JP2889728B2 - Photovoltaic device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photovoltaic device using a semiconductor thin film formed on a substrate having an uneven surface.

[0002]

2. Description of the Related Art In recent years, photovoltaic devices using thin film semiconductors such as amorphous semiconductors and polycrystalline semiconductors have been actively developed. In particular, this amorphous semiconductor is widely used because it can be easily formed in a large area as compared with a crystalline semiconductor, and has high light sensitivity in terms of physical properties.

FIG. 4 is a diagram showing the element structure of a conventional amorphous semiconductor photovoltaic device using amorphous silicon.
In the figure, (41) is a light-transmitting insulating substrate, and (42) is a transparent electrode made of a transparent conductive film, and the surface of this film is provided with irregularities. (43) is a p-type conductive semiconductor layer, (44) is a buffer layer made of intrinsic amorphous silicon, (45) is an intrinsic amorphous semiconductor layer, and (46) is an n-type conductive semiconductor layer. And (47) are back electrodes made of a metal film.

In this structure, the reason why the transparent electrode (42) is provided with an uneven shape is that the light incident from the insulating substrate (41) side is scattered by the unevenness and passes through the intrinsic amorphous semiconductor layer (45). Increasing the length reduces emission of light to the outside, and is particularly effective for absorbing long-wavelength light.

In order to make this function work effectively, irregularities having a height of about 1000 ° to 5000 ° are usually provided as the irregularities, and this effect is generally called an optical confinement effect. Hereinafter, this term will be used in the meaning described above.

A buffer layer (44) interposed between a p-type conductive semiconductor layer (43) and an intrinsic amorphous semiconductor layer (45).
Prevents diffusion of the conductivity-type determining impurities contained in the conductive semiconductor layer (43) from the p-type conductive semiconductor layer (43) to the intrinsic amorphous semiconductor layer (45), and This is intended to reduce carrier recombination between them.

All of these structures effectively contribute to improving the photoelectric conversion efficiency of the photovoltaic device.

[0008]

However, although this uneven shape provided on the transparent electrode is effective for the light confinement effect, on the other hand, on the other hand, the p-type of the photovoltaic device made of a thin film semiconductor is used. The conductive semiconductor layer is usually 100
Since a thin film of the order Å is used as a constituent material, it is difficult to form a uniform thin film on a surface having such an uneven shape, and unevenness in film thickness occurs.

In particular, an amorphous semiconductor typified by amorphous silicon is formed by a plasma CVD method, a sputtering method, or an electron beam evaporation method instead of a forming method based only on a surface reaction such as a thermal CVD method. It is particularly difficult to be done.

FIG. 5 shows, as an example for explaining this difficulty, an amorphous silicon (RF) film formed on a substrate (52) having a transparent electrode (51) having an uneven shape by a parallel plate electrode type high-frequency plasma CVD method. 53 shows the state of film formation when 53) is formed. The two discharge electrodes (54) and (55) are arranged so as to be parallel to each other, and one of the discharge electrodes (54) is attached to the substrate (52).
Is installed. The film formed by the plasma CVD method usually grows in a direction perpendicular to the discharge electrode (54). However, when the surface of the substrate has an uneven shape, the vertical direction of the surface is different from the vertical direction of the discharge electrode. , Depending on the inclination of the surface.

In general, the smaller the angle (α) between the irregular surface and the perpendicular of the surface of the discharge electrode (54), the smaller the angle (α).
The film growth rate in the vertical direction on the surface of the uneven shape is reduced. That is, as the angle approaches 90 degrees, the film growth rate increases.

Usually, in the film formation by the plasma CVD method, it is said that the degree of covering of the film with respect to the surface step is good, but an extremely thin film of 150 ° or less is insufficient. According to the measurement by an electron microscope, α = 45
It is known that the film thickness is only about 70% as compared with the case where α = 90 degrees.

In the case of the conventional photovoltaic device (FIG. 4), the influence of such unevenness in film thickness is due to the conductive semiconductor layers (43) and (46) and the buffer layer (46) constituting the semiconductor junction of this device. 44)
Appears remarkably about. This is because these usually have a film thickness of only about 100 °, and have
This is because it is extremely thin compared to 000 °.

In particular, in the case of the above-described conventional photovoltaic device, the p-type conductive semiconductor layer (43) having a small film thickness and directly formed on the surface having the irregularities has the following problems.
In particular, unevenness in the film thickness tends to occur.

[0015] For this reason, in the photovoltaic device, a characteristic defect such as a decrease in the open circuit voltage conventionally occurs. Regarding this issue, for example, the Technical Digest of the International
PVSEC-5, Kyoto, Japan, 1990, pp. 261-264. However, even if the thickness of the p-type conductive semiconductor layer (43) is increased for the purpose of compensating for this, light absorption by the p-type conductive semiconductor layer (43) increases and a short circuit occurs. This results in a decrease in the current, which is not a fundamental solution.

Therefore, the photovoltaic device of the present invention is to provide a photovoltaic device which can eliminate the inconvenience of having such an uneven shape.

[0017]

The feature of the photovoltaic device of the present invention is that it is formed on a substrate having a surface having an uneven shape or a substrate having a transparent electrode having a surface having an uneven shape. The conductive semiconductor layer and / or the buffer semiconductor layer have a superlattice structure of semiconductor thin films having two different optical band gaps, and the thickness of the semiconductor thin film having a small optical band gap is: It is characterized in that it locally changes on the surface of the uneven shape.

[0018]

According to the present invention, the quantum effect caused by the use of the superlattice structure is utilized.
The generation of unevenness in film thickness due to the above-mentioned uneven shape is used.

This superlattice structure refers to a laminated structure in which two kinds of semiconductors having different optical band gaps are periodically superimposed and formed in a thin film state. Such a laminate generally exhibits a quantum effect when the thickness of a semiconductor having a small optical band gap, which is sandwiched between semiconductors having a large optical band gap, is reduced to about 100 ° or less. Has been confirmed. Usually, the semiconductor having the smaller optical band gap is called a well layer.

The quantum effect means that electrons and holes are confined in a narrow optical bandgap portion of a well layer due to the periodic superposition of semiconductors having different optical bandgap. This is a phenomenon in which the optical bandgap substantially changes as a laminate, and in particular, has a feature that the optical bandgap as the laminate increases as the thickness of the well layer decreases. . Such technical matters are
Journal of AppliedPhysics Vol.26 No.1, January, 198
7, pp. 28-32.

On the other hand, on a substrate having the uneven shape,
When a laminate having such a superlattice structure is formed, at a portion where the angle α is small, that is, at a portion where the surface inclination is steep,
As described above, since the thickness of each layer is reduced, the optical band gap is significantly increased locally in this portion due to the quantum effect.

On the other hand, when the angle α is close to 90 degrees, the film thickness to be formed is almost the same as that formed on a flat surface, so that the optical band gap increases due to the quantum effect. Is smaller than the portion where the angle α is small.

That is, in a portion where the inclination of the surface is steep, if the laminate having the super lattice structure is not used, a local decrease in the open-circuit voltage occurs due to the thin film thickness in such a portion. However, according to the superlattice structure according to the present invention, the effect of the quantum effect based on the thinness of the film thickness causes an increase in the optical band gap, and the open voltage as a photovoltaic device can be increased. Become. The reason that the increase in the optical bandgap acts as an increase in the open-circuit voltage of the photovoltaic device is as follows.
It is considered that this increase reduces the loss due to the recombination of photocarriers, particularly electrons, generated in the photovoltaic device.

On the other hand, in a portion where the inclination of the surface is not steep, a film thickness similar to that of a film formed on a flat surface can be obtained. Only the forbidden bandwidth can be obtained.

Therefore, the optical bandgap that fluctuates due to the inclination of the surface can be controlled by the difference in the degree of local occurrence of the quantum effect. For such an effect, the thickness of the well layer may be locally changed along the uneven shape.

[0026]

FIG. 1 is a sectional view showing the element structure of a first embodiment of the photovoltaic device of the present invention. In the figure, (1) is a transparent insulating substrate made of glass, quartz, or the like, (2) is a transparent electrode made of tin oxide, indium oxide, or the like having a surface with an uneven shape having a haze ratio of 20%, ) Is a p-type conductive semiconductor layer having a superlattice structure which is a feature of the present invention, (4) is a buffer semiconductor layer made of amorphous silicon carbide, (5)
Is an intrinsic semiconductor layer made of amorphous silicon, (6) is an n-type conductive semiconductor layer, and (7) is a back electrode made of aluminum, titanium, chromium, or the like.

The components other than the p-type conductive semiconductor layer (3) are conventionally known. The p-type conductive semiconductor layer (3) has a superlattice structure in which two semiconductor layers of amorphous silicon p1 containing carbide and amorphous silicon p2 not containing it are alternately stacked. In each of these semiconductor layers, boron is added so as to form a p-type semiconductor layer. Table 1 shows the conditions for forming the two semiconductor layers p1 and p2 and the optical bandgap. However, the optical bandgap shown in the same table shows a value when formed on a flat substrate, and all are measured in a single layer state.

[0028]

[Table 1]

The table also shows conditions for forming the semiconductor layers used in the following description.

The superlattice structure used in the embodiment is composed of five layers (p1, p2, p1, p2, p2,
p1) is formed by lamination, and the optical band gap when this lamination is formed on a flat substrate is 2.0 eV.
It is.

By the way, assuming that the laminated body is in a thin film state formed in the uneven portion, the thickness of each layer is 1
As a result of evaluating the laminate when the thickness was reduced to 4 mm,
The optical band gap was larger than 2.0 eV, and a value of 2.1 eV was obtained.

The thickness of the other layers is 1 in the buffer layer (4).
The film was formed under the conditions of 00 °, the intrinsic semiconductor layer (5) was 4000 °, and the n-type semiconductor layer (6) was 300 °, with b3, i and n layers shown in Table 1, respectively.

Table 2 shows the photovoltaic characteristics of the photovoltaic device of this example.
Shown in The table also shows, for comparison, the characteristics of the photovoltaic device that differs only in that a conductive semiconductor layer that does not depend on the superlattice structure is used. In this conventional photovoltaic device, the p-type conductive semiconductor layer uses only one p3 layer (100 °) shown in Table 1 and has the same optical band gap as that of the laminate of this example. Value, ie 2.0e
V was formed.

[0034]

[Table 2]

As is clear from Table 2, in the case of this example, the open-circuit voltage is remarkably improved, and the effect of the superlattice structure is sufficiently exhibited.

According to an experiment conducted by the present inventor, a photovoltaic device including a conductive semiconductor layer having a superlattice structure according to the embodiment on a flat transparent electrode, and a photovoltaic device including the superlattice structure, The characteristics of a conventional photovoltaic device using a conductive semiconductor layer composed of a single layer having the same optical band gap value as the conductive semiconductor layer were also compared, but the difference was little. It has been found that the effect of the present invention can exhibit a sufficient effect only by the substrate having the uneven shape.

Next, a photovoltaic device according to a second embodiment will be described with reference to FIG. In the figure, the same parts as those in FIG. 1 are denoted by the same reference numerals. The difference between the present embodiment and the first embodiment is that, while the conductive semiconductor layer (23) uses one layer of p3 (150 °) in Table 1 and does not have a superlattice structure, the buffer semiconductor layer (24 ) To b1 and b2 in Table 1.
Have a superlattice structure in which five layers (b1, b2, b1, b2, b1 from the substrate side) are alternately formed while maintaining a film thickness of 20 °.

Table 3 shows the photovoltaic characteristics of the photovoltaic device of this example. The table also shows, for comparison, the characteristics when a single layer without a superlattice structure is used for the buffer semiconductor layer. As this single layer, a b3 layer (100 °) shown in Table 1 was used. The optical band gap of the b3 layer is designed to have the same value as that of the buffer semiconductor layer (24) having a superlattice structure when formed on a flat substrate.

[0039]

[Table 3]

According to Table 3, the characteristics of the photovoltaic device of this example show an improvement in the open-circuit voltage as in the first embodiment.

Next, a third embodiment of the photovoltaic device of the present invention is shown in FIG. In this example, both the conductive semiconductor layer (3) and the buffer semiconductor layer (24) adopt a super lattice structure. The conditions for forming these two layers are the same as those in the first and second embodiments.

Table 4 shows the photovoltaic characteristics of this example. As can be seen from the table, the open circuit voltage is higher than in the case of the first embodiment shown in Table 2, while the increase in short-circuit current can be confirmed as compared with the second embodiment shown in Table 3. This is considered to be because in the first embodiment, the thickness of the buffer semiconductor layer was uneven, and the open-circuit voltage was reduced due to the unevenness.

[0043]

[Table 4]

In the superlattice structure used in the present invention,
The characteristics can be effectively improved by locally changing the film thickness of the semiconductor thin film serving as the well layer on the uneven surface. This is because the quantum effect is remarkably exhibited when the semiconductor thin film having a small optical band gap, which is the well layer, is formed as a thin film due to its uneven shape.

In the embodiment, the semiconductor layer constituting the superlattice structure is formed by using amorphous silicon containing carbide and amorphous silicon not containing it. However, the present invention is not limited to this. Various superlattice structures can be formed by combining amorphous silicon germanium and amorphous silicon nitride having different optical band gaps.

Although the uneven shape provided on the substrate is formed by using the uneven shape of the transparent electrode in any of the above-described embodiments, the substrate provided with the uneven shape used in the present invention is not limited to this. is not. For example, quartz or glass,
A substrate having an uneven surface such as a ceramic substrate may be used. In this case, as a method for manufacturing the uneven shape, physical polishing or chemical etching may be performed.

[0047]

According to the photovoltaic device of the present invention, good photovoltaic characteristics can be obtained based on the improvement of the open circuit voltage and the like.

Further, in the present invention, since the formation of the thin film for forming the superlattice structure utilizes the uneven shape of the substrate, it is possible to form the element with good reproducibility.

Further, according to the structure of the present invention, the method of forming the concavo-convex shape may be based on the substrate itself, or may be provided by a transparent electrode or the like formed on the substrate. The degree of freedom is great in designing.

[Brief description of the drawings]

FIG. 1 is a sectional view of an element structure for explaining a first embodiment of a photovoltaic device of the present invention.

FIG. 2 is a sectional view of an element structure for explaining a second embodiment of the photovoltaic device.

FIG. 3 is a sectional view of an element structure for explaining a third embodiment of the photovoltaic device.

FIG. 4 is a sectional view of an element structure of a conventional photovoltaic device.

FIG. 5 is a schematic cross-sectional view showing a film formation state when amorphous silicon is formed on a substrate having an uneven shape.

[Explanation of symbols]

 1 substrate 3 conductive semiconductor layer 4 buffer semiconductor layer 5 intrinsic semiconductor layer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-29374 (JP, A) JP-A-62-29872 (JP, A) JP-A-61-218178 (JP, A) JP-A-3-29374 38068 (JP, A) JP-A-63-318170 (JP, A) JP-A-63-2385 (JP, A) JP-A-62-181476 (JP, A) (58) Fields investigated (Int. ⁶ , DB name) H01L 31/04

Claims (2)

(57) [Claims] 1. A superlattice comprising a semiconductor thin film in which a conductive semiconductor layer and / or a buffer semiconductor layer formed on a substrate having a transparent electrode having an uneven surface has two different optical band gaps. A photovoltaic device having a structure, wherein the thickness of the semiconductor thin film having a small optical band gap locally changes on the surface of the uneven shape. 2. The method according to claim 1, wherein the conductive semiconductor layer and / or the buffer semiconductor layer formed on the substrate having an uneven surface has two or more conductive semiconductor layers.
Along with a superlattice structure of semiconductor thin films having different types of optical band gaps, the film thickness of the semiconductor thin film having a small optical band gap locally changes on the surface of the uneven shape. A photovoltaic device.