Patent Application
20090314349
This invention relates to a microcrystalline silicon film forming method and a solar cell, especially to a microcrystalline silicon film forming method where a flow rate of hydrogen in material gas is reduced.
Conventionally, amorphous silicon (a-Si) films formed by a plasma CVD (chemical vapor deposition) method have been used for large-area solar cells. However, a tandem-structure solar cell where a-Si film and a microcrystalline silicon (μc-Si) film are stacked to improve the conversion efficiency by effectively absorbing solar spectra ranging from infrared to ultraviolet, has been gathering much attention and partially commercialized.
This microcrystalline silicon film is formed mostly using a parallel-plate type (capacitive-coupling type) plasma CVD apparatus under a condition different from that for an a-Si film. In general, it is formed by supplying a RF (radio-frequency) power higher than that in a-Si film formation, under a higher hydrogen gas flow rate (i.e., higher hydrogen/silane flow ratio) than that in a-Si film formation. Specifically, compared to silane gas, a larger volume of hydrogen gas (e.g., not less than twenty times) is introduced to create a larger volume of hydrogen radicals required for crystallization, and a higher power is supplied to decompose hydrogen gas. Substrate temperature, which is preferably between 300 and 400° C. in normal film formation, has to be lowered between 200 and 250° C. when an underlying a-Si film has been already formed, such as in the tandem structure. Moreover, it has been pointed out that a much larger volume of hydrogen gas flow is required for crystallization when substrate temperature is lowered.
The plasma CVD apparatus releases toxic or dangerous gas such as unreacted or reacted gas of silane and hydrogen. Normally, the exhaust gas is diluted with an incombustible gas, such as nitrogen, to make the hydrogen gas concentration below the explosive limit. Then, it is released out to atmosphere after processing silane-series gases by an abatement system.
Patent Reference 1: Japanese Publication No. 2003-158276
Patent Reference 2: Japanese Publication No. 2004-143592
Non-patent Reference 1: Solar Energy Materials & Solar Cells, 62, 97-108(2000)
Non-patent Reference 2: Researches of the Electrotechnical Laboratory No. 864, pp. 46-57
As described, nitrogen gas, which is comparatively cheap, has been normally used to dilute hydrogen gas. In forming a microcrystalline silicon film, however, cost of nitrogen gas could be a problem, because a large volume of hydrogen gas is used, and most of it is released out in an unreacted state. For example, when silane gas of 1 L/min. is used as material gas to fabricate a large-area solar cell, hydrogen gas of not less than 20 L(liters)/min. is required generally. Use of large-volume hydrogen gas requires a large-scale evacuation pump and increases the gas cost, boosting the total cost for a microcrystalline silicon solar cell. In addition, a large volume of nitrogen gas, i.e., 500 L/min., is required to dilute hydrogen gas of 20 L/min. in the exhaust gas below the explosive limit concentration (4%). This is another factor boosting the cost of the microcrystalline silicon solar cell.
For a solar cell with tandem structure, moreover, substrate temperature in forming a microcrystalline silicon film has to be approximately as low as in forming an amorphous silicon film. Therefore, the hydrogen gas flow rate has been much more increased, boosting the cost of the solar cell further.
In the situation as described, the inventors investigated many kinds of film formation methods and conditions thereof, not limited to the conventional parallel-plate-type plasma CVD, pursuing reduction of hydrogen gas flow rate. In this process, the inventors had discovered a method suiting for forming a microcrystalline silicon film. This method enables formation of a microcrystalline silicon film suiting for a solar cell even at a low flow rate of hydrogen gas. In this method, a multiplicity of antennas is disposed wholly covering a substrate to generate plasma, where one end of each antenna is connected to an high frequency power source, and another end of each antenna is grounded.
On a basis of this discovery, the invention had been completed by further investigation to realize stable formation of a microcrystalline silicon film suiting for a solar cell. That is, an object of the invention is to provide a plasma CVD method capable of forming a microcrystalline silicon film at a lower hydrogen gas flow rate and at a lower substrate temperature, compared to the prior art. A further object of the invention is to provide microcrystalline silicon solar cell at moderate price.
The method of this invention is one for forming a microcrystalline silicon film by a plasma CVD. In this method, plural antennas are arranged to form an antenna array structure in a vacuum chamber. One end of each antenna is connected to a high frequency power source and anther end is grounded. Substrates are placed facing the antenna array, and the substrate temperature is between 150 and 250° C. Plasma is generated by introducing gas mixture of hydrogen and silane to the chamber, and by introducing high frequency power to the antennas. When hydrogen/silane gas flow ratio is controlled in the range from 1 to 10, microcrystalline silicon films are formed on the substrates, with the ratio Ic/Ia between 2 and 6, whereas Ic is the Raman scattering intensity of a peak at around 520 cm−1 related to crystalline silicon, and Ia is the Raman scattering intensity at around 480 cm−1 related to amorphous silicon.
The microcrystalline silicon film of this invention is one where the ratio Ic/Ia is between 2 and 6, whereas Ic and Ia are the Raman scattering intensity at around 520 cm−1 and at 480 cm−1, related to crystalline silicon and amorphous silicon, respectively. By combining this microcrystalline silicon film with an amorphous silicon film, it is enabled to fabricate a solar cell capable of more effective utilization of sun light in spite of being thin layers.
It is preferable to form the microcrystalline silicon film by controlling the hydrogen/silane gas flow ratio in range from 1 to 7.
In this invention, it is preferable that each antenna has a U-shaped configuration, with phase control between the antennas next to each other. By these, it is possible to form a microcrystalline silicon film having more uniform thickness distribution on a larger-area substrate, compared to, for example, a method using rod-shaped antennas.
In this invention, it is preferable to dispose three or more arrays of the antennas and cause a discharge in three or more regions simultaneously. In this case, two substrates are disposed between each array of the antennas, resulting in increase of the productivity. In addition, it is enabled to make the hydrogen gas flow rate much lower, compared to a method using one or two arrays of the antennas for one or two regions.
Further, the microcrystalline silicon film forming method of this invention is the method to fabricate microcrystalline silicon by inductively coupled plasma CVD method, comprising disposing substrates in a vacuum chamber, making temperature of the substrate between 150 and 250° C., introducing a gas mixture of hydrogen and silane, generating plasma by applying an high frequency power, controlling the hydrogen/silane gas flow ratio in a range from 1 to 10, and thus forming a microcrystalline silicon film on the substrate, wherein the ratio Ic/Ia of the film is between 2 and 6. It is further preferable to form a microcrystalline silicon film with hydrogen/silane gas flow ratio controlled in a range from 1 to 7.
Because the invention is by an inductively-coupled type plasma CVD, especially by an array-antenna type plasma CVD, it enables stable formation of a microcrystalline silicon film with a lesser hydrogen gas flow rate, compared to film formation by the parallel-plate type (capacitively-coupled type) plasma CVD. For example, even when substrate temperature is low, e.g., 200° C., it is possible to make the hydrogen/silane gas flow ratio not more than 10, further not more than 4. Although it has been impossible by the prior-art method, the invention enables to stably form a microcrystalline silicon film even under such a low hydrogen gas flow rate with the ratio Ic/Ia between 2 and 6, suitable to improve conversion efficiency of solar cells. As a result, it is enabled to reduce the cost of inert gas for diluting hydrogen and the apparatus cost drastically, which contributes to lowering prices of solar cells.
The microcrystalline silicon film formation method of this invention, which uses the plasma CVD apparatus shown in
As shown in
The material gas source comprises cylinders of silane gas and hydrogen gas, mass flow controllers and other components, so that the gases of required flow rates and flow ratio can be introduced into the vacuum chamber through the gas introduction port. Beside the material gas introduction structure shown in
The pumping device in the example of
An a-Si/μc-Si tandem-type solar cell generally has the structure where p-type a-Si, i-type a-Si, n-type a-Si, p-type μc-Si, i-type μc-Si, n-type μc-Si and backside electrode are formed in this order on a glass substrate having a transparent electrode thereon. As an example, a method for manufacturing an i-type μc-Si tandem-type solar cell by the plasma CVD apparatus shown in
Opening a gate valve (not shown), a carrier 15 for the holders 14 holding the substrates 13 is transferred to the i-type μc-Si film formation room (vacuum chamber) 1. Each substrate 13 is placed facing to each antenna array. As shown in
By carrying out the described steps, it is possible to form the microcrystalline silicon film where the ratio Ic/Ia is 2 to 6, even when the substrate temperature is as low as 200° C. , and even when the hydrogen/silane gas flow ratio is as low as, e.g., 1. Therefore, the flow rate of nitrogen for dilution can be reduced drastically, which enables to reduce the cost for the solar cell manufacturing.
After forming the i-layers, i.e., i-type μc-Si films, the substrates 13 are transferred to a n-type μc-Si film forming apparatus, in which n-layers, i.e., n-type μc-Si films, are formed. Afterward, solar cells are completed by forming backside electrodes and other required components.
For improving uniformity of the plasma density, it is effective to form dielectric films on surfaces of the antennas, corresponding to the plasma density distribution. Otherwise, it is effective to coordinate thickness of each dielectric film or to vary diameter of the antennas, corresponding to the plasma density distribution. Moreover, the plasma density can be made more uniform on the whole area of each substrate 13 by controlling phase of the high frequency power applied to each antenna, specifically by controlling high frequency phase difference between the antennas neighboring to each other. By this, the homogeneity of microcrystalline silicon film thickness and the film properties can be improved furthermore.
The inventors had carried out concrete formation of silicon films under various conditions, and evaluated them regarding to crystallinity and photoelectrical property. In forming the silicon films, the same plasma CVD apparatus as shown in
U-shaped antennas made of a stainless-steel pipes were used, with 8 mm in diameter and with many gas diffusion holes (50 mm in pitch) therein. The antenna was 1.6 mm in length and the distance between the centers of two pipes were 35 mm. Each antenna array was formed of twenty five antennas where the distance between the centers of adjacent pipes of the same side were 70 mm. Three rows of the antenna array were disposed, from which the substrates were distant 35 mm.
Each gas introduction port was provided at the grounded port of each antenna in the plasma CVD apparatus used in this embodiment. A gas mixture of hydrogen and silane was introduced into the vacuum chamber from the gas diffusion holes of the antennas.
As the substrate, a 1.2×1.6 meters sized glass substrate was used. An 85 MHz source was used as the high frequency power source.
Film formation conditions were; the silane gas flow rate between 250 and 1,500 L/min., the hydrogen gas flow rate between 0 and 40,000 L/min., the hydrogen/silane gas flow ratio between 0 and 40, pressure between 2 and 29 Pa, the input power per one antenna between 20 and 428 W, and the substrate temperature between 150 and 250° C. Varying the conditions in these ranges, silicon films were formed. On each sample, Raman spectra, photoelectric current and dark current were measured. These results are shown in
When the flow ratio was 10 or below, formation of microcrystalline silicon having the Ic/Ia between 2 and 6 was rather difficult, as same as in the case of parallel-plate type plasma CVD apparatus. On the other hand, the result shown in the figure demonstrates formation of microcrystalline silicon even when the flow ratio was 10, further even 1. Although the reason has not been clear, a microcrystalline silicon film can be formed at a lesser hydrogen gas flow rate when the number of the discharge region is three.
Although the inductive-coupling plasma CVD method using the antenna arrays was described, the invention is not limited to this. The invention can be applied to an outside antenna type or an inside antenna type, such as disclosed in Japanese publication H10-265212 or Japanese publication 2001-35697.
Although the above description was on the PIN type solar cell where the i-layer is the tandem structure of an a-Si film and μc-Si film, the invention is not limited to this. Beside the PIN type, the invention can be applied to formation of microcrystalline silicon films of any types for PN type solar cells, Schottky type solar cells and others.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-092481 | Mar 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/000337 | 3/29/2007 | WO | 00 | 6/16/2009 |
