JP2826232B2 - Amorphous solar cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an amorphous solar cell.

[0002]

2. Description of the Related Art In recent years, in order to improve the efficiency of an amorphous solar cell using amorphous silicon and its alloy material as a main raw material, a solar cell having a band gap in an i-layer has been proposed. Such a method is disclosed in, for example, JP-A-01-71182. As shown in FIG. 3, the band gap profile technique changes the band gap in the i layer continuously, the band gap is wider at both ends of the p layer and the n layer, and the band gap is wider than that of the p layer on the light receiving surface side. This is to form a continuous band gap structure in which the narrowest portion is provided. For example, a-SiC, a-SiGe
It can be formed by changing the composition ratio of such materials. With such a structure,
The characteristics are improved by three kinds of effects. First, on the light-receiving side,
That is, generation of carriers on the p-layer side is increased, and collection of holes is facilitated. Next, by placing a low-quality film on the p-layer side and a high-quality film on the n-layer side, hole transport characteristics are improved. Finally, carrier collection is improved because the internal electric field is enhanced by the bandgap slope.

[0003]

Even if such a bandgap profile technique is used, there is a limit to the improvement of its characteristics.

[0004] One of the methods for improving the characteristics of a solar cell using a bandgap profile is to reduce the minimum bandgap, narrow the bandgap of the entire i-layer, increase the light absorption, and reduce the short-circuit current. There is a method that aims to improve. When the band gap is reduced in such a manner, the short-circuit current increases accordingly, but when the band gap is reduced below a predetermined band gap, the short-circuit current starts to decrease due to a decrease in the fill factor. In addition, the open circuit voltage decreases as the band gap decreases. In order to improve the characteristics of the solar cell, it is necessary to improve only the short-circuit current without lowering the fill factor and the open circuit voltage. For example, a-SiGe film
In the bandgap profile solar cell used for the layer,
The band gap of the minimum part was limited to about 1.4 eV.

Next, in order to improve the reliability, it is effective to increase the generation of carriers on the P layer side as much as possible. That is, by making the generation region of holes that are difficult to collect closer to the light receiving surface side, that is, closer to the p layer side, the traveling distance of the hole traveling toward the p layer can be shortened. For this purpose, a means has been conventionally used in which the minimum band gap portion is brought closer to the p-layer side. However, although this improves reliability, it has the drawback that the open-circuit voltage decreases and the efficiency also decreases. For example, in a solar cell using an a-SiGe film for the i-layer, the minimum band gap is 4 from the p-layer side.
The optimum value at which the efficiency becomes maximum is about 0 nm, but there is a drawback that the open-circuit voltage decreases when the distance is further closer to the p-layer side in order to improve reliability. This improvement in reliability is described in Japanese Patent Application No. 02-408745.

It is an object of the present invention to improve the shortcoming and reliability of a bandgap profile structure by improving the shortcomings of the bandgap profile structure.

[0007]

An amorphous solar cell having a pin structure using the amorphous silicon of the present invention and its alloy material as main raw materials and using a bandgap profile for an i-layer has a high light absorption layer on the p-layer side. Characterized by being provided in an i-layer within 80 nm from. Here, the high light absorption layer is an i-type layer having a predetermined thickness and a band gap smaller than that of the surrounding i layer.

[0008]

In such a structure, since the high light absorption layer exists near the p-layer side, holes traveling toward the p-layer side are easily collected. Here, the importance of collecting holes is emphasized because the hole has inferior transport properties to electrons. That is, if the collection of holes is improved, the recombination of carriers in the i-layer can be reduced, and the fill factor can be improved. By forming a film having a narrow band gap and a large amount of light absorption in the vicinity of the P layer side with a predetermined film thickness, the amount of light absorption is increased and the short-circuit current is improved. On the other hand, this high light absorption layer increases the amount of absorption only in the high light absorption layer instead of lowering the band gap of the entire i-layer, so that the fill factor and the open circuit voltage are suppressed from decreasing.

By forming the high light-absorbing layer within 80 nm from the P layer side, the region where holes are difficult to collect can be made closer to the p layer side than before, and the traveling distance of the hole traveling toward the p layer can be increased. And the reliability is improved.

[0010]

The present invention will be specifically described below with reference to examples. An example relating to a single-layer type a-SiGe element which is an example of the present invention will be described. FIG. 1 is a schematic view of the structure of a single-layer a-SiGe element of the embodiment, and FIG. 2 is a schematic view of a band gap structure. First, the structure of the single-layer a-SiGe device of the embodiment will be described. On the stainless steel substrate 1, a Ti layer 2 of 50 nm, an Ag layer 3 of 100 nm, a ZnO layer 4 of 100 nm, an amorphous silicon n layer 5 of 100 nm, an amorphous silicon germanium i layer 6 of 250 nm, and high light absorption in the amorphous silicon germanium i layer 6 Layer 7
2 nm, amorphous silicon carbon p-layer 8 at 10 nm, ITO as transparent conductive film 9 at 60 nm, and metal collector electrode 10 as Al.
Are sequentially laminated in a thickness of 600 nm. In the embodiment, the substrate 1 is not particularly limited to stainless steel, but can be formed using a substrate such as glass. When a light-transmitting substrate such as glass is used, a structure in which light is incident from the light-transmitting substrate side may be employed. The Ti layer 2 is used for improving the adhesion to the substrate. Although the Ag layer 3 is used as a light reflection layer on the back surface, another metal having a high reflectance can be used instead. It is desirable that the surface has an uneven structure. That is, the light reflected by the back surface is scattered by the uneven structure, and the optical path length of the light in the film is extended, so that the light absorption increases. This is because the insertion of the ZnO layer 4 also has the effect of further increasing light scattering. The amorphous layers are stacked in the order of nip, but when the light-transmitting substrate side is used, the order is pin. In each case, the light incident side is p
Preferably, it is a layer. The transparent conductive film 9 is not limited to ITO,
It is possible to use nO ₂ and ZnO. The metal collecting electrode 10
Other than Al, a metal having good electric conductivity is preferable.

Next, the amorphous silicon germanium i-layer 6 of the device of the embodiment will be described. In the device of the embodiment, on the light-receiving surface side to improve the short-circuit current
Band gap 1.34e from the position 15nm from the end of the i layer on the p layer side
A high light absorption layer 7 having a width of 2 nm and a width of 2 nm is provided. The band gap structure other than this region is the same as the conventional band gap profile structure shown in FIG. The band gap on the p-layer side and the n-layer side is 1.75 eV, and the minimum portion 12 excluding the high light absorption layer 7 in the band gap profile is located at 40 nm from the amorphous silicon carbon p-layer 8 on the light-receiving surface side of 1.42 eV. The film thickness 11 is 250 nm. Both ends of the high light absorption layer 7 are structured to be smoothly connected to a band gap profile.

In the present embodiment, the position of the high light absorption layer is i on the p layer side.
The position is 15 nm from the end of the layer. The reason will be described below. In order to improve the reliability by this high light absorption layer, it is necessary to provide it within 80 nm from the end of the i layer on the p layer side. This is indirectly derived from the experimental fact shown in FIG. 5 that if the minimum band gap position of the band gap structure shown in FIG. 3 is provided within 80 nm, the reliability is improved. FIG.
Is a comparison of the initial characteristics の and the characteristics 光 after light degradation of a uniform bandgap element and a bandgap profile element under long-wavelength light of 660 nm or more in AM 1,100 mW / cm ² . As the position of the bandgap minimum part approaches the p-layer side, that is, approaches 0 nm, the short-circuit current slightly increases, the open-circuit voltage decreases monotonically, and the fill factor reaches its maximum at 10 nm before and after photodegradation. As a result, the maximum output has a maximum value at 10 to 40 nm. At the maximum output after photo-deterioration, the position of the minimum bandgap at which characteristics almost equivalent to those of the uniform bandgap element can be obtained is 80 nm, and it is clear that the bandgap must be provided on the p-layer side. The reason for this tendency is that collection of holes becomes easier as the light absorption increases near the p-layer. This effect is exactly the same for the high light absorption layer. Therefore, even when the high light absorption layer is provided, it is necessary to provide the high light absorption layer within 80 nm, preferably within 40 nm from the p layer. At this time, the minimum part of the band gap in FIG. 2 needs to be provided within 80 nm, preferably within 40 nm from the p-layer. However, in the conventional bandgap profile structure shown in FIG. 3, as the position of the bandgap minimum portion approaches the p-layer side, the open-circuit voltage gradually decreases correspondingly. If the width is about 2 nm, the open-circuit voltage does not decrease unless it is too close to the p-layer side. Therefore, it is considered desirable to set the position of the band gap minimum portion to about 40 nm and to form the high light absorption layer further closer to the p layer side. Further, in FIG. 5, when the position of the band gap minimum portion is 0 nm, that is, when the band gap is in contact with the p layer, the maximum output before and after light degradation is greatly reduced. It is considered that recombination at the interface increased due to the contact. Therefore, it is not desirable that the high light absorption layer is in contact with the p layer.

The width of the high light absorption layer is 2n in this embodiment.
It can be seen that about m is the optimum value of the efficiency, but this does not mean that it cannot be used with a film thickness larger than that. For example, when used as a lower element of a stacked element, increasing the short-circuit current may increase the efficiency of the stacked element even if the open-circuit voltage and fill factor are slightly reduced. On the other hand, if the thickness is set to 2 nm or less, there is no advantage, only the short-circuit current decreases. That is, it is desirable that the width of the high light absorption layer is equal to or greater than the width at which the efficiency is maximized. Specifically, it may be selected in a range of 1 nm to 5 nm.

In this embodiment, the value of the high light absorption layer is 1.
Although it is set to 34 eV, there is no problem with a band gap larger than that. However, the efficiency decreases because the amount of light absorption decreases. Therefore, when the gap is increased, it is necessary to provide a plurality of high light absorption layers in order to obtain characteristics equal to or higher than those of the present embodiment. If the band gap is equal to or less than the band gap of this embodiment, the amount of light absorption increases, but the quality of the film deteriorates, so that it is difficult to collect generated carriers. Therefore, it is necessary to reduce the width of the high light absorption layer.

As described above, the parameters of the present invention include:
There are the position of the high light absorption layer from the p-layer side, the width, the value of the band gap, the number thereof, and the like. Optimization of each parameter is necessary to obtain the optimum value. However, it is relatively easy to obtain better characteristics than the conventional device shown in FIG.

Next, the manufacturing method of the embodiment will be described. First, a Ti layer 2 is formed to a thickness of 50 nm on a stainless steel substrate 1 by electron beam evaporation. After that, the Ag layer 3 is deposited at 100 ° C. at a substrate temperature of 350 ° C. to form irregularities. A ZnO layer 4 is formed thereon to a thickness of 100 nm. On the substrate thus formed, an amorphous silicon n layer 5, an amorphous silicon germanium i layer 6, and an amorphous silicon carbon p layer 8 were formed by plasma C
The layers are sequentially laminated by the VD method. As the conditions for forming the amorphous layer, the RF power was 10 W, the pressure was 0.1 to 0.3 Torr, and the gas flow conditions were as shown in Table 1.

[0017]

[Table 1]

After forming the amorphous silicon n layer 5 to 100 nm, an amorphous silicon germanium i layer 6 is formed. At this time, it is formed by controlling the flow ratio of the GeH ₄ gas and the SiH ₄ gas so as to form a band gap structure as shown in FIG. From the amorphous silicon n-layer 5 side to the band gap minimum portion 12, the profile is set so that the band gap changes linearly by increasing the SiH ₄ gas flow rate to 5 sccm and the GeH ₄ gas flow rate from 0 to 1.2 sccm. Form. The film thickness up to this minimum portion is 210 nm. Next, the GeH ₄ gas flow rate is reduced from 1.2 sccm to 0 sccm so that the band gap changes linearly, and a profile is formed with a thickness of 40 nm. 7 is formed. The high light absorbing layer 7
The GeH ₄ gas flow rate is set at a constant 1.6 sccm to form 2 nm. At this time, in order to form continuously without stopping the discharge, a transient region having a thickness of about 0.2 nm where the GeH ₄ gas flow rate changes rapidly is formed at both ends of the high light absorption layer 7. When the formation of the high light absorption layer 7 is completed, the flow rate of the GeH ₄ gas is reduced so that the bandgap changes linearly again and returns to the original profile. Then, 15 nm from the end of the high light absorption layer 7 and Ge
It is formed by reducing the H ₄ gas flow rate to zero. An amorphous silicon carbon p layer 8 is formed to a thickness of 10 nm on the amorphous silicon germanium i layer 6 thus formed. Thereafter, 60 nm of ITO, which is a transparent conductive film 9, is formed by magnetron sputtering, and Al10 is formed by electron beam evaporation.
Is formed to a thickness of 600 nm.

A comparison of the characteristics between the device of the embodiment thus formed and the conventional device shown in FIG. 3 is shown. FIG. 4 shows that the distance from the p / i interface to the end of the high light absorption layer is 15 nm, and the thickness of the high light absorption layer 7 is changed to obtain an initial characteristic under long wavelength light of 660 nm or more of 1,100 mW / cm ² of AM. And the characteristic △ after light degradation. The conventional device has a high light absorption layer 7 of 0 nm.
Is the case. In the initial characteristics, the element provided with the high light absorption layer 7 shows a higher short circuit current, and tends to increase as the thickness of the high light absorption layer 7 increases. This tends to coincide well with the increase in light absorption. The open circuit voltage tends to decrease as the thickness of the high light absorption layer 7 increases. However, when the thickness is 2 nm, the open-circuit voltage is exactly the same value as that of the conventional device, and it can be seen that there is no effect if the film thickness is about this level. The fill factor tends to decrease as the thickness of the high light absorption layer 7 increases. But also in the fill factor,
If the thickness is about 2 nm, it is almost the same as the conventional device.
As a result, the maximum output takes the maximum value in the device in which the film thickness of the high light absorption layer 7 is 2 nm, and has the same value as in the conventional device at 3 and 4 nm.

After the light irradiation for 200 hours under the light of AM 1.5 and 100 mW / cm ^2, the element provided with the high light-absorbing layer 7 has less decrease in the fill factor than the conventional element, and the initial characteristics Of the conventional device, which had the highest value in the above, has the lowest value in the characteristic の after light degradation. In addition, the maximum output after light degradation was almost the same at any film thickness in the element provided with the high light absorption layer 7, and was about 25% larger than that of the conventional element. In the device provided with the high light absorption layer 7 as described above, not only the initial characteristics but also the characteristics were significantly improved in the reliability against light irradiation.

In this embodiment, the case where the number of the high light absorption layers 7 is one is shown as an example, but a plurality of high light absorption layers 7 may be formed in order to further increase the light absorption.

Further, instead of the well type such as the band gap structure shown in FIG. 1 shown in the present invention, a shape having an inclined band gap may be used.

Further, the present invention is not limited to a single-layer solar cell, but is also applicable to two-layer and three-layer solar cells.

[0024]

As described above, according to the present invention, the efficiency and reliability of a solar cell can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic structural diagram illustrating an element structure of an amorphous solar cell according to an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a band gap structure of an amorphous solar cell according to an embodiment of the present invention.

FIG. 3 is a schematic diagram illustrating a band gap structure of a conventional amorphous solar cell.

FIG. 4 is a diagram illustrating initial characteristics and characteristics after light degradation of an amorphous solar cell according to an example of the present invention.

FIG. 5 is a diagram showing a correlation between a position of a minimum band gap portion and element characteristics.

[Explanation of symbols]

 Reference Signs List 1 stainless steel substrate 2 Ti layer 3 Ag layer 4 ZnO layer 5 amorphous silicon n layer 6 amorphous silicon germanium i layer 7 high light absorption layer 8 amorphous silicon carbon p layer 9 transparent conductive film 10 metal collecting electrode 11 i layer thickness 12 band gap minimum part

Continued on the front page (72) Inventor Yoshihiro Yamamoto 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Jun Junda 22-22 Nagaikecho, Abeno-ku, Osaka-shi Osaka Prefecture Inside Sharp Corporation (72 ) Inventor Katsuhiko Nomoto 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (56) References JP-A-58-215083 (JP, A) JP-A-3-208376 (JP, A) (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 31/04

Claims (1)

(57) [Claims] An amorphous solar cell having a pin structure in which amorphous silicon and its alloy material are used as main raw materials and a band gap profile is provided in an i layer, wherein a high light absorption layer is formed in an i layer within 80 nm from the p layer side. An amorphous solar cell, provided therein.