JP6415918B2 - Silicon surface passivation method, surface passivated silicon manufacturing method, and solar cell manufacturing method - Google Patents

Description

  The present invention relates to a method for inactivating a defect that becomes a carrier recombination center on a silicon (Si) surface at a low temperature by utilizing ozone treatment. The invention further relates to silicon thus surface passivated.

  At present, silicon crystals are used as the mainstream of solar cell materials from the viewpoints of safety of raw materials, resource abundance, manufacturing cost and conversion efficiency. Even if the conversion efficiency is relatively high, the theoretically derived maximum conversion efficiency is about 28.5%, and it can be said that 70% or more of energy is wasted.

Various attempts have been made to increase the conversion efficiency of silicon-based solar cells. One of these is surface passivation technology. As methods often used, formation of a silicon nitride (SiN ₃ ) film (Patent Document 1), an aluminum oxide (Al ₂ O ₃ ) film, and surface passivation with hydrogen (Patent Document 2) are often performed. Further, passivation with a silicon oxide (SiO ₂ ) film is also performed, but there is a problem that a temperature for forming a high quality oxide film is high.

  Recently, research on the application of nanostructures to solar cell materials has been actively conducted, and there is an urgent need to develop new surface passivation technology for nanostructures with a significantly increased surface area ratio.

  An object of the present invention is to effectively inactivate defects existing on the silicon surface under mild conditions, and to provide silicon subjected to such surface treatment.

According to one aspect of the present invention, a silicon surface passivation method is provided by holding a silicon surface in an atmosphere containing ozone.
Here, the temperature at the time of holding | maintenance in the said atmosphere may be the range of 200 to 400 degreeC.
Moreover, the time hold | maintained in the said atmosphere may be 10 minutes or more.
The pressure of the atmosphere may be in the range of 8 to 12 Torr.
The atmosphere containing ozone may be produced by irradiating a gas containing oxygen with ultraviolet rays from a light source.
The ultraviolet light may be vacuum ultraviolet light.
The light source may be a xenon excimer lamp.
An object that shields light may be disposed between the light source and the silicon.
According to another aspect of the present invention, silicon that has been surface passivated by any of the above methods is provided.
According to still another aspect of the present invention, there is provided a solar cell using the surface passivated silicon.

  Ozone treatment terminates dangling bonds (unbonded hands) on the surface defects that become carrier recombination centers and electrically inactivates them, thereby suppressing carrier recombination with high efficiency with a simple treatment. Can do.

The figure which shows notionally the apparatus structure for performing the low temperature ozone treatment of an Example. A scanning photomicrograph of an n-type silicon nanowire array on a silicon substrate formed by electroless etching. A scanning electron micrograph of an n-type core / p-type shell Si nanowire in which a p-type silicon layer is formed on the surface of an n-type Si nanowire array by CVD. 2B is a photograph of a solar cell using the n-type core / p-type shell Si nanowire shown in FIG. 2B. The figure which shows the ozone treatment temperature and time dependence to the IV characteristic in an n-type core / p-type shell Si nanowire photovoltaic cell. The figure which shows the change of the defect signal strength before and behind ozone treatment of the n-type core / p-type shell nanowire sample using electron spin resonance.

  The present invention inactivates defects on the silicon surface. Silicon surface defects, for example, serve as a recombination center for photo-induced carriers in silicon-based solar cells, which is one of the causes for reducing the energy conversion efficiency. In the present invention, dangling bonds of surface defects are terminated by performing ozone treatment on the silicon surface at a low temperature. Therefore, when recombination of carriers due to surface defects is applied to a silicon solar cell, the energy conversion efficiency is reduced. Can be improved. The surface referred to in the present application is not only a “macroscopic” surface that humans can directly visually recognize from the outside, but ozone molecules such as grain boundaries and the back of voids in nanostructures are accessed from the outside. It should be noted that this is a broad concept that includes a surface in a broad sense in the sense that it can be (invaded), that is, where ozone molecules can be reached from the outside.

  The ozone treatment of the present invention can be realized by various techniques. For example, the oxygen target is changed into ozone by irradiating oxygen introduced into the chamber with an ultraviolet light source, and this ozone is placed in the chamber. It is done by acting on. The ultraviolet light source is not limited to this. For example, a vacuum ultraviolet light VUV (wavelength λ = 172 nm) light source using a xenon excimer lamp can be used.

  In application to a silicon solar cell, defects existing on the surface of the silicon solar cell are inactivated by the ozone, thereby suppressing carrier recombination on the surface and improving solar cell characteristics. Although damage may occur if solar cell samples are directly exposed to vacuum ultraviolet light during ozone treatment, for example, a shielding block may be placed between the sample to be processed and an ultraviolet light source such as a xenon excimer lamp. This problem can be avoided.

  Examples of the low-temperature ozone treatment of the present invention will be described below, but it should be noted that the present invention is not limited thereto and is defined only by the claims.

FIG. 1 conceptually shows an apparatus configuration for performing low-temperature ozone treatment according to an embodiment of the present invention. In FIG. 1, ozone O ₃ is irradiated by irradiating oxygen molecules O ₂ in a vacuum chamber 6 through a quartz glass window 2 with a light source of vacuum ultraviolet light VUV (wavelength λ = 172 nm) using a xenon excimer lamp 1. generate. Oxygen molecules are decomposed into oxygen atoms by the energy of the vacuum ultraviolet light VUV. The decomposed oxygen atoms react with oxygen molecules to become further ozone. The generated ozone is highly reactive and easily reacts with the silicon surface of the sample 4 such as a silicon substrate even at a low temperature. Further, the sample 4 is heated by the heater 5 in order to maintain the sample surface at an appropriate temperature (described later). The vacuum chamber 6 is maintained at an appropriate degree of vacuum by an exhaust system shown at the lower right of the drawing, and an appropriate amount of oxygen is supplied from a pipe shown at the upper left. When the sample 4 is irradiated with VUV light, a defect may be formed. Therefore, in order to prevent the sample 4 from being directly irradiated with the VUV light during the ozone treatment, a shielding plate is provided between the xenon excimer lamp 1 and the sample 4. 3 is installed. Further, the shape, orientation and the like of the table on which the sample is placed may be adjusted.

  The pressure in the apparatus during ozone treatment is suitably 8 to 12 Torr, and the surface treatment can be controlled by adjusting the pressure, treatment time, and treatment temperature. An n-type core / p-type shell Si nanowire shown in FIG. 2B was produced by forming a p-type Si shell on the surface of the n-type Si nanowire shown in FIG. 2A. For the Si nanowire having the core / shell structure, the treatment temperature (the temperature of the sample in the present example) and the time conditions are changed and the ozone treatment described above and the one not subjected to the ozone treatment are used. An n-type core / p-type shell Si nanowire solar cell was prepared. One photograph of the produced n-type core / p-type shell Si nanowire solar cell is shown in FIG. 2C. In the solar cell shown in FIG. 2C, Ag was used for the lower layer electrode and Al was used for the upper layer electrode. In addition, a portion that appears slightly white in the photograph is a portion that functions as a solar cell (that is, a region where the n-type core / p-type shell Si nanowire shown in FIG. 2B is visible). Since the general structure and the like of the Si nanowire solar cell are well known to those skilled in the art, further detailed description is omitted, but if necessary, refer to Non-Patent Document 1 or the like.

  The measurement results of the IV characteristics of these solar cells are shown in FIG. Further, Table 1 shows physical property evaluation values such as conversion efficiency of solar cells in the n-type core / p-type shell Si nanowire solar cells shown in FIG. 2C.

  As shown in FIG. 3 and Table 1, the effect of ozone treatment is the highest in the treatment at 300 ° C., and the short-circuit current is about three times that of the solar cell using the n-type core / p-type shell Si nanowires before the ozone treatment. It can be seen that it can be supplied. Regarding the treatment time, solar cell characteristics are hardly improved after 20 minutes, and it can be said that the inactivation of recombination centers existing on the surface is almost completed in 20 minutes. With respect to the treatment temperature, experiments were conducted in the range of 200 to 600 ° C. As a result, it was found that the solar cell characteristics were improved at 200 to 400 ° C., and 300 ° C. was the most efficient. Including the treatment time, the treatment at 300 ° C. for 10 minutes to 20 minutes showed the highest efficiency. When ozone treatment is performed at 500 ° C., excessive thermal oxidation proceeds, so that the characteristics of the solar cell are considered to be deteriorated. In addition, in the solar cell using the sample which performed the ozone treatment at 400 degreeC or more, since the dispersion | variation in a measurement result became large, it has not described / displayed in Table 1 and FIG. Further, FIG. 4 shows the result of examining the defect signal intensity before and after the ozone treatment of the n-type core / p-type shell Si nanowire sample using electron spin resonance (ESR). As shown in the figure, since the signal intensity of the defect was decreased by the ozone treatment, a sufficient passivation effect by the ozone treatment was confirmed.

Incidentally, it is conceivable that the SiO ₂ film is formed on the Si surface in the case of performing excessive ozone treatment. However, in view of the measurement results shown in FIG. 4, it can be said that almost no oxide film is formed here. That is, even when a SiO ₂ film is newly formed, the same signal should be observed at the same position because the defects formed at the interface are the same. However, as can be seen from FIG. 4, in this embodiment, since a decrease in signal can be observed, it can be said that almost no new oxide film layer is formed. Further, when the quality of the newly formed oxide film is poor, naturally defects in the oxide film are observed. The signal appears at a different location from that observed this time. In that case, another peak should appear in FIG. 4, but such a peak was not actually observed. In conclusion, it was confirmed that the SiO ₂ film was not formed at least within the range detectable by ESR.

  In the above embodiment, the case where the surface of the nanostructured Si is inactivated by ozone has been described as an example. However, the same inactivation effect can be obtained even in a bulk state Si instead of the nanostructure. In addition, in the passivation by film formation among those listed as background art, it is difficult or even if it is possible to uniformly cover the entire surface of the nanostructure. It should be noted that such a problem does not occur in the present invention. In the embodiment, the direct ozone treatment target is the surface of the p-type shell. However, since the defect formation is the same between p-type Si and n-type Si, the present invention describes how to treat Si as a treatment target. It can be applied regardless of whether or not doping is performed. Therefore, it should be noted that in the present invention, Si includes even a doped one.

  INDUSTRIAL APPLICABILITY The present invention is effective, for example, for inactivating defects that become recombination centers of photoinduced carriers existing on the surface of solar cell materials using silicon, and can be an effective method for increasing the energy conversion efficiency of solar cells. . In order to increase the efficiency of solar cells, the use of nanostructures is also considered. In order to take advantage of the effect of increasing the surface area of nanostructures, surface defect inactivation and passivation are more essential than ever. It is thought that it becomes. Therefore, the present invention is expected to become a technology that contributes to the high efficiency of innovative silicon solar cells in the 21st century in the industrial field, industrial field, and other fields.

1 Xenon excimer lamp 2 Quartz glass window 3 Shield plate 4 Sample 5 Heater 6 Vacuum chamber

JP-T 01-503743 JP 2011-1992277 A

"Semiconductor Nanowires for Energy Conversion", A. Hochbaum, P. Yang, Chem. Rev. 110, 527, 2010.

Claims (8)

A silicon surface passivation method by maintaining the silicon surface in an atmosphere containing ozone ,
The temperature during holding in the atmosphere is in the range of 200 ° C to 400 ° C,
The atmosphere containing ozone is produced by irradiating a gas containing oxygen with ultraviolet rays from a light source,
An object that shields light is disposed between the light source and the silicon.
Silicon surface passivation method. The silicon surface passivation method according to claim 1, wherein the time for holding in the atmosphere is 10 minutes or more. The silicon surface passivation method according to claim 1 or 2 , wherein the pressure of the atmosphere is in the range of 8 to 12 Torr. The silicon surface passivation method according to claim 1, wherein the ultraviolet rays are vacuum ultraviolet rays. The silicon surface passivation method according to claim 1, wherein the light source is a xenon excimer lamp. Having a silicon surface passivation method according to any one of claims 1 to 5, surface passivated process for the preparation of silicon. Obtaining a surface-passivated silicon using the silicon surface passivation method according to claim 1;
A step of obtaining a solar cell using the surface-passivated silicon.   The silicon surface passivation method according to claim 1, wherein the silicon is for a solar cell.