JPH07283429A - Method for manufacturing thin-film solar cell - Google Patents

Abstract

PURPOSE:To form a high-quality crystal thin film at a lower temperature than a thermal crystallizability temperature by forming one portion of a crystal semiconductor thin film as an amorphous layer and applying X ray to the amorphous layer. CONSTITUTION:P<+> type polycrystalline silicon 3 is formed on a metal substrate 2 and p-type amorphous silicon 4 is formed by monosilane and hydrogen-diluted diborane by the plasma CVD method. Then, X ray is applied in vacuum to crystallize amorphous silicon. The crystal evaluation by the Raman spectral method reveals that only a peak peculiar to a crystal according to Si-Si crystal is recognizable, a crystal particle diameter is 3mum on average, and the specific resistance of the thin film is 2OMEGAcm, thus forming a high-quality crystal thin film. X ray is photon, does not have mass, and does not disturb the atom array due to the application of ions and electrons, and hence crystallizability proceeds due to the application of X ray at a lower temperature than a thermal crystallizability temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of a solar cell, which is one of photoelectric conversion elements, and in particular it is possible to form a high quality crystalline thin film at a temperature lower than the thermal crystallization temperature. The present invention relates to a method for manufacturing a thin film crystal solar cell.

[0002]

2. Description of the Related Art Generally, a crystalline solar cell has a high photoelectric conversion efficiency, but it requires a high temperature process and a thickness of several hundreds of μm in order to form an element on a crystalline substrate.
It is relatively expensive. For this reason, attempts have been made to form thin film crystals on a low-priced another substrate and use them as solar cells. For example, as a method of forming high quality thin film crystalline silicon, a method of zone melting on a SiO ₂ film is described in H.
Naomoto, S.Hamamoto, A.Takami, S.Arimoto, T.Ishihara,
H.Kumabe, T.Murotani and S.Mitsui; TechnicalDigest
of the International PVSEC-7, Nagoya, Japan,
Reported in 1993. However, since it is necessary to heat the substrate to 1200 ° C. or higher, the substrate is mostly limited to crystalline silicon. Therefore, in order to reduce the cost, it has been attempted to form a thin film crystal layer on an inexpensive substrate such as a glass substrate and make it an element. But high quality crystals,
In particular, it is extremely difficult to directly form a thin film of a group IV or III-V semiconductor such as silicon or GaAs on a substrate other than a crystal substrate.

As one method for solving such a problem, a method of depositing a semiconductor in an amorphous state and crystallizing it by a thermal process such as a solid phase epitaxial technique or a laser annealing technique has been studied. There is. However, since these methods also cause crystallization by thermal means, they are always exposed to a considerably high temperature. For example, T.Matsuyama, T.Ba
ba, T.Tanaka, S.Tsuda and S.Nakano; Technical Diges
t of the International PVSEC-7, Nagoya, Japan,
In 1993, a method of forming a thin film crystalline silicon solar cell by solid phase crystallization of amorphous silicon is shown. However, even in this case, 500 to 600 is required for crystallization.
Heat treatment at ℃ is required.

As a method of crystallizing an amorphous material at a lower temperature to form an element. Shiro Sato, using X-ray irradiation
By Yoshiyuki Hirano, Katsuyuki Goto, Hiroshi Kawada, Junichi Chikawa, Proceedings of the 54th JSAP Academic Lecture, Second Volume, 1993,
It is shown on page 838. However, nothing is mentioned about the case of having a film thickness of about 10 μm or more and requiring high quality crystallinity, such as a crystalline silicon solar cell formed on an insulator.

[0005]

In the method of crystallizing the amorphous layer by the thermal process, as described above, it is necessary to always produce 600
Since it is exposed to a high temperature of about ° C or higher, there is a problem that an inexpensive substrate such as a glass substrate cannot be used. In addition, since the thermal properties differ depending on the material, film peeling becomes a problem in the process where the temperature difference is large. In particular, it becomes an important problem when a low cost and a large area are required like a solar cell.

The object of the present invention is to solve the problems of the prior art described above, and to provide a thin film crystalline solar cell capable of forming a high quality crystalline thin film at a temperature lower than the thermal crystallization temperature. It is to provide a manufacturing method.

[0007]

The above object is to manufacture a thin film solar cell comprising a crystalline semiconductor thin film and an electrode on at least a main surface of a substrate, and first, a part of the crystalline semiconductor thin film is an amorphous layer. Alternatively, it can be achieved by providing a method for producing a thin-film solar cell, which is characterized in that it is formed as a microcrystalline layer and then the layer is irradiated with X-rays for crystallization. This makes it possible to obtain a highly efficient and low cost solar cell comparable to a crystalline solar cell using a single crystal or polycrystalline substrate.

[0008]

Although the fact has been confirmed regarding the crystallization by X-ray irradiation, the mechanism is not always clear. The following are some of the strong arguments. X-ray irradiation has the effect of lowering the activation energy for forming atomic vacancies. That is, the effect of X-ray irradiation is to generate high-density atomic vacancies, and crystallization proceeds through this state. The advantages of crystallization by X-ray irradiation are that X-rays are photons, have no mass, and do not act to disturb the atomic arrangement that occurs with irradiation of ions or electrons. Therefore, crystallization will proceed by irradiation. This means that there is little thermal damage, and that problems such as peeling from the substrate are unlikely to occur.

As the X-ray source, it is preferable to use synchrotron radiation having a high dose. In synchrotron radiation, X-rays are emitted from the electron storage ring in a close direction. Therefore, by setting an arbitrary number of X-ray extraction positions, a plurality of (for example, 10 or more) simultaneous processes can be performed. This indicates that the process is suitable for mass production.

However, the peak energy of such a continuous light source is in the vicinity of several keV, and since absorption in a semiconductor (eg, silicon) is large in this region, it becomes uniform in the film thickness direction at a film thickness of about 10 μm. X-ray irradiation becomes difficult,
A region with poor crystallinity partially occurs. This is especially fatal in a device such as a solar cell in which the entire thin film is used as an active region and carriers are transported in the film thickness direction.

Therefore, in the present invention, in order to avoid this, crystallization by X-ray irradiation is performed in a thin film state having a thickness of about 5 μm or less. As a result, a high quality and uniform crystal thin film is formed. After that, it has been clarified that a crystalline thin film with good orientation can be obtained by growing a semiconductor by a chemical vapor deposition method or the like, which makes it possible to form a highly efficient thin film crystalline solar cell.

[0012]

EXAMPLES The constitution of the method for producing a thin film crystal solar cell of the present invention will be specifically described below with reference to examples.

Example 1 FIG. 1 is a flow chart of an example of the method of the present invention, and FIG. 2 is a sectional view showing a schematic structure of a thin film crystal solar cell obtained by the method. First, p + type (B: 8 × 10 ²⁰ / cm 2 is formed on the metal substrate 2 by the thermal CVD method.
^The polycrystalline silicon of ³ ) was formed to a thickness of 200 nm. For this film formation, monosilane and diborane diluted with hydrogen as a p-type doping gas were used. Next, p-type amorphous silicon 4 was formed to a thickness of 1 μm from monosilane and diborane diluted with hydrogen by plasma CVD. At this time, Si in gas
: B was set to 1:10 ^-5 . Subsequently, X-ray irradiation was performed in vacuum at 400 ° C. for 7 minutes to crystallize amorphous silicon. X
The line was a continuous X-ray having a peak near 3 keV, and the number of irradiated photons was 5 × 10 ¹⁶ / cm ² near 3 keV.
When the crystals were evaluated by Raman spectroscopy, only peaks peculiar to the crystals due to the Si-Si bond were observed. Also,
The average crystal grain size was 3 μm. The specific resistance of the thin film at this time was 2 Ωcm.

Next, by a thermal CVD method under reduced pressure, 550 ° C.
Then, a p-type crystal semiconductor layer 5 having a thickness of 20 μm was formed. For film formation, disilane and 10 ppm diborane (diluted with hydrogen) as a doping gas were used. At this time, Si: B in the film
The gas flow rate was set so that the ratio was 1: 2 × 10 ⁻⁶ . Further, by the same film forming method, a 200 nm n-layer 6 was formed with monosilane and 10 ppm phosphine (diluted with hydrogen) as a doping gas. At this time, Si: P in the gas is 1:
The gas flow rate was set to be 5 × 10 ⁻⁴ . Next, silicon tetraethoxide is converted into Si by plasma CVD.
TiO ₂ was continuously formed from O ₂ and titanium tetraisopropoxide, and the passivation film 7 and the antireflection coating film 8 were formed to have predetermined thicknesses. Next, an opening was formed in the electrode lead-out portion by a known photoetching method, and then the electrode layer 9 was formed using a metal mask.

When the obtained thin film crystal solar cell was irradiated with sunlight 1, a short circuit current of 25 mA / cm ² , an open circuit voltage of 0.62 V and a fill factor of 0.71 were obtained. When the surface texture etching was performed before the formation of the n-layer 6, the short-circuit current was further increased. Further, when hydrogen gas was introduced during X-ray irradiation and plasma discharge treatment was performed under reduced pressure, an improvement in fill factor was observed. Usually, an amorphous film formed by the plasma CVD method contains a large amount of hydrogen, and film peeling easily occurs when heat-treated, but the maximum heat-treatment temperature in the method of the present invention is
The temperature was 550 ° C, and many thin film forming steps were required, but no problems such as film peeling occurred. This is also a great advantage of crystallization by X-ray irradiation.

[Embodiment 2] A second embodiment of the method of the present invention will be described with reference to FIG. First, a glass substrate was subjected to surface irregularity processing to form a V groove having an apex angle of about 70 degrees, and a glass processed substrate 10 was obtained. The difference in height of the uneven portion is about 40 μm
Met. An aluminum layer containing silicon was formed as a back electrode layer 11 on the surface by a vapor deposition method. Next, 100 nm of p-type microcrystalline silicon was formed from monosilane and diborane diluted with hydrogen by a plasma CVD method. At this time, Si: B in the gas was set to 1:10 ^-2 . Subsequently, X-ray irradiation was performed in vacuum at 300 ° C. for 7 minutes to crystallize silicon. Next, using the same plasma CVD method,
Si: B in gas is set to 1:10 ^-2 to 100 nm, and further 1:
The ^thickness was set to 10 ⁻⁵ and formed to 300 nm. Subsequently, X-ray irradiation was performed in vacuum at 300 ° C. for 7 minutes to crystallize silicon. Furthermore, Si: B in the gas is 1: 1 by the plasma CVD method.
Deposition at 10 ⁻⁵ and X-ray irradiation were repeated three times.
As a result, a p + layer 12 having a thickness of 200 nm and a p-type crystalline silicon layer 13 having a thickness of about 2 μm were formed. After that, in the same manner as in Example 1, the crystalline semiconductor thin film 14 and the n layer 15 are deposited,
A bond was formed. Then, a passivation film and a non-reflective coating film were formed, and an opening was formed in the electrode extraction portion by a well-known photoetching method, and then an electrode layer was formed by a vapor deposition method using a metal mask.

The sample thus obtained is the same as in Example 1.
The short-circuit current was increased to 28 mA / cm ² and the fill factor was also improved to 0.76 compared to the case of. As described above, even when the surface unevenness exceeds the film thickness, no film peeling was observed.

[Embodiment 3] A third embodiment of the method of the present invention will be described with reference to FIG. On the ceramic substrate 16,
A metal layer was partially formed as the back surface electrode 17 for serial connection. At this time, fine unevenness due to crystallization of 100 nm or less was formed on the surface of the metal layer by controlling the forming conditions.
Then, n-type amorphous silicon is removed by a molecular beam deposition method.
It was formed to 0 nm. At this time, set Si: P to 1:10 ^-2 to 100
An n-type layer having a thickness of 400 nm was formed as a high concentration n-type layer 18 having a thickness of 1:10 ^-5 . Subsequently, X-ray irradiation was performed in vacuum at 300 ° C. for 7 minutes to crystallize amorphous silicon. At this time, crystallization was promoted by the unevenness of the metal surface as compared with the case of a flat surface. The same effect was obtained by forming a thin film of ZnO or SnO ₂ . Then, in that case, the film thickness and the refractive index of the material can be appropriately selected, so that a state convenient for reflection on the back surface can be set. Further, 1: m deposition and X-ray irradiation were repeated 4 times with Si: P of 1:10 ^-5 to form a 4.4 µm crystalline silicon n-type layer 19. next,
20 nm of p-type microcrystalline silicon 20 was formed from disilane and diborane diluted with hydrogen by the plasma CVD method. At this time, Si: B in the gas was set to 1:10 ^-2 . This formed a thin film crystalline silicon solar cell.

Next, by the plasma CVD method, n of 30 nm
Type microcrystalline silicon, 100 nm i-type amorphous silicon, 15 nm
P-type amorphous silicon carbide was deposited and the thin film amorphous silicon 21 was used to form a solar cell structure. Finally, ITO (indium tin oxide) was formed as the transparent electrode 22 and aluminum was formed as the collector electrode and the take-out electrode 23 by the vapor deposition method so that the respective elements on the substrate were connected in series.

As a result, when solar light irradiation 1 was performed in 10 stages in series, a short circuit current of 12 mA / cm ² , an open circuit voltage of 14 V and a fill factor of 0.75 were obtained. In addition, the solar cell characteristics were deteriorated by 3% or less even after irradiation of sunlight for one month, and a highly stable solar cell could be manufactured.

In the above examples, only the Si type was described as the thin film crystal solar cell, but similar results can be obtained with an alloy type mainly containing silicon, for example, SiC or SiGe type. Further, it goes without saying that a crystalline thin film solar cell can be similarly produced from a binary or more multi-component amorphous semiconductor material such as GaAs or InP.

[0022]

As described above, the method of manufacturing a thin film crystal solar cell is the method of the present invention,
It has been possible to provide a method for manufacturing a thin film crystalline solar cell capable of forming a high quality crystalline thin film at a temperature lower than a thermal crystallization temperature by solving the problems of the conventional techniques. In particular, since it has a large coordination number, it is extremely effective as a technique for directly forming a thin film of a group IV semiconductor such as silicon, which is likely to have dangling bonds during crystallization, on a substrate other than a crystalline substrate.

[Brief description of drawings]

FIG. 1 is a flowchart illustrating a manufacturing process of a first embodiment.

2 is a cross-sectional view showing a schematic configuration of a thin film crystal solar cell formed by the manufacturing method of Example 1. FIG.

3 is a cross-sectional view showing a schematic configuration of a thin film crystal solar cell formed by the manufacturing method of Example 2. FIG.

FIG. 4 is a cross-sectional view showing a schematic configuration of a thin film crystal solar cell formed by the manufacturing method of Example 3.

[Explanation of symbols]

1 ... Solar irradiation, 2 ... Metal substrate, 3 ... Polycrystalline silicon,
4 ... Amorphous silicon, 5 ... Crystal semiconductor thin film, 6 ... N layer,
7 ... passivation film, 8 ... non-reflective coating film, 9 ... electrode layer, 10 ... glass processed substrate, 11 ... reverse electrode layer, 12 ... p +
Layer, 13 ... p-type crystalline silicon layer, 14 ... crystalline semiconductor thin film, 15
... n layer, 16 ... ceramic substrate, 17 ... back electrode, 18 ... high concentration n-type layer, 19 ... crystalline silicon n-type layer, 20 ... p-type microcrystalline silicon, 21 ... thin film amorphous silicon, 22 ... transparent Electrode, 23
... collection electrode and extraction electrode.

Claims (8)

[Claims] 1. In the production of a thin film solar cell comprising a crystalline semiconductor thin film and an electrode on at least a main surface of a substrate, first, a part of the crystalline semiconductor thin film is formed as an amorphous layer or a microcrystalline layer, Next, a method for producing a thin-film solar cell, which comprises crystallizing the layer by irradiating it with X-rays. 2. In the production of a thin film solar cell comprising a crystalline semiconductor thin film and an electrode on at least the main surface of a substrate, first, a part of the crystalline semiconductor thin film is an amorphous layer or a fine layer having a thickness of 0.1 to 5 μm. A method for producing a thin-film solar cell, which comprises forming a crystalline layer, crystallizing the layer by irradiating the layer with X-rays, and then further growing a crystallized semiconductor layer. 3. A step of 1) forming a semiconductor layer made of n-type or p-type polycrystalline or microcrystalline on at least the main surface of the substrate, and 2) the amorphous layer or microscopic layer having a thickness of 0.1-5 μm. A method for manufacturing a thin-film solar cell, comprising the steps of forming a crystal layer and then crystallizing the layer by irradiating X-rays, and 3) growing a crystal semiconductor layer by a growth method. 4. The method for manufacturing a thin film solar cell according to claim 1, wherein the amorphous layer or the microcrystalline layer is a layer containing silicon as a main component. 5. The method for producing a thin film solar cell according to claim 1, wherein the surface of the substrate on which the crystalline semiconductor thin film is formed is made uneven. 6. The crystallization by the above X-ray irradiation is performed on the substrate 45.
The method for producing a thin-film solar cell according to claim 1, wherein the irradiation is X-ray irradiation while maintaining the temperature at 0 ° C or lower. 7. The crystallization by X-ray irradiation is X-ray irradiation performed while introducing any one of hydrogen gas, helium gas, halogen gas, halogen hydride gas or a mixture thereof. Item 2. A method for manufacturing a thin-film solar cell according to Item 1. 8. The method of manufacturing a thin-film solar cell according to claim 7, wherein the X-ray irradiation performed while introducing the gas is X-ray irradiation performed while plasma-discharging the introduced gas.