JPS609178A - Photovoltaic device - Google Patents

Abstract

PURPOSE:To improve conversion efficiency by forming at least one layer in a P type layer and an N type layer forming a P-N junction as the connecting section of a unit photovoltaic cell into a crystallite semiconductor layer. CONSTITUTION:Unit photovoltaic cells using amorphous semiconductor layers with P-I- N type junctions are laminated on a conductive substrate 21 in succession, and the P-N junctions as the connecting sections of the unit photovoltaic cells in a photovoltaic device, on the surface thereof a conductive layer is formed, are shaped in crystallite semiconductor layers. Consequently, when the crystallite semiconductor layers are used as the P-N junctions, junctions, rectification factors thereof are smaller than any P-N junctions by an amorphous semiconductor and recombination currents in junction surfaces thereof are large, are obtained. Accordingly, the short-circuiting photocurrents JSC of the photovoltaic device can be increased. When forming the P-N junctions, optical forbidden band width also extends over 1.7eV or more and the quantity of beams being absorbed is reduced sufficiently in a P type layer even when the film thickness of the P type layer and an N type layer is selected properly within a range of 50Angstrom or more in which the junctions of the unit cells can be formed appropriately because an impurity need not be doped in high concentration. High release voltage can be obtained by said effect, and conversion efficiency is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to photovoltaic devices such as solar cells and photodetectors.

[Prior art and its problems] Amorphous semiconductors obtained by glow discharge decomposition of silane (8'iH+) and fluorosilicon (SiF4) compound gas have an average localized level in the forbidden band width. Density is 131
In recent years, it has been confirmed that amorphous semiconductors can be made small to 76m" or less, making it possible to control P-type and N-type impurities. Since then, amorphous semiconductors have attracted attention as solar cell materials that are low-cost and easy to mass-produce. .

One of the advantages of the amorphous semiconductor formation method using the glow discharge decomposition method is that P-type, I-type, and N-type amorphous semiconductor layers can be formed by simply changing the type of gas introduced into the glow discharge atmosphere. A desired number of layers can be formed easily and continuously in any order.

Taking advantage of this advantage, it is possible to stack a plurality of unit photovoltaic cells composed of P-type, ■-type, and N-type amorphous silicon layers, that is, pin/pin/p...
A photovoltaic device that independently generates a high voltage by forming a multilayer thin film with an n/pin structure has been proposed (
JP-A-55-125680).

This takes advantage of the fact that in the P-N junction, which is the junction of the unit photovoltaic cells that make up the photovoltaic device, the recombination current at the junction surface is extremely large and the rectification characteristics of the junction are poor. It is. When light enters each unit photovoltaic cell, the generated electron-hole pairs are separated by the built-in electric field,
They move in opposite directions and recombine at the P-N junction,
The recombination current often forms the only current flowing between adjacent unit photovoltaic cells.

Therefore, the equivalent circuit of the photovoltaic device described above can be written as shown in FIG.

In FIG. 1, 11, 12, 13 are DC current sources,
14゜15, 16 are diodes, 17, 18, 1
9 is resistance.

As a result, the open-circuit voltage VOC becomes the sum of the open-circuit voltages of each unit cell due to the series connection effect, while the short-circuit photocurrent Jsc is constrained by the minimum value among the short-circuit photocurrents generated by each unit cell. It turns out.

Furthermore, the smaller the degree of dominance of the recombination process at the P-N junction surface, that is, the better the rectification characteristics of the junction, the smaller the recombination current becomes, resulting in a loss of the short-circuit photocurrent Jsc.

However, in the conventional photovoltaic device, P
-N junction is formed only by an amorphous semiconductor layer (particularly an amorphous silicon layer), and in that case, the rectification ratio is about one order of magnitude or more at an applied voltage of ±1V, so short circuits occur inside each unit sale. There was a drawback that the photocurrent could not be sufficiently converted into a recombination current θ.Also, as a way to compensate for this drawback, P-N was doped at a very high concentration (dopant concentration > 1at5J).
Although it is possible to increase the recombination current using junctions,
In that case, the optical bandgap width of the P-type layer must be narrow, especially less than 1.7e■, and the amount of light absorbed there increases, so the mass reaching the unit cell located on the back side with respect to the incident light As a result, the optical short circuit current decreases. On the other hand, if the thickness of the P1 type layer is made thin < (201~50X), the amount of light absorbed there can be reduced, but in this case, the junction of the unit cell will not be formed properly, and as a result, the open circuit voltage VOC will be lowered. The problem arises that something happens.

As mentioned above, in the conventional photovoltaic device,
When a P-N junction is formed using K only in the amorphous semiconductor layer,
Due to the above-mentioned problems, there is a drawback that a loss in conversion efficiency occurs.

[Object of the Invention] Therefore, an object of the present invention is to improve the conversion efficiency in a multilayer thin film photovoltaic device capable of outputting high voltage compared to the conventional one.

[Summary of the Invention] The present invention is configured such that at least one layer of a P-type layer and an N-type layer forming a P-N junction, which is a connecting portion of a unit photovoltaic cell, is a microcrystalline semiconductor layer. It is something.

[Effect of the invention] The activation energy △E of an amorphous semiconductor is P type.

Both N type and very high concentration doping (dopant concentration) 1
At%), the doping efficiency is about 0.2 eV, but the doping efficiency is extremely low in -Machicrystalline semiconductors because microcrystals with a grain size of several 10X to several IooX exist in the amorphous network. is high, and ΔE can be easily reduced to about Q, leV or less without high-concentration doping.

Therefore, if a microcrystalline semiconductor layer is used for the P-N junction, the rectification ratio will be smaller than any P-N junction made of an amorphous semiconductor.
In addition, a joint with a large recombination basket at the joint surface can be obtained.

Therefore, with the configuration of the present invention, the short-circuit photocurrent Jsc of the photovoltaic device can be increased.

When forming a P-N junction, it is not necessary to do high concentration doping, so the optical bandgap must be 1.7 eV or more. Even if you choose appropriately within the range of 501 or more that is sufficient to form
The amount of light absorbed in the mold layer is sufficiently small. This effect makes it possible to obtain a higher open circuit voltage than in the past.

That is, due to the above effects, an improvement in conversion efficiency is realized compared to the conventional method.

[Embodiments of the invention] An embodiment of the present invention will be described below.

The raw material gas introduced into the vacuum container is 2 to 100 SiH.
4/H2, 2500p9mB2H6/H2, 2500p
pm PH3/H2, and when forming a doping layer, B2H6/81H4 = 0.1-2%, PH3
The mixing ratio of these gases was adjusted so that /S 1H4 = 0.1 to 3. After introducing the raw material gas into a vacuum reaction vessel, the gas pressure was 0.3 to 5 Torr, and the applied high frequency power was 5 to 200 W at a frequency of 13.56 MHz.
The substrate temperature was adjusted to 200 to 350°C to form a thin film. At this time, the growth rate of the film is 1000X-10000X7hr for p-type and n-type layers, and 2x for i-type layer.
000A to 15000X/hr.

Therefore, the thickness of each layer constituting the photovoltaic device can be arbitrarily and easily set by adjusting the formation time.

The microcrystalline semiconductor film, which is a microcrystalline silicon film in this example, is formed under the following conditions:
High gas pressure of 5 Torr, applied high frequency power is 50~2
The power was as high as 00 W, and the hydrogen dilution of silane gas (S 1H4) was in a relatively large range of 2% to 10%. However, it was easily confirmed by the X-ray diffraction image that the film formed under these conditions was a microcrystalline semiconductor film.

FIG. 2 is a sectional view of a photovoltaic device in which the number of unit cells is three, for example. Figure 2 shows an example in which the P-type and N-type layers constituting the PN junction are both microcrystalline silicon films, but even if only one is a microcrystalline silicon film, similar or equivalent effects can be obtained. can get.

In Figure 2, 21 is a stainless steel base! , 32.35
.

Wings are amorphous silicon i-type 2 layers, 33, :36,3
9 is a microcrystalline silicon n-type layer, 31 is an amorphous silicon P-type layer, 34°37 is a microcrystalline silicon P-type layer, 31
, 32, 33 are the 1st P of 22. Otsu's 2nd P is 34, 35, 36, and Sae's 3rd P is 37, 38, 39.
It constitutes an IN-type unit photovoltaic cell, and 5 is a conductive layer film on the surface such as ITO.

Incident light such as sunlight is irradiated from 25. □Of course, the present invention is also effective even if the unit cell structure is an NIP type, contrary to that shown in FIG.

Figure 2 shows the case where the number of unit cells is n = 3, but
Other than this, similar structures were formed with n=1.2 and 4.5. In these five samples, P type,
A microcrystalline silicon layer is used as the N-type layer, and the thickness of each layer is set so that the unit cell junction is not insufficient, and the effect of the microcrystalline layer is to minimize light absorption at the PN junction. It was set to 60-120X.

The i-type layer is an amorphous silicon layer, and its thickness is determined so that the photocurrent generated in each unit cell is equal and maximum, and is designed to become thicker as it goes deeper from the incident light side. However, they were within the range of 100 to 10,000 A.

On the other hand, for comparison, there is also a photovoltaic device in which one layer is exactly the same and the PN junction is formed of amorphous silicon, n=1.2.
Five sets of 3, 4.5 were formed.

At this time, in order to increase the recombination current, a heavily doped P+N+ junction was formed, but in order to reduce light absorption in the P+ type layer, the thickness of the same layer was reduced to 20 to 50 K, and The thickness of the n-type layer was 60 to 100X.

Figure 3 summarizes the above experimental results.
A M −1,1 when formed using bonding or microcrystalline silicon (solid line) and when formed using amorphous silicon (broken line)
This is a comparison of conversion efficiency under irradiation of 00 mw/m.

In Figure 3, the horizontal axis represents the number of unit cells and the vertical axis represents the conversion efficiency in arbitrary units. By Jsc, Vo
It can be clearly seen that the conversion efficiency is certainly improved since both c and c become larger.

[Other embodiments of the invention] In addition to forming a PN junction, which is a junction of a unit photovoltaic cell, with a microcrystalline silicon layer (hereinafter microcrystalline is referred to as μC), pc-8ic, 1tc-8i (3e,
Even if it is formed using μc-81GeC, μC-Ge or ttc-GeC film alone or in combination,
The present invention is effective. Also, for example, the layers 34 and 37 may be
-8iC, P with 33 and 36 as μc-8i:H-
The N junction may be a heterojunction. If this is done, many interface states will be generated at the junction due to the difference in lattice constant, and the recombination current will become stronger, making it possible to further increase Jsc. Of course, a heterojunction may be realized by a combination of μc-8iGe/μc-8i:H or the like.

In order to contain germanium Ge, a four-coordinate element, in the film, germane gas (GeH4) is mixed in the introduced gas, and in order to contain carbon C, a hydrocarbon gas such as CH4 is mixed. good.

In the above embodiments, an example was shown in which the i-type layer was formed of an amorphous silicon layer. If a silicon germanium layer is used as the type 1 layer, the efficiency can be further improved. In this case, the structure may be such that the optical forbidden band width of the i-type layer becomes narrower as the depth increases from the incident light side.

In addition to forming a PN junction with a single P-type layer and an N-type layer, it is also possible to form a P-type layer with a combination of an amorphous semiconductor layer and a microcrystalline semiconductor layer and connect it to the N layer. There is no problem as long as the P-type layer to be used is a microcrystalline layer.

This also applies to the N-type layer. Although the embodiments have been described with reference to the case where the substrate is a stainless steel substrate, it goes without saying that a glass plate or an organic film coated with an interconductive film such as ITO may also be used as the substrate.

[Brief explanation of the drawing]

Fig. 1 is an equivalent circuit diagram of a photovoltaic device, Fig. 2 is a sectional view showing an example of the photovoltaic device of the present invention, and Fig. 3 is a comparison of characteristics of the photovoltaic device of the present invention with conventional examples. It is a diagram. In the figure, 11, 12, 13... DC current sources, 14, 15
, 16...diode, 17, 18, 19...
・Resistance, 21... Board, 22, 23, 24...
Unit photovoltaic cell, 5... conductive layer. Agent Patent attorney Kensuke Chika (1 other person)

Claims (1)

[Claims] (]) A photovoltaic device in which a plurality of unit photovoltaic cells using an amorphous semiconductor layer having a PIN type junction are sequentially stacked on a conductive substrate, and a conductive layer is provided on the surface of the unit photovoltaic cells. In the device, a
A photovoltaic device characterized in that at least one of the type layer and the N-type layer is a microcrystalline semiconductor layer. (2) A patent characterized in that the four-coordinated element constituting the amorphous semiconductor layer contains at least one of silicon, germanium, tin, lead, and carbon. (3) Constituting a microcrystalline semiconductor layer 2. The photovoltaic device according to claim 1, wherein the four-coordinated element is silicon. The photovoltaic device according to item 1. (5) The photovoltaic device according to claim 1, wherein the four-coordinate elements constituting the microcrystalline semiconductor layer are silicon and germanium. (6) The photovoltaic device according to claim 1, wherein the four-coordinated elements constituting the microcrystalline semiconductor layer are silicon, germanium, and carbon. (Photovoltaic device according to claim 1, characterized in that the 4-coordination element constituting the microcrystalline semiconductor layer is germanium. (8) The 4-coordination element constituting the microcrystalline semiconductor layer The photovoltaic device according to claim 1, characterized in that the elements are germanium and carbon. (9) The P-type layer and the N-type layer are constituted by a microcrystalline semiconductor layer,
The photovoltaic device 0 according to claim 1, characterized in that the PN junction is a Peter junction.