JPH10125941A - Chalcopyrite type solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the leak current, without the precise compsn. control by placing a compd. M(INx Ga1-x )(Sy Se1-y )2 at the crystal grain boundary of Cu(Inx Ga1-x )(Sy Se1-y )2 in a photoabsorptive layer of a solar cell. SOLUTION: In a Cu(Inx Ga1-x )(Sy Se1-y )2 thin film an M(Inx Ga1-x )(Sy Se1-y )2 (0<=X, Y<1) layer exists; M is at least one of Li, Ga and K. The relative compsn. ratio at the surface side indicates Na and In are much and Cu is little while that in the film indicates Na is below the detection limit and In and Cu have the same ratio. The mixed film is etched to remove the M(Inx Ga1-x )(Sy Se1-y )2 phase. After etching the M(Inx Ga1-x )(Sy Se1-y )2 phase exists at the crystal grain boundary of the Cu(Inx Ga1-x )(Sy Se1-y )2 thin film, this reducing the leak current and improve the conversion efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention, Cu (In _X G
The present invention relates to a chalcopyrite solar cell represented by a _1-X ) (S _Y Se _1-Y ) ₂ (0 ≦ X, Y ≦ 1).

[0002]

_2. Description of the Related Art A chalcopyrite type compound (hereinafter abbreviated as CIS type) represented by Cu (In, Ga) (S, Se) ₂ is a direct transition type semiconductor and therefore has a large light absorption coefficient. Since the band gap matches the solar spectrum, application as a thin film solar cell material is expected. Solar cells of this material are generally composed of a CIS-based thin film, a semiconductor layer, and a transparent electrode layer laminated on a glass substrate coated with a metal electrode, and a CIS-based thin film formed to use a glass substrate. Is a polycrystal. The development of solar cells using CIS-based thin films is described in Boei
ng is a solar cell using CuInSe ₂ as a light absorbing layer.
It became very active in the early 1980s when it achieved a conversion efficiency exceeding 0%, and since the 1990s, Eur
oCIS group achieved efficiency of 14% or more (Ap
pl. Phys. Lett. 62 (6). 1993. p
597-599). However, as described above, since the CIS-based thin film is polycrystalline, there are many defects at crystal grain boundaries and the like, which causes recombination of generated photocarriers and leakage current through the grain boundaries. . As an improvement measure, NREL has proposed a Cu (In, Ga) Se ₂ solar cell having a double-inclined structure having a large amount of Ga on the metal electrode side and the n-type semiconductor side. Recently, a solar cell having a conversion efficiency of more than 17% has been proposed. (Prog. Photoovo)
lt. . 3.235-238 (1995)). The Cu (In, Ga) Se ₂ thin film having this structure changes the band gap in the depth direction and accelerates the optical carriers by the generated internal electric field, thereby making it difficult for traps due to defects to occur. Although the improvement of the conversion efficiency by the double-inclined structure is remarkable, it does not fundamentally solve the problem of crystal grain boundaries, and the method of precisely controlling the composition change in the depth direction is industrial. Is not preferable.

[0003]

DISCLOSURE OF THE INVENTION The present invention is directed to a method for reducing recombination and defects caused by defects existing at crystal grain boundaries of a CIS-based thin film.
It is an object of the present invention to provide a solar cell using a Cu (In, Ga) (S, Se) ₂ thin film capable of suppressing a leak without precise composition control in a film depth direction.

[0004]

In such a situation,
The present inventors have conducted intensive studies in order to solve the above-mentioned problems, and as a result, when forming a Cu (In, Ga) (Se, S) ₂ thin film, the thin film is formed by adding an alkali element such as Li, Na, and K. M (In, Ga) (S, Se) ₂ (where M is at least one element of Li, Na and K)
Is mixed to reduce the leakage current at the crystal grain boundary, and is effective for improving the conversion efficiency of a solar cell using the mixed thin film as a light absorbing layer. Cu (In, Ga) (Se,
The present inventors have found the optimum range of the mixing ratio of S) ₂ and M (In, Ga) (S, Se) ₂ under specific conditions, and have accomplished the present invention.

That is, the present invention is as follows. That is, Cu (In _x Ga _1-x ) (S _Y Se _1-y ) ₂ (0 ≦
In the chalcopyrite solar cell represented by X, Y ≦ 1), M (In _X G) is contained in the light absorbing layer of the solar cell.
a _1-X ) (S _Y Se _1-Y ) ₂ (where M is Li, N
a) at least one element selected from the group consisting of a and K), wherein M (In _x Ga ₁ -x) (S _Y Se _{1 -Y} ) ₂
Are present at the crystal grain boundaries of Cu (In _x Ga _1-x ) (S _Y Se _1-y ) _{2 in} the chalcopyrite solar cell.

[0006] A second aspect of the present invention is a calcopara according to claim 1.
In the unit-type solar cell, the M (In)_XGa_1-X)
(S_YSe_1-Y)_TwoAnd Cu (In)_XGa_1-X) (S
_YSe _1-Y)_Two-Ray diffraction measurement by θ / 2θ scan method
The peak intensity m of the (003) plane obtained by
2) The ratio m / c to the peak intensity c of the surface is 0.002 or more.
Chalcopalite-type thick, characterized by being 0.1 or less
It is a positive battery.

[0007]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in further detail.
You. For example, Cu (In_XGa_1-X) (S_YSe_1-Y)
_TwoThe layer is CuInS_TwoAnd M (In_XGa_1-X) (S_YS
e_1-Y)_TwoIs NaInS_TwoThe case of is described. Ma
, CuInS_TwoAnd NaInS_TwoTo form a mixed thin film of
As a method, Cu, In, Na_TwoAfter stacking S
Sulfidation method with heat treatment in an atmosphere, CuInS _TwoPowder and N
aInS_TwoThe target made by mixing and pressing the powder
Sputtering method used, Cu, In, S, Na_TwoS
Four-source simultaneous evaporation method used as a raw material, etc.
CuInS method such as MOCVD method, spray method, electrodeposition method_Two
And NaInS_TwoAs long as a mixed thin film of
It is not specified. Here, the raw material of the alkali element is N
a_TwoS is for compounds compared to metallic sodium
Is easy to handle.
There is no particular limitation on oxides other than the product.

Next, the CuInS ₂ formed above and Na
The state of mixing of InS ₂ was examined. That is, it was examined by an X-ray diffraction pattern by the θ / 2θ scanning method. θ / 2
The X-ray diffraction measurement by the θ scan method means that when the angle between the incident X-ray and the sample is θ, scanning is performed so that the position of the counter tube (detector) is at an angle of 2θ with the extension of the incident X-ray. This is a measurement method in which the line diffraction intensity is displayed as a function of 2θ.

Before examining the X-ray diffraction pattern of a mixed thin film
CuInS_TwoAnd NaInS_TwoEach single diffraction
I checked the pattern. The results are shown in FIGS.
InS_TwoIs (112) face, NaInS_TwoIs the (003) plane
It turns out that it is orientated. Next, CuInS_TwoAnd Na
InS_TwoThe X-ray diffraction pattern of the mixed thin film of
The results are shown in FIG. CuInS_Two(112) peak and N
aInS_Two(003) peak exists. Therefore, N
aInS_TwoIs CuInS_TwoWhat is described above
CuInS is also used in the mixed thin film prepared by these methods._TwoConclusion
Na, which is an alkali element during crystal growth, is NaInS_Twophase
(Layer) and segregated CuInS_TwoDifficult to mix in crystals
I understand that Furthermore, the composition distribution in the depth direction of the mixed thin film
Was evaluated by Auger electron spectroscopy. As a result, the surface
The relative composition ratio on the side was high in Na and In and low in Cu
On the other hand, Na became below the detection limit as it became
Below, In and Cu had almost the same ratio. Therefore, N
aInS_TwoPhase is CuInS _TwoExists on the surface of the thin film
It can be said. As the amount of Na added increases,
NaInS in existence_TwoIt is also clear that the phases increase.
Also, the mixed thin film is etched with an acid (for example, hydrochloric acid).
NaInS_TwoSelectively removing only phases
Pre-etching, dense film with no gap between crystal grains
However, after etching, there is no gap between the crystal grains.
It was observed. Therefore, NaInS_TwoPhase is CuInS_TwoThin
It can be said that it exists at the crystal grain boundary of the film.

Here, as an index indicating the degree of mixing of the NaInS ₂ phase, the peak intensity of NaInS ₂ (003);
The ratio m / c of the peak intensity; c of uInS ₂ (112) is defined. FIG. 1 shows the relationship between m / c and the conversion efficiency of the solar cell. As shown in FIG. 1, almost no efficiency was obtained when m / c = 0, and the efficiency was significantly improved as the ratio increased. Further, as the ratio increases, the efficiency tends to gradually decrease, and an optimal amount of NaInS ₂ exists. In order to obtain a sufficiently high efficiency, m / c is preferably in the range of 0.002 to 0.1, and more preferably 0.01 to 0.04.

NaInS_TwoOf efficiency by mixing degree
The causes are as follows. NaInS_TwoIs CuInS
_TwoSince the band gap is larger and the resistance is higher, Cu
InS_TwoNaInS at the crystal grain boundary_TwoLeak if present
The current is suppressed. . However, due to the high resistance,
If it becomes too high, the series resistance component of the solar cell will increase and the conversion efficiency will increase.
Will decrease. In addition, general CIS based thin-film solar
Pond is CIS-based thin film / n-type semiconductor such as CdS / ZnO: A
1 has a transparent electrode structure such as
NaInS_TwoThe layer is CuInS_TwoIf present on a layer
A structure excluding an n-type semiconductor such as CdS is also possible. why
Then, C formed between the CIS-based thin film layer and the transparent electrode layer
The semiconductor layer (buffer layer) such as dS generally has
Gap is larger than CIS based thin film and smaller than transparent electrode
Is selected, but NaInS _TwoThe band gap of
ZnO: Al or ITO used as a transparent electrode
Suitable for a buffer layer
You. Another requirement for the buffer layer is high
This is because the resistance is also satisfied.

[0012] In addition to Na as an alkali element, K, Li
Of the mixed thin film in the case of Na
It is clear that the same effect can be obtained as
A similar tendency was obtained for. Then, CuInS
_TwoOther than, that is, Cu (In) of X ≠ 1 and Y ≠ 1_XG
a_1-X) (S_YSe_1-Y)_TwoAbout the mixed thin film
You. CuInSe_TwoAnd Cu (In, Ga) Se_TwoTo
As a result of the examination, the effect of the addition of the alkali element was CuI
nS _TwoThe same tendency as in the case of was shown. But alkali
Alkali element compared to the case without element addition (m / c = 0)
Conversion efficiency is improved by adding CuInS_TwoIt's a solar cell
It was smaller. From these results, Cu (In
_XGa_1-X) (S_YSe_1-Y)_Two(0 ≦ X, Y <1)
M (In) in the thin film represented_XGa_1-X) (S_YS
e_1-Y)_TwoMixing layers is effective, but Cu = X = 1
InS_TwoThe invention is particularly effective in thin films
You.

[0013]

[Embodiment 1] An embodiment of the present invention will be specifically described below.
I do. First, the lower electrode Mo layer is used for sputtering.
Formed SiO_TwoOn soda lime glass substrate with layer
First, a thin layer of In was sputtered in an Ar gas atmosphere.
Deposited by the plating method and then Na_TwoS layer is deposited by resistance heating
And then deposit the Cu layer and the In-S layer respectively.
In an Ar gas atmosphere and H_TwoIn S / Ar gas atmosphere
Were laminated by the sputtering method described above. When laminating,
Did not heat. Sputtering of Cu and In
8mTorr sputter gas using metal target
Performed under pressure. Also, H_TwoH of S / Ar gas_TwoS concentration
7.5 at. %. Next, this laminated film is
t. % H_TwoIn an S / Ar gas atmosphere (atmospheric pressure)
Heat treatment was performed at 550 ° C. for 2 hours. X-ray rotation of film after heat treatment
When the fold measurement was performed, NaInS_TwoAnd CuInS_TwoBlend of
NaInS_Two(003) and CuInS
_TwoThe peak intensity ratio of (112) was 0.015. X
The line diffraction is as follows by RU-200B manufactured by Rigaku Corporation.
It was measured under the conditions. << Measurement conditions >> Target: Cu (wavelength: 1.5405Å) Acceleration voltage: 40 [kV] X-ray tube current: 110 [mA] Sampling width: 0.02 [°] Scanning speed: 4 [° / min] Is etched with hydrochloric acid as described above.
Then, NaInS _TwoOnly phases are selectively removed, as well
CuInS_TwoThere are gaps between the crystal grains of the thin film and NaIn
S_TwoPhase is CuInS_TwoThat it existed at the grain boundaries of the thin film
It could be confirmed.

A CdS layer and an ITO (indium tin oxide) layer were laminated on the mixed film of NaInS ₂ and CuInS _{2 by} a solution growth method and an RF sputtering method, respectively, to produce a solar cell. FIG. 2 shows a cross-sectional view of a solar cell manufactured according to this example. The thickness of each layer is 1 μm for the Mo layer (2), ₂ μm for the NaInS ₂ / CuInS ₂ mixed layer (3), 80 nm for the CdS layer (4), and 0.8 μm for the ITO layer (5).

When the dark IV characteristic of the solar cell having this structure was evaluated, the reverse current was 1.7 × 10 ⁻⁶ A / cm ² when a voltage of −0.5 (V) was applied. The conversion efficiency of this battery was 9.2% at AM1.5. NaInS ₂ (003) and CuInS ₂ (112)
A solar cell using a CuInS ₂ film as a light absorbing layer was produced in the same manner as described above except that the peak intensity ratio was 0.15.

The leakage current of the solar cell having this structure is dar
When evaluated by the kI-V characteristic, the reverse current was 2.0 × 10 ⁻⁷ A / cm ² when a voltage of −0.5 (V) was applied. Here, the reverse current is measured as the sum of the saturation current and the leak current (the same applies hereinafter). The conversion efficiency of this device was 4.2% at AM1.5. Compared with the above-mentioned characteristics, the short-circuit current and the fill factor were reduced, and particularly the fill factor was remarkable, indicating that the series resistance of the solar cell was increased.

[0017]

Comparative Example 1 A solar cell using a CuInS ₂ film as a light absorbing layer was produced in the same manner as in Example 1 except that no Na ₂ S layer was laminated. When the darkIV characteristics of the solar cell having this structure were evaluated, the reverse current was 6.1 × 10 ⁻⁵ A / cm ² when a voltage of −0.5 (V) was applied. The conversion efficiency of this solar cell was 1.5% at AM1.5.

[0018]

Embodiment 2 Na_TwoK instead of S_TwoAfter using S
Outside is the same as in the first embodiment. _TwoAnd CuInS_Twoof
A mixed film was prepared. X-ray diffraction measurement of the film after heat treatment
KInS_Two(003) and CuInS_Two(112) pea
The strength ratio was 0.015. X-ray diffraction was the same as in Example 1.
The measurement was performed under the same conditions. This KInS_TwoAnd CuInS_TwoBlend of
A CdS layer and an ITO layer were formed on the composite film in the same manner as in Example 1.
Were laminated to produce a solar cell.

When the dark IV characteristic of this solar cell was evaluated, the reverse current was 0.9 × 10 ⁻⁶ A / cm ² when a voltage of −0.5 (V) was applied. The conversion efficiency of this solar cell was 9.2% at AM1.5.

[0020]

Embodiment 3 Sputter film formation was performed using a powder mixture target of CuInS ₂ and NaInS ₂ . The target was produced in the following procedure. Cu ₂ S powder and In ₂ S ₃
After mixing the powder, the Na ₂ S powder and the In ₂ S ₃ powder, respectively, the mixture was calcined at 850 ° C. for 4 hours in an Ar atmosphere to synthesize a CuInS ₂ powder and a NaInS ₂ powder. After mixing these synthetic powders, pressing was performed to produce a target.

The formation of a mixed film of CuInS ₂ and NaInS ₂ was performed as follows. First, a substrate temperature of 250 ° C. was placed on Mo-coated soda lime glass coated with Mo by a sputtering method using the prepared press target.
To form a mixed thin film. Next, this thin film was coated at 5 at. % In a H ₂ S / Ar gas atmosphere (atmospheric pressure).
C. for 2 hours. When the X-ray diffraction measurement of the film after the heat treatment was performed, it was a mixed film of NaInS ₂ and CuInS ₂ , and NaInS ₂ (003) and CuInS ₂ (11
The peak intensity ratio in 2) was 0.025. X-ray diffraction was measured under the same conditions as in Example 1. This NaInS ₂ and Cu
CdS was formed on the InS ₂ mixed film by the same manufacturing method as in Example 1.
The solar cell was manufactured by laminating the layers and the ITO layer.

When the dark IV characteristic of this solar cell was evaluated, the reverse current was 0.6 × 10 ⁻⁶ A / cm ² when a voltage of −0.5 (V) was applied. The conversion efficiency of this battery was 8.0% at AM1.5.

[0023]

Embodiment 4 First, a Mo layer as a lower electrode is formed on a soda lime glass substrate with a SiO ₂ layer by sputtering, and then a Na ₂ Se layer is deposited by a resistance heating evaporation method, and thereafter, a Cu layer and an In layer are deposited. Were sequentially laminated by a sputtering method in an Ar gas atmosphere. Sputtering is performed using a metal target of Cu and In at 8 mTo.
This was performed at a sputtering gas pressure of rr. Next, selenization was performed by irradiating Se vapor while maintaining the laminated film at 500 ° C. in a vacuum. When the film after selenization was subjected to X-ray diffraction measurement, it was a mixed film of NaInSe ₂ and CuInSe ₂ , and NaInSe ₂ (003) and CuInSe ₂ (11
The peak intensity ratio in 2) was 0.018. X-ray diffraction was measured under the same conditions as in Example 1. This NaInSe ₂ and C
_On the mixed film of uInSe ₂ , C was produced in the same manner as in Example 1.
A solar cell was manufactured by laminating the dS layer and the ITO layer.

When the dark IV characteristic of this solar cell was evaluated, the reverse current was 2.1 × 10 ⁻⁶ A / cm ² when a voltage of −0.5 (V) was applied. The conversion efficiency of this battery was 7.8% at AM1.5.

[0025]

Embodiment 5 First, a Mo layer as a lower electrode is formed on a soda lime glass substrate provided with an SiO ₂ layer by sputtering, and then a Na ₂ O ₂ layer is deposited in a vacuum evaporation apparatus, followed by Cu, In, and Se. A CuInSe ₂ layer was deposited by a three-source simultaneous evaporation method using as an evaporation source. The substrate temperature is kept at 200 ° C. during the deposition of the Na ₂ O ₂ layer, and Cu,
The deposition of In and Se was started, and then the temperature was raised to 500 ° C. and held. If the temperature was kept high during the deposition of Na ₂ O ₂ , re-evaporation occurred, and a temperature profile was devised to prevent this. X-ray diffraction measurement of this film revealed that NaInS
e ₂ and a mixed film of CuInSe ₂ , and NaInSe ₂
The peak intensity ratio between (003) and CuInSe ₂ (112) was 0.012. X-ray diffraction was measured under the same conditions as in Example 1. On this mixed film of NaInSe ₂ and CuInSe ₂ , a CdS layer and an ITO layer were laminated in the same manner as in Example 1 to produce a solar cell.

When the dark IV characteristic of this solar cell was evaluated, the reverse current was 0.7 × 10 ⁻⁶ A / cm ² when a voltage of −0.5 (V) was applied. The conversion efficiency of this battery was 11.3% at AM1.5.

[0027]

Comparative Example 2 A solar cell using a CuInSe ₂ film as a light absorbing layer was produced in the same manner as in Example 5, except that no Na ₂ O ₂ layer was deposited. When the darkIV characteristics of this solar cell were evaluated, the reverse current was 9.0 × 10 ⁻⁶ A / cm ² when a voltage of −0.5 (V) was applied. The conversion efficiency of this battery was 7.0% at AM1.5.

[0028]

According to the present invention, Cu (In _X G
In a solar cell in which a semiconductor thin film represented by a _1-X ) (S _Y Se _1-Y ) ₂ (0 ≦ X, Y ≦ 1) is used as a light absorbing layer, the composition distribution in the depth direction of the light absorbing layer is Without precise control, leakage current in the element in the reverse direction can be reduced, and high conversion efficiency can be realized.

[Brief description of the drawings]

FIG. 1 shows the relationship between conversion efficiency and m / c intensity ratio.

FIG. 2 is a cross-sectional view of an example of a solar cell using a mixed thin film of CuInS ₂ and NaInS ₂ according to the present invention.

FIG. 3 is an XRD pattern of a mixed thin film of CuInS ₂ and NaInS ₂ according to the present invention.

FIG. 4 is an XRD pattern of a CuInS ₂ thin film.

FIG. 5 is an XRD pattern of a NaInS ₂ thin film.

[Explanation of symbols]

1. Glass substrate 2. 2. Lower metal electrode layer (Mo) 3. Mixed thin film of CuInS ₂ and NaInS ₂ 4. n-type semiconductor layer (CdS) Transparent electrode layer (ITO)

Claims (2)

[Claims] 1. The method according to claim 1, wherein Cu (In _x Ga _{1 -x} ) (S _Y S
e _1-Y ) ₂ (0 ≦ X, Y ≦ 1) In a chalcopyrite solar cell, M is contained in a light absorbing layer of the solar cell.
_{(In X Ga 1-X)} (S Y Se 1-Y) 2 ( However, M is Li, Na, at least one element of K) has a compound represented by the M (In _X Ga _1-X ) (S _Y Se
_1-Y ) ₂ is Cu (In _x Ga _1-x ) (S _Y S
chalcopyrite solar cell characterized by being present in e _{1-Y) 2} of the grain boundary. 2. The chalcopyrite solar cell according to claim 1, wherein the M (In _x Ga ₁ -x) (S _Y S
e _1-Y ) ₂ and the above-mentioned Cu (In _x Ga _1-x ) (S _Y Se
_1-Y) and the peak intensity m of the obtained (003) plane in X-ray diffraction measurement using ₂ theta / 2 [Theta] scan method, (112) the ratio m / c and the peak intensity c of surfaces 0.002 more 0 0.1 or less, chalcopyrite solar cell.