KR101157555B1 - fabrication method of high efficiency multiple junction solar cell using II-VI compound semiconductor layers - Google Patents

Abstract

The present invention relates to a method for fabricating a high efficiency multi-junction solar cell, comprising a step of growing a II-VI compound semiconductor thin film containing no In on a GaSb substrate. The manufacturing method of the thin film for a multi-junction high efficiency solar cell using a semiconductor is made into a technical subject. As a result, it is possible to efficiently absorb light in the electric field of the solar light without using a material such as In, which is expected to be depleted and exhibit a high price increase rate, thereby significantly lowering the manufacturing cost of the solar cell, thereby providing high efficiency at low cost. There is an advantage in that the thin film for solar cells can be implemented.

Description

Fabrication method of high efficiency multiple junction solar cell using II-VI compound semiconductor layers}

The present invention relates to a method for fabricating a high efficiency multi-junction solar cell, which is capable of absorbing light efficiently in various wavelength ranges of sunlight using conventional compound semiconductor materials without using In. A thin film for multi-junction high efficiency solar cells using a -VI compound semiconductor.

The development of solar cells, one of renewable green energy, is very important to solve the global environment and energy problems caused by global warming due to CO ₂ emission. Globally, solar cells generate 5GW of electricity per year, which accounts for only a small percentage of the total energy, but is growing at an average annual rate of 30%. At present, the solar cell market accounts for more than 80% of silicon-based solar cells, and it is very important to develop solar cells that can replace them due to rising silicon prices, supply problems, and low conversion efficiency. In addition, in the case of liquid fuel, solar fuel cell is low efficiency (~ 16%) / low price product, high efficiency (> 30%) / high price considering that various fuels are used according to the purpose of use, such as jet oil, gasoline, diesel oil, and bunker C oil. A variety of products are expected to emerge, including products, mid-efficiency (~ 25%) and low-priced products.

Recently, low-efficiency solar cells are Si-based, but technologies developed for intermediate efficiency and high-efficiency products include CdTe, CIGS, GaAs-based multi-junction thin film solar cells, and GaAs-based compound semiconductor multi-junction thin-film solar cells. Is most suitable for high efficiency solar cell purposes.

Among the GaAs-based compound semiconductor multi-junction thin-film solar cell technology, the most efficient solar cell is Ga _0.35 In on a Ge substrate in a 5.09 mm ² cell area announced by the Fraunhofer Institute for Solar Energy Systems in August 2009. Multi-junction solar cell with conversion efficiency of 41.1% using _0.65 P / Ga _0.83 In _0.17 As. To date, ultra high efficiency solar cells have been using compound semiconductors of GaInP and GaAs series.

Also, after it found that the band gap of InN 0.7eV in 2002 by Cornell University and the Lawrence Berkeley National Laboratory Walukiewicz with the help of researchers from the University of Ritsumeikan MBE crystal growth, In _x Ga _{1 - x} N of 0.7 ~ 3.4eV Since it can have a bandgap energy, it can be found to be the most suitable bandgap (1.1 / 1.7 eV) in the two-junction structure, so it is found to be suitable as a material for an ultra-high efficiency solar cell.

Since InGaN has a direct transition band gap, there is no thermal loss of the indirect transition band gap, and its absorption coefficient (~ 10 ⁵ cm ^-1 ) is high and its radioactivity resistance is high, so it is not only used for ground but also as a spacecraft solar cell. It is possible.

FIG. 1 (a) shows the structure of a conventional high efficiency InGaAs / InGaP / Ge triple junction solar cell and FIG. 1 (b) shows the structure of a conventional high efficiency InGaP / AlGaInAs / InGaP / Ge quadruple junction solar cell. .

However, conventional GaAs-based and GaN-based multi-junction solar cells both require a compound thin film layer containing In to maximize light absorption. However, In has a disadvantage in that production cost is very expensive, so special purpose such as satellite solar cell Except for solar cells, there are limitations in commercialization, and there are serious problems such as depletion of materials such as In, which are necessarily used as thin film materials for solar cells. Therefore, there is an urgent need for a method of manufacturing a solar cell using another material to replace the existing high efficiency solar cell.

While the present invention utilizes a multijunction of GaAs-based compound semiconductor thin films containing In on a Ge substrate, the present invention grows a group II-VI compound semiconductor thin film on a GaSb substrate to form a rare compound such as In. An object of the present invention is to provide a method for manufacturing a thin film for multi-junction high efficiency solar cells using a group II-VI compound semiconductor which proposes a structure capable of efficiently absorbing light in various wavelengths of sunlight while avoiding the use of elements.

In order to achieve the above object, the present invention includes a step of growing a II-VI compound semiconductor thin film containing no In on a GaSb substrate. The manufacturing method of a battery thin film is made into a technical summary.

In addition, it is preferable to irradiate the Sb molecular beam on the surface of the GaSb substrate, the irradiation amount of the Sb molecular beam is a molecular equivalent beam (beam equivalent pressure) of 4.5x10 ^-7 Torr ~ 5.5x10 ^-7 Torr on the basis of 500 ℃ ~ 600 ℃ It is desirable to maintain.

In addition, the present invention includes a first step of removing contamination of the GaSb substrate surface; Attaching the GaSb substrate to a sample holder and inserting the GaSb substrate into a crystal growth apparatus, and increasing the temperature of the GaSb substrate surface to 500 ° C. to 600 ° C .; Irradiating Sb molecular beams onto the GaSb substrate to perform Sb treatment on the substrate; A fourth step of double or triple depositing different types of II-VI compound semiconductor thin films not including In on the Sb-treated GaSb substrate surface; A fifth step of injecting impurities into the group II-VI compound semiconductor thin film; a method for manufacturing a multi-junction high efficiency solar cell thin film using a group II-VI compound semiconductor, characterized in that it comprises another technical gist.

Here, the crystal growth apparatus, it is preferable to use a molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD).

In addition, the irradiation amount of the Sb molecular beam of the third step is preferably maintained at the molecular equivalent weight (beam equivalent pressure) of 4.5x10 ^-7 Torr ~ 5.5x10 ^-7 Torr on the basis of 500 ℃ ~ 600 ℃.

In addition, the group II-VI compound semiconductor thin film is preferably deposited on the GaSb substrate in the order of AlGaSb, CdSe, ZnTe, In addition, the deposition temperature of the AlGaSb is 500 ℃ ~ 600 ℃, CdSe is 280 ℃ ~ 320 C and ZnTe are preferably 250 ° C to 300 ° C.

In the fifth step, the AlGaSb is a p-type impurity, a Group II or IV element, an n-type impurity, a Group VI or Group IV element, and CdSe is a p-type impurity, a Group I or V element It is preferable to inject a Group III element or Group VII element into n-type impurities, ZnTe into Group I or Group elements as p-type impurities, and Group III element or Group VII element into n-type impurities.

The present invention is expected to be depleted, and it is possible to implement a thin film for a high efficiency solar cell that can absorb light efficiently in the full-wavelength region of sunlight without using a material that has already exhibited a very high price increase rate every year.

In addition, compared with the prior art, the mismatch between the lattice constants of the materials constituting each layer is almost negligible, so that crystal growth is facilitated, and the manufacturing cost of the solar cell can be greatly reduced, thereby enabling the implementation of low-cost and high-efficiency solar cells. There is.

Figure 1 (a) shows the structure of a conventional high efficiency InGaAs / InGaP / Ge triple junction solar cell thin film and Figure 1 (b) shows the structure of a conventional high efficiency InGaP / AlGaInAs / InGaP / Ge quadruple thin film for solar cells It is shown.
2 shows the structure of a thin film for quadruple bonded high efficiency solar cells using a II-VI compound semiconductor according to an embodiment of the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high efficiency solar cell thin film using a compound semiconductor material containing no In, and to growing a II-VI compound semiconductor thin film on a GaSb substrate.

Thin film for multi-junction solar cell with Ga _0.35 In _0.65 P (1.7eV) / Ga _0.83 In _0.17 As (1.1eV) / Ge (0.67eV) structure on Ge substrate which has the highest efficiency from previous research. . The solar cells can absorb sunlight efficiently in the wavelength range of 1.7 / 1.1 / 0.67eV, respectively, and have a multi-layered structure with a small energy gap from the surface of the solar cell. It has a structure that can absorb electricity efficiently and generate electricity.

The present invention also has an equivalent structure, as shown in Figure 2 has a structure of ZnTe (2.3eV) / CdSe (1.74eV) / AlGaSb (1.58 ~ 0.72eV) / GaSb (0.72eV). Therefore, it is expected that the effect of absorbing light efficiently in the same wavelength range as the previous research results without using In at all, and the group II-VI material is generally light absorption coefficient compared to the group III-V material High efficiency can be expected.

In addition, great advantages can be expected in the manufacturing process. In the prior art, since the lattice constant changes greatly according to the composition change of In, very accurate composition control is required. This is a very difficult task to ensure reproducibility and uniformity in crystal engineering. In contrast, the materials used in the present invention, as shown in Table 1, all materials have a lattice constant very close to 6.1 Å, which facilitates crystal growth and minimizes the decrease in efficiency due to defects during thin film crystal growth. There is an advantage.

The following Table 1 compares the physical properties of Group II-VI and III-V materials for solar cells.

matter Lattice constant (10 ^-10 m)  Bandgap (eV) Transition method GaAs 5.65 1.42 Direct transition InAs 6.05 0.35 Direct transition GaP 5.45 2.27 Indirect transition InP 5.86 1.35 Direct transition GaSb 6.09 0.72 Direct transition AlSb 6.14 1.58 Indirect transition ZnTe 6.10 2.3 Direct transition CdSe 6.06 1.69 Direct transition

As shown in Table 1, Group II-VI materials (GaSb, AlSb, ZnTe, CdSe) used in the present invention have a lattice constant very close to 6.1 Å, which facilitates crystal growth, and the same without using In. It can be seen that the light of the wavelength band can be efficiently absorbed.

In addition, by irradiating the GaSb substrate to the surface of the GaSb substrate by irradiating Sb molecular beams, atoms of the substrate surface are rearranged in the form of 2x4 to lower the surface energy to achieve good crystal growth of the compound semiconductor thin film deposited later. Here, the irradiation amount of the Sb molecular beam is to be made in the state of maintaining the molecular equivalent pressure (beam equivalent pressure) of 4.5x10 ^-7 Torr ~ 5.5x10 ^-7 Torr at 500 ℃ ~ 600 ℃.

Hereinafter will be described in detail for the preferred embodiment of the present invention.

First, a chemical etching process is performed to remove contamination of the GaSb substrate and natural oxide film. Acetone and methanol are used to remove organic contaminants on the surface. Each of the substrates is immersed in 100 ml of solution and subjected to ultrasonic cleaning for at least 10 minutes. The substrate surface is etched using a solution obtained by diluting sulfuric acid (H ₂ SO ₄ ) and hydrogen peroxide (H ₂ O ₂ ) in distilled water at a ratio of 1: 1: 50. To completely remove the etching solution remaining on the surface.

The surface-treated substrate is mounted on a sample holder and put into a crystal growth apparatus to enter crystal growth. In this case, various crystal growth methods such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) are applicable, but MBE is known to be most suitable in consideration of crystallinity and convenience of deposition. Use MBE.

The MBE equipment for crystal growth consists of a sample exchange chamber and a crystal growth chamber. The sample holder is placed in the sample exchange chamber. When the vacuum of the sample exchange chamber becomes sufficiently close to that of the crystal growth chamber, the sample is moved to the crystal growth chamber.

The sample moved to the crystal growth chamber increases the temperature of the substrate surface to a temperature of about 550 ° C. to remove the oxide film and residual impurities remaining on the surface. At this time, in order to protect the substrate surface from roughening, the Sb molecular beam should be irradiated to the surface by the molecular beam epitaxy method. The Sb molecular beam increases the vapor pressure of Sb when Sb is heated to a high temperature to obtain Sb vapor, which is composed of Sb atoms or molecules, which is called an Sb molecular beam. The amount of Sb molecular beam to be irradiated is to be at least 5x10 ^-7 Torr (Beam Equvalent Pressure) based on 550 ° C.

The substrate surface subjected to heat treatment for 30 minutes under these conditions can be identified by the high-speed electron diffraction method (RHEED) to determine whether the surface is (2X4) rearrangement. This rearrangement occurs when Sb is treated on the surface. This is very important for good crystal growth.

On the GaSb substrate having the above process, different types of II-VI compound semiconductor thin films containing no In are deposited in duplicate or triple.
In the exemplary embodiment of the present invention, a case of triple deposition is illustrated, and it is illustrated in FIG. 2 as a quadrature junction including a substrate. Therefore, the GaSb substrate is preferably deposited in the order of AlGaSb, CdSe, ZnTe. The materials are small enough that the mismatch of lattice constants is almost negligible (both close to ~ 6.1 Å), which facilitates crystal growth and significantly lowers the manufacturing cost of the solar cell. Can be implemented.

Deposition temperature of the AlGaSb is 500 ℃ ~ 600 ℃ preferably 540 ℃, CdSe is 280 ℃ ~ 320 ℃ preferably 300 ℃, ZnTe is 250 ℃ ~ 300 ℃ preferably 280 ℃, the optimum crystal at this temperature Showed growth.

In addition, since the thin film for a solar cell is composed of a junction of a p-type thin film and an n-type thin film, in each case, it is necessary to form a junction through the doping of an impurity into an n-type or p-type.

In the case of AlGaSb, group II elements or group IV elements such as Zn may be used as the p-type impurity, but group II elements are used in the present invention, and group VI elements or group IV elements may be used as the n-type impurity. Te was used in the invention.

In the case of CdSe, a group I element or a group V element may be used as the p-type impurity, but in the present invention, Sb, which is a group V element, may be used, and a group III element or a group VII element may be used as the n-type impurity. Ga was used.

In the case of ZnTe, a p-type impurity may use a Group I element or a Group V element, but in the present invention, Sb, a Group V element, is used, and an n-type impurity may use a Group III element or a Group VII element. Al was used.

The impurity implantation yielded a charge concentration of ~ 5X10 ¹⁷ / cm ³ in each thin film, which is sufficient for solar cell construction.

After the crystal growth is finished, an anti-reflective protective film treatment and an electrode (window) are formed on the surface, and then a rear electrode is formed on the back of the substrate to complete the fabrication of a multi-quad junction solar cell.

Claims (9)

delete delete delete A first step of removing contamination of the GaSb substrate surface;
Attaching the GaSb substrate to a sample holder and inserting the GaSb substrate into a crystal growth apparatus, and increasing the temperature of the GaSb substrate surface to 500 ° C. to 600 ° C .;
Irradiating a Sb molecular beam on a surface of the GaSb substrate to perform Sb treatment on an upper layer of the substrate;
A fourth step of double or triple depositing different types of II-VI compound semiconductor thin films not including In on the Sb-treated GaSb substrate surface;
And a fifth step of injecting impurities into the group II-VI compound semiconductor thin film. The method of claim 4, wherein the crystal growth equipment, MBE (Molecular beam epitaxy) or MOCVD (Metal Organic Chemical Vapor Deposition) using a multi-junction high efficiency solar cell thin film using a group II-VI compound semiconductor, characterized in that Way. The method of claim 4, wherein the irradiation amount of the Sb molecular beam (Sb molecular beam) of the third step is to maintain a beam equivalent pressure of 4.5x10 ^-7 Torr ~ 5.5x10 ^-7 Torr at 500 ℃ ~ 600 ℃ A method of manufacturing a thin film for a multi-junction high efficiency solar cell using a II-VI compound semiconductor. The method of claim 4, wherein the II-VI compound semiconductor thin film,
A method of manufacturing a thin film for a multi-junction high efficiency solar cell using a II-VI compound semiconductor, characterized in that the triple deposition in the order of AlGaSb, CdSe, ZnTe. The method of claim 7, wherein
Deposition temperature of the AlGaSb is 500 ℃ ~ 600 ℃,
The deposition temperature of the CdSe is 280 ℃ ~ 320 ℃,
The deposition temperature of the ZnTe is a manufacturing method of a thin film for multi-junction high efficiency solar cell using a II-VI compound semiconductor, characterized in that 250 ℃ ~ 300 ℃. The method of claim 7, wherein the impurity of the fifth step,
AlGaSb injects a group II or group element into the p-type impurity, and implants a group VI or group IV element into the n-type impurity,
The CdSe is a group I or V element is injected into the p-type impurities, a Group III element or a Group VII element is injected into the n-type impurities,
The ZnTe is a thin film for a multi-junction high efficiency solar cell using a II-VI compound semiconductor, characterized in that the group I or V element is injected into the p-type impurity, and the Group III or Group VII element is injected into the n-type impurity Manufacturing method.

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