JPH05110125A - Photovoltaic element - Google Patents

Abstract

PURPOSE:To provide a photovoltaic element improved in the photoelectric conversion efficiency of the element by lowering the value of resistance while maintaining a transmittance at a high value. CONSTITUTION:A photovoltaic element having a transparent conductive layer 103 added between semiconductor layer and rear electrode 102 being on the opposite side of the plane of incidence of light by changing a chemical element such as metal for the change of conductivity in the direction of film thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photovoltaic element, and in particular, it stabilizes output characteristics by increasing the light absorbed by a semiconductor layer by reflecting and scattering incident light on the back surface. Photovoltaic device

[0002]

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

The most important 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.

The solar cells can be roughly classified into those using a single crystal, those using a polycrystal, those using an amorphous, and those using a compound thin film. Among them, a solar cell using an amorphous film or a 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, since it should be calculated per unit power, 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
Ag, Al, Cu, A on the side opposite to the light incident side of the solar cell
A technique of forming a reflective layer of a metal film having a high reflectance such as u is known. In this technique, light transmitted through a semiconductor layer that generates carriers is reflected by a reflective layer so that the light is absorbed again in the semiconductor layer to increase output current and improve photoelectric conversion efficiency. ..

However, as the back electrode, Ag, A
When a metal film having a high reflectance, such as l, Cu, or Au, is used, it diffuses into the semiconductor layer to deteriorate 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 Ag, Al, Cu, Au
It is considered that a metal having a high reflectance, such as, is easily diffused in the semiconductor layer.

This is particularly noticeable 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 of improving the surface property of the substrate by interposing a transparent conductive layer between the back electrode and the semiconductor layer is disclosed in Japanese Patent Publication Nos. 59-43101 and 60-4.
It is disclosed in Japanese Patent No. 1878. In these publications, as an effect of interposing a transparent conductive layer between the back electrode and the semiconductor layer, the flatness of the back electrode is improved, the adhesion of the semiconductor layer is improved, or the metal of the back electrode and the alloy of the semiconductor layer are alloyed. The prevention of is mentioned.

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

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

[0012]

Technical Records to be Solved 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 a high transmittance for light having a wavelength that can be absorbed by the semiconductor layer and a high conductivity. However, the conductive oxide film that is preferably 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 surface metal, if priority is given to the transmittance, 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 manufacturing conditions of 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 an n-type, and a barrier may be formed at the interface between the transparent conductive layer and the semiconductor layer depending on the combination thereof.

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, and some elements are subjected to conductive oxidation. It has been found to be effective as a film dopant. However, if it is attempted to apply this technique to the transparent conductive layer, if the amount of addition of the element is increased in order to increase the conductivity, the light transmittance will decrease, or the structure of the conductive oxide film will decrease. It is expected that it will be difficult to apply the doping technique, because adverse effects such as disordering of the element and diffusion of the element into the semiconductor layer are expected.

[0017]

It is an object of the present invention to solve the above problems of the transparent conductive layer interposed between the back electrode and the semiconductor layer, maintain the high reflectance of the back electrode, and increase the long wavelength sensitivity of the solar cell. Moreover, it is an object 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 In order to achieve the above-mentioned object, the present inventor has earnestly studied and, as a result, achieved the object by a photovoltaic device having the following structure.

In a photovoltaic element having a transparent conductive layer between a back electrode provided on the side opposite to the light incident surface and a photoelectric conversion layer made of a semiconductor, the transparent conductive layer contains an element that changes the conductivity. The photovoltaic element is characterized in that the added amount of the element changes in the film thickness direction.

Here, various patterns can be considered as a method of changing the addition amount of the element that changes the conductivity, but 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, a barrier is not formed at the interface between the conductive oxide and the semiconductor layer, and the photovoltaic power is reduced. The series resistance of the device decreased and the photoelectric conversion efficiency increased.

Further, by increasing the addition amount of the element near the interface with the back electrode, the conductive oxide near the interface with the back electrode has a high resistance due to the diffusion of the metal forming the back electrode. And the series resistance of the photovoltaic element was reduced, and the photoelectric conversion efficiency was increased.

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

With respect to this effect, the addition amount of the element is monotonically decreased as it approaches the interface with the back electrode, so that the refractive index of the conductive oxide monotonically approaches the interface with the back electrode. It is considered that the amount of the long-wavelength light incident on the semiconductor layer increased due to the decrease in the reflection at the interface between the transparent conductive layer and the semiconductor layer.

In any of the above cases, since the above-mentioned elements are not added to the entire transparent conductive layer, the transmittance of the transparent conductive layer can be maintained at a high value even if the added amount is partially increased. The sagging and the disorder of the structure were suppressed, and the semiconductor layer was not adversely affected by 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 that is preferably 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 and the} like are preferably used.

As a method for forming the transparent conductive layer, a vapor deposition method,
A sputtering method, a CVD method, a spray method, a spin-on method, a dip method and the like are preferably used.

The following method is used to change the amount of the element added to the conductive oxide in the film thickness direction of the transparent conductive layer 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 partial pressure of the source gas of the additive element introduced during film formation are changed with time, or various parameters of the film formation conditions are changed with time. The thin film of the element to be changed or added is laminated on or under the transparent conductive layer to diffuse the added element inside the transparent conductive layer, or the added element is vapor-deposited at the same time. Made by changing the speed,
In the case of a spray method, a spirion method, a dip method, etc., the film is divided into layers with different addition amounts of the above elements, or a thin film depending on the element to be added is laminated on or under the transparent conductive layer and heat-treated. Done by things.

As the element (dopant) for changing the conductivity, which is added to the compound forming the transparent conductive layer, A
l, In, B, Ga, Si, F, Sn, Te, Ti, S
b, Pb, P, As, Tl, Cl, Br, I are used, and particularly when the transparent conductive layer is ZnO, Al,
In, B, Ga, Si, F, etc., and in the case of In ₂ O ₃ , Sn, F, Te, Ti, Sb, Pb, etc., and particularly, in the case of SnO ₂ , 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 the photovoltaic element of the present invention.

In FIG. 1, reference numeral 101 denotes a substrate which serves as a support, and the material may be either a conductive material or an insulating material. As the conductive material,
Molybdenum, titanium, cobalt, chromium, nickel,
Examples thereof include plates, films made of iron, copper, tantalum, niobium, zirconium, aluminum metal or alloys thereof. 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 synthetic resin such as polyester, polyethylene, 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 may be used.

In FIG. 1, 102 is a back electrode,
Materials include gold, silver, copper, aluminum, nickel,
Examples thereof include metals such as iron, chromium, molybdenum, titanium, cobalt, tantalum, niobium, and zirconium, and alloys such as stainless steel. Among them, aluminum, copper, silver,
A metal having a high reflectance such as gold is particularly preferable.

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

In FIG. 1, 103 is a transparent conductive layer, and its material and characteristics 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), or amorphous silicon carbide (a-Sic),
Alternatively, polycrystalline silicon, II-VI group semiconductors such as CdS and CdTe, or I such as CuInSe ₂
An example is one using a compound semiconductor of the group-III-VI ₂ . At least a part of the semiconductor layer is p-type or n-type.
It is doped in the mold and has at least one PN junction or PIN junction.

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

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

In FIG. 1, 108 is a collector electrode,
When the resistivity of the transparent electrode 107 cannot be made sufficiently low, it is formed on a part of the transparent electrode 107 as necessary to reduce the low efficiency of the electrode and the series resistance of the photovoltaic element. The material is gold, silver, copper, aluminum, nickel, iron, chromium, molybdenum, titanium, cobalt,
Examples thereof include metals such as tantalum, niobium, and zirconium, alloys such as stainless steel, and conductive paste using powder metal. 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.

The method of manufacturing the photovoltaic element of the present invention will be described below.

First, a back electrode is formed on the substrate to be the support, using the above-mentioned materials as the substrate to be the support. However, when the substrate serving as a support also serves as the back electrode, the back electrode may not be required to be formed in some cases. A vapor deposition method, a sputtering method, a plating method, a printing method, or the like is used for forming the back electrode. Further, when the back electrode is formed in a concavo-convex shape that scatters light, it is formed by dry-etching, wet-etching, sandblasting, or heating the formed metal or alloy curtain. Further, it is also possible to form a concavo-convex shape that scatters light by vapor-depositing the above-mentioned metal or alloy while heating the substrate serving as a support.

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

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 such as plasma CVD method, optical CVD method, thermal CVD method in the case of amorphous semiconductor or polycrystalline Si.
II-VI group, or I-III-VI, depending on the method.
_In the case of a Group ₂ compound semiconductor, it is formed by a vapor deposition 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 collector electrode is formed in the same manner as the back electrode, but in a pattern that does not block incident light on the semiconductor layer as much as possible.

Further, when a solar cell module is manufactured by the photoelectric conversion element of the present invention, photoelectric conversion elements formed in a desired area are connected in series so as to obtain a desired output voltage, and output of The extraction electrode is formed, and the 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 The solar cell of one example of the present invention shown in FIG. 1 was produced by the following steps.

First, the surface of the substrate 101 is Rmax.
And a thickness of 1.0 mm and a thickness of 1.0 mm and a 10 cm square non-alkali glass substrate are washed, and as the back surface electrode 102, Al is averaged to 0.
4 μm was formed.

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

In FIG. 3, 301 is a vacuum container,
The heating plate 303 is supported by the supporting portion 302 having an insulating property. A heater 306 and a thermocouple 3 are provided on the heating plate 303.
04 is embedded and is controlled to a predetermined temperature by the temperature controller 305. A heat transfer plate 307 is a substrate 30.
8 is supported by a substrate holder 309. The target 310 is arranged facing the substrate 308.
10 is installed on the 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. Cooling water is introduced from the cooling water introducing pipe 313 to the back surface of the target in order to cool the target heated during sputtering. The introduced water cools the target and then is discharged from the cooling water discharge pipe.

The target 310 is obtained by mixing zinc oxide powder with zinc and sintering it. It is also possible to use a target made of metallic zinc.

The target base 3 is attached to the target 310.
A DC voltage is applied from the sputter power source 314 via 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 conducted by the present inventor, the larger the current supplied to the sputter, the less the absorption of light by the zinc oxide layer to be produced, and the larger the photoelectric conversion efficiency of the solar cell, especially the generated current. This is also the case when the zinc oxide layer is formed by using the RF type sputtering method, and the solar cell manufactured by increasing the RF power has R
It was more advantageous than the solar cell in the case of smaller F power than in the generated current.

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

Argon gas and oxygen gas are supplied as the sputtering gas through the mass flow controller 316 or 317, respectively. Of course, it is also possible to dope fluorine with the zinc oxide layer formed by mixing the sputtering gas with another gas such as SiF ₄ or 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 the vacuum gauge 318 attached to the vacuum container 301. Vacuum container 3
01 Main valve 3 connected to an exhaust system not shown
A vacuum state is established via 19. The background internal pressure before starting the sputter 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 considered as follows.

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

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 vapor-deposited was heated at 220 ° C. for 1 hour in an enzyme atmosphere, so that In was converted to Z.
Diffused in nO. Secondary Ion Mass Spectrometer (SIM
The depth direction analysis by S) revealed that In had a distribution as shown in FIG. 2 in the film thickness direction.

After this, a known so-called glow discharge method (GD) in which an RF high frequency of 13.56 MHz is applied to the electrodes to decompose the raw material gas into a plasma state under reduced pressure
Method), the following semiconductor layers were formed.

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

Next, while heating the substrate to 250 ° C., H ₂
Monosilane (SiH ₄ ) diluted with was decomposed to form an intrinsic a-Si layer 105 on it by 400 nm.

Next, while heating the substrate to 200 ° C., H ₂
The monosilane (SiH ₄ ) and boron triboride (BF ₃ ) diluted with are decomposed to form the p-type microcrystalline silicon layer 106.
With a thickness of 5 nm.

Next, the substrate is heated to 170 by resistance heating vapor deposition.
ITO was vapor-deposited with a thickness of 70 nm while heating at 0 ° C. to form the transparent electrode 107.

Next, Al was vapor-deposited in a pattern as shown in FIG. 4 using an electron beam vapor deposition method to form a collector electrode 108.

Through the above steps, 10 so-called single layer type a-Si solar cells of 10 cm square were produced.

The shunt resistance is 1K per cm ^2.
A solar cell of Ω or more is irradiated with pseudo sunlight of AM1.5, 100 mw / cm ² by a solar simulator at 25 ° C. to open voltage (Voc), short circuit current (Js).
c), fill factor (FF), photoelectric conversion efficiency (η), series resistance (Rse), shunt resistance (Rsh)
The solar cell characteristics such as the above were measured, and the average value was obtained.

The results of solar cell characteristics are summarized in Table 1.

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

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

This is because ZnO is changed by changing the amount of In added to ZnO in the film thickness direction as shown in FIG.
However, in the vicinity of the interface with the n-type a-Si layer 104,
It is considered that the series resistance becomes small because a barrier is not formed at the interface between the transparent conductive layer 103 and the n-type a-Si layer 104 or tunneling is possible due to the formation of a type. Further, by increasing the amount of In to be added only in the vicinity of the interface with the n-type a-Si layer 104, the ZnO transmittance 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 with In added on the glass substrate.
O was formed and measured by the method described above.

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

In the same manner as in Example 1, the solar cell characteristics 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 the target obtained by adding 5% In to ZnO. Except for that, in the same manner as in Example 1, ten 10 cm square so-called single-layer a-Si solar cells were produced.

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

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

[0077]

[Table 1]

Example 2 A solar cell of another example of the present invention shown in FIG. 1 was produced by the following steps.

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

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

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

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

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

Next, while heating the substrate to 280 ° C., H ₂
Diluted with monosilane (SiH ₄ ) and germane (Ge
H ₄₎ by decomposing and 250nm forming an 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.

Next, while heating the substrate to 200 ° C., H ₂
P-type microcrystalline silicon layer 106 is decomposed by decomposing monosilane (SiH ₄ ) and trifluoroboron (BF ₃ ) diluted with
With a thickness of 5 nm.

Then, the substrate is heated to 170 by resistance heating vapor deposition.
ITO was vapor-deposited with a thickness of 70 nm while heating at 0 ° C. to form the transparent electrode 107.

Next, Al was vapor-deposited in a pattern as shown in FIG. 4 by electron beam vapor deposition using a mask to form a collector electrode 108.

Through the above steps, 10 so-called single layer type a-siGe solar cells each having 10 cm square were prepared.

Also, the shunt resistance is 1K per cm ^2.
A solar cell of Ω or more is irradiated with pseudo sunlight of AM1.5, 100 mw / cm ² by a solar simulator at 25 ° C. to open voltage (Voc), short circuit current (Js).
c), fill factor (FF), photoelectric conversion efficiency (η), series resistance (Rse), shunt resistance (Rsh)
The solar cell characteristics such as the above were measured, and the average value was obtained.

The results of the solar cell characteristics are summarized in Table 2.

However, the solar cell characteristics are standardized with 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 a higher open circuit voltage (Voc) and fill factor (FF) than Comparative Example 3. However, the photoelectric conversion efficiency (η) was improved.

In Comparative Example 3, Cu or Ag can serve as an acceptor when Cu or Ag diffuses into ZnO which is a defective n-type semiconductor. Therefore, ZnO has a high resistance in the vicinity of the interface with Ag. In contrast to the present invention, the amount of F added to ZnO is changed in the film thickness direction as shown in FIG. It is considered that the resistance becomes low in the vicinity of the interface with 102, the high resistance of ZnO as described above does not occur, and the series resistance of the solar cell decreases. Further, the amount of F to be added is set to the back electrode 102.
By increasing only in the vicinity of the interface of ZnO, the transmittance of ZnO does not decrease by the addition of F, and the wavelength of 400 nm
To 1000 nm, the transmittance was 85% or more.
Here, the transmittance was measured by forming ZnO with F added on a glass substrate by the above-described method.

(Comparative Example 3) In Example 2, no In was added to ZnO which is the transparent conductive layer 103, and otherwise the same as in Example 2 except that a 10 cm square so-called single layer type a was used.
10 SiGe solar cells were produced.

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 in which the amount of F added was constant in the film thickness direction was formed at a constant μmol / min. Except for that, ten so-called single-layer a-SiGe solar cells each having 10 cm square were produced in exactly the same manner as in Example 2.

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

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

[0099]

[Table 2]

Example 3 A solar cell of another example of the present invention shown in FIG. 7 was produced by the following steps.

FIG. 7 shows a stack type solar cell in which two sets of PIN junctions are laminated.

First, the surface has a Rmax of 0.1 μm or less,
A sheet-shaped stainless steel substrate having a thickness of 0.15 mm, a width of 32 cm and a length of 15 m is washed, and the treatment is performed while continuously moving the substrate between a roll for delivery and a roll for winding. 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 of Rmax of 0.6 μm was produced.

Next, Ag was deposited by the above-mentioned RF sputtering method.
50 nm of Al was laminated on the above.

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

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

After the ZnO film is formed, Al is again formed to a thickness of 5 nm by the RF sputtering method described above, and then N _{2 is} formed.
A thermal annealing process was performed at 300 ° C. for 20 minutes in the 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 spectrometry (SI
It is a graph of the result of the depth direction analysis of the Al peak by MS).

Next, the glow discharge method (GD method)
Then, the following semiconductor layers were formed.

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

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

The intrinsic a-SiGe layer 705 is composed of an n layer and ap layer.
A so-called buffer layer having a composition that continuously changes from a-SiGe to a-Si in 30 nm increments near the layer is provided.

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

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

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 having a thickness of 4 nm was formed.

Next, the substrate is heated at 170 ° C. by resistance heating vapor deposition.
While heating to 70 nm, ITO is vapor-deposited to a thickness of 70 nm to form a transparent electrode 7.
Formed 10.

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

Next, Al was vapor-deposited in a pattern as shown in FIG. 4 by an electron beam vapor deposition method to form a collector electrode 711.

Through the above steps, so-called Si /
300 SiGe two-layer stack type solar cells were produced.

The shunt resistance is 1K per cm ^2.
A solar cell of Ω or more is irradiated with pseudo sunlight of AM1.5, 100 mw / cm ² by a solar simulator at 25 ° C. to open voltage (Voc), short circuit current (Js).
c), fill factor (FF), photoelectric conversion efficiency (η), series resistance (Rse), shunt resistance (Rsh)
The solar cell characteristics such as the above were measured, and the average value was obtained.

The results of the solar cell characteristics are summarized in Table 3.

However, the solar cell characteristics are standardized with 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 reduces the series resistance, and the open circuit voltage (Voc), the short circuit current (Jsc) and the fill factor (FF) are respectively reduced. The photoelectric conversion efficiency (η) was improved.

In addition to the effects of the first and second embodiments, the addition amount of Al monotonically increases as it goes from the n-type a-Si layer 704 to the back surface electrode 702 except for the vicinity of the interface. The refractive index gradually decreases due to
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 trapping effect due to the scattering of long wavelength light was increased.

(Comparative Example 5) In Example 3, the same procedure as in Example 3 was performed except that Al was not added to ZnO which is the transparent conductive layer 703.
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, 1 was added to Al.
By performing sputtering using a ZnO target added with 0%, a transparent conductive layer 103 of ZnO having a constant Al addition amount in the film thickness direction was formed. In this case ZnO
Al was not vapor-deposited before and after the film formation. Other than that is exactly the same as the third embodiment, 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 transparent conductive layer 703 has a low resistance, the series resistance is reduced as compared with Comparative Example 5, and the open circuit voltage (Voc) and the fill factor (FF) are improved. However, since Al is added over the entire thickness direction of ZnO, the transmittance of ZnO is reduced, the reflection from the back electrode is reduced, and the short-circuit current (J
sc) was decreased, and the photoelectric conversion efficiency (η) was not improved as compared with Comparative Example 5.

[0130]

[Table 3]

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

First, as the substrate 901, the surface has an Rmax of 0.1 μm or less, a thickness of 0.18 mm, and a width of 32 cm.
A sheet-shaped polyimide film having a length of 10 m was used as a substrate for cleaning, and the following treatment was carried out by the so-called roll-to-roll method similar to that in Example 3.

First, with 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 form a concavo-convex shape that scatters light having a Rmax of 0.6 μm. Next, an O ₂ gas was introduced while keeping the substrate temperature at 180 ° C. by an unillustrated so-called ion plating device equipped with two electron beam vapor deposition devices and two dielectric coils for applying RF high frequency in the chamber, ITO was vapor-deposited to form a transparent conductive layer 903 having a thickness of 0.3 μm.

At this time, I
By independently evaporating n and Sn, the amount of Sn added to In ₂ O ₃ can be freely controlled. Then, by changing the amount of electron beam current in the vapor deposition of Sn as shown in FIG. 10, the amount of Sn added to In ₂ O _{3 is} changed in the film thickness direction of the transparent conductive layer 903 as shown in FIG. I was able to do something. At this time, the addition amount of Sn was obtained by repeating Auger electron spectroscopy and sputtering.

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

First, while heating the substrate at 180 ° C., a p-type CdTe film 904 having a thickness of 0.35 μm was formed by vapor deposition.

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

Next, the substrate is heated to 170 ° C. by resistance heating vapor deposition.
While heating to 90 nm, ITO is vapor-deposited to a thickness of 70 nm to form a transparent electrode 9.
06 was formed.

Next, Ag paste was printed by screen printing in a pattern as shown in FIG. 4 to form a collecting electrode 907.

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

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

Through the above steps, so-called CdS of 10 cm square
200 CdTe solar cells were produced. And, the more solar cells 1KΩ per 1cm ² shunt resistance, 2
AM1.
Irradiation with 5, 100 mw / cm ² of pseudo sunlight, open circuit 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 an average value was obtained.

The results of the solar cell characteristics are summarized in Table 4.

However, the solar cell characteristics are standardized with 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 a small series resistance, and has an open circuit voltage (Voc), a short circuit current (Jsc) and a fill factor (FF), respectively. The photoelectric conversion efficiency (η) was improved.

This is because the addition amount of Sn to In ₂ O ₃ is changed in the film thickness direction as shown in FIG.
n ₂ O ₃ becomes n-type with low resistance in the vicinity of the interface with the p-type CdTe layer 904, and n ₂ O ₃ and the transparent conductive layer 903 and p-type CdTe layer 904.
It is considered that the series resistance was reduced by forming the tunnel junction at the interface with the layer 904. In addition, it is considered that the series resistance was reduced due to the low resistance of In ₂ O ₃ even in the vicinity of the interface between the transparent conductive layer 903 and the back electrode 902. Furthermore, the addition amount of Sn is
As it goes from the p-type CdTe layer 904 to the back surface electrode 902, it monotonically decreases except near the interface, so that the reflection of light at the interface between the p-type CdTe layer 904 and the transparent conductive layer 903 decreases and the long wavelength It is considered that the light trapping effect due to the scattering of light is increased and the short-circuit current (Jsc) is improved.

(Comparative Example 7) In Example 4, In ₂ O ₃ containing no Sn was used as the transparent conductive layer 903.
Except for that, in the same manner as in Example 4, 200 so-called Cds / CdTe solar cells of 10 cm square were prepared.

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 was used.}
An ITO transparent conductive layer 903 having a constant addition amount of Sn of 10% was formed. Except for that, 200 so-called CdS / CdTe solar cells of 10 cm square were manufactured in exactly the same manner as in Example 4.

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

By adding Sn to In ₂ O ₃ by 10%, the transparent conductive layer 903 became low in resistance, but the series resistance was not sufficiently reduced, and the photoelectric conversion efficiency (η) was lower than that in Example 4. It was low. This is probably because the barrier at the interface between the p-type CdTe layer 904 and the transparent conductive layer 903 could not be sufficiently reduced. Further, when the addition amount of Sn was kept constant at 40%, the series resistance was sufficiently reduced, but the transmittance was lowered.

[0152]

[Table 4]

[0153]

Since the present invention is constructed as described above, it has the following effects.

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

[Brief description of drawings]

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

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

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

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

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

FIG. 6 is a graph showing a second example of changes in film thickness of an element added to the compound of the transparent conductive layer of the photovoltaic device according to the example of the present invention.

FIG. 7 is a cross-sectional view of another example of the photovoltaic element of the present invention.

FIG. 8 is a graph showing a third example of changes in film thickness of an element added to the compound of the transparent conductive layer of the photovoltaic device according to the example of the present invention.

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

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

FIG. 11 is a graph showing a fourth example of changes in film thickness of an element added to the compound of the transparent conductive layer of the photovoltaic device according to the example of the present invention.

[Explanation of symbols]

 101, 701, 901 Substrate 102, 702, 902 Back surface 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 Layer 107,710,906 Transparent electrode 108,711,907 Current collecting electrode 301 Vacuum container 302 Support part 303 Heating plate 304 Thermocouple 305 Temperature controller 306 Heater 307 Heat transfer plate 308 Substrate 309 Substrate retainer 310 Target 311 Target stand 312 Magnet 313 Cooling water introduction pipe 314 Sputtering 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 (5)

[Claims] 1. In a photovoltaic device having a transparent conductive layer between a back electrode provided on the side opposite to a light incident surface and a photoelectric conversion layer made of a semiconductor, the conductivity of the transparent conductive layer is changed. A photovoltaic element comprising an element, the addition amount of the element varying in the film thickness direction. 2. The photovoltaic element according to claim 1, wherein the compound forming the transparent conductive layer is a conductive oxide. 3. The photovoltaic element according to claim 1, wherein the added amount of the element that changes the conductivity is increased in the vicinity of the interface with the semiconductor layer. 4. The photovoltaic element according to claim 1, wherein the added amount of the element that changes the conductivity is increased near the interface with the back electrode. 5. The additive amount of the element that changes the conductivity is monotonically decreased over at least a certain film thickness range from the interface with the semiconductor layer to the interface with the back electrode. Claims 3 and 4
Photovoltaic device according to item 1.