JP2009182290A - Solar cell and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar cell that prevents the occurrence of dilation or ball-like protrusions of aluminum in a backside electrode portion of a solar cell substrate after sintering and reduces a warpage of the substrate caused by the backside electrode portion. <P>SOLUTION: The solar cell includes a silicon substrate 1 having a light receiving surface, a first electrode layer 7 containing aluminum formed on a surface opposite to the light receiving surface of the silicon substrate 1, a second electrode layer 6 containing aluminum and silicon formed between the first electrode layer 7 and the silicon substrate 1, and a BSF layer 9 formed between the second electrode layer 6 and the silicon substrate 1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a solar cell using a silicon substrate and a manufacturing method thereof.

  In order to prevent the generated carriers from disappearing due to recombination on the surface of the substrate of the solar cell, a reverse electric field region is formed in the vicinity of the surface, and minority carriers that have moved to the back surface in this reverse electric field region The function to repel is given. This reverse electric field region is called a BSF layer. In a solar cell, a BSF layer is formed on the back surface (substrate surface opposite to the light receiving surface) of the solar cell in order to efficiently capture carriers generated by the incidence of light. In order to form such a BSF layer (reverse electric field region), there is a method in which a region having a p + type conductivity type is simultaneously formed on a substrate of a p type solar cell when a back electrode is formed. Has been taken.

  In the case of a polycrystalline silicon type solar cell, an aluminum paste is formed on the back surface of the polycrystalline silicon substrate by screen printing, and the substrate is made from the aluminum paste during firing. A method of simultaneously forming a p + type conductive layer (BSF layer) by the diffusion of aluminum into the interior and the back electrode using aluminum is used as a low cost manufacturing method.

  In addition, as an invention related to a solar cell, a solar cell in which a layer containing silicon atoms and carbon atoms is formed to prevent recombination of photoexcited carriers and the band gap of the layer changes smoothly in the layer thickness direction is proposed. (See Patent Documents 1 to 3). In these documents, a method of using AlSi as a backside ohmic electrode is described.

  Further, Patent Document 4 proposes a method for optimizing the aperture ratio of a plurality of openings in a back insulating layer formed to electrically connect a solar cell silicon substrate and a back electrode so as to be suitable for space applications. ing. This document describes a method of forming an electrode and a BSF layer by laminating Al and Ti—Pd—Ag. Moreover, in order to eliminate the curvature of a photovoltaic cell, the method of laminating and heat-treating Al in electrode formation (see Patent Document 5), or in the formation of an electrode to prevent cracking of the photovoltaic cell, Al and inorganic fibers are used. A method of stacking has been proposed (see Patent Document 6).

  Further, Patent Documents 7 to 9 propose methods for improving the electrical performance of the solar cell and the adhesiveness of the solder. In these documents, a method of forming an electrode and a BSF layer by a process of laminating Al and Ag and performing heat treatment is described.

  As described above, various studies have been made on obtaining the ohmic contact between the substrate and the back electrode and improving the solderability between the back electrode and the connecting member as the back electrode material.

JP-A-6-77512 JP-A-6-163656 JP-A-6-204514 JP 2000-174304 A JP 2002-217435 A JP 2004-128411 A JP 2006-302890 A JP 2006-302891 A JP 2006-313744 A

  However, as a process problem in the back electrode, in the baking step after the back electrode paste is formed by screen printing, if the baking conditions or the printed film thickness of the electrode paste is not properly selected, the sun after baking It can be mentioned that bulges and defects in which aluminum protrudes in a ball shape occur in the battery substrate back electrode part. In addition to hindering the function as an electrode, the occurrence of such a defect causes a crack in the cell due to a protrusion when a plurality of solar cells are tiled to form a single solar panel. In addition, the back film to be attached to the back surface of the cell may break through the back film, leading to a failure in mounting of the substrate such as an increase in the possibility of poor insulation, resulting in a decrease in product performance and a decrease in production yield.

  As for the electrode portion during electrode firing, the majority of the back surface electrode on the air side becomes a porous portion formed by carrying over the shape of the aluminum particles contained in the paste, and the aluminum particles in the electrode paste and the substrate However, there is an aluminum silicon compound liquid portion generated by lowering the melting point by eutectic reaction at the boundary between the back electrode and the substrate. The defects described above are considered to occur when the generated aluminum silicon melt breaks through a mechanically weak portion of the porous portion of the back electrode and blows out to the air surface side.

  In recent years, the thickness of a polycrystalline substrate has been reduced for cost reduction. However, when the substrate thickness is reduced, it is necessary to reduce the warpage of the substrate caused by the back electrode. . For this reason, in order to reduce the generation | occurrence | production stress, the back surface electrode makes thin formation thickness or uses the electrode paste with little stress generation | occurrence | production. However, reducing the electrode thickness reduces the electrode strength, and low-stress pastes often have poor sinterability of aluminum particles, which tends to reduce the strength of the electrode itself. Therefore, many of the defects may occur. In addition, the suppression of sintering for reducing the stress is likely to lead to a decrease in Al diffusibility, the formation state of the BSF layer formed by the diffusion of Al into the silicon substrate is deteriorated, and the expected power generation efficiency is improved. It may be difficult to obtain.

  The amount of warpage of the solar cell substrate depends on the electrode thickness. If the electrode thickness is increased, the warpage of the substrate is increased, and if it is decreased, the warpage of the substrate can be decreased. As the formation state of the BSF layer, a sufficient formation state cannot be obtained if the thickness of the back electrode is thin, and a sufficient BSF layer can be obtained by increasing the thickness. The conditions for balancing the two tending in this way become narrower as the substrate thickness becomes thinner, and furthermore, it is extremely difficult to find a configuration and process conditions that satisfy all of them by suppressing the generation of electrode defects. . In addition, the contents of the above-mentioned documents have different purposes and do not solve the problem.

  The present invention has been made to solve the above-described problem, and suppresses the occurrence of blisters or defects in which aluminum protrudes in a ball shape in the back electrode portion of the solar cell substrate after firing, and the back electrode. An object of the present invention is to obtain a solar cell in which the warpage of the substrate caused by the above is reduced.

  A solar cell according to the present invention includes a silicon substrate having a light receiving surface, a first electrode layer containing aluminum formed on the opposite side to the light receiving surface of the silicon substrate, and between the first electrode layer and the silicon substrate. And a second electrode layer containing aluminum and silicon, and a BSF layer formed between the second electrode layer and the silicon substrate.

  Regarding the configuration of the back electrode, the first electrode layer is mainly made of aluminum, and particularly a paste having low stress characteristics. Further, as the second electrode layer which is an intermediate layer, a layer containing aluminum and silicon, which is a material having a low melting point and a high BSF forming ability, is disposed with respect to the first electrode layer. The second electrode layer can suppress generation of excessive aluminum silicon melt during substrate baking, thereby suppressing generation of defects such as protrusions on the back electrode, and at the same time, forming an effective BSF layer. Therefore, the sun using a low-thickness silicon substrate that suppresses the occurrence of swelling and defects in which aluminum protrudes in a ball shape in the back electrode part and reduces the warpage of the substrate caused by the back electrode. Battery cells can be produced with high efficiency and high yield.

  Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof.

<Embodiment 1>
(Constitution)
FIG. 5 shows a configuration of the solar cell according to the present embodiment. The solar cell includes a silicon substrate 1 made of polycrystalline silicon, and has an n + conductive layer 3 on the surface of the light incident surface of the silicon substrate 1, that is, the light receiving surface, and the surface of the n + conductive layer 3 is covered with an antireflection film 4. ing. On the antireflection film 4, a plurality of electrodes 5 made of, for example, silver are provided.

  In addition, the first electrode layer 7 is formed on the side opposite to the light receiving surface of the silicon substrate 1 and contains aluminum. The first electrode layer 7 is formed between the first electrode layer 7 and the silicon substrate 1. A second electrode layer 6 containing aluminum and silicon having a composition of 12 wt% silicon is provided. The melting point of the first electrode layer 7 is higher than the melting point of the second electrode layer 6. Further, a BSF layer 9 is provided on the silicon substrate 1 side and an alloy layer 8 is provided on the second electrode layer 6 side between the second electrode layer 6 and the silicon substrate 1.

(Manufacturing method)
1-5 is sectional drawing for demonstrating the manufacturing process of the solar cell substrate in Embodiment 1 for implementing this invention. As shown in FIG. 1, the silicon substrate 1 made of polycrystalline silicon has a substrate first surface 1a serving as a light incident surface, that is, a light receiving surface, and a substrate second surface 1b serving as a back surface thereof. A silicon substrate 1 having a thickness of about 200 μm is prepared.

  The silicon substrate 1 has p-type conductivity, and an etching layer 2 is formed on its surface as shown in FIG. 2 to prevent reflection. The substrate second surface 1b of the silicon substrate 1 constitutes a P-type surface layer. Further, the surface layer of the substrate first surface 1 a has the n + conductive layer 3, and the antireflection film 4 is formed on the surface of the n + conductive layer 3. A plurality of electrodes 5 made of, for example, silver are formed on the first substrate surface 1a.

  With respect to the light incident surface side structure having such a configuration, an electrode is formed on the second substrate surface 1b as shown in FIG. A layer containing particles having a composition of aluminum-12 wt% silicon having a film thickness of about 10 μm to be the second electrode layer 6 is formed on the surface of the second substrate surface 1b by screen printing. That is, the second electrode layer 6 containing aluminum and silicon is formed on the surface opposite to the light receiving surface of the silicon substrate 1. The particles at this time may be alloy particles of aluminum and silicon, or a mixture of aluminum particles and silicon particles in a predetermined ratio.

  Next, as shown in FIG. 4, the surface of the second electrode layer 6 is printed on the surface of the second electrode layer 6 using a printing paste containing aluminum particles having a small stress generation by a screen printing method. 1 electrode layer 7 is formed. That is, the first electrode layer 7 containing aluminum is formed on the surface of the second electrode layer 6. At this time, the melting point of the first electrode layer 7 is higher than the melting point of the second electrode layer 6.

  Furthermore, this laminated body is heat-treated at about 800 ° C. for 30 seconds in the atmosphere. During this heat treatment, the second electrode layer 6 and the front surface (back surface 1b) of the silicon substrate 1 are reacted and melted to form an alloy layer 8 as shown in FIG. 7 is diffused into the alloy layer 8 and the silicon substrate 1, and a BSF layer 9 is formed between the silicon substrate 1 and the second electrode layer.

  At the same time, when the second electrode layer 6 and the first electrode layer 7 cause mutual diffusion, silicon in the second electrode layer 6 or the second electrode layer 6 and the substrate surface react and melt. The silicon diffused from the silicon substrate 1 also diffuses into the first electrode layer 7 during and after firing. At the same time, the aluminum in the first electrode layer 7 also diffuses from the second electrode layer 6 during and after firing into the substrate melted portion, and is thus involved in the formation of the reaction portion and the formation of the BSF layer 9 as a whole. In other words, it functions as a source of aluminum. Furthermore, the electrode 5 on the first surface of the substrate penetrates the antireflection film 4 and is electrically connected to the n + conductive layer 3.

  The low-stress paste forming the first electrode layer 7 suppresses volume shrinkage by suppressing the densification of aluminum particles that constitute the electrode by processing of the aluminum particles and additives, Many of them suppress the generation of stress on the substrate. In this case, suppressing the sintering often leads to a decrease in the strength of the electrode at the same time. When a low-stress paste is used alone for the back electrode, the back electrode swells or cracks occur. Then, a defect in which an aluminum silicon alloy melted by reaction is exposed is likely to occur (see the back electrode defect 12 in FIG. 6).

(effect)
The formation of the BSF layer 9 depends greatly on the state of the electrode / substrate interface, and the thickness of the electrode is important but indirectly related to the state of the electrode substrate interface, such as the atmosphere during firing and the supply of aluminum. It is presumed that it is influenced by. Further, the defects of the back electrode such as the Al protrusions described above are caused by a low melting point due to a eutectic reaction between the aluminum particles in the electrode forming paste and silicon as the substrate material, and the aluminum silicon melt locally for some reason. It is presumed that this occurs in a balance between the generation of the electrode and the electrode strength.

  By adopting a multilayer configuration in which electrode materials having a plurality of different performances as described above are optimally arranged, the generation state of the reaction molten layer at the interface between the substrate that is the source of the back surface defect and the electrode layer is not completely. It becomes possible to perform a certain control.

  That is, a material having a high BSF forming ability is disposed on the electrode portion in contact with the substrate, and the mechanical strength of the second electrode layer 6 itself is higher than that of the first electrode layer 7 alone. In addition, the strength of the electrode itself can be increased. On the other hand, since the generated stress is large due to the high electrode strength, it is necessary to form it thinly, and as it is, it is difficult to prepare an environment for forming a sufficient BSF layer 9, but directly outside this region. Is not involved in the formation of BSF, so that the first electrode layer 7 is formed on the second electrode layer 6 with a low-stress aluminum paste to assist the formation of the BSF, thereby supplying aluminum for forming the BSF layer. The oxidation state can be adjusted, and both low stress characteristics and high BSF formation ability can be achieved.

  The high BSF capability forming capability means that the p + conductivity can be effectively imparted to the diffusion portion by diffusing into the silicon substrate having p-type conductivity. For example, as the intermediate layer on the substrate, as described above Further, if aluminum, an alloy of silicon, or a mixed material is used as the main constituent layer, the intermediate layer has a lower melting point than the main constituent layer, so that diffusion of aluminum can be secured.

  In the back electrode using the above-described configuration, the low resistance portion formed from the end of the back electrode seen as the BSF layer 9 into the substrate is 5 as an average depth in the measurement by the spreading resistance method using the two-terminal direct contact with the measurement probe. The number of back surface defects at this time was almost zero per 5 cm square of the substrate.

  For comparison, a comparative example in which the back electrode of a solar cell, which is a conventional technique, is produced by screen printing with a commercially available low-stress aluminum paste will be described. The steps are almost the same as described above, except that the electrode layer is printed once, that is, the electrode configuration is one type. The average thickness of the electrode after printing and drying was about 35 μm. As the state of the back electrode in this case, when the low resistance portion in the substrate was measured in the same manner as described above, the depth was 5.2 μm as an average depth.

  Although the formation thickness is slightly shallower than that of the sample, the formation state itself is considered to be almost the same. Regarding the back surface defects, 10 to 20 back surface electrode bulges, spherical aluminum silicon alloy protrusions, and defects were observed per 5 cm square of the substrate. These defects are not preferable in terms of solar cell reliability and mounting productivity. In addition, it is possible to suppress the occurrence to some extent by adjusting the film thickness of the back electrode and the firing conditions, but the limited production conditions are disadvantageous for stable production or high yield. It is desirable to apply a wide range of electrode configurations and electrode configurations and materials with less defects under firing conditions.

  These second electrode layers 6, that is, the intermediate layers can exhibit an effect if they are present at the boundary between the substrate and the electrode in the initial stage of the heat treatment, and therefore do not need to be formed thicker and are thinner than the main constituent layers. A thickness of about 3 μm is sufficient. Although there is no particular limitation on the forming method, a spin coating method or spray coating may be performed in the case of a screen printing method or when forming a thin film.

<Embodiment 2>
(Constitution)
FIG. 5 shows a configuration of the solar cell according to the present embodiment. In the present embodiment, a plurality of solar cells are prepared, and aluminum and silicon in each second electrode layer 6 have different compositions. Note that the silicon content with respect to aluminum in the second electrode layer 6 is in a suitable range in the present embodiment of 2 wt% or more and 15 wt% or less.

  Since other configurations are the same as those of the first embodiment, detailed description thereof is omitted here.

(Manufacturing method)
In the present embodiment, the same aluminum paste as in the first embodiment is used for the first electrode layer 7, but the formation of the second electrode layer 6 is performed using the aluminum silicon paste described in samples 1 to 7 in FIG. Used. Each paste differs in the composition of aluminum and silicon, or the kind of metal powder (mixed powder or alloy powder). The silicon content with respect to aluminum in the second electrode layer 6 before firing is in a preferred range in the present embodiment of 2 wt% or more and 15 wt% or less.

  FIG. 7 also shows the formation thickness of the second electrode layer 6 in the substrate, the formation depth of the low resistance region, and the amount of back electrode defects generated at this time. The thickness of the second electrode layer 6 is the thickness after printing and drying.

  Since other manufacturing methods are the same as those in the first embodiment, detailed description thereof is omitted here.

(effect)
From FIG. 7, even if the composition is the same, a difference can be seen depending on whether the mixed powder or the alloy powder is used. From the viewpoint of the thickness of the low resistance layer in the substrate, which is regarded as the BSF layer 6, the second electrode 6 is used. 15% by weight or less of the aluminum silicon composition of the layer is substantially effective, and when the silicon content is 20% by weight, a low resistance layer is formed, but a thin generation depth is obtained. There is a high possibility that some parts are not sufficient, which raises concerns about stable production.

  Further, when the silicon amount is 1% by weight, there is no difference in the amount of back surface defects compared to the sample shown as a comparison in the first embodiment, and the effectiveness is reduced. Therefore, it is effective that the content of silicon with respect to aluminum in the second electrode layer 6 is 2% by weight or more and 15% by weight or less.

  The allowable number of defects that can be generated cannot be determined uniquely because it depends on the size of the defect, but it is best if there are no defects. .

  Thus, by limiting the composition of the aluminum silicon of the intermediate layer from the time of formation, it is possible to control the formation state of the aluminum silicon melt at the interface portion of the electrode and the substrate related to the generation of the back surface electrode defect. Occurrence can be suppressed. As an effective composition range, an aluminum silicon composition having a melting point lower than the aluminum melting point is effective.

  From the above result, that is, from the viewpoint of the melting point, the silicon content with respect to aluminum is effective from about 2% by weight where the melting point is clearly reduced to about 15% by weight close to the melting point of aluminum. As a range, an aluminum-enriched composition region in which silicon on the substrate side can be slightly dissolved from 12% silicon which is the eutectic composition with the lowest melting point is more effective.

  There is a difference between the mixed powder and the alloy powder with respect to the change in the formation depth of the low resistance region with respect to the silicon content, but in the case of the alloy powder, when viewed locally, it is an aluminum silicon paste. However, since there is a part where the aluminum powder and the silicon substrate are in direct contact with each other, it behaves like a 100% aluminum paste. It is considered difficult.

<Embodiment 3>
(Constitution)
FIG. 5 shows a configuration of the solar cell according to the present embodiment. In the present embodiment, the composition of the second electrode layer 6 of the solar cell, that is, the silicon content relative to the aluminum of the second electrode layer 6 is 15% by weight, and sodium borate is based on the metal component weight. 2% is added. That is, it further contains a boron compound.

  Since other configurations are the same as those of the first embodiment, detailed description thereof is omitted here.

(Manufacturing method)
In the present embodiment, the same aluminum paste as in the first embodiment is used for the first electrode layer 7 before firing, but the second electrode layer 6 is formed by using an aluminum alloy powder containing 15% by weight of silicon. The paste was used, and 2% of sodium borate was added to the weight of the metal component in the paste. That is, the one further containing a boron compound was used.

  Since other manufacturing methods are the same as those in the first embodiment, detailed description thereof is omitted here.

(effect)
By adding sodium borate to the second electrode layer 6, the state of generation of electrode defects does not change as compared with the case where it is not added, but the thickness of the low resistance layer formed in the substrate is the same as in the case where it is not added. Compared to 3.5 μm, the added sample can be as deep as 4.6 μm. The increase in the thickness of the low resistance layer due to the addition is advantageous in terms of characteristic stability in production.

  As described above, in addition to aluminum, boron is also effective as an ability to form the p + conductive layer, and it is also effective to have a composition containing a boron compound in addition to the above structure. As the boron compound, it is simple and effective to add boron oxide containing boron oxide such as boron oxide, sodium borate, borosilicate glass having a high boron concentration, or boron having a high concentration such as Al—Ca—B—O system. However, sodium borate, high boron borosilicate glass, and borate glass are easy to react, stable, and easy to apply.

  Since the second electrode layer 6 can control the generation of a melt due to a eutectic reaction between the substrate and an electrode mainly composed of aluminum to some extent, the ability to form the BSF layer 9 formed in the substrate is reduced. In this case, it is effective to add an element such as a boron compound that is advantageous for forming the BSF layer 9.

It is a figure which shows the manufacturing process of the solar cell which concerns on this invention. It is a figure which shows the manufacturing process of the solar cell which concerns on this invention. It is a figure which shows the manufacturing process of the solar cell which concerns on this invention. It is a figure which shows the manufacturing process of the solar cell which concerns on this invention. It is a figure which shows the structure of the solar cell which concerns on this invention. It is a figure which shows the problem which this invention should solve. It is a figure which shows the effect of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Silicon substrate, 1a Substrate 1st surface, 1b Substrate 2nd surface, 2 Etching layer, 3 n + conduction layer, 4 Antireflection film, 5 Electrode, 6 2nd electrode layer, 7 1st electrode layer, 8 Alloy layer , 9 BSF layer, 12 Back electrode defect.

Claims (8)

A silicon substrate having a light receiving surface;
A first electrode layer formed on the opposite side of the light receiving surface of the silicon substrate and containing aluminum;
A second electrode layer formed between the first electrode layer and the silicon substrate and containing aluminum and silicon;
A BSF layer formed between the second electrode layer and the silicon substrate,
Solar cell. The melting point of the first electrode layer is higher than the melting point of the second electrode layer;
The solar cell according to claim 1. The silicon content with respect to aluminum in the second electrode layer is 2 wt% or more and 15 wt% or less.
The solar cell according to claim 1 or claim 2. The second electrode layer further contains a boron compound;
The solar cell according to any one of claims 1 to 3. (A) forming a light receiving surface on one surface of the silicon substrate;
(B) forming a second electrode layer containing aluminum and silicon on the surface of the silicon substrate opposite to the light receiving surface;
(C) forming a first electrode layer containing aluminum on the surface of the second electrode layer;
(D) heat-treating the laminate formed by the steps (a) to (c) to form a BSF layer between the silicon substrate and the second electrode layer,
A method for manufacturing a solar cell. The melting point of the first electrode layer formed in the step (c) is higher than the melting point of the second electrode layer formed in the step (b).
The manufacturing method of the solar cell of Claim 5. The content of silicon with respect to aluminum in the second electrode layer formed in the step (b) is 2 wt% or more and 15 wt% or less.
The manufacturing method of the solar cell of Claim 5 or Claim 6. The second electrode layer formed in the step (b) further contains a boron compound;
The manufacturing method of the solar cell in any one of Claims 5-7.

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