JPH0644638B2 - Stacked photovoltaic device with different unit cells - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improvement of a stack type photovoltaic device used for a solar cell, a detection device, etc., which is a combination of a crystalline semiconductor and an amorphous semiconductor.

2. Description of the Related Art Conventionally, in a photovoltaic element centered on a solar cell using a semiconductor material, only P-type and N-type single crystal or polycrystal crystalline semiconductors such as Si, GaAs, CdS, and CdTe are used. In addition to crystalline photovoltaic elements, N type, I
Type and P type amorphous semiconductors having a silicon layer of amorphous semiconductor deposited thereon are commercially available. The former element has a high photoelectric conversion efficiency, but its manufacturing method involves a high temperature process, so that there is a problem that the cost is high.
Therefore, in order to reduce the cost, a single photovoltaic element having a heterojunction made of amorphous silicon and crystalline silicon has been developed. However, although this device can realize cost reduction, in terms of conversion efficiency, it is at the same level as the conventional crystal-based device, and has not yet reached a sufficient value.

On the other hand, the latter element can be manufactured by a low-temperature process called plasma CVD method and can be thinned to a thickness of only 0.5 μm as compared with 300 μm of single crystal silicon, so that there is an advantage that it can be manufactured at low cost. As shown in
The area surrounded by the carrier collection efficiency curve a-Si of the amorphous silicon solar cell for each wavelength of the incident light is about half of the area surrounded by the carrier collection efficiency curve c-Si of the single crystal silicon solar cell. The conversion efficiency of the cell is lower than that of the single crystal silicon solar cell. That is, FIG. 1 shows the wavelength λ [μm] on the horizontal axis and the carrier collection efficiency η [%] on the vertical axis, and the carrier collection efficiency curve c-Si of the single crystal silicon solar cell is a light having a relatively long wavelength. However, the carrier collection efficiency curve a-Si of the amorphous silicon solar cell is biased toward light having a relatively short wavelength. Therefore, since the practically used amorphous silicon solar cell cannot effectively utilize the incident light as much as the crystalline semiconductor solar cell, its conversion efficiency is 8 to
It is as low as 9%.

Therefore, recently, for sunlight with a wide spectrum,
Research has also been conducted on a-Si solar cells having a multilayer unit cell structure in which unit cells wavelength-divided with a maximum value of carrier collection efficiency for each wavelength range within a certain range are stacked. In the amorphous semiconductor, the optical bandgap that influences the carrier collection efficiency for each wavelength can be changed by adding the kind of the supply gas or the mixing ratio thereof at the time of manufacturing. As the amorphous semiconductor for short wavelength, hydrogenated amorphous silicon carbide a-SiC: H,
Hydrogenated amorphous silicon nitride a-SiN: H, for long wavelength, hydrogenated amorphous silicon germanium a-SiGe: H, hydrogenated amorphous silicon tin a-SiSn: H, etc. are used. However, even in these amorphous solar cells having a multi-layer unit cell structure, as described above, the amorphous semiconductor is essentially unable to sufficiently absorb light for long wavelengths as compared with the crystalline semiconductor. There is a problem that the drawback cannot be sufficiently compensated.

In order to solve this problem, technical development is being carried out to further promote effective use of light by combining an amorphous semiconductor and a crystalline semiconductor. However, if the unit cell of the crystalline photovoltaic element and the unit cell of the amorphous photovoltaic element described above are simply stacked, the electromotive force of both unit elements will cause a PN junction formed at the interface between both unit elements. Since they are canceled by the internal potential and do not operate as an effective stack type electromotive force element, the majority of carriers that become the photocurrent generated in each unit element are recombined by some method at the interface between both unit elements and carrier exchange is performed. By doing so, it is necessary to cancel the PN internal electromotive force effect at the interface between both unit elements. For example, there is an example in which an oxide such as indium tin (ITO), which is called a transparent conductive film, is purposely used for the purpose of fulfilling the function of carrier exchange recombination. In this case, an amorphous silicon photovoltaic element is formed on the oxide transparent conductive film, and the components of the transparent conductive film are diffused at this time to deteriorate the element, resulting in a conversion efficiency of the element. It was the cause of the decrease. In addition, in order to form a transparent conductive film in the process of forming an element, a means different from the means for forming the next amorphous semiconductor, that is, an electron beam vapor deposition method or a spray method, is required. It is disadvantageous in terms of running cost. Therefore, a photovoltaic device having a good carrier exchange / recombination function without using a transparent conductive film is desired.

An object of the present invention is to form an amorphous unit cell made of an amorphous semiconductor on a crystalline unit cell of an amorphous semiconductor heterojunction with a single crystal or polycrystal crystalline semiconductor, and to form an amorphous unit cell at the interface between these two unit cells. By performing the carrier exchange recombination function by utilizing the PN junction of the amorphous semiconductor, the unit cell of the amorphous semiconductor absorbs the light on the short wavelength side of the incident light, and the crystalline unit cell is changed to the amorphous semiconductor. A solar cell in which a crystalline semiconductor and an amorphous semiconductor are heterojunctioned by absorbing light on the long wavelength side, which cannot be absorbed by the unit cell, and effectively utilizing a wide range of light with a single photovoltaic element. Stacked solar cells consisting of batteries and amorphous semiconductors alone, and crystalline unit cells using a transparent conductive film Another object of the present invention is to provide a stack type photovoltaic device that is more efficient and less expensive than a stack type device of a fass unit cell.

Examples Hereinafter, examples of the present invention will be described in detail. Figure 2 (A)
1 is a configuration diagram of a first embodiment of the present invention, in which 1 is an aluminum electrode, 11p is a P-type polycrystalline silicon substrate (P-
Poly ・ Si), 20n is N-type amorphous silicon (N-aSi) or N-type microcrystalline silicon (N-μC ・
Si), the P-type polycrystalline semiconductor 11p and the amorphous or microcrystalline semiconductor 20n are heterojunction HJ,
The crystal unit cell 11 is formed. 21p is P-type amorphous silicon (P-aSi), 21i is I-type amorphous silicon (I-aSi), 21n is N-type amorphous silicon (N-aSi) or N-type microcrystalline silicon (N-μC · Si). 21p, 21i and 2
1n is an amorphous unit cell 2 having a P-I-N junction
Make up one. Further, the transparent conductive film 10 as an electrode is formed on the N-type semiconductor 21n. The charge-controlled microcrystalline semiconductor described above has optically the same characteristics as an amorphous semiconductor, and electrically has the same characteristics as a crystalline semiconductor. The embodiment of FIG. 2 (A) is a case where a crystalline semiconductor and an amorphous semiconductor are heterojunctionally formed to form a unit cell, and the unit cell of the amorphous semiconductor is ohmic-junctioned. These amorphous semiconductors 20n, 21p, 21i, 21n
And the thickness of the transparent conductive film 10 is 3000Å, 500 respectively
Å, 5000 Å, 100 Å and 700 Å. Fig. 2 (B)
FIG. 3 is an energy band diagram of the photovoltaic element of FIG. 2 (A). In the figure, Egcl is about 1.1 [eV] in the forbidden band width of the polycrystalline silicon semiconductor 11p, and Egcl
gal is a band gap of an amorphous silicon semiconductor, which is about 1.7 to 1.8 [eV], and Egal> Eg.
It is cl. In the figure, Ec represents the lower limit of the conduction band, Ef represents the Fermi level, and Ev represents the upper limit of the valence band.

FIG. 3 shows the carrier collection efficiency of the solar cell with respect to changes in the wavelength λ of the incident light, with the horizontal axis representing the wavelength λ [μm] of the incident light and the vertical axis representing the carrier collection efficiency η [rel · u].
In the figure, the alternate long and short dash line Sun-S indicates the spectrum curve of sunlight. Further, the carrier collection efficiency η [rel · u] for each wavelength λ [μm] of the light incident on the amorphous unit cell having a large forbidden band is shown by the solid line a-S in FIG.
It becomes as shown in i.

Next, the polycrystalline semiconductor 11p having a small bandgap absorbs light on the long wavelength side that has passed through the amorphous semiconductor, and the carrier collection efficiency of the polycrystalline semiconductor is shown by the dotted line Poly-
As shown in Si. As a result, a single photovoltaic element in which a unit cell made of an amorphous semiconductor is ohmic-bonded to a crystalline unit cell in which a crystalline semiconductor and an amorphous semiconductor are hetero-junctioned, and a wide range of light from a short wavelength to a long wavelength is generated. Can be used effectively.

By the way, in order to effectively convert the light absorbed in each unit cell into electricity and extract it to the outside, the majority carriers generated in each unit cell are recombined at the interface between the two unit cells, and the internal electromotive force is generated. You need to cancel the effect.

In the embodiment of the present invention, electrons generated in a crystal unit cell composed of a crystal system and an amorphous semiconductor by light irradiation and holes generated in an amorphous unit cell composed of an amorphous semiconductor are generated in the vicinity of the interface between both unit cells. By recombining via the localized level in the forbidden band, a recombination current flows and a good carrier exchange recombination function is obtained. That is, by taking advantage of the generally disadvantageous property that the number of localized levels in the band gap of an amorphous semiconductor is large,
Amorphous semiconductor PN formed at the interface between both unit cells
It is characterized in that the carrier exchange and recombination function can be easily obtained by joining without inserting a special layer between the completely different crystal system cell and the amorphous cell.

FIG. 4 is a diagram showing the measured values of the IV characteristics of the photovoltaic element of FIG. 2 (A). In the figure, the horizontal axis is the second
The output voltage Vout [V] of the photovoltaic element of FIG. 6A is shown, and the vertical axis represents the output current Iout [mA / cm of the photovoltaic element.
² ] and the open circuit voltage in this case is about 1.3.
[V]. The conversion efficiency of the photovoltaic element of FIG. 2 (A) obtained from the IV characteristics of the same figure is significantly higher than the announced amorphous solar cell of 8 to 9%.
2% was obtained.

FIG. 5 (A) shows innumerable minute tetrahedral surfaces on the surface of a crystalline semiconductor substrate or an inorganic solid thin plate or an organic solid film substrate in order to increase the absorption of incident light in the photovoltaic element. It is a model figure which formed the unevenness | corrugation which consists of a body. In the figure (B), by providing the unevenness shown in the figure (A), the absorption of light is increased by the optical multiple reflection refraction between the incident light and the pyramid surface of the surface, and the crystalline semiconductor of the present invention and the amorphous semiconductor are formed. The conversion efficiency of the photovoltaic element can be improved by combining the structure in which the crystalline unit cell made of a heterojunction with a semiconductor and the amorphous unit cell made of an amorphous semiconductor are joined. In FIG.
Is a transparent conductive film, 21 is an amorphous semiconductor, 11 is a crystalline semiconductor, and incident light is an interface between the transparent conductive film 10 and the amorphous semiconductor 21 and an interface between the amorphous semiconductor 21 and the crystalline semiconductor 11, as indicated by a solid line, Multiple reflections occur one after another, and light absorption can be increased due to the light confinement effect.

EFFECTS OF THE INVENTION As described above, according to the photovoltaic element of the present invention, the amorphous semiconductor unit cell can absorb the light on the short wavelength side of the incident light, and further can absorb the light in these amorphous semiconductors. Long-wavelength side light that could not be transmitted is absorbed by the crystal unit cell consisting of a crystal semiconductor and an amorphous semiconductor, so that a single photovoltaic element can cover a wide range from short wavelength to long wavelength. It absorbs light and has a higher conversion efficiency than a single solar cell in which a crystalline semiconductor and an amorphous semiconductor are heterojunctioned or a stacked solar cell in which an amorphous semiconductor is alone.
For the purpose of fulfilling the function of carrier exchange and recombination,
Since it is not necessary to purposely use a transparent conductive film, the performance of the crystal system and the amorphous cell is improved, and it is possible to manufacture at a higher efficiency and at a lower cost.

Further, as described above, since the photovoltaic device of the present invention is formed by laminating materials having different spectral sensitivity characteristics, it can be used as a photo-detecting device that also detects light of a specific wavelength.

[Brief description of drawings]

FIG. 1 shows the wavelength λ [μ of incident light of a conventional crystalline solar cell (c-Si) and an amorphous solar cell (a-Si).
m] (horizontal axis) and carrier collection efficiency η [%] (vertical axis), and FIGS. 2 (A) and 2 (B) are respectively the first embodiment of the photovoltaic device of the present invention. The configuration diagram and energy band diagram of the example, FIG. 3 shows the configuration of the embodiment of FIG. 2 (A), and the amorphous semiconductor (solid line curve aS
i) and polycrystalline semiconductors (dotted curve Poly-Si)
Incident light λ [μm] (horizontal axis) and carrier collection efficiency η
FIG. 4 is a diagram showing a relationship with [rel · u] (vertical axis), and FIG. 4 is an IV characteristic of the photovoltaic element of the embodiment of FIG. 2 (A) (output of the photovoltaic element on the horizontal axis). Voltage Vout [V], a diagram showing the output current Iout [mA / cm ² ] of the photovoltaic element on the vertical axis, and FIGS. 5A and 5B are the substrate of the photovoltaic element of the present invention. FIG. 9 is a model diagram and an operation explanatory diagram in which a texture pattern that greatly absorbs incident light is formed. 1 ... Electrode (ohmic electrode or conductive substrate), 2 ...
... inorganic solid thin plate substrate or organic solid film substrate, 11 ... Crystalline unit cell, 11p ... Crystalline semiconductor, 21 ... Amorphous unit cell, 20n, 21p,
21i, 21n ... Amorphous semiconductor, Egcl ...
Forbidden band width of crystalline semiconductor, Egal ... Forbidden band width of amorphous semiconductor, HJ ... Heterojunction between crystalline semiconductor and amorphous semiconductor.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroaki Okamoto 779, Kamishinji, Hirano, Kawanishi-shi, Hyogo 1 (72) Inventor Koji Okuda 2-11, Tagawa, Yodogawa-ku, Osaka-shi, Osaka Osaka Transformer Co., Ltd. (56) Reference JP-A-57-79674 (JP, A) JP-A-56-33888 (JP, A)

Claims (6)

[Claims] 1. A crystal unit cell that is PN-heterojunction by forming an amorphous semiconductor on a single crystal or polycrystal crystal semiconductor, and a PIN junction amorphous on the amorphous semiconductor of the crystal unit cell. A stack type photovoltaic device in which heterogeneous unit cells are formed by laminating amorphous unit cells made of a semiconductor into an integrated structure, and in which the amorphous semiconductor film of the crystalline unit cell has a thickness of 3000 Å . 2. The stacked photovoltaic element according to claim 1, wherein at least one of the amorphous semiconductors is a charge-controlled microcrystalline semiconductor. 3. The stacked photovoltaic element according to claim 1, wherein the crystalline semiconductor is used as a substrate, and an amorphous semiconductor is deposited on the substrate. 4. The stacked photovoltaic element according to claim 1, wherein the crystalline semiconductor is deposited on a thin plate of an inorganic solid or a substrate of a film of an organic solid. 5. The stacked photovoltaic element according to claim 3, wherein the crystalline semiconductor substrate is formed with a texture pattern for increasing absorption of incident light. 6. A stack type of different unit cells according to claim 4, wherein a texture pattern for increasing absorption of incident light is formed on the substrate made of a thin plate of inorganic solid or a film of organic solid. Photovoltaic device.