JP6048920B2 - Light scattering film, method for producing the same, and solar cell - Google Patents

Description

  The present invention relates to a light scattering film used for solar cells and the like and used for scattering light and a method for manufacturing the same. Moreover, it is related with the solar cell using this.

  Various types of solar cells are known, and many thin-film solar cells have been developed especially for cost reduction. The solar cell includes an absorption layer that absorbs light (visible light) and converts the light into electric energy. An electrode is formed in order to extract electric power from the absorption layer, but the presence of this electrode may reduce the amount of light incident on the absorption layer. In order to suppress this, a low-resistance transparent electrode that has a high light transmittance and can extract electric energy with low loss is used.

  In particular, in a thin-film solar cell, it is necessary to form a thin absorption layer, so that it is difficult to sufficiently absorb light in the absorption layer. For this reason, the ratio of absorption is increased by forming a structure (texture structure) having many fine irregularities on the surface of a transparent electrode or the like to scatter light and substantially increasing the optical path length of light in the absorption layer. Configuration is being used. As an amount indicating the efficiency of light scattering, there is a haze ratio defined as diffuse transmittance / total light transmittance. Here, the total light transmittance indicates a ratio of all the light beams transmitted with respect to the incident light, and the diffuse transmittance is a ratio at which the directly incident light beam is transmitted in a changed direction (diffused). For the above purpose, the layer having a texture structure is required to have a high total light transmittance and a high haze ratio.

  Various techniques have been proposed as a configuration for obtaining a high haze ratio. For example, Patent Documents 1 and 2 describe a technique of forming a scattering layer in which particles serving as a scatterer are dispersed in a transparent resin material in a laminated structure constituting a solar cell. Patent Document 3 describes a configuration in which a texture structure is provided on a reflective electrode provided on the side facing a transparent electrode.

  Moreover, it is requested | required that the material which comprises a solar cell can manufacture a large area thing cheaply. In this respect, zinc oxide (ZnO) is promising. Zinc oxide is a preferable material in that it does not contain elements harmful to the human body. Although ZnO can be used as a semiconductor, it can be used as a transparent electrode particularly when high concentration doping is performed. Patent Document 4 describes a technique for forming a texture structure on the surface of a ZnO layer by repeatedly interrupting and resuming growth when the ZnO layer is formed by a CVD method.

  In addition to this, it is possible to form irregularities on the surface of the glass substrate before the transparent electrode is formed by machining, or by pressing the resin substrate before the transparent electrode is formed. Techniques for forming are known. Even in such a case, a texture structure can be formed on the surface of the transparent electrode.

  By using such a light scattering film, even in a thin solar cell, light can be sufficiently absorbed by the absorption layer, and high photoelectric conversion efficiency can be obtained.

JP 2009-16553 A JP 2011-40525 A JP 2009-194289 A JP 2011-129288 A

  However, in the configurations of Patent Documents 1 and 2, a light scattering layer is newly inserted into the laminated structure constituting the solar cell. At this time, it is substantially difficult to increase the transmittance of the light scattering layer, particularly the total light transmittance. That is, light is absorbed also in the light scattering layer, and thus the photoelectric conversion efficiency is lowered. For this reason, it has been difficult to obtain sufficient characteristics in view of the original purpose of the light scattering film which causes only scattering without causing absorption.

  On the other hand, in the configuration described in Patent Document 3, only the configuration of the reflective electrode is improved. Usually, in a solar cell, one electrode is a reflective electrode and the other electrode is a transparent electrode. At this time, although it is preferable to provide the texture structure with both electrodes, in the configuration described in Patent Document 3, the texture structure is formed only on the reflective electrode. For this reason, it has been difficult to obtain high photoelectric conversion efficiency even using this technique.

  In the technique described in Patent Document 4, it is possible to form a texture structure on the transparent electrode. For this reason, for example, if this technique is combined with the technique described in Patent Document 3, high scattering efficiency can be obtained on both electrode sides. However, in the technique described in Patent Document 4, since the substantial film formation rate of the transparent electrode (ZnO) is slow, the mass productivity is lowered, and the process becomes complicated. Moreover, since the electrical resistance of the transparent electrode provided with such irregularities is usually high, the loss when taking out the output through the transparent electrode is increased. In addition, when a semiconductor layer (for example, amorphous Si) serving as an absorption layer is formed on the transparent electrode, it is difficult to obtain a semiconductor layer with good and uniform characteristics due to the unevenness of the surface of the transparent electrode. there were.

  Also, when forming irregularities on the surface of the substrate on which the transparent electrode is formed, it is still difficult to obtain desired electrical characteristics and optical characteristics of the transparent electrode, and there is a problem in reproducibility. The characteristics of the semiconductor layer formed thereon are the same as described above.

  As described above, it has been difficult to obtain a light scattering film having an excellent light scattering characteristic and a sufficient light scattering characteristic by a simple manufacturing method.

  The present invention has been made in view of such problems, and an object thereof is to provide an invention that solves the above problems.

In order to solve the above problems, the present invention has the following configurations.
The light scattering film of the present invention is a light scattering film that scatters incident light, and includes a substrate, silica fine particles having an average particle diameter in a range of 300 nm to 5 μm, and silica as a main component, and an average particle diameter of 50 to And a fine particle layer formed on the substrate by binding a plurality of types of fine particles containing at least 500 nm ZnO fine particles through a binder, and a haze ratio of 29.94% to 68.97%. It is the range of these.
In the light scattering film of the present invention, the binder is SOG (Spin On Glass).
In the light scattering film of the present invention, the substrate is a glass substrate.
The light scattering film of the present invention is characterized by comprising a transparent conductive film mainly formed of zinc oxide (ZnO) formed on the fine particle layer.
Method for producing a light-scattering film of the present invention is a manufacturing method of the light scattering layer, and the silica fine particles, wherein the ZnO particles, a coating solution but at least a plurality of types of fine particles contained is mixed with the binder It is characterized by comprising a fine particle layer forming step of forming a fine particle layer on the substrate by baking after coating on the substrate.
The method for producing a light-scattering film of the present invention is characterized in that the ZnO fine particles are produced by granulating a zinc material from a state evaporated by arc discharge in a vacuum atmosphere.
In the method for producing a light scattering film of the present invention, the binder contains SOG (Spin On Glass).
The method for producing a light scattering film of the present invention is characterized in that, in the fine particle layer forming step, the firing temperature is 300 ° C. or lower.
In the method for producing a light scattering film of the present invention, the substrate is a glass substrate.
The manufacturing method of the light-scattering film | membrane of this invention comprises the electrically conductive film formation process of forming the transparent conductive film which has a zinc oxide (ZnO) as a main component on the said fine particle layer.
The solar cell of the present invention is characterized in that the light scattering film is used.

  Since the present invention is configured as described above, an element having good characteristics can be formed thereon, and a light scattering film having sufficient light scattering characteristics can be obtained by a simple manufacturing method. .

It is sectional drawing of the light-scattering film | membrane used as embodiment of this invention. It is process sectional drawing which shows the manufacturing method of the light-scattering film | membrane used as embodiment of this invention. It is an example of the particle size distribution of the ZnO fine particle used in the light-scattering film | membrane used as embodiment of this invention. It is a SEM photograph of the surface of the fine particle layer when only ZnO fine particles are used (a) when only silica particles are used (b). It is a measured value of the total light transmittance in an Example and a comparative example. It is the measured value of the haze rate in an Example and a comparative example.

  The light scattering film of the present invention is a light scattering film using a fine particle layer formed by sintering a plurality of types of fine particles, at least one of which includes silica fine particles. This light scattering film has a configuration in which fine particles are dispersed and sintered in a binder on a substrate. Hereinafter, the light-scattering film | membrane used as embodiment of this invention is demonstrated.

  FIG. 1 is a cross-sectional view showing the configuration of the light scattering film 10. The light scattering film 10 is configured by laminating a fine particle layer 12 and a transparent conductive film 13 on a substrate 11.

  As the substrate 11, for example, a glass substrate can be used. In this case, light (visible light) can be transmitted through the substrate 11 from the lower side in FIG.

  The fine particle layer 12 is a layer formed by dispersing and sintering a plurality of types of fine particles (first fine particles 121 and second fine particles 122) on the substrate 11. Here, the plurality of types correspond to the case where the main components are different or the manufacturing method thereof is different. However, one of them includes silica fine particles (first fine particles 121) whose average particle diameter is in the range of 100 nm to 5 μm and whose main component is silica. As other components mixed with this, ZnO fine particles (second fine particles 122) having an average particle diameter of 50 to 500 nm are preferable. Moreover, these can be sintered using the binder 123, and it is preferable to use SOG (Spin On Glass) as the binder 123 in this case.

As the transparent conductive film 13, a material containing ZnO as a main component is used. In this case, for example, a Ga-doped ZnO film can be used. Here, ZnO is doped with a large amount (for example, 5%) of Ga to provide conductivity, and has high transmittance for visible light. As impurities, a group III element other than Ga (for example, Al) or a group VII element (halogen: Cl or the like) may be used. Further, a Mg _x Zn _1-x O mixed crystal thin film (where X = 0 to 0.3) containing these impurities can also be used. In this case, since the absorption edge of the transparent conductive film is shifted to the short wavelength side due to the addition of Mg, light absorption can be further reduced. The transparent conductive film 13 can be used as a transparent electrode when the light scattering film 10 is used in a solar cell, for example. That is, the output of the solar cell can be obtained through the transparent conductive film 13.

  In this configuration, since the texture structure is formed by irregularities corresponding to the surface of the fine particles in the fine particle layer 12, high diffuse transmittance can be obtained. Moreover, since the fine particles constituting the fine particle layer 12 are transparent to visible light, the total light transmittance can be kept high. Further, the light scattering film 10 can be manufactured particularly easily by the manufacturing method described below.

  2A to 2D are process sectional views showing this manufacturing method. First, as shown in FIG. 2A, a flat substrate 11 (for example, a glass substrate) is prepared.

  Next, as shown in FIG. 2B, the first fine particles 121 and the second fine particles 122 are dispersed on the substrate 11. At this time, if SOG is used as the binder 123, the space between the fine particles can be filled and planarized. As the first fine particles 121, silica fine particles can be used. Silica fine particles are fine particles composed of silica having an average particle diameter of about 100 nm to 5 μm, and are produced by a generally known production method.

  As the second fine particles 122, ZnO fine particles having an average particle diameter of 50 to 500 nm are particularly preferably used. The manufacturing method is described in, for example, JP-A-2005-60145. The fine particles can be produced by a gas evaporation method. Here, as shown in FIG. 1, a target made of zinc (Zn) is installed in the chamber. Zn is evaporated from the target surface by generating an arc discharge between the electrode placed in a vacuum in the vicinity of the target and the target in a reduced pressure atmosphere containing oxygen in the chamber. The evaporated Zn is oxidized by oxygen in the atmosphere to become ZnO, and attaches as fine particles to the inner wall of the chamber, which becomes ZnO fine particles. As a target raw material, a zinc ingot having a low concentration, for example, 4N (purity 99.99%) can be used. Even when such an inexpensive ingot with low purity is used, high-quality ZnO crystals can be obtained in the ZnO fine particles.

Specifically, as the atmosphere in the chamber, for example, an oxygen gas and nitrogen gas having a molar ratio of 4: 1 similar to air can be used. The gas atmosphere is depressurized to about 20 × 10 ³ Pa where arc discharge is likely to occur. Further, as described in JP-A-2005-60145, the crystallinity of ZnO in the fine particles is high. For this reason, the light transmittance of the ZnO fine particles can be increased.

  At this time, the particle diameter of the ZnO fine particles can be controlled by the gas component of the atmosphere, the pressure, the current value of the arc discharge, and the like. Alternatively, the particle size can be reduced by further pulverizing using a ball mill or the like. The unevenness of the texture structure can be set by this particle size. An example of the particle size distribution (histogram) of the ZnO fine particles obtained by the above method is shown in FIG.

  In order to disperse the first fine particles 121 and the second fine particles on the substrate 11, it is preferable to apply a coating liquid in which these and the binder 123 are mixed in a solvent onto the substrate 11. In this case, first, the first fine particles 121 and the second fine particles are mixed with a binder 123 (SOG liquid) at a predetermined mixing ratio, and further mixed in a solvent (for example, an organic solvent such as alcohol) using a ball mill or the like. Liquid (coating solution) is produced. This coating solution is applied on the entire surface of the substrate 11 with a uniform film thickness using a method such as spin coating, dip coating, spraying, or the like. Alternatively, when a printing method, an inkjet method, or the like is used, it is possible to apply and pattern only in a desired region. The kind and viscosity of the solvent can be appropriately selected according to the method of application and the form of the binder after firing described later.

  Thereafter, as shown in FIG. 2C, by firing, the dispersed first fine particles 121 and second fine particles 122 are sintered to form the fine particle layer 12 (fine particle layer forming step). ). For example, when SOG is used as the binder 123, this baking can be performed at 200 to 300 ° C. At this time, ZnO in the form of fine particles, unlike bulk crystals, agglomerates and sinters even at such a low temperature. Further, by adjusting the concentration and viscosity of the binder 123, the voids between the fine particles can be filled and flattened.

  However, when SOG is used as the binder 123, the refractive indexes of SOG and silica are substantially equal. For this reason, the probability of causing reflection or refraction at the SOG / silica fine particle interface is low. Even in such a case, the second fine particles 122 (ZnO fine particles), the silica particles, and the SOG have significantly different refractive indexes, so that reflection or refraction occurs at the interface between them. At this time, in order to increase the scattering probability in the texture structure, it is preferable that the depth of the unevenness in the texture structure is larger than the wavelength of the scattered light. On the other hand, as described above, the average particle diameter of the ZnO fine particles is 50 to 500 nm, which is smaller than the wavelength of visible light. However, at this time, ZnO made into fine particles is aggregated and sintered even at a low temperature of 300 ° C. or lower, unlike bulk crystals. Thereby, the unevenness | corrugation comprised with ZnO in the fine particle layer 12 can be made larger than the wavelength of visible light, and it is possible to obtain a high diffuse transmittance. On the other hand, in the above configuration, the silica fine particles and SOG contribute to increasing the total light transmittance of the fine particle layer 12.

  When, for example, a resin substrate having low heat resistance is used as the substrate 11, this firing can be performed by locally increasing the surface temperature only by irradiating the surface with pulsed laser light. For example, when application is performed by spraying, application and baking can be performed simultaneously. In this case, it is possible to set the thickness of the fine particle layer 12 by repeating coating and baking a plurality of times.

  Further, a photocurable resin can be used as the binder 123. In this case, the fine particle layer 12 can be formed at a lower temperature.

  Next, as shown in FIG. 2D, a transparent conductive film 13 is formed on the fine particle layer 12 by, for example, a magnetron sputtering method (sputtering method) (conductive film forming step). In this case, a sputtering target made of the same material as that of the transparent conductive film 13 can be used. At this time, as described above, since the surface of the fine particle layer 12 can be flattened, the physical unevenness of the surface serving as the base of the transparent conductive film 13 can be reduced. In this case, since the film quality of the transparent conductive film 13 can be improved, the transparent conductive film 13 having high conductivity and high light transmittance can be obtained with good reproducibility. Furthermore, it is possible to improve the uniformity of an amorphous Si layer or the like serving as a light absorption layer in the solar cell, and it is easy to increase the area of the solar cell.

  On the other hand, since the optically functioning irregularities are secured by the second fine particles 122 (ZnO fine particles), the function of scattering light is maintained. For this reason, high light scattering efficiency is obtained.

  That is, the light scattering film 10 can form an element having good characteristics on the light scattering film 10 and has high light scattering characteristics. For this reason, the solar cell with high photoelectric conversion efficiency can be obtained using this.

  Furthermore, in the manufacturing method described above, the fine particle layer 12 can be formed by a simple method of coating and baking and a method that does not require a large-scale apparatus. That is, the light scattering film can be easily obtained using a simple manufacturing method.

(Example)
The results of actually manufacturing the light scattering film will be described below.

  4A is an SEM photograph of an example in which a fine particle layer using ZnO fine particles having an average particle diameter of 100 to 200 nm is formed on a glass substrate, and FIG. 4B is an image having an average particle diameter of 3 to 5 μm. It is a SEM photograph of the example which formed the fine particle layer using a silica particle on the glass substrate. Here, a photo-curable resin (trade name: OX-SQ (Toagosei Co., Ltd.)) is used as a binder, and fine particles are sintered but not flattened. In both cases, it is clear that a texture structure is formed. However, in both cases, aggregation of fine particles occurs. In particular, although the ZnO fine particles have a smaller particle size than the silica particles, the cohesion is strong, and therefore, in the SEM photograph (FIG. 4A), irregularities having the same size as the silica particles are observed. The aggregation of the ZnO fine particles is maintained in the following examples.

  Next, the total light transmittance and haze ratio of the light scattering film were actually used using SOG (trade name: Si-05S (manufactured by Kojundo Chemical Laboratory Co., Ltd.)) as a binder. The measurement was performed on the structure formed up to the film. At this time, irregularities on the surface of the transparent conductive film were also measured.

  Here, the silica fine particles used have an average particle diameter of 300 nm. Further, the used ZnO fine particles were produced by using a gas having an arc current of 50 A, a pressure of 150 Torr, and nitrogen: oxygen = 4: 1 in the method described in JP-A-2005-60145.

  As a transparent conductive film formed on the fine particle layer, ZnO doped with 5% Ga was formed to a thickness of 300 nm by a magnetron sputtering method at a deposition temperature of 300 ° C.

Table 1 shows reference examples, examples in which silica fine particles and ZnO fine particles are mixed, and comparative examples using only ZnO fine particles, the average total light transmittance at a wavelength of 400 nm to 700 nm, the haze ratio, and the sheet resistance of the transparent conductive film The results of measuring the unevenness (average roughness measured with a stylus type measuring instrument) are shown. Here, Reference Example 1 and Example 2 (and Comparative Examples 1 and 2) are applied by spin coating, Examples 3 and 4 are applied by spray application, and Example 5 is applied by dropping a coating solution. The composition of the coating liquid in the reference examples and the examples is the same. For example , the coating conditions are different between the reference examples 1 and 2 and the examples 3 and 4. Further, in Comparative Example 2, the concentration of ZnO fine particles in the coating solution is halved compared to Comparative Example 1. FIG. 5 shows the wavelength dependence of the total light transmittance, and FIG. 6 shows the wavelength dependence of the haze ratio.

  From this result, it can be seen that the same total light transmittance and haze ratio are obtained in the example and the comparative example, and good light scattering characteristics are obtained. However, from the results in Table 1, in Comparative Examples 1 and 2, the sheet resistance of the transparent conductive film is high. For this reason, when the light scattering film of the comparative example is used, although good light scattering characteristics can be obtained, it is difficult to form a solar cell or the like having good characteristics thereon. On the other hand, in all of the examples, the sheet resistance of the transparent conductive film is low, so that a solar cell or the like having good characteristics can be formed thereon.

  From the results of FIG. 5, in Comparative Examples 1 and 2, the total light transmittance is lower than that of the example. Further, from the results of FIG. 6, although the haze ratio of the comparative example is high, in Example 4 (spray coating) and Example 5 (dropping), the haze ratio is equal to or higher than that of the comparative example. .

  The reason why the total light transmittance is low when only ZnO fine particles are used is that ZnO having a hexagonal crystal type is likely to have a hexagonal columnar shape, and the shape when aggregated is a shape that hardly transmits light. This is thought to be due to the fact that In addition, the refractive index of silica is about 1.45, whereas the refractive index of ZnO is as large as about 2.0, which is one reason for the low total light transmittance. Further, when only silica fine particles are used, as described above, the diffuse transmittance is lowered because the refractive index of the SOG and the silica particles as the binder are equal.

  From the above, it was confirmed that particularly good results were obtained when both silica fine particles and ZnO fine particles were used.

  In addition to the ZnO fine particles, it is also possible to use a material that is transparent to visible light and has a refractive index significantly different from that of silica. Similarly, as the binder, a material that is transparent to visible light and can form the same structure as described above can be used.

  From the above results, for example, if a solar cell absorption layer (for example, a semiconductor layer in which a pn junction is formed) is formed on the transparent conductive layer in the above structure, and another electrode is further formed thereon, the solar cell Can be formed. In this case, a solar cell in which visible light is incident from the glass substrate side is obtained, and the light absorption efficiency in the absorption layer can be increased due to the light scattering layer. That is, high photoelectric conversion efficiency can be obtained.

  In addition to solar cells, it is obvious that they can be used in the same manner for elements that require the property of scattering without absorbing light. For example, this light scattering film can be used for a light emitting element, and in this case, high intensity divergent light can be obtained. In this case, the light scattering film having a small surface irregularity and high scattering efficiency can be used without using a transparent electrode (transparent conductive film) in the light scattering film. In this case, the substrate is not necessarily required to be transparent. That is, as long as the above-described fine particle layer is formed on the substrate, other points can be appropriately set according to the element in which the light scattering film is used.

10 Light Scattering Film 11 Substrate 12 Fine Particle Layer 13 Transparent Conductive Film 121 First Fine Particle (Silica Fine Particle)
122 Second fine particles (ZnO fine particles)
123 binder

Claims (11)

A light scattering film for scattering incident light,
A substrate,
A plurality of types of fine particles including at least a silica fine particle having an average particle size in a range of 300 nm to 5 μm and containing silica as a main component and a ZnO fine particle having an average particle size of 50 to 500 nm are bonded together via a binder. A fine particle layer formed on a substrate;
And a haze ratio in the range of 29.94% to 68.97% .   The light scattering film according to claim 1, wherein the binder is SOG (Spin On Glass).   The light scattering film according to claim 1, wherein the substrate is a glass substrate.   The light scattering film according to any one of claims 1 to 3, further comprising a transparent conductive film mainly composed of zinc oxide (ZnO) formed on the fine particle layer. . It is a manufacturing method of the light-scattering film of Claim 1 ,
It said silica fine particles, wherein the ZnO particles, but by at least a plurality of types of fine particles in the baking after coating the mixed coating solution onto a substrate together with the binder, fine particle layer to form a fine particle layer on a substrate The manufacturing method of the light-scattering film | membrane characterized by including a formation process.   6. The method for producing a light scattering film according to claim 5, wherein the ZnO fine particles are produced by making particles of zinc material from a state evaporated by arc discharge in a vacuum oxygen atmosphere.   The method for producing a light scattering film according to claim 5, wherein the binder contains SOG (Spin On Glass).   The method for producing a light scattering film according to claim 7, wherein, in the fine particle layer forming step, the firing temperature is 300 ° C. or lower.   The method for producing a light scattering film according to any one of claims 5 to 8, wherein the substrate is a glass substrate.   The conductive film formation process which forms the transparent conductive film which has a zinc oxide (ZnO) as a main component on the said fine particle layer is comprised, The any one of Claim 5 to 9 characterized by the above-mentioned. Manufacturing method of light emitting element.   A solar cell, wherein the light scattering film according to any one of claims 1 to 4 is used.