JP4804571B2 - Compound solar cell and method for producing the same - Google Patents

Description

  The present invention relates to a compound solar cell and a manufacturing method thereof, and more particularly to a multi-junction type compound solar cell and a manufacturing method thereof.

  Among solar cells, there is a multi-junction group III-V compound solar cell as a solar cell having the highest efficiency and suitable for a space solar cell. An example of a method for producing such a multi-junction type III-V compound solar cell will be described.

First, as shown in FIG. 26, a Ge substrate 101 (or GaAs substrate) is used as a substrate. A bottom cell BB including a Ge pn junction is formed by epitaxially growing Ge on the surface of the Ge substrate 101 and the like and adding AsH ₃ or PH ₃ to thermally diffuse As or P.

  A middle cell MM including a GaAs pn junction is formed by epitaxially growing GaAs on the bottom cell BB. A top cell TT including an InGaP pn junction is formed by epitaxially growing InGaP on the middle cell MM.

  In this manner, a three-junction type III-V group compound solar cell 110 having a cell body CC in which three pn junctions of Ge / GaAs / InGaP are connected in series on the Ge substrate 101 is manufactured.

  The forbidden band width of InGaP forming the top cell TT is about 1.7 to 2.1 eV, the forbidden band width of GaAs forming the middle cell MM is about 1.3 to 1.6 eV, and the forbidden band width of Ge forming the bottom cell BB. The band width is about 0.7 eV or less.

  Sun rays are incident from the top cell TT (InGaP) side and travel toward the bottom cell BB (Ge). Meanwhile, light of a predetermined wavelength based on the forbidden bandwidth of each of the top cell TT, the middle cell MM, and the bottom cell BB is absorbed and converted into electric energy.

  However, the value of the forbidden band width (about 0.7 eV or less) of Ge forming the bottom cell BB is relatively small in terms of converting light energy into electric energy, and thus 0.9 to 0.9 as a material having higher conversion efficiency. Application of a material having a forbidden bandwidth of about 1.1 eV is being studied.

  Non-Patent Document 1 proposes InGaAs as one of such materials. In the multi-junction solar cell 110 using InGaAs instead of Ge, as shown in FIG. 27, a bottom cell NN including an InGaAs pn junction is formed on the surface of the Ge substrate 101 (or GaAs substrate) by epitaxial growth. .

  On the bottom cell NN, a middle cell MM including a GaAs pn junction and a top cell TT including an InGaP pn junction are formed by epitaxial growth.

  In addition to InGaAs, InGaAsN is proposed in Non-Patent Document 2 as a material replacing Ge.

M. Tamura et al, 'Threading doislocations in InXGa1-XAs / GaAs heterostructures', J. Appl. Phys., 72 (8), 15 October, (1992) p. 3398. J.F.Geisz et al, 'Photocurrent of 1eV GaInNAs lattice-matched to GaAs', J. Crystal Growth, 195 (1998) p.401.

  However, the multi-junction solar cell 110 having the bottom cell NN using InGaAs or InGaAsN instead of Ge has the following problems.

  First, in a multi-junction solar cell using InGaAs (0.9 to 1.1 eV) as the bottom cell NN, the lattice constant of the Ge substrate 101 (or GaAs substrate) is different from the lattice constant of InGaAs. Therefore, in InGaAs grown epitaxially, dislocations (hereinafter referred to as “misfit dislocations”) due to the difference from the lattice constant of the base (such as a GaAs substrate) occur.

  In a multi-junction solar cell using InGaAsN as the bottom cell, the composition of N atoms is controlled so that the lattice constant of InGaAsN matches the lattice constant of the base. This prevents misfit dislocations from occurring in epitaxially grown InGaAsN.

  However, vacancies or the like of the added N atoms themselves are generated. As a result, defects caused by N atoms are generated in epitaxially grown InGaAsN.

  As described above, in the bottom cell to which InGaAs or InGaAsN is applied, misfit dislocations and defects are generated, so that the cell quality is not good and a desired power generation amount cannot be achieved.

  Since misfit dislocations and defects are generated in the bottom cell NN, the influence is exerted on the GaAs of the middle cell MM epitaxially grown on the bottom cell NN and further on the InGaP of the top cell TT.

  Therefore, the quality of GaAs and InGaP as a cell deteriorates, and there is a problem that improvement in conversion efficiency to electric energy is hindered.

  Further, as described above, while sunlight enters from the top cell T side and travels toward the bottom cell B, light having a predetermined wavelength is absorbed and converted into electric energy.

  At this time, the component of the sunlight that was not absorbed in the top cell TT to the bottom cell BB was absorbed by the Ge substrate 101 (or GaAs substrate), and could not contribute effectively to power generation. Therefore, there has been a problem that improvement in the efficiency of conversion to electric energy is hindered.

  The present invention has been made to solve the above problems, and one object is to provide a compound solar cell that can improve the conversion efficiency into electric energy, and the other object is to provide such a compound solar cell. It is providing the manufacturing method of a compound solar cell.

  The compound solar battery according to one aspect of the present invention includes a cell body, a first electrode portion, and a second electrode portion. The cell body has at least one pn junction layer made of a single crystal and receives sunlight. The first electrode portion is directly formed on the surface of the cell body opposite to the side on which the sunlight is incident, and has a predetermined thickness for supporting the cell body. The second electrode portion is formed on the surface of the cell body on the side on which sunlight is incident.

  According to this structure, the 1st electrode part is directly formed in the surface on the opposite side to the side into which a solar ray injects in a cell main body. As a result, it is incident on the cell body and is not absorbed by the cell body as compared to the conventional structure in which a predetermined substrate for epitaxial growth is disposed on the surface opposite to the side on which the solar light is incident in the cell body. The component of the sunlight is reflected by the first electrode part. As a result, the light confinement effect is improved, and the conversion efficiency of the compound solar cell can be increased.

  Moreover, it is preferable that a cell main body and a 1st electrode part are flexible, or the 1st electrode part has the thickness which can bend.

Thereby, the freedom degree of the shape of a compound solar cell improves.
More specifically, as the structure of the cell body, the cell body includes a plurality of pn junction layers made of compounds having different forbidden band widths, and each of the plurality of pn junction layers receives sunlight from the first electrode portion side. It is preferable that the forbidden band width is increased toward the incident side.

  More specifically, the plurality of pn junction layers are formed on the first electrode portion, and are formed on the first pn junction layer of the III-V group compound having the first forbidden band width and the first pn junction layer. And a second pn junction layer made of a III-III-V group compound having a second forbidden band width higher than the first forbidden band width.

  Alternatively, the plurality of pn junction layers are formed on the first electrode portion, and are formed on the first pn junction layer of the III-III-V group compound having the first forbidden band width and the first pn junction layer, A second pn junction layer made of a III-V group compound having a second forbidden band width higher than the first forbidden band width, and a third forbidden band width formed on the second pn junction layer and higher than the second forbidden band width It is preferable to include a third pn junction layer made of a III-III-V group compound having:

  Alternatively, the plurality of pn junction layers are formed on the first electrode portion, and are formed on the first pn junction layer including the I-III-VI group compound having the first forbidden band width and the first pn junction layer. , A second pn junction layer made of a III-III-V group compound having a second forbidden band width higher than the first forbidden band width, and a third pn junction layer formed on the second pn junction layer and higher than the second forbidden band width. It is preferable to include a third pn junction layer made of a III-III-V group compound having a forbidden band width. Note that Group I, Group III, Group V and Group VI represent groups in the periodic table of elements.

  The method for producing a compound solar cell according to another aspect of the present invention includes the following steps. A layer to be a first cell having a first forbidden band width is formed on the surface of the semiconductor substrate by epitaxial growth. A layer to be a second cell having a second forbidden band width lower than the first forbidden band width is formed on the layer to be the first cell. On the layer to be the second cell, the first electrode portion having a predetermined thickness for supporting the layer to be the first cell and the layer to be the second cell is directly formed. A support plate for supporting the first electrode part is attached to the surface of the first electrode part. The layer serving as the first cell is separated from the semiconductor substrate. A second electrode portion is formed on the surface of the layer to be the first cell that is separated and exposed from the semiconductor substrate. The support plate is removed after the second electrode portion is formed.

  According to this manufacturing method, the layer to be the first cell that will be located on the side on which sunlight is incident in the completed state is formed on the semiconductor substrate first, on the side opposite to the side on which sunlight is incident. The layer that will be the second cell to be located is formed later. Accordingly, even when a material having a relatively high forbidden band width is used as the second forbidden band width of the layer serving as the second cell, the quality of the layer serving as the second cell does not reach the layer serving as the first cell. . In addition, since the first electrode portion is directly formed on the layer serving as the second cell, the component of sunlight that has not been absorbed by the layer serving as the first cell and the layer serving as the second cell is reflected by the first electrode portion. Will be. Thereby, the light confinement effect is improved. As a result, the conversion efficiency as a compound solar cell can be improved.

  Further, a third forbidden band width that is lower than the first forbidden band width and higher than the second forbidden band width is provided between the step of forming the layer that becomes the first cell and the step of forming the layer that becomes the second cell. It is preferable to provide the process of forming the layer used as a 3rd cell.

  Thereby, in the layer used as each cell, the component of the sunlight of the predetermined wavelength according to each forbidden band width is absorbed, and conversion efficiency can be improved more.

  Further, in order to separate the semiconductor substrate, specifically, a step of forming a predetermined intermediate layer by epitaxial growth between the layer to be the first cell and the semiconductor substrate, and the layer to be the first cell and the semiconductor The step of separating the substrate preferably includes a step of removing the semiconductor substrate by etching and further removing the intermediate layer.

  Alternatively, the method includes a step of forming a predetermined intermediate layer by epitaxial growth between the layer serving as the first cell and the semiconductor substrate, and the step of separating the layer serving as the first cell from the semiconductor substrate is performed by removing the intermediate layer by etching. It is preferable to include a step of removing the semiconductor substrate.

  In particular, in this case, the semiconductor substrate can be reused.

It is sectional drawing which shows 1 process of the manufacturing method of the compound solar cell which concerns on the related technique 1 relevant to this invention. It is a sectional view showing a step performed after the step shown in FIG. FIG. 3 is a cross-sectional view showing a step performed after the step shown in FIG. 2. FIG. 4 is a cross-sectional view showing a step performed after the step shown in FIG. 3. It is a sectional view showing a step performed after the step shown in FIG. FIG. 6 is a cross-sectional view showing a step performed after the step shown in FIG. 5. It is a perspective view which shows the external appearance of the completed compound solar cell based on the related technique 1. FIG. It is a fragmentary sectional view for explaining an effect of a compound solar cell concerning related technology 1 . It is a fragmentary sectional view which shows the effect | action of the compound solar cell used as the comparison for demonstrating the effect of the compound solar cell which concerns on related technology 1. FIG. It is a figure which shows the current-voltage characteristic of the compound solar cell by the solar simulator based on the related technique 1. FIG. It is sectional drawing which shows the compound solar cell which concerns on the related technique 2 relevant to this invention. It is sectional drawing which shows the compound solar cell used as the comparison for demonstrating the effect of the compound solar cell which concerns on the related technique 2. FIG. It is sectional drawing which shows 1 process of the manufacturing method of the compound solar cell which concerns on Embodiment 1 of this invention. FIG. 14 is a cross-sectional view showing a step performed after the step shown in FIG. 13 in the same embodiment. FIG. 15 is a cross-sectional view showing a step performed after the step shown in FIG. 14 in the same embodiment. FIG. 16 is a cross-sectional view showing a step performed after the step shown in FIG. 15 in the same embodiment. FIG. 17 is a cross-sectional view showing a step performed after the step shown in FIG. 16 in the same embodiment. FIG. 18 is a cross-sectional view showing a step performed after the step shown in FIG. 17 in the same embodiment. In the same embodiment, it is a figure which shows the current voltage characteristic of the compound solar cell by a solar simulator. It is sectional drawing which shows 1 process of the manufacturing method of the compound solar cell which concerns on the related technique 3 relevant to this invention. FIG. 21 is a cross-sectional view showing a step performed after the step shown in FIG. 20. FIG. 22 is a cross-sectional view showing a step performed after the step shown in FIG. 21. FIG. 23 is a cross-sectional view showing a step performed after the step shown in FIG. 22. FIG. 24 is a cross-sectional view showing a step performed after the step shown in FIG. 23. FIG. 25 is a cross-sectional view showing a step performed after the step shown in FIG. 24. It is sectional drawing which shows the conventional compound solar cell. It is sectional drawing which shows the other conventional compound solar cell.

Related Technology 1
A compound solar cell according to Related Art 1 relating to the present invention will be described. Here, a two-junction type compound solar battery having a bottom cell and a top cell is taken as an example of the cell body of the compound solar battery.

First, the manufacturing method will be described. As a substrate, a GaAs substrate (1 × 10 ¹⁸ cm ⁻³ , Si-doped, diameter 50 mm) is prepared. The GaAs substrate is put into, for example, a vertical MOCVD (Metal Organic Chemical Vapor Deposition) apparatus.

  Next, as shown in FIG. 1, an n-type InGaP layer 3 having a thickness of about 0.5 μm is formed on the surface of the GaAs substrate 1 by an epitaxial growth method. The InGaP layer 3 serves as an intermediate layer between the cell body formed on the InGaP layer 3 and the GaAs substrate 1.

  Next, each single-crystal layer serving as the top cell T is formed on the InGaP layer 3 by epitaxial growth. Specifically, an n-type GaAs layer T1, an n-type AlInP layer T2, an n-type InGaP layer T3, a p-type InGaP layer T4, and a p-type AlInP layer T5 are sequentially formed.

  Next, on the AlInP layer T5, a p-type AlGaAs layer 5 and an n-type InGaP layer 7 are sequentially formed as an epitaxial growth method as a tunnel junction.

  Next, each single-crystal layer serving as the bottom cell B is formed on the n-type InGaP layer 7 by an epitaxial growth method. Specifically, an n-type AlInP layer B1, an n-type GaAs layer B2, a p-type GaAs layer B3, a p-type InGaP layer B4, and a p-type GaAs layer B5 are sequentially formed.

As a condition for epitaxial growth, the temperature is about 700 ° C. TMG (trimethylgallium) and AsH ₃ (arsine) are used as raw materials for growing the GaAs layer.

As materials for growing the InGaP layer, TMI (trimethylindium), TMG and PH ₃ (phosphine) are used. As raw materials for growing the AlInP layer, TMA (trimethylaluminum), TMI and PH ₃ are used.

In addition, SiH ₄ (monosilane) is used as an impurity for forming the n-type GaAs layer, InGaP layer, and AlInP layer, respectively. On the other hand, DEZn (diethyl zinc) is used as an impurity for forming the p-type GaAs layer, InGaP layer, and AlInP layer, respectively.

Further, TMI, TMG, and AsH ₃ are used as raw materials for growing the AlGaAs layer, and CBr ₄ (carbon tetrabromide) is used as an impurity for forming the p-type AlGaAs layer.

  In this way, the cell body C in the compound solar battery composed of the top cell T and the bottom cell B is formed.

  Next, an Au—Zn film (not shown) is deposited on the surface of the cell body C (p-type GaAs layer of the bottom cell). Thereafter, heat treatment is performed for about 1 minute at a temperature of about 400 ° C. in a nitrogen atmosphere.

  Next, a resist (not shown) is applied to the back surface of the GaAs substrate 1 and thermally cured. Next, an Au plating film having a film thickness of about 30 μm is formed on the Au—Zn film by electroplating.

  In this way, as shown in FIG. 2, the back electrode 9 made of the Au plating film is formed on the cell body C. Thereafter, the resist formed on the back surface of the GaAs substrate 1 is removed.

  Next, for example, wax 11 is applied onto the back electrode 9 as a temporary adhesive, and the glass plate 13 and the back electrode 9 are bonded together. Next, the GaAs substrate 1 is removed by immersing the GaAs substrate 1 supported by the glass substrate 13 in an alkaline solution such as ammonia water.

  Here, for example, the GaAs substrate 1 having a thickness of about 350 μm is completely etched away by being immersed in an alkaline solution for about 300 minutes. Etching is stopped when the InGaP layer 3 as an intermediate layer is exposed.

  At this time, the InGaP layer 3 as an intermediate layer is etched by immersing the GaAs substrate 1 supported on the glass substrate 13 in an acid solution such as HCl to separate the GaAs substrate 1. Also good.

  Next, by etching with an acid solution, the exposed InGaP layer 3 as an intermediate layer is removed, and the n-type GaAs layer T1 of the top cell T is exposed. In this way, as shown in FIG. 3, the surface of the cell main body C (particularly, the top cell T) is exposed.

  Next, a predetermined resist pattern (not shown) for forming a surface electrode is formed on the exposed surface of the cell body C (top cell) by photolithography.

  Next, the cell main body C on which the resist pattern is formed is introduced into a vacuum deposition apparatus (not shown) together with the glass substrate 13. An Au (containing 12 wt% Ge) film (not shown) having a film thickness of about 100 nm is formed by a resistance heating method so as to cover the resist pattern.

  Thereafter, an Ni layer having a thickness of about 20 nm and an Au layer having a thickness of about 5000 nm (both not shown) are successively formed by EB (Electron Beam) vapor deposition.

  Next, the resist pattern and the Au film formed on the resist pattern are removed by a lift-off method. In this way, the surface electrode 15 is formed as shown in FIG.

  Next, etching with an alkaline aqueous solution is performed using the surface electrode 15 as a mask, thereby removing the exposed GaAs layer and exposing the AlInP layer (see FIG. 6).

  Next, a predetermined resist pattern (not shown) for mesa etching is formed so as to cover the surface electrode 15. Etching with an aqueous alkali solution and an acid solution is performed using the resist pattern as a mask, so that the Au plating film forming the back electrode 9 is exposed.

Next, a TiO ₂ film having a film thickness of about 55 nm and a MgF ₂ film having a film thickness of about 100 nm (both not shown) are continuously formed as an antireflection film on the surface (front surface) on which sunlight is incident by EB vapor deposition. Formed. Next, the glass substrate 13 is separated from the back electrode 9 as shown in FIG.

  Then, by cutting the Au plating film along the exposed line-shaped Au plating film, for example, 12 compound solar cells having a size of 10 mm × 10 mm are manufactured.

  FIG. 6 shows a cross-sectional structure of the compound solar cell thus manufactured. As shown in FIG. 6, compared with the structure of a conventional compound solar cell in which a bottom cell is formed on a predetermined substrate for epitaxial growth (see, for example, FIG. 26 or FIG. 27), in the above-described compound solar cell, The back electrode 9 is directly formed on the bottom cell B of the cell body C. On the other hand, a surface electrode 15 is formed on the surface of the top cell T of the cell body C.

  The cell body C includes a bottom cell B having a pn junction made of GaAs (III-V group compound) and a top cell A having a pn junction made of InGaP (III-III-V group compound).

  The cell body C has a thickness L1 of about 4 μm, and the back electrode 9 has a thickness L2 of about 30 μm. Thereby, the cell main body C and the back surface electrode 9 have flexibility, and for example, as shown in FIG. 7, the compound solar battery 10 can be flexed freely.

  In the compound solar cell described above, each layer that becomes the top cell T is sequentially formed on the GaAs substrate 1 for epitaxial growth by the epitaxial growth method, and each layer that becomes the bottom cell B is sequentially formed on the top cell T.

  Then, the GaAs substrate 1 is separated from the cell body C, the back electrode 9 is directly formed on the bottom cell B, and the cell body C is supported by the back electrode 9. By forming the back electrode 9 directly on the surface of the bottom cell B, it is possible to improve the light confinement effect when sunlight is incident.

  That is, as shown in FIG. 8, in the above-described compound solar battery, a component of sunlight that is not absorbed by the cell body C when passing through the cell body C is reflected by the back electrode 9. Thereby, the light confinement effect to the cell main body C is improved, and the component of the sunlight reflected by the back electrode 9 contributes to power generation. As a result, the conversion efficiency of the solar battery cell can be improved.

  On the other hand, in the conventional compound solar battery, as shown in FIG. 9, the substrate 101 for epitaxial growth is located on the bottom cell side of the cell body CC. For this reason, the components of sunlight that are not absorbed by the cell body CC when passing through the cell body CC are absorbed by the substrate 101 and cannot contribute to power generation.

  Next, the solar cell was evaluated with a solar simulator. This will be described. A solar simulator is an irradiation light source used to perform solar cell characteristic tests and reliability tests indoors. It satisfies the irradiance, uniformity and spectrum matching required for the test purpose. The

  First, a reference solar ray of air mass (AM) 1.5G was used as an irradiation light source. And the current-voltage characteristic at the time of irradiation was measured. Based on the current-voltage characteristics, the short circuit current, the release voltage, the fill factor and the conversion efficiency were determined.

  Here, the air mass refers to a ratio of a path through which direct sunlight incident on the earth passes to a path when sunlight enters the normal atmosphere (standard atmospheric pressure 1013 hPa) vertically.

  A short circuit current means the electric current which flows between both output terminals when the output terminal of a photovoltaic cell (module) is short-circuited. The release voltage refers to a voltage between the output terminals when the output terminal of the solar battery cell (module) is released.

  The fill factor is a value obtained by dividing the maximum output by the product of the release voltage and the short-circuit current. The conversion efficiency is a value (%) obtained by dividing the maximum output by the product of the area of the solar cell (module) and the irradiance.

  FIG. 10 shows the measured current-voltage characteristics (IV curve). In this case, the short-circuit current was 10.1 mA, the release voltage was 2.39 V, the fill factor was 0.85, and the conversion efficiency was 20.5%.

  From these results, the above-described compound solar cell has the same level or better than the conventional two-junction compound solar cell having two pn junctions of InGaAs and GaAs formed on a GaAs substrate. It was found that a satisfactory result was obtained.

Related technology 2
Here, in order to confirm the light confinement effect by the back electrode, a compound solar cell having a structure different from the structure of the cell body described above will be described as an example, and the evaluation performed on this will be described.

First, as shown in FIG. 11, in the compound solar battery according to the related technology , the cell body C is directly formed on the surface of the back electrode 9. In the cell body C, a p-type InGaP layer 21 is formed on the back electrode 9. A p-type GaAs layer 22 is formed on the InGaP layer 21.

  An n-type GaAs layer 23 is formed on the GaAs layer 22. An n-type InGaP layer 24 is formed on the GaAs layer 23. Then, a surface electrode 15 is formed at a predetermined position on the InGaP layer 24 through a contact by the n-type GaAs layer 25.

  This compound solar cell is formed by the same method as the compound solar cell described above. That is, first, layers from the n-type GaAs layer 25 to the p-type InGaP layer 21 are sequentially formed on a predetermined substrate (not shown). Next, the back electrode 9 is formed on the bottom cell side, and the substrate is separated.

  On the other hand, as shown in FIG. 12, in a comparative compound solar cell, a p-type InGaP layer 121 is formed on the surface of a p-type GaAs substrate 101. A p-type GaAs layer 122 is formed on the InGaP layer 121.

  An n-type GaAs layer 123 is formed on the GaAs layer 122. An n-type InGaP layer 124 is formed on the GaAs layer 123. Then, a surface electrode 115 is formed at a predetermined position on the InGaP layer 124 through a contact by the n-type GaAs layer 125.

  A comparative compound solar cell is formed by sequentially growing each layer on a p-type GaAs substrate 101 by an epitaxial growth method.

  When the above-described compound solar cell was compared with the above-described compound solar cell by the solar simulator, the short circuit current was 19 mA, the release voltage was 1.03 V, and the fill factor in the compound solar cell according to the present embodiment. Was found to be 0.84 and the conversion efficiency was 16.4%.

  On the other hand, in the compound solar cell to be compared, the short-circuit current was 15 mA, the release voltage was 1.03 V, the fill factor was 0.84, and the conversion efficiency was 13.0%.

  Thus, in the compound solar cell according to the present embodiment, it has been found that the conversion efficiency is particularly increased as compared with the comparative compound solar cell, and the light confinement effect by the back electrode 9 can be improved.

Embodiment 1
The compound solar cell according to Embodiment 1 of the present invention will be described. Here, an example of a three-junction type compound solar cell having a bottom cell, a middle cell, and a top cell is given as the cell body of the compound solar cell.

First, the manufacturing method will be described. First, a GaAs substrate (1 × 10 ¹⁸ cm ⁻³ , Si-doped, diameter 50 mm) is prepared as a substrate. The GaAs substrate is put into a vertical MOCVD apparatus.

  Next, as shown in FIG. 13, an n-type AlAs layer 4 serving as an intermediate layer having a thickness of about 0.5 μm is formed on the surface of the GaAs substrate 1 by an epitaxial growth method.

  Next, each layer to be the top cell T is formed on the AlAs layer 4 by an epitaxial growth method. Specifically, an n-type GaAs layer T1, an n-type AlInP layer T2, an n-type InGaP layer T3, a p-type InGaP layer T4, and a p-type AlInP layer T5 are sequentially formed.

  Next, a p-type AlGaAs layer 5 and an n-type InGaP layer 7 are sequentially formed as a tunnel junction on the AlInP layer T5 by epitaxial growth.

  Next, each layer to be the middle cell M is formed on the n-type InGaP layer 7 by an epitaxial growth method. Specifically, an n-type AlInP layer M1, an n-type GaAs layer M2, a p-type GaAs layer M3, and a p-type InGaP layer M4 are sequentially formed.

  Next, a p-type GaAs layer 6 and an n-type GaAs layer 8 are sequentially formed as a tunnel junction on the p-type InGaP layer M4 by an epitaxial growth method.

  Next, each layer to be the bottom cell B is formed on the n-type GaAs layer 8 by an epitaxial growth method. Specifically, an n-type InP layer B6, an n-type InGaAs layer B7, a p-type InGaAs layer B8, a p-type InP layer B9, and a p-type GaAs layer B10 are sequentially formed.

As a condition for epitaxial growth, the temperature is about 700 ° C. TMG (trimethylgallium) and AsH ₃ (arsine) are used as raw materials for growing the GaAs layer.

As materials for growing the InGaP layer, TMI (trimethylindium), TMG and PH ₃ (phosphine) are used. As raw materials for growing the AlInP layer, TMA (trimethylaluminum), TMI and PH ₃ are used.

In addition, SiH ₄ (monosilane) is used as an impurity for forming the n-type GaAs layer, InGaP layer, and AlInP layer, respectively. On the other hand, DEZn (diethyl zinc) is used as an impurity for forming the p-type GaAs layer, InGaP layer, and AlInP layer, respectively.

Further, TMI, TMG, and AsH ₃ are used as raw materials for growing the AlGaAs layer, and CBr ₄ (carbon tetrabromide) is used as an impurity for forming the p-type AlGaAs layer.

  The In composition ratio in the InGaAs layer was 0.25, and a cross-hatch pattern morphology indicating the presence of misfit dislocations was observed in the InGaAs layer.

  In this manner, the cell body C in the three-junction type compound solar battery including the top cell T, the middle cell M, and the bottom cell B is formed.

  Next, a predetermined resist pattern (not shown) for forming a back electrode is formed on the surface of the cell body C (p-type GaAs layer of the bottom cell). An Au—Zn film (not shown) is deposited so as to cover the resist pattern.

  Next, the resist pattern and the Au—Zn film located on the resist pattern are removed by a lift-off method. Thereafter, heat treatment is performed for about 1 minute at a temperature of about 400 ° C. in a nitrogen atmosphere.

  Next, a predetermined resist pattern (not shown) is formed except for the region where the Au—Zn film is formed. Further, a resist (not shown) is applied to the surface of the GaAs substrate 1 on the side where the cell body C is not formed.

  Next, an Au plating film (not shown) having a thickness of about 30 μm is formed on the Au—Zn film by electroplating. Thereafter, the resist pattern and the Au plating film located on the resist pattern are removed by a lift-off method. Thereby, as shown in FIG. 14, the back surface electrode 9 by the Au plating film is formed on the cell body.

  Next, as shown in FIG. 15, a predetermined resist is formed so as to cover the back electrode 9 in the region where the back electrode 9 is formed and to expose the surface of the cell body C in the region where the back electrode 9 is not formed. A pattern 17 is formed.

  By etching the alkaline solution and the acid solution using the resist pattern 17 as a mask, the exposed portion of the cell body C is removed and the AlAs layer 3 as an intermediate layer is exposed. Thereafter, the resist pattern 17 is removed.

  Next, a wax 11 is interposed on the surface of the back electrode 9, and a mesh-shaped resin plate 19 having chemical resistance is attached to the back electrode 9 side (see FIG. 16). With the resin plate 19 attached to the back electrode 9 side, the cell body C and the back electrode 9 are immersed in the hydrofluoric acid solution.

  The AlAs layer 4 is removed by being immersed in the hydrofluoric acid solution, and the cell body C and the GaAs substrate 1 are separated as shown in FIG. In this way, the GaAs substrate 1 is separated, and the n-type GaAs layer of the top cell T in the cell body C is exposed.

  Next, a predetermined resist pattern (not shown) for forming a surface electrode is formed on the exposed surface of the GaAs layer. Next, the cell body C on which the resist pattern is formed is introduced into a vacuum vapor deposition apparatus (not shown) together with the resin plate 19.

  An Au (containing 12 wt% Ge) film having a thickness of about 100 nm is formed by resistance heating so as to cover the resist pattern. Thereafter, an Ni layer having a thickness of about 20 nm and an Au layer having a thickness of about 5000 nm (both not shown) are successively formed by EB vapor deposition.

  Thereafter, the resist pattern and the Au film formed on the resist pattern are removed by a lift-off method. In this way, the surface electrode 15 is formed as shown in FIG.

  Next, etching with an alkaline aqueous solution is performed using the surface electrode 15 as a mask, thereby removing the exposed GaAs layer and exposing the AlInP layer (see FIG. 18).

Next, a TiO ₂ film having a film thickness of about 55 nm and a MgF ₂ film having a film thickness of about 100 nm (both not shown) are continuously formed as an antireflection film on the surface (front surface) on which sunlight is incident by EB vapor deposition. Formed. Next, for example, by removing the wax 11 with toluene, the resin plate 19 is separated from the back electrode 9 as shown in FIG.

  Then, by cutting the Au plating film along the exposed line-shaped Au plating film, for example, 12 compound solar cells having a size of 10 mm × 10 mm are manufactured.

  FIG. 18 shows a cross-sectional structure of the compound solar cell thus manufactured. As shown in FIG. 18, when compared with the structure of a conventional compound solar cell in which a bottom cell is formed on a predetermined substrate for epitaxial growth (see, for example, FIG. 26 or FIG. 27), in the above-described compound solar cell, The back electrode 9 is directly formed on the bottom cell B of the cell body C.

  On the other hand, a surface electrode 15 is formed on the surface of the top cell T of the cell body C. A middle cell M is formed between the top cell T and the bottom cell B.

  The cell body C includes a bottom cell having a pn junction made of InGaAs (III-III-V compound), a middle cell M having a pn junction made of GaAs (III-V compound), and a pn made of InGaP (III-III-V compound). A top cell A having a junction is provided.

  The cell body C has a thickness L1 of about 6 μm, and the back electrode 9 has a thickness L2 of about 30 μm. Thereby, like the compound solar battery described above, the cell body C and the back electrode 9 have flexibility, and the compound solar battery can be flexed freely.

  In the above-described solar battery cell, first, each layer to be the top cell T having a forbidden band width of about 1.7 to 2.1 eV is sequentially formed on the GaAs substrate 1 for epitaxial growth by the epitaxial growth method.

  Next, each layer to be a middle cell M having a forbidden band width of about 1.3 to 1.6 eV is sequentially formed on the top cell T. Further, on the middle cell M, the layers to be the bottom cell B having a forbidden band width of about 0.9 to 1.1 eV are sequentially formed.

  Thus, in the compound solar cell described above, each layer forming the top cell T is formed first, and the layer forming the bottom cell B is formed last.

  Thus, even if a material having a high forbidden band width (about 0.9 to 1.1 eV) is used as the bottom cell B compared to the conventional material (up to 0.7 eV), the quality of the bottom cell B is the middle cell M and the top cell. The conversion efficiency as a compound solar cell can be improved without reaching T. This will be described in more detail.

  In a conventional method for manufacturing a compound solar cell, each layer forming a bottom cell is formed on a Ge substrate (or GaAs substrate) for epitaxial growth first, and finally each layer forming a top cell is formed.

  At this time, as described above, when InGaAs having a relatively high forbidden band width (about 0.9 to 1.1 eV) is applied as the material forming the bottom cell, the lattice constant of the Ge substrate (GaAs substrate) Since the lattice constant of InGaAs is different, misfit dislocations occur in the InGaAs layer.

  In addition, when InGaAsN is applied as the bottom cell, defects caused by N atoms occur in InGaAsN.

  Since misfit dislocations and defects are generated in the bottom cell, the influence is exerted on the GaAs layer forming the middle cell epitaxially grown on the bottom cell and further on the InGaP layer forming the top cell.

  Therefore, the quality of the middle cell and the top cell is also deteriorated, and the improvement of the conversion efficiency to electric energy as a compound solar cell is hindered.

  On the other hand, in the above-described compound solar battery, each layer forming the top cell T and each layer forming the middle cell M are sequentially formed on the surface of the GaAs substrate 1, and finally, a layer forming the bottom cell B is formed.

  At this time, the lattice constant of InGaAs forming the bottom cell B is different from the lattice constant of GaAs forming the middle cell M. Therefore, the quality of the bottom cell B formed on the middle cell M is the same level as the quality of the conventional compound solar cell.

  On the other hand, the lattice constants of InGaP forming the top cell T and GaAs forming the middle cell M are the same as the lattice constant of the GaAs substrate 1 for epitaxial growth. Therefore, dislocations, defects, and the like do not occur in the InGaP layer and the GaAs layer that are sequentially epitaxially grown on the GaAs substrate 1.

  That is, in the above-described compound solar battery, even if the quality of the bottom cell B is the same level as the quality of the bottom cell in the conventional compound solar battery, the top cell T and the middle cell M are formed first, so that the bottom cell B The degradation of quality does not reach the middle cell M and the top cell T at all.

  As a result, even when a material having a relatively high forbidden bandwidth is applied as the bottom cell B such as InGaAs, the quality of the middle cell M and the top cell T is not deteriorated, and the conversion efficiency as a compound solar cell is improved. be able to.

  Next, about the compound solar cell mentioned above, the evaluation by the solar simulator mentioned above is demonstrated. First, FIG. 19 shows measured current-voltage characteristics (IV curve). In this case, the short-circuit current was 10.2 mA, the release voltage was 2.49 V, the fill factor was 0.85, and the conversion efficiency was 21.6%.

  From these results, the above-described compound solar cell has a higher release voltage and higher than the conventional two-junction type compound solar cell having two pn junctions of InGaAs and GaAs formed on a GaAs substrate. It was found that conversion efficiency can be obtained.

Related technology 3
A compound solar cell according to Related Art 3 relating to the present invention will be described. Here, another example of a three-junction compound solar battery having a bottom cell, a middle cell, and a top cell as a cell body of the compound solar battery will be described.

  First, the manufacturing method will be described. First, each layer forming the top cell T and each layer forming the middle cell M are sequentially formed on the surface of the GaAs substrate 1 by a method similar to the method described in the first embodiment and the like, as shown in FIG.

  Next, a p-type GaAs layer 6 and an n-type GaAs layer 8 are sequentially formed as a tunnel junction on the InGaP layer M4 of the middle cell M. Next, each layer to be the bottom cell B is formed on the GaAs layer 8.

Specifically, an ITO (Indium Tin Oxide) film 10, a CdS film B11, and a CuInSe ₂ film B12 are sequentially formed. The ITO film 12 is formed by sputtering, for example. The CdS film B11 is formed by, for example, a vapor deposition method. The CuInSe ₂ film is formed by, for example, a vapor deposition method.

  In this manner, the cell body C in the three-junction type compound solar battery including the top cell T, the middle cell M, and the bottom cell B is formed.

Next, a predetermined resist pattern (not shown) for forming a back electrode is formed on the surface of the cell body C (p-type CuInSe ₂ film of the bottom cell). A Mo film (not shown) is deposited so as to cover the resist pattern.

  Next, the resist film and the Mo film located on the resist pattern are removed by a lift-off method. Thereafter, heat treatment is performed for about 1 minute at a temperature of about 400 ° C. in a nitrogen atmosphere.

  Next, a predetermined resist pattern (not shown) is formed except for the region where the Mo film is formed. Further, a resist (not shown) is applied to the surface of the GaAs substrate 1 on the side where the cell body C is not formed.

  Next, an Au plating film (not shown) having a thickness of about 30 μm is formed on the Mo film by electroplating. Thereafter, the resist pattern and the Au plating film located on the resist pattern are removed by a lift-off method. Thereby, as shown in FIG. 21, the back electrode 9 by the Au plating film is formed on the cell body.

  Next, as shown in FIG. 22, a predetermined resist is formed so as to cover the back electrode 9 in the region where the back electrode 9 is formed and to expose the surface of the cell body C in the region where the back electrode 9 is not formed. A pattern 17 is formed.

  By performing predetermined etching using the resist pattern 17 as a mask, the exposed portion of the cell body C is removed, and the AlAs layer 3 as an intermediate layer is exposed. Thereafter, the resist pattern 17 is removed.

  Next, as shown in FIG. 23, a resin plate 19 having a mesh-like chemical resistance is attached to the back electrode 9 side with wax 11 interposed on the surface of the back electrode 9. With the resin plate 19 attached to the back electrode 9 side, the cell body C and the back electrode 9 are immersed in the hydrofluoric acid solution.

  The AlAs layer 3 is removed by being immersed in the hydrofluoric acid solution, and the cell body C and the GaAs substrate 1 are separated. By separating the GaAs substrate 1 in this manner, the n-type GaAs layer of the top cell in the cell body C is exposed.

  Next, a predetermined resist pattern (not shown) for forming a surface electrode is formed on the exposed surface of the GaAs layer. Next, the cell body C on which the resist pattern is formed is introduced into a vacuum vapor deposition apparatus (not shown) together with the resin plate 19.

  An Au (containing 12 wt% Ge) film having a thickness of about 100 nm is formed by resistance heating so as to cover the resist pattern. Thereafter, an Ni layer having a thickness of about 20 nm and an Au layer having a thickness of about 5000 nm (both not shown) are successively formed by EB vapor deposition.

  Thereafter, the resist pattern and the Au film formed on the resist pattern are removed by a lift-off method. In this way, the surface electrode 15 is formed as shown in FIG.

  Next, etching with an alkaline aqueous solution is performed using the surface electrode 15 as a mask, thereby removing the exposed GaAs layer and exposing the AlInP layer (see FIG. 25).

Next, a TiO ₂ film having a film thickness of about 55 nm and a MgF ₂ film having a film thickness of about 100 nm (both not shown) are continuously formed as an antireflection film on the surface (front surface) on which sunlight is incident by EB vapor deposition. Formed. Next, by removing the wax 11 with, for example, toluene, the resin plate 19 is separated from the back electrode as shown in FIG.

  Thereafter, the Au plating film is cut along the exposed line-shaped Au plating film, whereby a plurality of solar cells having a predetermined size are produced.

  FIG. 25 shows a cross-sectional structure of the compound solar cell thus manufactured. As shown in FIG. 25, when compared with the structure of a conventional compound solar cell in which a bottom cell is formed on a predetermined substrate for epitaxial growth (see, for example, FIG. 26 or FIG. 27), in the above-described compound solar cell, The back electrode 9 is directly formed on the bottom cell B of the cell body C.

  On the other hand, a surface electrode 15 is formed on the surface of the top cell T of the cell body C. A middle cell M is formed between the top cell T and the bottom cell B. The cell body C is a three-junction compound solar cell having a bottom cell B, a middle cell M, and a top cell T.

In particular, unlike the top cell T and middle cell M formed by epitaxial growth, the bottom cell B has a CdS film B11 and a CuInSe ₂ film B12 formed by vapor deposition.

Accordingly, the cell body C includes a bottom cell having a pn junction made of CuInSe ₂ (group I-III-VI compound) and CdS (group II-VI compound), a middle cell M having a pn junction made of GaAs (group III-V compound), and A top cell T having a pn junction made of InGaP (III-III-V group compound) is provided.

  In the above-described solar battery cell, first, each layer to be the top cell T having a forbidden band width of about 1.7 to 2.1 eV is sequentially formed on the GaAs substrate 1 for epitaxial growth by the epitaxial growth method.

  Next, each layer to be a middle cell M having a forbidden band width of about 1.3 to 1.6 eV is sequentially formed on the top cell T. Then, on the middle cell M, each layer to be the bottom cell B having a forbidden band width of about 0.9 to 1.1 eV is sequentially formed by a sputtering method and an evaporation method other than the epitaxial growth method.

  Thus, in the above-described compound solar battery, each layer forming the top cell T is formed first, and the layer forming the bottom cell B is formed last, so that the conventional material (˜0.7 eV) as the bottom cell B Even if a material having a high forbidden bandwidth (about 0.9 to 1.1 eV) is used, the quality of the bottom cell B does not reach the middle cell M and the top cell T, and the conversion efficiency as a compound solar cell is improved. Can be improved.

  Moreover, since the quality of the bottom cell B does not affect the quality of the top cell T and the middle cell M, each layer forming the bottom cell B can be formed by a method other than the epitaxial growth method.

Thereby, for example, the above-described polycrystalline CuInSe ₂ film B12 other than the single crystal can be applied as the material of the layer forming the bottom cell having a relatively high forbidden band width (0.9 to 1.1 eV). In addition, the choice of materials for each layer forming the bottom cell B and the method of forming the same is expanded.

  In addition, in the compound solar cell in each embodiment mentioned above, the back electrode by the Au plating film | membrane mainly about 30 micrometers thick was mentioned as an example as the back electrode 9, and was demonstrated. The thickness of the back electrode 9 is not limited to this thickness as long as it has a thickness that supports the cell body C.

  Therefore, the thickness may be set such that the back electrode 9 can be bent. Or depending on the material of the back surface electrode 9, you may set to the thickness which can bend.

  In this case, the compound solar cell in which the cell body C is formed on the back electrode 9 can be flexed freely, and the degree of freedom in shape is improved.

  Moreover, as a formation method of the back surface electrode 9, it can form by printing or spraying other than the method by the plating mentioned above, for example. Also, a conductive polymer film can be applied instead of metal.

  Assuming the above-described case as the form of the back electrode, the thickness of the back electrode 9 is preferably about 2 to 500 μm.

  Moreover, in each compound solar cell mentioned above, the thermal conductivity between a cell main body and a heat sink improves because the board | substrate for making it grow epitaxially is removed finally. As a result, the temperature rise of the cell body in the compound solar battery can be suppressed.

  Further, the removed substrate for epitaxial growth can be reused, and the cost can be reduced.

  The embodiment disclosed this time is to be considered as illustrative in all points and not restrictive. The present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

1 GaAs substrate, 3 InGaP layer, 4 AlAs layer, 5 AlGaAs layer, 6 GaAs layer, 7 InGaP layer, 8 GaAs layer, 9 back electrode, 10 solar cell, 11 wax, 12 ITO film, 13 glass substrate, 15 surface electrode , 17 Resist pattern, 19 Resin plate, T1 GaAs layer, T2 AlInP layer, T3 InGaP layer, T4 InGaP layer, T5 AlInP layer, B1, M1 AlInP layer, B2, M2 GaAs layer, B3, M3 GaAs layer, B4, M4 InGaP layer, B5 GaAs layer, B6 InP layer, B7 InGaAs layer, B8 InGaAs layer, B9 InP layer, B10 GaAs layer, B11 CdS film, B12 CuInSe ₂ film, T top cell, M middle cell, B bottom cell, C cell body.

Claims (8)

A cell body;
In the cell body, a surface electrode formed on the side on which sunlight is incident;
In the cell body, having a back electrode formed on the side opposite to the side on which sunlight is incident,
The cell body is
A top cell that is formed on the side on which sunlight is incident and includes a single crystal pn junction layer;
A middle cell that is formed on the opposite side of the top cell from the side on which sunlight is incident and includes a single crystal pn junction layer;
A bottom cell including a single crystal pn junction layer formed on a side opposite to a side on which sunlight is incident on the middle cell;
The forbidden band width of the pn junction layer of the top cell, the forbidden band width of the pn junction layer of the middle cell, and the forbidden band width of the pn junction layer of the bottom cell are different from each other,
Misfit dislocations exist in the bottom cell,
A compound solar battery in which the misfit dislocation does not exist in the middle cell and the top cell . A cell body;
In the cell body, a surface electrode formed on the side on which sunlight is incident;
In the cell body, a back electrode formed on the side opposite to the side on which sunlight is incident;
Have
The cell body is
A top cell that is formed on the side on which sunlight is incident and includes a single crystal pn junction layer;
A middle cell that is formed on the opposite side of the top cell from the side on which sunlight is incident and includes a single crystal pn junction layer;
A bottom cell formed on a side opposite to the side on which the solar light is incident on the middle cell and including a single crystal pn junction layer;
With
Misfit dislocations exist in the bottom cell,
A compound solar cell in which the middle cell and the top cell are free from defects due to the misfit dislocations . The top cell and the middle cell are lattice matched,
The bottom cell, the middle cell is not lattice-matched, compound solar cell of claim 2 Symbol placement. A tunnel junction layer formed between the middle cell and the bottom cell;
The compound solar cell according to claim 2, wherein the tunnel junction layer is not lattice-matched with the bottom cell. The forbidden band width of the pn junction layer of the middle cell is set higher than the forbidden band width of the pn junction layer of the bottom cell;
5. The compound solar cell according to claim 2 , wherein the forbidden band width of the pn junction layer of the top cell is set higher than the forbidden band width of the pn junction layer of the middle cell. Forming a top cell including a single crystal pn junction layer on a main surface of a semiconductor substrate by an epitaxial growth method;
Forming a middle cell that includes a single crystal pn junction layer on the surface of the top cell and is lattice-matched to the top cell by an epitaxial growth method;
Forming a bottom cell that includes a single crystal pn junction layer on the surface of the middle cell by epitaxial growth and does not lattice match with the middle cell;
Forming a back electrode on the surface of the bottom cell;
Separating the top cell and the semiconductor substrate after the back electrode is formed;
Forming a surface electrode on a surface of the top cell from which the semiconductor substrate is separated;
A method for producing a compound solar cell , comprising: The manufacturing method of the compound solar cell of Claim 6 provided with the process of forming a tunnel junction layer between the said middle cell and the said bottom cell by the epitaxial growth method . In the step of forming the top cell, the step of forming the middle cell, and the step of forming the bottom cell, the forbidden band width of the pn junction layer of the middle cell is higher than the forbidden band width of the pn junction layer of the bottom cell. is formed such that, the band gap of the pn junction layer of the top cell, wherein said middle cell is formed to be higher than the band gap of pn junction layer a compound according to claim 6 or 7 A method for manufacturing a solar cell.

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