JP2014220351A - Multi-junction solar cell - Google Patents

Abstract

Higher photoelectric conversion efficiency is obtained in a multi-junction solar battery including a solar battery cell made of silicon and a solar battery cell made of a III-V group compound semiconductor excluding As. At least a first solar cell 131 serving as a bottom cell and a second solar cell 132 serving as a top cell are provided. The second solar cell 132 includes a base layer 107 made of p-type InGaPSb, and an n-type. An emitter layer 108 made of InGaPSb is provided as a light absorption layer. [Selection] Figure 1

Description

  The present invention relates to a multi-junction solar cell formed by bonding a solar cell composed of silicon and a solar cell composed of a III-V compound semiconductor.

  In recent years, in order to solve the serious energy shortage, interest in solar cells, which are clean and safe power sources, is increasing. Recently, a photovoltaic power plant with an overall output of 1 megawatt or more has been studied, and it is expected to be a new power plant that replaces the existing power plant using fossil fuel. One effective means of increasing the output of this solar cell is to increase the conversion efficiency from sunlight to electricity.

  Currently, the most popular solar cell is a single solar cell including only one pn junction made of Si. However, high conversion efficiency exceeding 30% cannot be obtained with this solar cell. This is due to the small band gap of Si. FIG. 9 shows the distribution of irradiance for each wavelength of sunlight on the ground surface. The spectrum of sunlight is distributed around a wavelength of about 0.6 μm (2.05 eV in terms of energy). On the other hand, the band gap of Si is about 1.1 eV.

  For this reason, although sunlight has energy larger than the band gap of Si, carriers (electrons and holes) generated by absorption of sunlight in Si are caused by heat in the crystal. Energy is lost, and ultimately only a voltage corresponding to the band gap of Si can be generated. This is a major factor in that high conversion efficiency cannot be obtained in a solar cell using Si as described above.

  On the other hand, there is a technique for forming a solar cell with a high conversion efficiency exceeding 30% by using a III-V group compound semiconductor. In this technology, two or more cell structures with different band gaps of light absorption layers made of III-V compound semiconductors are combined to form a tandem solar cell structure, and light is absorbed in multiple stages according to the energy distribution of the solar spectrum. Trying to get high conversion efficiency.

  FIG. 10 is a cross-sectional view showing the basic structure of a two-junction solar cell as an example of the tandem solar cell described above. In this two-junction solar cell, the bottom cell 1001 is joined to the top cell 1003 by a tunnel junction 1002. Sunlight enters from the antireflection film 1007 and light absorption occurs first in the top cell 1003. Next, light that is not absorbed because the energy is smaller than the band gap of the top cell 1003 is absorbed by the bottom cell 1001.

  As will be described later, by adjusting the band gap of the light absorption layer of the top cell 1003 and the band gap of the light absorption layer of the bottom cell 1001, the energy of sunlight can be used effectively and the conversion efficiency can be increased. Can do. Furthermore, the top cell 1003 and the bottom cell 1001 can be integrated by a tunnel junction 1002 using a group III-V compound semiconductor, and a current can be taken out by providing a front finger electrode 1005 and a back electrode 1006 thereon. Note that the surface finger electrodes 1005 are connected via a contact layer 1004. As described above, the manufacturing process in making the element of the tandem solar cell is not so different from that of a single solar cell and has a feature that it is simple.

K. Tanabe et al., "III-V / Si hybrid photonic devices by direct fusion bonding", Scientic Reports, Vol.2, 349, 2012. S. Kurtz et al., "Modeling of two-junction series-connected tandem solar cells using top-cell thickness as an adjustable parameter", Jourrnal of Applied Physics, Vol.68, No.4, pp.1890-1895, 1990 .

  Incidentally, as described above, since Si has a small band gap, a high conversion efficiency cannot be obtained with a solar cell using only Si. On the other hand, as described above, in a multijunction solar cell using a III-V group compound semiconductor, high conversion efficiency is realized by combining cells having different band gaps into a tandem structure.

  Most of the two-junction solar cells using III-V compound semiconductors that have been studied so far use GaAs for the bottom cell. In a multi-junction solar cell having three or more junctions, Ge is often used for the bottom cell. In this case, GaAs or InGaAs containing a small amount of In is used as a material for the middle cell. Thus, in a conventional multi-junction solar cell using a group III-V compound semiconductor, a material mainly containing As as a group V element such as GaAs, InGaAs, or AlGaAs is used in any of the constituent cells. .

  In order to increase the ratio of electric power energy by solar cells, it is necessary to consider not only the improvement of conversion efficiency but also the impact on the environment including the disposal of solar cells after use. However, as described above, in a solar cell using a group III-V compound semiconductor, a group III-V compound semiconductor having As as a main group V element is used.

The working environment allowable concentration of As recommended by the American Conference on Government Industrial Hygienists (ACGIH) is 0.01 mg / m ^{3 as a} Time Weighted Average (TWA). Is not easy. Even in semiconductor lasers and electronic devices that are currently in practical use, As is included as an element, the size of these elements is from several μm to at most several hundred μm, and the amount of As contained in the element itself is small. Therefore, there is little possibility of becoming a problem.

  On the other hand, in the case of a solar cell, in order to increase the amount of power generation, even in the case of a concentrating module that can relatively reduce the element area, the solar cell element needs an area of about 1 mm × 1 mm to 10 mm × 10 mm. . For this reason, the solar cell using the above-described III-V group compound semiconductor contains an amount of As that cannot be compared with a semiconductor laser or an electronic device.

  Considering the environmental impact including the disposal of solar cells, it is desirable that the elements contained in the solar cells are also composed of easily toxic elements that are easy to handle. However, so far, in multijunction solar cells using III-V group compound semiconductors, materials having As as the main group V element have been used in any of the constituent cells.

  In the two-junction solar cell, the layer structure in which the material containing As is not contained in either the top cell or the bottom cell is an absorption layer. A method using a group V compound semiconductor is conceivable. Conventionally, it has not been easy to realize a semiconductor device in which Si and a III-V group compound semiconductor are combined. However, due to recent progress in semiconductor process technology, Si and a III-V group compound semiconductor are connected via a metal, a dielectric, or the like. However, it is also possible to directly join and further energize.

  This direct bonding technique is also called “Direct Wafer Bonding” or “Wafer Fusion”, and can directly bond Si and various III-V compound semiconductors. As a method of direct bonding, first, a hydroxyl group (expressed as —OH, also referred to as a hydroxyl group) is attached to the bonding surface of the wafer, and two wafers are adsorbed via this to be bonded by thermal annealing. There is a way. Further, various direct bonding methods such as a method of bonding wafers in a vacuum after activating the bonding surface of each wafer by Ar plasma have been studied. This direct bonding technique between Si and a III-V group compound semiconductor is also applied to the production of solar cells (see, for example, Non-Patent Document 1).

  A method for manufacturing a two-junction solar cell including a top cell using a Si bottom cell and a III-V group compound semiconductor using the direct bonding technique described above will be described with reference to FIGS. 11A to 11D. First, as shown in FIG. 11A, a Si solar battery cell 1101 serving as a bottom cell is manufactured. Next, as shown in FIG. 11B, a solar battery cell 1103 made of a III-V group compound semiconductor grown on a GaAs substrate 1102 serving as a top cell is manufactured.

  Next, as shown in FIG. 11C, the Si solar cells 1101 and the solar cells 1103 are directly bonded and bonded together. Next, in order to enable energization between the bonded Si solar cells 1101 and 1103, a treatment such as thermal annealing is performed as necessary. Next, as shown in FIG. 11D, the GaAs substrate 1102 is separated (removed) from the solar battery cell 1103 using selective etching or the like. Finally, an electrode, an antireflection film, and the like are formed on the integrated two-junction solar cell.

In this two-junction solar cell, high conversion efficiency can be obtained by adjusting the band gap of the top cell in accordance with the bottom cell. Such a change in conversion efficiency due to the combination of the band gaps of the top cell and the bottom cell can be estimated by calculation (see, for example, Non-Patent Document 2). FIG. 12 is a characteristic diagram showing the change in conversion efficiency by the combination of the band gap of the bottom cell and the top cell in the two-junction solar cell by calculation. As the incident light, it is assumed that air mass 1.5 direct (AM-1.5Direct, incident light intensity 768 W / m ² ) is condensed 500 times.

  As described above, the band gap of Si is about 1.1 eV, and this is indicated by a broken line in FIG. As shown in FIG. 12, it can be seen that the band gap of the top cell that can maximize the conversion efficiency when Si is used for the bottom cell is around 1.65 eV. Specifically, if the band gap of the top cell is set in the range of 1.3 eV to 1.8 eV, a high conversion efficiency of 33% or more can be expected at a light condensing magnification of 500 times.

  In addition to the spectral distribution of sunlight, the actual solar cell conversion efficiency varies greatly depending on physical parameters such as cell absorption coefficient and film thickness, carrier (electron and hole) mobility, lifetime, and effective mass. To do. Furthermore, various loss factors need to be considered. Therefore, it is difficult to strictly discuss the absolute value of the conversion efficiency. However, since the band gap range of the top cell where high conversion efficiency can be obtained does not change greatly even if the physical property parameter changes, the band gap range where high conversion efficiency can be expected is from 1.3 eV as described above. 1.8 eV.

  A top cell made of a III-V group compound semiconductor can be lattice-matched to a high-quality substrate such as a GaAs substrate. As a material not containing As, InGaP, InGaAlP, and InAlP are used as described above. Conceivable. Among these materials, the material having the smallest band gap is InGaP, and the band gap under the condition of lattice matching with GaAs is about 1.9 eV.

  On the other hand, in order to increase the conversion efficiency in the two-junction solar cell using Si as the bottom cell as described above, it is desirable that the band gap of the top cell be in the range of 1.3 eV to 1.8 eV. For this reason, in a two-junction solar cell using Si as the bottom cell, it is difficult to obtain high conversion efficiency even when InGaP is used for the light absorption layer of the top cell.

  The present invention has been made to solve the above-described problems, and is a multi-junction solar battery including a solar battery cell made of silicon and a solar battery cell made of a III-V group compound semiconductor excluding As. An object is to obtain higher photoelectric conversion efficiency.

  The multi-junction solar cell according to the present invention includes a first solar cell including a p-type silicon layer made of p-type silicon and an n-type silicon layer made of n-type silicon, and a light absorption layer made of InGaPSb, And a second solar cell composed of a compound semiconductor layer made of a III-V group compound semiconductor and a bonding layer made of InGaP, and the n-type silicon layer and the bonding layer are bonded together, The second solar battery cell is integrated.

  In the multi-junction solar cell, the light absorption layer may be made of InGaPSb having a composition with a band gap of 1.55 to 1.8 eV. Moreover, the light absorption layer should just be comprised from InGaPSb of the composition from which the lattice mismatch with respect to GaAs will be -0.1 to + 0.1%.

  As described above, according to the present invention, higher photoelectric conversion efficiency can be obtained in a multi-junction solar battery including a solar battery cell made of silicon and a solar battery cell made of a III-V group compound semiconductor excluding As. An excellent effect is obtained.

FIG. 1 is a configuration diagram showing the configuration of the multijunction solar cell according to Embodiment 1 of the present invention. FIG. 2 is a characteristic diagram showing the results obtained by calculating the band gap contours of the group III composition and the group V composition for InGaPSb. FIG. 3 is a characteristic diagram showing changes in the band gap due to the Sb composition by extracting only the composition lattice-matched to GaAs from the InGaPSb shown in FIG. FIG. 4 is a cross-sectional view showing the configuration of the sample element used for investigating the band gap of the InGaPSb layer. FIG. 5 is a characteristic diagram showing an X-ray diffraction pattern of the sample element described with reference to FIG. FIG. 6 is a characteristic diagram showing a photoluminescence spectrum of the InGaPSb layer 403 in the sample element described with reference to FIG. FIG. 7 is a cross-sectional view schematically showing the layer structure of the top cell in the multijunction solar cell according to Embodiment 2 of the present invention. FIG. 8 is a cross-sectional view showing a layer structure of a top cell for direct bonding to a Si bottom cell manufactured by ion-implanting P into a p-Si substrate to form an n-type emitter layer. FIG. 9 is a distribution diagram showing the distribution of irradiance for each wavelength of sunlight on the ground surface. FIG. 10 is a cross-sectional view showing the basic structure of a two-junction solar cell as an example of a tandem solar cell. FIG. 11A is a cross-sectional view schematically illustrating a state in a manufacturing process of a two-junction solar cell including a top cell using a Si bottom cell and a group III-V compound semiconductor. FIG. 11B is a cross-sectional view schematically showing a state in a manufacturing process of a two-junction solar cell including a top cell using a Si bottom cell and a III-V group compound semiconductor. FIG. 11C is a cross-sectional view schematically showing a state in a manufacturing process of a two-junction solar cell including a top cell using a Si bottom cell and a III-V group compound semiconductor. FIG. 11D is a cross-sectional view schematically showing a state in a manufacturing process of a two-junction solar cell including a top cell using a Si bottom cell and a III-V group compound semiconductor. FIG. 12 is a characteristic diagram showing a change in conversion efficiency due to the band gap between the bottom cell and the top cell in a two-junction solar cell obtained by calculation.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1]
First, Embodiment 1 of the present invention will be described. FIG. 1 is a configuration diagram showing the configuration of the multijunction solar cell according to Embodiment 1 of the present invention. FIG. 1 schematically shows a cross section. This multi-junction solar cell includes at least a first solar cell 131 serving as a bottom cell and a second solar cell 132 serving as a top cell.

  The first solar cell 131 includes a p-type Si substrate 101 and an emitter layer 102 made of n-type Si formed on the Si substrate 101. Each layer has a conductivity obtained by introducing a predetermined impurity by ion implantation or the like.

The second solar cell 132 includes a base layer 107 made of p-type InGaPSb and an emitter layer 108 made of n-type InGaPSb as light absorption layers. In addition, a BSF (Back Surface Field) layer 106 made of p-type InGaAlP is formed on the first solar cell 131 side (lower side) of the base layer 107. Further, below the BSF layer 106, a p ⁺ -InGaP layer 105 made of InGaP doped with p-type impurities at a high concentration, and an n ⁺ -InGaP layer made of InGaP doped with n-type impurities at a high concentration. 104, and an n-InGaP layer 103 made of n-type InGaP is formed.

  On the other hand, a BSF layer 109 made of n-type InGaAlP, a window layer 110 made of n-type InAlP, and a contact layer 111 made of n-type InGaP are formed on the emitter layer 108. In addition, a front finger electrode 112 is connected to the contact layer 111, and a back electrode 113 is formed on the back side of the Si substrate 101. An antireflection film 114 is formed in the region of the window layer 110 where the surface finger electrode 112 is not formed. Each BSF layer is used to suppress the diffusion of decimal carriers.

The first solar cell 131 and the second solar cell 132 are physically and electrically connected by bonding the emitter layer 102 and the n-InGaP layer 103 together. Further, a tunnel junction composed of the n ⁺ -InGaP layer 104 and the p ⁺ -InGaP layer 105 is provided so that the first solar cell 131 and the second solar cell 132 can be connected in series with a small electric resistance. It has become. For this reason, electricity can be taken out simply by installing two electrodes, the front finger electrode 112 and the back electrode 113.

  The first embodiment is characterized in that the emitter layer 102 and the base layer 107 which are light absorption layers of the second solar battery cell 132 are made of InGaPSb. This feature is characterized in that a band gap in the range of 1.55 eV to 1.8 eV, which cannot be realized with an InGaP layer formed on GaAs, is realized. A multijunction solar in which a second solar cell 132 having a light absorption layer of InGaPSb having a band gap in the range of 1.55 eV to 1.8 eV and a first solar cell 131 using Si are combined. By using a battery, high conversion efficiency can be obtained.

  In the multijunction solar cell in the first embodiment, it is important that InGaPSb can obtain a band gap that cannot be realized under the condition that InGaP is lattice-matched to GaAs. For this reason, the band gap of InGaPSb fabricated on GaAs will be described below.

  FIG. 2 is a characteristic diagram showing the results obtained by calculating the band gap contours of the group III composition and the group V composition for InGaPSb. In FIG. 2, the composition that is lattice-matched with GaAs is shown as line 1. The composition in which the Sb composition on the vertical axis is 0 on this line is InGaP lattice-matched with GaAs, and the band gap is about 1.9 eV. However, as described above with reference to FIG. 12, the band gap of the top cell that can obtain high conversion efficiency in combination with the Si bottom cell is in the range of 1.3 eV to 1.8 eV, and this band gap can be obtained with InGaP. Absent.

  On the other hand, InGaPSb can obtain a band gap of 1.55 eV to 1.8 eV, which is 1.9 eV or less, which is smaller than InGaP, on the line 1 having a composition lattice-matched with GaAs. Furthermore, as will be described later, since the solar cell operates even when there is a lattice mismatch that does not cause the occurrence of crystal defects, the band gap can be set to 1.55 eV or less in this case. .

  FIG. 3 is a characteristic diagram showing changes in the band gap due to the Sb composition by extracting only the composition lattice-matched to GaAs from the InGaPSb shown in FIG. In this calculation, the In composition of InGaPSb is changed to satisfy the lattice matching condition with GaAs as the Sb composition changes. As shown in FIG. 3, it can be seen that the band gap of InGaPSb monotonously decreases as the Sb composition increases. Specifically, the band gap can be decreased from 1.9 eV to 1.55 eV by increasing the Sb composition of InGaPSb from 0 to 0.3.

  As can be seen from FIG. 2, InGaPSb cannot be lattice-matched to GaAs when the Sb composition is 0.3 or more. For this reason, the lower limit of the band gap that can be realized with InGaPSb is 1.55 eV under conditions close to lattice matching with GaAs.

  As described above, InGaPSb is used for the base layer 107 and the emitter layer 108 (light absorption layer) of the second solar cell 132 (top cell) in the multi-junction solar cell using Si as the first solar cell 131 (bottom cell). By using it, it is possible to realize a multi-junction solar cell with high conversion efficiency that cannot be obtained with a top cell using InGaP. InGaPSb having a composition with a band gap of 1.55 to 1.8 eV may be used.

Next, the result of investigating the band gap of the produced InGaPSb layer after producing the InGaPSb layer will be described. Epitaxial growth of the InGaPSb layer was performed by an organometallic molecular beam epitaxy method using trimethylindium (TMIn), triethylgallium (TEGa) as a group III source gas, and phosphine (PH ₃ ) or arsine (AsH ₃ ) as a group V source material. .

  FIG. 4 is a cross-sectional view showing the configuration of the sample element used for investigating the band gap of the InGaPSb layer. In this sample element, an undoped GaAs buffer layer 402 having a layer thickness of 0.3 μm is epitaxially grown on a semi-insulating GaAs substrate 401, and an undoped InGaPSb layer 403 having a layer thickness of 0.2 μm is epitaxially grown thereon. ing.

  FIG. 5 is a characteristic diagram showing an X-ray diffraction pattern of the sample element described above. The angle of the diffraction peak position of the InGaPSb layer 403 is in good agreement with the angle of GaAs, and it can be seen that InGaPSb is almost lattice-matched with GaAs.

  Next, the band gap of the InGaPSb layer 403 was investigated by measuring the photoluminescence of the sample element described above. The measurement was performed at room temperature (about 25 ° C.) using a laser having a wavelength of 532 nm as an excitation light source. FIG. 6 is a characteristic diagram showing a photoluminescence spectrum of the InGaPSb layer 403. The energy peak around 1.65 eV is due to light emission from the InGaPSb layer 403. The peak near 1.42 eV is due to light emission from the GaAs buffer layer 402. These results correspond to the band gap of each layer.

  From the results shown in FIGS. 5 and 6, it can be seen that a band gap of 1.65 eV can be obtained even when InGaPSb is lattice-matched to GaAs. In addition, the Sb composition of InGaPSb in this case is 0.18 when the results shown in FIG. 3 are combined.

  In the above description, InGaPSb having a band gap of 1.65 eV is shown. However, as shown in FIG. 3, the band gap of InGaPSb can be changed by changing the Sb composition. This composition can be controlled by adjusting the amount of raw material supplied during the crystal growth of InGaPSb. For this reason, InGaPSb can change the band gap from 1.55 eV to 1.8 eV near the condition of lattice matching with GaAs. This is an easily feasible matter.

  In the above description, undoped InGaPSb having a layer thickness of 0.2 μm is used. However, when applied to an actual solar cell, doping is performed to form p-type and n-type as in the structure shown in FIG. There is a need. In p-type doping of InGaPSb, Be, Zn, or the like may be used as a dopant, and in n-type doping, Si, Sn, or the like may be used as a dopant. It goes without saying that InGaPSb can easily obtain a p-type layer and an n-type layer.

  Regarding the layer thickness, in the first embodiment, the base layer 107 may have a thickness of about 0.5 to 1.0 μm, and the emitter layer 108 may have a thickness of about 0.05 to 0.1 μm. Needless to say, these layer thicknesses can be easily controlled, and the layer thickness of InGaPSb can be easily controlled by adjusting the growth time.

  In the above description, the characteristics of InGaPSb lattice-matched to GaAs are shown, but the layer thickness of InGaPSb used for the emitter layer and the base layer of the solar cell is about 1 μm even when these two layers are combined. Therefore, it is not necessary to strictly match the lattice with GaAs. Specifically, when the lattice mismatch with respect to GaAs is in the range of −0.1% to + 0.1%, crystal defects caused by the lattice mismatch caused by relaxation can be suppressed, and the device characteristics deteriorate. Can also be suppressed. InGaPSb can be applied to the emitter layer 108 and the base layer 107 of the second solar cell 132 if the lattice mismatch with respect to GaAs is in the range of −0.1% to + 0.1%.

  In the above description, the case where organometallic molecular beam epitaxy is used as the crystal growth method has been described. However, the crystal growth method is not limited to this. Needless to say, the applicable growth technique is that each compound semiconductor layer can be formed, and techniques such as metal organic vapor phase epitaxy, gas source molecular beam epitaxy, and molecular beam epitaxy may be used.

In the multijunction solar cell of the first embodiment, the example in which the n ⁺ -InGaP layer 104 and the p ⁺ -InGaP layer 105 are combined as a tunnel junction is shown, but the present invention is not limited to this. For example, as shown in Non-Patent Document 1, a tunnel junction is formed by directly bonding Si doped with a high concentration of impurities and a III-V group compound semiconductor doped with a high concentration of impurities. May be.

  Moreover, in the multijunction solar cell in Embodiment 1, p-type Si, n-type Si, n-type III-V group compound semiconductor, p-type III-V group compound semiconductor, n-type III- from the back electrode 113 side. Although they are stacked in the order of the group V compound semiconductor, it is possible to reverse the stacking order.

  Specifically, a bottom cell is formed by forming a p-type Si layer on an n-type Si substrate, and a p-type III-V group compound semiconductor, an n-type III-V group compound semiconductor layer, and a p-type are formed on the GaAs substrate. A top cell is produced by laminating in the order of the III-V group compound semiconductor layer, and a multi-junction solar cell is produced by directly bonding the p-type Si layer and the p-type III-V group compound semiconductor layer. Is possible. As described above, many variations and combinations of the stacking order can be realized by those having ordinary knowledge in the art.

[Embodiment 2]
Next, a second embodiment of the present invention will be described. FIG. 7 is a cross-sectional view schematically showing the layer structure of the top cell in the multijunction solar cell according to Embodiment 2 of the present invention. The top cell (second solar cell) according to the second embodiment includes a p-type GaAs buffer layer 702 having a layer thickness of 0.5 μm and a p-type layer having a thickness of 0.03 μm on a substrate 701 made of p-type GaAs. A BSF layer 703 made of InGaAlP is grown, and a base layer 704 made of p-type InGaP having a thickness of 0.7 μm and an emitter layer 705 made of n-type InGaP having a thickness of 0.05 μm are subsequently grown. .

The Sb composition of InGaPSb used for the base layer 704 and the emitter layer 705 is both 0.14 and the band gap is 1.70 eV. On the emitter layer 705, a BSF layer 706 made of n-type InGaAlP having a thickness of 0.03 μm and a window layer 707 made of p-type InAlP having a thickness of 0.2 μm were grown, and finally a contact made of n-type InGaP. Layer 708 is grown. For the epitaxial growth of each of these layers, a gas source molecular beam epitaxy method using In, Ga, Al as a group III material and phosphine (PH ₃ ) or arsine (AsH ₃ ) as a group V material is used. The growth temperature is 530 ° C.

Further, a surface finger electrode 709 is formed by depositing a metal on a part of the contact layer 708 and performing heat treatment. In a region without an electrode, the contact layer 708 made of n-type InGaP is selectively removed by dry etching using BBr ₃ . The back electrode 710 is formed by thinning the substrate 701 by polishing from the back surface and then depositing and heat-treating metal on the back surface of the thinned substrate 701. Finally, an antireflection film 711 is formed on the window layer 707 on which sunlight is incident.

Using the top cell in the second embodiment, current-voltage characteristics are measured under the conditions of air mass 1.5 global (AM-1.5 Global, incident light intensity 1000 W / m ² ) and non-condensing. In the case of this top cell, the release voltage is 1.25 V, the short-circuit current density is 13.0 mA / cm ² , the fill factor is 0.83, and the conversion efficiency is 13.5%. On the other hand, in the solar cell produced for comparison, the release voltage is 1.40 V, the short-circuit current density is 11.0 mA / cm ² , the fill factor is 0.84, and the conversion efficiency is 12.9%. The solar cell for comparison was manufactured using InGaP (band gap: 1.91 eV) instead of InGaPSb with the base layer and the emitter layer having the same layer thickness.

  The reason why InGaPSb is higher in conversion efficiency than InGaP is that InGaPSb has a smaller band gap than InGaP, can absorb sunlight with a small energy that cannot be absorbed by InGaP, and can increase the short-circuit current density. In addition, the short circuit current density of a solar cell can be adjusted by increasing / decreasing the layer thickness of a base layer and an emitter layer. For this reason, the layer thickness of InGaPSb of the base layer and the emitter layer is not limited to the above-described values. Actually, in a multi-junction solar cell in which a top cell containing InGaPSb and a Si bottom cell described above are combined, the layer thickness of InGaPSb may be adjusted so as to be current-matched with the Si bottom cell.

When the top cell containing InGaPSb described with reference to FIG. 7 is directly joined to the Si bottom cell to produce a multi-junction solar cell, the layer configuration needs to be changed. FIG. 8 shows a top cell layer structure for direct bonding to a Si bottom cell fabricated by ion implantation of P into a p-Si substrate to form an n-type emitter layer. In this structure, in order to pass an electric current between the Si bottom cell, a p ⁺ -InGaP layer 808 doped with a p-type impurity at a high concentration on a BSF layer 807 made of p-type InGaAlP and an n-type impurity at a high concentration. A tunnel junction composed of an n ⁺ -InGaP layer 809 doped with is formed.

  Note that a buffer layer 802 made of undoped GaAs is formed on a substrate 801 made of p-type GaAs, and a contact layer 803 made of n-type InGaP is formed on the buffer layer 802, and the contact layer 803 is made of Are stacked with a BSF layer 804 made of n-type InGaAlP, an emitter layer 805 made of n-type InGaPSb is formed on the BSF layer 804, and made of p-type InGaPSb on the emitter layer 805. A base layer 806 is formed, and a BSF layer 807 is formed on the base layer 806.

In addition, a junction layer 810 made of n-type InGaP for bonding to the n-type Si emitter layer constituting the bottom cell is formed on the n ⁺ -InGaP layer 809 when viewed from the substrate 801 side. A multi-junction solar cell can be manufactured through the manufacturing process as shown in FIG. 11 using the top cell formation substrate having such a configuration.

  As described above, in the present invention, the top cell made of a III-V compound semiconductor includes a light absorption layer made of InGaPSb, and is made of a compound semiconductor layer made of a group III-V compound semiconductor other than As and InGaP. Since it comprised from the joining layer, higher photoelectric conversion efficiency comes to be obtained in the multijunction solar cell by the photovoltaic cell which consists of silicon | silicone, and the photovoltaic cell which consists of a III-V group compound semiconductor except As.

  The present invention described above is suitable as a solar cell that efficiently converts sunlight from light to electricity and has low toxicity and is easy to handle. As a result, it is possible to meet the social demand for increasing the proportion of electric power that is currently required to be generated by solar cells.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, in the above description, a case where one solar battery cell formed of a III-V group compound semiconductor is combined with a solar battery cell formed of silicon has been described as an example, but the present invention is not limited thereto. It goes without saying that a plurality of solar cells made of a III-V group compound semiconductor may be combined with a solar cell made of silicon.

DESCRIPTION OF SYMBOLS 101 ... Si substrate, 102 ... Emitter layer, 103 ... n-InGaP layer, 104 ... n ⁺ -InGaP layer, 105 ... p ⁺ -InGaP layer, 106 ... BSF layer, 107 ... Base layer, 108 ... Emitter layer, 109 ... BSF layer, 110 ... window layer, 111 ... contact layer, 112 ... front finger electrode, 113 ... back electrode, 114 ... antireflection film, 131 ... first solar cell, 132 ... second solar cell.

Claims (3)

a first solar cell comprising a p-type silicon layer made of p-type silicon and an n-type silicon layer made of n-type silicon;
A second solar battery cell including a light absorbing layer made of InGaPSb, a compound semiconductor layer made of a group III-V compound semiconductor other than As, and a bonding layer made of InGaP, and
The n-type silicon layer and the bonding layer are bonded to each other, and the first solar cell and the second solar cell are integrated. The multijunction solar cell according to claim 1,
The said light absorption layer is comprised from InGaPSb of the composition whose band gap is 1.55-1.8 eV, The multijunction solar cell characterized by the above-mentioned. The multi-junction solar cell according to claim 1 or 2,
The multi-junction solar cell, wherein the light absorption layer is made of InGaPSb having a composition of −0.1 to + 0.1% of lattice mismatch with respect to GaAs.