KR20140081970A - Method to enhance the fill factor of CIGS thin film solar cells using Cd-free buffer layers - Google Patents

Abstract

The present invention relates to a method for enhancing a fill factor of a Cu(In,Ga)Se^2 (referred to as CIGS, hereinafter) thin film solar cell using a Cd-free buffer layer such as an oxide or a sulfide having wide band gap energy. In detail, when oxide and sulfide thin films used as a buffer layer are manufactured by using an atomic layer deposition method, compositions within the thin films are adjusted to fabricate a buffer layer having a dual-layer structure having different electrical characteristics. Thus, a relatively low fill factor generated in a CIGS thin film solar cell using an oxide or a sulfice having low electrical conductivity in a buffer layer can be resolved. The structure of the CIGS thin film solar cell using the dual-layer buffer layer fabricated according to the present invention, and a fill factor and light conversion efficiency of the solar cell can be enhanced.

Description

[0001] The present invention relates to a method for improving the fidelity of a CIGS thin film solar cell using a cadmium buffer layer,

The present invention relates to a method for improving the fidelity of Cu (In, Ga) Se ₂ (hereinafter abbreviated as "CIGS") thin film solar cell using a cadmium buffer layer such as oxide and sulfide having a wide band gap energy . More specifically, in order to solve the problem of relatively low fidelity in a CIGS thin film solar cell using an oxide or a sulfide having a low electric conductivity as a buffer layer, oxide and sulfide thin films used as a buffer layer are formed by atomic layer deposition to manufacture a CIGS solar cell A buffer layer having a bilayer structure having different electrical characteristics was prepared by controlling the composition of the inside of the thin film.

The present invention is characterized by the structure of the CIGS thin film solar cell using the thus-fabricated double-layer buffer layer and the improvement of the fidelity and the light conversion efficiency of the solar cell.

The present invention relates to a method for producing a double-layer buffer layer for improving the fidelity of a cadmium-free CIGS thin film solar cell and a photovoltaic conversion efficiency of a solar cell manufactured therefrom. More particularly, the present invention relates to a buffer layer applied to a CIGS thin- The present invention relates to a buffer layer having a bilayer structure with different conductivity by controlling the composition of the buffer layer during the buffer layer deposition using an atomic layer deposition method and a method for manufacturing the CIGS thin film solar cell using the buffer layer.

Among the various types of thin film solar cells reported so far, CIGS thin film solar cells are the most efficient solar cells with a maximum efficiency of about 20%. The basic structure of the CIGS thin film solar cell is composed of a soda lime glass substrate coated with molybdenum (Mo), which is a lower electrode, and a p-type semiconductor CIGS, a cadmium sulfide (CdS) buffer layer, . In general, CIGS thin film solar cell, which has high efficiency, uses a cadmium sulfide layer deposited by solution growth method as a buffer layer. The cadmium sulfide buffer layer improves the band structure and bonding property between the absorption layer and window layer, When the film is deposited by sputtering, it serves to protect the absorbing layer from physical impacts that may occur in a plasma environment.

However, the cadmium sulfide buffer layer contains cadmium (Cd), which is a toxic substance, and has been an obstacle to commercialization. In addition, the bandgap energy of cadmium sulfide is 2.42 eV. Since the buffer layer absorbs incident light having energy below this energy, the loss of quantum efficiency caused by the light in the short wavelength region not reaching the absorption layer causes a decrease in the efficiency of the solar cell. In addition, the solution growth method, which is a vapor deposition method of cadmium sulfide, is not suitable for mass production by the non-vacuum synthesis method and a large amount of chemical substance is discharged after the production of the desired substance. Therefore, in order to replace the cadmium sulfide buffer layer in various solar cell research groups, attempts have been made to fabricate a material having a wide band gap energy through a new vapor deposition method instead of a solution growth method. Accordingly, sputtering, ion gas reaction, evaporation New materials have begun to be used as buffer layers by various methods such as chemical vapor deposition, atomic layer deposition and chemical vapor deposition. Among the above deposition methods, the study on the preparation of the cadmium buffer layer for CIGS solar cells using the atomic layer deposition method has been intensively studied since the mid 1990s and intensively in the early 2000s and is now in the stage of mass production. The atomic layer deposition method is advantageous for uniform deposition of a thin film on a large area, easy composition control of the thin film, and a vacuum deposition method, so that the actual mass production line is suitable. Therefore, the CIGS solar cell buffer layer deposition method is very advantageous Have. Thus, various materials ranging from sulphides such as ZnS and In ₂ S ₃ to oxides such as ZnO, Zn _1-x Mg _x O, and Zn _1-x Sn _x O, Was fabricated using the atomic layer deposition method and actually applied to the CIGS solar cell, showing excellent efficiency [cf. Reference 1-9].

Among the buffer materials that have been deposited using ALD to date, the most effective buffer materials are Zn (O, S), Zn _1-x Mg _x O, Zn _1-x Sn _x O In ₂ (O, S) ₃ . All of the above materials show high solar cell efficiency of over 16%. Source materials suitable for use in atomic layer deposition processes are well known and have the advantage of varying the bandgap energy from 3.2 eV to 3.8 eV depending on the film composition. In the case of atomic layer deposition, the number of atomic layer deposition cycles of various thin films such as zinc oxide, magnesium oxide, tin oxide, zinc sulfide, indium oxide and indium sulfide, which are unit films in the production of sulfide and oxide thin films, It is advantageous to fine-tune the composition of the buffer layer to optimize the conduction band structure at the junction interface with CIGS.

Generally, the conduction band structure at the interface between the buffer layer and the light absorption layer in the CIGS thin film solar cell greatly affects the efficiency of the solar cell. When the conduction band of the buffer layer is located lower than the conduction band of the light absorption layer, the band structure is called a cliff type band. At this time, the cliff formed on the interface forms an electron injection The recombination at the interface becomes large and the open circuit voltage (Voc) of the solar cell is lowered. Conversely, when the conduction band of the buffer layer is located above the conduction band of the light absorption layer, a spike type band structure is formed. If the size of the spike is too large, the photoelectrons generated by the absorption of the incident light in the light absorption layer pass through the buffer layer The short circuit current (Isc) is lowered because it interferes with movement when moving to the negative electrode. Therefore, it is known to have a spike type band structure of a suitable size (0 to 0.3 eV) in order not to lower both the open-circuit voltage and the short-circuit current.

In the present invention, the conduction band structure at the interface between the zinc-magnesium oxide buffer layer and the CIGS light absorption layer was analyzed by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) As a result, it was confirmed that the zinc-magnesium oxide had a band structure having a proper spike type when the content of magnesium was about 20% and the band gap energy was 3.67 eV. However, oxide and sulfide buffer layers having such a wide bandgap energy have a lower electrical conductivity than cadmium sulfide, so that the series resistance of manufactured solar cells increases, resulting in lowering of fidelity and lowering of efficiency. The relation between the fidelity and the series resistance of the solar cell is as follows.

(F.F= fill factor, 충실도)

(FF = fill factor, fidelity)

As can be seen from the above equation, in order to improve the fidelity of the solar cell, the series resistance of the solar cell should be reduced. In order to reduce the series resistance, it is necessary to make the thickness of the low-conductivity buffer layer as thin as possible. However, if the buffer layer becomes too thin, the damage of the absorption layer due to the plasma can not be prevented in the sputtering process, It is also difficult to reduce the thickness.

In USP 2006/0180200 A1 (thin film solar cell), a second original layer deposition (ALD) ZnO buffer layer is formed on the first Zn (O, S) buffer layer, and a CIGS layer To a CIGS thin film solar cell in which a first Zn (O, S) buffer layer is completely formed as a buffer layer. In US 2010/0233841 A1 (thin film solar cell), a first ALD Zn (O, S) buffer layer is formed on the CIGS layer, and a second ALD ZnO-Buffer layer The present invention relates to a CIGS thin film solar cell formed with a complete buffer layer. However, these prior arts have different technical constructions from those of the present invention.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems by providing a buffer layer structure in which the thickness of a buffer layer is sufficiently secured to prevent physical damage due to a plasma environment during deposition of a transparent conductive film, It is intended to solve this problem with a double layer. The buffer layer deposited close to the bonding interface with the CIGS layer as the light absorbing layer is deposited to a thickness of half the optimum thickness by adjusting the composition of the buffer layer so as to have a high band gap energy so as to have an optimum conduction band structure, To improve the conductivity of the entire buffer layer and to improve the fidelity and efficiency of the CIGS thin film solar cell by ultimately providing a conduction band structure that facilitates photoelectric transfer.

The technical objects to be achieved by the present invention are not limited to the above-mentioned technical problems, and other technical subjects which are not mentioned can be clearly understood by those skilled in the art from the description of the present invention .

In order to achieve the above object, the present invention provides an organic metal precursor containing zinc (Zn) and metal elements such as indium (In), tin (Sn), and magnesium (Mg) Wherein a double layer buffer layer having a desired thickness is grown on the CIGS light absorption layer by controlling the number of atomic layer deposition cycles of the metal oxide and the metal sulfide unit film by atomic layer deposition using a gas reactor do.

Preferably, the thickness of the double layer buffer layer is adjusted so that the half thickness of the bonding interface of the CIGS light absorbing layer has a high band gap energy for an optimal conduction band structure, and then the thickness of the remaining half of the buffer layer And the composition is adjusted so as to have a low band gap energy.

More preferably, the thickness of the double-layer buffer layer is about 40 nm, the high band gap energy has a spike shape with a small conduction band structure to the CIGS light absorption layer through analysis, and the low band gap energy is a zinc oxide transparent conduction film layer And the band gap energy value is smaller than that of the buffer layer having the high band gap energy and the base energy.

Preferably, the thickness and the composition of the double-layer buffer layer can be controlled through the number of cycles of atomic layer deposition of the metal oxide and the metal sulfide, which are unit films, by determining the composition by controlling the ratio of the number of cycles of the two unit films constituting the desired substance , The total number of cycles of the two unit films is adjusted to deposit the bilayer buffer layer to a desired thickness. For example, in the case of zinc-magnesium oxide, the unit film is a zinc oxide layer and a magnesium oxide layer. When the magnesium content of the zinc-magnesium oxide thin film is 20%, the magnesium oxide unit film process The total magnesium unit film process proceeds at a rate of 1 per 5 times of the unit film of the whole atomic layer deposition process, so that the magnesium content in the finally deposited thin film is 20%.

Preferably, the organometallic precursor comprising the metal element is comprised of a well-known metal organic chemical precursor such as dimethylzinc, diethylzinc, indiumacetylacetonate, bisethylcyclopentadienylmagnesium, tinacetylacetonate, etc. And the like.

Preferably, the oxygen-containing gas reactor is any one selected from the group consisting of water vapor, O ₂ , and the like.

Preferably, the sulfur-containing gas reactor is hydrogen sulfide.

Preferably, in the atomic layer deposition process, the substrate temperature is 100 to 120 ° C to prevent thermal energy supply for thin film growth and thermal deterioration of the thin film solar cell.

The double layer buffer layer made of oxide and sulfide according to the present invention improves the overall electrical conductivity of the buffer layer by the thin film portion having low band gap energy compared with the single layer buffer layer deposited with the conventional constant composition. In addition, the structure of the conduction band in the manufacturing of the solar cell can reduce the series resistance of the solar cell because the conduction band structure formed in the CIGS light absorption layer is more smoothly transferred. In addition, the reduction of the series resistance of the solar cell has the effect of improving the fidelity of the solar cell and consequently the photoconversion efficiency of the solar cell.

FIG. 1 is a schematic cross-sectional view showing the construction of a multi-layered CIGS thin film solar cell to which a double-layer buffer layer (a) and a conventional single-layer buffer layer (b) according to the present invention are applied.
FIG. 2 is a schematic cross-sectional view illustrating a multi-layer CIGS thin film solar cell electronic band structure using a double layer buffer layer and a conventional single layer buffer layer according to a manufacturing method of the present invention.
FIG. 3 is a photograph of a microstructure observed by a scanning microscope after a double-layer buffer layer according to the present invention is deposited on a CIGS light absorption layer.
FIG. 4 is a graph showing changes in band gap energy according to the magnesium content of the zinc-magnesium buffer layer according to the manufacturing method of the present invention. FIG.
5 is a graph comparing the electrical conductivities of the double-layer buffer layer and the conventional single-layer buffer layer according to the manufacturing method of the present invention
6A is a current density-voltage curve of a CIGS thin film solar cell using a dual-layer buffer layer and a single-layer buffer layer of different thicknesses according to an embodiment of the present invention.
FIG. 6B is a resistance-current density curve of a CIGS thin film solar cell using a dual-layer buffer layer and a single-layer buffer layer of different thicknesses according to an embodiment of the present invention.

The present invention shows a method for improving the fidelity of a CIGS thin film solar cell.

A CIGS thin film solar cell using a cadmium buffer layer having a low series resistance and high fidelity by applying an oxide and a sulfide buffer layer of a bilayer structure having different band gap energies in a high efficiency CIGS thin film solar cell improving method This shows how to improve the fidelity.

The buffer layer of oxide and sulfide was prepared by atomic layer deposition, and a buffer layer having a high band gap energy was grown to a thickness of 20 to 25 nm for the spike type conduction band structure near the CIGS light absorption layer junction by controlling the composition during the manufacturing process And the remaining 20 to 25 nm thick may be a double layer buffer layer structure having a thickness within a range of 50 nm or less by preparing a buffer layer having a relatively low band gap energy.

The oxide and sulfide buffer layer may have a band gap energy of 3.2 eV to 4.0 eV.

The oxide and the sulfide may be zinc oxide, zinc sulfide, indium oxide, indium sulfide, magnesium oxide, tin oxide, and / or a compound thereof.

The deposition temperature at the time of depositing the double-layer buffer layer by the atomic layer deposition method may be 120 ° C or lower, preferably 100-120 ° C to prevent thermal deterioration of the solar cell.

In order to achieve the above object, in the present invention, a buffer layer having a bilayer structure is prepared by adjusting the ratio of the number of cycles of the zinc oxide and magnesium oxide thin films, which are unit films, by atomic layer deposition using a zinc-magnesium oxide thin film as a buffer layer . At this time, the temperature of the substrate is preferably 100 to 120 캜 in consideration of the reaction of the source materials for forming the thin film and thermal degradation of the solar cell.

Hereinafter, the contents of the present invention will be described in more detail with reference to the drawings.

FIG. 1A is a schematic cross-sectional view of a dual-layer buffer layer and a multi-layered CIGS thin-film solar cell including the buffer layer according to a manufacturing method of the present invention, and FIG. 1B is a cross-sectional view of a CIGS thin- Fig. The CIGS thin film solar cell consists of a substrate 1, molybdenum 2 as a back electrode layer, CIGS 3 as a light absorbing layer, zinc-magnesium oxide 4 having a high band gap energy and having a magnesium content of 20% A zinc-magnesium oxide 5 having a magnesium content of 10% with a low band gap energy, and a zinc oxide transparent conductive film 6 are sequentially laminated. The substrate may be made of glass or stainless steel having flexibility, polyimide, or the like. The rear electrode layer may be formed of a metal such as molybdenum, tantalum or the like, but molybdenum is preferably used. The CIGS thin film can be produced by vacuum evaporation or selenization after precursor deposition.

Deposition of the buffer layer of FIGS. 1 (4) and (5) is carried out by atomic layer deposition, using diethylzinc as the zinc source, bisethylcyclopentadienyl magnesium as the magnesium source, and using steam as the oxygen source Respectively. The gas source and the oxygen source containing the respective metal atoms were charged with argon gas as the transport gas and argon gas as the metal precursor and the water vapor at the flow rate of 100 sccm to inject the sources into the reactor. When the magnesium content of the zinc-magnesium oxide is 20% to control the composition of the buffer layer, the magnesium oxide unit film cycle is performed once every four times of the zinc oxide unit film cycle, and the magnesium content of the zinc- %, The magnesium oxide unit film cycle was performed once every 9 times of the zinc oxide unit film cycle, and the buffer layer having the optimum thickness of 40 nm was deposited by 300 cycles of the entire atomic layer deposition process. At this time, in order to deposit the buffer layer with a thickness of 20 nm, the total atomic layer deposition process cycle number is 150 cycles.

The zinc oxide layer 6 of FIG. 1 (6) The transparent conductive film is deposited by sputtering using a pure zinc oxide target and a zinc oxide target doped with aluminum.

2 is a schematic view showing an electronic band structure of a CIGS thin film solar cell including the double-layer buffer layer and the conventional single-layer buffer layer. It can be seen that the CIGS thin film solar cell using the double layer buffer layer has an electron band structure which is easier to move the photoelectrons generated by the absorption of the incident light in the light absorption layer than the CIGS thin film solar cell using the single layer buffer layer.

FIG. 3 is a microstructure photograph of a double-layer buffer layer and a CIGS junction interface using zinc-magnesium oxide, which is observed by scanning electron microscopy. It can be seen from the figure that the double layer buffer layer is uniformly deposited on the surface of the CIGS light absorbing layer with a thickness of about 40 nm.

FIG. 4 is a graph showing the band gap energy change according to the magnesium content of zinc-magnesium oxide. In the case of pure zinc oxide, the band gap energy is about 3.2 eV and the pure magnesium oxide has a band gap energy of about 7.7 eV. When the zinc-magnesium oxide is prepared by mixing magnesium oxide with zinc, the band gap energy of the oxide increases from 3.25 eV to 3.76 eV as the magnesium content increases.

FIG. 5 shows a table comparing electrical characteristics of a zinc-magnesium oxide double layer buffer layer and a single layer buffer layer deposited on a glass substrate. It can be seen that the electric conductivity is higher than that of the single-layer buffer layer when the double-layer buffer layer is used.

FIG. 6 shows current density-voltage curves and resistance-current density curves of a CIGS thin film solar cell using a CIGS thin film solar cell and a single layer buffer layer using a dual layer buffer layer according to the manufacturing method of the present invention.

Referring to FIG. 6A, the CIGS thin film solar cell using the double-layer buffer layer has a significantly improved open-circuit voltage and short-circuit current density compared to a single-layer solar cell, while the fidelity improves from 60% to over 70% Finally, the solar cell efficiency reached 15.41%. If the thickness of the buffer layer is too small, the open-circuit voltage, short-circuit current density, and fidelity are all lowered, thereby deteriorating the performance of the solar cell.

The fidelity enhancement is the series resistance value than the resistance of 0.527Ωcm ² solar cell using a single-layer buffer layer in the series resistance of the solar cell ² 0.329Ωcm for CIGS thin film solar cell using the buffer layer, double-layer Referring to Figure 6b . Since the CIGS thin film solar cell using the buffer layer having the above-described two-layer structure is manufactured using the vacuum evaporation method without using the conventional non-vacuum solution growing method, it is easy to form the CIGS thin film solar cell and high reproducibility can be secured, The performance of the battery can also be improved. Other processes may be added to the manufacturing method of the present invention within the scope of not damaging the effects of the present invention, and these should be construed as falling within the scope of the present invention.

Hereinafter, the present invention will be described in detail with reference to examples. However, these are for the purpose of illustrating the present invention in more detail, and the scope of the present invention is not limited thereto.

<Examples>

In order to manufacture the CIGS thin film solar cell of FIG. 1, soda lime glass was used as a substrate.

The molybdenum metal thin film deposited directly on the substrate is deposited to a thickness of about 1 μm by a DC sputtering method using a molybdenum target. On top of that, the CIGS layer is deposited by vacuum evaporation, and the copper, indium, gallium, and selenium metal elements constituting the CIGS layer are placed in the respective crucibles at a pressure of about 10 to 6 Torr in the chamber, The thin film was deposited on molybdenum by heating each element to a gas by heating.

Next, the buffer layer is formed by atomic layer deposition. Diethylzinc is used as a zinc source, bisethylcyclopentadienyl magnesium is used as a magnesium source, and water vapor is used as an oxygen source. The gas source and the oxygen source containing the respective metal atoms were charged with argon gas as the transport gas and argon gas as the metal precursor and the water vapor at the flow rate of 100 sccm to inject the sources into the reactor. The process temperature was 120 ° C. and the composition of the zinc-magnesium oxide deposited was controlled by the ratio of the number of atomic layer deposition cycles of the zinc oxide film and the magnesium oxide film, which are unit films. Thus, a buffer layer of about 40 nm thickness is finally deposited. After the buffer layer deposition, the transparent conductive film is composed of a 50 nm thick undoped pure zinc oxide layer and a 400 nm thick aluminum-doped zinc oxide layer and is deposited by rf-sputtering. CIGS thin film solar cells are finally manufactured by applying these various deposition methods sequentially.

After the CIGS thin film solar cell was fabricated, the thickness of the formed zinc-magnesium oxide buffer layer was about 40 nm as measured by a scanning electron microscope. It was confirmed that the thin film was uniformly formed on the CIGS layer to a uniform thickness. The bandgap energy of the zinc-magnesium oxide buffer layer was analyzed using the change of extinction coefficient according to the incident laser wavelength using ellipsometry, and the electrical properties of the zinc-magnesium oxide thin film were analyzed by the 4-point probe method . Finally, the performance of the CIGS solar cell was analyzed using a solar simulator using a white light source of AM1.5.

1) Cross-section and surface analysis by scanning electron microscope

As a result of analyzing the cross section and surface of the sample using a scanning electron microscope after the buffer layer deposition, it was confirmed that the thickness of the buffer layer was about 40 nm and the thickness was uniformly deposited on the CIGS surface as shown in FIG.

2) Evaluation of optical characteristics using ellipsometry

The change of the bandgap energy according to the magnesium content of the zinc-magnesium oxide was determined by analyzing the absorption coefficient of the buffer layer by measuring the extinction coefficient in a wavelength range of 300 nm to 1000 nm by using ellipsometry. The following relation holds between the extinction coefficient (k) and the absorption coefficient (?) Of the thin film.

α = 4πk / λ where λ is the wavelength.

By extrapolating α2 vs photon energy graph with this relation, the optical band gap energy of the thin film can be obtained. As a result, it was confirmed that the band gap energy of the zinc-magnesium oxide varied from 3.25 eV to 3.76 eV according to the magnesium content as shown in FIG.

3) Evaluation of electrical characteristics using 4-point probe analysis

As a result of analyzing the electrical characteristics using the 4-point probe method after depositing the double-layer buffer layer and the conventional single-layer buffer layer on the glass substrate, it was found that the double- The resistivity of the film was measured to be low. This is due to the fact that in the case of the double-layer buffer layer, about 20 nm of the thickness of 40 nm is composed of a portion having a small magnesium content, that is, a portion having a small band gap energy.

4) Performance evaluation of solar cell using solar simulator

The solar simulator is a device designed to evaluate the performance of solar cells. It consists of an artificial light source similar to sunlight reaching the surface of the earth and a meter for measuring electrical characteristics. The open-circuit voltage, short-circuit current density, and fidelity of the solar cell were evaluated through a solar simulator, and finally the photovoltaic efficiency of the solar cell was measured.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. It will be understood that the invention may be variously modified and changed.

In the CIGS thin film solar cell using the double layer buffer layer of the present invention, the open voltage and the short circuit current density are improved slightly compared with the solar cell using a single layer, but the fidelity is greatly improved to 60% to 70% 15.41%, which is likely to be used industrially.

1: substrate
2: Molybdenum back electrode
3: Cu (In, Ga) Se ₂ light absorbing layer
4: buffer layer having a large band gap energy with a magnesium content of 20%
5: buffer layer having a small band gap energy with a magnesium content of 10%
6: zinc oxide transparent conductive film

Claims (5)

In a method for improving the high-efficiency CIGS thin film solar cell fidelity,
Improving the Fidelity of CIGS Thin Film Solar Cells Using a Cadmium-Free Buffer Layer with Low Series Resistance and High Fidelity by Applying Double-Layered Oxide and Sulfide Buffer Layers with Different Band Gap Energies 2. The method of claim 1, wherein the oxide and sulfide buffer layer is prepared by atomic layer deposition, wherein a buffer layer having a high band gap energy for a spike type conduction band structure is formed in the vicinity of the CIGS light absorption layer junction, CIGS thin film solar cell using a cadmium buffer layer, wherein the buffer layer has a thickness of 20 to 25 nm and the buffer layer has a relatively low band gap energy. The buffer layer has a thickness of 40 to 50 nm. How to improve fidelity The method for improving the fidelity of a CIGS thin film solar cell using a Cadmium-free buffer layer according to claim 1, wherein the oxide and sulfide buffer layer has a band gap energy of 3.2 eV to 4.0 eV The CIGS thin film solar cell according to claim 1, wherein the oxide and the sulfide are zinc oxide, zinc sulfide, indium oxide, indium sulfide, magnesium oxide, tin oxide and / or a compound thereof. How to improve fidelity The method for improving the fidelity of a CIGS thin film solar cell using a cadmium buffer layer according to claim 1, wherein the deposition temperature at the time of depositing the double layer buffer layer by atomic layer deposition is set to 120 ° C or less to prevent thermal deterioration of the solar cell

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