JPH11195801A - Photovoltaic element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent reflection of light on the surface of a transparent electrode layer for higher transmissivity as well as the reflection of light on the boundary surface between the transparent electrode layer and a semiconductor layer so as to make the transparent layer to be at low resistance, by allowing the surface shape of the cross section of the transparent electrode layer formed together with a metallic layer and a semiconductor layer to have a specific fractal dimension. SOLUTION: A surface shape of the cross section of a transparent electrode layer 101 has a fractal property, and the fractal dimension D is in a image between 1.001 and 1.250. The fractal structure has a visually characteristic shape that is a similar figure to the original one when it is magnified to various sizes, and its shape is a self-similar figure. Further, the surface of a metallic layer 112 has a fractal structure, and the transparent electrode layer 101 has a fractal dimension reflecting the fractal structure of boundary surface of a semiconductor layer 114, and the inplane direction of the layer 101 preferably has a fractal structure of 10 nm or more for example.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photovoltaic device using a non-single-crystal semiconductor such as hydrogenated amorphous silicon (abbreviated as a-Si: H) or a compound semiconductor such as CdS / CuInSe. In particular, the present invention relates to a transparent electrode layer for a solar cell element.

[0002]

2. Description of the Related Art In recent years, in order to make effective use of solar energy, a-Si has been used as a solar cell device in which a semiconductor substrate can be obtained by a relatively simple manufacturing process and a panel-like element assembling operation is easy. : An apparatus using an H thin film has been developed and put to practical use.

[0003] In this type of solar cell element, as shown in FIG. 1, a back electrode layer comprising a metal layer 112 and a back conductive layer 111 is formed on a conductive substrate 113 made of, for example, stainless steel. A-Si: H is grown over the back electrode layer, and an impurity such as phosphorus or boron is appropriately introduced to form a semiconductor layer 114 having photovoltaic power. The solar cell is formed by forming the transparent electrode layer 101 on the semiconductor layer 114.

In an amorphous solar cell, a transparent electrode layer 10 is used.
Reference numeral 1 denotes an electrode and also serves as an antireflection film, and the thickness of the transparent electrode layer 101 must be determined in consideration of the surface reflection of the semiconductor layer 114. For example, as shown in “Amorphous solar cell (by Kiyoshi Takahashi)” and the like, since the collected spectrum of the amorphous solar cell has a peak at 500 to 550 nm, the thickness of the transparent electrode layer 101 is optimally around 70 nm to 200 nm. It has been.

As the transparent electrode layer 101, there are those using a metal thin film such as Au or an oxide film such as In ₂ O _3. However, in addition to appropriate conductivity and high transmittance, excellent film strength is obtained. Oxide films are often used for transparent electrode layers for solar cells. In ₂ O ₃ film doped with tin oxide (typically ITO
Has a low specific resistance of about 2 × 10 ⁻⁴ Ωcm, and has a resistivity of 3 to 100 Ω in a thickness range of 50 nm to 600 nm.
/ □ is obtained.

However, the transmittance of the film is 80 to 90% when the substrate is made of ordinary soda glass, and the transmission loss is mostly due to the surface reflection of the film. Since a normal transparent electrode shows much higher film reflection than bare glass, the visible region (300 to 1200 n) used for photoelectric conversion is used.
m) does not sufficiently reach the lower a-Si: H layer,
There is a problem that the photoelectric conversion efficiency is reduced. For example, in the case of ITO, as the particle size increases, the influence of light scattering appears, and at the same time, unoxidized metal In is generated, which causes absorption. As a result, the permeability of the membrane is significantly reduced due to scattering and absorption.

Therefore, the particle size of ITO is set to about 200 nm,
There has been proposed a method for preventing a large scattering on the surface of the transparent electrode layer by making the particle size considerably smaller than the wavelength of visible light.

Further, as the transparent electrode layer, SiO ₂ , Sn
When O ₂ , Al ₂ O _3, or the like is used, since silicon has a higher refractive index, reflection on the surface of the semiconductor layer proceeds, and there is a problem that light cannot be absorbed and photoelectric conversion can be performed effectively.

As means for preventing reflection on the surface of the semiconductor layer, at least one of a magnesium fluoride film, zinc oxide and zinc sulfide is laminated on the In ₂ O ₃ film.
Alternatively, there has been proposed a method in which an In ₂ O ₃ film is sandwiched between an aluminum oxide film and a magnesium fluoride film to form an antireflection film.

A current collecting electrode and a protective layer are formed on the transparent electrode layer. However, if the adhesion between the transparent electrode layer and the current collecting electrode is poor, the series resistance is too high and the When the solar cell is used outdoors for a long period of time, the characteristics may be deteriorated, and peeling between the transparent electrode layer and the current collecting electrode or the protective layer may become a problem.

When a protective layer having a refractive index of 1.3 or more is provided, there is a problem that light loss due to reflection at the interface between the transparent electrode layer and the protective layer cannot be sufficiently suppressed.

[0012]

SUMMARY OF THE INVENTION The present invention solves the above problems, suppresses surface light reflection of a transparent electrode layer, improves transmittance, and suppresses light reflection at the interface between the transparent electrode layer and the semiconductor layer. Another object is to obtain a transparent electrode layer having a low resistance.

[0013]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above problems, and as a result, the transparent electrode layer whose surface shape has a specific fractal dimension optimizes the transmitted light reaching the solar cell. The present inventors have found that an ideal anti-reflection film necessary for the formation can be formed, and have reached the present invention.

That is, the present invention relates to a photovoltaic device having at least a metal layer, a semiconductor layer, and a transparent electrode layer, wherein the cross-sectional surface of the transparent electrode layer has a fractal property and a fractal dimension. D is 1.001 ≦ D ≦ 1.25
0 is a photovoltaic element.

[0015]

Since the surface of the transparent electrode layer of the present invention has a self-similar shape, it is a relatively uniform film having fine irregularities on the surface of the transparent electrode layer, and does not cause large scattering with respect to the wavelength of visible light.
Therefore, the incident light can be collected in the solar cell without being reflected. Further, in the transparent electrode layer of the present invention, the growth of the film particles is suppressed, the filling rate of the film is increased, the refractive index is increased, and the reflection at the transparent electrode layer / semiconductor layer interface is suppressed. The amount of light absorbed is increased, and a good anti-reflection film is formed as a solar cell transparent electrode layer.

The details of the film structure of the transparent electrode when it has a fractal structure are unknown, but can be considered as follows. The back reflection layer of a solar cell is generally formed of a metal layer and a transparent conductive layer. The metal layer is made of a metal having high conductivity and high reflectivity, is formed by metal bonding, and atoms are regularly arranged in atomic size. In general, metal has a size of several nanometers to several millimeters or more, and metal grains have grain boundaries. Therefore, it is considered that the surface of the metal layer has a fractal structure having a basic shape of metal particles of several nm to several mm.

It can be inferred that the transparent conductive layer formed on the metal layer also has a fractal structure based on crystal grains of usually several tens nm to several mm when polycrystal is used.

Further, the semiconductor layer as a base of the transparent electrode layer has a short-range order even if it is amorphous,
The entire semiconductor layer is configured with a set of atoms having a size of several nm as a basic shape. Therefore, it is considered that the semiconductor layer has a fractal structure whose basic shape is the shape of an atomic aggregate of several nm.

Since the metal layer and the transparent conductive layer reflect more light to the semiconductor layer, the outermost surface of the transparent conductive layer generally has an uneven structure of about the wavelength of light (several hundred nm to several μm). . Therefore, the semiconductor layer also has an uneven structure on the surface of the transparent conductive layer.

In the solar cell, a transparent electrode layer is formed on the above-described metal layer, transparent conductive layer, and semiconductor layer which are sequentially laminated. Therefore, the surface shape of the transparent electrode layer is inherited from the shape of the outermost surface of the semiconductor layer, and is randomly spread on the surface of the transparent electrode layer based on an uneven structure having a height and a width of several μm.

Furthermore, the transparent electrode layer may have a polycrystalline structure and form crystal grains of several tens of nm or more. Even if the crystal grains of the transparent electrode layer itself are used as the basic shape, the shape of the surface of the transparent electrode layer is affected. Is receiving.

The transparent electrode of the present invention has a self-similar fractal structure. In order to reduce surface reflection by suppressing irregularities having a size of several μm or more, the range of the fractal structure is reduced from the wavelength of light to several tens of wavelengths. By providing a fractal structure over the range of nm, it is expected that the surface scattering will be reduced as a transparent electrode to provide a good antireflection film.

The transparent electrode of the present invention may be produced by, for example, a sputtering apparatus as shown in FIG. The raw material gas at that time is Ar, Ne, Xe, He, Kr, O ₂ , H
₂ S, CH _{4 and the} like are often used. Changing the gas flow at this time changes the mean free path of the plasma reaction. Therefore, the speed of the ions colliding with the target changes, and the sputtering speed changes.

For example, when Ar and H ₂ S are used as source gases,
Considering the case where ITO is used as the target, the sputtering rate is generally slow because H ₂ S ions are lighter than heavy atoms such as Ar ions, and the deposition rate of the formed transparent electrode film is reduced. For this reason, when H ₂ S is used as compared with Ar, radicals fly at a lower rate to the substrate, and act in a direction to promote the growth of crystal grains of the transparent electrode ITO. Bring. When the ions collide with the target, they are inelastic collisions, and the sputtering rate of each element is different from the element composition of the target. Therefore, the type, density, and composition of radicals jumping out of the target change, so that the composition of the sputter target is different from the composition of the transparent electrode formed by sputtering, which also affects the crystal growth of ITO.

In the case of ITO, Sn for improving conductivity is used.
Is increased to increase the carrier concentration, or the crystal orientation is improved to improve the traveling properties of the carrier. However, in the former case, simply increasing the Sn concentration increases light absorption in the transparent electrode, further deteriorating the crystallinity of ITO, and lowering the refractive index of the film. Alternatively, if the crystallinity is too high, the refractive index increases and the reflection on the transparent electrode surface increases.

In the transparent electrode having a fractal structure according to the present invention, even when an oxide target such as ITO is used,
Pre-sputtering the target to control the Sn concentration and crystal grain size, suppress light reflection on the electrode surface, and improve the substantial transmittance of the film, including some scattered light commensurate with the reduction in reflection. Hope that will happen.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described.

FIG. 1 is a sectional view showing an example of the photovoltaic element of the present invention. However, the present invention is not limited to the photovoltaic element having the configuration shown in FIG.

In FIG. 1, 113 is a substrate, 112 is a metal layer, 111 is a transparent conductive layer, 104, 107 and 110
Is an n-type semiconductor layer, 103, 106 and 109 are i-type semiconductor layers, 102, 105 and 108 are p-type semiconductor layers, 10
1 is a transparent electrode layer.

The photovoltaic element shown in FIG. 1 has a configuration in which light is incident from the side opposite to the substrate 113. However, the photovoltaic element having a configuration in which light is incident from the substrate side is the same as FIG. Each layer may be laminated in the reverse order.

Hereinafter, each layer of the photovoltaic device of the present invention will be described in detail.

(Transparent Electrode Layer 101) The transparent electrode layer is an electrode on the light incident side that transmits light, and also functions as an anti-reflection film by optimizing its thickness.

The transparent electrode layer is required to have a high transmittance in a wavelength region where the semiconductor layer can absorb light and to have a low resistivity. Preferably, the transmittance at 550 nm is at least 80%, more preferably at least 85%. Further, the resistivity is preferably 5 × 10 ⁻³ Ωc.
m, more preferably 1 × 10 ⁻³ Ωcm or less.

The refractive index of light having a wavelength of 1000 nm is preferably 1.91 or more, and the refractive index is preferably increased in the depth direction toward the semiconductor layer 114.

The thickness of the transparent electrode layer is not particularly limited.
It is preferably 70 nm or less.

The transparent electrode layer of the present invention has a fractal property in the surface shape of the cross section and a fractal dimension D.
Is 1.001 ≦ D ≦ 1.250.

Here, the fractal structure means that the shape is visually similar to the original shape obtained by expanding them into various sizes with one characteristic shape. Say they are self-similar. When something is a fractal, there are generally upper and lower bounds, and dimensions can be defined by positive non-integer values. In the case of a transparent electrode layer, the number N (r) of squares that divide the surface shape of the transparent electrode layer when viewed from an arbitrary cut plane by a lattice with an interval r and include a part of the cut plane
Count. When r is changed so that N (r) becomes at least 1 or more, when the relationship of N (r) ∝r ^−D is satisfied, the surface of the transparent electrode layer has a D-dimensional fractal within a range where r is changed. It is said that.

Further, the surface of the metal layer 112 has a fractal structure, the transparent electrode layer has a fractal dimension reflecting the fractal structure at the interface of the semiconductor layer 114, and the fractal structure is within a range of 10 nm or more in the in-plane direction of the transparent electrode layer. It is preferable to have

Further, it is preferable that the deposition is performed so that the fractal dimension of the shape of the outermost surface of the deposition gradually increases. That is, the fractal dimension is low at the interface between the semiconductor layer 114 and the transparent electrode layer, and the fractal dimension is increased as the transparent electrode layer grows. Therefore, the surface of the formed transparent electrode layer has a high fractal dimension, a large surface area of the transparent electrode layer, and good adhesion between the transparent electrode layer and the protective layer.

It is preferable that the crystal grains constituting the transparent electrode layer have fractal properties.

The average of the inclination angles of the surface irregularities of the transparent electrode layer is preferably 15 degrees or more. The average tilt angle of the surface irregularities is obtained by calculating the following formula with 1mm square is represented by the following formula when the height Y _i from the reference plane at a point X _i.

[0042]

(Equation 1)

As the material of the transparent electrode layer, In ₂ O ₃ , S
nO ₂ , ITO (In ₂ O ₃ + SnO ₂ ), ZnO, Cd
O, Cd ₂ SnO ₄ , TiO ₂ , Ta ₂ O ₅ , Bi ₂ O ₃ , M
Conductive oxides such as oO ₃ and Na _x WO ₃ or mixtures thereof are preferably used.

Of these, In ₂ O ₃ , SnO ₂ ,
At least one selected from ITO, CdO, and ZnO is preferable.

Further, an element (dopant) that changes the conductivity may be added to these compounds. For example, when the transparent electrode layer is ZnO, Al, In, B, Ga, S
When i, F, etc. are In ₂ O ₃ , Sn, F, T
e, Ti, Sb, Pb, etc., and in the case of SnO ₂ ,
F, Sb, P, As, In, Tl, Te, W, Cl, B
r, I, etc. are preferably used.

As a method for forming the transparent electrode layer, a spray method, a CVD method, a plasma CVD method, a plating method, a vacuum evaporation method, an ionization evaporation method, a sputtering method, a spin-on method, a dipping method and the like are suitably used.

Of these, Ar, H ₂ , He, N
e, Xe, Kr, sputtering using at least one as an introduction gas selected from H ₂ S, ion deposition, CV
The D method or the plasma CVD method is preferred.

The transparent electrode layer may have a multilayer structure of two or more layers.

(Metal Layer 112) The metal layer used in the present invention is an electrode arranged on the back surface of the semiconductor layer 114 in the light incident direction.

As the material of the metal layer, for example, gold, silver,
Examples include metals such as copper, aluminum, nickel, iron, chromium, molybdenum, tungsten, titanium, cobalt, tantalum, niobium, and zirconium, and alloys such as stainless steel. Among them, metals having high reflectivity, such as aluminum, copper, silver, and gold, can serve also as a light reflection layer that reflects light that could not be absorbed by the semiconductor layer to the metal layer again to the semiconductor layer. .

The metal layer may be formed by laminating two or more kinds of materials in two or more layers.

The shape of the metal layer may be flat. However, the metal layer has a concave and convex shape for scattering light, so that long-wavelength light that cannot be absorbed by the semiconductor layer 114 is scattered, and It is possible to extend the optical path length of the photovoltaic element, improve the long-wavelength sensitivity of the photovoltaic element, increase the short-circuit current, and improve the photoelectric conversion efficiency. In the uneven shape for scattering light, the height difference between the peaks and valleys of the unevenness is 0.2 μm to 2.
Desirably, it is 0 μm.

When the substrate 113 also serves as a metal layer, it may not be necessary to form a metal layer.

The metal layer is formed by a vapor deposition method, a sputtering method,
A plating method, a printing method, or the like is used. In such a case, the metal layer can be formed into an uneven shape that scatters light by dry etching, wet etching, sand blasting, heating, or the like of the formed metal or alloy film. Also, by depositing the above-mentioned metal or alloy while heating the substrate 113, it is possible to form an uneven shape for scattering light.

(Transparent Conductive Layer 111) The transparent conductive layer is disposed between the metal layer 112 and the semiconductor layer 114 mainly for the following purposes.

That is, the irregular reflection on the back surface of the photovoltaic element is improved, light is confined in the photovoltaic element by multiple interference by the thin film, the optical path length in the semiconductor layer is extended, and the short-circuit current of the photovoltaic element is reduced. (Jsc), then
By preventing the metal of the metal layer 112 from diffusing or migrating into the semiconductor layer 114 and causing shunting of the photovoltaic element, and by providing the transparent conductive layer with a slight resistance value, Is to prevent a short circuit generated by a defect such as a pinhole in the semiconductor layer 114 between the metal layer 112 and the transparent electrode layer 101 provided therebetween.

The transparent conductive layer is required to have a high transmittance in a wavelength region that can be absorbed by the semiconductor layer 114 and to have an appropriate resistivity. Preferably, the transmittance at 650 nm or more is 80% or more, more preferably 85% or more, and most preferably 90% or more. Further, the resistivity is preferably 1 × 10 ⁻⁴ Ωcm to 1 × 10 ⁶ Ωcm, and more preferably 1 × 10 ⁻² Ωcm to 5 × 10 ⁴ Ωcm.

As the material of the transparent conductive layer, In ₂ O ₃ , S
nO ₂ , ITO (In ₂ O ₃ + SnO ₂ ), ZnO, Cd
O, Cd ₂ SnO ₄ , TiO ₂ , Ta ₂ O ₅ , Bi ₂ O ₃ , M
Conductive oxides such as oO ₃ and Na _x WO ₃ or mixtures thereof are preferably used. Further, an element (dopant) that changes the conductivity may be added to these compounds.

As an element (dopant) for changing the conductivity, for example, when the transparent conductive layer 111 is ZnO,
Al, In, B, Ga, Si, F, etc., In the case of In ₂ O ₃ , Sn, F, Te, Ti, Sb, Pb, etc., and in the case of SnO ₂ , F, Sb, P, As, In, T
1, Te, W, Cl, Br, I and the like are preferably used.

As the method for forming the transparent conductive layer, various vapor deposition methods such as EB vapor deposition and sputter vapor deposition, various CVD methods, spray methods, spin-on methods, and dipping methods are suitably used.

(Semiconductor Layer 114) The material of the semiconductor layer used in the present invention is, for example, an IV such as Si, C, Ge or the like.
Using group elements, or SiGe, SiC, Si
An alloy using a group IV alloy such as Sn is used. Among these, semiconductor materials particularly preferably used for the photovoltaic device of the present invention include a-Si: H (abbreviation for crystallized amorphous silicon), a-Si: F, and a-Si: H :
F, a-SiGe: H, a-SiGe: F, a-SiG
e: H: F, a-SiC: H, a-SiC: F, a-S
Group IV and Group IV alloy-based non-single-crystal semiconductor materials such as iC: H: F.

The semiconductor layer can perform valence electron control and forbidden band width control. Specifically, a raw material compound containing an element serving as a valence electron controlling agent or a forbidden band width controlling agent when forming a semiconductor layer is used alone, or mixed with the deposited film forming raw material gas or the diluent gas to form a film. You only have to introduce it in the space.

The semiconductor layer is controlled by valence electrons.
At least a portion is doped p-type and n-type to form at least one set of pin junctions. And
By stacking a plurality of pin junctions, a so-called stack cell configuration is obtained.

As a method of forming a semiconductor layer, microwave plasma CVD, RF plasma CVD, photo CVD
Examples thereof include various methods such as CVD, thermal CVD, and MOCVD, various methods such as EB evaporation, MBE, ion plating, and ion beam, sputtering, spraying, and printing. As a method adopted industrially, a plasma CVD method in which a raw material gas is decomposed by plasma and deposited on a substrate is preferably used.
In addition, as the reaction device, a batch type device, a continuous film forming device, or the like can be used as desired.

[0065]

EXAMPLES [Examples 1 to 7, Comparative Example 1] First, a substrate 1 was prepared using a DC magnetron sputtering apparatus shown in FIG.
A metal layer 112 of silver (Ag) containing a carbon atom, an oxygen atom, and a nitrogen atom was formed on 13.

As the substrate 113, a 50 mm square, 0.2 mm thick stainless steel (SUS430) substrate was subjected to ultrasonic cleaning with acetone (CH ₃ OCH ₃ ) for 10 minutes and isopropanol (CH ₃ CHOCH ₃ ) for 10 minutes. Substrate 203 which was subjected to hot air drying at 110 ° C. for 30 minutes.
Was used.

The target 207 has a purity of 99.999%.
Silver (Ag), and is insulated from the deposition chamber 201 by an insulating support.

Reference numeral 217 denotes a gas introduction line, which is an argon (Ar) cylinder (purity: 99.9999%) and methane (CH ₄ ) diluted to 50 ppm with an argon (Ar) gas.
/ Ar) gas cylinder, 50 pp with argon (Ar) gas
m), an oxygen (O ₂ / Ar) gas cylinder diluted to 50 ppm, and a nitrogen (N ₂ /
Ar) connected to a gas cylinder.

First, the inside of the deposition chamber 201 is evacuated by a vacuum pump, and when the degree of vacuum reaches about 1 × 10 ⁻⁶ Torr,
From the gas introduction line 217, the Ar gas flow rate gradually becomes 10
sccm, CH ₄ / Ar gas flow rate is 10 sccm, O ₂ /
Ar gas flow rate is 5 sccm, N ₂ / Ar gas flow rate is 20
The pressure in the deposition chamber 201 is adjusted to 7 sccm.
The opening of the conductance valve (butterfly type) was adjusted while watching the vacuum gauge so that the pressure became mTorr.

Thereafter, the voltage of the DC power supply 210 is reduced to -480.
V, introduce DC power to the target 207,
A DC glow discharge was generated. After elapse of 5 minutes, the shutter 213 is opened to start manufacturing the metal layer 112 on the substrate 203. When the metal layer 112 having a thickness of 0.06 μm is manufactured, the shutter 213 is closed, and the output of the DC power supply 210 is turned off. The DC glow discharge was stopped.

Next, a transparent conductive layer 113 of zinc oxide (ZnO) containing carbon, oxygen and nitrogen atoms was formed on the metal layer 112.

The target 208 is made of zinc oxide (ZnO) having a purity of 99.99%, and is made of an insulating support.
1 is insulated.

Using the gas introduction line 217, the Ar gas flow rate is 10 sccm, and the CH ₄ / Ar gas flow rate is 5 sccc.
m, the conductance valve (butterfly type) while adjusting the O ₂ / Ar gas flow rate to 20 sccm and the N ₂ / Ar gas flow rate to 5 sccm, and watching the vacuum gauge so that the pressure in the deposition chamber 201 becomes 9 mTorr. The opening of was adjusted.

Thereafter, the voltage of the DC power supply 211 is reduced to -420.
V, introduce DC power to the target 208,
A DC glow discharge was generated. After a lapse of 7 minutes, the shutter 214 is opened to start production of the transparent conductive layer 111 on the substrate. When the transparent conductive layer 111 having a thickness of 3 μm is produced, the shutter 214 is closed, and the output of the DC power supply 211 is turned off. Glow discharge was stopped.

Next, a semiconductor layer was formed on the back surface conductive layer 111.

First, a first n-type a-Si: H layer 110 was formed using an RF glow discharge decomposition apparatus. At this time, the substrate temperature was maintained at 250 ° C., and the flow rates of the SiH ₄ gas and the Si ₂ H ₆ gas as the source gases were 0.5 sccm and 0.8 s, respectively.
ccm, the flow rate of the SiF ₄ gas was adjusted to 0.1 sccm, and the flow rate of the PH ₃ gas was adjusted to 1000 ppm, and the pressure in the deposition chamber was controlled by the conductance valve so as to be 0.6 Torr. Then, rf power density 0.13 W / c
m ² was deposited.

Next, a first i-type a-Si: H layer 109 was deposited on the first n-type a-Si: H layer 110 by using a microwave plasma CVD apparatus shown in FIG.

First, the reaction chamber is once evacuated to 3 × 10 ⁻³ Torr. Then, the H ₂ gas flow rate is 200 scc.
m, Ar gas flow rate is 50 sccm, He gas flow rate is 10
The gas was introduced from the gas inlet 315 so as to be sccm. After that, the substrate 302 was heated using the heater 303.

When the substrate temperature of the substrate 302 is stabilized at 200 ° C., the flow rate of SiH ₄ gas is 100 sccm, the flow rate of Ge gas is 2 sccm, and the flow rate of SiF ₄ gas is 316 from the gas line 316.
Gas flow rate is 10 sccm, He gas flow rate is 6 sccm,
The flow rate of the Ne gas was adjusted to 2 sccm, and the pressure of the deposition chamber 301 was adjusted to 30 mT using the butterfly valve 304.
orr was adjusted.

Thereafter, the microwave 30 is introduced through the introduction window 306.
0 W is introduced, RF power 10 W is applied from the rf power supply 309, and after 4 minutes, the shutter 311 is opened and the first i-type a-
When the Si: H layer 109 was formed to a thickness of 100 nm, the shutter 311 was closed, RF power and microwave output were turned off, and discharge was stopped.

Next, a first p-type a-Si: H layer 108 was formed using an RF glow discharge decomposition apparatus.

[0082] At this time, the substrate temperature was held at 200 ° C., SiH ₄ gas flow rate 1.0sccm as source gases, Si ₂
The H ₆ gas flow rate is 0.4 sccm, and the SiF ₄ gas flow rate is 0.
The flow rate was adjusted to ₂ sccm and the B ₂ H ₆ gas flow rate to 500 ppm, and the pressure in the deposition chamber was controlled by a conductance valve so as to be 0.8 Torr. Thereafter, deposition was performed with an rf power density of 0.12 W / cm ² .

Similarly, semiconductor bonding layers 102 to 107 were formed.

Finally, a transparent electrode layer 101 was deposited on the substrate formed up to the third p-layer using a DC magnetron sputtering apparatus shown in FIG.

The substrate manufactured up to the third p-layer 102 was set on the substrate holder 202. The target 215 is indium tin oxide (ITO) having a purity of 99.99%, and is insulated from the deposition chamber 201 by an insulating support.
A gas introduction line 221 is connected to a hydrogen sulfide (H ₂ S) cylinder (purity: 99.99%) and a krypton (Kr / Ar) cylinder diluted to 60 ppm with Ar.

First, the inside of the deposition chamber 201 is evacuated from the exhaust port 216 by a vacuum pump, and the degree of vacuum is about 5 × 10 ⁻³ Torr.
, The gas is gradually introduced from the gas introduction line 217 to A.
r gas flow rate is 300 sccm, CH ₄ / Ar gas flow rate is ₂ sccm, O ₂ / Ar gas flow rate is 3 sccm, N ₂ / A
The opening of the conductance valve (butterfly type) was adjusted while watching the vacuum gauge so that the flow rate of the r gas was 20 sccm and the pressure in the deposition chamber 201 was 7 mTorr.

Thereafter, the power of the DC power supply 212 is increased by 5 kW /
cm ² , DC power was introduced to the target 209 to generate a DC glow discharge. After pre-sputter cleaning of the ITO surface by DC plasma for 5 minutes using the gas shown in Table 1, the shutter 215 was opened, and the DC bias was gradually changed to the desired input power. The start-up time at that time was 1.0 s.

Then, the formation of the transparent electrode layer 101 on the substrate was started. Open the shutter and make the input power 4kW,
When the transparent electrode layer 101 having a thickness of 69 nm was formed, the shutter 215 was closed, and the output of the DC power supply 209 was turned off.
The DC glow discharge was stopped.

[0089]

[Table 1]

When the solar cell element of Comparative Example 1 fabricated up to the transparent electrode layer 101 was cut as shown in FIG. 1 and a cross-sectional TEM observation of the surface of the transparent electrode layer 101 was performed, the surface shape shown in FIG. 4 was observed. Was.

In order to check whether the transparent electrode layer 101 is fractal, as shown in FIG.
One cross section is divided by the lattice spacing r, and the number N (r) of squares including a part of the cut surface is counted. Next, the number of squares is counted again by changing the length of r. In addition, observation was performed at various TEM magnifications to determine the relationship between r and N (r), and this is shown in FIG. As shown in FIG. 6, in the cross section of the transparent electrode layer 101 of Comparative Example 1, N (r) ∝r ^−D (D = 1.00) is satisfied, and the surface of the transparent electrode layer 101 has a fractal structure. , The fractal dimension is 1.00.

The transparent electrode layer 10 of the solar cell element of Comparative Example 1
FIG. 7 shows the reflectivity of light on one surface. Light wavelength 550n
m was about 8.7%, but at a wavelength of 700 n
At m, as much as 34% of the light is reflected and is not effectively collected by the solar cell.

FIG. 8 shows the relationship between the fractal dimension and the conversion efficiency. As shown in FIG. 8, the conversion efficiency is high when the fractal dimension is in the range of 1.02 to 1.09, and the conversion efficiency is 6% or more. In particular, the transparent electrode layers 10 of Examples 1, 3, 5 to 7 and Comparative Example 1 whose conversion efficiency is 6% or more.
1 was analyzed by X-ray diffraction.
On the other hand, the transparent electrode layer 101 of the example is formed of crystal grains of 70 to 120 nm, while the denseness of the film is higher than that of the comparative example 1.
The refractive index of Comparative Example 1 was 1.8, whereas that of Example was 2.2. Therefore, the thickness of the transparent electrode layer 101 as an anti-reflection film was changed from 69 nm of Comparative Example 1 to 66 nm.
And the transmittance could be improved by about 5%.

[Examples 8 to 14] In forming the transparent electrode layer 101, after opening the shutter 215 for ITO sputtering, a desired DC power of 5 k was applied for a rise time shown in Table 2.
W / cm ² , the DC power was reduced from 5 kW / cm ² to 1 kW / cm ^{2 at} the fall time shown in Table 2 from the vicinity of the ITO layer thickness of 65 nm, and at the time of DC sputtering as shown in Table 2, A solar cell element was prepared in the same manner as in Example 1, except that whether the substrate side was grounded or not was changed. Table 2 shows the results.

[0095]

[Table 2]

The reflectivity in Table 2 is a relative value of the surface reflectivity of light having a wavelength of 550 nm when light having a wavelength of 300 to 2000 nm is incident on a solar cell element.

The transparent electrode layer 101 of the sample of Example 10 having the highest reflectance was analyzed by X-ray diffraction.
The signal height from the c-axis of TO increased, and the signal width decreased. From this, it was found that the crystallinity of ITO was improved. Further, the fractal dimension was 1.07, and it is considered that the resistance was reduced and the conversion efficiency of the solar cell was improved due to the improvement in the phototacticity of carriers due to the improvement in crystallinity.

Example 15 A solar cell element was fabricated in the same manner as in Example 1 except that ITO was deposited to a thickness of 65 nm and then ZnO was deposited to a thickness of 5 nm using a target 214 in forming the transparent electrode layer 101. did. The thickness of ZnO was changed so that reflection of incident light at 550 nm from the transparent electrode layer 101 ITO / ZnO at that time was minimized.

As a result, the shape of the ZnO surface was finer than that of the ITO surface, and the surface reflection was reduced. Transparent electrode layer 1
The fractal dimension of 01 was 1.10. The average of the inclination angles at that time was 22 °.

Further, a current collecting electrode was provided on the transparent electrode layer 101, lamination was performed, and the adhesion between the transparent electrode and the lamination was tested under high humidity. Also improved the characteristics.

[0102] In the formation of Example 16 transparent electrode layer 101, while the input power during ITO film deposition was changed from 1 kW / cm ² to 5 kW / cm ^2, other conditions Example 1
Under the same conditions as in Example 5, a solar cell element was produced.

[0102] The transparent electrode layer 10 was
The fractal dimension of 1 was found to be 1.2.
The average of the inclination angles at that time was 25 °.

Silver was formed thereon as a current collecting electrode by a vacuum evaporation method using resistance heating.
When a durability test was performed under the conditions of 5 ° C. and 85% humidity,
It remained in the usable area for 120 hours. In Comparative Example 1, peeling occurred between the transparent electrode layer 101 and the current collecting electrode in 20 hours, making it unusable.

Example 17 In forming the transparent electrode layer 101, the flow rate of the Kr / Ar gas was set to 0.5 sccm during the film formation.
While the other conditions were changed from Example 1 to 5 sccm.
Under the same conditions as in Example 5, a solar cell element was produced. Transparent electrode layer 1
During the film formation of 01, the refractive index near the growth surface was measured with an ellipsometer every 5 nm, and as the film was formed, the refractive index decreased and approached 1.4. The average of the inclination angles at that time was 32 °.

When the sample was irradiated with light and the reflectance of light at 550 nm was measured, the amount of light reflected at the interface between the semiconductor layer and the transparent electrode layer and returned to the transparent electrode layer was smaller than that in Comparative Example 1. Was. Therefore, light was sufficiently absorbed by the semiconductor layer, and photoelectric conversion could be efficiently generated.

Example 18 A solar cell element was produced in the same manner as in Example 15 except that the flow rate of H ₂ S was changed to 1 sccm in forming the transparent electrode layer 101. Cross-sectional TEM observation revealed that the ITO crystal grains had fractal shape. The fractal dimension was 1.05 and the surface was smooth. The surface area of each fine crystal grain was small and very good. The average of the inclination angles at that time was 28 °.

After attaching the current collecting electrode, lamination was performed. Then, together with Comparative Example 1, a temperature of 85 ° C. and a humidity of 85
%, After 100 hours, a film peeling test was performed. As a result, the film was harder to peel than Comparative Example 1.

[0108]

According to the present invention, a transparent electrode having a high refractive index and a small film thickness can be produced. Furthermore, this transparent electrode had high crystallinity and low resistance.
By using the transparent electrode of the present invention, a photovoltaic element having high conversion efficiency could be manufactured. Especially,
In the case of a transparent electrode that is fractal within the range defined by claims 2 to 4, a photovoltaic element with little reflection on the surface and high conversion efficiency was obtained. Further, the fractal dimension transparent electrode as described in claim 5 has finer irregularities on the surface, and is a transparent electrode excellent as an antireflection film. According to claim 6, by mixing various impurities, the plasma radical is changed, the crystallinity of the transparent electrode is improved, the resistance is further reduced, the refractive index is controlled to reduce the film thickness, and the transmittance is good. An excellent transparent electrode could be formed. According to claim 7, the adhesion to the lamination is improved, and an excellent solar cell which is strong in the peeling test can be produced. According to Claims 8 and 9, a transparent electrode excellent in reducing reflection at the semiconductor / transparent electrode interface and improving photoelectric conversion efficiency could be formed. Claim 11
Thus, the adhesion to the lamination for surface protection was improved, the reflection of light at the transparent electrode / lamination interface was suppressed, and the light collection efficiency of the solar cell could be increased.
From the above, it was found that the transparent electrode of the present invention was excellent.

[Brief description of the drawings]

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

FIG. 2 is a schematic diagram of a DC magnetron sputtering device used in an example.

FIG. 3 is a schematic view of a microwave plasma CVD apparatus used in an example.

FIG. 4 is a view showing a surface shape of a transparent electrode of Comparative Example 1.

FIG. 5 is a diagram in which the surface shape of a transparent electrode of Comparative Example 1 is divided by a grid interval r.

FIG. 6 is a graph showing the relationship between r and N (r) of the transparent electrode of Comparative Example 1.

FIG. 7 is a graph showing the reflectance of the solar cell element of Comparative Example 1.

FIG. 8 is a graph showing a relationship between a fractal dimension and a conversion efficiency.

[Explanation of symbols]

 101 Transparent electrode layer 102 Third p layer 103 Third i layer 104 Third n layer 105 Second p layer 106 Second i layer 107 Second n layer 108 First p layer 109 First i-layer 110 first n-layer 111 transparent conductive layer 112 metal layer 113 substrate 201 deposition chamber 202 substrate holder 203 substrate 204 heating heater 205 rf matching box 206 rf power supply 207-209 target 210-212 DC power supply 213-215 shutter 216 exhaust Port 217, 221 Gas introduction line 218 Holder support shaft 219 Plasma space 220, 222 Introduced gas 301 Deposition chamber 302 Substrate 303 Heater 304 Butterfly valve 305 Microwave waveguide 306 Introducing window 307 Microwave introducing ceramic window 308 Bias rod 309 f Power 310 plasma space 311 shutter 312 produced gas 313 exhaust gases 314 outlet 315 gas inlet 316 gas lines

Claims (12)

[Claims] 1. A photovoltaic device having at least a metal layer, a semiconductor layer, and a transparent electrode layer, wherein the cross-sectional surface of the transparent electrode layer has a fractal property and a fractal dimension D is 1. A photovoltaic device, wherein 001 ≦ D ≦ 1.250. 2. The method according to claim 1, wherein the transparent electrode layer is made of In ₂ O ₃ , SnO ₂ , I
mixtures nO ₂ -SnO _2, CdO, photovoltaic element according to claim 1, characterized in that comprises at least one selected from ZnO. 3. The photovoltaic device according to claim 1, wherein the transparent electrode layer has a refractive index of 1.91 or more with respect to light having a wavelength of 1000 nm. 4. The photovoltaic device according to claim 1, wherein the transparent electrode layer has a thickness of 70 nm or less. 5. The transparent electrode layer is made of Ar, H ₂ , He, N
e, Xe, Kr, sputtering using at least one as an introduction gas selected from H ₂ S, ion deposition, CV
The photovoltaic device according to any one of claims 1 to 4, wherein the photovoltaic device is deposited by a D method or a plasma CVD method. 6. The metal layer surface has a fractal structure,
The transparent electrode layer has a fractal dimension reflecting the fractal structure of the interface of the semiconductor layer, and has a 10
The photovoltaic device according to any one of claims 1 to 5, wherein the photovoltaic device has a fractal structure in a range of not less than nm. 7. The photovoltaic device according to claim 1, wherein the transparent electrode layer has a multilayer structure of two or more layers. 8. The photovoltaic device according to claim 1, wherein the transparent electrode layer is deposited such that the fractal dimension of the shape of the outermost surface of the deposition is gradually increased. 9. The semiconductor device according to claim 1, wherein the refractive index of the transparent electrode layer increases in the depth direction toward the semiconductor layer.
3. The photovoltaic device according to claim 1. 10. The photovoltaic device according to claim 1, wherein the crystal grains constituting the transparent electrode layer have fractal properties. 11. The method according to claim 1, wherein the average of the inclination angles of the surface irregularities of the transparent electrode layer is 15 degrees or more.
3. The photovoltaic device according to claim 1. 12. The photovoltaic device according to claim 1, wherein the semiconductor layer is a non-single-crystal semiconductor layer.