JPH11274527A - Photovoltaic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable application of microcrystal silicon wherein band gap is widened and optical deterioration is little to a photovoltaic layer, by containing nitrogen element in the whole of a photovoltaic layer or at least a region being in contact with a light incidence side doped layer. SOLUTION: In this photovoltaic device, a transparent electrode 2 is formed on an insulating substrate 1 composed of glass. On the electrode 2, a P-type microcrystal Si layer 3, an I-type microcrystal Si: N layer 4 and N-type microcrystal 5 are laminated in order. On the N-type microcrystal Si layer 5, a rear electrode 7 composed of aluminum is formed. A light enters from a translucent substrate 1. The I-type microcrystal Si: N layer 4 turns to somewhat of an N-type in which nitrogen acts as donor. In this case, when film thickness is increased, the level due to nitrogen acts as recombination center, and gathering effect is decreased. In order to cancel the nitrogen donor, it is desirable to introduce group III element like boron.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photovoltaic device using microcrystalline silicon for a photovoltaic layer.

[0002]

2. Description of the Related Art Conventionally, glow discharge decomposition of raw material gas and light C
Amorphous silicon (hereinafter a-S) formed by the VD method
Write i. The photovoltaic device using () as a main material has features that it can be easily formed into a thin film and has a large area, and is expected as a low-cost photovoltaic device.

[0003] The structure of this type of photovoltaic device includes p
A pin-type a-Si photovoltaic device having an in-junction is common. FIG. 11 shows the structure of such a photovoltaic device, in which a transparent electrode 22 and a p-type a-Si
It is formed by sequentially laminating a layer 23, an i-type a-Si layer 24, an n-type a-Si layer 25, and a metal electrode 26. In this photovoltaic device, photovoltaic power is generated by light incident through the glass substrate 21.

It is known that the a-Si photovoltaic device described above undergoes photodegradation after light irradiation. Therefore, microcrystalline silicon is a thin film and has high stability to light irradiation, and a photovoltaic device using this microcrystalline silicon for a photovoltaic layer has been proposed (see, for example, Japanese Patent Application Laid-Open No. H05-1993).
See No. 10055. ). This microcrystalline silicon is a thin film in which a microcrystalline Si phase and an a-Si phase are mixed.

Further, in order to use this microcrystalline silicon for a window layer of a photovoltaic device, it has been proposed to include oxygen at the time of forming the microcrystalline silicon (for example, see Japanese Unexamined Patent Publication No.
See 267868. ). Further, there has been proposed a photovoltaic device in which a microcrystalline silicon layer is inserted into a photovoltaic layer to obtain high light scattering (for example, Japanese Patent Application Laid-Open No.
See Japanese Patent Publication No. 31435. ).

As described above, microcrystalline silicon is a film in which a microcrystalline Si phase and an a-Si phase are mixed, and the ratio of the two, that is, the crystallization ratio, is such as a condition for forming a microcrystalline silicon film. It changes with. It is known that microcrystalline silicon has different photodegradation in photoconductivity due to a difference in crystallization rate.
Here, FIG. 3 shows a result of measuring a change in photoconductivity of the microcrystalline silicon film and the amorphous silicon film having different crystallization rates before and after light irradiation. Light irradiation is light intensity 500mW / c
m ² for 5 hours at 25 ° C. The change in the photoconductivity before and after the light irradiation is, as shown in FIG.
%, The photoconductivity hardly changes, whereas microcrystalline silicon having a low crystallization rate of 47% shows a decrease in photoconductivity similarly to amorphous silicon. Therefore, it is understood that when microcrystalline silicon having a low crystallization rate is used for a photovoltaic device, photodegradation occurs. Therefore, in order to prevent light degradation, it is preferable that the crystallization ratio of microcrystalline silicon be as high as possible.

Here, a method of calculating the crystallization ratio will be described. The peaks of the signals is crystalline component by Raman spectroscopy of the microcrystalline silicon in the vicinity of 520 cm ^-1 and 510 cm ^-1,
It has been found that the amorphous component exists around 480 cm ^-1 . Then, fitting is performed for each peak by the sum of Gaussian distribution as shown in FIG. At this time, the ratio of the crystal component to the total area, that is, the value calculated based on the equation (1) is defined as the crystallization ratio.

[0008]

(Equation 1)

[0009]

By the way, when the above-mentioned microcrystalline silicon is used as a photovoltaic layer of a photovoltaic device, the short-circuit current is higher than that of amorphous silicon because of its photosensitivity over a wide wavelength range. Is obtained. However, since the band gap is small, the open-circuit voltage is low, and a very high photoelectric conversion efficiency has not been obtained.

On the other hand, when oxygen is mixed into microcrystalline silicon, the band gap tends to widen, but the crystallization rate is remarkably reduced, and the photosensitivity is lost particularly in a long wavelength region. Did not improve much. Furthermore, photostability has been lost as the crystallization rate decreases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and provides a photovoltaic device using microcrystalline silicon for a photovoltaic layer without widening the band gap and light degradation. The purpose is to do.

[0012]

SUMMARY OF THE INVENTION The present invention provides a photovoltaic device in which a semiconductor layer in which microcrystalline silicon and amorphous silicon coexist is used as a photovoltaic layer. Is characterized by containing a nitrogen element in a region in contact with.

As described above, by introducing nitrogen atoms into the microcrystalline silicon film, microcrystalline silicon having a wide band gap can be obtained, and the open-circuit voltage can be improved.

[0014] The semiconductor layer is formed with a crystallization ratio higher than 47%, and preferably, the crystallization ratio of the semiconductor layer is 52%.
It is good to do above.

When the crystallization ratio of the semiconductor layer exceeds 47%, the slope of the characteristic curve of the deterioration rate of the photoconductivity changes, and by setting the crystallization rate to about 52% or more, the deterioration rate is greatly increased. Can be improved.

Further, the semiconductor layer can be formed by decomposing a mixed gas having a flow rate ratio of nitrogen gas to silane of less than 30%.

The concentration of nitrogen contained in the semiconductor layer is about 1
× 10 ²¹ cm ^-3 or less, preferably about 0.8 × 10 ²¹ cm
^It is good to be ^-3 or less.

By controlling the nitrogen concentration of the semiconductor layer film as described above, a film having a reduced rate of photoconductivity deterioration can be obtained.

It is preferable to introduce a Group 3 element into the region containing the nitrogen atom.

By introducing a Group 3 element, a nitrogen donor is offset, and a decrease in collection efficiency can be prevented.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. The microcrystalline silicon containing nitrogen atoms used in the present invention (hereinafter, microcrystalline Si: N
That. ) The film is formed by a plasma CVD method.
Table 1 shows the forming conditions. For comparison, conditions for forming a microcrystalline silicon (hereinafter, microcrystalline Si) film containing neither oxygen nor nitrogen and a microcrystalline silicon (hereinafter, microcrystalline Si: O) film containing oxygen are shown in the table. It is shown in FIG. However, oxygen is also contain 5 × 10 ¹⁸ cm ^-3 or less of oxygen and 5 × 10 ¹⁸ cm ^-3 or less of nitrogen even in the microcrystalline Si film that does not contain nitrogen.

[0022]

[Table 1]

[0023]

[Table 2]

As described above, the microcrystalline Si refers to a microcrystalline Si phase in which silicon grains having a crystal structure with a diameter of several tens to several hundreds of angstroms are gathered and a-
It is a mixture of Si phases.

FIG. 1 shows the relationship between the flow rate ratio (N ₂ / SiH ₄ ) of nitrogen (N ₂ ) gas to silane (SiH ₄ ) of the microcrystalline Si: N film prepared under the conditions shown in Table 1 and the nitrogen concentration in the film. FIG. 2 shows the relationship between the flow rate ratio of nitrogen gas to silane (N ₂ / SiH ₄ ) and the crystallization ratio.

As can be seen from FIG. 1, the nitrogen concentration in the film can be controlled by changing the nitrogen flow ratio, that is, by controlling the nitrogen flow. As can be seen from FIG. 2, the crystallization rate decreases as the nitrogen flow rate increases. Similarly, it has been found that the crystallinity of microcrystalline Si: O decreases with an increase in the amount of carbon dioxide.

As described above, FIG. 3 shows the results of measuring the change in photoconductivity before and after light irradiation in microcrystalline Si and a-Si having different crystallization ratios. As shown in FIG. 3, the change in photoconductivity before and after light irradiation is that microcrystal Si having a crystallization rate of 64% has almost no change in photoconductivity and no photodegradation. In microcrystalline Si having a low crystallization ratio of 47%, a decrease in photoconductivity is observed as in the case of a-Si, and it can be seen that photodeterioration occurs.

Next, in order to compare the spread of the band gap, FIG. 4 shows the results of measuring the absorption coefficient (α) when the nitrogen flow rate (N ₂ / SiH ₄ ) for silane was changed. As can be seen from FIG. 4, it can be seen that the decrease in the absorption coefficient shifts to the higher energy side as the percentage of the nitrogen flow rate increases. In particular, the shift after exceeding 8% is more remarkable. This indicates that the band gap is widened by increasing the nitrogen flow rate.

In the present invention, the band gap is represented by a straight line α = 10 in the light absorption characteristic diagram shown in FIG.
It is defined as the X coordinate of the intersection of 0 and the absorption coefficient curve.

FIG. 5 is a graph showing the relationship between the crystallization ratio of microcrystalline Si and the deterioration ratio of photoconductivity. According to FIG. 5, it can be seen that the gradient of the characteristic curve changes when the crystallization ratio exceeds 47%, and that the deterioration ratio sharply increases after exceeding 50%. Further, it can be seen that by setting the crystallization rate to about 52%, the deterioration rate can be reduced to half or less as compared with the case where the crystallization rate is 47%.

FIG. 6 shows microcrystalline Si: O and microcrystalline Si:
FIG. 3 is a diagram showing a relationship between each band gap of N and a crystallization ratio. From this figure, it can be seen that even with the same band gap, microcrystalline Si: N can provide a microcrystalline film with a higher crystallization ratio. Therefore, it can be seen from FIGS. 5 and 6 that the microcrystalline Si: N can reduce the deterioration rate of the photoconductivity as compared with the microcrystalline Si: O even at the same band gap.

FIG. 6 shows that these crystallization rates
On the other hand, the band gap is 1.4 when the crystallization rate is 50%.
At the time of 9 eV and the crystallization rate of 52%, the value is 1.48 eV. N
_Two/ SiH_FourFIG. 7 showing the relationship between the flow ratio and the band gap
At this time, the flow ratio of nitrogen gas to silane (N_Two/ S
iH_Four) Are about 30% and about 25%, respectively. In FIG.
According to this, the nitrogen concentration at this time is about 1 × 10^{twenty one}cm
^-3, About 0.8 × 10^{twenty one}cm^-3It is. Therefore, the microcrystal S
The concentration of nitrogen contained in the i: N film is about 1 × 10 ^{twenty one}cm^-3
And more preferably about 0.8 × 10^{twenty one}cm^-3Less than
Below.

Although the lower limit of the concentration of nitrogen contained in the microcrystalline Si: N film is not particularly limited, it is necessary to contain nitrogen in order to widen the band gap. Even if a gas serving as a nitrogen source such as (N ₂ ) is not added, 5 × 10 ¹⁸
Since nitrogen of about cm ^-3 is contained, a higher nitrogen concentration is required.

As described above, by introducing nitrogen atoms into the microcrystalline silicon film, microcrystalline silicon having a wide band gap can be obtained, and the open-circuit voltage can be improved. Also,
As the content of both oxygen atoms and nitrogen atoms increases, the crystallization rate of the microcrystalline silicon film decreases, but the crystallization rate of nitrogen atoms becomes higher when compared at the same band gap. Therefore, deterioration due to light irradiation is small, and the microcrystalline silicon film containing nitrogen atoms can be used as a film effective as a photovoltaic layer.

Next, an embodiment of a photovoltaic device using a microcrystalline Si: N film as a photovoltaic layer according to the present invention will be described with reference to FIG. FIG. 9 is a sectional view showing a photovoltaic device according to one embodiment of the present invention.

As shown in FIG. 9, a transparent electrode 2 made of SnO ₂ and having a thickness of 6000 Å is provided on an insulating light-transmitting substrate 1 made of glass. On this transparent electrode 2, a 100-Å-thick p-type microcrystal S
An i layer 3, an i-type microcrystalline Si: N layer 4 having a thickness of 3 μm, and an n-type microcrystalline Si layer 5 having a thickness of 200 Å are sequentially laminated. A back electrode 6 made of aluminum (Al) is provided on the n-type microcrystalline Si layer 5. Light enters from the translucent substrate 1 side. p-type microcrystalline Si
Layer 3, i-type microcrystalline Si: N layer 4, and n-type microcrystalline Si layer 5
Was formed by a plasma CVD method using high hydrogen dilution.

Here, the i-type microcrystalline Si: N layer 4 is slightly n-type with nitrogen (N) as a donor. In this case, when the film thickness is large, the level due to nitrogen acts as a recombination center, which causes a decrease in collection efficiency. Therefore, a Group 3 element such as boron may be introduced so that the nitrogen donor is offset.
By introducing a Group 3 element such as boron, a decrease in collection efficiency can be prevented.

Next, an i-type microcrystalline Si film and an i-type microcrystalline
Photovoltaic device using crystalline Si: O film as photovoltaic layer
Formed. Oxygen (O) also acts as a donor for microcrystalline Si: O.
In the same manner, a Group 3 element was introduced in the same manner. In addition, microcrystal S
i: Nitrogen concentration in N is 6 × 10 ²⁰cm^-3As the result
A film having a crystallization ratio of about 60% was used. Microcrystalline Si: O
The film is made to match the crystallization rate of microcrystalline Si: N, and the crystallization rate is about
The oxygen concentration was controlled to be 60%. Furthermore, microcrystals
The crystallization ratio of the Si film is 70%.

Table 3 shows the results of comparing the solar cell characteristics of cells using the microcrystalline silicon material as a photovoltaic layer. still,
Table 3 standardizes the case where a photovoltaic layer is an i-type microcrystalline Si film having a thickness of 3 μm.

[0040]

[Table 3]

As shown in Table 3, when the microcrystalline Si: N film was used for the photovoltaic layer, the open-circuit voltage (V
oc), the microcrystalline Si film or microcrystalline Si:
It exceeds the photoelectric conversion efficiency of a photovoltaic device using an O film as a photovoltaic layer. In this embodiment, i-type microcrystal S
Although the thickness of the i: N layer is 3 μm, the thickness is not limited to this and can be changed by the absorption coefficient or diffusion length of the film.

In the above embodiment, the entire photovoltaic layer is a microcrystalline Si: N film. However, the photovoltaic layer may be applied to at least a region in contact with the light incident side doped layer. The ratio of the microcrystalline Si: N film applied to the region in contact with the light incident side doped layer to the photovoltaic layer is 30%, and the other regions are microcrystalline Si.
Table 4 shows the results of comparison between the photovoltaic layer formed as a film and the photovoltaic layer formed entirely of a microcrystalline Si film. In addition, microcrystal S
The crystallization ratio of the i: N film is 60%, and the crystallization ratio of the microcrystalline Si film is 70%.

[0043]

[Table 4]

In the case where a part is made of a microcrystalline Si: N film, the conversion efficiency is improved more than in the case where the whole is made of a microcrystalline Si: N film. This is probably because the conversion efficiency was further improved by suppressing the increase in the resistance component in the film thickness direction due to the decrease in the crystallization ratio.

FIG. 10 is a sectional view showing a photovoltaic device according to another embodiment of the present invention. Note that the same parts as those in the above-described embodiment are denoted by the same reference numerals, and description thereof will be omitted. This embodiment has a structure in which semiconductor layers having a pin structure are stacked in several stages. That is, the glass substrate 1
A transparent conductive film 2 is provided thereon, and a p-type semiconductor layer 7, i
The type semiconductor layer 8 and the n-type semiconductor layer 9 are stacked in several steps in this order.

In the case of such a photovoltaic device, the band gap of each photovoltaic layer material is maximized on the light incident side,
Thereafter, the wavelength region that can be absorbed by each layer is determined by sequentially reducing the size, and a photovoltaic device having high sensitivity in a wide wavelength region can be obtained. Microcrystalline Si is often used as the last layer from the light incident side of these stacked photovoltaic devices. By employing the i-type microcrystalline Si: N according to the present invention as the last photovoltaic layer, the same effect as in the first embodiment can be obtained.

[0047]

As described above, by using microcrystalline Si: N containing nitrogen in microcrystalline Si as a photovoltaic layer, the band gap can be made wider than that of microcrystalline Si or microcrystalline Si: O. It is possible to improve the conversion efficiency and to prevent light deterioration. The same effect can be obtained by using microcrystalline Si: N in a region in contact with the doped layer on the light incident side.

[Brief description of the drawings]

FIG. 1 shows silane (SiH) of a microcrystalline Si: N film._FourAgainst)
Nitrogen (N_Two) Gas flow ratio (N _Two/ SiH_Four) And nitrogen in the membrane
FIG. 4 is a characteristic diagram illustrating a relationship between element concentrations.

FIG. 2 shows silane (SiH) of a microcrystalline Si: N film._FourAgainst)
Nitrogen (N_Two) Gas flow ratio (N _Two/ SiH_Four) And crystallization
It is a characteristic view which shows the relationship of a rate.

FIG. 3 is a characteristic diagram showing a relationship between crystallization rate and photodegradation in photoconductivity.

FIG. 4 shows silane (SiH) of a microcrystalline Si: N film._FourAgainst)
Nitrogen (N_Two) Gas flow ratio (N _Two/ SiH_Four) And absorber
It is a characteristic view which shows the relationship of a number.

FIG. 5 is a characteristic diagram showing a relationship between a crystallization rate of microcrystalline Si and a deterioration rate of photoconductivity.

FIG. 6 is a characteristic diagram showing the relationship between the respective band gaps and the crystallization ratio of microcrystalline Si: O and microcrystalline Si: N.

FIG. 7 shows silane (SiH) of a microcrystalline Si: N film._FourAgainst)
Nitrogen (N_Two) Gas flow ratio (N _Two/ SiH_Four) And band
FIG. 4 is a characteristic diagram illustrating a relationship between gaps.

FIG. 8 is a distribution diagram for calculating a crystallization ratio.

FIG. 9 is a sectional view showing a photovoltaic device according to one embodiment of the present invention.

FIG. 10 is a sectional view showing a photovoltaic device according to another embodiment of the present invention.

FIG. 11 shows a pin-type a-Si having a conventional pin junction.
It is an outline sectional view showing the structure of a photovoltaic device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Insulating translucent substrate 2 Transparent electrode 3 p-type microcrystalline Si 4 i-type microcrystalline Si: N 5 n-type microcrystalline Si 6 Back electrode

Claims (7)

[Claims] 1. A photovoltaic device in which a semiconductor layer in which microcrystalline silicon and amorphous silicon coexist is used as a photovoltaic layer.
A photovoltaic device, characterized in that a nitrogen element is contained in the entire photovoltaic layer or at least in a region in contact with the light incident side doped layer. 2. The photovoltaic device according to claim 1, wherein the semiconductor layer has a crystallization ratio higher than 47%. 3. The photovoltaic device according to claim 1, wherein the crystallization ratio of the semiconductor layer is 52% or more. 4. The photovoltaic device according to claim 1, wherein the semiconductor layer is formed by decomposing a mixed gas having a flow rate ratio of nitrogen gas to silane of less than 30%. Power equipment. 5. The semiconductor device according to claim 1, wherein the concentration of nitrogen contained in the semiconductor layer is about 1 × 10 ²¹ cm ⁻³ or less.
5. The photovoltaic device according to any one of claims 4 to 4. 6. The method according to claim 1, wherein the concentration of nitrogen contained in said semiconductor layer is about
0.8 × 10^{twenty one}cm ^-3Claims characterized by the following
Item 6. A photovoltaic device according to Item 5. 7. The photovoltaic device according to claim 1, wherein a group III element is introduced into the region containing the nitrogen atom.