KR101733438B1

KR101733438B1 - Compositionally graded CZTSSe thin film and its preparation method

Info

Publication number: KR101733438B1
Application number: KR1020150020601A
Authority: KR
Inventors: 김진영; 이도권; 김홍곤; 전종옥; 서세원; 손해정; 고민재
Original assignee: 한국과학기술연구원
Priority date: 2015-02-11
Filing date: 2015-02-11
Publication date: 2017-05-11
Also published as: KR20160098695A

Abstract

We fabricated a Cu ₂ ZnSn (S, Se) ₄ (CZTSSe) thin film solar cell with high efficiency of about 10% by electrochemical deposition and subsequent annealing process. A copper-zinc-tin (Cu-Zn-Sn) alloy was formed on soda lime glass on which molybdenum (Mo) was deposited by using one-pot electrochemical vapor deposition, Was annealed in a mixed atmosphere of sulfur (S), selenium (Se) and argon (Ar) to form a CZTSSe thin film having a band gap of 1.13 eV. It was confirmed by X-ray diffraction and electron spectroscopy that a thin film with a high crystallinity of about 2.5 袖 m was successfully formed and a composition / band gap slope in the vertical direction of the CZTSSe thin film in element depth profiling . As a result of bandgap engineering such as adjustment of bandgap energy and formation of bandgap gradients, the optimum CZTSSe thin film solar cell showed a light conversion efficiency of 9.9%. This is the highest efficiency among the CZTSSe thin film solar cells using the electrochemical vapor deposition method so far reported.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a thin film and a preparation method thereof,

The present invention relates to a sloping composition type chalcogenide thin film, a thin film solar cell including the same, and a manufacturing method thereof.

The Cu ₂ ZnSn (S, Se) ₄ (CZTSSe) thin film is a light absorbing layer of the next generation thin film solar cell and has received considerable research / industrial interest. In particular, CZTSSe is expected to significantly reduce power generation costs due to its abundant constituents, low cost, and high efficiency. Currently, the hydrazine-based solution process shows a maximum efficiency of 12.7%.

Although the hydrazine-based solution process is an inexpensive process as compared with the vacuum process, it is problematic for commercialization and mass production due to the specificity (explosiveness) of hydrazine. In the non-vacuum solution process instead of the hydrazine process, electrochemical vapor deposition is used in several research groups because of its low initial cost of capital, effective utilization of materials, ease of facing, environmental friendliness, and mass production.

Until now, efficiency of CZTSSe thin film solar cell using electrochemical vapor deposition has not been reported more than 8% due to technological difficulties so far.

Korea Patent No. 10-1339874

K. Woo et al. Band-gap-graded Cu2ZnSn (S1-x, Sex) 4 Solar Cells Fabricated by an Ethanol-based, Particulated Precursor Ink Route, Sci. Rep. 2013, 3.

The present invention relates to a Cu ₂ ZnSn (S, Se) ₄ (CZTSSe) thin film solar cell and a Cu ₂ ZnSn (S, Se) ₄ (CZTSSe) thin film solar cell which can be applied thereto by using the electrochemical vapor deposition method and a subsequent heat treatment process. , And a method for producing the same.

One aspect of the present invention is a solar _{cell-Cu 2 ZnSn (S, Se)} 4 thin film, (S _s - S _v) is a positive number and, (Se _s - Se _v) is negative, and the S _s and S _v are each sulfur composition value from the surface and the composition gradient branch point (%), and the Se _s and Se _v is a solar cell-Cu ₂ ZnSn (s, Se), characterized in that the composition value (%) of selenium in each of the surface and the composition gradient of the fork ₄ thin film.

In the present invention, a CZTSSe thin film having a band gap of 1.13 eV, which shows a high efficiency of 9.9%, has been successfully achieved by using electrochemical deposition and a subsequent heat treatment process in a mixed selenium-sulfur atmosphere. As compared with the conventional CZTSe having a band gap of 1.02 eV, since the increase of the band gap energy due to the formation of the band gap difference in the vertical direction minimizes the photocurrent reduction and the light voltage and the filling rate are greatly improved, Seems to be.

Figure 1 shows a schematic representation of a heat treatment system.
2 (a) compares X-ray diffraction patterns of CZTSSe (band gap 1.13 eV) and CZTSe (band gap 1.02 eV), which is an enlarged view around the diffraction peak of the 112 plane. FIG. 2 (b) is a cross-sectional clear-sky image using a transmission electron microscope (TEM), and the illustration is a selected area electron diffraction of the corresponding part. Figure 2 (c) shows the results of EDS line scanning.
FIG. 3 shows the horizontal ratio (left side) and the corresponding band gap energy (right side) of the sulfur composition through TEM EDS analysis of the CZTSSe thin film.
4 (a) compares light and dark current-voltage curvature of CZTSSe with CZTSe, and black line means CZTSSe gray line means CZTSe. 4 (b) is an external quantum efficiency (EQE) curve.
FIG. 5 shows (a) Voc-Tplot and (b) admittance spectroscopic characteristics of the temperature dependent characteristics of the CZTSSe thin film solar cell in the 1 sun state.

Hereinafter, various aspects and various embodiments of the present invention will be described in more detail.

In the present invention, the composition gradient point means a point where the composition of sulfur is increasing and the composition of selenium is decreasing from this point to the surface.

According to one embodiment of the present invention, the composition gradient point may be a point at a depth of about 300 to 1,000 nm, preferably 400 to 600 nm at the surface, or may be a specific point in the upper section, depending on the situation Or may be any point within the above section.

For example, in the right-hand elliptical dashed area in Fig. 2c, sulfur decreases from about 30% to about 50% from the surface to about 500 nm (position about 2,700 nm) to the surface (position about 3,200 nm) And selenium decreased from 30% to 25%.

In FIG. 2C, since Mo and S can not be distinguished from each other in the EDS analysis, pink is indicated as a composition of (Mo + S), and there is no possibility that Mo exists in the region from the surface to about 500 nm , And the pink line in the elliptic dotted line area in Fig. 2C may be interpreted as the composition of S.

In one implementation, the larger the value of S / (S + Se) from the surface than the composition gradient of the branch point, the S and Se is a solar cell, characterized in that the elemental composition values (%) of each of sulfur and selenium, Cu ₂ ZnSn (S, Se) ₄ thin film is disclosed.

In another embodiment, the value of S / (S + Se) from the concentration gradient branch point to the surface is increasing, and S and Se are elemental compositional values (%) of sulfur and selenium, respectively ₂ ZnSn (S, Se) ₄ thin film is disclosed.

Therefore, in the present invention, as defined above, the composition gradient point means a point where the composition of sulfur is increasing and the composition of selenium is decreasing from this point to the surface, or S / (S + Se) composition ratio may indicate an increasing trend.

In the present invention, the tendency to increase or increase indicates a tendency to increase as a whole although there are some decreasing intervals. Specifically, in FIG. 3, when the position is about 2,750 nm to 3,200 nm, about 2,900 the S / (S + Se) value is increasing at a position of approximately 2,750 nm to 3,200 nm according to the above definition of the present invention, although an extremely narrow region that is partially reduced in the vicinity of nm is observed have.

In yet another embodiment, S / (S + Se) value at the surface is 0.3 to 0.5, and the S and Se is a solar cell-Cu ₂ ZnSn characterized in that the elemental composition values (%) of each of sulfur and selenium, (S, Se) ₄ thin film is disclosed.

2C is a TEM EDS analytical value that is not calibrated and corresponds to raw data. FIG. 3C is a graph in which FIG. 3 is calibrated. In FIG. 2C, it is meaningful to compare composition ratios according to the depth of the same material, It is difficult to compare the composition values of materials (eg, S and Se) with each other. It is preferable to use the graph of Fig. 3 in order to compare the composition values of different materials (e.g., S and Se) with each other.

As shown in FIG. 3, S / (S + Se) is less than 0.5 over the entire depth range. As shown in FIG. 3, The desired band gap can be obtained.

In another embodiment, a Cu ₂ ZnSn (S, Se) ₄ thin film for a solar cell is disclosed, wherein the bandgap of the thin film is increasing from the concentration gradient bifurcation point to the surface.

In still other embodiments, the band gap of the thin film is a solar _{cell-Cu 2 ZnSn (S, Se)} 4 thin film, it characterized in that the value of the surface larger 0.08 to 0.12 eV than the lowest point of the interior of the thin-film is disclosed.

Another aspect of the present invention is a method for manufacturing a solar cell comprising the steps of: (A) heat treating a metal alloy thin film composed of a substrate, Mo located on the substrate, and Cu-Zn-Sn alloy located on the Mo, Cu ₂ ZnSn (S, Se) ₄ thin film.

In one embodiment, the S is supplied by providing H ₂ S together with the carrier gas, and the Se is supplied by heating and vaporizing the Se powder to form a Cu ₂ ZnSn (S, Se) ₄ thin film A method for producing the same is disclosed.

In other embodiments, the (A) step of the solar _{cell-Cu 2 ZnSn (S, Se)} 4 thin film, characterized in that over time, keeping the amount of supply of H ₂ S being carried out in the supply amount of Se is reduced atmosphere A manufacturing method is disclosed.

In another embodiment, the step (A) continuously supplies H ₂ S together with the carrier gas, stops the heating of the Se powder, and naturally cools to the temperature below the melting point of Se, thereby reducing the amount of Se supplied the method of manufacturing a solar _{cell-Cu 2 ZnSn (S, Se)} 4 thin film, characterized in that is carried out is disclosed.

In the present invention, the natural cooling means that the natural cooling is left without being heated. For example, it can be performed by heating the Se powder to 500 to 600 ° C, holding it for 10 minutes to 1 hour, and then stopping the heating.

In another embodiment, the step (A) is performed in an atmosphere having a partial pressure of S and Se of 0.01 to 0.1 atm in the total pressure (1 atm) of S and Se and the carrier gas. ₂ ZnSn (S, Se) ₄ thin film is disclosed.

If the upper limit is exceeded, the molybdenum compound is excessively formed on the rear electrode layer, thereby decreasing the conductivity of the rear electrode and increasing the series resistance value. The characteristics of the solar cell can be reduced.

Also, by adjusting the value within the above range, it is possible to control the composition gradient or the degree of inclination according to the depth of sulfur and selenium, and as a result, the gradient or the degree of inclination of the band gap depending on the depth can be controlled. That is, by increasing the pressure value, it is possible to increase the composition gradient or the degree of inclination according to the depth of sulfur and selenium, and to increase the gradient or inclination of the band gap depending on the depth.

In a further embodiment, the (A) step is a method for manufacturing a solar cell ₂ ZnSn Cu (S, Se) ₄ thin film, it characterized in that the partial pressure ratio of S and Se is 0.2 to 2.0 is disclosed.

If the value is less than the lower limit of the above range, the composition ratio of S and Se in the thin film becomes excessively low, so that the band gap becomes excessively small in the direction of 1.0 eV and the composition gradient does not occur. The composition ratio of S and Se is excessively increased, the band gap may be excessively increased in the direction of 1.5 eV, or a secondary phase may be generated and the problem of increasing the roughness of the thin film may occur.

Another aspect of the present invention relates to a solar cell comprising Cu ₂ ZnSn (S, Se) ₄ thin film according to various embodiments of the present invention.

Another aspect of the invention (A) the selenium vaporization zone, (B) a metal alloy thin film heating _{zone, (C-1) H 2} S gas _{inlet, (C-2) H 2} S for solar cells comprising a gas outlet Cu ₂ ZnSn (S, Se) ₄ thin film manufacturing apparatus.

H ₂ S gas is introduced with the carrier gas through the H ₂ S gas inlet and flows into the substrate heating zone together with selenium vapor vaporized in the selenium vaporization zone, and in the metal alloy thin film heating zone, (b- 1) substrate, (b-2) Mo disposed on the substrate, and (b-3) a Cu-Zn-Sn alloy located on the Mo phase.

Hereinafter, the present invention will be described in more detail with reference to Examples and the like, but the scope and content of the present invention can not be construed to be limited or limited by the following Examples. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the present invention as set forth in the following claims. It is natural that it belongs to the claims.

Example

Example 1

Copper-zinc-tin alloy thin films were prepared by electrochemical deposition using potentiostat (model: PARSTAT MC, Princeton Applied Research, USA). The aqueous electrolytes used included copper, zinc, tin cations and additional additives (0.02 M CuSO ₄ .5H ₂ O, 0.032 M ZnSO ₄ .7H ₂ O, 0.014 M SnCl ₂ , 0.5 M tri-sodium citrate as complexing agent).

Since the conventional three-electrode method is difficult to apply to commercialization, it was deposited using a two-electrode method. Soda lime glass (3 x 4 cm ² ) deposited with Mo was used as a working electrode, And used as an electrode. Alloy film deposition started with a constant current density of about -1.18 mA / cm ² for 1,200 seconds. Before deposition, the substrate was cleaned with acetone, ethanol, distilled water, and then dried with nitrogen. The precursor films thus deposited were heat treated in a two-zone cylindrical furnace (FIG. 1), at which time an atmospheric condition of sulfur and selenium was formed and heated to 550 ° C at a rate of 5 ° C per minute and held for 15 minutes. Selenium vapor was formed by evaporating selenium powder at 380 ° C and sulfur vapor H ₂ S gas was supplied (200 sccm) in a 1% dilution to Ar gas. While supplying H ₂ S, the heating of the selenium powder was stopped and the CZTSSe thin film was formed by natural cooling until the selenium melting point was reached.

At this time, the substrate heating zone may stop heating only the selenium vaporization zone without stopping the heating and naturally cooling, or both the selenium vaporization zone and the substrate heating zone may stop heating and naturally cool. Preferably, both the selenium vaporization zone and the substrate heating zone are more advantageously interrupted and cooled naturally, in that the composition gradient and the bandgap composition gradient can be further increased to further improve the final performance.

The selenium vapor is generated by heating the elemental selenium powder in the selenium vaporization zone and the sulfur vapor is used as the carrier gas by using H ₂ S gas mixed with argon, do. For this reason, the sulfur vapor is continuously supplied to the thin film until the heat treatment system is completely cooled, and the selenium vapor is supplied to the thin film until the selenium vaporization zone is cooled below the melting point of selenium.

For reference, selenium is vaporized at the melting point and vaporized together with steam, and the vapor generated in the selenium vaporization zone is transferred to the next zone.

After the formation of the thin film by heat treatment, the second phase of Cu _x (S, Se) system was etched for 2 minutes in KCN solution. Then, a 60 nm CdS buffer layer was formed using a chemical wet process to form a pn junction. An i-ZnO / AZO layer (60 nm / 550 nm, RF sputtering), a Ni / Al upper electrode (50 nm / ) Was deposited to fabricate a solar cell. The antireflection film was not used and the area of each cell was about 0.4 cm ² .

Test Example 1

The crystallographic properties of the prepared CZTSSe thin films were determined by X-ray diffraction spectroscopy (XRD model: D8 advanced, Bruker, Cu-Kα source λ = 1.54060 Å, 40 kV, 30 mA, θ-2θ mode). (EDS: model: PV9761, EDAX) equipped with a transmission electron microscope (TEM: Tecnai F20 G2, FEI, 200 kV) and a limited field electron diffraction pattern and EDS line scanning Respectively. The photocurrent density-voltage characteristics were measured using a solar cell measurement system (Keithley model 2400, solar simulator = model; YSS-50A, Yamashita Denso equipped with a 1000 W Xenon lamp and AM 1.5 filter). The light amount was adjusted to 1 SUN light amount by using the Si photodiode. The external quantum efficiency of the photovoltaic cell was measured using a photovoltaic current conversion efficiency measurement system (Model G1218A, PV Measurements). Capacitance-voltage measurement and admittance spectroscopy were performed using an impedance analyzer (Model: Agilent 4284, Hewlett- Packard). The temperature of the sample was adjusted using a liquid nitrogen-cooled cryostat (Model: LTS420E-P, LINKAM).

Test result

2 (a) compares X-ray diffraction patterns of CZTSSe (band gap 1.13 eV) and CZTSe (band gap 1.02 eV), which is an enlarged view around the diffraction peak of the 112 plane. In both samples, except for Mo and MoSe ₂ , no other secondary phases were identified. The figure shows that the diffraction peak of CZTSSe is shifted to an elevation angle, which is replaced with a sulfur atom (covalent radius of 0.102 nm) of smaller selenium atom (covalent radius of 0.116 nm) and a unit cell of CZTSSe ) Is reduced in size. In the diffraction pattern, the sulfur content calculation using the lever rule was 23%, which was very similar to the 24% shown by energy dispersive X-ray spectroscopy (EDS).

FIG. 2 (b) is a cross-sectional clear-sky image using a transmission electron microscope (TEM), and the illustration is a selected area electron diffraction of the corresponding part. Transmission electron microscopy image shows the lamination structure of CZTSSe thin film solar cell, micron - sized crystal grains are distributed in the thin film of 2.5 μm thickness, and cavity is observed on the Mo electrode side. These cavities are found in several reported CZTSSe layers. It was confirmed that the limited field electron diffraction pattern was a single crystal of Kesterites through the clear one, which confirmed that it agrees well with the result of the X-ray diffraction analysis.

FIG. 2 (c) shows the vertical composition distribution of CZTSSe by EDS line scanning. As a result, it can be seen that the remaining elements except sulfur and selenium are relatively uniformly distributed in the CZTSSe thin film. In case of sulfur, And the content of selenium was decreased within the same range.

FIG. 3 shows the sulfur atom composition ratio and the corresponding band gap energy of the CZTSSe thin film according to the vertical direction. It should be noted that the corrected TEM EDS data were corrected through the SEM EDS results. The envelope calculation shows that the surface bandgap is about 0.1 eV larger than the lowest bandgap inside the CZTSSe thin film.

For reference, Cd is the CdS layer immediately above the CZTSSe layer, and it can be seen that the region is about 500 nm deep from the surface.

Thus, the compositional change near the surface appears to have been caused by a post-heat treatment process. During the heat treatment process, selenium vapor is supplied through heating of selenium powder and sulfur vapor is supplied through H ₂ S gas diluted in Ar gas. In the case of Se vapor after the heat treatment process, the selenium at a relatively high melting point (about 220 ° C) is cut off relatively quickly due to the temperature drop. In the case of sulfur vapor, however, due to the continuous flow of H ₂ S, . For this reason, it is easy for sulfur atoms to substitute for selenium atoms during cooling. The composition gradient of the sulfur / selenium atoms along the vertical direction creates a bandgap gradient in the CZTSSe film, which appears to affect the photoelectric properties of this film.

4 (a) compares light and dark current-voltage curvature of CZTSSe with CZTSe, and black line means CZTSSe gray line means CZTSe. Detailed solar cell parameters and electrical characteristics are summarized in Table 1. In the case of CZTSSe, which showed the highest efficiency, the short circuit current ( J _sc ) was 33.7 mA / cm ² , the open circuit voltage ( V _oc ) was 0.48 V and the fill factor (FF) was 0.61 at 9.9 eV. A 24% improvement in efficiency was observed due to the addition of S atoms when compared to a CZTSe thin film solar cell with a bandgap of 1.02 eV, with an increase in open-circuit voltage of 23%, an increase in filling rate of 7%, and a shorting current reduction of 5%.

The factors affecting the short-circuit current were examined through the external quantum efficiency (EQE) curve (Fig. 4 (b)). It was confirmed that the absorption edge of the external quantum efficiency curve was shifted to the shorter wavelength due to the increase of the bandgap due to the addition of the sulfur atom. In the calculation of the band gap using the external quantum efficiency curve, the band gap showed 1.13 eV, which is similar to the 1.12 eV shown in the X-ray diffraction peak analysis and EDS analysis. Although the absorption spectrum is smaller than that of CZTSe, the short-circuit current shows a small decrease (33.4 mA / cm ² -> 31.6 mA / cm ² ). This is thought to be due to an increase in the light response at the longer wavelength side. The reason why the light response near the long wavelength is increased seems to be that the probability of collecting the minority carriers generated near the back electrode is increased due to the decrease in the thickness of the thin film. The shortcircuit current can be increased sufficiently by improving the CdS layer and the TCO layer and using the reflection ring film. The increased open-circuit voltage seems to have been mainly caused by the increase in band gap because the open-circuit voltage shortage ( V _oc deficit = E _g / q- V _oc ) of CZTSSe is 0.65 V which is similar to 0.63 V of CZTSe Because. The increase in bandgap due to the addition of sulfur atoms may have resulted in a decrease in the reverse saturation current ( J ₀ ) and shunt conductance, which leads to an improvement in the diode ideality factor and filling factor .

Cell Eff.
(%) V _oc
(V) E _g
(eV) E _g / q ?? V _oc
(eV) J _sc ^a
(mA
/ cm ² ) FF
R _s
(Ωcm ² ) A J ₀
(mA
/ cm ² ) CZTSSe 9.9 0.48 1.13 0.65 33.7 0.61 0.72 1.55 1.6e ?? 4 CZTSe ^b 8.0 0.39 1.02 0.63 35.3 0.58 0.76 1.66 1.3e

FIG. 5 shows the effect of temperature on the open-circuit voltage of a CZTSSe thin-film solar cell, which gives information on the dark current and the electron-recombination method. The temperature dependence of the open-circuit voltage is expressed by the following relationship.

[Equation 1]

In this case, E _A , A , k , T , J ₀₀ , and J _L represent the activation energy, the ideal diode factor, the absolute temperature, the reverse saturation current density factor, and the photocurrent density of the main recombination mechanism, respectively. If the open-circuit voltage exhibits a linear dependence on temperature, it can be said that A , J ₀₀ , J _L do not depend on the temperature. Therefore, the activation energies EA / q are calculated by using data extrapolation in the Voc-T plot and V _oc Value, and it has an activation energy of about 0.99 eV. This is the largest value of CZTSSe with similar band gaps reported previously. The difference between the relatively small activation energy and the band gap energy (0.14 eV) suggests that the interface recombination between the buffer layer and the photoabsorption layer may be reduced. This is a good explanation for the effect of increasing the content of sulfur atoms on the aforementioned surface. Figure 3b shows the admittance spectral characteristics of CZTSSe thin film solar cell. Due to the relaxation behavior of the capacitance with respect to the measurement frequency, the characteristic frequency ( ω ₀ ) is as shown in Figure 3b. d C ( ? ) / d ln ? the maximum value of ω plot. The characteristic frequency is determined by the following equations: the attempt-to-escape frequency (ν ₀ ), the emission factor ( ξ ₀ ), the defective energy ( E _d : the energy distance difference between the valence band and the defect level), the Boltzmann constant ( k ) T ).

&Quot; (2) "

The Ed value is determined by the slope of ln (ω ₀ / T ² ) vs. 1 / Tplot as shown in Figure 3b and is confirmed to be 105 mV. It can be seen that this is somewhat similar to the activation energy difference (140 mV) of the previously measured band gap and main recombination mechanism.

As shown above, CZTSSe thin film solar cell through electrochemical deposition and post heat treatment process showed high efficiency of 9.9%. Band gap adjustment and bandgap inclination are important factors. When compared with pure CZTSe with a bandgap of 1.02 eV, CZTSSe thin film solar cell with a band gap of 1.13 eV has a bandgap increase, a small reverse saturation current and an increased shunt resistance The open-circuit voltage was greatly increased. On the other hand, the decrease in the light absorption spectrum due to the increase in the band gap has resulted in a relatively small decrease in the open current due to the improvement in the optical response near the band edge, which is presumably related to the band gap slope. In addition, this CZTSSe thin film solar cell exhibited a relatively high primary recombination mechanism activation energy, which appears to be due to an increase in the content of sulfur atoms near the surface.

Example 2

In the production of CZTSSe thin film using the heat treatment system shown in FIG. 1, the internal partial pressure of sulfur or selenium is closely related to the final CZTSSe thin film solar cell. When the internal partial pressure is insufficient, the anion sulfur or selenium (MoSe _x or MoS _x or MoSe _x S _y ) is excessively formed on the rear electrode layer and the conductivity of the back electrode is excessively high, It is confirmed that the series resistance of the solar cell can be increased and the solar cell characteristics can be further reduced.

In addition, the thin film produced in the region where the internal partial pressure is too high is difficult to have a flat surface due to excessive growth of particles, and when the surface is not flat, the overall efficiency of the solar cell is deteriorated due to the possibility of recombination due to the increase of the interface. It was confirmed that maintaining proper internal partial pressure is very important because it is imported.

Example 3

Sulfur and selenium ratio and bandgap of the CZTSSe thin film were determined by the ratio of sulfur and selenium in the atmosphere in the heat treated tube. It was confirmed that not only this ratio but also the total pressure inside the heat treatment tube had an effect of CZTSSe. The system according to one embodiment of the present invention produces selenium vapor through particle vaporization, whereas sulfur provides through an H ₂ S gas supply. At this time, when the exhaust pressure of the heat treatment tube is increased to increase the internal total pressure (for example, 1.2 atm), the selenium supply stops more rapidly than when the internal total pressure of 1 atm is formed in a general heat treatment, The slope of the slope of the photovoltaic device can be rapidly increased to form a graded thin film whose surface sulfur ratio is increased compared with the inside. This can reduce the surface recombination and selectively increase the bandgap to improve the rectifying characteristic of the solar cell device As a result, it was confirmed that solar cell characteristics can be improved.

Claims

As Cu ₂ ZnSn (S, Se) ₄ thin film for solar cell,
(S _s - S _v ) is a positive number, (Se _s - Se _v ) is a negative number,
S _s and S _v are the compositional values (atomic%) of sulfur at the surface and composition gradient point, respectively,
The Se Se _v _s and the solar cell ₂ ZnSn Cu (S, Se), characterized in that the composition value (atomic%) of Se in the respective surface of the fork ₄ and the composition gradient film.

The method according to claim 1, wherein the value of S / (S + Se) at the surface is larger than the composition gradient junction,
Wherein S and Se are elemental compositional values (atomic%) of sulfur and selenium, respectively, for Cu ₂ ZnSn (S, Se) ₄ thin film for solar cells.

2. The method of claim 1, wherein the S / (S + Se) value is increasing from the composition gradient point to the surface,
Wherein S and Se are elemental compositional values (atomic%) of sulfur and selenium, respectively, for Cu ₂ ZnSn (S, Se) ₄ thin film for solar cells.

The method of claim 1 wherein the S / (S + Se) value at the surface is from 0.3 to 0.5,
Wherein S and Se are elemental compositional values (atomic%) of sulfur and selenium, respectively, for Cu ₂ ZnSn (S, Se) ₄ thin film for solar cells.

The method of claim 1, wherein the band gap of the thin film solar cell, characterized in that the increase up to the surface of the trend in the composition gradient fork _{Cu 2 ZnSn (S, Se)} 4 film.

The method of claim 1, wherein the band gap of the thin film solar cell, characterized in that the value of the surface 0.08 to 0.12 eV greater than the lowest point of the interior of the thin-film _{Cu 2 ZnSn (S, Se)} 4 film.

(A) located on the substrate, the substrate Mo, solar cell for a metal alloy thin film consisting of a Cu-ZnSn alloy located on the Mo comprising the step of heat treatment in an atmosphere which is S and Se is supplied ZnSn Cu ₂ (S, a method for producing a Se) ₄ thin film,
In the Cu ₂ ZnSn (S, Se) ₄ thin film, (S _s - S _v ) is a positive number, (Se _s - Se _v )
S _s and S _v are the compositional values (atomic%) of sulfur at the surface and composition gradient point, respectively,
The Se Se _s and _v are each surface and the solar cell manufacturing method of Cu ₂ ZnSn (S, Se) ₄ thin film, characterized in that the composition value (atomic%) of selenium in the composition gradient of the fork.

8. The method of claim 7 wherein the S is supplied by providing H ₂ S with a carrier gas,
The method for manufacturing a solar cell is Se _{Cu 2 ZnSn (S, Se)} 4 thin film, characterized in that supplied by vaporization by heating the Se powder.

The method of claim 8, wherein the (A) step of the solar _{cell-Cu 2 ZnSn (S, Se)} 4 thin film, characterized in that over time, keeping the amount of supply of H ₂ S being carried out in the supply amount of Se is reduced atmosphere Gt;

The method as claimed in claim 9, wherein the step (A) comprises continuously supplying H ₂ S together with the carrier gas, stopping the heating of the Se powder, and naturally cooling to a temperature below the melting point of Se, solar cell characterized in that the Cu ₂ ZnSn (S, Se) ₄ the method of the thin film.

11. The method of claim 10, wherein the (A) step is S and Se and the carrier the total pressure of the gas such that a total partial pressure of S and Se is carried out at 0.01 to 0.1 atm atmosphere for solar cells Cu ₂ ZnSn (S, the method of Se) ₄ film.

12. The method according to claim 11, wherein in step (A), Cu ₂ ZnSn (S, Se) ₄ for a solar cell is characterized in that the ratio of the partial pressures of S and Se in the total pressures of S and Se and the carrier gas is 0.2 to 2.0. A method for producing a thin film.

A solar cell comprising the Cu ₂ ZnSn (S, Se) ₄ thin film according to any one of claims 1 to 6.

(A) selenium vaporization zone, (B) a metal alloy thin film heating _{zone, (C-1) H 2} S gas inlet, _(C-2) H 2 for solar cells comprising a S gas outlet Cu ₂ ZnSn (S, Se ) ₄ thin film manufacturing apparatus,
H ₂ S gas is introduced into the metal alloy thin film heating zone together with the vaporized selenium vapor in the selenium vaporization zone through the H ₂ S gas inlet,
(B-1) substrate, (b-2) Mo disposed on the substrate, and (b-3) a Cu-Zn-Sn alloy disposed on the Mo phase in the metal alloy thin film heating zone,
In the Cu ₂ ZnSn (S, Se) ₄ thin film, (S _s - S _v ) is a positive number, (Se _s - Se _v )
S _s and S _v are the compositional values (atomic%) of sulfur at the surface and composition gradient point, respectively,
The Se Se _s and _v is the solar cell ₂ ZnSn Cu (S, Se), it characterized in that the composition value (atomic%) of Se in the respective surface and the composition gradient produced the fork ₄ of the thin film device.