JPS6284571A - Multilayer structure type amorphous solar cell - Google Patents

Abstract

PURPOSE:To utilize long wavelength beams effectively, and to obtain high conversion efficiency by forming electrode layers shaped onto a substrate in succession, amorphous thin-film layers constituting a plural pair of PIN junctions and electrode layers pairing with said electrodes and composing at least one of said I layers by an a-Ge:H film. CONSTITUTION:An a-Ge:H film, film quality thereof does not lower on the basis of alloying, etc., is substituted for a conventional a-SiGe:H film in an I layer controlling long wavelength sensitivity (conversion efficiency), and forbidden band width E9 is lowered. Glass is used as a substrate 1, ITO (2,000Angstrom ) as a transparent electrode 2 and an a-Si:H film (400Angstrom ), an a-SiGe:H film (1,500Angstrom ) and an a-Ge:H film (5,000Angstrom ) as I1, I2 and I3 respectively, and other P1-P3 and N1-N3 are each shaped by a-Si:H films, and film thickness is brought respectively to 100Angstrom . The forbidden band width E9 of said I1-I3 is each brought to 1.8eV (I1), 1.6eV (I2) and 1.1eV (I3).

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to amorphous silicon solar cells. More specifically, the present invention relates to a multilayer amorphous solar cell that can convert light in a particularly long wavelength range with high efficiency.

Conventional technology Amorphous semiconductors can be made thinner and larger in area.
It is used in various devices because it has a wide degree of freedom in composition and can adjust electrical and optical properties over a wide range.However, because of its low carrier mobility, crystalline semiconductors are was not noticed. But 19
Around 1968, it was announced that chalcogenide-based amorphous semiconductors could be applied to high-speed switching devices, etc., and since then they have attracted a great deal of attention, and various application examples have already been submitted and considerable results are being achieved.

In recent years, solar cells that utilize clean, non-depletable solar energy have attracted attention, and amorphous silicon as described above is expected to be a low-cost solar cell material. However, compared to crystalline semiconductor solar cells such as Si and GaAs, photoelectric conversion efficiency is low, and they have not been widely used as a power source.

The reason why amorphous semiconductors have poor photoelectric conversion efficiency is that there are many localized levels in the forbidden band of amorphous semiconductors, which have low mobility and bind each other. This method has drawbacks such as the need for a large electric field in order for the hole pairs to move freely.

In addition, in order to increase the photoelectric conversion efficiency of solar cells, it is important to effectively utilize light energy over a wide wavelength range, and for this purpose, it is important to use modifiers, such as silicon, to easily form four-coordinate bonds. It is being considered to change the forbidden band width of amorphous silicon by adding group (2) elements.

For example, when carbon is used as a modifier, the forbidden band width increases, and by using carbon as a p-type layer on the light incident side of a solar cell, it becomes possible to use shorter wavelength light more effectively; Ge5Sn, P
It is known that absorption of long wavelength light is improved by using an element such as b as a type 1 layer.

However, especially in the latter case, the forbidden band width becomes small and the film quality deteriorates, making it difficult to sufficiently convert long wavelength light into electrical energy.

Furthermore, recently hydrogenated amorphous silicon (hereinafter a-3
i : H) is attracting attention as a thin-film solar cell material, and its absorption coefficient for light near the peak of the solar energy distribution is about an order of magnitude larger than that of crystalline Si. It has various advantages such as being able to form a film by direct glow discharge decomposition and forming a bond easily.

The most important issue in designing and manufacturing such amorphous solar cells and putting them into practical use is achieving high conversion efficiency. Therefore, recently, Ge was used as an additive element, and a-3i:H (hereinafter referred to as a
-3iGe: 1 which absorbs long wavelength light (referred to as H)
Multilayer type used as a layer, i.e. pin/pinSpin
The development of two-layer and three-layer solar cells such as /p, i'n/pi"n, etc. is being actively carried out. In addition, a band gap of 1.4 to 1.6 eV has been used.

Problems to be Solved by the Invention As mentioned above, amorphous silicon is attracting attention as a promising low-cost material for solar cells that utilize clean and non-depletable solar energy, and efforts are being made to increase its photoelectric conversion efficiency. being done. In order to achieve high conversion efficiency, it is necessary to have a structure that can effectively utilize light in a wide wavelength range, and thus a multilayer structure solar cell was proposed.

In conventional multilayer amorphous silicon solar cells,
a-3i as a type 1 layer that particularly advantageously absorbs long wavelength light.
A Ge:H film was used, but the photoelectric properties of this film were insufficient to function as a component for solar cells;
When incorporated into a solar cell with a multilayer structure, its efficiency was almost the same as that of a single layer structure of amorphous silicon (a-3i), and the intended purpose could not be achieved. Although the a-3iGe:H film has the interesting property that the bandgap can be changed by changing the element ratio of Si and Ge, This is thought to be due to the fact that since it is an amorphous material, it is difficult to reduce defects caused by the combination of two elements, and the film quality cannot be improved. That is, since the film quality of a-3iGe:H film deteriorates due to the addition of Ge, it was thought that the limit for applying this type of film to solar cells was a Ge-added film with a band gap of about 1.6 eV. .

Under these circumstances, the above conventional a-3iGe:
Improving multilayer amorphous solar cells using H film and developing more efficient multilayer amorphous solar cells will realize the effective use of clean, non-depletable solar energy, and will make it possible to use it as a future fossil fuel source. This is of great significance in terms of its practical application as an alternative energy source in the event of depletion.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a multilayer structure type amorphous solar cell that can effectively utilize long wavelength light and obtain high conversion efficiency.

Means for Solving the Problems In view of the above-mentioned current state of the multilayer structure type amorphous solar cell using an a-3iGe:H film as one layer, the present inventor has conducted various studies in order to overcome the various difficulties it has. ,
As a result of our research, we found that the reduction in film quality due to the addition of Ge in the a-3iGe:H film or the limit in the conversion efficiency of solar cells using this film is due to the difficulty in eliminating defects that occur as a result of combining different elements. Focusing on the fact that Completed the invention.

That is, the multilayer structure amorphous solar cell of the present invention includes a substrate, electrode layers sequentially provided on a core laminate, and a plurality of sets of P.I.
It includes an amorphous thin film layer constituting an n-junction, and an electrode layer that pairs with the above-mentioned electrode, and at least one of the above-mentioned layers is a
-Ge: It is characterized by being composed of an H film.

The solar cell of the present invention has a configuration as shown in FIG. 1, for example. That is, a substrate 1, an electrode 2 provided thereon, and a plurality of unit batteries (
In other words, it is composed of one set of pln junctions) 3.4.5 (here, an example including three sets of unit batteries) and a second electrode layer 6, and each of the unit batteries 3.4.5 has a p+1
It is composed of three thin layers: +nzp212n2 and p* 13 n3, which may of course be in the order of nlp.

Here, as the substrate 1, any of glass, ceramics, plastic, mild steel, metals or alloys such as A1 and Cu, stainless steel, molybdenum, etc. can be used.

In addition, as an electrode, ITOC indium tin oxide (
In addition to transparent electrodes such as Indium Tin Oxide) and Sn○2, metals such as A1 can be suitably selected from publicly known electrodes.

The substrate material and electrode material vary somewhat depending on the type of solar cell. For example, in the example shown in FIG. 1, since light is incident from the substrate side, both the substrate 1 and the electrode 2 are formed of transparent materials. On the other hand, when an opaque material is used as the material for the substrate 1, a configuration is adopted in which the second electrode 6 is formed of a transparent electrode material and light is incident from the second electrode side.

Furthermore, it is naturally possible to form the substrate from a conductive material so that the substrate also functions as an electrode for extracting photocurrent and charge voltage.

Note that it is also possible to apply an antireflection coating to the sunlight incident side to further increase the light absorption efficiency.

In this case, by forming the electrode on the light incident side with 1TO, it is possible to have both the functions of an electrode and an antireflection coating.

Although the example in FIG. 1 shows an embodiment including three pin junctions, it is also possible to combine two or four or more pin junctions.

Such a multilayer structure type amorphous solar cell of the present invention can be produced according to the following method. First, an electrode layer, a microcrystalline Si layer constituting a pin junction, an a-3i layer, an a-3iGe
: ) 1 layer or a-Ge : 8 layers are formed in a predetermined order, and a second electrode layer to be paired with the above electrode is sequentially formed by an appropriate film forming method.

In the present invention, for example, in the case of a solar cell including three sets of pin junctions, the transparent electrode /I)+i+n+/p2i
2n2/I) 31 n37 metal electrode (Figure 1) or similarly transparent electrode/n1i+p+/n21zp from the light incident side
When the order is 2/Tl+1:+p*/metal electrode,
In both cases, it is advantageous to use an a-Ge:H film as the 13th layer.

For p1 to p3 it is advantageous to use a-3i:H or microcrystalline Si:H films, which are n.

The same applies to ~n3. Also, for l, a-3i
:H and a-3iGe:H film for 12 is advantageous, and the film thickness of each layer is adjusted to use the optimally designed film thickness so that the currents generated in 11.12.13 are equal. It is advantageous to do so.

First of all, the a-3i:H film is formed using SiH,
, SiF, , 5xHs, or a mixed gas thereof, etc., can be performed by direct current or high frequency glow discharge, and a
-3i: Control the conductivity type (p type, n type) and electrical conductivity of the H film by adding PH3, etc. to SiH, gas, etc. at 0.1 to 1.
This can be achieved by adding a trace amount (about one layer) of 6% by volume or B 2 Hs gas. This a-3i
: The standard conditions for H film are a total pressure of 0.1 to 1 O during discharge.
Torr.

Gas concentration of SiH, etc. is 10% or more (this is Ar or H
2), the gas flow rate is 50 to 200 cc/min, the discharge output is several watts to several tens of watts, and the process is performed at a relatively low temperature of 300° C. or lower. In addition to the glow discharge method, it is also possible to use various methods such as a vacuum evaporation method, a sputtering method, an ion blating method, etc., with appropriate modifications.

On the other hand, the formation of the microcrystalline Si:H film is performed in accordance with the above a-3i.
A microcrystalline Si:H film can be obtained by performing the same operation as in the formation of the :H film and increasing the applied electric field for glow discharge or increasing the hydrogen dilution rate of the source gas.

Further, the a-3iGe:H film can be obtained, for example, according to a method for which the applicant has separately filed a patent application.

That is, 82/(Ge source) is added to the raw material gas containing the additive source (Ge source).
Hydrogen is added so that the ratio of source + Si source + 82) falls within the range of 0.25 to 0.60, and the film can be formed by a glow discharge method, a photo-CVD method, or the like. As a Si source, SiH, 5I2F6,
SiF< or a mixture thereof can be used, and GeH<, GeF< or a mixture thereof can be used as the Ge source. Moreover, as the excitation light wavelength for photo-CVD, ultraviolet rays of 0.3 μm or less are advantageously used. This addition of Ge can be carried out by adding a Ge hydride or a fluorine compound into the amorphous silicon raw material gas as described above, and simultaneously subjecting the resulting mixture to glow discharge decomposition, or during glow discharge decomposition of the amorphous silicon raw material gas. This can be done by sputtering a solid Ge raw material.

The a-Ge:H film specific to the amorphous solar cell of the present invention can be formed according to a method similar to the formation of the a-3i:H film, using Ge as the Ge source instead of the Sl source.
Use Hl, GeF, or a mixture thereof. As such a film forming method, a glow discharge method, a photo-CVD method, etc. are known. For example, in the glow discharge method, GeH4 is used as a raw material gas, the pressure is 0.1 to 10'l'orr, and the gas flow rate is 10 to LOOO. ,SCCM, RF output 5~50W
.

The film can be formed at a substrate temperature of 100 to 250°C. Also, light C
In the VD method, a mercury sensitization method is used, GeH is used as a raw material gas, and U light is irradiated with a low-pressure mercury lamp at a pressure of 0.1 to 10 Torr, a gas flow rate of 10 to 100 SCCM, and a temperature of 40 to 80°C. An a-Ge:H film can be produced by irradiating with .

Note that a trace amount of boron (B) is added to the layers used to form one layer such as the a-3i:H film, the a-3iGe:H film, and the a-Ge:H film to make them intrinsic. However, this can be easily achieved by mixing a B source, for example B2H6, with 1FFN in the layer forming raw material gas.

Furthermore, the electrode layer can be formed by various conventionally known methods. For example, metals such as AI can be formed by vapor deposition, reduction of halides, thermal decomposition of metal carbonyl, thermal decomposition of organometallic complexes, etc. by CVD, etc. In addition, ITO can be produced by, for example, a vacuum evaporation method using Sn-doped indium oxide as a starting material, an RF sputtering method, a reactive evaporation method in an oxidizing atmosphere using an alloy of indium and tin as a starting material, or a reactive sputtering method. Both the ion brating method and the ion beam sputtering method can be used.

In order to make an amorphous solar cell with a psychological multilayer structure sufficiently suitable for practical use, it was necessary to increase the photoelectric conversion efficiency more than ever before. It is believed that this improvement in conversion efficiency can be achieved by effectively utilizing solar energy over a wide wavelength range. As already mentioned, the absorption efficiency on the short wavelength side can be increased by using carbon as a modifier in the p-type layer. On the other hand, in order to increase the conversion efficiency on the long wavelength side, Ge, Sn, and pb are used as modifiers.
Although it is known to use this as a type 1 layer using the above method, conventional methods and configurations have not yet achieved conversion efficiency sufficient for practical use.

Therefore, in the present invention, a plurality of unit batteries (one set of pins)
a-3i as at least one layer in bonding):
By using H (B addition), it has become possible to greatly improve the conversion efficiency on the long wavelength side.

That is, in the conventional 5iGe alloy system, it was not possible to simultaneously prevent the inherent defects of two different components, Sl and Ge, resulting in a single layer with poor film quality, but as in the solar cell of the present invention, hydrogenation of Ge alone When one layer is composed of an amorphous film, the defects are not complicated, and as a result, the defect density is small and the electrical properties are considered to be excellent.

The bandgap of this a-Ge:H is 1.1e
It is near V, therefore 1. It is possible to absorb light with large energy of leV or more and convert it into electricity. By using a -Ge:H film as one layer, it is possible to photoelectrically convert light in the long wavelength range of 600 to 11,000 nm, and by using two or three or more layers of such a unit cell, it is more efficient than conventional solar cells. Conversion efficiency is greatly improved. In the resulting solar cell, this a-Ge:H film is preferably used as a type 1 layer forming material of the unit cell located at the farthest position from the light incident side. This is based on the property that this l-type layer absorbs long wavelength light.

Further, for the same reason, it is preferable that the a-3iGe:H film be one layer of the pin junction located next away from the light incident side.

Further, in the multilayer structure type amorphous solar cell of the present invention, the thickness of each p, i, and n layer is 11.12, ... i,
(n is an integer of 3 or more) The conversion efficiency can be optimized by optimally designing so that the current generated in each is equal. Generally, the p-type layer has 50 to 200 people, and the l-type layer has 1000-8000 people, n-type layer 100-5
The above object can be achieved by setting the thickness within the range of 0.00 mm. In addition, the thickness of the electrode layer is 800 to 600
Generally, it is about 0.08.

Furthermore, the conversion efficiency can also be improved by providing an antireflection coating on the light incident side to increase the sunlight absorption efficiency. Materials for such coatings include S+ 0.5102, ZrO2, Zn5SSi
3N, Ta205, Al2O3,5b20*, TlO
2 or a multilayer structure such as Ta205/Si02 can be used. These films can be formed by various known methods, such as vacuum evaporation and sputtering, depending on the characteristics of each material.

It has been found that it is advantageous to select the thickness d of this anti-reflection film so as to satisfy the following condition: d=λ/4ni, where λ is the wavelength of light and n is the refractive index of the material. It is being Therefore, by forming two or more layers of materials with different refractive indexes, it is possible to obtain an antireflection film that is effective against a wider range of light.

The structural features of the invention can be advantageously utilized in various structures of amorphous solar cells that require a narrow gap (E, = 1.1 eV).

EXAMPLES Hereinafter, the multilayer structure type amorphous solar cell of the present invention will be explained in more detail using examples.

A multilayer amorphous solar cell including three sets of PLN junctions having a configuration similar to that shown in FIG. 1 was fabricated.

The substrate 1 is made of glass, and the transparent electrode 2 is made of IT○(
2000 people), and a-3i: H film (400A) and a-3i as 11s12 and j3, respectively.
Ge:H film (1500A) and a-Ge:H
membrane (5000 people) and other pI"" p3
and n1 to n3 were each formed of an a-3i:H film, each having a film thickness of 100. The bandgap E in the above 11 to 13 is 1.8 eV (i +), ■, 6 eν (12
) and 1.1 eV (i 3).

300~ for each layer of the amorphous solar cell thus obtained.
The relative spectral sensitivity for light in the range of 1l100n gave the results shown in Figure 2 (spectral sensitivity, that is, conversion efficiency η =
14%).

On the other hand, in the same way, lZ~1+3 was formed into a-3i:H film (1°r; E, = 1.8 eV) and a-3iGe, respectively.
: H film (i12; E, = 1,65 eV) and a
-3iGe: H film (i'a; Eg=1.4e
A conventional multilayer structure type amorphous solar cell was prepared in exactly the same manner as described above except for the configuration described in v), and its spectral sensitivity was measured and plotted in FIG.

In addition, in Figures 2 and 3, the relative sensitivity on the vertical axis is a
-3i: indicates a relative value when the peak sensitivity of H (1.8 eV) is set to 1.

Comparing FIG. 2 and FIG. 3, it is found that a-Ge: H with E9=1.1 eV as in the present invention as one layer and three layers.
When a film is used, its relative sensitivity peak is around 900 nm. It can be seen that the sensitivity (or absorption efficiency) on the long wavelength side has been significantly improved. By the way, the conventional one is as shown in Figure 3! 'The relative sensitivity peak of the 3 layers is 70
It can be seen that it is near 0 nm and almost 0 at 900 nm.

Furthermore, the configuration of the second set from the light incidence side is made the same, and the third set is made the same.
Several types of solar cells with different forbidden widths of coarse unit cells were fabricated, their efficiencies were measured, and the relative efficiency of each cell was calculated based on the efficiency of the 1.8 eV A-31 cell. The results shown in FIG. 4 were obtained. In FIG. 4, E on the horizontal axis represents the forbidden zone of the type 1 layer of the third coarse battery. Looking at this result, when E is 1.8 eV, the efficiency at point A shows a maximum point at E9-1.6 eV as at point B as the gap becomes narrower, but even further. When E is decreased, the quality of the a-3iGe:H film decreases as alloying progresses. 0 point (E, =' 1.3eV)
It takes a minimum value at and rises again, and as in the present invention, E,
It can be seen that high efficiency (higher than the maximum point B) can be achieved again by using an a-Ge:H film of = 1.1 eV. In other words, it is considered that the efficiency of the amorphous film made only of Ge without the inclusion of Sl is improved because there is no deterioration in film quality due to alloying or the like.

Effects of the Invention As explained in detail above, in the multilayer structure type amorphous solar cell of the present invention, the one layer that is dominant for its long wavelength sensitivity (conversion efficiency) is a conventional a-3iGe:
From H film to a-G, which shows no deterioration in film quality due to alloying etc.
By replacing it with an e:H film and lowering the forbidden band width E9, the photoelectric conversion efficiency was greatly improved, and it became possible to obtain an amorphous solar cell that could withstand practical use.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view schematically showing a preferred embodiment of a multilayer amorphous solar cell according to the present invention, and FIG.
This is a diagram plotting the relative sensitivity of the mold layer to wavelength. Figure 3 is a diagram similar to Figure 2 for a conventional solar cell, and Figure 4 is a diagram for each solar cell when E of the 13 layers is changed. 2 is a graph showing changes in conversion efficiency in terms of relative efficiency. (Main reference numbers) 1...Substrate, 2...Electrode 3.4.5...Unit battery 6...Second electrode

Claims (7)

[Claims] (1) The i-type multilayer amorphous solar cell including a substrate, an electrode layer sequentially provided on the substrate, an amorphous thin film layer constituting a plurality of pin junctions, and an electrode layer paired with the electrode, The above multilayer amorphous solar cell, wherein at least one of the layers is composed of a hydrogenated amorphous germanium layer. (2) The amorphous solar cell according to claim 1, wherein the i-type layer composed of the hydrogenated amorphous germanium layer is a pin-junction i-type layer farthest from the light incidence side. (3) The p-type layer and the n-type layer are each composed of an impurity-doped hydrogenated amorphous silicon film, a microcrystalline hydrogenated silicon film, or a germanium-doped hydrogenated amorphous silicon film. An amorphous solar cell according to claim 1 or 2. (4) The germanium-doped hydrogenated amorphous silicon film is prepared by diluting the raw material gas containing the doping source with hydrogen such that the ratio of the hydrogen gas flow rate to the total gas flow rate is in the range of 0.25 to 0.6. 4. The amorphous solar cell according to claim 3, wherein the amorphous solar cell is formed as a film in a state in which the amorphous solar cell is formed into a film. (5) Claims characterized in that the i-type layer other than the i-type layer made of hydrogenated amorphous germanium is made of a hydrogenated amorphous silicon film or a hydrogenated amorphous silicon film added with germanium. The amorphous solar cell according to any one of items 1 to 4. (6) The i-type layer composed of the germanium-doped hydrogenated amorphous silicon film is an i-type layer in a pin junction located as far away from the light incident side as possible. The amorphous solar cell according to item 5. (7) Any one of claims 1 to 6, wherein the i-type layer has boron as an additive for making it intrinsic.
The amorphous solar cell described in section.