KR20130070687A - Solar cell and method of fabricating the same - Google Patents

Abstract

PURPOSE: A solar cell and a method for fabricating the same are provided to increase short wavelength transmittance by using a buffer layer having a cubic crystalline structure. CONSTITUTION: A back electrode layer is arranged on a support substrate. A light absorption layer(300) is arranged on the back electrode layer. A buffer layer(400) is arranged on the light absorption layer. A buffer layer has a cubic crystalline structure. A front electrode layer(600) is arranged on the buffer layer.

Description

SOLAR CELL AND METHOD OF FABRICATING THE SAME

An embodiment relates to a solar cell and a manufacturing method thereof.

A solar cell can be defined as a device that converts light energy into electric energy by using a photovoltaic effect that generates electrons when light is applied to a p-n junction diode. The solar cell can be classified into a silicon solar cell, a compound semiconductor solar cell represented by group I-III-VI or III-V, a dye-sensitized solar cell, and an organic solar cell, depending on materials used as a junction diode.

CIGS (CuInGaSe) solar cell, which is one of the I-III-VI family chalcopyrite compound semiconductors, has excellent light absorption, high photoelectric conversion efficiency even at a thin thickness, and excellent electro- It is emerging as an alternative solar cell.

In general, CIGS thin film solar cells are manufactured by sequentially forming a substrate containing a sodium, a back electrode layer, a light absorbing layer, a buffer layer and a front electrode layer. Among these, the buffer layer is disposed between the light absorbing layer having a large difference in lattice constant and energy band gap and the front electrode layer to form a good junction. Conventionally, cadmium sulfide (CdS) is mainly used as a buffer layer, but there are disadvantages in that cadmium (Cd) itself is toxic and manufactured by a wet process called chemical bath deposition (CBD).

In order to solve such a problem, recently, a Cd-free solar cell using zinc sulfide (ZnS) having a bandgap energy greater than cadmium sulfide as a buffer layer has been highlighted as an alternative. The zinc sulfide (ZnS) layer has a wide bandgap energy (E _g ), and thus has an advantage of improving efficiency due to excellent short wavelength transmittance. However, zinc sulfide (ZnS) the band gap energy (E _g) of the layer due to the alignment problems (alignment) of the band gap energy (E _g) of light absorption and a large difference in I-bar, a band gap with cadmium sulfide as a buffer layer There is a problem of lower efficiency than the case.

The embodiment provides a solar cell having a bandgap energy (E _g ) aligned between layers and a method of manufacturing the same.

The solar cell according to the first embodiment includes a rear electrode layer disposed on a support substrate; A light absorbing layer disposed on the rear electrode layer; A buffer layer disposed on the light absorbing layer and mainly composed of a cubic crystal structure; And a front electrode layer disposed on the buffer layer.

A solar cell according to a second embodiment includes a front electrode layer disposed on a support substrate, the front electrode layer mainly comprising a cubic crystal structure; A buffer layer disposed on the front electrode layer, the buffer layer mainly comprising a cubic crystal structure; A light absorbing layer disposed on the buffer layer; And a rear electrode layer disposed on the light absorbing layer.

A method of manufacturing a solar cell module according to an embodiment includes forming a rear electrode layer on a supporting substrate; Forming a light absorbing layer on the back electrode layer; Forming a buffer layer mainly composed of a cubic crystal structure on the light absorbing layer; And forming a front electrode layer on the buffer layer.

The solar cell according to the embodiment provides a buffer layer mainly composed of a cubic crystal structure. The buffer layer mainly composed of the cubic crystal structure may have a reduced bandgap energy than the buffer layer mainly composed of the hexagonal crystal structure. Accordingly, the solar cell according to the embodiment enables bandgap alignment between the buffer layer and adjacent layers, thereby minimizing recombination of electrons and holes, and improving photoelectric conversion efficiency.

1 to 5 are cross-sectional views illustrating a method of manufacturing a solar cell according to an embodiment.

In the description of the embodiments, where each substrate, layer, film, or electrode is described as being formed "on" or "under" of each substrate, layer, film, or electrode, etc. , “On” and “under” include both “directly” or “indirectly” other components. In addition, the upper or lower reference of each component is described with reference to the drawings. The size of each component in the drawings may be exaggerated for the sake of explanation and does not mean the size actually applied.

1 to 5 are cross-sectional views illustrating a method of manufacturing the solar cell according to the first embodiment. Hereinafter, the solar cell and the manufacturing method thereof according to the first embodiment will be described in detail with reference to FIGS. 1 to 5.

Referring to FIG. 1, a rear electrode layer 200 is formed on the support substrate 100. The support substrate 100 has a plate shape and supports the rear electrode layer 200, the light absorbing layer 300, the buffer layer 400, the high resistance buffer layer 500, and the front electrode layer 600.

The support substrate 100 may be an insulator. The support substrate 100 may be a glass substrate, a plastic substrate, or a metal substrate. In more detail, the support substrate 100 may be a soda lime glass substrate. The supporting substrate 100 may be transparent. The support substrate 100 may be rigid or flexible.

The back electrode layer 200 may be formed on the support substrate 100 by a method of physical vapor deposition (PVD) or plating.

The back electrode layer 200 is a conductive layer. The back electrode layer 200 may include at least one of molybdenum (Mo), gold (Au), aluminum (Al), chromium (Cr), tungsten (W), and copper (Cu). Among these, molybdenum (Mo) has a small difference between the support substrate 100 and the coefficient of thermal expansion compared to other elements, so that it is possible to prevent the occurrence of peeling due to excellent adhesiveness, and to the rear electrode layer 200 described above. Overall required properties can be met.

Referring to FIG. 2, the light absorbing layer 300 is disposed on the back electrode layer 200. The light absorbing layer 300 includes a?-?-? Group compound. For example, the light absorbing layer 300 is copper-indium-gallium-selenide-based (Cu (In, Ga) Se _2; CIGS-based) crystal structure, a copper-indium-selenide-based or copper-gallium-selenide Crystal structure. The band gap of the light absorption layer 300 may be about 1 eV to about 1.8 eV.

The light absorbing layer 300 may be, for example, copper-indium-gallium-selenide-based (Cu (In, Ga) Se ₂ ; CIGS-based) while simultaneously evaporating copper, indium, gallium, and selenium. The method of forming (300) and the method of forming a metal precursor film and forming it by the selenization process are used widely.

When the metal precursor film is formed and selenization is subdivided, a metal precursor film is formed on the back electrode 300 by a sputtering process using a copper target, an indium target, and a gallium target. Thereafter, the metal precursor film is formed of a copper-indium-gallium-selenide-based (Cu (In, Ga) Se ₂ ; CIGS-based) light absorbing layer by a selenization process.

Alternatively, the copper target, the indium target, the sputtering process using the gallium target, and the selenization process may be performed simultaneously.

Alternatively, the CIS-based or CIG-based optical absorption layer 300 can be formed by using only a copper target and an indium target, or by a sputtering process and a selenization process using a copper target and a gallium target.

Referring to FIG. 3, a buffer layer 400 is formed on the light absorbing layer 300. The solar cell of the present invention forms a pn junction between the light absorbing layer 300 of the CIGS or CIGSS compound thin film of the p-type semiconductor and the front electrode layer 600 of the n-type semiconductor. However, since the two materials have a large difference in lattice constant and band gap energy, a buffer layer having a band gap in between the two materials is required to form a good junction. The thickness of the buffer layer 400 may be about 10 nm to about 30 nm, but is not limited thereto.

The buffer layer 400 may include cadmium sulfide (CdS), zinc sulfide (ZnS), In _X S _Y and In _X Se _Y Zn (O, OH). More preferably, the buffer layer 400 may be a zinc sulfide (ZnS) layer.

In more detail, the zinc sulfide (ZnS) layer according to the embodiment mainly consists of a cubic crystal structure. At this time, the ratio of the cubic crystal in the crystal structure of the zinc sulfide layer is preferably 50% or more, and more preferably, the ratio of the cubic crystal is about 80% or more, but is not limited thereto. In particular, it is preferable to make substantially all crystal structures of a zinc sulfide layer into a cubic crystal.

The zinc sulfide layer mainly composed of a cubic crystal structure according to the embodiment may have a reduced bandgap energy than the zinc sulfide layer mainly composed of a hexagonal crystal structure. For example, a zinc sulfide layer mainly composed of a cubic crystal structure according to the embodiment may have a bandgap energy of about 0.1 eV to about 0.2 eV lower than a zinc sulfide layer mainly composed of a hexagonal crystal structure. However, it is not limited thereto.

Accordingly, the difference between the bandgap energy of the zinc sulfide layer and the bandgap energy of the light absorbing layer 300 may be reduced. Accordingly, the solar cell according to the embodiment may improve the bandgap alignment between the buffer layer 400 and adjacent layers, thereby minimizing recombination of electrons and holes, and improving photoelectric conversion efficiency. The band gap energy of the zinc sulfide layer mainly composed of the cubic crystal structure may be about 3.4 eV to about 3.5 eV, but is not limited thereto.

The zinc sulfide layer mainly composed of the cubic crystal structure may be formed by atomic layer deposition (ALD), metal-organic chemical vapor deposition (MOCVD), chemical bath deposition (CBD), or the like. Can be formed. More preferably, the zinc sulfide layer mainly composed of the cubic crystal structure may be formed by chemical bath deposition (CBD) as described below.

First, a zinc complex is reacted with a zinc ion source to form a metal complex. A zinc acetate solution may be used as a zinc ion source, and Na ₂ EDTA (ethylene diamine tetra acetic acid disodium salt) may be used as a complex compound. By the above process, a ZnEDTA metal complex may be formed.

The metal complex and the sulfur source are then mixed to form a reaction solution. As the sulfur source, thioacetamide or thiourea (SC (NH ₂ ) ₂ ) or the like can be used. Thereafter, sodium hydroxide (NaOH) or the like may be further used as a pH adjuster.

The above-mentioned process can be summarized by the following Chemical Formulas 1 to 5. In addition, the chemical solution deposition method for producing a zinc sulfide layer according to the embodiment does not require ammonia or hydrazine to promote zinc ion production unlike the prior art.

Formula 1 to Formula 5

Finally, a zinc sulfide layer mainly having a cubic crystal structure is deposited on the light absorbing layer 300 by immersing the support substrate 100 having the light absorbing layer 300 formed therein. In the chemical solution deposition process, the temperature may be performed at a condition of about 65 ° C to about 80 ° C.

The crystal structure of the zinc sulfide layer produced by the above method can be measured by X-ray diffraction which is generally used. That is, in the X-ray diffraction peak, the hexagonal peaks of the (100) plane and the (002) plane appear, whereas the cubic system shows the peaks of the (111) plane. By the crystal orientation in which this peak appears, it can be confirmed whether the zinc sulfide layer has a hexagonal crystal structure. Specifically, using an amorphous glass sample case (plate shape) having a recessed portion of about 0.5 mm, the zinc sulfide layer is filled in the recessed portion, and the X-rays are irradiated to make the surface flat. Thereby, it becomes possible to measure with a small amount of samples.

Referring to FIG. 4, a high resistance buffer layer 500 is formed on the buffer layer 400. As the high resistance buffer layer 500, zinc oxide (i-ZnO) or the like which does not contain impurities may be used. In addition, the high resistance buffer layer 500 may be formed on the buffer layer 400 by a sputtering process or the like.

Since the high resistance buffer layer 500 is formed on the buffer layer 400 mainly having a cubic crystal structure, the high resistance buffer layer 500 may also be predominantly formed with a cubic crystal structure. Alternatively, in order to convert the high resistance buffer layer 500 of the hexagonal crystal structure into the high resistance buffer layer 500 of the cubic crystal structure, a high temperature heat treatment process of about 700 ° C. may be additionally performed, but is not limited thereto.

Referring to FIG. 5, a front electrode layer 600 is formed on the high resistance buffer layer 500. The front electrode layer 600 is transparent and is a conductive layer. For example, the front electrode layer 600 may be formed of boron doped zinc oxide (ZnO: B, BZO), aluminum doped zinc oxide (AZO), or gallium doped zinc oxide (Ga doped zinc oxide); GZO) and the like. In more detail, the front electrode layer 600 may include aluminum doped zinc oxide (AZO) or boron-doped zinc oxide (ZnO: B, BZO) in consideration of a band gap and contact with the buffer layer 400. ), But is not limited thereto.

The front electrode layer 600 may be formed by depositing a transparent conductive material on the high resistance buffer layer 500. In detail, the front electrode layer 600 may be formed by sputtering or organic metal chemical vapor deposition (MOCVD). For example, the front electrode layer 600 may be deposited by a sputtering process.

Since the front electrode layer 600 is formed on the buffer layer 400 mainly having a cubic crystal structure and the high resistance buffer layer 500 mainly having a cubic crystal structure, the front electrode layer 600 also has a cubic crystal structure. Can be formed.

Alternatively, in order to convert the front electrode layer 600 of the hexagonal crystal structure into the front electrode layer 600 of the cubic crystal structure, a high temperature and high pressure process may be further performed. For example, the front electrode layer 600 having a hexagonal crystal structure may be formed at a high pressure of about 15 GPa at about 550 K to form the front electrode layer 600 having a cubic crystal structure.

Meanwhile, only a solar cell having a substrate structure has been described so far, but the embodiment is not limited thereto. That is, embodiments of the present disclosure may also include a solar cell of a superstrate structure.

In one embodiment, the solar cell according to the second embodiment is disposed on the support substrate 100, the front electrode layer 600 mainly having a cubic crystal structure; A buffer layer 400 disposed on the front electrode layer 600 and mainly comprising a cubic crystal structure; A light absorbing layer 300 disposed on the buffer layer 400; And a rear electrode layer 200 disposed on the light absorbing layer 300. The solar cell according to the second embodiment may include all of the contents disclosed in the solar cell according to the first embodiment mentioned above, and the description thereof will not be repeated.

Features, structures, effects, and the like described in the above embodiments are included in at least one embodiment of the present invention, and are not necessarily limited to only one embodiment. Furthermore, the features, structures, effects, and the like illustrated in the embodiments may be combined or modified with respect to other embodiments by those skilled in the art to which the embodiments belong. Therefore, it should be understood that the present invention is not limited to these combinations and modifications.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of illustration, It can be seen that various modifications and applications are possible. For example, each component specifically shown in the embodiments can be modified and implemented. It is to be understood that all changes and modifications that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (13)

A rear electrode layer disposed on the supporting substrate;
A light absorbing layer disposed on the rear electrode layer;
A buffer layer disposed on the light absorbing layer and mainly composed of a cubic crystal structure; And
A solar cell comprising a front electrode layer disposed on the buffer layer.
The method of claim 1,
The buffer layer is a solar cell containing zinc sulfide (ZnS).
The method of claim 1,
The front electrode layer is a solar cell mainly having a cubic crystal structure.
The method of claim 1,
Bandgap energy of the buffer layer is a solar cell of 3.4 eV to 3.5 eV.
The method of claim 1,
The solar cell further comprises a high resistance buffer layer between the buffer layer and the front electrode layer.
The method of claim 5, wherein
The high resistance buffer layer includes a zinc oxide (ZnO) layer containing no impurities,
The zinc oxide layer has a cubic crystal structure mainly solar cell.
A front electrode layer disposed on the support substrate, the front electrode layer mainly comprising a cubic crystal structure;
A buffer layer disposed on the front electrode layer, the buffer layer mainly comprising a cubic crystal structure;
A light absorbing layer disposed on the buffer layer; And
A solar cell comprising a back electrode layer disposed on the light absorbing layer.
The method of claim 7, wherein
The buffer layer includes a zinc sulfide (ZnS) layer, and the band gap energy of the zinc sulfide layer is 3.4 eV to 3.5 eV.
Forming a back electrode layer on the support substrate;
Forming a light absorbing layer on the back electrode layer;
Forming a buffer layer mainly composed of a cubic crystal structure on the light absorbing layer; And
Forming a front electrode layer on the buffer layer manufacturing method of a solar cell.
The method of claim 9,
The buffer layer includes a zinc sulfide (ZnS) layer, and the zinc sulfide layer is formed by chemical bath deposition (CBD).
11. The method of claim 10,
The chemical solution deposition method is a method of manufacturing a solar cell is carried out under the conditions of 65 ℃ to 80 ℃.
11. The method of claim 10,
The cadmium sulfide layer,
Reacting the complex with a zinc ion source to form a metal complex,
Mixing the metal complex with a sulfur source to form a reaction solution,
A method of manufacturing a solar cell comprising immersing a support substrate having a light absorbing layer formed in the reaction solution.
13. The method of claim 12,
The zinc ion source is an aqueous zinc acetate solution,
The complex is Na 2 EDTA (ethylene diamine tetra acetic acid disodium salt),
The sulfur source is a method for producing a solar cell comprising thioacetamide or thiourea (SC (NH ₂ ) ₂ ).

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