JPH07105520B2 - Amorphous semiconductor solar cell - Google Patents

Description

TECHNICAL FIELD The present invention relates to an amorphous semiconductor solar cell having a PIN structure, and more particularly to an amorphous semiconductor solar cell having an I layer with a changed band gap. is there.

[Conventional technology]

In order to improve the photoelectric conversion efficiency of an amorphous semiconductor solar cell, amorphous silicon / germanium (a-SiGe) having a narrower band gap than amorphous silicon (a-Si) or amorphous having a wide band gap is used. Quality silicon carbon (a-Si
Research on solar cells using an a-Si based alloy such as C) for the I layer is underway.

The reason is that a material having a narrower bandgap than a-Si is effective for improving sensitivity to long-wavelength light, while a material having a wider bandgap than a-Si is effective for reducing voltage factor loss for short-wavelength light. Because there is. Further, a stacked solar cell in which a plurality of solar cells using these materials are stacked is being developed as a solar cell capable of achieving high efficiency by utilizing the effectiveness of these materials.

In the past, solar cells using these a-si alloy materials for the I layer had only lower conversion efficiencies than solar cells using a-Si for the I layer. Problems such as are clarified and improvements have been made.

First, there is a problem at the interface between the I layer and the P layer. For example I
When a-SiGe having a narrow band gap is used as the layer and a-SiC is used as the P layer, discontinuity of the band gap occurs at the interface, which increases recombination at the interface and causes an open circuit voltage (Voc) or It was responsible for the reduction of fill factor (FF). On the other hand, Voc and FF can be improved by introducing an interfacial layer at the interface as shown in the band structure diagram shown in FIG. 7 and grading the band gap to eliminate the discontinuity of the band gap. It has already been reported.

Secondly, the film quality of the a-Si alloy is inferior to that of a-Si. Therefore, in the solar cells using the a-Si alloy material and the entire I layer, the FF is significantly reduced. On the other hand, as shown in the band structure diagram of FIG. 8, the composition ratio of the alloy material in the I layer is changed to change the band gap, so that the film quality of the I layer is partially improved with a-Si having a good film quality. It has recently been reported that the FF can be improved while maintaining the characteristics of the a-Si alloy material such as high sensitivity to long-wavelength light and reduction of voltage factor loss, which is close to the film quality of JP-A No. 6-7118.
2). It is also shown that at this time, an electric field is generated by continuously changing the bandgap of the I layer to form a gradient, and the electric field has a function of promoting the movement of carriers.

As described above, although the solar cell using the a-Si alloy material has been improved in recent years, it has not been possible to obtain a high-efficiency solar cell that makes full use of its effectiveness. Was needed.

[Problems to be Solved by the Invention]

In a conventional solar cell using an a-Si alloy material for the I layer, the FF is particularly lower than the FF of the a-Si solar cell,
Also, Voc was obtained only at a lower value than the value expected from the band gap of the I layer. It is considered that this is due to insufficient separation of photogenerated carriers and collection to the electrode, and large recombination and reverse diffusion of carriers at the interface between the I layer and the doped layer.

The present invention was devised in view of the above points, and by making a new improvement in the change of the band gap in the I layer, the advantages of the a-Si alloy material are utilized to improve the photoelectric conversion efficiency. It is intended to provide an intended amorphous semiconductor solar cell.

[Means for Solving the Problems] In order to achieve the above object, an amorphous semiconductor solar cell of the present invention comprises an amorphous semiconductor film, and from the light incident side, a p-type impurity doped layer, an i layer, PIN in order of n-type impurity doped layer
In the amorphous semiconductor solar cell having a structure, a bandgap of the i layer in contact with a side end of the p-type impurity doped layer is wider and discontinuous than a bandgap of the p-type impurity doped layer, It is characterized in that it has a region that is gradually reduced from the impurity-doped layer side and is narrower than the band gaps of the p-type impurity-doped layer and the n-type impurity-doped layer.

Further, in the amorphous semiconductor solar cell, the band gap of the i layer between the region and the n-type impurity doped layer is discontinuously changed.

[Action]

As described above, by making the bandgap at the end of the P-type impurity doped layer of the I layer wider than the bandgap of the P-type impurity doped layer, it is possible to reduce the recombination of carriers and the backward diffusion. And Voc can be greatly improved. Conventionally, to continuously connect a bandgap between an I-layer and a P-type impurity-doped layer reduces recombination via an interface level due to a discontinuity of the bandgap at the interface, and a conduction band. Alternatively, it is intended that movement of carriers existing in the valence band is not blocked by the band gap barrier. The inventor has found that recombination via the interface state does not increase even if the band gap of the I layer is larger than that of the P type impurity doped layer at the edge of the P type impurity doped layer. It has been found that the backward diffusion is suppressed by the effect of expanding.

Further, due to the above structure of the band gap of the I layer, the difference in band gap in the I layer becomes large,
The electric field due to the band gap gradient can be strengthened. As a result, the separation of the photo-generated carriers and the movement to the electrode are promoted, and the FF can be improved.

On the other hand, the fact that the band gap in the I layer has a region narrower than both band gaps of the I layer has the following effect in addition to the effect of grading the band gap as described above.

That is, when a material having a wider bandgap than a-Si such as a-SiC is applied to the I layer, the bandgap is narrowed to
By approaching the film quality of -Si, it is possible to improve FF while suppressing voltage factor loss.

When a material having a narrower bandgap than a-Si such as a-SiGe is applied to the I layer, long wavelength light can be effectively absorbed in the narrow bandgap region. Of course, it is also possible to combine the above two effects by continuously changing the band gap from a-SiC to a-SiGe in the I layer. The present invention applies the above-described actions to a solar cell.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view schematically showing the structure of an amorphous semiconductor solar cell as one embodiment of the present invention. In FIG.
1 is a stainless steel substrate. N on this stainless steel substrate
I-type a-SiGe layers 3a, 3a, I each having a thickness of about 1000Å are deposited on the type a-Si layer 2 and the composition of the intrinsic layer 3 is gradually changed.
The type a-SiC layer 3b is deposited together to a thickness of about 3000Å, and the P-type a-SiC layer 4 is further deposited thereon to a film thickness of about 100Å to form a PIN structure. A transparent conductive film 5 is formed thereon with a thickness of about 600Å, and finally a collector electrode 6 of Al is formed. At this time, the intrinsic layer 3
In the formation of a, first, a-Si is formed at the N / I interface, the composition of Ge in the film is gradually increased to a film thickness of 2200Å, and then the composition of Ge in the film is reduced to form a Ge film at a film thickness of 2600Å. The composition is set to 0, and then the composition of carbon is gradually increased from 0 to form a film thickness of 3000 Å. In order to change the composition of such elements, the intrinsic layer 3 is plasma-treated.
It is realized by changing the flow rate ratios of silane gas (SiH ₄ ) and germane gas (GeH ₄ ) and SiH ₄ and methane gas (CH ₄ ) as shown in FIG. 2 when forming by the CVD method.

That flow of GeH ₄ at N / I interface is 0, GeH ₄ / at the time of gradually thickness increases the flow rate of _{GeH 4 2200Å (SiH 4 + GeH} 4)
= 15%, and then the flow rate of GeH ₄ is gradually reduced.
At the time of 00Å, only SiH ₄ was used, then the flow rate of CH ₄ was gradually increased from 0, and at the time of film thickness of 3000Å CH ₄ / (CH ₄ + SiH ₄ ) = 5
It is formed to be 0%. Where CH ₄ / (CH ₄ + SiH ₄ ) =
The bandgap at 50% is 2.1 eV, and P-type a
-The band gap of SiC is 1.9 eV.

By changing the flow rate ratio of SiH ₄ and GeH ₄ or SiH ₄ and CH ₄ in this way, it is possible to form the band gap of the I layer with a gradient as shown in FIG.

FIG. 4 shows the current-voltage characteristics of the a-SiGe solar cell configured as described above and the solar cell having the conventional structure. Here, the conventional structure means that the change in the flow rate of CH ₄ gas at the time of producing the I layer shown in the example is CH ₄ / (CH ₄ + Si at the end of the P layer.
H ₄ ) = 25% and the band gap was 1.9 eV, which is the same as that of the P layer. The band structure diagram at that time is as shown in FIG. The light source at the time of measurement was a long-wavelength light through a filter that transmits light with an AMI spectrum of 100 mW / cm ² only at a wavelength of 660 nm or more. The curve shown by the broken line in FIG. 4 is the current-voltage characteristic of the conventional a-SiGe solar cell, and the curve shown by the solid line is a according to the embodiment of the present invention.
-It is a current-voltage characteristic of a SiGe solar cell. In addition, these characteristics are shown in the table below. As shown in the table below, the open circuit voltage and fill factor are improved, and the maximum output is improved by about 13%. This indicates that by making the bandgap of the I layer at the edge of the P layer wider than that of the P layer, diffusion of carriers in the opposite direction and recombination can be reduced.

Next, another embodiment according to the present invention will be described. A cross-sectional view showing the structure of the amorphous solar cell is similar to that shown in FIG.
This is a modification of the method of changing the composition of Ge in the layer. Flow rate ratio of SiH ₄ and GeH ₄ and SiH ₄ and CH ₄ in forming I layer
The flow rate ratio is changed as shown in FIG.
Half of the I layer on the N / I interface side is a-Si, and on the P / I interface side,
GeH ₄ (GeH ₄ / SiH ₄ ) = 7.5% to 15% and then GeH ₄
The flow rate ratio is gradually reduced to only SiH ₄ when the film thickness is 2600Å, and then the CH ₄ flow rate is gradually increased from 0 to a film thickness of 300
It is formed so that CH ₄ / (CH ₄ + SiH ₄ ) = 50% at the time of 0Å. The band gap at this time is as shown in FIG.

The current-voltage characteristic of the a-SiGe solar cell configured as described above is shown by the alternate long and short dash line in FIG. The characteristics are shown in the table below. As shown in the table below, in this embodiment, although the short-circuit current was reduced as compared with the conventional one, the open-circuit voltage and the fill factor were improved, and the maximum output was improved by about 8%. Thus, even if the band gap of the I layer changes stepwise, the characteristics are not significantly deteriorated.
Rather, it was found that the open-circuit voltage and fill factor are improved by using an a-Si film having good characteristics.

As described above, according to the present invention, the band gap of the I layer is made wider than the band gap of the P type impurity doped layer at the interface between the I layer and the P type impurity doped layer. A highly efficient solar cell can be obtained by changing the band gap inside.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing the structure of an a-SiGe solar cell as one embodiment of the present invention, and FIG. 2 is GeH ₄ / (SiH ₄ at the time of producing an I layer in one embodiment of the present invention. ＋ Ge
H ₄ ) and CH ₄ / (CH ₄ + SiH ₄ ) with time, FIG. 3 is a band structure diagram of the solar cell produced by the example, and FIG. 4 is a diagram of the a-SiGe solar cell according to the present invention. FIG. 5 is a diagram showing an example of current-voltage characteristics, FIG. 5 is a temporal change diagram of GeH ₄ / (SiH ₄ + GeH ₄ ) and CH ₄ / (SiH ₄ + CH ₄ ) in another embodiment according to the present invention, FIG. FIG. 7 is a band structure diagram of a solar cell manufactured according to the example, and FIGS. 7 and 8 are band structure diagrams of a solar cell having a conventional structure compared in the examples. 1 ... Stainless substrate, 2 ... N-type a-Si layer, 3 ... Intrinsic layer, 3a ... I-type a-SiGe layer, 3b ... I-type a-SiC layer,
4 ... P-type a-SiC layer, 5 ... Transparent conductive layer, 6 ... Al
Collector electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasumi Inoue 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Within Sharp Corporation (72) Inventor Hitoshi Sannomiya 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Incorporated (72) Inventor Manabu Ito 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (56) References JP-A-59-55077 (JP, A) JP-A-63-199466 (JP) , A) JP 64-71182 (JP, A) JP 60-258975 (JP, A)

Claims (2)

[Claims] 1. An amorphous semiconductor film, which comprises:
p-type impurity-doped layer, i-layer, n-type impurity-doped layer
In the amorphous semiconductor solar cell having a PIN structure, a band gap of the i layer in contact with a side end of the p-type impurity doped layer is wider and discontinuous than a band gap of the p-type impurity doped layer, An amorphous semiconductor solar cell, which has a region that is gradually reduced from the type impurity doped layer side and is narrower than the band gaps of the p type impurity doped layer and the n type impurity doped layer. 2. The amorphous semiconductor solar cell according to claim 1, wherein the band gap of the i layer between the region and the n-type impurity doped layer is discontinuously changed. .