JPH0526354B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to single crystal or polycrystalline semiconductors (hereinafter referred to as
A crystalline unit cell in which a PN heterojunction is formed by depositing an amorphous or microcrystalline semiconductor (hereinafter referred to as an amorphous semiconductor) on top of a crystalline semiconductor (hereinafter referred to as a crystalline semiconductor); This invention relates to a tandem solar cell in which a plurality of amorphous unit cells made of semiconductor PIN junctions are stacked in series.

Conventional technology Conventionally, photovoltaic elements mainly used in solar cells using semiconductor materials include Si, GaAs, CdS, and CdTe.
In addition to crystalline photovoltaic devices that utilize only P-type and N-type single-crystal or polycrystalline semiconductors such as photovoltaic devices are commercially available. The former device has a high photoelectric conversion efficiency, but its manufacturing method involves a high-temperature process, resulting in a high cost. Therefore, in order to reduce costs, a single photovoltaic element having a heterojunction made of amorphous silicon and crystalline silicon has been developed. However, although this element can achieve low cost,
In terms of conversion efficiency, it is on the same level as conventional crystalline elements, and has not yet reached a sufficient value.

On the other hand, the latter device uses the glow discharge decomposition method (GD
It can be produced using a low-temperature process known as the method (method), and can be made thinner to a thickness of only 0.5 μm compared to 300 μm for single crystal silicon, so it has the advantage of being cheaper to manufacture. The area covered by the carrier collection efficiency curve of is approximately half of the area covered by the carrier collection efficiency curve of a single crystal silicon solar cell, so
The conversion efficiency of amorphous silicon solar cells is lower than that of single crystal silicon solar cells. In other words, while the carrier collection efficiency curve of single-crystal silicon solar cells extends to light with relatively long wavelengths, the carrier collection efficiency curve of amorphous silicon solar cells is biased towards light with relatively short wavelengths. There is. Therefore, the amorphous silicon solar cells that are in practical use cannot utilize incident light as effectively as crystalline semiconductor solar cells, so their conversion efficiency is as low as 8 to 9%. It's summery.

Therefore, recently, a-Si solar cells with a multilayer unit cell structure in which wavelength-divided unit cells are laminated to have the maximum value of carrier collection efficiency for each wavelength range for sunlight with a wide spectrum have been developed. being researched. In the case of amorphous semiconductors, the optical band gap, which influences the carrier collection efficiency for each wavelength, can be changed by changing the type of gas supplied or the mixing ratio thereof during manufacture. Hydrogenated amorphous silicon carbide a-
SiC: H, hydrogenated amorphous silicon nitride a-
SiN:H, and for long wavelengths, hydrogenated amorphous silicon germanium a-SiGe:H, hydrogenated amorphous silicon tin a-SiSn:H, etc. are used. However, even in these amorphous solar cells with a multilayer unit cell structure, as mentioned above, amorphous semiconductors are essentially unable to absorb long wavelength light as well as crystalline semiconductors. There is a problem in that the drawbacks cannot be sufficiently compensated for.

In order to solve this problem, technological developments are underway to further advance the effective use of light by combining amorphous semiconductors and crystalline semiconductors. However, if the unit cell of the crystalline photovoltaic device and the unit cell of the amorphous photovoltaic device described above are simply stacked, the electromotive force of both unit devices will be reduced by the PN junction formed at the interface between the two unit devices. Since they are canceled by the internal potential and do not operate as an effective stacked electromotive force element, the multiple carriers that become the photocurrent generated in each unit element are recombined by some method at the interface between both unit elements. By exchanging carriers, it is necessary to cancel the PN internal electromotive force effect at the interface between both unit elements. For example, there are cases where oxides such as indium-tin (ITO), which are called transparent conductive films, are purposely used to perform this carrier exchange and recombination function. In this case, an amorphous silicon photovoltaic device is created on a transparent conductive oxide film, and at this time, the components of the transparent conductive film diffuse and deteriorate the device, resulting in the conversion efficiency of the device. was causing a decline in In addition, in order to form a transparent conductive film during device fabrication, a method other than the method for forming the next amorphous semiconductor, such as electron beam evaporation or spraying, is required, which increases equipment costs and This is disadvantageous in terms of running costs. Therefore, a photovoltaic element is desired that has a good carrier exchange and recombination function without using a transparent conductive film.

Purpose of the Invention The present invention involves depositing an amorphous semiconductor on a crystalline semiconductor using a low-temperature process, for example, at 200 to 300°C in the case of a glow discharge decomposition method (GD method).
By forming a PN junction or a PIN junction,
To provide solar cells that can achieve higher efficiency by reducing the manufacturing cost of solar cells in terms of manufacturing technology, manufacturing equipment, etc., and by using tandem manufacturing that takes advantage of the characteristics of amorphous materials. The purpose is to

Embodiments The present invention will now be described in detail with reference to illustrated embodiments.

FIG. 1A is a block diagram of a first embodiment of the present invention, showing a solar cell in which a wafer is used as the crystalline semiconductor. In the figure, 2 is a wafer (thin plate) of P-type crystalline semiconductor, for example, P-type polycrystalline silicon, 1 is an electrode made of aluminum deposited on the back side of the P-type polycrystalline silicon wafer, and 3 is P-type polycrystalline silicon. N-type thin film semiconductor, for example, a thin film of N-type microcrystalline silicon, deposited by glow discharge decomposition method (GD method) on a wafer 2, 4, indium oxide and tin oxide formed on a thin film of N-type microcrystalline silicon. 5 is a comb-shaped electrode formed on the transparent conductive film of the compound (ITO).

The above N-type microcrystalline silicon has optical properties similar to those of an amorphous semiconductor, and electrical properties similar to those of a crystalline semiconductor.
The formation temperature of this N-type microcrystalline silicon is 200 to 300℃
The conventional crystalline semiconductor PN junction formation temperature is 1000℃.
The processing time for forming a PN junction is only a few minutes, which is shorter than the time required to form a PN junction in conventional crystalline semiconductors. Can be lowered.

In this first embodiment, a wafer of P-type polycrystalline silicon and a wafer of N-type microcrystalline silicon form a heterojunction HJ.
Since the N-type semiconductor deposited on the P-type polycrystalline silicon is a thin film of microcrystalline silicon, low-temperature processes such as glow discharge decomposition are not possible. Since a PN junction can be formed in this way, it is possible to reduce the manufacturing cost of the solar cell. The above-mentioned glow discharge decomposition method is a method of growing a thin film at a low temperature by bringing a compound of atoms constituting the thin film to be grown into a plasma state and decomposing it into chemically active ions and radicals.

FIG. 1B is an energy band diagram of the solar cell of FIG. 1A. In the same figure, the energy gap Ega of N-type microcrystalline silicon is approximately 1.8 [eV], and the energy gap Ega of P-type polycrystalline silicon is approximately 1.8 [eV].
Since it is larger than about 1.1 [eV] of Egc, the PN junction becomes a heterojunction HJ, which can reduce the window effect, that is, optical loss on the short wavelength side of sunlight. In the figure, Ec indicates the lower limit of the conduction band, Ef indicates the Fermi level, and Ev indicates the upper limit of the valence band.

FIG. 2 is a diagram showing actually measured values of the IV characteristics of the solar cell shown in FIG. 1A. In the figure, the horizontal axis shows the output voltage Vout [V] of the solar cell in FIG. 1A, and the vertical axis shows the output current Iout [mA/
Indicates ^cm2 . Figure 1 A obtained from the IV characteristics in the same figure
The conversion efficiency of the solar cell is about 11%,
Using the same wafer, it is possible to obtain the same efficiency as a solar cell in which a PN junction is formed by a high-temperature process such as thermal diffusion or ion implantation. Therefore, according to the heterojunction solar cell and the manufacturing method thereof of the present invention, it is possible to maintain substantially the same efficiency as a solar cell manufactured using a conventional high-temperature process while reducing the manufacturing cost of the solar cell. can.

In the first embodiment shown in FIG. 1A, the crystalline semiconductor is P type and the amorphous semiconductor is N type; however, conversely, the crystalline semiconductor is N type and the amorphous semiconductor is P type. may also be the code 2
Crystalline semiconductors include single crystal or polycrystalline semiconductors.
It may be Si, GaAs, Ge, etc. Furthermore, the amorphous semiconductor 3 may be hydrogenated or fluorinated microcrystalline or amorphous silicon carbide, silicon nitride, silicon germanium, etc.

In addition to the GD method mentioned above, methods for forming amorphous semiconductors include heating a substance in a vacuum, evaporating it, and depositing the evaporated substance on the surface of another substance to form a film. Vacuum evaporation method, sputtering method in which the deposited material is released by collision of gas ions with glow discharge to form a film on the surface of another material, and vacuum evaporation method in which the evaporated material is ionized.
Ion plating is a method in which a film is formed by depositing it on the surface of another substance. A mixed reaction gas containing the elements for forming the film is fed onto a substrate that has been preheated in a reaction chamber, and is irradiated with light. A low-temperature process such as photo-CVD, which forms a film using a thermochemical reaction on a substrate, can be used.

FIG. 3 is a block diagram of a second embodiment of the present invention, in which a thin film is used as the crystalline semiconductor. In the figure, reference numeral 6 denotes a thin plate-like substrate made of an inorganic solid such as metal, glass, or ceramic, or a film-like substrate made of an organic solid such as polyimide.
1' is an ohmic electrode formed on the substrate 6;
2' is vapor phase deposition (CVD) on the ohmic electrode,
It is a thin film of crystalline semiconductor deposited by methods such as metal-organic thermal decomposition (MOCVD), molecular beam epitaxy (MBE), sputtering, and ion plating. 3 is on this thin film 2',
This is a thin film of N-type amorphous or N-type microcrystalline silicon deposited by the same low-temperature process as in FIG. 1A, and 4 and 5 are transparent conductive films and comb-shaped electrodes similar to the embodiment in FIG. 1A. .

In the above embodiment, if a conductive material is used as the substrate 6 and it also serves as an electrode, there is no particular need to form the ohmic electrode 1' separately. Also,
In this second embodiment, by also making the crystalline semiconductor thinner, manufacturing costs are further reduced than in the first embodiment, and the CVD method and
Using techniques such as MOCVD, it is possible to increase the area.

FIG. 4A is a block diagram of a third embodiment of the present invention, in which a crystalline semiconductor 2 and an amorphous semiconductor 3 similar to those in FIG. 1A are formed into a heterojunction HJ.
The energy gap formed by the junction is formed in the crystalline unit cell C1 of Egc1, and furthermore, the crystalline semiconductor 8 and the amorphous semiconductor 9 form a heterojunction HJ.
The energy gap formed by the PN junction is Egc2
This is a tandem solar cell in which crystalline unit cells C2 (Egc2>Egc1) are stacked via an amorphous semiconductor 7 whose valence is controlled to be the same as the crystalline semiconductor 8.

In the figure, numerals 1 to 5 are the same as those in FIG. 9 is an amorphous N-type thin film semiconductor further deposited on the semiconductor 8, and 9 is a crystal in which the crystalline semiconductor 8 and the amorphous semiconductor 9 are heterojunction HJ. A system unit cell C2 is formed.

In order to form a tandem structure, it is necessary to form an automic junction at the interface between the crystalline unit cells C1 and C2. Therefore, as in the present invention, an amorphous semiconductor 7
By providing this, the amorphous semiconductors 3 and 7 form a good ohmic junction OJ.

FIG. 4B shows an energy band diagram of the solar cell of FIG. 4A when irradiated with light. The electrons e generated in the crystalline unit cell C1 and the holes h generated in the unit cell C2 by light irradiation recombine through the localized level Es in the forbidden band near the junction, resulting in a rejunction current. flows, resulting in a good ohmic connection. In other words, in the third embodiment of the present invention, by using an amorphous semiconductor in the junction part, the property of the amorphous semiconductor, which is generally considered to have a disadvantage of having many localized levels in the forbidden band, can be taken advantage of. By using the PN junction of amorphous semiconductor,
The feature is that ohmic junctions are easily obtained.

In this third embodiment, the light on the short wavelength side of sunlight is absorbed by the crystalline unit cell C2 with the larger energy gap, and the light on the long wavelength side that has passed through it is absorbed by the lower part of the unit cell C2. Since the energy can be absorbed by the crystalline unit cell C1 with a smaller energy gap, in addition to reducing the manufacturing cost of the solar cells of the first and second embodiments,
Highly efficient solar cells can be obtained.

FIG. 5 is a block diagram of a fourth embodiment of the present invention, in which an energy gap formed by a PN junction in which a crystalline semiconductor and an amorphous semiconductor shown in FIG. 1A are formed into a heterojunction HJ is Egc. In addition to the crystalline unit cell C, by the GD method, an amorphous unit cell A with two units of amorphous PIN junction is formed.
This is a tandem solar cell in which solar cells A1 and A2 are stacked.

In the figure, numerals 1', 2', 3' to 6 are the same as those in FIG. 3, and between the amorphous semiconductor 3 and the transparent conductive film 4 in FIG. The energy gap composed of the amorphous semiconductors 11 to 13 is Ega1 (Ega
1>Egc) amorphous unit cell A1
and an amorphous unit cell A2 consisting of 14 to 16 and having an energy gap of Ega2 (Ega2>Ega1). The interface between the semiconductors 3 and 11 forming the junction between the crystalline unit cell C and the amorphous unit cell A1 is the third
A good automatic connection is achieved for the same reason as in the example of
It has become O.J.

In this fourth embodiment, light on the shorter wavelength side of sunlight is absorbed by the amorphous unit cells A2 and A1 of the upper PIN junction, and light on the longer wavelength side is absorbed by the lower heterojunction PN junction. Since it can be absorbed by the crystalline unit cell C, in addition to reducing the manufacturing cost of the solar cell of the first or second embodiment, a highly efficient solar cell can be obtained.

FIG. 6 is a block diagram of a fifth embodiment of the present invention, in which two units of amorphous PIN are added on top of the amorphous semiconductor 9 shown in FIG. 4 by the GD method in the same manner as in FIG. This is a tandem solar cell in which junction unit cells A1 and A2 are stacked. In the figure, the numbers 1 to 5 and 7 to 9 are the fourth
It is the same as the figure, and numerals 11 to 16 are the fifth
This is the same as the case shown in the figure.

In this fifth embodiment, amorphous unit cells A2, Ega1 (Ega2 ＞Ega1)
PIN junction amorphous unit cell A1,
Since it can be absorbed by the crystalline unit cell C2 of the PN junction where Egc2 (Ega1>Egc2) and the crystalline unit cell C1 of the PN junction where Egc1 (Egc2>Egc1), the first or second implementation In addition to reducing the manufacturing cost of the example solar cells, highly efficient solar cells can be obtained.

Effects of the Invention As described above, according to the solar cell of the present invention, an amorphous or microcrystalline amorphous semiconductor is deposited on a single crystal or polycrystalline semiconductor using a low-temperature method such as a glow discharge decomposition method (GD method). Since the PN junction or PIN junction is formed by deposition by a process, the junction formation temperature is extremely low, and the junction formation and processing time are shortened, simplifying the manufacturing equipment and manufacturing time. It is possible to reduce the equipment costs and running costs of the entire manufacturing process by shortening the time.Moreover, the amorphous unit cell absorbs the short wavelength side of the incident light, and the crystalline unit cell absorbs the short wavelength side of the incident light. In addition to absorbing light on the wavelength side and making it possible to effectively utilize a wide range of light with a single photovoltaic element, the interface between unit cells necessary to construct a tandem element is made of amorphous amorphous. Since an optically transparent ohmic junction is formed at the interface by the PN layer of the system semiconductor, internal electromotive force is not generated at the interface between different types of unit cells, making it possible for the tandem device to operate well. In combination with the above-mentioned effects, high efficiency can be achieved, which is of great industrial benefit.

[Brief explanation of the drawing]

FIGS. 1A and 1B are a block diagram and an energy band diagram, respectively, of a first embodiment of the solar cell of the present invention, and FIG. 2 is a diagram of the solar cell shown in FIG. 1.
Characteristics (the horizontal axis is the output voltage Vout [V] of the solar cell,
The vertical axis is a diagram showing the output current Iout [mA/cm ² ]) of the solar cell, FIG. 3 is a block diagram of the second embodiment of the solar cell of the present invention, and FIG. 4 A and B are respectively The block diagram and energy band diagram of the third embodiment of the present invention, and FIGS. 5 and 6 are block diagrams of the fourth and fifth embodiments of the solar cell of the present invention, respectively. 1, 1'... Electrode, 2, 2', 8... Crystal semiconductor, 3, 7, 11 to 16... Amorphous semiconductor, 4... Transparent conductive film, 5... Comb-shaped electrode,
C, C1, C2...Unit cell of PN junction, A, A
1, A2...Unit cell of PIN junction, Ega, Ega
1, Ega2... Energy gap of amorphous semiconductor, Egc, Egc1, Egc2... Energy gap of crystalline semiconductor, HJ... Heterojunction between crystalline semiconductor and amorphous semiconductor,
OJ: Ohmic junction between unit cells.

Claims (1)

[Claims] 1. A single-crystal or polycrystalline crystalline semiconductor and an amorphous or microcrystalline amorphous semiconductor.
A plurality of PN heterojunction crystalline unit cells are stacked in series via an amorphous semiconductor whose valence is controlled to be the same as that of the crystalline semiconductor, and two amorphous semiconductors are stacked at the interface of each crystalline unit cell. This is a tandem solar cell that forms an optically transparent ohmic junction between each crystalline unit cell. 2. The tandem solar cell according to claim 1, wherein among the crystalline semiconductors, the outermost semiconductor opposite to the incident light side is a wafer. 3. The tandem solar cell according to claim 1, wherein the crystalline semiconductor is a thin film semiconductor. 4 Single crystal or polycrystalline crystalline semiconductor and amorphous or microcrystalline amorphous semiconductor
A plurality of PN heterojunction crystalline unit cells are stacked in series via an amorphous semiconductor whose valence is controlled to be the same as that of the crystalline semiconductor, and two amorphous semiconductors are stacked at the interface of each crystalline unit cell. By forming an optically transparent ohmic junction between each crystalline unit cell, a crystalline tandem cell is formed, and one or more units of PIN-junctioned amorphous unit cells made of amorphous or microcrystalline semiconductor are formed. An optically transparent ohmic is formed by stacking an amorphous cell formed by stacking in series on the crystalline tandem cell, and two amorphous semiconductors contacting each other at the interface between the crystalline tandem cell and the amorphous cell. Tandem solar cell with junction formed. 5. The tandem solar cell according to claim 4, wherein among the crystalline semiconductors, the outermost semiconductor opposite to the incident light side is a wafer. 6. The tandem solar cell according to claim 4, wherein the crystalline semiconductor is a thin film semiconductor.