JP2962897B2 - Photovoltaic element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photovoltaic device, and more particularly to a photovoltaic device in which incident light is reflected and scattered on a back surface to increase light absorbed by a semiconductor layer, thereby stably improving output characteristics. It relates to a photovoltaic element.

[0002]

2. Description of the Related Art A solar cell, which is a photoelectric conversion element for converting sunlight into electric energy, is expected to be a clean energy alternative to fossil energy such as petroleum or nuclear energy.

[0003] The biggest problem to be solved in order to put a solar cell into practical use as an energy source for electric power is to reduce the manufacturing cost.

[0004] Solar cells can be roughly classified into materials using a single crystal, those using a polycrystal, those using an amorphous material, and those using a thin film of a compound. Among them, a solar cell using an amorphous or compound thin film has been actively developed as a material having a high possibility of significantly reducing the manufacturing cost.

When considering the manufacturing cost, the calculation should be performed per unit power. Therefore, it is important to improve the photoelectric conversion efficiency of the solar cell in addition to reducing the cost of the manufacturing process.

One of the means for improving the photoelectric conversion efficiency is as follows:
Ag, Al, Cu, A on the side opposite to the light incident side of the solar cell
A technique for forming a reflective layer of a metal film having a high reflectivity such as u is known. According to this technique, light transmitted through a semiconductor layer that generates carriers is reflected by a reflective layer, so that the light is absorbed by the semiconductor layer again, thereby increasing the output current and improving the photoelectric conversion efficiency. .

[0007] However, Ag, A
When a metal film having a high reflectance, such as l, Cu, or Au, is used, the metal film diffuses into the semiconductor layer to lower the characteristics of the solar cell, or conducts with the electrode on the light incident side to cause a short circuit (shunt). Easy to do. This is because Ag, Al, Cu, Au
It is considered that a metal having a high reflectance, such as a metal, easily diffuses in the semiconductor layer.

This is particularly remarkable when the back electrode is formed as a rough surface having irregularities in order to scatter light transmitted through the semiconductor layer and increase the optical path length of long wavelength light having a small absorption coefficient. there were.

On the other hand, a method for improving the surface properties of a substrate by interposing a transparent conductive layer between a back electrode and a semiconductor layer has been disclosed in Japanese Patent Publication No. 59-43101 and Japanese Patent Publication No. Sho 60-4.
1878. In these publications, the effect of interposing a transparent conductive layer between the back electrode and the semiconductor layer is to improve the flatness of the back electrode, improve the adhesion of the semiconductor layer, or alloy the metal of the back electrode with the semiconductor layer. Prevention and so on.

Japanese Patent Application Laid-Open No. 60-84888 discloses a technique for reducing a current flowing in a defect region of a semiconductor layer by interposing a transparent conductive layer as a barrier layer between the back electrode and the semiconductor layer. It has been disclosed.

Furthermore, although the principle is not clear, it has been reported that the sensitivity of a solar cell in the long wavelength region is increased by interposing a transparent conductive layer between the back electrode and the semiconductor layer. I have.

[0012]

As described above, some effects are expected by interposing the transparent conductive layer between the back electrode and the semiconductor layer, but there are still problems to be solved.

The transparent conductive layer interposed between the back electrode and the semiconductor layer is required to have high transmittance and high conductivity for light having a wavelength that can be absorbed by the semiconductor layer. However, the conductive oxide film suitably used as the transparent conductive layer has a property that the light transmittance decreases when the conductivity is increased. Therefore, in order to take advantage of the case of using a metal having a high reflectance as the back metal, if the transmittance is prioritized, the conductivity cannot be sufficiently increased, and the series resistance of the solar cell increases. The expected increase in photoelectric conversion efficiency cannot be obtained.

Further, depending on the conditions for forming the transparent conductive layer,
In some cases, the reflectivity of the back electrode was impaired.

Further, the conductive oxide film preferably used as the transparent conductive layer is often of the n-type, and depending on the combination of the transparent conductive layer and the semiconductor layer, a barrier may be formed at the interface between them.

On the other hand, in order to improve the conductivity of the conductive oxide film, a so-called doping technique of adding an element that changes the conductivity to the conductive oxide film has been studied. It has been found to be effective as a film dopant. However, if this technology is to be applied to the transparent conductive layer, if the added amount of the element is increased in order to increase the conductivity, the light transmittance is reduced, or the structure of the conductive oxide film is reduced. Adverse effects such as disturbing the diffusion of the element into the semiconductor layer and the like, and it was considered difficult to apply the doping technique.

[0017]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the transparent conductive layer interposed between the back electrode and the semiconductor layer, maintains a high reflectance of the back electrode, and increases the long wavelength sensitivity of the solar cell. It is another object of the present invention to increase the conductivity of the transparent conductive layer, eliminate the barrier at the interface with the semiconductor layer, reduce the series resistance of the solar cell, and further improve the photoelectric conversion efficiency.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have made intensive studies to achieve the above-mentioned object, and as a result, have achieved the object by a photovoltaic element having the following structure.

That is, the photovoltaic element of the present invention is provided with a back electrode provided on the side opposite to the light incident surface and a half- side for photoelectric conversion.
In a photovoltaic element having a transparent conductive layer formed of ZnO between the conductive layer and a conductive layer , the transparent conductive layer may include Al, In, B, Ga, Si, or F as an element that changes the conductivity.
Rutotomoni is comprises an element chosen, the amount of said element is characterized in that it varies in thickness direction so as to increase in the vicinity of the interface with the semiconductor layer. Alternatively,
The bright photovoltaic element is located on the back of the
ZnO, between the plane electrode and the semiconductor layer for photoelectric conversion
Having a transparent conductive layer formed of In ₂ O ₃ or SnO ₂
A photovoltaic element, wherein the conductivity of the transparent conductive layer is changed.
When the transparent electrode layer is ZnO,
An element selected from In, B, Ga, Si, and F
When the bright electrode layer is In ₂ O ₃ , Sn, F, Te, Ti,
An element selected from Sb and Pb is used as the transparent electrode layer.
In the case of O ₂ , F, Sb, P, As, In, Tl, T
e, an element selected from Cl, Br, and I
In both cases, the amount of the element added is close to the interface with the back electrode.
And it changes in the film thickness direction to increase
And

Here, various patterns can be considered as a method of changing the addition amount of the element for changing the electric conductivity, and a remarkable effect was found in the following patterns.

That is, by increasing the addition amount of the element in the vicinity of the interface with the semiconductor layer, no barrier is formed at the interface between the conductive oxide and the semiconductor layer. The series resistance of the device decreased, and the photoelectric conversion efficiency increased.

Further, by increasing the amount of the element added in the vicinity of the interface with the back electrode, the conductive oxide near the interface with the back electrode has a high resistance due to diffusion of the metal constituting the back electrode. , The series resistance of the photovoltaic element decreased, and the photoelectric conversion efficiency increased.

Further, the long-wavelength sensitivity of the photovoltaic element is increased by monotonically decreasing the addition amount of the element from the interface with the semiconductor layer to the interface with the back electrode over at least a certain thickness range. However, the short-circuit current increased, and the photoelectric conversion efficiency increased.

[0024] With regard to this effect, the addition amount of the element is monotonously decreased as approaching the interface with the back electrode, so that the refractive index of the conductive oxide is monotonically approaching as approaching the interface with the back electrode. It is considered that the reflection decreased at the interface between the transparent conductive layer and the semiconductor layer, and the incidence of long-wavelength light on the semiconductor layer increased.

In any of the above cases, the above-mentioned elements are not added to the entire transparent conductive layer. Therefore, even if the addition amount is partially increased, the transmittance of the transparent conductive layer is maintained at a high value. Structural disturbance was suppressed, and there was no adverse effect on the semiconductor layer due to the diffusion of the element.

A remarkable effect was also found by combining some of the above patterns.

In the present invention, examples of the conductive oxide suitably used as the transparent conductive layer include ZnO and SnO.
₂ , In ₂ O ₃ , ITO (In ₂ O ₃ + SnO ₂ ), Ti
O ₂ , CdO, Cd ₂ SnO ₄ , Bi ₂ O ₃ , MoO ₃ , Na
_X WO ₃ or the like is preferably used.

The transparent conductive layer may be formed by a vapor deposition method,
Sputtering, CVD, spraying, spin-on, dipping, and the like are preferably used.

The following method is used to change the amount of the element to be added to the conductive oxide in the thickness direction of the transparent conductive layer by using these film forming methods.
That is, in the case of the vapor deposition method, the sputtering method, the CVD method, etc., the flow rate and the partial pressure of the source gas of the additional element introduced during the film formation are changed with time, or various parameters of the film formation conditions are changed with time. Either change or add a thin film of the element to be added on or under the transparent conductive layer and heat-treat to diffuse the element to be added inside the transparent conductive layer, or vapor-deposit the element to be added at the same time By changing the speed,
In the case of a spray method, a spirion method, a dipping method, or the like, a film is formed by dividing the layers into layers having different amounts of the above elements, or a thin film of the elements to be added is laminated on or below the transparent conductive layer and heat-treated. Done by things.

The element (dopant) added to the compound forming the transparent conductive layer to change the electrical conductivity is A
1, In, B, Ga, Si, F, Sn, Te, Ti, S
b, Pb, P, As, Tl, Cl, Br, I are used. For example, when the transparent conductive layer is ZnO, Al,
In, B, Ga, Si, F, etc., In the case of In ₂ O ₃ , particularly Sn, F, Te, Ti, Sb, Pb, etc., and in the case of SnO ₂ , particularly, F, Sb, P, As, In,
Tl, Te, Cl, Br, I and the like are preferably used.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view showing an example of a photovoltaic device according to the present invention.

In FIG. 1, reference numeral 101 denotes a substrate serving as a support, and any of a conductive material and an insulating material can be used as a material. As a conductive material,
Molybdenum, titanium, cobalt, chromium, nickel,
Plates and films made of iron, copper, tantalum, niobium, zirconium, aluminum metal or alloys thereof are included. Among them, stainless steel, nickel chromium alloy and nickel, tantalum, niobium, zirconium, titanium metal and / or alloy are particularly preferable from the viewpoint of contact resistance. As the insulating material, a film or sheet of a synthetic resin such as polyester, polyethene, polycarbonate, cellulose acetate, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, or polyamide, or a plate-like body such as glass or ceramic is used. You can also. Alternatively, a conductive material coated with an insulating material can be used.

In FIG. 1, reference numeral 102 denotes a back electrode.
Materials include gold, silver, copper, aluminum, nickel,
Examples include metals such as iron, chromium, molybdenum, titanium, cobalt, tantalum, niobium, and zirconium, and alloys such as stainless steel. Among them, aluminum, copper, silver,
Highly reflective metals such as gold are particularly preferred.

The shape of the back electrode may be flat, but it is more preferable that the back electrode has an uneven shape for scattering light.
By having an uneven shape that scatters light, it scatters long-wavelength light that could not be absorbed by the semiconductor layer, extends the optical path length in the semiconductor layer, improves the long-wavelength sensitivity of the photovoltaic element, and increases the short-circuit current. And the photoelectric conversion efficiency can be improved. In the uneven shape for scattering light, it is desirable that the difference between the heights of the peaks and valleys of the unevenness is 0.2 to 2.0 μm in Rmax.

In FIG. 1, reference numeral 103 denotes a transparent conductive layer, the material and characteristics of which are as described above.

As the semiconductor layer of the photovoltaic element of the present invention, an amorphous semiconductor such as amorphous silicon (a-Si), amorphous silicon germanium (a-SiGe), amorphous silicon carbide (a-Sic), etc.
Alternatively, polycrystalline silicon, or a II-VI group semiconductor such as CdS or CdTe, or an I-type semiconductor such as CuInSe ₂
-III-VI Group ₂ compound semiconductors are used. At least a part of the semiconductor layer is p-type or n-type.
It is doped in a mold and has at least one PN junction or PIN junction.

In FIG. 1, reference numeral 104 denotes an n-type semiconductor layer, 105 denotes an intrinsic (i-type) semiconductor layer, and 106 denotes a p-type semiconductor layer.
A PIN junction is formed at 104 to 106, and a photoelectromotive force is generated by light transmitted through the transparent electrode 107 and incident.

In FIG. 1, 107 is a transparent electrode,
In addition to being an electrode on the light incident side, by optimizing its film thickness, it also serves as an antireflection film. The transparent electrode 107 is required to have a high transmittance in a wavelength region where the semiconductor layer can absorb light and to have low efficiency.
The same material as the transparent conductive layer 103 is used as the material.

In FIG. 1, reference numeral 108 denotes a current collecting electrode;
If the resistivity of the transparent electrode 107 cannot be made sufficiently low, it is formed on a part of the transparent electrode 107 as needed, and serves to reduce the low efficiency of the electrode and reduce the series resistance of the photovoltaic element. Its materials include gold, silver, copper, aluminum, nickel, iron, chromium, molybdenum, titanium, cobalt,
Examples of the conductive paste include metals such as tantalum, niobium, and zirconium, alloys such as stainless steel, and powdered metals. Then, the shape is formed in a branch shape as shown in FIG. 4, for example, so as not to block incident light on the semiconductor layer as much as possible.

Hereinafter, a method for manufacturing the photovoltaic element of the present invention will be described.

First, a back electrode is formed on a substrate serving as a support by using the above-described material as a substrate serving as a support. However, in the case where the substrate serving as the support also serves as the back electrode, the formation of the back electrode may not be necessary. For forming the back surface electrode, a vapor deposition method, a sputtering method, a plating method, a printing method, or the like is used. In the case where the back electrode is formed to have an uneven shape that scatters light, the formed metal or alloy curtain is formed by dry etching, wet etching, sand blasting, heating, or the like. In addition, the above-described metal or alloy is deposited while heating the substrate serving as the support, so that an uneven shape that scatters light can be formed.

Next, a transparent conductive layer is formed by the above-described method.

Next, a semiconductor layer having at least one PN junction or PIN junction is formed. The semiconductor layer is formed by various kinds of CVD methods such as a plasma CVD method, an optical CVD method, and a thermal CVD method in the case of an amorphous semiconductor or polycrystalline Si.
Depending on the method, II-VI or I-III-VI
_In the case of a Group ₂ compound semiconductor, it is formed by an evaporation method, a sputtering method, a spray method, a printing method, or the like.

Next, a transparent electrode is formed in the same manner as the transparent conductive layer.

Finally, the current collecting electrode is formed in the same manner as the back electrode, but in a pattern so as to block incident light to the semiconductor layer as little as possible.

When a solar cell module is manufactured using the photoelectric conversion element of the present invention, the photoelectric conversion elements formed in a desired area are connected in series so as to obtain a desired output voltage, and the output of An extraction electrode is formed, and protective layers are formed on both surfaces of the photoelectric conversion element.

[0047]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited thereto.

Example 1 A solar cell according to an example of the present invention shown in FIG. 1 was manufactured in the following steps.

First, the surface of the substrate 101 is Rmax.
A 0.1 cm thick, 1.0 mm thick, 10 cm square, non-alkali glass substrate is washed, and as the back electrode 102, an average of 0.1 μm of Al is deposited on the substrate by electron beam evaporation.
4 μm was formed.

Next, 0.4 μm of zinc oxide (ZnO) was formed by the DC magnetron sputtering apparatus shown in FIG.

In FIG. 3, reference numeral 301 denotes a vacuum vessel;
The heating plate 303 is supported by a supporting portion 302 having an insulating property. The heating plate 303 includes a heater 306 and a thermocouple 3
04 is buried and controlled to a predetermined temperature by a temperature controller 305. Reference numeral 307 denotes a heat transfer plate,
8 is supported by a substrate holder 309. A target 310 is disposed to face the substrate 308, and the target 3
Numeral 10 is provided on a target table 311 and has a magnet 312 on the back surface so that a magnetic field can be formed in the plasma space 320. In order to cool the target heated during sputtering, cooling water is introduced from the cooling water introduction pipe 313 to the back surface of the target. After the introduced water cools the target, it is discharged from a cooling water discharge pipe.

The target 310 is made by mixing zinc oxide powder with zinc and sintering the mixture. Alternatively, a target made of metal zinc can be used.

The target 310 is attached to the target table 3.
A DC voltage is applied from a sputter power supply 314 via the power supply 11. The DC current supplied from the sputtering power source is preferably set to 0.01 A or more, more preferably 0.1 A or more. According to the experiments performed by the present inventors, the larger the current supplied to the sputter, the smaller the absorption of light by the produced zinc oxide layer, and the higher the photoelectric conversion efficiency of the solar cell, especially, the larger the generated current. This is the same even when the zinc oxide layer is formed by using the RF type sputtering method.
It was more advantageous in terms of generated current than a solar cell when the power was smaller than F power.

Reference numeral 315 denotes an RF high-frequency power supply, which is used for applying a high frequency to the substrate side as necessary to roughen the substrate surface. Roughening of the substrate surface is performed before DC sputtering, and is intended to improve the adhesion of a film formed by DC sputtering to the substrate.

As a sputtering gas, an argon gas and an oxygen gas are supplied via a mass flow controller 316 or 317, respectively. Of course, it is also possible to perform fluorine doping on the zinc oxide layer formed by mixing the sputtering gas with another gas, for example, a SiF ₄ gas or an NF ₃ gas. The flow rate of the argon gas is preferably 1 sccm to 1 slm, and the flow rate of the oxygen gas is preferably 0.1 sccm to 100 sccm.

The internal pressure can be monitored by a vacuum gauge 318 attached to the vacuum vessel 301. Vacuum container 3
01 is a main valve 3 connected to an exhaust system (not shown).
A vacuum state is established via 19. The background internal pressure before starting the sputtering is preferably 10 ^-4 To
rr or less, more preferably 10 ^-5 Torr or less,
Internal pressure during sputtering is 1 mTorr or more and 1 Torr
It is as follows.

The formation of the zinc oxide layer is started while maintaining the above conditions, and after the thickness of the zinc oxide layer reaches a desired value, the supply of power from the sputtering power supply and the supply of the sputtering gas are appropriately performed. After stopping and cooling the substrate appropriately, the inside of the vacuum vessel is leaked to the atmosphere, and the substrate on which the zinc oxide layer is formed is taken out.

Next, 5 nm of indium (In) was vapor-deposited on the substrate on which ZnO was formed by the DC magnetron sputtering apparatus described above by a known vacuum vapor deposition apparatus.

After that, the substrate on which In was deposited was heated at 220 ° C. for 1 hour in an enzyme atmosphere to convert In into Z.
Diffusion in nO. Secondary ion mass spectrometer (SIM
As a result of S), in the depth direction analysis, it was found that In had a distribution as shown in FIG. 2 in the film thickness direction.

Thereafter, a known so-called glow discharge method (GD) in which a source gas is decomposed into a plasma state under reduced pressure by applying an RF radio frequency of 13.56 MHz to the electrode to decompose the material gas.
), The following semiconductor layers were formed.

[0061] First, while heating the substrate to 300 ° C., H ₂
The monosilane (SiH ₄ ) and phosphine (PH ₃ ) diluted by the above were decomposed to form an n-type a-Si layer 104 of 20 nm on the solar cell substrate of the present invention.

[0062] Next, while heating the substrate to 250 ° C., H ₂
The monosilane (SiH ₄ ) diluted with the above was decomposed to form an intrinsic a-Si layer 105 thereon of 400 nm.

[0063] Next, while heating the substrate to 200 ° C., H ₂
Decompose monosilane (SiH ₄ ) and boron trifluoride (BF ₃ ) diluted with
Was formed to a thickness of 5 nm.

Next, the substrate was heated to 170 by a resistance heating evaporation method.
While heating to 70 ° C., ITO was deposited to a thickness of 70 nm to form a transparent electrode 107.

Next, Al was deposited in a pattern as shown in FIG. 4 using a mask by electron beam evaporation to form a current collecting electrode 108.

Through the above steps, ten 10 cm square so-called single-layer a-Si solar cells were manufactured.

The shunt resistance is 1K per 1 cm ²
A solar cell of Ω or more was irradiated with a solar simulator at 25 ° C. and a solar simulator of AM 1.5, 100 mw / cm ² , and the open-circuit voltage (Voc) and short-circuit current (Js)
c), fill factor (FF), photoelectric conversion efficiency (η), series resistance (Rse), shunt resistance (Rsh)
And the like, and the average value was determined.

Table 1 summarizes the results of the solar cell characteristics.

However, the solar cell characteristics are standardized by the values of Comparative Example 1 described later.

As is apparent from Table 1, the solar cell using the transparent conductive layer 103 of the present invention has a smaller series resistance and an improved open-circuit voltage (Voc) and fill factor (FF) as compared with Comparative Example 1. However, the photoelectric conversion efficiency (η) was improved.

This is achieved by changing the amount of In added to ZnO in the film thickness direction as shown in FIG.
Near the interface with the n-type a-Si layer 104,
It is considered that no barrier was formed at the interface between the transparent conductive layer 103 and the n-type a-Si layer 104 or tunneling was possible, and the series resistance was reduced. Further, by increasing the amount of In added only near the interface with the n-type a-Si layer 104, the transmittance of ZnO does not decrease due to the addition of In, and the wavelength of 400 n
The transmittance of light from m to 1000 nm was 85% or more. Here, the transmittance is Zn on which In is added on a glass substrate.
O was formed and measured by the method described above.

(Comparative Example 1) In Example 1, a 10 cm square so-called single layer type a was used in the same manner as in Example 1 except that In was not added to ZnO as the transparent conductive layer 103.
Ten -Si solar cells were produced.

In the same manner as in Example 1, the characteristics of the solar cell were measured, and the average value was obtained.

(Comparative Example 2) In Example 1, a ZnO transparent conductive layer 103 in which the amount of In added was constant in the film thickness direction was formed using a target in which 5% of In was added to ZnO. Except for this, ten 10 cm square so-called single-layer a-Si solar cells were manufactured in exactly the same manner as in Example 1.

In the same manner as in Example 1, the characteristics of the solar cell were measured, and the average value was obtained.

By adding In to ZnO, the resistance of the transparent conductive layer 103 was reduced, the series resistance was reduced, and the open-circuit voltage (Voc) and the fill factor (FF) were improved as compared with Comparative Example 1. However, since In is added to the entire thickness direction of ZnO, the transmittance of ZnO decreases, the reflection from the back electrode decreases, and the short-circuit current (J
sc) decreased, and the photoelectric conversion efficiency (η) did not improve as compared with Comparative Example 1.

[0077]

[Table 1]

Example 2 In the following steps, another example of the solar cell of the present invention shown in FIG. 1 was manufactured.

First, as a substrate 101, a 10 cm square SU having a surface with an Rmax of 0.1 μm or less and a thickness of 0.7 mm is used.
In step S304, the stainless steel substrate was washed, and an average of 0.4 μm of Ag was formed as the back electrode 102 by RF sputtering. At this time, by performing sputtering while heating the substrate to 380 ° C., a concavo-convex shape for scattering light having a Rmax of 0.6 μm was produced.

Next, while heating the substrate to 180 ° C. in a chamber by MOCVD, diethylzinc (D
EZ) and H ₂ O were vaporized and introduced, SiF ₄ diluted to 10% with H ₂ was introduced and mixed as a doping gas, and 0.4 μm of F-doped ZnO was formed as the transparent conductive layer 103.

Here, by changing the flow rate of SiF ₄ diluted to 10% with H ₂ introduced during the film formation with time as shown in FIG. 5, the amount of F added in ZnO is shown in the film thickness direction. 6 was changed. It is a graph of the result of the depth direction analysis of the peak of F by secondary ion mass spectrometry (SIMS) of ZnO.

Thereafter, the following semiconductor layers were formed by the so-called glow discharge method (GD method) in which a source gas was decomposed into a plasma state under reduced pressure by applying an RF radio frequency of 13.56 MHz to the electrodes.

[0083] First, while heating the substrate to 300 ° C., H ₂
The monosilane (SiH ₄ ) and phosphine (PH ₃ ) diluted by the above were decomposed to form an n-type a-Si layer 104 of 20 nm on the solar cell substrate of the present invention.

Next, while heating the substrate to 280 ° C., H ₂
Monosilane (SiH ₄ ) and germane (Ge
H ₄ ) was decomposed to form a 250 nm intrinsic a-SiGe layer 105 thereon. At this time, a so-called buffer layer was formed near the interface with the n-layer and near the interface with the p-layer.

[0085] Next, while heating the substrate to 200 ° C., H ₂
Decompose monosilane (SiH ₄ ) and 3-fluoroboron (BF ₃ ) diluted with p-type to form a p-type microcrystalline silicon layer 106.
Was formed to a thickness of 5 nm.

Next, the substrate was heated to 170 by a resistance heating evaporation method.
While heating to 70 ° C., ITO was deposited to a thickness of 70 nm to form a transparent electrode 107.

Next, a current collecting electrode 108 was formed by evaporating Al in a pattern as shown in FIG. 4 using a mask by an electron beam evaporation method.

Through the above steps, ten 10 cm square so-called single-layer a-siGe solar cells were manufactured.

The shunt resistance is 1K per 1 cm ²
A solar cell of Ω or more was irradiated with a solar simulator at 25 ° C. and a solar simulator of AM 1.5, 100 mw / cm ² , and the open-circuit voltage (Voc) and short-circuit current (Js)
c), fill factor (FF), photoelectric conversion efficiency (η), series resistance (Rse), shunt resistance (Rsh)
And the like, and the average value was determined.

Table 2 summarizes the results of the solar cell characteristics.

However, the solar cell characteristics are standardized by the values of Comparative Example 3 described later.

As is clear from Table 2, the solar cell using the transparent conductive layer 103 of the present invention has a smaller series resistance and improved open-circuit voltage (Voc) and fill factor (FF) as compared with Comparative Example 3. However, the photoelectric conversion efficiency (η) was improved.

This is because, in the case of Comparative Example 3, when Cu or Ag diffuses into ZnO, which is a defective n-type semiconductor, it can act as an acceptor. And increasing the series resistance of the solar cell, as in the present invention, by changing the amount of F added to ZnO in the film thickness direction as shown in FIG. It is considered that the resistance became low near the interface with the substrate 102, and the above-described increase in the resistance of ZnO did not occur, and the series resistance of the solar cell decreased. In addition, the amount of F to be added
, The transmittance of ZnO is not reduced by the addition of F, and the wavelength is 400 nm.
The transmittance of light having a wavelength of from to 1000 nm was 85% or more.
Here, the transmittance was measured by forming ZnO to which F was added on a glass substrate by the above-described method.

(Comparative Example 3) In Example 2, ZnO as the transparent conductive layer 103
Without the addition of 10
Ten so-called single-layer a-SiGe solar cells of cm square were manufactured.

In the same manner as in Example 2, the solar cell characteristics were measured, and the average value was obtained.

Comparative Example 4 In Example 2, the flow rate of SiF ₄ diluted to 10% with H ₂ introduced during film formation was 8.0.
The ZnO transparent conductive layer 103 was formed at a constant rate of μmol / min and the amount of F added was constant in the film thickness direction. Except for this, 10 so-called single-layer a-SiGe solar cells of 10 cm square were manufactured in exactly the same manner as in Example 2.

In the same manner as in Example 2, the characteristics of the solar cell were measured, and the average value was obtained.

By adding F to ZnO, the resistance of the transparent conductive layer 103 was reduced, the series resistance was reduced, and the open-circuit voltage (Voc) and the fill factor (FF) were improved as compared with Comparative Example 3. However, since F is added throughout the thickness direction of ZnO, the transmittance of ZnO decreases, the reflection from the back electrode decreases, and the short-circuit current (Js
c) decreased, and the photoelectric conversion efficiency (η) did not improve as compared with Comparative Example 3.

[0099]

[Table 2]

Example 3 In the following steps, another example of the solar cell of the present invention shown in FIG. 7 was manufactured.

FIG. 7 shows a stacked solar cell in which two sets of PIN junctions are stacked.

First, when the surface is Rmax of 0.1 μm or less,
A sheet-like stainless steel substrate having a thickness of 0.15 mm, a width of 32 cm, and a length of 15 m is washed, and a process is performed while continuously moving the substrate between a feeding roll and a winding roll. The following processing was performed by the roll-to-roll method.

First, Ag was formed as an average of 0.4 μm as the back electrode 702 by a known RF magnetron sputtering apparatus using a high frequency of 13.56 MHz. At this time, by performing sputtering while heating the substrate to 380 ° C., a concavo-convex shape for scattering light having a Rmax of 0.6 μm was produced.

Next, Ag was formed by the above-mentioned RF sputtering method.
A 50 nm layer of Al was laminated on the substrate.

Next, 0.4 μm of zinc oxide (ZnO) was formed by the RF sputtering method described above.

At this time, before forming ZnO, the substrate is sputtered by applying RF high frequency to the substrate side to roughen the substrate surface, and at the same time, Al on the substrate surface is sputtered on the ZnO target. A thin film was formed. Then, by starting the sputtering of ZnO,
Al could be doped only at the initial stage of ZnO film formation.

[0107] Further, after forming the ZnO, again Al was 5nm formed by an RF sputtering method described above, then N ₂
Thermal annealing was performed at 300 ° C. for 20 minutes in an atmosphere.

By the above method, the amount of Al added in ZnO could be changed in the film thickness direction as shown in FIG. FIG. 8 shows secondary ion mass spectroscopy (SI
10 is a graph showing the results of analysis of the Al peak in the depth direction by (MS).

Next, glow discharge method (GD method)
As a result, the following semiconductor layers were formed.

First, while heating the substrate to 300 ° C., an n-type a-Si layer 704 was formed to a thickness of 20 nm.

Next, while heating the substrate to 280 ° C., an intrinsic a-SiGe layer 705 was formed thereon to a thickness of 250 nm.
At this time, the intrinsic a-SiG
When e was deposited at 1 μm and evaluated, the optical band gap (Eg) was 1.48 eV.

The intrinsic a-SiGe layer 705 is composed of the n layer and the p layer.
A so-called buffer layer is provided in which the composition changes continuously from a-SiGe to a-Si in the vicinity of the layer at 30 nm each.

Next, while heating the substrate to 260 ° C., a p-type microcrystalline silicon layer 706 was formed to a thickness of 5 nm.

Next, while heating the substrate to 240 ° C., an n-type a-Si layer 707 was formed to a thickness of 20 nm.

Next, while heating the substrate to 240 ° C., an intrinsic a-Si layer 708 was formed thereon to a thickness of 220 nm. Next, while heating the substrate to 200 ° C., a p-type microcrystalline silicon layer 709 was formed to a thickness of 4 nm.

Next, the substrate was heated to 170 ° C. by resistance heating evaporation.
While heating to a thickness of 70 nm, ITO was deposited to a thickness of 70
10 was formed.

Next, the solar cell is etched by 10 cm.
The substrate was cut into corners and cut along the etching line.

Next, a current collecting electrode 711 was formed by evaporating Al in a pattern as shown in FIG. 4 by an electron beam evaporation method.

In the above steps, a 10 cm square so-called Si /
300 SiGe two-layer stack type solar cells were produced.

The shunt resistance is 1K per 1 cm ²
A solar cell of Ω or more was irradiated with a solar simulator at 25 ° C. and a solar simulator of AM 1.5, 100 mw / cm ² , and the open-circuit voltage (Voc) and short-circuit current (Js)
c), fill factor (FF), photoelectric conversion efficiency (η), series resistance (Rse), shunt resistance (Rsh)
And the like, and the average value was determined.

Table 3 summarizes the results of the solar cell characteristics.

However, the solar cell characteristics are standardized by the values of Comparative Example 5 described later.

As is clear from Table 3, the solar cell using the transparent conductive layer 703 of the present invention has a reduced series resistance, and has an open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF), respectively. And the photoelectric conversion efficiency (η) was improved.

This means that, in addition to the effects of Embodiments 1 and 2, the amount of Al monotonically increases from the n-type a-Si layer 704 toward the back electrode 702 except for the vicinity of the interface. , The refractive index gradually decreases, and the n-type a
It is considered that the reflection of light at the interface between the -Si layer 704 and the transparent conductive layer 703 was reduced, and the light confinement effect by scattering of long-wavelength light was increased.

Comparative Example 5 A 10 cm square so-called Si / S was prepared in the same manner as in Example 3 except that Al was not added to ZnO as the transparent conductive layer 703 except for Al.
300 iGe two-layer stack type solar cells were produced.

In the same manner as in Example 3, the solar cell characteristics were measured, and the average value was obtained.

(Comparative Example 6) In Example 3, Al was changed to 1
By performing sputtering using a ZnO target to which 0% was added, a transparent conductive layer 103 of ZnO in which the addition amount of Al was constant in the film thickness direction was formed. In this case ZnO
Al deposition was not performed before and after the film formation. Other than that, exactly as in Example 3, a 10 cm square so-called Si /
300 SiGe two-layer stack type solar cells were produced.

In the same manner as in Example 3, the solar cell characteristics were measured, and the average value was obtained.

By adding Al to ZnO, the resistance of the transparent conductive layer 703 was reduced, the series resistance was reduced, and the open-circuit voltage (Voc) and the fill factor (FF) were improved as compared with Comparative Example 5. However, since Al was added throughout the thickness direction of ZnO, the transmittance of ZnO was reduced, the reflection from the back electrode was reduced, and the short-circuit current (J
sc) decreased, and the photoelectric conversion efficiency (η) did not improve as compared with Comparative Example 5.

[0130]

[Table 3]

Example 4 A further example of the solar cell of the present invention shown in FIG. 9 was manufactured by the following steps.

First, the substrate 901 has a surface with an Rmax of 0.1 μm or less, a thickness of 0.18 mm, a width of 32 cm,
The sheet-like polyimide film having a length of 10 m was washed as a substrate and subjected to the following treatment by a so-called roll-to-roll method similar to that of Example 3.

First, Cu was used as the back surface electrode 902 by using the DC magnetron sputtering apparatus shown in FIG.
4 μm was formed. Next, while heating the substrate to 150 ° C.
An RF high frequency was applied to the substrate side to generate plasma of Ar gas, and the substrate was sputtered to produce an irregular shape that scatters light having a Rmax of 0.6 μm. Next, an O ₂ gas was introduced while maintaining the substrate temperature at 180 ° C. by using a so-called ion plating device (not shown) provided with two electron coils each for applying an RF beam and an electron beam vapor deposition device in a chamber. ITO was deposited to form a transparent conductive layer 903 having a thickness of 0.3 μm.

At this time, the two electron beam evaporation devices
By independently evaporating n and Sn, the amount of Sn added to In ₂ O ₃ can be freely controlled. Then, the amount of Sn added to In ₂ O _{3 is} changed in the thickness direction of the transparent conductive layer 903 as shown in FIG. 11 by changing the current amount of the electron beam in the deposition of Sn as shown in FIG. I was able to do things. At this time, the addition amount of Sn was determined by repeating Auger electron spectroscopy and sputtering.

Then, the semiconductor layer of the solar cell shown in FIG. 9 was manufactured in the following steps.

First, while heating the substrate to 180 ° C., a p-type CdTe film 904 was formed to a thickness of 0.35 μm by an evaporation method.

Next, while heating the substrate to 170 ° C., an n-type CdS film 905 was formed to a thickness of 0.1 μm by vapor deposition.

Next, the substrate was heated to 170 ° C. by resistance heating evaporation.
While heating to a thickness of 70 nm, ITO was evaporated to a thickness of 70
06 was formed.

Next, a current collector electrode 907 was formed by printing an Ag paste by screen printing in a pattern as shown in FIG.

Thereafter, a heat treatment was performed in an N ₂ atmosphere at 120 ° C. for 1 hour.

Next, the solar cell was etched by 10 cm.
The substrate was cut into corners and cut along the etching line.

In the above steps, a so-called CdS of 10 cm square is formed.
/ 200 CdTe solar cells were produced. And, the more solar cells 1KΩ per 1cm ² shunt resistance, 2
At 5 ° C., the AM1.
5, irradiate 100 mw / cm ² of simulated sunlight, open voltage (Voc), short circuit current (Jsc), fill factor (FF), photoelectric conversion efficiency (η), series resistance (Rs)
e) Solar cell characteristics such as shunt resistance (Rsh) were measured and averaged.

Table 4 summarizes the results of the solar cell characteristics.

However, the solar cell characteristics are standardized by the values of Comparative Example 7 described later.

As is clear from Table 4, the solar cell using the transparent conductive layer 903 of the present invention has reduced series resistance, and has an open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF), respectively. And the photoelectric conversion efficiency (η) was improved.

This is achieved by changing the amount of Sn added to In ₂ O ₃ in the film thickness direction as shown in FIG.
Near the interface with the p-type CdTe layer 904, the n ₂ O ₃ becomes a low-resistance n-type, and the transparent conductive layer 903 and the p-type CdTe layer 904.
It is considered that the formation of the tunnel junction at the interface with the layer 904 reduced the series resistance. In addition, it is considered that the series resistance was also reduced due to the low resistance of In ₂ O ₃ near the interface between the transparent conductive layer 903 and the back electrode 902. Further, the amount of Sn added is
By decreasing monotonically from the p-type CdTe layer 904 to the back surface electrode 902 except for the vicinity of the interface, light reflection at the interface between the p-type CdTe layer 904 and the transparent conductive layer 903 is reduced, and a long wavelength It is considered that the light confinement effect due to light scattering increased and the short-circuit current (Jsc) improved.

(Comparative Example 7) In Example 4, In ₂ O ₃ to which Sn was not added was used as the transparent conductive layer 903.
Otherwise, exactly the same as in Example 4, 200 so-called Cds / CdTe solar cells of 10 cm square were manufactured.

In the same manner as in Example 4, the solar cell characteristics were measured, and the average value was obtained.

(Comparative Example 8) In Example 4, In ₂ O ₃
A transparent conductive layer 903 of ITO was formed, in which the amount of Sn added to was constant at 10%. Except for this, exactly the same as in Example 4, 200 so-called CdS / CdTe solar cells of 10 cm square were manufactured.

In the same manner as in Example 4, the solar cell characteristics were measured, and the average value was obtained.

By adding 10% of Sn to In ₂ O ₃ , the resistance of the transparent conductive layer 903 was reduced, but the series resistance was not sufficiently reduced, and the photoelectric conversion efficiency (η) was lower than that of Example 4. Was low. This is presumably because the barrier at the interface between the p-type CdTe layer 904 and the transparent conductive layer 903 could not be sufficiently reduced. When the amount of Sn added was fixed at 40%, the series resistance was sufficiently reduced, but the transmittance was reduced.

[0152]

[Table 4]

[0153]

The present invention has the following effects because it is configured as described above.

An element for changing the conductivity is added to the compound forming the transparent conductive layer, and the amount of the element is changed in the film thickness direction. Therefore, the series resistance of the photovoltaic element decreases, and the open-circuit voltage ( Voc) and the fill factor (FF) are improved, and the absorption of long-wavelength light in the semiconductor layer is increased, thereby improving the short-circuit current (Jsc), thereby improving the photoelectric conversion efficiency of the photovoltaic device. I was able to.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an example of a photovoltaic device of the present invention.

FIG. 2 is a graph showing a first example of a change with respect to a film thickness of an element added to a compound of a transparent conductive layer of a photovoltaic device according to an example of the present invention.

FIG. 3 is a cross-sectional view of an example of a manufacturing apparatus for forming a transparent conductive layer of the photovoltaic device of the present invention.

FIG. 4 is a schematic view of an example of the photovoltaic device of the present invention.

FIG. 5 is a graph showing how a flow rate of a gas introduced when forming a transparent conductive layer of a photovoltaic element according to an example of the present invention is changed with time.

FIG. 6 is a graph showing a second example of a change with respect to the film thickness of an element added to the compound of the transparent conductive layer of the photovoltaic device of one example of the present invention.

FIG. 7 is a sectional view of another example of the photovoltaic device of the present invention.

FIG. 8 is a graph showing a third example of a change with respect to the film thickness of an element added to the compound of the transparent conductive layer of the photovoltaic device of one example of the present invention.

FIG. 9 is a sectional view of still another example of the photovoltaic device of the present invention.

FIG. 10 is a graph showing how to change the formation conditions with time when forming a transparent conductive layer of a photovoltaic element according to an example of the present invention.

FIG. 11 is a graph showing a fourth example of the change with respect to the film thickness of the element added to the compound of the transparent conductive layer of the photovoltaic device of one example of the present invention.

[Explanation of symbols]

 101, 701, 901 Substrate 102, 702, 902 Back electrode 103, 703, 903 Transparent conductive layer 104, 704, 707, 905 N-type semiconductor layer 105, 705, 708 Intrinsic semiconductor layer 106, 706, 709, 904 P-type semiconductor Layers 107, 710, 906 Transparent electrodes 108, 711, 907 Collector electrodes 301 Vacuum container 302 Support 303 Heating plate 304 Thermocouple 305 Temperature controller 306 Heater 307 Heat transfer plate 308 Substrate 309 Substrate holder 310 Target 311 Target base 312 Magnet 313 Cooling water introduction pipe 314 Sputter power supply 315 High frequency power supply 316, 317 Mass flow controller 318 Vacuum gauge 319 Main valve 320 Plasma space 401 Solar cell light incident surface 402 Extraction electrode 7 21 First cell 722 Second cell

Claims (4)

(57) [Claims] A ZnO film is formed between a back electrode provided on a side opposite to a light incident surface and a semiconductor layer for photoelectric conversion.
In a photovoltaic element having a transparent conductive layer, Al, In,
B, Ga, Si, when Ru is contained an element selected from F
In both cases, the photovoltaic device is characterized in that the addition amount of the element changes in the film thickness direction so as to increase near the interface with the semiconductor layer. A back electrode provided on the opposite side of the light incident surface;
ZnO, In ₂ O ₃ between the semiconductor layer for photoelectric conversion
Or a photovoltaic device having a transparent conductive layer formed of SnO ₂
An element that changes the electrical conductivity of the transparent conductive layer.
When the transparent electrode layer is made of ZnO, Al, In,
An element selected from B, Ga, Si, and F is added to the transparent electrode
If the layer is In ₂ O ₃ , Sn, F, Te, Ti, Sb,
An element selected from Pb, transparent electrode layer is SnO ₂
In the case, F, Sb, P, As, In, Tl, Te, C
containing an element selected from l, Br and I
In the vicinity of the interface with the back electrode,
Characterized in that it changes in the film thickness direction so as to increase.
Photovoltaic element. 3. The photovoltaic device according to claim 2 , wherein the amount of the element that changes the conductivity is increased near the interface with the semiconductor layer. 4. The method according to claim 1, wherein the amount of the element that changes the electric conductivity is:
Monotonously decreasing from the semiconductor layer side to the back electrode side
4. The method as claimed in claim 2, wherein the portion has a reduced portion.
The photovoltaic device according to any one of the preceding claims.