained by the simulation of Experiment 3. Further, FIG. 11 is a graph showing the relationship between the band gap and the maximum energy conversion efficiency when the light is collected, obtained by the simulation of Experiment 3.

In the simulation of Experiment 3, similarly to the simulation of Experiment 1 and Experiment 2, calculation is performed with Ts = 6000K and T0 = 300K, and the condensing magnification is C ₀ = 1 for C ₀ in Equation (7) and C ₀ Two patterns were used when ₀ = 1000. In these cases, “Non-condensing” is displayed when C ₀ = 1 (FIG. 10), and “1000 times condensing” is displayed when C ₀ = 1000 (FIG. 11).

Referring to FIG. 10, in the case of non-condensing, the four-level intermediate band solar cell is compared with the conventional intermediate band solar cell when the band gap of the barrier layer (also referred to as a parent semiconductor) is Eg <1.2 eV. It can be seen that there is almost no difference in the energy conversion efficiency.
On the other hand, when the band gap of the barrier layer is Eg ≧ 1.2 eV, the 4-level intermediate band solar cell is more optimized than the conventional intermediate band solar cell by optimizing the band gap and the intermediate band energy level. There is a possibility that the energy conversion efficiency is increased (FIG. 10).
Referring to FIG. 10, a four-level intermediate band solar cell has energy that cannot be obtained by a conventional intermediate band solar cell when the band gap of the barrier layer is in the region of Eg = 1.8 to 3.5 eV. It can be seen that the conversion efficiency is achieved. For example, the maximum energy conversion efficiency of the conventional intermediate band solar cell is about 46.7%, whereas the maximum energy conversion efficiency of the four-level intermediate band solar cell is that the band gap of the barrier layer is Eg = 2. At 6 eV, it is about 53.0%.
Thus, it can be seen from the results of FIG. 10 that, by selecting the optimum band lineup, the 4-level intermediate band solar cell achieves energy conversion efficiency that cannot be obtained with the conventional intermediate band solar cell. (The result of FIG. 10 can also be understood by referring to Table 1 described later.)

Referring to FIG. 11, when the band gap of the barrier layer is Eg ≧ 1.0 eV in the case of 1000 times condensing, the intermediate band energy level of the 4-level intermediate band solar cell is higher than that of the conventional intermediate band solar cell. By optimizing the position, the energy conversion efficiency may be increased (FIG. 11).
Referring to FIG. 11, when the band gap of the barrier layer is in the region of Eg = 1.6 to 3.2 eV, the energy of a four level intermediate band solar cell cannot be obtained by a conventional intermediate band solar cell. It can be seen that the conversion efficiency is achieved. For example, the maximum energy conversion efficiency of the conventional intermediate band solar cell is about 57.3%, whereas the maximum energy conversion efficiency of the four-level intermediate band solar cell is that the band gap of the barrier layer is Eg = 2. At 3 eV, it is about 63.8%.
Thus, from the result of FIG. 11, even in the case of 1000 times condensing, by selecting the optimum band lineup, the energy conversion efficiency that cannot be obtained by the conventional intermediate band solar cell is obtained by the four-level intermediate band solar. You can see that the battery achieves. (The results in FIG. 11 can also be understood by referring to Table 2 described later.)

The results of Experiment 1 are shown in Tables 5 and 6. Tables 5 and 6 compare the energy conversion efficiency and energy levels of the 4-level intermediate band solar cell and the conventional intermediate band solar cell, and are a table showing a part of the results of Experiment 1. Table 5 shows the case where the condensing condition is “non-condensing”, and Table 6 shows the case where the condensing condition is “1000 times condensing”.
Tables 5 and 6 have a column of ΔEci1 (ΔEvi2) and a column of ΔEvi2 (ΔEci1), which indicates that if one is a value of ΔEci1, the other is a value of ΔEvi2, and one is ΔEvi2 Is the value of ΔEci1.

  Referring to Tables 5 and 6, when comparing Eg of the same size, the optimum band lineup of the four-level intermediate band solar cell exceeding the conversion efficiency of the conventional intermediate band solar cell is ΔEci1 ≧ 0.1 eV, or ΔEvi2 It can be seen that ≧ 0.1 eV. It can also be seen that this optimal band lineup satisfies | (ΔEci1−ΔEvi2) | ≧ 0.25 eV. It can also be seen that ΔEii ≧ 0.25 eV.

  In addition, referring to Tables 5 and 6, when comparing the maximum conversion efficiency of the conventional intermediate band solar cell, the four-level intermediate band solar cell is optimal so as to exceed the maximum conversion efficiency of the conventional intermediate band solar cell. It can be seen that the band lineup satisfies ΔEci1 ≧ 0.325 eV or ΔEvi2 ≧ 0.325 eV. It can also be seen that this optimal band lineup satisfies | (ΔEci1−ΔEvi2) | ≧ 0.325 eV. Further, it can be seen that ΔEii ≧ 0.325 eV.

For example, when a four-level intermediate band solar cell is constituted by quantum dots and the band offset between the quantum dots and the barrier layer in the valence band is 0, or the quantum level formed in the valence band is 1 When it can be regarded as one band (that is, when two intermediate band levels are produced using the potential due to the conduction band offset), the higher the ΔEvi2 (the smaller the ΔEci1 + ΔEii), the lowest intermediate band energy level Ei2 Is close to the bottom of the conduction band of the barrier layer, and the electron wave function of the quantum dot layer is likely to interact greatly with the wave function of the adjacent quantum dot layer. Therefore, an intermediate band of Ei1 and Ei2 is easily formed, and carrier movement is more likely to occur. Examining Table 1 and Table 2 from such a viewpoint, the optimum band lineup of the four-level intermediate band solar cell is preferably ΔEvi2 ≧ (Eg / 2 + 0.125) eV.
For example, in the case of Eg = 2.3 eV in a 4-level intermediate band solar cell under 1000 times focusing, the band lineup that results in an energy conversion efficiency of 63.8%, that is, the combination of ΔEci1 and ΔEvi2 is (ΔEci1, ΔEvi2) = (1.30 eV, 0.65 eV) (0.65 eV, 1.30 eV), (1.00 eV, 0.65 eV), (0.65 ev, 1.00 eV). This satisfies the band lineup, and a more preferable combination than the above condition ΔEvi2 ≧ (Eg / 2 + 0.125) eV is (ΔEci1, ΔEvi2) = (0.65 eV, 1.30 eV).

  From the above experiment, it can be seen that the 6-level intermediate band solar cell, the 5-level intermediate band solar cell, and the 4-level intermediate band solar cell have high energy conversion efficiency.

  The energy levels other than Eg in Tables 1 to 6 only show some examples for a certain Eg. That is, other combinations of energy levels other than Eg satisfying the same efficiency for a certain Eg are conceivable from the symmetry of the energy interval. Moreover, Tables 1-6 only showed the combination of the optimal energy level which gives the maximum energy conversion efficiency with respect to a certain Eg, Even if it is a combination other than these, the energy conversion efficiency of the conventional intermediate | middle band solar cell Can be exceeded. Therefore, the technical scope of the present invention is not limited to these examples.

Simulation experiment 2
[Experiment 4]
Next, with respect to the solar cell having the energy level (4 to 6 level intermediate band solar cell, also referred to as multilevel intermediate band solar cell) clarified in Experiments 1 to 3, focusing on a specific structure, A simulation experiment was conducted.

  The band structure calculation was performed by solving the Schrödinger equation using MATLAB software. In this simulation experiment, we focused on “a structure in which the valence band band offset is zero” and “a structure in which the valence band band offset is not zero” as structures that can realize a multilevel intermediate band solar cell. Further, the shape of the quantum dots was considered to be a cube, and the size of the three sides was set to (a, b, c) nm.

1. Structure in which the valence band offset is zero If the combination of InAs _1-x Sb _x / AlSb is used, the valence band offset can be made zero (the valence band offset here is InAs _x Sb _1-x and AlSb valence band peak energy level difference is zero). InAs _1-x Sb _x has the smallest energy difference between the lower end of the conductor at the Γ point and the upper end of the valence band at the Γ point (direct band gap), and AlSb has the lower end of the conduction band at the X point and the valence band at the Γ point. The energy difference at the top is the smallest (indirect band gap). However, in a solar cell, absorption at the Γ point is the most important and dominant, and the band structure at the Γ point is considered below for both InAs _1-x Sb _x and AlSb. Here, the following calculation was performed with x = 0.3 from the Vegard rule. The band gap at the Γ point of AlSb is 2.3 eV.

The results of the band structure calculation relating to the structure in which the valence band offset is zero are shown in FIGS. 12, FIG. 14 and FIG. 16 are diagrams showing the band structure calculation results of the intermediate band solar cell. The multilevel intermediate band solar cells in these figures are a four level intermediate band, a five level intermediate band, and a six level intermediate band in the order of FIGS. In the solar cells in these figures, the valence band offset between the quantum dot layer and the barrier layer is zero, and the quantum dot layer and the barrier layer are composed of InAs _0.7 Sb _0.3 and AlSb, respectively. FIGS. 13, 15 and 17 are diagrams showing the results of simulating the relationship between voltage and current when the solar cells in FIGS. 12, 14 and 16 are irradiated with light, respectively.

In FIG. 12, “InAs _0.7 Sb _0.3 /AlSb=2.7+2.7+9/3 nm” indicates that the quantum dot layer is InAs _0.7 Sb _0.3 and the barrier layer is AlSb. + 2.7 + 9 ”indicates the size of a quantum dot composed of InAs _0.7 Sb _0.3 (when the size of three sides of a rectangular parallelepiped quantum dot is (a, b, c), (Shown as a + b + c)). Further, “3 nm” in the figure indicates the layer thickness of AlSb. Such a notation is the same in FIGS.

(1-1) 4-level intermediate band solar cell InAs _0.7 Sb _0.3 /AlSb=(2.7, 2.7, 9) / 3 nm
As shown in FIG. 12, in the case of such a quantum dot size, each energy level becomes 0 at the upper end of the valence band due to confinement from three directions, and (Ev, Ei2, Ei1, Ec) = (0, 1 .29, 1.64, 2.32) eV. Using these energy levels, the energy conversion efficiency of the four-level intermediate band solar cell was 51.9% in the case of non-condensing, and 63.4% in 1000 times condensing (see FIG. 13).

(1-2) 5-level intermediate band solar cell InAs _0.7 Sb _0.3 /AlSb=(2.7, 2.7, 13) / 3 nm
As shown in FIG. 14, in the case of such a quantum dot size, each energy level becomes 0 at the upper end of the valence band due to confinement from three directions, and (Ev, Ei13, Ei12, Ei11, Ec) = (0 1.25, 1.44, 1.80, 2.3) eV. Using these energy levels, the energy conversion efficiency of the five-level intermediate band solar cell was 52.1% in the case of non-condensing, and 63.6% in 1000 times condensing (see FIG. 15).

(1-3) 6-level intermediate band solar cell InAs _0.7 Sb _0.3 /AlSb=(2.7, 2.7, 17) / 3 nm
As shown in FIG. 16, in the case of such a quantum dot size, each energy level becomes 0 at the upper end of the valence band due to confinement from three directions, and (Ev, Ei24, Ei23, Ei22, Ei21, Ec) = (0, 1.23, 1.35, 1.57, 1.90, 2.3) eV. Using these energy levels, the energy conversion efficiency of the 6-level intermediate band solar cell was 53.2% in the case of non-condensing, and 65.0% in 1000 times condensing (see FIG. 17).

2. Structure in which valence band offset is not zero The combination of InAs / AlSb _1-x As _x cannot make the valence band offset zero. However, the valence band offset is relatively small, and there are heavy holes and light holes in the valence band. Since the effective mass of the heavy holes is relatively large, the valence band has many quantum energy levels. These multiple levels are formed and can be regarded as one valence band. From the upper end of the valence band regarded as one to the lower end of the conduction band can be considered as an effective band gap, a multi-level intermediate band solar cell can be realized.

On the other hand, in this combination as well, InAs has the smallest energy difference between the lower end of the conductor at the Γ point and the upper end of the valence band at the Γ point (direct band gap), and AlSb _1-x As _x is X The energy difference between the lower end of the conduction band at the point and the upper end of the valence band at the Γ point is the smallest (indirect band gap). However, in solar cells, absorption at the Γ point is the most important and dominant, and the band structure at the Γ point is considered below for both InAs and AlSb _1-x As _x . Here, the following calculation was performed with x = 0.5 from the Vegard rule.

The results of the band structure calculation regarding the structure where the valence band offset is not zero are shown in FIGS. 18, FIG. 20 and FIG. 22 are diagrams showing the band structure calculation results of the intermediate band solar cell. The intermediate bands of the solar cell in these figures are a four-level intermediate band, a five-level intermediate band, and a six-level intermediate band in the order of FIGS. 18, 20, and 22. In the solar cells in these figures, the valence band offset of the quantum dot layer and the barrier layer is not zero, and the quantum dot layer and the barrier layer are composed of InAs and AlSb _0.5 As _0.5 , respectively. FIGS. 19, 21 and 23 are diagrams showing the results of simulating the relationship between voltage and current when the solar cells in FIGS. 18, 20 and 22 are irradiated with light, respectively.

(1-1) 4-level intermediate band solar cell InAs / AlSb _0.5 As _0.5 = (2.5, 2.5, 8.5) / 3 nm
As shown in FIG. 18, in the case of such a quantum dot size, each energy level becomes 0 at the upper end of the valence band due to confinement from three directions, and (Ev, Ei2, Ei1, Ec) = (0, 1 .54, 1.90, 2.52) eV. Here, Ev is the energy level at the upper end of the valence band considered as one, Ec is the lower end of the conductor of the barrier layer, and therefore (Ec−Ev) is the band gap of the barrier layer. Absent. Using these energy levels, the energy conversion efficiency of the four-level intermediate band solar cell was 47.7% in the case of non-condensing, and 56.8% in 1000 times condensing (see FIG. 19).

(2-2) 5-level intermediate band solar cell InAs / AlSb _0.5 As _0.5 = (2.7, 2.7, 12) / 3 nm
As shown in FIG. 20, in the case of such a quantum dot size, each energy level becomes 0 at the upper end of the valence band due to confinement from three directions, and (Ev, Ei13, Ei12, Ei11, Ec) = (0 1.41, 1.61, 1.98, 2.50) eV. Using these energy levels, the energy conversion efficiency of the four-level intermediate band solar cell was 51.3% in the case of non-condensing and 61.4% in the case of 1000 times condensing (see FIG. 21).

(2-3) 6-level intermediate band solar cell InAs / AlSb _0.5 As _0.5 = (3.0, 3.0, 15) / 3 nm
As shown in FIG. 22, in the case of such a quantum dot size, each energy level becomes 0 at the upper end of the valence band due to confinement from three directions, and (Ev, Ei24, Ei23, Ei22, Ei21, Ec) = (0, 1.28, 1.42, 1.67, 2.03, 2.49) eV. Using these energy levels, the energy conversion efficiency of the four-level intermediate band solar cell was 52.8% in the case of non-condensing, and 63.4% in the case of 1000 times condensing (see FIG. 23).

  As described above, it can be understood from the results of Experiment 4 that the 4-6 level intermediate band solar cell exhibits high energy efficiency.

  While the present invention has been described with reference to the embodiments, the present invention is not limited to these embodiments.

  For example, a resonant tunneling effect between quantum levels is generated by electronic coupling of wave functions between quantum dots in a superlattice structure, and an intermediate band in which quantum levels are connected to one another is formed from the viewpoint of carrier movement. More preferably, an intermediate band need not necessarily be formed. As shown in Non-Patent Document 3, the quantum energy levels formed from each quantum dot may exist independently without resonating, and even in such a configuration, as an intermediate band solar cell Function. For this reason, the energy level which each existed independently in the superlattice semiconductor layer may be sufficient as the intermediate | middle band of said embodiment (and experiment 1-4).

  In the above-described embodiments (and Experiments 1 to 4), the superlattice structure mainly formed of quantum dots has been described. For example, several kinds of impurities may be contained in an intermediate band or a compound semiconductor material formed by a quantum well structure. The present invention may be applied to an intermediate band (intermediate band using impurities) formed by doping and the present invention is not limited to an intermediate band solar cell using quantum dots.

  As described above, the present invention can be variously modified within the scope of the claims. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

1: p-type semiconductor substrate 3: buffer layer 4: base layer (p-type semiconductor layer) 6: quantum dot layer 7: quantum dot 8: barrier layer 10: superlattice semiconductor layer 12: emitter-layer (n-type semiconductor layer)
14: window layer 15: contact layer 17: n-type electrode 18: p-type electrode 20: solar cell 50: energy level at the top of valence band (Ev)
60, 61, 62, 63, 64: Intermediate band (Ei21, Ei22, Ei23, Ei24, Ei11, Ei12, Ei13, Ei1, Ei2, Ei3)
70: Energy level at the bottom of the conduction band (Ec)

Claims (7)

a p-type semiconductor layer, an n-type semiconductor layer, and a light-absorbing semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer,
The light absorption semiconductor layer includes at least three or more intermediate bands in the forbidden band of the light absorption semiconductor layer,
The light absorbing semiconductor layer is a superlattice semiconductor layer having a superlattice structure in which barrier layers and quantum layers are alternately and repeatedly stacked .
Before Symbol intermediate band is formed between the conduction band of the barrier layer and the conduction band of the quantum layer,
The quantum layer is a quantum dot layer composed of quantum dots ,
Below the width of the forbidden band of the previous SL barrier layer Eg, a top energy level of the valence band in the barrier layer, the energy difference between the highest energy level is lower intermediate band of the intermediate band is taken as ΔEvi The solar cell characterized by satisfying the formula (1).
ΔEvi ≧ (Eg / 2) (unit: eV) (1)   The solar cell according to claim 1, wherein Eg ≧ 1.5 eV is satisfied. The solar cell according to claim 1 or 2, wherein there are at least four intermediate bands . There are three intermediate bands ,
2. The solar cell according to claim 1, wherein the forbidden band of the light absorbing semiconductor layer or the barrier layer has a width of 1.1 eV or more and 3.8 eV or less. There are four intermediate bands ,
The solar cell according to claim 1, wherein the forbidden band of the light absorbing semiconductor layer or the barrier layer has a width of 1.3 eV or more and 3.8 eV or less. There are three intermediate bands ,
The energy difference between the energy level Ec at the lower end of the conduction band of the barrier layer and the intermediate band Ei11 closest to Ec is ΔEci11, and the energy level Ev at the upper end of the valence band and the intermediate band Ei13 third closest to Ec are The energy difference between Ei11 and the second intermediate band Ei12 closest to Ec is ΔEii112, the energy difference between Ei12 and Ei13 is ΔEii123, and the smallest value among ΔEii112 and ΔEii123 is MIN (ΔEii112, ΔEii123)
ΔEci11 ≧ 0.05 eV, or ΔEvi13 ≧ 0.05 eV,
| (ΔEci11−ΔEvi13) | ≧ 0.625 eV,
The solar cell according to claim 1, wherein MIN (ΔEii112, ΔEii123) ≧ 0.15 eV. There are four intermediate bands ,
The energy difference between the energy level Ec at the lower end of the conduction band of the barrier layer and the intermediate band Ei21 closest to Ec is ΔEci21, and the energy level Ev at the upper end of the valence band and the intermediate band Ei24 fourth closest to Ec are ΔEvi24, the energy difference between Ei21 and the second intermediate band Ei22 closest to Ec is ΔEii212, the energy difference between Ei22 and the third intermediate band Ei23 closest to Ec is ΔEii223, and Ei23 and Ei24 When the energy difference is ΔEii234 and the smallest numerical value among ΔEii212, ΔEii223, and ΔEii234 is MIN (ΔEii212, ΔEii223, ΔEii234),
ΔEci21 ≧ 0.05 eV, or ΔEvi24 ≧ 0.05 eV,
| (ΔEci21−ΔEvi24) | ≧ 0.65 eV,
The solar cell according to claim 1, wherein MIN (ΔEii212, ΔEii223, ΔEii234) ≧ 0.10 eV.

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