KR20140031351A - Solar cell - Google Patents

Abstract

A solar cell according to one embodiment comprises: a rear electrode layer arranged on a support substrate; a light absorption layer arranged on the rear electrode layer; a first buffer layer arranged on the light absorption layer; a second buffer layer arranged on the first buffer layer and indicated as Chemical Formula 1 below; and a front electrode layer arranged on the buffer layer, wherein band gap energy of the second buffer layer is greater than band gap energy of the first buffer layer, and the band gap energy of the second buffer layer becomes smaller from the interface of the second buffer layer and the front electrode layer to the interface of the second buffer layer and the first buffer layer.

Description

Solar cell {SOLAR CELL}

Embodiments relate to solar cells.

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.

Conventional CIGS thin film solar cells generally have a structure of Soda lime glass / Mo / CIGS / CdS (ZnS) / ZnO / ITO / Al. Among these, the CdS layer is a buffer layer, which minimizes damage to the CIGS layer when the ZnO layer is deposited by the sputtering process, and reduces the difference in lattice constant and band gap between the two layers, thereby increasing the efficiency of the CIGS thin film solar cell. It plays a role.

However, the severe toxicity of Cd and its low bandgap (2.4eV) have led to the need for a new buffer layer to replace CdS by reducing the light absorption in the CIGS layer. As an alternative, materials such as ZnS, Zn (O, S, OH), (Zn, Mg) O, and In2S3 have been studied, but the band gap of ZnS material itself is high, about 3.7 eV.

Embodiments provide a solar cell having improved photoelectric conversion efficiency.

Solar cell according to the embodiment, the back electrode layer disposed on the support substrate; A light absorbing layer disposed on the rear electrode layer; A first buffer layer disposed on the light absorbing layer; A second buffer layer disposed on the first buffer layer and represented by Chemical Formula 1; And a front electrode layer disposed on the buffer layer, wherein a band gap energy of the second buffer layer is greater than a band gap energy of the first buffer layer, and the second buffer layer and the second buffer layer are formed from an interface between the second buffer layer and the front electrode layer. The bandgap energy of the second buffer layer decreases toward the interface of the first buffer layer.

The embodiment provides a solar cell including a buffer layer in which bandgap energy is sequentially changed. Therefore, in the embodiment, the solar cell can easily transport electrons and / or holes formed by external sunlight to the back electrode layer and the front electrode layer, and can have improved power generation efficiency. In addition, the buffer layer according to the embodiment has a higher bandgap than the conventionally used buffer layer, so that the transmittance of sunlight may be improved.

1 is a cross-sectional view showing a cross section of a solar cell according to the first embodiment.
2 is a graph showing the bandgap energy of each layer according to the first embodiment.
3 is a graph showing the change in the band gap energy of the buffer layer according to the content of oxygen and sulfur.
4 is a graph showing the sulfur content of the buffer layer according to the first embodiment.
5 is a cross-sectional view showing a cross section of the solar cell according to the second embodiment.
6 to 10 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.

As used herein, the term “The Highest Occupied Molecular Orbital (HOMO) level” refers to the highest energy level of the valence band. As used herein, the term “The Highest Occupied Molecular Orbital (LUMO) level” refers to the lowest energy level of the conduction band. As used herein, the term “bandgap” means the difference between HOMO level energy and LUMO level energy.

1 is a cross-sectional view showing a cross section of a solar cell according to the first embodiment. Referring to FIG. 1, a solar cell according to an embodiment includes a support substrate 100, a rear electrode layer 200, a light absorbing layer 300, a buffer layer 400, a high resistance buffer layer 500, and a front electrode layer 600. Include.

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 support substrate 100 may be rigid or flexible. In more detail, the support substrate 100 may be a flexible substrate. For example, a flexible polymer or the like may be used as a material of the support substrate 100. The front electrode layer 600 of the solar cell according to the embodiment not only has excellent mechanical properties but also excellent flexibility, and when the support substrate 100 is a flexible material, the solar cell according to the embodiment has flexible characteristics. It can be used easily in the required area.

The back electrode layer 200 is disposed on the support substrate 100. The back electrode layer 200 is a conductive layer. The rear electrode layer 200 may be formed of any one of molybdenum (Mo), gold (Au), aluminum (Al), chromium (Cr), tungsten (W), and copper (Cu). Among them, molybdenum (Mo) has a smaller difference in thermal expansion coefficient than the support substrate (100) in comparison with other elements, so that it is possible to prevent peeling phenomenon from occurring due to its excellent adhesion.

The light absorption layer 300 is disposed on the rear electrode layer 200. The light absorption layer 300 includes an I-III-VI compound. For example, the light absorbing layer 300 may be formed of a copper-indium-gallium-selenide-based (Cu (In, Ga) (Se, S) ₂ ; CIGSS-based) crystal structure, copper-indium-selenide-based, or copper- It may have a gallium-selenide-based crystal structure. In addition, the band gap of the light absorbing layer 300 may be about 1.0 eV to about 1.8 eV, but is not limited thereto.

The buffer layer 400 is disposed on the light absorbing layer 300. The thickness of the buffer layer 400 may be about 20 nm to about 70 nm, but is not limited thereto. The buffer layer 400 may be represented by the following Chemical Formula 1.

[Chemical Formula 1]

ZnO _1-X S _X (0.2≤ X ≤0.8)

In addition, the bandgap energy of the buffer layer 400 may have a value of about 2.83 eV to about 3.3 Ev. Accordingly, the buffer layer 400 according to the embodiment may have a value larger than the band gap energy (about 2.2 eV to 2.4 eV) of the buffer layer, which is conventionally used, and the transmittance of the buffer layer 400 may be improved.

The bandgap energy of the buffer layer 400 may be sequentially adjusted according to a position in the buffer layer 400. In more detail, the bandgap energy of the buffer layer 400 may decrease from the interface of the buffer layer 400 to the front electrode layer 600 toward the interface of the buffer layer 400 and the light absorbing layer 300. For example, the bandgap energy of the buffer layer 400 is about 3.3 eV at the interface between the buffer layer 400 and the front electrode layer 600, and gradually decreases as the closer to the light absorbing layer 300. The interface between the buffer layer 400 and the light absorbing layer 300 may have a value of about 2.8 eV, but is not limited thereto.

Referring to FIG. 2, the bandgap energy of the buffer layer 400 is smaller than the bandgap energy of the front electrode layer 600 and has a value greater than the bandgap energy of the light absorbing layer 300. For example, the bandgap energy of the buffer layer 400 is the second bandgap energy, the bandgap energy of the front electrode layer 600 is the first bandgap energy, the bandgap energy of the light absorbing layer 300 is a third When referred to as bandgap energy, the second bandgap energy is higher than the third bandgap energy and lower than the first bandgap energy.

That is, the bandgap energy of the buffer layer 400 according to the embodiment has a value between the front electrode layer 600 and the light absorbing layer 300, and a band gradually goes from the front electrode layer 600 to the light absorbing layer 300. The gap energy decreases sequentially. Accordingly, in the solar cell according to the embodiment, a buffer layer 400 having a sequential potential barrier is formed. Therefore, in the solar cell according to the embodiment, electrons and / or holes formed by sunlight may be easily transported to the back electrode layer 200 and the front electrode layer 600, and may have improved power generation efficiency.

The solar cell according to the embodiment may manufacture the buffer layer 400 having the sequential band gap energy by controlling the content of oxygen and sulfur in the buffer layer 400. 3 and 4 are graphs showing the band gap energy change of the buffer layer 400 according to the oxygen and sulfur contents.

In one embodiment, the sulfur content in the buffer layer 400 may increase from the interface of the buffer layer 400 to the front electrode layer 600 toward the interface of the buffer layer 400 and the light absorbing layer 300. For example, as the sulfur content (value of X) in Formula 1 is sequentially increased from about 0.2 to about 0.5 (a), the oxygen content may be sequentially reduced from about 0.8 to about 0.5. Accordingly, the band gap energy of the buffer layer 400 may decrease from the interface of the buffer layer 400 to the front electrode layer 600 toward the interface of the buffer layer 400 and the light absorbing layer 300.

In another embodiment, the sulfur content in the buffer layer 400 may decrease from the interface of the buffer layer 400 to the front electrode layer 600 toward the interface of the buffer layer 400 and the light absorbing layer 300. For example, in Formula 1, the sulfur content (value of X) is sequentially decreased from about 0.8 to about 0.5 (b), so that the oxygen content may be sequentially increased from about 0.2 to about 0.5. Accordingly, the band gap energy of the buffer layer 400 may decrease from the interface of the buffer layer 400 to the front electrode layer 600 toward the interface of the buffer layer 400 and the light absorbing layer 300.

Meanwhile, only the buffer layer 400 formed as a single layer has been mentioned so far, but embodiments are not limited thereto. That is, the buffer layer 400 according to the embodiment may include a plurality of buffer layers. For example, the buffer layer 400 may include a first 'buffer layer disposed on the light absorbing layer 300 and a first ″ buffer layer disposed on the first' buffer layer. In this case, the bandgap energy of the first '' buffer layer is greater than the bandgap energy of the first 'buffer layer, and the bandgap energy of the first''buffer layer and the bandgap energy of the first' buffer layer are sequential values. Can have

The high resistance buffer layer 500 is disposed on the buffer layer 400. The high-resistance buffer layer 500 includes zinc oxide (i-ZnO) that is not doped with impurities. The bandgap energy of the high resistance buffer layer 500 may be about 3.1 eV to about 3.3 eV, but is not limited thereto. In addition, the high-resistance buffer layer 500 may be omitted.

The front electrode layer 600 may be disposed on the light absorbing layer 300. For example, the front electrode layer 600 may be disposed in direct contact with the high-resistance buffer layer 500 on the light absorption layer 300.

The front electrode layer 600 may be formed of a light-transmitting conductive material. In addition, the front electrode layer 600 may have characteristics of an n-type semiconductor. At this time, the front electrode layer 600 may form an n-type semiconductor layer together with the buffer layer 400 to form a pn junction with the light absorbing layer 300 which is a p-type semiconductor layer. The front electrode layer 600 may be formed of, for example, aluminum-doped zinc oxide (AZO). The thickness of the front electrode layer 600 may be about 100 nm to about 500 nm.

5 is a cross-sectional view illustrating a cross section of the solar cell according to the second embodiment. Referring to FIG. 5, the solar cell according to the second embodiment includes a first buffer layer 410 disposed on the light absorbing layer 300 and a second buffer layer 420 disposed on the first buffer layer 410. Include. The bandgap energy of the second buffer layer 420 has a value greater than the bandgap energy of the first buffer layer 410, and the second buffer layer 420 may be represented by Chemical Formula 1 below. In addition, as mentioned in the first embodiment, the bandgap energy of the second buffer layer 420 is formed from the interface between the second buffer layer 420 and the front electrode layer 600. It may be sequentially reduced toward the interface of the first buffer layer 410.

[Chemical Formula 1]

ZnO _1-X S _X (0.2≤ X ≤0.8)

As mentioned above, the first buffer layer 410 has a smaller bandgap energy value than the second buffer layer 420. For example, the bandgap energy of the first buffer layer 410 may be less than about 2.83 eV. In more detail, the first buffer layer 410 may have a bandgap energy of about 2.0 eV to about 2.83 eV, but is not limited thereto.

The first buffer layer 410 may be a cadmium sulfide (CdS) layer. The cadmium sulfide layer may be formed to a thickness of about 1 nm to about 10 nm, to minimize the toxicity of the cadmium sulfide layer. In addition, a part of cadmium ions (Cd ²⁺ ) may be diffused into the light absorbing layer 300 when the cadmium sulfide layer is manufactured. That is, the cadmium ions (Cd ²⁺ ) may be diffused into the copper vacancy formed in the light absorbing layer 300. Accordingly, the surface vicinity of the light absorbing layer 300 may include the cadmium ion (Cd ²⁺ ). By the diffusion of the cadmium ion (Cd ²⁺ ) in the place of the copper vacancy (Cu vacancy), the defect of the light absorbing layer 300 can be removed, the efficiency of the light absorbing layer 300 is increased Can be.

In addition, a third buffer layer 430 may be further formed between the first buffer layer 410 and the second buffer layer 440. For example, the third buffer layer 430 may be a CdZnS layer. That is, a part of the first buffer layer 410 and the second buffer layer 420 react to form a CdZnS layer, so that band gap energy can be easily adjusted.

6 to 10 are cross-sectional views illustrating a method of manufacturing the solar cell according to the embodiment. For a description of the present manufacturing method, refer to the description of the solar cell described above.

Referring to FIG. 6, the back electrode layer 200 is formed on the support substrate 100. The back electrode layer 200 may be formed by physical vapor deposition (PVD) or plating.

Subsequently, referring to FIG. 7, a light absorbing layer 300 is formed on the back electrode layer 200. 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.

Thereafter, referring to FIG. 8, a buffer layer 400 is formed on the light absorbing layer 300. The method of manufacturing the buffer layer 400 is not particularly limited as long as it is used in the art for manufacturing a buffer layer of a solar cell. For example, the buffer layer 400 may be formed by sputtering, evaporation, chemical vapor deposition, organometallic chemical vapor deposition (MOCVD), and close-spaced sublimation (CSS). Spray pyrolysis, chemical spraying, screen printing, non-vacuum liquid film deposition, chemical bath deposition, chemical transport deposition, vapor transport deposition, atomic layer deposition (Atomic layer deposition: ALD) and electrodeposition (electrodeposition) can be formed by any one of the methods selected. In more detail, the buffer layer 400 may be manufactured by chemical bath deposition (CBD), atomic layer deposition (ALD), or organometallic chemical vapor deposition (MOCVD).

In one embodiment, the buffer layer 400 may be manufactured by the solution growth method as follows. As the source of zinc and sulfur, zinc sulfuric acid (ZnSO ₄ ) and thiourea (NH ₂ ) ₂ CS in aqueous solution were used, respectively, and ammonia (ammonia, NH) as a complex and a pH regulator were used. ₃ ). In addition, an appropriate amount of hydrazine hydrate solution may be added to promote the production of zinc ions in the reaction solution. That is, to grow a zinc sulfide (ZnS) thin film, a reagent in an aqueous state is added to a reaction vessel containing an appropriate amount of deionized water in the order of zinc sulfate, ammonia, hydrazine hydrate, and thiourea. In this case, the temperature of the support substrate 100 on which the light absorbing layer 300 is formed may be adjusted to about 50 ° C. to about 90 ° C. using a heater installed in the reaction vessel.

In addition, by adjusting the reaction temperature in two or more steps, the buffer layer 400 having sequential band gap energy may be manufactured. For example, the buffer layer 400 having a sequential band gap energy may be manufactured by mainly reacting oxygen at a temperature of about 50 ° C. to about 60 ° C. and mainly reacting sulfur at about 70 ° C. to about 90 ° C. .

In another embodiment, when manufacturing the buffer layer 400 using atomic layer deposition (ALD) or organometallic chemical vapor deposition (MOCVD), the band of the buffer layer 400 by adjusting the partial pressure of the reaction gas The gap energy can be adjusted sequentially.

9 and 10, a high resistance buffer layer 500 is sequentially formed on the buffer layer 400. The high resistance buffer layer 500 may be deposited on the buffer layer 400 by a sputtering process. In addition, the front electrode layer 600 may be formed by depositing using a ZnO target by RF sputtering, reactive sputtering using a Zn target, and organometallic chemical vapor deposition.

The features, structures, effects and the like described in the 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 can be combined and modified by other persons 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 (7)

A rear electrode layer disposed on the supporting substrate;
A light absorbing layer disposed on the rear electrode layer;
A first buffer layer disposed on the light absorbing layer;
A second buffer layer disposed on the first buffer layer and represented by Chemical Formula 1; And
A front electrode layer disposed on the buffer layer, wherein a band gap energy of the second buffer layer is greater than a band gap energy of the first buffer layer,
The band gap energy of the second buffer layer decreases from the interface between the second buffer layer and the front electrode layer to the interface between the second buffer layer and the first buffer layer.
[Chemical Formula 1]
ZnO1-XSX (0.2≤ X ≤0.8) The method of claim 1,
The first buffer layer is a cadmium sulfide (CdS) layer solar cell. 3. The method of claim 2,
The first buffer layer is a solar cell formed to a thickness of 1nm to 10nm. 3. The method of claim 2,
A portion of the cadmium sulfide (CdS) of the cadmium sulfide (CdS) in the portion of the light absorbing layer in contact with the first buffer layer comprising a diffusion of copper in the copper vacancy (Cu Vacancy) site of the light absorbing layer. The method of claim 1,
Between the first buffer layer and the second buffer layer, the first buffer layer and the second buffer layer react to form a third buffer layer,
The third buffer layer is a solar cell comprising a CdZnS layer. The method of claim 1,
The bandgap energy of the second buffer layer is sequentially reduced in the range of 2.83 eV to 3.3 eV from the interface between the second buffer layer and the front electrode layer to the interface between the second buffer layer and the first buffer layer. The method of claim 1,
The x increases from 0.2 to 0.5 from the interface between the second buffer layer and the front electrode layer toward the interface between the second buffer layer and the first buffer layer.

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