JP2004111687A - Tandem type solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tandem type solar cell which has a two-terminal structure and is highly efficient. <P>SOLUTION: A tandem type solar cell 10 has a high-energy conversion rate, since it is equipped with a bottom cell 12, a middle cell 14, and a top cell 16, with band gap energy different among others. Since the band gap energies are set so that the generated current values of the bottom cell 12, the middle cell 14, and the top cell 16 agree among others, it is not required that a wiring be set for each to extract current nor an inverter be converted into a common voltage. So only one inverter is required to be provided between an upper part electrode 34 and a lower part electrode 38, after two wirings are installed between them. Thus, the tandem type solar cell 10 of 2-terminal structure is provided, whose efficiency and practicality are high. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an improvement of a tandem solar cell in which a plurality of types of semiconductor solar cells are monolithically connected in series.
[0002]
[Prior art]
2. Description of the Related Art A tandem solar cell in which a plurality of types of semiconductor solar cells having different band gaps (forbidden band widths) are monolithically connected in series is known. For example, a high-efficiency solar cell described in Patent Document 1 is such. In such a tandem-type solar cell, each semiconductor solar cell receives light in a different wavelength range and performs photoelectric conversion on the solar spectrum, so that solar energy can be effectively used and energy conversion efficiency can be improved. It is possible to generate a large amount of electric power, and to obtain the same level of electric power.
[0003]
[Patent Document 1]
JP-A-3-272185
[0004]
[Problems to be solved by the invention]
By the way, in the tandem solar cell described in Patent Document 1, as shown in FIG. 11, a solar cell 120 made of a GaAsP semiconductor having a different mixed crystal ratio is added to a solar cell 120 made of a silicon (Si) semiconductor. , 124 are provided, and electrodes 126, 128, 130, and 132 are provided between each other and on the entire upper and lower surfaces. According to this structure, since the magnitudes of the generated currents of the respective solar cells 120, 122, and 124 are different from each other, a wiring for extracting current is connected to each of the electrodes 126 to 132, and the respective cells 120, 122, and 124 are connected. Inverters 134, 136, and 138 provided for each output a common voltage. For this reason, the number of wirings for extracting current is increased, and the number of inverters corresponding to the number of stacked solar cells is required.
[0005]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-efficiency tandem solar cell having a two-terminal structure requiring a small number of wirings.
[0006]
[First means for solving the problem]
In order to achieve such an object, the gist of the first invention is a tandem-type solar cell in which a plurality of types of semiconductor solar cells having different band gap energies are monolithically stacked via a tunnel junction layer. (A) The bandgap energy of each of the plurality of types of semiconductor solar cells is determined so that current values generated from the respective types become equal to each other.
[0007]
[Effect of the first invention]
With this configuration, in a tandem solar cell having high energy conversion efficiency because a plurality of semiconductor solar cells having different band gap energies are provided, each of the semiconductor solar cells constituting the tandem solar cell has a generated current. Since the bandgap energies are determined so that the values coincide with each other, it is not necessary to provide a wiring for each of them and take out a current and convert it to a common voltage by an inverter. Therefore, a pair of electrodes may be provided above and below the whole battery and one inverter may be provided between them, so that a highly practical two-terminal structure can be adopted. In the present application, “the current values coincide with each other” is not limited to the case where the current values are completely coincident. In the case where there is a difference in the current value, the difference is such that the current can be taken out by the pair of electrodes as described above. Including those that stay.
[0008]
Incidentally, in the conventional tandem solar cell as shown in FIG. 11 described above, the structure is determined solely from the viewpoint of improving the energy conversion efficiency, and a plurality of semiconductors are provided so as to absorb sunlight over a wide wavelength range. The bandgap energy of each of the solar cells was set. For this reason, the mutual relation between the band gap energies is not particularly taken into consideration, and thus the problem that the generated current values are different from each other occurs as described above. The present inventors have paid attention to the fact that the generated current value of the semiconductor solar cell changes according to the band gap energy, and match the generated current value within the band gap energy range where high efficiency can be obtained for various materials. As a result, they have found a relationship between bandgap energies that satisfies such requirements in an extremely narrow range. The present invention has been made based on such findings.
[0009]
Moreover, according to the present invention, since it is not necessary to adopt a metal interconnect structure in which electrodes are provided between semiconductor solar cells, the semiconductor constituting each solar cell can be epitaxially grown on the semiconductor. There is also an advantage that a tandem solar cell in which the crystallinity of each layer is good and hardly deteriorates is obtained. Incidentally, in the conventional tandem-type solar cell shown in FIG. 11, since it is necessary to provide metal electrodes 128 and 130 for extracting current between the solar cells 120, 122 and 124, the solar cells 122 and 124 are formed. Semiconductors are grown on the metal film. Therefore, in this structure, the crystallinity of the semiconductor solar cells 122 and 124 is poor, so that there is a problem that the semiconductor solar cells 122 and 124 are easily deteriorated.
[0010]
[Second means for solving the problem]
The gist of the tandem solar cell of the second invention for achieving the above object is as follows: (a) a bottom cell made of a semiconductor solar cell having a band gap energy of 1.1 (eV); A middle cell made of a semiconductor solar cell having a band gap energy of 1.4 to 1.5 (eV) provided on the bottom cell through the tunnel junction layer, and (c) a middle cell on the middle cell through the tunnel junction layer. And a top cell composed of a semiconductor solar cell having a band gap energy of 1.9 to 2.0 (eV).
[0011]
[Effect of the second invention]
In this manner, the tandem solar cell has a high energy conversion efficiency because the bottom cell, the middle cell, and the top cell have a three-junction structure in which the band gap energies are different from each other. Since the band gap energies of the three semiconductor solar cells are set to 1.1 (eV), 1.4 to 1.5 (eV), and 1.9 to 2.0 (eV), respectively, The generated current values match each other. That is, these band gap energies are determined in a range where the current values generated from each other match each other. Therefore, it is not necessary to provide a wiring for each of the semiconductor solar cells, take out the current, and convert the current to a common voltage by an inverter, so that a pair of electrodes are provided on the upper and lower sides of the whole battery, that is, on the bottom surface of the bottom cell and the top surface of the top cell and Since only one inverter needs to be provided between them, a highly practical two-terminal structure can be adopted.
[0012]
[Third Means for Solving the Problems]
Further, the gist of the tandem solar cell of the third invention for achieving the above object is as follows: (a) a bottom cell made of a semiconductor solar cell having a band gap energy of 1.1 (eV); And a top cell made of a semiconductor solar cell having a band gap energy of 1.65 to 1.85 (eV) provided on the bottom cell via the tunnel junction layer.
[0013]
[Effect of the third invention]
In this way, the tandem solar cell has a high energy conversion efficiency because it has a two-junction structure in which a bottom cell and a top cell having different band gap energies are stacked. Since the band gap energies of the solar cells are set to 1.1 (eV) and 1.65 to 1.85 (eV), respectively, the current values generated from each match each other. That is, these band gap energies are determined in a range where the current values generated from each other match each other. Therefore, it is not necessary to provide a wiring for each of the semiconductor solar cells, take out the current, and convert the current to a common voltage by an inverter, so that a pair of electrodes are provided on the upper and lower sides of the whole battery, that is, on the bottom surface of the bottom cell and the top surface of the top cell and Since only one inverter needs to be provided between them, a highly practical two-terminal structure can be adopted. Thus, the present invention is preferably implemented with a three-joint or two-joint structure.
[0014]
Other aspects of the invention
Here, preferably, the semiconductor solar cell (bottom cell in the second and third inventions) constituting the lowermost layer of the tandem solar cell is made of single-crystal silicon. In this case, since silicon has stable characteristics and can be obtained at low cost, there is an advantage that a tandem solar cell having high characteristics can be manufactured at low cost. In addition, silicon has high strength, so it is difficult to break during mounting, handling is easy, wafer process is stable, rigidity is high, and there is little occurrence of thermal stress or external stress, which leads to deterioration of characteristics. is there.
[0015]
Preferably, when the bottom cell is made of single-crystal silicon, and in the case of the three-junction structure, the middle cell and the top cell are made of SiGeC / SiGeC, ZnSiSb / ZnSiP, CdSiAs / CdSiP, InGaAsP / It is made of AlGaInP (all are middle cells / top cells), and in the case of the two-junction structure, the top cell is made of GaNPAs. In this manner, since these compound semiconductors have a lattice constant similar to that of silicon, a high-performance tandem solar cell with less distortion and defects due to lattice mismatch can be obtained.
[0016]
Preferably, a plurality of types of semiconductor solar cells constituting the tandem type solar cell, that is, the bottom cell, the middle cell, and the top cell in the second invention, and the bottom cell and the top cell in the third invention, It has an area. This has the advantage that the difference in the generated current value of each of the semiconductor solar cells can be further reduced in combination with the band gap energy being determined as described above.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
[0018]
FIG. 1 is a schematic diagram illustrating a cross-sectional structure of a tandem solar cell 10 according to one embodiment of the present invention. In the figure, a tandem solar cell 10 has a three-junction structure in which a middle cell 14 and a top cell 16 are monolithically stacked on a bottom cell 12 via bonding layers 18 and 20. Each of the bottom cell 12 to the top cell 16 and the bonding layers 18 and 20 has the same area of, for example, about 7 × 7 (mm). That is, the light receiving areas of the bottom cell 12, the middle cell 14, and the top cell 16 are similar to each other.
[0019]
The bottom cell 12 is made of, for example, single-crystal silicon having a band gap energy of about 1.1 (eV) and a lattice constant of about 0.543 (nm), and is a p-type layer made of p-Si. 22 and an n-type layer 24 made of n-Si. The overall thickness of the bottom cell 12 is, for example, 100 (μm) or more, for example, about 350 (μm). The thickness of the n-type layer 24 is, for example, about 300 (nm). To position. The n-type layer 24 is formed by diffusing an n-type dopant from the upper surface of a p-type silicon substrate into a p-type silicon substrate and inverting the surface portion thereof to n-type. It consists of a part maintained in p-type. The bottom cell 12 is configured as a semiconductor solar cell by providing such a pn junction structure.
[0020]
The middle cell 14 is formed of, for example, Si having a band gap energy of about 1.5 (eV) and a lattice constant of about 0.49 (nm). _0.45 Ge _0.45 C _0.1 Consisting of p-Si _0.45 Ge _0.45 C _0.1 P-type layer 26 of n-Si _0.45 Ge _0.45 C _0.1 And an n-type layer 28 composed of The overall thickness of the middle cell 14 is, for example, in the range of about 0.5 to 20 (μm), for example, about 2 (μm), and the thickness of the n-type layer 28 is, for example, about 300 (nm). The middle cell 14 is also configured as a semiconductor solar cell by providing such a pn junction structure.
[0021]
The top cell 16 is made of, for example, Si having a band gap energy of about 2.0 (eV) and a lattice constant of about 0.44 (nm). _0.3 Ge _0.4 C _0.3 Consisting of p-Si _0.3 Ge _0.4 C _0.3 P-type layer 30 and n-Si _0.3 Ge _0.4 C _0.3 And an n-type layer 32 composed of The overall thickness of the top cell 16 is, for example, in the range of about 0.5 to 20 (μm), for example, about 2 (μm), similar to that of the middle cell 14, and the thickness of the n-type layer 32 is, for example, 300. (Nm). The top cell 16 is also configured as a semiconductor solar cell by providing such a pn junction structure.
[0022]
Further, as shown in FIG. 2, for example, each of the bonding layers 18 is formed of n-AlInP, p ⁺⁺ -InGaP, n ⁺⁺ -AlGaAs and p-AlInP are stacked and formed of a four-layer semiconductor having a double heterostructure. Each layer has a thickness of about 100 (nm), and the entire thickness of the bonding layer 18 is about 400 (nm). The bonding layer 20 is configured similarly to the bonding layer 18. For this reason, the bottom cell 12, the middle cell 14, and the top cell 16 are mutually connected by a tunnel junction. The junction layers 18 and 20 are for electrically connecting the cells 12, 14 and 16 in series with low resistance by confining minority carriers and allowing majority carriers to pass through the tunnel barrier. The constituent materials of the bonding layers 18 and 20 have a lattice constant similar to that of silicon, and are connected to each other while alleviating distortion caused by lattice mismatch between the cells 12, 14, and 16. I have.
[0023]
Returning to FIG. 1, for example, a comb-shaped upper electrode 34 made of, for example, Au—Sb is fixed to a portion other than the light receiving surface 36 on the upper surface of the top cell 16, and aluminum ( A lower electrode 38 made of Al) is fixed on the entire surface. That is, the tandem solar cell 10 has a two-terminal structure in which the electrodes 34 and 38 are provided only on the upper and lower surfaces. The exposed portion of the upper surface of the top cell 16, that is, the light receiving surface 36, ₂ Is provided.
[0024]
The tandem solar cell 10 configured as described above is used, for example, as shown in FIG. 3, by connecting the wirings 42 and 44 to the upper electrode 34 and the lower electrode 38 and providing an inverter 46 therebetween. . When sunlight is received by the light receiving surface 36, the current generated in the bottom cell 12, the middle cell 14, and the top cell 16 flows between the upper electrode 34 and the lower electrode 38 through the bonding layers 18 and 20. Converted and retrieved. At this time, the theoretical energy conversion efficiency is, for example, about 41 (%), and an extremely high efficiency that has not been obtained in the past can be obtained despite the two-terminal structure.
[0025]
That is, in the present embodiment, the top cell 16 having a band gap energy of about 2.0 (eV) absorbs light having a wavelength of 560 (nm) or more out of sunlight and has a wavelength of about 1.2 (V). Is taken out as In the middle cell 14 having a bandgap energy of about 1.5 (eV), light having a wavelength of 560 to 750 (nm) among sunlight is absorbed and extracted as about 0.9 (V). In the bottom cell 12 having a band gap energy of about 1.1 (eV), light having a wavelength of 750 to 1130 (nm) is absorbed and extracted as about 0.65 (V). Since these are connected in series, a current of a voltage of about 2.75 (V) is taken out as a whole. As described above, in the tandem solar cell 10 in which a plurality of semiconductor solar cells having different band gap energies are stacked, sunlight can be absorbed over a wide wavelength range and converted into electric energy, so that high conversion efficiency can be obtained. Can be obtained. At this time, since the current value generated in each cell is substantially the same, the current generated in each of the three semiconductor solar cells can be extracted without waste only by the upper electrode 34 and the lower electrode 38 provided above and below.
[0026]
As described above, since the top cell 16 and the middle cell 14 are configured to have a small thickness of about 2 (μm), not all the light in the above wavelength range can be necessarily absorbed. However, light that has reached the lower cell without being absorbed is absorbed by that cell, so there is no particular problem. The reason why the bottom cell 12 is configured to have a relatively thick thickness of 100 (μm) or more as described above is to lengthen a path for sufficiently absorbing light on the long wavelength side of sunlight. When the thickness is set to a sufficient thickness, light that has passed without being absorbed by the top cell 16 and the middle cell 14 is absorbed. That is, high conversion efficiency is ensured.
[0027]
Incidentally, in the present embodiment, the constituent materials of the tandem solar cell 10 are determined in consideration of the band gap energy and the lattice constant. FIG. 4 is a diagram showing the theoretical energy conversion efficiency when the band gap energy of the bottom cell 12 is set to 1.1 (eV) in relation to the band gap energies of the middle cell 14 and the top cell 16. This figure shows the efficiency under the condition that the mass of solar radiation passing through the air is AM (Air Mass) -1 and the temperature is 27 ° C. Each of the plurality of closed curves shown in FIG. The numerical value attached indicates the conversion efficiency on the curve, and a higher efficiency is obtained inside the conversion efficiency. For example, a high conversion efficiency of 41.1 (%) or more can be obtained in the region shown at the innermost periphery. Therefore, from the viewpoint of increasing the energy conversion efficiency as much as possible, a material having a band gap energy within a range surrounded by a closed curve indicating a desired conversion efficiency may be selected as a constituent material of the middle cell 14 and the top cell 16. Good.
[0028]
However, since the generated current value of the semiconductor solar cell is determined according to the amount of photons absorbed, if an arbitrary material is selected in the above-mentioned region, the current values generated in the middle cell 14 and the top cell 16 may be mutually or the bottom cell 12. Will be different. If there is such a difference, a structure in which only a pair of electrodes of the upper electrode 34 and the lower electrode 38 is provided, the extracted current value is limited by the minimum cell, and the actual conversion efficiency is reduced. On the contrary, in order to secure the conversion efficiency, there is a disadvantage that the structure as shown in FIG. 11 must be adopted in which an electrode and an inverter are provided for each cell to take out a current.
[0029]
Therefore, in the present embodiment, the band gap energy of the middle cell 14 and the top cell 16 is adjusted so that the current value generated in the middle cell 14 and the top cell 16 in the high conversion efficiency region (combination) matches that of the bottom cell 12. Stipulated. In the range where the current values coincide, the band gap energy of the top cell 16 is 1.9 to 2.0 (eV) and the band gap energy of the middle cell 14 is 1.4 to 1.5 (FIG. 4). eV), which is a rectangular range (indicated by oblique lines in the figure) and substantially overlaps with a range in which high efficiency can be obtained, but does not completely match. From the viewpoint of matching the current values, the constituent materials of the middle cell 14 and the top cell 16 can be appropriately selected from those having characteristics within this range. However, specific material selection depends on manufacturing stability and cost. In consideration of the above, a device that can obtain the highest possible efficiency is selected.
[0030]
FIG. 5 is a diagram for examining candidate materials for the middle cell 14 and the top cell 16 for actually configuring the tandem solar cell 10 within the band gap energy range determined from the above viewpoint. Represents the relationship between the lattice constant and the band gap energy. In the present embodiment, since silicon is used as a constituent material of the bottom cell 12, the constituent materials of the middle cell 14 and the top cell 16 bonded thereon have distortion as much as possible due to lattice mismatch. It is desirable to have a lattice constant close to that of silicon so as to be small. Examples of the material system satisfying such conditions include a SiC-Si-Ge system, a ZnSiP-ZnSiSb system, and a CdSiP-CdSiAs system as shown in the drawing. Among them, in the tandem solar cell 10, The SiC-Si-Ge system is selected. In this figure, the line gap between SiC and Si satisfies the value of the above band gap energy when the constituent material of the middle cell 14 has a composition (mixed crystal ratio) shown in FIG. _0.45 Ge _0.45 C _0.1 And the constituent material of the top cell 16 has a composition (mixed crystal ratio) range indicated by A in the figure, for example, Si _0.3 Ge _0.4 C _0.3 It is.
[0031]
As described above, in the present embodiment, assuming that the bottom cell 12 is made of silicon, the constituent materials of the middle cell 14 and the top cell 16 can obtain high efficiency, match current values, and reduce lattice mismatch. It has been chosen to be smaller. Therefore, high conversion efficiency can be obtained as described above, while taking a simple and practical two-terminal structure.
[0032]
As is apparent from FIG. 5, there is a slight lattice mismatch between the bottom cell 12, the middle cell 14, and the top cell 16. However, since these are tunnel-joined to each other via the bonding layers 18 and 20, lattice mismatch is almost alleviated in the bonding layers 18 and 20, so that the bottom cell 12, the middle cell 14, and the top cell 16 Has almost no distortion due to lattice mismatch.
[0033]
The tandem solar cell 10 is manufactured, for example, according to the process shown in FIG. First, in the n-type layer forming step 48, the n-type layer 24 is formed by diffusing an n-type dopant such as phosphorus (P) by thermal diffusion or implantation on the upper surface of the p-Si substrate, for example. Thus, the bottom cell 12 is formed. Next, in a bonding layer forming step 50, n-AlInP, p-type is formed on the n-type layer 24 by using, for example, MOCVD (metal organic chemical vapor deposition). ⁺⁺ -InGaP, n ⁺⁺ -AlGaAs and p-AlInP are sequentially laminated to form the bonding layer 18. Next, in the middle cell forming step 52, the p-type layer 26 and the n-type layer 28, that is, p-Si _0.45 Ge _0.45 C _0.1 And n-Si _0.45 Ge _0.45 C _0.1 Are successively grown to form the middle cell 14 described above. Next, in the bonding layer forming step 54, the above-described respective semiconductor layers are stacked on the n-type layer 28 in the same manner as in the case of forming the bonding layer 18 to form the bonding layer 20 described above. In the top cell forming step 56, the p-type layer 30 and the n-type layer 32, that is, p-Si _0.3 Ge _0.4 C _0.3 And n-Si _0.3 Ge _0.4 C _0.3 Are successively grown to form the above-mentioned top cell 16. Also in the middle cell forming step 52 to the top cell forming step 56, for example, MOCVD is used for crystal growth.
[0034]
After the bottom cell 12 to the top cell 16 are formed as described above, in an electrode forming step 58, an electrode material is formed on the front surface of the top cell 16 and the back surface of the bottom cell 12 by using a vapor deposition method, a sputtering method, or the like. Thus, the upper electrode 34 and the lower electrode 38 are formed. Thereafter, in a protective layer forming step 60, the SiO 2 is covered so as to cover the light receiving surface 36. ₂ By forming a film and providing the protective layer 40, the tandem solar cell 10 is obtained. Note that the lower electrode 38 may be formed by using a thick film printing method.
[0035]
In the present embodiment, the middle cell 14 and the top cell 16 are formed on the semiconductor layer that has been sequentially crystal-grown on the bottom cell 12 as described above. Therefore, compared to a tandem solar cell having a metal interconnect structure in which metal electrodes are provided between semiconductor solar cells as shown in FIG. 11, crystal disorder such as when a film is formed on metal is reduced. Moreover, since recombination hardly occurs even during use, there is an advantage that higher conversion efficiency can be obtained.
[0036]
In short, in the present embodiment, since three semiconductor solar cells having different band gap energies, ie, the bottom cell 12, the middle cell 14, and the top cell 16, are provided, the tandem solar cell 10 having high energy conversion efficiency is provided. Since the band gap energies are determined so that the generated current values of the bottom cell 12, middle cell 14, and top cell 16 match each other, that is, the band gap energy is 1.1 (eV), respectively. Since they are set to 1.5 (eV) and 2.0 (eV), there is no need to provide a wiring for each of them and take out a current and convert it to a common voltage by an inverter. That is, it is sufficient to provide two wirings 42 and 44 between the upper electrode 34 and the lower electrode 38 and provide one inverter 46 between them, so that a tandem solar cell having a two-terminal structure with high efficiency and high practicability is sufficient. Battery 10 is obtained.
[0037]
Further, according to the present embodiment, since the bottom cell 12 of the tandem solar cell 10 is made of single crystal silicon, silicon has stable characteristics and can be obtained at low cost. There is an advantage that it can be manufactured at low cost.
[0038]
Next, another embodiment of the present invention will be described. In the following embodiments, portions common to the above-described embodiments will be denoted by the same reference numerals and description thereof will be omitted.
[0039]
The tandem solar cell 70 shown in FIG. 7 is configured substantially in the same manner as the tandem solar cell 10 described above, except that the constituent materials of the middle cell 72 and the top cell 74 provided on the bottom cell 12 are different. That is, the middle cell 72 has a band gap energy of about 1.5 (eV) and a lattice constant of about 0.57 (nm). _0.31 Si _0.38 Sb _0.31 Semiconductor solar cell comprising: p-Zn _0.31 Si _0.38 Sb _0.31 Layer 76 of n-Zn _0.31 Si _0.38 Sb _0.31 And an n-type layer 78 composed of The overall thickness of the middle cell 72 is, for example, in the range of about 100 (nm) to 10 (μm), for example, about 2 (μm), and the thickness of the n-type layer 78 is, for example, about 300 (nm). .
[0040]
The top cell 74 is made of, for example, Zn having a band gap energy of about 2.0 (eV) and a lattice constant of about 0.54 (nm). _0.33 Si _0.34 P _0.33 Semiconductor solar cell comprising: p-Zn _0.33 Si _0.34 P _0.33 P-type layer 80 of n-Zn _0.33 Si _0.34 P _0.33 And an n-type layer 82 composed of The overall thickness of the top cell 74 is, for example, in the range of about 100 (nm) to 10 (μm) similar to that of the middle cell 72, for example, about 2 (μm), and the thickness of the n-type layer 82 is, for example, about It is about 300 (nm).
[0041]
In the tandem-type solar cell 70 configured as described above, similarly to the above-described tandem-type solar cell 10, the above-described semiconductor is selected as a constituent material of the middle cell 72 and the top cell 74. As shown in FIG. 5, since the lattice constant is similar to that of the bottom cell 12 made of silicon, distortion caused by lattice mismatch is suppressed. In addition, since it has the bandgap energy as described above, the high efficiency shown in FIG. 4 is obtained in relation to the bottom cell 12 and the current value is matched. Even if it is configured with a two-terminal structure including only the lower electrode 38, current can be efficiently extracted. That is, a conversion efficiency of, for example, about 36 (%) can be obtained.
[0042]
The tandem solar cell 90 shown in FIG. 8 is also different from the tandem solar cell 10 in the constituent materials of the middle cell 92 and the top cell 94. The middle cell 92 has, for example, Cd having a band gap energy of about 1.6 (eV) and a lattice constant of about 0.58 (nm). _0.33 Si _0.34 As _0.33 A semiconductor solar cell comprising: p-Cd _0.33 Si _0.34 As _0.33 P-type layer 96 comprising n-Cd _0.33 Si _0.34 As _0.33 And an n-type layer 98 made of The overall thickness of the middle cell 92 is, for example, in the range of about 100 (nm) to 10 (μm), for example, about 2 (μm), and the thickness of the n-type layer 98 is, for example, about 300 (nm). .
[0043]
The top cell 94 is formed of, for example, Cd having a band gap energy of about 2.0 (eV) and a lattice constant of about 0.57 (nm). _0.33 Si _0.34 P _0.33 A semiconductor solar cell comprising: p-Cd _0.33 Si _0.34 P _0.33 P-type layer 100 comprising n-Cd _0.33 Si _0.34 P _0.33 And an n-type layer 102 composed of The overall thickness of the top cell 94 is, for example, in the range of about 100 (nm) to 10 (μm), for example, about 2 (μm), similar to that of the middle cell 92, and the thickness of the n-type layer 102 is, for example, about 2 (μm). It is about 300 (nm).
[0044]
The tandem solar cell 90 including the middle cell 92 and the top cell 94 has a lattice constant similar to that of the bottom cell 12, as shown in FIG. 5, so that distortion due to lattice mismatch is suppressed. In addition, since it has the bandgap energy as described above, the high efficiency shown in FIG. 4 is obtained in relation to the bottom cell 12 and the current value is matched. Even if it is configured with a two-terminal structure including only the lower electrode 38, current can be efficiently extracted. That is, for example, a conversion efficiency of about 34 (%) can be obtained.
[0045]
FIG. 9 is a diagram schematically illustrating a cross-sectional structure of a tandem solar cell 110 according to still another embodiment. The tandem solar cell 110 has a two-junction structure in which a top cell 112 is provided on a bottom cell 12 via a bonding layer 18. The top cell 112 is made of, for example, GaN having a band gap energy of about 1.75 (eV) and a lattice constant of about 0.55 (nm). _0.04 P _0.76 As _0.2 A semiconductor solar cell comprising: p-GaN _0.04 P _0.76 As _0.2 P-type layer 114 comprising n-GaN _0.04 P _0.76 As _0.2 And an n-type layer 116 composed of The overall thickness of the top cell 112 is, for example, about 0.2 to 2 (μm), for example, about 0.8 (μm), and the thickness of the n-type layer 116 is, for example, about 300 (nm). It is.
[0046]
Also in the tandem solar cell 110 having the two-junction structure thus configured, the top cell 112 having the band gap energy of 1.75 (eV) is placed on the bottom cell 12 having the band gap energy of 1.1 (eV). Is provided, a high conversion efficiency can be obtained by absorbing sunlight over a wide wavelength range based on the difference, and the current generated from each of them is related to the magnitude of their band gap energy. Are similar to each other on the basis of the above-described formula, there is an advantage that a two-terminal structure can be adopted while obtaining high conversion efficiency as in the case of the tandem solar cell 10. The conversion efficiency is, for example, about 34.5 (%).
[0047]
Note that, also in the present embodiment, the constituent material of the top cell 112 is determined based on the band gap energy and the lattice constant. That is, in relation to the theoretical energy conversion efficiency and the band gap energies of the bottom cell and the top cell as shown in FIG. 10, the band gap energy is a value at which high efficiency is obtained and the generated current values are similar to each other. It is determined in the range. This figure also shows the efficiency under the conditions of AM-1 and 27 ° C. In this embodiment, since the bandgap energy of the bottom cell 12 is 1.1 (eV), the constituent material of the top cell 112 has a bandgap energy of 1.65 to 1.85 (eV) for obtaining high efficiency. Materials within the range of ()) are selected. This range is almost the same as the range in which high efficiency is obtained, as in the case of three junctions, but does not completely match.
[0048]
Then, among the materials having the band gap energy as described above, the material having the mixed crystal ratio as described above is selected as a material having a lattice constant similar to that of silicon constituting the bottom cell 12.
[0049]
As mentioned above, although one Example of this invention was described in detail based on drawing, this invention can be implemented in another aspect.
[0050]
For example, in the above embodiment, the case where the present invention is applied to the tandem solar cells 10 and 70 having three junctions and two junctions has been described. If joined, a structure having four or more joints may be used.
[0051]
In the embodiment, the middle cell 14 and the top cell 16 and the like are made of SiGeC / SiGeC, ZnSiSb / ZnSiP, CdSiAs / CdSiP, and the like, and the bottom cell 12 and the top cell 112 are made of Si / GaNPAs. Although the case where the present invention is applied to the tandem solar cell has been described, the present invention can be similarly applied to a tandem solar cell having another semiconductor solar cell.
[0052]
In the above embodiment, the case where the middle cell 14, the top cell 16 and the like are formed by using the MOCVD apparatus has been described. It is possible to do.
[0053]
In the above-described embodiment, the light receiving areas of the middle cell 14, the top cell 16, and the like are set to the same size as the bottom cell 12. However, the sizes are appropriately changed within a range where the current values are maintained at the same value. It may be.
[0054]
Further, in the embodiment, the bottom cell 12 is configured by inverting the surface of the p-Si substrate to the n-type, but these conductivity types may be opposite to each other.
[0055]
Further, the values of the band gap energies of the middle cell 14 and the top cell 16 can be appropriately selected from the ranges described above, and can be arbitrarily combined within this range.
[0056]
Although not specifically exemplified, the present invention can be variously modified without departing from the gist thereof.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing a cross-sectional structure of a tandem solar cell according to one embodiment of the present invention.
FIG. 2 is a diagram illustrating a configuration of a bonding layer provided between cells in the tandem solar cell of FIG.
FIG. 3 is a schematic diagram illustrating a use state of the tandem solar cell of FIG.
FIG. 4 is a diagram illustrating the theoretical energy conversion efficiency of a tandem solar cell having a three-junction structure in relation to the band gap energy of a middle cell and a top cell.
FIG. 5 is a diagram illustrating the relationship between the lattice constant of the constituent material of the tandem solar cell of FIG. 1 and the band gap energy.
6 is a process chart illustrating a method for manufacturing the tandem solar cell of FIG.
FIG. 7 is a diagram schematically showing a cross-sectional structure of a tandem solar cell according to another embodiment of the present invention.
FIG. 8 is a view schematically showing a cross-sectional structure of a tandem solar cell according to still another embodiment of the present invention.
FIG. 9 is a diagram schematically showing a cross-sectional structure of a tandem solar cell having a two-junction structure according to still another embodiment of the present invention.
FIG. 10 is a diagram illustrating the theoretical energy conversion efficiency of a tandem solar cell having a two-junction structure in relation to the band gap energy of the bottom cell and the top cell.
FIG. 11 is a diagram illustrating a method of using a conventional tandem solar cell.
[Explanation of symbols]
10: Tandem solar cell
12: Bottom cell
14: Middle cell
16: Top cell
34: Upper electrode
38: Lower electrode

Claims (4)

A tandem solar cell in which a plurality of types of semiconductor solar cells having different band gap energies are monolithically stacked via a tunnel junction layer,
The bandgap energy of each of the plurality of types of semiconductor solar cells is determined so that current values generated from the respective types become mutually coincident values. A bottom cell made of a semiconductor solar cell having a band gap energy of 1.1 (eV);
A middle cell comprising a semiconductor solar cell having a band gap energy of 1.4 to 1.5 (eV) provided on the bottom cell via a tunnel junction layer;
A top cell comprising a semiconductor solar cell having a band gap energy of 1.9 to 2.0 (eV) provided on the middle cell through a tunnel junction layer. A bottom cell made of a semiconductor solar cell having a band gap energy of 1.1 (eV);
A top cell comprising a semiconductor solar cell having a band gap energy of 1.65 to 1.85 (eV) provided on the bottom cell through a tunnel junction layer. 4. The tandem solar cell according to claim 2, wherein said bottom cell is made of single-crystal silicon.

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