JP4467218B2 - Surface roughening method for solar cell substrates - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for roughening a solar cell substrate, and more particularly, to a method for roughening a solar cell substrate that is suitable when the total area of substrates to be processed at one time by dry etching is large.
[0002]
[Background Art and Problems to be Solved by the Invention]
The solar cell converts light energy such as sunlight incident on the surface into electric energy. Various attempts have been made to improve the conversion efficiency into electric energy. One of them is a technique for reducing the reflection of light incident on the surface of the substrate. By reducing the reflection of incident light, the conversion efficiency to electric energy can be increased.
[0003]
Major solar cells are classified into crystalline, amorphous, and compound types depending on the type of materials used. Of these, most of the crystalline silicon solar cells currently on the market are in the market. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. Single crystal silicon solar cells have the advantage that the substrate quality is good, and thus the efficiency is easy, while the substrate manufacturing cost is high. On the other hand, a polycrystalline silicon solar cell has the merit that it can be manufactured at a low cost although it has a weak point that it is difficult to achieve high efficiency due to poor substrate quality. In recent years, conversion efficiency of about 18% has been achieved at the research level due to the improvement of the quality of the polycrystalline silicon substrate and the advancement of cell technology.
[0004]
On the other hand, mass-produced polycrystalline silicon solar cells have been distributed in the market because of their low cost. However, in recent years, demand has increased further as environmental issues have been addressed, and low cost and higher conversion are required. Efficiency has been demanded.
[0005]
When a solar cell element is formed using a silicon substrate, if the substrate surface is etched with an alkaline aqueous solution such as sodium hydroxide, fine irregularities are formed on the surface, and reflection on the substrate surface can be reduced to some extent.
[0006]
When a single crystal silicon substrate having a (100) plane orientation is used, a pyramid structure called a texture structure can be uniformly formed on the substrate surface by such a method. Since it depends on the orientation, when a solar cell element is formed from a polycrystalline silicon substrate, there is a problem that the pyramid structure cannot be formed uniformly, and therefore the overall reflectance cannot be reduced effectively.
[0007]
In order to solve such problems, it has been proposed that when a solar cell element is formed of a polycrystalline silicon substrate, fine protrusions are formed on the substrate surface by a reactive ion etching method. (For example, see Japanese Patent Publication No. 60-27195, Japanese Patent Laid-Open No. 5-75152, Japanese Patent Laid-Open No. 9-102625). That is, fine protrusions are uniformly formed regardless of the plane orientation of the irregular crystal in the polycrystalline silicon, and the reflectance is more effectively reduced particularly in the solar cell element using the polycrystalline silicon. To do.
[0008]
Thin film polycrystalline silicon solar cells are expected to be the next-generation low-cost solar cells, compared to the current mainstream crystalline silicon solar cells. In this thin-film polycrystalline silicon solar cell, an electrode layer is formed on a substrate such as glass, and a silicon layer is sequentially laminated thereon to form an element. A polycrystalline silicon film having a small light absorption coefficient is used as an active layer. In such a thin film polycrystalline silicon solar cell, formation of a light confinement structure is extremely important in order to efficiently absorb incident light.
[0009]
As one means for obtaining this light confinement effect, an uneven structure is formed on the light incident surface or the back surface.
[0010]
For example, after forming a transparent layer made of zinc oxide or the like on a glass substrate, a method of forming irregularities on the transparent layer by immersing it in an acetic acid solution and applying an electric field treatment (for example, see JP-A-6-140649), A method of forming a transparent conductive film having a haze of 2 to 48% by applying a coating liquid in which conductive oxide ultrafine particles are dispersed in a dispersion medium on a glass substrate and curing the coating liquid (for example, JP-A-10-12059) Etc.) are known. However, all of these methods require the formation of a transparent layer, and thus problems remain in terms of cost and throughput.
[0011]
In addition, for example, a method of forming irregularities by forming an insulating fine particle thin film having an average particle diameter of 0.1 to 1.0 μm on a glass substrate (see Japanese Patent Application Laid-Open No. 11-274536), while applying ultrasonic vibration There is also a method of supplying a slurry containing loose abrasive grains and pressing a tool having a plurality of V grooves on the surface of the glass substrate to form irregularities (see JP-A-6-350114, etc.). It is difficult to obtain an uneven shape, which induces a situation in which the quality of the polycrystalline silicon film formed on the glass substrate is deteriorated.
[0012]
In addition, as a method of directly forming a concavo-convex structure on a glass substrate, a method of spraying abrasive grains having a count of # 2000 or more on the surface of the glass substrate (see JP-A-9-199745) is also known. Concavities and convexities to be formed have a large aspect ratio, and microcracks are generated in the glass substrate due to blast damage, and there is a concern that leakage may occur when an element is formed.
[0013]
In addition, as with crystalline silicon, it has been proposed that a reactive ion etching process is performed on a glass substrate to form irregularities (see, for example, Japanese Patent Application No. 2000-301419).
[0014]
However, the conditions for forming irregularities on the glass substrate are very subtle, as in the case of forming on the crystalline silicon substrate, and also vary depending on the structure of the apparatus. If the fine protrusions cannot be formed uniformly, the photoelectric conversion efficiency of the solar cell will decrease, and the value of each solar cell will be determined by its power generation efficiency, so in order to reduce its cost, the conversion efficiency of the solar cell Must be improved.
[0015]
Moreover, the reactive ion etching apparatus used in the reactive ion etching method is generally a parallel plate electrode type, and an RF voltage is applied to the electrode side on which the substrate is installed, The side wall is connected to ground. The inside of the container is evacuated by a vacuum pump, and after the evacuation is completed, an etching gas is introduced, and the substrate to be etched is etched while keeping the pressure constant. After the etching is completed, the inside of the container is returned to atmospheric pressure.
[0016]
Since such a procedure is followed, the reactive ion etching apparatus has a long waiting time for evacuation and air leakage. Reactive ion etching apparatuses are often used for precision small semiconductor elements such as LSIs, but when used for solar cells, the area of the solar cell itself is large, so the number of treatments per process is small and the cost is low. There was a problem that became high. Therefore, when a reactive ion etching apparatus is used for the manufacturing process of a solar cell, it is also an important point how to process with high tact.
[0017]
In order to achieve a high tact by the reactive ion etching method, the number of processed sheets per apparatus is important. In order to increase the number of processed sheets per apparatus, it is necessary to increase the area of the etching electrodes and trays of the apparatus. However, as the area increases, a problem of uniformity in the batch occurs. Specifically, there arises a problem that unevenness cannot be formed in an optimum shape at the same time both in the periphery and in the central portion of the etching batch.
[0018]
The roughening of the substrate surface is performed using a residue formed on the surface during etching as a mask. Using the residue as a mask, etching of the substrate proceeds by the action of both ions and radicals accelerated by the electric field. The substrate side has a negative potential because of a blocking capacitor provided between the RF electrode and the RF power source. For this reason, the greater the effect of positive ions on the etching, the easier the etching in the direction perpendicular to the substrate proceeds, and the concavo-convex aspect ratio increases. However, as the etching area becomes larger, the ratio of the positive ion effect to the whole gas is not uniform in the batch plane, and the aspect ratio of the uneven shape in the peripheral part becomes smaller than the central part, and the ratio in the batch becomes smaller. There was a problem that sufficient unevenness could not be formed uniformly.
[0019]
The present invention has been made in view of the problems of the prior art associated with the increase in the area of such an apparatus, and unevenness on the surface of a substrate such as silicon used for solar cells is uniformly formed in a batch of an etching processing apparatus. Thus, an object of the present invention is to provide a method that enables tact-up of the device (improves yield).
[0020]
[Means for Solving the Problems]
The surface roughening method for a solar cell substrate of the present invention is the method for roughening a solar cell substrate in which the surfaces of the plurality of solar cell substrates are roughened by a dry etching method. Is made of silicon, and etches the surfaces of the plurality of solar cell substrates with a gas containing fluorine-based gas or bromine-based gas, oxygen-based gas, and chlorine-based gas, Alone Form fine irregularities Have etching conditions that can Etching the surfaces of the plurality of solar cell substrates with a first step and a gas containing a fluorine-based gas or bromine-based gas, an oxygen-based gas, and a chlorine-based gas; Alone Forming fine irregularities having an aspect ratio larger than the irregularities formed in the first step; Have etching conditions that can A second step, wherein the second step is performed after the first step.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings by taking a bulk silicon solar cell as an example. FIG. 1 is a view showing the structure of a bulk silicon solar cell formed by using the method for roughening a solar cell substrate according to the present invention. In FIG. 1, 1 is a silicon substrate, 1a is a surface uneven structure (rough surface portion), 1b is a light-receiving surface side impurity diffusion layer, 1c is a back surface side impurity diffusion layer (BSF), 1d is a surface antireflection film, 1e is The front electrode and 1f are back electrodes.
[0022]
The silicon substrate 1 is a monocrystalline or polycrystalline silicon substrate. The substrate 1 may be either p-type or n-type. In the case of monocrystalline silicon, it is formed by a pulling method or the like, and in the case of polycrystalline silicon, it is formed by a casting method or the like. Polycrystalline silicon can be mass-produced and is extremely advantageous over single-crystal silicon in terms of manufacturing cost. A silicon block formed by a pulling method or a casting method is cut into a size of about 10 cm × 10 cm or 15 cm × 15 cm to form an ingot, and sliced to a thickness of about 300 μm to form a silicon substrate.
[0023]
On the surface side of the silicon substrate 1, fine irregularities 1a are formed in order to effectively capture incident light without reflecting it. This is because a gas is introduced into a vacuumed chamber, held at a constant pressure, and plasma is generated by applying RF power to an electrode provided in the chamber. The substrate surface is etched by the action of radicals or the like.
[0024]
This method, called the reactive ion etching (RIE) method, is generally performed using an apparatus as shown in FIGS. 2 and 3, 2a is a mass flow controller, 2b is a silicon substrate, 2c is an RF electrode, 2d is a pressure regulator, 2e is a vacuum pump, and 2f is an RF power source. Gas is introduced into the apparatus from the mass flow controller 2a portion, and plasma is generated by the RF electrode 2c to excite and activate ions and radicals to act on the surface of the silicon substrate 2b installed on the RF electrode 2c. Etch. In the apparatus shown in FIG. 2, the RF electrode 2c is installed in the apparatus to etch the surface of one silicon substrate 2b. In the apparatus shown in FIG. 3, a plurality of RF electrodes 2c are installed on the outer wall of the apparatus. The surface of the silicon substrate 2b is simultaneously etched.
[0025]
Of the generated active species, a method that increases the effect of ions on etching is generally called a reactive ion etching method. Plasma etching is a similar method, but the principle of plasma generation is basically the same, and the distribution of the active species acting on the substrate 2b is changed to a different distribution depending on the chamber structure, electrode structure, generation frequency, etc. It ’s just that. Therefore, the present invention is effective not only for the reactive ion etching method but also for the plasma etching method in general.
[0026]
For example, Cl ₂ 0.1 slm, O ₂ 0.6 slm, SF ₆ Etching for about 5 minutes at a reaction pressure of 7 Pa and an RF power of 5 kW for generating plasma. As a result, a concavo-convex structure is formed on the surface of the silicon substrate 2b to form a rough surface. During the etching, silicon is etched and basically vaporized, but a part of the silicon is not vaporized and molecules are adsorbed and remain as a residue on the surface of the substrate 2b.
[0027]
Further, if the gas conditions, reaction pressure, RF power, etc. are set to such conditions that a residue whose main component is silicon remains on the surface of the silicon substrate 2b after etching under the concavo-convex forming gas conditions, the concavo-convex formation can be reliably performed. . However, the aspect ratio of the irregularities needs to be optimized depending on conditions. On the contrary, it is impossible to form unevenness under the condition that no residue remains on the surface after etching for forming unevenness. Concavity and convexity formation is performed by etching using this residue as a mask.
[0028]
A solar cell requires a large area to obtain a large output, and the cost is very important. Therefore, when forming a concavo-convex structure on the surface of the solar cell, it is necessary to etch a large number of substrates 2b at a time in order to carry out at low cost. Therefore, the electrode 2c for installing the substrate 2b needs a large area. However, when etching was actually performed in a large area, it was found that there was a problem that the uneven shape on the surface of the silicon substrate 2b placed on the electrode 2c (on the substrate tray) was different depending on the location.
[0029]
For example, when etching is performed at the above gas ratio, the surface of the substrate 1b installed in the vicinity of the center of the electrode 2c has an aspect ratio (height / width of each unevenness) = 1, but in the vicinity of the periphery of the electrode 2c. The aspect ratio of the surface of the substrate 2b placed on the substrate 2b was about 0.5 at the central portion of the substrate 2b, and the aspect ratio decreased to a level that could not be measured in the vicinity of the periphery of the electrode 2c in the substrate 2b. This difference in aspect ratio can be alleviated to some extent by changing gas conditions, RF power, pressure, etc., but as a result of examining various conditions, the final solar cell characteristics are almost the same. It turned out that it was difficult to form uniform unevenness.
[0030]
Further, this uniformity is a problem because the installation area of the substrate to be etched 2b is 2500 cm. ² It was found to be more prominent when larger than.
[0031]
Therefore, as a result of further examination of the etching conditions, the aspect ratio of the surface of the central substrate 2b on the substrate tray on the electrode 2c is equal to the aspect ratio of the surface of the peripheral substrate 2b regardless of the unevenness formation conditions. It turned out to be larger than the ratio. This schematic diagram is shown in FIG. In FIG. 4, 4a is a central portion having a large aspect ratio, 4b is a peripheral portion having a small aspect ratio, and 4c is a substrate tray. Further, when the etching amount was examined, it was found that the etching amount of the peripheral substrate was larger than the etching amount of the central substrate.
[0032]
In order to solve this problem, the present invention provides a surface structure of a solar cell in which uniform characteristics are obtained over the entire surface by performing etching by combining a plurality of etching conditions.
[0033]
In the present invention, at least two or more etching conditions capable of forming concavo-convex independently are continuously performed, and the etching performed later is performed on the aspect ratio of the concavo-convex shape obtained under the first etching condition of the two etching conditions. These problems are solved by performing etching under conditions such that the aspect ratio of the concavo-convex shape obtained under the conditions becomes large when compared by performing independent etching on the substrate surface portion on the substrate tray on the same electrode.
[0034]
For example, the total surface area of the substrate to be etched is about 10,000 cm ² In a reactive ion etcher to etch the substrate surface on the substrate tray on the electrode ₂ 0.1 slm, O ₂ 0.6 slm, SF ₆ Is etched for 7 minutes at a pressure of 7 Pa and an RF power of 5 kW (etching condition A). As a result, irregularities having an aspect ratio of about 0.6 are formed near the center 4a of the entire substrate to be etched. At this time, slight irregularities having an aspect ratio of about 0.1 are formed in the vicinity 4b. On the other hand, the etching amount is large in the peripheral part 4b, and the size of each uneven base is about 0.5 μm in the central part 4a, whereas the peripheral part is about 1.0 μm.
[0035]
Next, Cl ₂ 0.1 slm, O ₂ 0.6 slm, SF ₆ Is performed at a pressure of 15 Pa and an RF power of 3 kW for 3 minutes (etching condition B). When this etching is performed alone, the central portion 4a has an aspect ratio of about 1.5 and the peripheral portion 4b has a ratio of about 1.1. However, some unevenness has already been formed in the etching condition A, and the final uneven shape is the sum of these two etching conditions. When viewed by location, in the central portion 4a, an etching with an aspect ratio of 0.6 is performed for 7 minutes under the etching condition A, and an etching with an aspect ratio of 1.5 is performed for 3 minutes under the etching condition B. An etched shape is obtained. In the peripheral portion 4b, etching with an aspect ratio of 0.1 is performed for 7 minutes under the etching condition A, and etching with an aspect ratio of 1.1 is performed for 3 minutes under the etching condition B, whereby an etching shape with an aspect ratio of 1.0 is finally obtained. It is done. As a result, moderate irregularities are formed on the entire surface of the substrate to be etched.
[0036]
In etching condition A, SF is compared with etching condition B. ₆ The flow rate ratio is large. SF ₆ Since the etching of is isotropic and the etching rate is very large, SF ₆ As the flow rate ratio increases, the aspect ratio decreases. That is, the surface shape under the etching condition A performed first has a smaller aspect ratio than the surface shape under the subsequent etching condition B. Thus, it is an important factor that the etching conditions performed after the first etching are conditions with a large aspect ratio. The gas for this isotropic etching is SF ₆ However, other gases can be used. Also, other gases can be used for other etching conditions regardless of this example. Thereby, moderate unevenness | corrugation can be formed in the peripheral part 4b.
[0037]
The uneven structure formed by this method has a feature that the unevenness itself is larger on the substrate surface near the peripheral portion 4b than on the substrate surface near the central portion 4a on the tray on the electrode.
[0038]
An example of the concavo-convex shape of the substrate surface etched according to the present invention is shown in FIG. In FIG. 5, (a) is a center part, (b) is a peripheral part. However, both reflectivity is low, and the surface structure produced by this method can improve the characteristics of the solar cell in the same manner.
[0039]
Moreover, it is desirable that the reaction pressure is higher in the etching condition B than in the etching condition A. The formation of unevenness on the substrate surface is performed for improving characteristics, and it is desirable that unevenness can be formed uniformly. However, when the polycrystalline silicon substrate is etched, the etching is performed under the influence of the plane orientation, although there is a difference in degree. If the influence of this plane orientation is large, the grain surface of each polycrystalline grain will have different irregularities, and depending on the grain, a shape with large surface reflection will also be formed, increasing the overall reflectivity and improving the characteristics. The contribution to becomes smaller. Moreover, although the surface reflection is small depending on the grain, the aspect ratio of the unevenness may be too large to contribute to the improvement of characteristics. Therefore, etching that is not affected by the plane orientation is desired. In the etching using plasma, when the reaction pressure is increased, the self-bias potential is reduced, so that the effect of ion incidence is reduced and the influence of the plane orientation tends to appear.
[0040]
In the present invention, when performing the concavo-convex formation using the etching conditions capable of forming the concavo-convex independently, the first etching is performed deeply using the etching conditions having a small aspect ratio. Unevenness with little influence of orientation can be formed. However, at this time, the shape of the unevenness is in a state where the aspect ratio is large at the central portion and small at the peripheral portion. In the subsequent etching, etching having an average aspect ratio larger than that of the first etching is performed on the whole, but in the present invention, this etching amount needs to be smaller than that of the first etching. When the RF power is large or the etching amount is large when the etching time is long, a relatively good shape (aspect ratio) can be obtained at the edge portion, but the aspect ratio becomes too large at the center portion, and the characteristics The shape can not contribute to.
[0041]
Therefore, in subsequent etching, it is necessary to stop the formation of irregularities when a uniform aspect ratio is obtained within the tray surface. More specifically, there is a method for realizing this, for example, reducing the RF power by etching after the beginning, or shortening the etching time by etching after the beginning. At this time, since the amount of etching in the subsequent etching is smaller than that in the first etching, even if the etching is performed under the condition that the influence of the plane orientation is large, the uniformity of the final entire surface is not greatly affected.
[0042]
Under such conditions, even if the reaction pressure in the later etching conditions is increased and the influence of the subsequent conditions alone on the surface orientation of the etching is increased, the influence on the surface orientation of the etching throughout is small. can do. Under the etching conditions that can form unevenness independently, if the self-bias potential during etching is large, the incident effect of the generated positive ions on the substrate tends to be larger in the central portion than in the peripheral portion of the batch. Formation is easy to proceed in the center. In the subsequent etching, it is desirable to suppress the formation of irregularities in the central portion as much as possible and promote the peripheral portion. As this method, in the present invention, this can be solved by increasing the reaction pressure under the subsequent etching conditions.
[0043]
When the reaction pressure under the etching conditions is increased, the self-bias potential can be reduced, so that unevenness formation in the batch by single etching can be performed more uniformly at the center and the peripheral portion. As a result, it is possible to suppress the formation of unevenness at the central part in the subsequent etching, and as a result, it is possible to form unevenness on the entire surface within the batch.
[0044]
Further, in the above example, the formation of irregularities by the etching conditions of two steps is taken as an example, but this etching step can be increased any number. For example, etching is performed in 10 steps, and the content is SF of the mixed gas. ₆ The same effect can be obtained by introducing a gas flow rate of 0.7 slm for the first 30 seconds and then gradually reducing the flow rate to 0.6 slm for the next 30 seconds and 0.5 slm for the next 30 seconds. be able to. Furthermore, this etching step is increased indefinitely, and SF ₆ The same effect can be obtained by performing the process under completely continuous conditions such as gradually changing the flow rate of the gas. Further, in the present invention, such a change in multi-stage or continuous etching conditions is caused by SF. ₆ Not only the flow rate but also any gas conditions can be applied. Further, if necessary, etching conditions such as reaction pressure and RF power can be changed at the same time.
[0045]
The fine irregularities 1a shown in FIG. 1 have a conical shape or a continuous shape, and the size can be changed by controlling the gas concentration or etching time by the RIE method. The width and height of the fine irregularities 1a are each set to 2 μm or less. In order to form the fine irregularities 1a uniformly and accurately over the entire necessary portion of the silicon substrate 1, a thickness of 1 μm or less is preferable. The aspect ratio of the fine irregularities 1a (height / width of the irregularities 1a) is desirably 2 or less. When this aspect ratio is 2 or more, fine irregularities 1a are damaged in the manufacturing process, and when a solar battery cell is formed, a leak current increases and good output characteristics cannot be obtained.
[0046]
Note that the definition of the aspect ratio used in the description of the present invention is a value obtained by the ratio of the height and the base of a triangle formed by approximately linear portions on both sides when the concavo-convex cross section is viewed as shown in FIG. 5c. Moreover, in the part where the cone of unevenness | corrugation continues, it is set as the value calculated | required similarly with the uneven | corrugated shape at the time of taking a cross section perpendicular to the continuous direction.
[0047]
After the unevenness 1a is formed by the above dry etching method, the etching residue remaining on the surface of the silicon substrate 1 is removed. Thereby, the characteristic of the solar cell produced can be improved. As a method for removing the etching residue, for example, an unevenness is formed by a dry etching method and the substrate 1a is taken out, and then ultrasonic waves are applied in a water tank. As the types of devices that apply ultrasonic waves, the frequency of main commercially available ultrasonic cleaning devices is several tens of kHz to several hundreds of kHz, and the vibrator to be applied is of various types such as material, shape, and output. However, this type of device can be selected depending on the ease of removal of residue on the surface. The ease of residue removal varies depending on the shape and size of the unevenness, the remaining amount of residue, the thickness of the substrate, etc., and also varies depending on the frequency of the ultrasonic wave, but even under conditions where residue removal is relatively difficult The residue can be removed by extending the application time.
[0048]
On the surface side of the silicon substrate 1, a layer 1b in which a reverse conductivity type semiconductor impurity is diffused is formed. The layer 1b in which the reverse conductivity type semiconductor impurity is diffused is provided to form a semiconductor junction in the silicon substrate 1. For example, when n-type impurity is diffused, POCl _Three Vapor phase diffusion method using P ₂ O _Five Coating diffusion method using P, and P ⁺ It is formed by an ion implantation method in which ions are directly introduced into the substrate by an electric field. The layer 1b containing the reverse conductivity type semiconductor impurity is formed to a depth of about 0.3 to 0.5 μm.
[0049]
An antireflection film 1 d is formed on the surface side of the silicon substrate 1. The antireflection film 1 d is provided to prevent light from being reflected from the surface of the silicon substrate 1 and to effectively take light into the silicon substrate 1. This antireflection film 1d is made of a material having a refractive index of about 2 in consideration of a difference in refractive index with respect to the silicon substrate 1, and a silicon nitride film or silicon oxide (SiO2) having a thickness of about 500 to 2000 mm. ₂ ) Consists of a film or the like.
[0050]
On the back side of the silicon substrate 1, it is desirable to form a layer 1c in which one conductivity type semiconductor impurity is diffused at a high concentration. The layer 1c in which this one-conductivity-type semiconductor impurity is diffused at a high concentration forms an internal electric field on the back surface side of the silicon substrate 1 in order to prevent a decrease in efficiency due to carrier recombination near the back surface of the silicon substrate 1. is there. In other words, as a result of the carriers generated near the back surface of the silicon substrate 1 being accelerated by this electric field, the electric power is effectively extracted, and particularly the long wavelength photosensitivity increases and the deterioration of the solar cell characteristics at high temperatures is reduced. it can. Thus, the sheet resistance on the back surface side of the silicon substrate 1 on which the layer 1c in which one conductivity type semiconductor impurity is diffused at a high concentration is formed is about 15Ω / □.
[0051]
A front surface electrode 1 e and a back surface electrode 1 f are formed on the front surface side and the back surface side of the silicon substrate 1. The front electrode 1e and the back electrode 1f are screen-printed and fired with an Ag paste mainly composed of Ag powder, binder, frit, etc., and a solder layer is formed thereon. The surface electrode 1e is composed of, for example, a large number of finger electrodes (not shown) formed with a width of about 200 μm and a pitch of about 3 mm, and two bus bar electrodes (1e) that connect the large number of finger electrodes to each other. The The back electrode 1f is composed of, for example, a large number of finger electrodes (not shown) formed with a width of about 300 μm and a pitch of about 5 mm, and two bus bar electrodes (1f) that connect the large number of finger electrodes to each other. The
[0052]
Next, another embodiment of the present invention will be described by taking a thin film polycrystalline silicon solar cell as an example. FIG. 6 is a view showing the structure of a substrate type thin film polycrystalline silicon solar cell formed by using the method for roughening a solar cell substrate according to the present invention.
[0053]
Although the case where the light incident side silicon layer is n-type will be described here, it can also be used when the light incident side is p-type. In that case, the conductivity type in the sentence may be read in reverse. Further, the present invention is not limited to a substrate type thin film polycrystalline silicon solar cell, but can also be applied to a super straight type solar cell, and is not limited to a thin film polycrystalline silicon solar cell. The present invention can be widely applied to solar cells and thin film solar cells using compound semiconductors. Furthermore, the present invention can also be applied to a multilayer (tandem) type thin film solar cell.
[0054]
In FIG. 6, 6a indicates a glass substrate, 6b indicates a back electrode, 6c indicates a semiconductor layer having a semiconductor junction in which the photoactive layer portion is formed of crystalline Si, 6d indicates a transparent conductive film, and 6e indicates a surface collecting electrode.
[0055]
A fine concavo-convex structure 6a is formed on the surface of the glass substrate 6a so that incident light is effectively scattered and reflected on the surface of a back electrode 6b described later. This is formed by using the reactive ion etching (RIE) method described in the embodiment of the bulk silicon solar cell.
[0056]
For example, Cl ₂ 0.2 slm, SF ₆ Etching for 5 minutes at a reaction pressure of 5 Pa and an RF power of 5 kW for generating plasma. Thereby, a fine concavo-convex structure 6 f is formed on the surface of the glass substrate 6. During the etching, the glass is etched and basically vaporized, but a part of the glass is not vaporized and molecules are adsorbed and remain on the surface of the substrate 6 as a residue.
[0057]
In addition, if the gas conditions, reaction pressure, RF power, etc. are set so that residues remain on the surface of the glass substrate, the irregularities can be reliably formed. However, the aspect ratio of the irregularities needs to be optimized depending on conditions. On the contrary, it is impossible to form irregularities under any conditions under which no residue remains on the surface. Concavity and convexity formation is performed by etching using this residue as a mask.
[0058]
In order to realize low cost in a thin film solar cell, it is common to perform integration up to a large area substrate. For this reason, the etching apparatus is required to be able to form unevenness uniformly on a large area substrate. However, when etching was actually performed on a large-area substrate, it was found that there was a problem that the uneven shape on the surface of the glass substrate 6a was different depending on the location. For example, when etching is performed at the above gas ratio, the aspect ratio (height / width of each unevenness) near the center of the substrate is about 0.07, but the aspect ratio of the periphery of the substrate is measured from about 0.03. The aspect ratio has been reduced to an impossible level. This difference in aspect ratio can be alleviated to some extent by changing gas conditions, RF power, pressure, etc., but as a result of various conditions, the final solar cell characteristics are almost the same. It has been found that it is difficult to form a rough surface. In addition, this uniformity is a problem because the area of the substrate to be etched is 2500 cm. ² It was found to be more prominent when larger than.
[0059]
As a result of further examination of the etching conditions, it has been found that the aspect ratio near the center of the substrate is larger than the aspect ratio near the periphery of the substrate, regardless of the irregularity formation conditions.
[0060]
In order to solve this problem, the present invention provides a method for roughening a solar cell substrate, in which uniform characteristics are obtained by performing etching in combination with a plurality of etching conditions.
[0061]
In the present invention, at least two or more etching conditions capable of forming concavo-convex independently are continuously performed, and the etching performed later is performed on the aspect ratio of the concavo-convex shape obtained under the first etching condition of the two etching conditions. These problems are solved by performing the etching under such conditions that the aspect ratio of the concavo-convex shape obtained under the conditions becomes large when compared by performing single etching.
[0062]
For example, the total surface area of the substrate to be etched is about 10,000 cm ² In order to etch the substrate surface which is ₂ 0.1 slm, SF ₆ For 7 minutes at a pressure of 5 Pa and an RF power of 5 kW (etching condition C). As a result, irregularities having an aspect ratio of about 0.05 are formed near the center of the substrate to be etched. At this time, slight irregularities having an aspect ratio of about 0.01 are formed in the vicinity of the peripheral portion. Next, chlorine (Cl ₂ ) 0.3 slm, SF ₆ For 3 minutes at a pressure of 10 Pa and an RF power of 4 kW (etching condition D). When this etching is performed alone, the central portion has an aspect ratio of about 0.16 and the peripheral portion has about 0.11. However, some unevenness has already been formed under the etching condition C, and the final uneven shape is the sum of these two etching conditions. Looking at each location, etching with an aspect ratio of 0.05 is performed for 7 minutes under the etching condition C in the central portion, and etching with an aspect ratio of 0.16 is performed for 3 minutes under the etching condition D. A shape is obtained. In the peripheral portion, etching with an aspect ratio of 0.01 is performed for 7 minutes under the etching condition C, and etching with an aspect ratio of 0.11 is performed for 3 minutes under the etching condition D, whereby an etching shape with an aspect ratio of 0.07 is finally obtained. . As a result, moderate irregularities are formed on the entire surface of the substrate to be etched.
[0063]
In etching condition C, SF is compared with etching condition D. ₆ The flow rate ratio is large. SF ₆ Etching Is SF is very large, so SF ₆ As the flow rate ratio increases, the aspect ratio decreases. In other words, the surface shape under the etching condition C performed first has an aspect ratio smaller than the surface shape under the subsequent etching condition D. Thus, it is an important factor that the etching conditions performed after the first etching are conditions with a large aspect ratio. This No The gas used for etching is SF ₆ However, other gases can be used. Also, other gases can be used for other etching conditions regardless of this example. Thereby, moderate unevenness | corrugation can be formed in a peripheral part. All the surface structures produced by this method can similarly improve the characteristics of the solar cell.
[0066]
When the RF power is large or the etching amount is large when the etching time is long, a relatively good shape (aspect ratio) can be obtained at the edge portion, but the aspect ratio becomes too large at the center portion, and the characteristics The shape can not contribute to.
[0067]
Therefore, in subsequent etching, it is necessary to stop the formation of irregularities when a uniform aspect ratio is obtained within the tray surface. More specifically, there are methods for realizing this, such as making the RF power smaller by etching after the beginning or shortening the etching time by etching after the beginning. . each Under the etching conditions that can form unevenness independently, if the self-bias potential during etching is large, the incident effect of the generated positive ions on the substrate tends to be larger at the center than at the periphery of the batch. Formation is easy to proceed in the center. In the subsequent etching, it is desirable to suppress the formation of irregularities in the central portion as much as possible and promote the peripheral portion. As this method, in the present invention, this can be solved by increasing the reaction pressure under the subsequent etching conditions. When the reaction pressure under the etching conditions is increased, the self-bias potential can be reduced, so that unevenness formation in the batch by single etching can be performed more uniformly at the center and the peripheral portion. As a result, it is possible to suppress the formation of unevenness at the central part in the subsequent etching, and as a result, it is possible to form unevenness on the entire surface within the batch.
[0068]
Further, in the above example, the formation of irregularities by the etching conditions of two steps is taken as an example, but this etching step can be increased any number. For example, etching is performed in 10 steps, and the content is SF of the mixed gas. ₆ Gas and Cl ₂ Introduce 0.7slm and 0.1slm of gas flow for the first 30 seconds, stepwise 0.6slm and 0.2slm for the next 30 seconds, 0.5slm and 0.3slm for the next 30 seconds, etc. The same effect can be obtained by a method of reducing the number of times. Furthermore, this etching step is increased indefinitely, and SF ₆ Gas and Cl ₂ The same effect can be obtained by performing the process under completely continuous conditions such as gradually changing the gas flow rate. Further, in the present invention, such a change in multi-stage or continuous etching conditions is caused by SF. ₆ Gas and Cl ₂ Applicable to any gas conditions, not just gas. Further, if necessary, etching conditions such as reaction pressure and RF power can be changed at the same time.
[0069]
Furthermore, in order to optimize the uneven shape, it has been proposed to add a wet etching process after the dry etching process (for example, Japanese Patent Application No. 200). 1 -257609).
[0070]
The fine concavo-convex structure 6f has a quasi-elliptical shape as viewed from the surface, and an average inclination angle of the concavo-convex portion in an arbitrary cross section perpendicular to the glass substrate is 5 to 10 ° (aspect ratio is 0). An uneven shape of about .04 to 0.1) is formed.
[0071]
Here, the average inclination angle of the concavo-convex portion in an arbitrary cross section perpendicular to the glass substrate is a ridge line substrate that forms the concavo-convex portion in an arbitrary cross-section cut in a direction perpendicular to the glass substrate 6a. The average value (in the entire cross section) of the inclination angle with respect to the horizontal direction.
[0072]
At this time, when the average inclination angle of the unevenness is 5 ° or less, when applied to a thin film solar cell, the probability that the reflected light reflected on the back surface side of the solar cell is totally reflected on the light receiving surface side decreases. The effective optical path length in the active layer is not effectively increased. Conversely, when the average inclination angle of the irregularities is 10 ° or more, the growth of crystals on the irregular inclined surfaces facing each other is extremely hindered, and the grains with voids in the vicinity of the interface where each crystal grain collides A field is generated and V _oc In addition, a situation in which device characteristics centering on FF are deteriorated is induced.
[0073]
After the unevenness is formed by this dry etching method, the residue remaining on the substrate surface is removed. Thereby, the characteristic of a solar cell can be improved. As a method for removing the etching residue, an unevenness is formed by dry etching and the substrate is taken out, and then ultrasonic waves are applied in a water tank. As the types of devices that apply ultrasonic waves, the frequency of main commercially available ultrasonic cleaning devices is several tens of kHz to several hundreds of kHz, and the vibrator to be applied is of various types such as material, shape, and output. However, this type of device can be selected depending on the ease of removal of residue on the surface. The ease of residue removal varies depending on the shape and size of the unevenness, the remaining amount of residue, the thickness of the substrate, etc., and also varies depending on the frequency of the ultrasonic wave, but even under conditions where residue removal is relatively difficult It is possible to remove the residue by lengthening the application time.
[0074]
In addition, when a wet etching process is added, it is desirable to perform ultrasonic cleaning in pure water or the like at least before or after the wet etching process. In order to further improve the throughput, etching and residue removal may be performed simultaneously by applying ultrasonic vibration during the wet etching process. It becomes possible to easily form a roughened glass substrate as described above.
[0075]
Next, a metal film to be the back electrode layer 6b is formed. As the metal film material, it is desirable to use Al, Ag, etc., which are excellent in light reflection characteristics. As a film forming method, known techniques such as vapor deposition, sputtering, and ion plating can be used. The film thickness is 0.05 to 2 μm, and the surface of the back electrode layer 6b has a fine uneven structure reflecting the fine uneven structure 6f on the surface of the glass substrate 6a. If the film thickness of the back electrode layer 6b is 0.05 to 2 μm, the uneven shape on the surface of the back electrode layer 6b almost reflects the uneven structure 6f on the surface of the substrate 6a.
[0076]
If the film thickness of the back electrode layer 6b is 0.05 μm or less, characteristic deterioration due to an increase in the series resistance component of the element cannot be ignored, and if it is 2 μm or more, the uneven structure 6f on the surface of the glass substrate 6a becomes the back electrode layer. It becomes difficult to be effectively reflected on the surface of 6b, and at the same time, it is not realistic in terms of cost.
[0077]
Further, in order to increase the adhesive strength between the glass substrate 6a and the back electrode layer 6b, for example, a metal layer (not shown) such as Ti is formed with a thickness of 0.5 to 200 nm between the glass substrate 6a and the back electrode layer 6b. It is described in Japanese Patent Application No. 2001-53290 that it may be inserted.
[0078]
Furthermore, when diffusion of metal components from the back electrode layer 6b to the semiconductor layer 6c described later becomes a problem, a diffusion barrier layer (not shown) may be inserted between the back electrode layer 6b and the semiconductor layer 6c. When a metal film such as Ti is used as the diffusion barrier layer, the thickness is 10 nm or less, ZnO, SnO ₂ When a transparent conductive film such as ITO is used for the diffusion barrier layer, the thickness may be 100 nm or less. When using a transparent conductive film, in addition to the function as a diffusion barrier layer, a function of improving the effective reflectance of the back electrode layer 6b can be provided. Furthermore, when the latter transparent conductive film is used, the average height difference of the uneven shape on the surface is made smaller than the average height difference of the uneven shape at the interface between the transparent conductive film and the back electrode layer 6b after the film formation. This makes it possible to form a high-quality film in which the generation of a leakage current is suppressed in the formation of the semiconductor layer 6c described later (see, for example, Japanese Patent Application No. 2001-20623).
[0079]
Next, a semiconductor layer 6c having a semiconductor junction in which the photoactive layer portion is formed of crystalline Si is formed. The semiconductor layer 6c is roughly divided into a base layer, a photoactive layer, and a bonding layer.
[0080]
First, as a base layer (not shown), a non-single crystal Si film is formed by a catalytic CVD method or a plasma CVD method. The film thickness is about 10 to 500 nm. The doping element concentration is 1E18 to 1E21 / cm ^Three P and ⁺ Type (or n ⁺ Type).
[0081]
Next, a crystalline Si film is formed as a photoactive layer (not shown) by catalytic CVD or plasma CVD. The film thickness is about 0.5 to 10 μm. Note that the conductivity type is the same conductivity type whose doping concentration is lower than that of the base layer, or a substantially i-type.
[0082]
Next, in order to form a semiconductor junction (not shown), a non-single-crystal Si film (bonding layer) is formed by a catalytic CVD method, a plasma CVD method, or the like. The film thickness is about 5 to 500 nm. Doping element concentration is 1E18-1E21 / cm ^Three The conductivity type (n ⁺ Type or p ⁺ Type). In order to further improve the bonding characteristics, a substantially i-type non-single-crystal Si layer may be inserted between the photoactive layer and the bonding layer. At this time, the thickness of the insertion layer is about 10 to 500 nm in the case of a crystalline Si layer, and about 1 to 20 nm in the case of amorphous Si.
[0083]
Next, the transparent conductive layer 6d is formed. As a material of the transparent conductive layer 6d, SnO ₂ Known materials such as ITO, ZnO can be used. As a film forming method, a known technique such as a vapor deposition method, a sputtering method, or an ion plating method can be used. This film thickness is preferably about 60 to 300 nm in consideration of the optical interference effect.
[0084]
Finally, a metal film to be the collection electrode 6e is formed. As the metal film material, it is desirable to use Al, Ag or the like having excellent conductivity. As a film forming method, known techniques such as vapor deposition, sputtering, and screen printing can be used. At this time, in the vapor deposition method and the sputtering method, a metal film can be formed in a desired pattern by using a masking method, a lift-off method, or the like. In order to enhance the adhesive strength with the transparent conductive layer 6d, it is effective to insert a metal material having excellent adhesive strength with an oxide material such as Ti between the transparent conductive layer 6d and the surface collecting electrode 6e. It is.
[0085]
As described above, a high-efficiency and low-cost thin-film crystalline Si solar cell having a concavo-convex structure that has high optical confinement efficiency and can sufficiently suppress leakage current can be obtained.
[0086]
The material of the substrate 6a is not limited to glass, and may be any of metal, plastic, and resin.
[0087]
【The invention's effect】
As described above, in the method for roughening a solar cell substrate according to the present invention, when the surface of the solar cell substrate is formed into a rough surface by dry etching, the etching conditions to be performed later are used. Etching is performed under at least two different etching conditions such that the in-plane average aspect ratio of the concavo-convex shape to be formed is larger than the in-plane average aspect ratio of the concavo-convex shape formed in the first etching condition. Therefore, 2500cm, which was impossible before ² The region of the substrate to be etched having a large area can be processed by dry etching using uniform plasma.
[Brief description of the drawings]
FIG. 1 is a view showing a bulk silicon solar cell formed by a surface roughening method for a solar cell substrate according to the present invention.
FIG. 2 is a diagram showing an example of a reactive ion etching apparatus used in the method for roughening a solar cell substrate according to the present invention.
FIG. 3 is a view showing another example of a reactive ion etching apparatus used in the method for roughening a silicon substrate for solar cells according to the present invention.
FIG. 4 is a schematic diagram showing a distribution of differences in etching shape depending on a substrate position on a tray when a conventional RIE process is performed.
FIG. 5 is an electron microscopic image showing the difference in structure depending on the location of the surface of the silicon substrate on which irregularities are formed according to the present invention.
FIG. 6 is a view showing a substrate-type thin film polycrystalline silicon solar cell formed by a method for roughening a solar cell substrate according to the present invention.

Claims (9)

In the method of roughening the solar cell substrate, the surface of the plurality of solar cell substrates is roughened by a dry etching method.
The plurality of solar cell substrates are made of silicon,
The etching conditions are such that the surface of the plurality of solar cell substrates is etched with a gas containing a fluorine-based gas or a bromine-based gas, an oxygen-based gas, and a chlorine-based gas, so that fine irregularities can be formed independently . And the process of
Etching the surfaces of the plurality of solar cell substrates with a gas containing a fluorine-based gas or a bromine-based gas, an oxygen-based gas, and a chlorine-based gas, and having an aspect ratio larger than the concavo-convex shape formed in the first step alone A second step having an etching condition capable of forming fine irregularities, and
The method for roughening a substrate for a solar cell, wherein the second step is performed after the first step. 2. The method for roughening a solar cell substrate according to claim 1, wherein a reaction pressure in the second step is higher than a reaction pressure in the first step. The ratio of the flow rate of the sum of the fluorine-based gas or bromine-based gas to the sum of oxygen-based gas and chlorine-based gas is smaller in the second step than in the first step. 2. A method for roughening a solar cell substrate according to 2. The gas used in the first step and the second step contains SF ₆ , O ₂ , Cl ₂ , and the ratio of the flow rate of the SF ₆ gas to the sum of the O ₂ gas and the Cl ₂ gas is as follows: 3. The method for roughening a solar cell substrate according to 1 or 2, wherein the second step is less than the first step. The ratio of the gas having anisotropic etching characteristics to the gas having isotropic etching characteristics with respect to the solar cell substrate is larger in the second process than in the first process. 3. A method for roughening a solar cell substrate according to 1 or 2. Preparing a solar cell substrate made of glass;
Etching the surface of the solar cell substrate with a gas containing a fluorine-based gas or a bromine-based gas, and a chlorine-based gas, and a first step having etching conditions capable of forming irregularities independently ;
Etching the surface of the solar cell substrate with a gas containing a fluorine-based gas or a bromine-based gas, and a chlorine-based gas to form a concavo-convex shape having an aspect ratio larger than the concavo-convex shape formed in the first step alone. A method of roughening a substrate for a solar cell, comprising: a second step having an etching condition capable of being performed , wherein the second step is performed after the first step. 7. The method for roughening a solar cell substrate according to claim 6, wherein the reaction pressure in the second step is higher than the reaction pressure in the first step. The ratio of the flow rate of the sum of the fluorine-based gas or bromine-based gas to the sum of chlorine-based gas is smaller in the second step than in the first step. A method for roughening a solar cell substrate. The gas used in the first step and the second step contains SF ₆ and Cl ₂ , and the ratio of the flow rate of the SF ₆ gas to the Cl ₂ gas is higher than the second step than the first step. 8. The method for roughening a solar cell substrate according to 6 or 7, wherein the number of steps is smaller.

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