JP5093502B2 - Thin film solar cell and surface electrode for thin film solar cell - Google Patents

Description

  The present invention relates to a thin film solar cell having excellent photoelectric conversion efficiency and a surface electrode for the thin film solar cell.

  In recent years, solar cells using crystalline silicon thin films such as polycrystalline silicon and microcrystalline silicon, and amorphous silicon thin films have been vigorously developed. In the development of these thin film solar cells, the objective is to achieve both low cost and high performance by forming a high-quality silicon thin film on an inexpensive substrate by a low temperature process.

As one of such thin-film solar cells, a photoelectric conversion semiconductor layer in which a surface electrode made of a transparent conductive film and a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer are stacked in this order on a light-transmitting substrate And a structure in which a back electrode including a light-reflective metal electrode is sequentially formed is known.
In this thin-film solar cell, the photoelectric conversion action mainly occurs in the i-type semiconductor layer. Therefore, if the i-type semiconductor layer is thin, light in a long wavelength region having a small light absorption coefficient is not sufficiently absorbed. In particular, it is restricted by the film thickness of the i-type semiconductor layer. Therefore, in order to use light incident on the photoelectric conversion semiconductor layer including the i-type semiconductor layer more effectively, a surface uneven structure is provided on the surface electrode on the light incident side to scatter light into the photoelectric conversion semiconductor layer, A device has been devised to diffusely reflect the light reflected by the back electrode.

  In such a silicon-based thin film solar cell having a surface uneven structure on the surface electrode on the light incident side, a tin oxide film is widely used as the surface electrode. However, a tin oxide film having a surface concavo-convex structure is expensive because a high temperature process of 500 ° C. or higher is required, and because the film resistance is high, increasing the film thickness decreases the transmittance, resulting in photoelectric conversion. There is a problem that the conversion efficiency is lowered.

  Therefore, a zinc oxide (AZO) film doped with Al or a zinc oxide (GZO) film doped with Ga is formed by sputtering on a base electrode made of a tin oxide film or an indium oxide (ITO) film doped with Sn. A method of forming a surface electrode having a surface concavo-convex structure by etching a zinc oxide film that is easily etched has been proposed (see Patent Document 1).

In addition, a method of forming a surface electrode having a surface uneven structure by providing unevenness on a glass substrate by sandblasting and forming a transparent conductive film made of an ITO film or a zinc oxide film on the glass substrate has been proposed (Patent Document 2). reference)
JP 2000-294812 A JP-A-9-199745

  However, when a tin oxide film is used for the surface electrode of a thin film solar cell, the film resistance is high, so the transmittance is reduced by increasing the film thickness, and when an ITO film is used, reflection in the near infrared region is caused by carriers. Since this occurred, there was a problem that the transmittance was lowered. If a zinc oxide film such as an AZO film or a GZO film is used, the transmittance in the near infrared region decreases when the resistance value is decreased and the film thickness is decreased, and the resistance is increased when the transmittance in the near infrared region is increased. There is also a problem that the film thickness increases because the value increases.

  In addition, when forming a film by sputtering, local defects may occur in the AZO film due to arcing, and local defects may occur in the GZO film due to generation of particles. Such a defective portion is not preferable because the film resistance becomes abnormal or becomes a scatterer.

  Under such circumstances, the present invention has a surface electrode for a thin film solar cell having a high transmittance in the near infrared region and a low film resistance value, and a higher photoelectric conversion efficiency than the conventional one using this surface electrode. The object is to provide a thin film solar cell.

  Therefore, an indium oxide film doped with Ti, which is excellent in light transmittance in the near infrared region, as a surface electrode on a translucent substrate with a concavo-convex structure formed on the substrate surface by blasting, and generation of arcing and particles during film formation The present inventors have found that the conversion efficiency of a thin film solar cell is improved by forming a transparent conductive film composed of a zinc oxide film doped with Al and Ga with a small amount of Al and Ga.

更に、その凹凸構造基板面は、ブラスト加工により形成された表面であることを特徴とするものである。 That is, the thin-film solar cell of the present invention includes a surface electrode made of a transparent conductive film on a translucent substrate, a photoelectric conversion semiconductor layer laminated in the order of a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer. In a thin film solar cell in which a back electrode having at least a light-reflective metal electrode is sequentially formed,
The translucent substrate has a concavo-convex structure on the substrate surface on which the surface electrode is provided,
On the concavo-convex structure substrate surface of the translucent substrate, an indium oxide film doped with Ti having a film thickness d1 calculated by the following formulas (1) to (4), Al and Ga having a film thickness of 5 to 20 nm are formed. The surface electrode of the transparent conductive film which consists of a laminated body provided in order of the doped zinc oxide film | membrane is provided, The film | membrane surface at the side of the photoelectric conversion semiconductor layer of the said zinc oxide film is an uneven structure, It is characterized by the above-mentioned.
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Furthermore, the uneven structure substrate surface is a surface formed by blasting.

The surface electrode for a thin film solar cell of the present invention is doped with Ti indium oxide film having a film thickness d1 calculated by the following formulas (1) to (4), and doped with Al and Ga having a film thickness of 5 to 20 nm. A transparent conductive film made of a laminate provided in the order of the zinc oxide films, wherein the film surface on the photoelectric conversion semiconductor layer side of the zinc oxide film has an uneven structure.
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  By providing, as a surface electrode, a transparent conductive film sequentially laminated on an indium oxide film doped with Ti and a zinc oxide film doped with Al and Ga on a transparent substrate surface exhibiting a concavo-convex structure as in the present invention. It is possible to reduce the thickness and increase the transmittance in the near-infrared region, and to obtain a thin film solar cell with photoelectric conversion efficiency superior to that of the conventional one.

FIG. 1 shows a schematic cross-sectional view of a thin film solar cell of the present invention, and the thin film solar cell and surface electrode of the present invention will be described.
In FIG. 1, 1 is a translucent substrate, 1a is an uneven structure on the surface of the translucent substrate, 2 is a surface electrode, and the surface electrode 2 is an indium oxide film 21 doped with Ti and a zinc oxide film doped with Al and Ga. 22 is comprised. Reference numeral 3 denotes a photoelectric conversion semiconductor layer in which a p-type semiconductor layer 31, an i-type semiconductor layer 32, and an n-type semiconductor layer 33 are stacked. Reference numeral 4 denotes a back electrode, and the thin film solar cell 10 of the present invention has a structure in which these are sequentially laminated. With respect to this thin film solar cell 10, the light which should be photoelectrically converted enters from the translucent substrate 1 side.

The concavo-convex structure 1a on the translucent substrate surface provided on the translucent substrate 1 forms a concavo-convex structure by blasting a flat substrate surface. It is preferable that the unevenness of the uneven structure has an average roughness (Ra) of 30 to 100 nm.
Furthermore, as the translucent substrate 1, a transparent substrate such as an organic film, ceramics, or low-melting-point inexpensive glass can be used.

  The surface electrode 2 provided on the translucent substrate 1 preferably has a high transmittance of 80% or more with respect to light having a wavelength of 400 to 800 nm. An indium oxide film doped with Ti (hereinafter referred to as indium oxide [ Ti] film 21 and a zinc oxide film doped with Al and Ga (hereinafter referred to as zinc oxide [Al, Ga] film) 22. The film thickness of the surface electrode 2 is adjusted so that the sheet resistance is 10Ω / □ or less.

  In the surface electrode 2, the thickness of the zinc oxide [Al, Ga] film 22 is 5 to 20 nm, preferably 5 to 10 nm. If the thickness is less than 5 nm, the film becomes uneven, and indium oxide [Ti] A portion where the film 21 is exposed is generated, and indium oxide is reduced by hydrogen plasma to cause blackening, resulting in a decrease in transmittance. When the film thickness is made thicker than 20 nm, the film thickness of the indium oxide [Ti] film 21, which is a lower resistance film, must be reduced in order to make the sheet resistance as a surface electrode 10Ω / □ or less. As a result, it is difficult to obtain high transmittance.

  The film thickness of the indium oxide [Ti] film 21 is obtained by calculation from the following formulas (1) to (4).

The photoelectric conversion semiconductor layer 3 formed on the surface electrode 2 is formed using a plasma CVD method in which the base temperature is set to 400 ° C. or lower.
The plasma CVD method to be used may be a generally well-known parallel plate type RF plasma CVD, or may be a plasma CVD method using a high-frequency power source having a frequency of 150 MHz or less from the RF band to the VHF band.

  The photoelectric conversion semiconductor layer 3 is formed by stacking a p-type semiconductor layer 31, an i-type semiconductor layer 32, and an n-type semiconductor layer 33. The p-type semiconductor layer 31 and the n-type semiconductor layer 33 are arranged in the reverse order. Normally, a p-type semiconductor layer is disposed on the light incident side in a solar cell.

  The p-type semiconductor layer 31 is made of, for example, a microcrystalline silicon thin film doped with B (boron) as an impurity atom. However, the impurity atoms are not particularly limited, and aluminum or the like may be used in the case of a p-type semiconductor layer. Instead of microcrystalline silicon, polycrystalline silicon, amorphous silicon, or an alloy material such as silicon carbide or silicon germanium can be used. If necessary, the crystallization fraction and the carrier concentration may be controlled by irradiating the deposited semiconductor layer with pulsed laser light (laser annealing).

  The i-type semiconductor layer 32 is made of an undoped microcrystalline silicon thin film, but has a sufficient photoelectric conversion function with polycrystalline silicon, amorphous silicon, or a weak p-type semiconductor or weak n-type semiconductor containing a small amount of impurities. The silicon-based thin film material provided in the above can be used. Further, the material is not limited to the above materials, and alloy materials such as silicon carbide and silicon germanium can be used in addition to microcrystalline silicon.

  The n-type semiconductor layer 33 formed on the i-type semiconductor layer 32 is an n-type microcrystalline silicon doped with P (phosphorus) as an impurity atom, polycrystalline silicon, amorphous silicon, silicon carbide, silicon germanium, or the like. The impurity atoms to be doped are not particularly limited and may be N (nitrogen) in the n-type semiconductor layer.

On the n-type semiconductor layer 33, a back electrode 4 composed of a transparent conductive oxide film 41 and a light reflective metal electrode 42 is formed.
Although this transparent conductive oxide film 41 is not necessarily required, by improving the adhesion between the n-type semiconductor layer 33 and the light reflective metal electrode 42, the reflection efficiency of the light reflective metal electrode 42 is improved, and n It has a function of preventing chemical changes to the type semiconductor layer 33.

The transparent conductive oxide film 41 is formed of at least one selected from a zinc oxide film, an indium oxide film, a tin oxide film, and the like. In particular, the zinc oxide film has at least one of Al and Ga, and the indium oxide film has at least one of Sn, Ti, W, Ce, Ga, and Mo. A conductive film is preferred. The specific resistance of the transparent conductive oxide film 41 adjacent to the n-type semiconductor layer 33 is preferably 1.5 × 10 ⁻³ Ωcm or less.

  The light reflective metal electrode 42 is formed by a method such as vacuum deposition or sputtering, and is preferably formed of one selected from Ag, Au, Al, Cu, and Pt, or an alloy containing these. For example, Ag having high light reflectivity may be formed by vacuum deposition at a temperature of 100 to 330 ° C., more preferably 200 to 300 ° C.

The present invention will be described below with reference to examples.
Example 1
A silicon-based thin film solar cell having the structure shown in FIG. 1 was produced by the following production method.
First, a glass substrate is used as the translucent substrate 1, and the substrate surface of the translucent substrate 1 is blasted so that the difference in surface irregularities is 88 nm in average roughness (Ra). Structure 1a was formed.

  A transparent conductive film in which a Ti-doped indium oxide film 21 and an Al and Ga-doped zinc oxide film 22 were stacked as the surface electrode 2 was formed on the substrate surface of this uneven structure. The indium oxide [Ti] film 21 is a film obtained by doping indium oxide with 1% by mass of titanium oxide, and the zinc oxide [Al, Ga] film 22 is 0.6% by mass of gallium oxide and 0.3% of aluminum oxide in zinc oxide. A film doped with mass% was used.

The surface electrode 2 is formed by sputtering using a base temperature of 300 ° C., using Ar gas as an introduction gas, an indium oxide [Ti] film 21 of 206 nm, and a zinc oxide [Al, Ga] film 22 of 10 nm. Formed. The film thickness of the final surface electrode 2 was 216 nm, and the surface roughness of the outermost zinc oxide [Al, Ga] film 22 was 83 nm in average roughness (Ra).
Next, by plasma CVD, a p-type semiconductor layer 31 composed of a p-type microcrystalline silicon layer doped with B having a thickness of 10 nm, an i-type semiconductor layer 32 composed of an i-type microcrystalline silicon layer having a thickness of 3 μm, and a P having a thickness of 15 nm. An n-type semiconductor layer 33 made of an n-type microcrystalline silicon layer doped with is sequentially formed to form a pin junction photoelectric conversion semiconductor layer 3.

Next, a 70 nm thick transparent conductive oxide film 41 made of Ga-doped zinc oxide film and a 300 nm thick light reflective metal electrode 42 are formed on the obtained photoelectric conversion semiconductor layer 3 by sputtering. A back electrode 4 was formed.
As the transparent conductive oxide film 41, a film obtained by doping zinc oxide with 2.3% by weight of gallium oxide and 1.2% by weight of aluminum oxide was used.
The thin film solar cell 10 thus obtained was irradiated with AM (air mass) 1.5 light at a light amount of 100 mW / cm ² and measured for characteristics at 25 ° C., and the photoelectric conversion efficiency was 8.6. %Met.

(Comparative Example 1)
Instead of the indium oxide film doped with Ti, an ITO film doped with 10% by mass of tin oxide was used, and instead of the zinc oxide film doped with Al and Ga, only Ga was doped with gallium oxide and 6% by mass was doped. A thin-film solar cell was produced in the same manner as in Example 1 except that the GZO film was used.
When the obtained thin-film solar cell was irradiated with AM (air mass) 1.5 light at a light amount of 100 mW / cm ² and the characteristics were measured at 25 ° C., the photoelectric conversion efficiency was 8.1%.

It is sectional drawing which shows an example of the thin film solar cell which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Translucent board | substrate 1a Uneven structure of a translucent board | substrate 2 Surface electrode (transparent conductive film)
10 Thin Film Solar Cell 21 Ti-doped indium oxide film (indium oxide [Ti] film)
22 Zinc oxide film doped with Al, Ga (zinc oxide [Al, Ga] film)
3 photoelectric conversion semiconductor layer 31 p-type semiconductor layer 32 i-type semiconductor layer 33 n-type semiconductor layer 4 back electrode 41 transparent conductive oxide 42 light reflective metal electrode

Claims (3)

A back surface comprising a surface electrode made of a transparent conductive film, a photoelectric conversion semiconductor layer laminated in the order of a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer on a translucent substrate, and at least a light-reflective metal electrode In a thin film solar cell in which electrodes are sequentially formed,
The translucent substrate has a concavo-convex structure on the substrate surface on which the surface electrode is provided,
On the concavo-convex structure substrate surface of the translucent substrate, an indium oxide film doped with Ti having a film thickness d1 calculated by the following formulas (1) to (4), Al and Ga having a film thickness of 5 to 20 nm are formed. A thin film characterized in that a surface electrode of a transparent conductive film composed of a laminate provided in the order of doped zinc oxide films is provided, and a film surface of the zinc oxide film on the photoelectric conversion semiconductor layer side has an uneven structure Solar cell.
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  2. The thin-film solar cell according to claim 1, wherein the concavo-convex structure substrate surface of the translucent substrate is a surface formed by blasting. A laminated body in which a Ti-doped indium oxide film having a film thickness d1 calculated from the following formulas (1) to (4) and a zinc oxide film doped with Al and Ga having a film thickness of 5 to 20 nm are provided. A surface electrode for a thin-film solar cell, characterized in that the surface of the zinc oxide film on the photoelectric conversion semiconductor layer side has an uneven structure.
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