JP6486219B2 - Manufacturing method of solar cell - Google Patents

Description

The present invention relates to the production how a solar cell, more particularly the formation of aluminum and aluminum alloy electrode.

  2. Description of the Related Art Conventionally, there is a solar cell module in which a plurality of solar cells having element electrodes on a light receiving surface side and a back surface side are arranged side by side, and the light receiving surface side and the back surface side are sealed with glass and a resin film. In order to connect in series a solar cell that is a photoelectric conversion element that receives light and generates power and another solar cell adjacent to the solar cell, the other solar cell adjacent to the light-receiving surface electrode of one solar cell An inter-element connector that electrically connects the back electrode is used.

  The photoelectric conversion element utilizes the internal photoelectric effect of the semiconductor substrate. Since a semiconductor substrate used for a photoelectric conversion element has a relatively low conductivity compared to a metal, resistance loss increases when a distance of current flowing through the semiconductor substrate is long. In addition, if the distance that the minority carriers move in the semiconductor substrate is long, the extraction current to the outside of the semiconductor decreases due to the deactivation of the photogenerated carriers.

  Therefore, a general photoelectric conversion element is formed on a semiconductor substrate using a metal electrode or a translucent electrode as an element electrode, thereby taking out carriers in the semiconductor substrate at a short distance and ensuring conductivity in the in-plane direction. It has a structure. However, in the structure where the metal electrode is used for carrier extraction and in-plane conduction, the light receiving surface side is separated by a certain distance so that the device electrode does not cover the entire semiconductor substrate in consideration of light loss due to the electrode shadow. The structure is widely distributed over the entire surface of the semiconductor substrate. Here, the structure in which the device electrodes are widely distributed is approximately equal to or less than the diffusion length of minority carriers in the substrate, and the current collecting resistance from the semiconductor substrate to the device electrode is comparable to the current collecting resistance of the device electrode itself. A structure in which element electrodes that are in direct contact with the semiconductor substrate are distributed over the entire surface of the semiconductor substrate at intervals as described below.

  In the above structure, since the semiconductor substrate has a much lower conductivity than the metal, in the part where there is no element electrode, the current flows through the element and reaches the element electrode part. Since the resistance of the semiconductor substrate itself is added and the resistance loss in the semiconductor substrate increases, it is preferable to reduce the distance between the element electrodes from the viewpoint of resistance. On the other hand, when the electrode interval is narrow, the electrode shadow area increases on the light receiving surface side, and the amount of incident light decreases.

  Even on the back surface side, the recombination speed of the carrier is increased at the portion where the semiconductor substrate and the metal are in contact with each other, so that the contact area between the element electrode and the semiconductor substrate is preferably small. Therefore, in terms of optical characteristics and recombination speed in the semiconductor substrate, it is necessary to reduce the distance between the device electrodes and the contact area between the device electrodes and the semiconductor substrate both on the light receiving surface side and on the back surface side. From the viewpoints of resistance, optical characteristics, and recombination speed, it is necessary to adjust the device electrode interval and the contact area between the device electrode and the semiconductor substrate to have appropriate values in order to maximize the photoelectric conversion efficiency. . However, as for the part other than the electrode, if the semiconductor substrate surface is left as it is, the dangling bond or crystal periodicity will be disturbed, and the recombination loss due to the presence of the defect levels accompanying these will occur. Covering with a passivation film that lowers the recombination rate is necessary for high efficiency. As described above, the portion other than the contact portion, which is a contact portion between the semiconductor substrate and the metal electrode for taking out current from the semiconductor substrate, covers the surface of the semiconductor substrate with a passivation film that sufficiently reduces the carrier recombination rate at the interface. The photoelectric conversion efficiency can be improved by covering.

  Due to these factors, the element electrode is widely distributed over the entire semiconductor substrate on the light-receiving surface and back surface side of the semiconductor substrate, and the double-sided passivation structure in which the surface of the semiconductor substrate other than the element electrode formation region is covered with a passivation film is more There was an advantage that the photoelectric conversion efficiency was higher than that of a structure in which element electrodes are formed on the entire back surface of the semiconductor substrate. However, on the other hand, the double-sided passivation type solar cell described above has a problem that the current collection resistance is large because the area of the electrode is small compared to the structure in which the electrode is provided on the entire back surface side of the semiconductor substrate.

  Therefore, Patent Document 1 discloses a structure in which metal is bonded to the back side of an element in a double-sided passivation type solar cell.

  Patent Document 2 discloses a method of forming a metal electrode on the back surface of an element by fusing metal particles with a laser in a double-sided passivation solar cell.

Special table 2010-502021 gazette Japanese Patent Application Laid-Open No. 2014-075505

Applied Physics Letters: APPLIED PHYSICS LETTERS Vol. 98 (2011) 153508 (titled “Silicon diffusion in aluminum for solar sir cell”, author Elias Urrejola, Kr.

  In the method of Patent Document 1, alignment is not required, but the reaction characteristics between the metal and the silicon substrate differ depending on the laser irradiation point, and there is a portion where local thermal damage remains in the semiconductor substrate, As a result, production was difficult.

  Further, when the metal foil is bonded to the back surface side of the element of the semiconductor substrate as in Patent Document 2, although the conductivity of the metal foil itself is high, the electrical connection between the element back electrode and the metal foil is not good. There was a fear of becoming perfect. In Patent Document 2, an aluminum electrode including glass frit is used to form a back surface electric field layer, and a metal foil is bonded to the outside thereof.

  The metal foil is bonded for electrical contact. In other words, the aluminum electrode is used to obtain the effect of contacting the semiconductor substrate, but it is difficult for the aluminum electrode to achieve both the erosion of the passivation film and the action of aluminum as a dopant. In other words, when the passivation film is eroded and contacted with the semiconductor layer in the firing step, it is difficult to form an aluminum doping layer having a sufficient thickness.

  For this reason, in the case of using aluminum as the element electrode, it is necessary to form the electrode by opening the passivation film with a laser and then aligning the opening with the ink jet or screen printing. The photoelectric conversion efficiency decreases due to an increase in contact resistance due to misalignment during alignment or an increase in the device electrode area due to securing a misalignment margin, and an accompanying increase in the amount of light transmitted through the device.

  The present invention has been made in view of the above, and a solar cell manufacturing method capable of suppressing recombination of generated carriers, improving photoelectric conversion efficiency, and reducing current collection resistance. The purpose is to obtain.

  In order to solve the above-described problems and achieve the object, a method for manufacturing a solar cell according to the present invention includes a step of coating a light-receiving surface side and a back surface side of a semiconductor substrate having a pn junction with a passivation film, A step of forming an opening in the passivation film on the back side, a step of forming a back electrode made of an aluminum-containing film in the opening, and a film for forming a light receiving surface electrode containing glass frit distributed over the light receiving surface on the light receiving surface. A printing process, a firing process in which a semiconductor substrate on which a film for forming a light-receiving surface electrode is formed is heated to penetrate the passivation film to form a light-receiving surface electrode containing glass frit, and a metal that covers the back surface of the back electrode Heating the foil in contact with the foil, and diffusing aluminum from the surface electrode directly under the back electrode.

  According to the present invention, it is an object to obtain a method for manufacturing a solar cell capable of suppressing recombination of generated carriers, improving photoelectric conversion efficiency, and reducing current collection resistance.

The top view which looked at the solar cell of Embodiment 1 from the light-receiving surface side The top view which looked at the state which removed the metal foil of the element back surface from the solar cell of Embodiment 1 from the back surface side The top view which shows the shape of the metal foil used for the solar cell back surface side of Embodiment 1 It is a top view which shows the structure of the solar cell back surface side of Embodiment 1, and is a figure which shows the state by which the metal foil shown by FIG. 3 was connected to the back surface side of the solar cell element of FIG. The top view which looked at the solar cell which shows the structure which connected the solar cell shown in FIG. 1 and FIG. 4 concerning Embodiment 1 in series, and made it a string from the light-receiving surface side. The top view which looked at the solar cell which shows the structure which connected the solar cell shown by FIG. 1 and FIG. 4 concerning Embodiment 1 in series, and made it a string from the back side. FIG. 1 is a cross-sectional view taken along line AB in FIG. CD sectional view in FIGS. 5 and 6 of the first embodiment EF sectional drawing in Embodiment 5 of FIG. 5 and FIG. (A) And (b) is principal part expansion explanatory drawing which shows the structure of the solar cell of Embodiment 1, and the solar cell of a comparative example. (A) to (j) are process cross-sectional views illustrating the method for manufacturing the solar cell of the first embodiment. The flowchart which shows the manufacturing method of the solar cell of Embodiment 1. FIG. The flowchart which shows the manufacturing method of the solar cell of Embodiment 1. FIG. It is a figure which shows the manufacturing method of the solar cell concerning Embodiment 1, and is a cross-sectional schematic diagram which shows the process of forming a solder layer in the element electrode of a solar cell. Sectional schematic diagram showing a jet formed in a part of the solder bath of the first embodiment It is a figure which shows the solar cell of Embodiment 2, and is a top view which shows the pattern of the element electrode of the back surface side of a solar cell It is a figure which shows the solar cell of Embodiment 3, and is a top view which shows the pattern of the element electrode of the back surface side of a solar cell

It will be described below in detail with reference to the production method of solar cell that written to the embodiment of the present invention with reference to the accompanying drawings. Note that the present invention is not limited to the embodiments. In addition, in each figure, the same code | symbol is attached | subjected about the same or similar component. In addition, in order to avoid the following description from becoming unnecessarily redundant and to facilitate understanding by those skilled in the art, a detailed description of already well-known matters and a redundant description of substantially the same configuration may be omitted. is there. Also, the contents of the following description and the accompanying drawings are not intended to limit the subject matter described in the claims.

Embodiment 1 FIG.
1 and 2 are plan views showing shapes of element electrodes on the light-receiving surface side and the back surface side of the solar cell according to Embodiment 1 of the present invention, showing a single element excluding a metal foil on the element back surface. Yes. FIG. 3 is a plan view showing the shape of the metal foil used on the back surface side of the solar cell of the first embodiment. 4 is a plan view showing the configuration of the back surface side of the solar cell of Embodiment 1, and shows a state in which the metal foil shown in FIG. 3 is connected to the back surface side of the solar cell of FIG. FIGS. 5 and 6 are plan views showing a configuration in which the solar cells shown in FIGS. 1 and 4 according to the first embodiment are connected in series to form a string, and show the light receiving surface side and the back surface side, respectively. Yes. FIG. 7 is a cross-sectional view of the solar cell of the first embodiment, and is a view corresponding to the cross section AB in FIG. FIG. 8 is a cross-sectional view of the solar cell of the first embodiment, which corresponds to the CD cross section in FIGS. 5 and 6. FIG. 9 is a cross-sectional view of the solar cell of the first embodiment, which corresponds to the EF cross section in FIGS. 5 and 6. FIG. 10A and FIG. 10B are enlarged views for explaining the main part of the structure of the solar cell of the first embodiment. FIG. 11 is a process cross-sectional view illustrating the method for manufacturing the solar cell according to the first embodiment. 12 and 13 are flowcharts showing the method for manufacturing the solar cell according to the first embodiment. FIG. 14 is a diagram illustrating the method for manufacturing the solar cell according to the first embodiment, and is a schematic cross-sectional view illustrating a process of forming a solder layer on the element electrode of the solar cell. 15 is a schematic cross-sectional view showing a jet formed in a part of the solder bath of FIG. Solar cell 100 of Embodiment 1 is a solar cell in which a passivation film is formed on both surfaces, and the formation of an oxide film is suppressed by bringing the back electrode into contact with molten aluminum by opening h formed in the passivation film. In addition to being formed as aluminum or an aluminum alloy, it is made to act as a p-type dopant in an n-type single crystal silicon substrate as the semiconductor substrate 110. Since the obtained aluminum electrode can be welded to other alloy foils, when forming a module, it is welded to the alloy foil to form an inter-element connection body, which is connected to an adjacent element by soldering or welding. As a result, a string is formed. Further, the back electrode 125 made of aluminum can be formed without being aligned with the opening h of the passivation film. Here, as shown in FIG. 6, the inter-element connector 130 covers the inter-element connection portion 131 extending from the back surface 100B side of the solar cell 100 to the light receiving surface 100A side of the adjacent solar cell 100 and the entire back surface 100B side. It is comprised with the main-body part 132 which consists of aluminum foil. The main body 132 is directly bonded to the back electrode 125 formed on the semiconductor substrate 110 and covers the entire back surface 110B. The inter-element connection portion 131 is connected to the back surface side of the main body portion 132 and connects the light receiving surface electrode 115 on the light receiving surface 100 </ b> A side of the adjacent solar cell 100. In the present embodiment, the light receiving surface electrode 115 includes the grid electrode 112 and the bus electrode 113. The back electrode 125 is actually composed of a back surface point electrode 122D.

  In the following description, the element electrode refers to a light receiving surface electrode and a back surface electrode formed on a semiconductor substrate forming a solar cell, that is, a silicon substrate.

  In the solar cell 100 of the first embodiment, the entire back surface 100B is covered with either the passivation film 121 or the back electrode 125 distributed over the entire back surface 100B. As shown in the cross-sectional view of FIG. 7, the back surface electrode 125 includes back surface point-like electrodes 122 </ b> D arranged and distributed in the form of dots over the entire surface. The back surface dotted electrode 122D is made of a lump of aluminum formed in contact with a molten aluminum bath through an opening h formed in the passivation film 121. Here, the lump means that each point-like electrode is not composed of a large number of particles, is formed by hardening molten aluminum, and is an integral aluminum containing almost no voids, resin, or aluminum oxide film inside. It shall be said. In the case of a linear electrode, it is preferable that one line segment of each linear electrode is integrated, but it is sufficient that at least some part of each linear electrode can be electrically connected to the inter-element connector. It suffices that the electrode has a portion that is a lump over a length of about 1% or more.

  In contrast, general aluminum paste is composed of a large number of fine aluminum particles of several tens of microns or less, so the electrode formed from the aluminum paste is a porous body in which a large number of aluminum particles are sintered, and the surface of each particle is The hole between the particles contains an aluminum oxide or resin residue. Of these, aluminum oxide can be removed with an etching solution such as an aqueous hydrofluoric acid solution even if it is somewhat contained inside each point-like electrode, but the resin or the residue of the resin after heating remains. If this is the case, it is difficult to remove, and there arises a problem that the welding between the point-like electrode and the metal of the inter-element connector is hindered.

  Moreover, the solar cell 100 of Embodiment 1 has the main-body part 132 of the inter-element connection body 130 comprised with the metal foil shown in FIG. 3, and as shown in FIG. Are joined. As shown in FIG. 6, the inter-element connector 130 includes an inter-element connection part 131 and a main body part 132, and the main body part 132 is connected and fixed to the back electrode 125 and covers the back surface of the back electrode 125. Specifically, as shown in FIG. 7, the main body portion 132 is connected to the back surface dotted electrode 122D and covers the entire back surface.

  As shown in FIGS. 1 and 2, solar cell 100 of Embodiment 1 is configured with a semiconductor substrate 110 having a substantially rectangular planar shape. In this embodiment mode, an n-type single crystal silicon substrate 101 in which a p-type diffusion layer is formed and a pn junction is formed is used as a semiconductor substrate 110. Here, the substantially rectangular shape means a quadrangular shape having two sets of parallel sides that are perpendicular to each other. In particular, in a solar cell using a silicon single crystal, when forming a rectangular substrate from a cylindrical single crystal ingot, in order to reduce a portion that is cut off from a circle to a rectangle and is wasted, as shown in FIG. In many cases, a substrate having a shape in which a part of the substrate is cut off is used. The shape shown in FIG. 1 is also included in the substantially rectangular shape. In FIG. 1, an example of a shape in which a part of a square corner is cut off is shown. However, it is also possible to use a shape that has a substantially rectangular shape by dividing it into half. The semiconductor substrate 110 has a pn junction and has a thin plate shape with a thickness of, for example, 0.05 mm to 0.5 mm. As a material for forming the semiconductor substrate 110, for example, an n-type single crystal silicon substrate or a p-type single crystal silicon substrate of about 1 Ωcm or more and about 30 Ωcm is used, and boron is doped on one surface and phosphorus is doped on the other surface. Can be used as the semiconductor substrate 110 constituting the solar cell substrate. Although the n-type single crystal silicon substrate is used in Embodiment 1, it goes without saying that a p-type single crystal silicon substrate, an n-type polycrystalline silicon substrate, and a p-type polycrystalline silicon substrate may be used. .

  Passivation films 111 and 121 made of a silicon nitride film are formed on the light receiving surface 100 </ b> A and the back surface 100 </ b> B of the solar cell 100 at least over the entire surface except the contact portion between the element electrode and the semiconductor substrate 110 and the end portion of the semiconductor substrate 110. The recombination of electrons and holes on the surface of the semiconductor substrate 110 can be reduced. As the passivation films 111 and 121, an amorphous silicon nitride film having a thickness of 40 nm or more, a laminated film of a silicon oxide film and an amorphous silicon nitride film, a laminated film of alumina and an amorphous silicon nitride film, or the like is used. be able to.

  In the first embodiment, the light receiving surface electrode 115 which is an element electrode formed on the light receiving surface 100A of the solar cell 100 includes a plurality of parallel thin linear light receiving surface grid electrodes 112 and a light receiving surface as shown in FIG. The grid electrode 112 and the four light-receiving surface bus electrodes 113 that intersect and intersect each other are formed. The number of light receiving surface bus electrodes 113 can be, for example, 2 to 5 in order to reduce the electrode shadow caused by the light receiving surface bus electrodes 113 and reduce the current collection distance of the light receiving surface grid electrodes 112.

  The light-receiving surface grid electrode 112 according to the first embodiment is an electrode that collects charges generated by the generation of photocarriers from the semiconductor substrate 110 and collects the charges, and each electrode is disposed at an appropriate interval. The width and spacing of the light-receiving surface grid electrode 112 vary depending on the sheet resistance of the surface of the semiconductor substrate 110 on the light-receiving surface 100A side, but the light-receiving surface bus electrode 113 extends at a cycle of 0.5 mm to 2.5 mm, for example. It is arranged in parallel to the direction orthogonal to the current direction, and the width of each line can be comprised between 0.01 mm and 0.2 mm. The length of the light receiving surface grid electrode 112 can be about the same as that of the semiconductor substrate 110, and the thickness can be about 5 μm to 50 μm. The light-receiving surface bus electrode 113 is connected to the light-receiving surface grid electrode 112 having one polarity, and functions as a bus electrode that extracts the current collected by the light-receiving surface grid electrode 112 to the outside of the solar cell 100. The light receiving surface bus electrode 113 may be formed in a separate process from the light receiving surface grid electrode 112, but is often formed in the same process as the light receiving surface grid electrode 112.

  The light-receiving surface grid electrode 112 is preferably made of a metal material mainly containing aluminum Al, silver Ag, copper Cu, nickel Ni, and tin Sn, and a laminate thereof. In solar cell 100 of the first embodiment, light receiving surface grid electrode 112 includes glass frit and silver, and the glass frit maintains the connection strength between semiconductor substrate 110 and light receiving surface grid electrode 112. The conductivity is secured by silver.

  The current collected by the light receiving surface grid electrode 112 as described above is taken out of the element through the light receiving surface bus electrode 113 and used as electric power. In addition, when connecting each line of the light-receiving surface grid electrode 112 by a conducting wire or the like when creating the solar cell module, the light-receiving surface bus electrode 113 may not be connected, and the island-like shape is discontinuous. Or it may not be connected at all.

  The light-receiving surface bus electrode 113 may not include glass frit, but in this embodiment, an electrode mainly made of silver and containing a small amount of glass frit is used. Frit generally refers to a material mixture that is melted and vitrified and then crushed to an appropriate particle size by water quenching or roll quenching, but in this embodiment, in the sintering aid, that is, in the sintering process, It is assumed that it can penetrate into the passivation film and reach the underlayer sufficiently. For example, as such a glass frit, what consists of oxides, such as lead, boron, bismuth, and silicon, can be used.

  The shape of the element electrode on the back surface 100B side of the solar cell 100, that is, the shape of the back electrode 125 is shown in FIG. As shown in the drawing, on the back surface side of the semiconductor substrate 110, a back surface dotted electrode 122D made of an aluminum layer formed by hardening molten aluminum as an electrode of the other polarity is formed in contact with the semiconductor substrate 110 and the passivation film 121. Has been. In this embodiment, the back surface electrode 125 is constituted by these back surface point-like electrodes 122D, and each of the point electrodes is electrically connected to the semiconductor substrate 110 through the opening of the passivation film 121 of the semiconductor substrate 110. In the present embodiment, the opening of the passivation film 121 has a circular shape with a diameter of about 30 μm, and is indicated by h in FIG. The back surface dotted electrode 122D of the first embodiment is an electrode for taking out the electric charge generated by the photocarrier generation from the semiconductor substrate 110, and each is arranged at an appropriate interval.

  As shown in FIG. 7, the pattern of the back surface dotted electrode 122D differs depending on the sheet resistance of the surface of the semiconductor substrate 110 made of an n-type single crystal silicon substrate, for example, 0.2 mm to 2 mm. They can be formed at equal intervals with a distance of about the following. The size of the back surface dotted electrode 122D varies depending on the electrode forming method, but may be, for example, 50 μm in diameter, 20 μm in thickness, or the like.

  Here, a case is shown in which a plurality of dot-like electrodes are separated from each other as the shape of the contact surface between the semiconductor substrate 110 and the back electrode 125 and the shape of the back electrode 125 itself. It is not necessary to have a shape, and may be another shape such as a thin line.

  As a material for these back surface dotted electrodes 122D, aluminum, an aluminum alloy, and a laminate thereof can be used. In this embodiment, a material obtained by curing molten aluminum is used. The details will be described later along with the process cross-sectional view of FIG. 11, but after the opening h is formed in the passivation film 121 formed on the back surface 100B side of the semiconductor substrate 110, by passing the semiconductor substrate 110 over the molten aluminum, A back surface dotted electrode 122D made of an aluminum layer is selectively formed in the opening h. Arranged in a state suitable for connection with the main body 132 made of metal foil constituting the inter-element connector 130 when modularized, with no oxide film formed on the outermost surface of the back surface dotted electrode 122D. Is done. For example, when the element electrode and the main body 132 of the inter-element connector 130 are connected using solder, the surface oxide film of the back surface dotted electrode 122D is removed by etching prior to the connection. In addition, aluminum is used as a material for the back surface dotted electrode 122D, which is an element electrode on the back surface 100B side. However, in order to facilitate soldering to aluminum, the back surface electrode 125 such as the back surface dotted electrode 122D is used. In the outermost part, a material suitable for connecting to the body part 132 made of the metal foil shown in FIGS. 3 and 4 covering the whole of the inter-element connection body such as a tab wire or the back side when arranging the module is arranged. May be. For example, when the main body part 132 made of metal foil and the back surface dotted electrode 122D are connected using solder, an electroplating layer of copper, tin, silver or the like, which is easy to solder, is attached to the main body part 132 made of metal foil or You may form on back surface dotted electrode 122D.

  In a usual solar cell, an electrode material made of a paste containing glass frit and aluminum mainly formed by screen printing is used as the back electrode. In the paste, aluminum has particles sufficiently smaller than the mesh size of the screen plate, and contains an organic substance for adjusting the viscosity for printing. For this reason, when the surface of the aluminum particles is oxidized by firing and a cross section larger than the particle diameter of the aluminum particles is taken, it is difficult to solder or braze because the aluminum oxide film exists in any cross section. It was. Therefore, in the present embodiment, a large-sized aluminum that does not include glass frit is formed for each back surface point-like electrode 122D as the back surface side aluminum electrode. The size of aluminum at each point of the back surface point-like electrode 122D can be, for example, a circle with a diameter of 50 μm and a thickness of about 20 μm. Because of the size and thickness described above, even when an oxide film is formed at a high temperature during bonding to the semiconductor substrate 110 and thereafter, the surface is oxidized by simply removing the surface oxide film by mechanical polishing or chemical etching. An aluminum surface without a film can be provided on the surface. In this way, each element electrode can be made of aluminum containing almost no oxide film or organic matter inside and on the surface without using vacuum deposition, so other metals can be used by electroplating, soldering, brazing, etc. Can be used to connect with. In addition, it is desirable to carry out in an inert gas atmosphere, such as nitrogen gas and argon gas, from an oxide film removal to the joining process with another metal. Further, in the present embodiment, brazing in which aluminum is melted is used for connection between the body portion 132 of the inter-element connection body and the back surface point-like electrode 122D. A structure in which the surface of the electrode is plated with tin may be used, and in this case, soldering between the main body portion 132 of the inter-element connector and the back surface dotted electrode 122D becomes easy.

  In the solar cell 100 of the first embodiment, the surface of the light-receiving surface bus electrode 113 on the light-receiving surface 100A side is covered with solder when the current is taken out from the solar cell 100 through the inter-element connector 130, and the inter-element connector and the element are connected through the solder. The light receiving surface grid electrode 112 is not covered with solder.

  The solar cell 100 of Embodiment 1 has a metal foil as the main body part 132 of the inter-element connector shown in FIG. 3 on the back side of the semiconductor substrate 110 constituting the solar cell. The main body 132 has the same size as the outer edge of the semiconductor substrate 110, and is connected to the back surface dotted electrode 122D, which is an element electrode on the back surface 100B side, and a plan view thereof is as shown in FIG. Become. The light receiving surface 100A and the back surface 100B of the solar cell 100 with the main body portion 132 mounted are the solar cells of FIGS. 1 and 4, and FIG. 2 is a plan view of the back surface 100B with the main body portion 132 removed therefrom. .

  The main body 132 is made of a flat plate made of a metal foil such as copper or aluminum, and preferably has a higher melting point than the material used for the back surface dotted electrode 122D, which is a device electrode on the back surface, depending on the manufacturing method. The main body 132 can be created, for example, by punching a metal foil. The metal foil is not limited to a single metal foil, but may be a film or foil in which a metal thin film such as a metal film deposited on glass or a polyimide film is connected in the same pattern shape as the back electrode. In the present embodiment, a large aluminum piece having approximately the same size as the semiconductor substrate, which does not include glass frit, is used as the back surface side aluminum electrode. In order to secure a region in which the aluminum oxide is not formed as much as possible in the region connected to the inter-element connection portion of the inter-element connection body, it is preferably made of a single aluminum foil and has a thickness of about 10 μm or more. Because of this size and thickness, resistance can be reduced, and even if the surface is oxidized at high temperatures, pure aluminum remains inside, and can be easily connected to other metals by soldering or brazing by mechanical polishing. It is possible to have a structure in which there is no oxide film on the surface of the aluminum foil over the entire area. The surface of the main body 132 of the inter-element connection body may have a structure plated with tin. With the above configuration, the inter-element connection portion 131 of the inter-element connection body and the main body portion 132 made of aluminum foil can be directly connected by a connection body such as solder.

  The back electrode 125 is not limited to the one formed by bringing a semiconductor substrate into contact with molten aluminum, but a bulk aluminum such as aluminum in a bulk state such as an aluminum ribbon, which is not a particle but larger than at least the opening h of the passivation film 121. It may be cured after being brought into contact with a semiconductor substrate and melted. In this way, it is desirable that each continuous back electrode electrically connected to the semiconductor substrate has a continuous aluminum portion that is at least larger than the opening h of the passivation film 121. With the above configuration, the back electrode 125 formed on the back surface on the element and the main body 132 of the inter-element connection body can be directly fused or soldered or brazed.

  The main body 132 made of metal foil has a function of reflecting light that has passed through the semiconductor substrate 110 and reached the back surface 100B side of the solar cell 100. For this reason, it is desirable that the material used for the main body portion 132 made of metal foil has a low resistance in the in-plane direction and a high light reflectance on the solar cell side surface. In the solar cell of the first embodiment, the element electrodes refer to the light receiving surface electrode 115 and the back surface electrode 125. The light receiving surface electrode 115 includes a light receiving surface grid electrode 112 and a light receiving surface bus electrode 113. The back electrode 125 is composed of the back surface point-like electrode 122D, refers to an electrode formed on the semiconductor substrate 110, and does not include the main body portion 132 made of metal foil. As shown in FIG. 6, the main body 132 constitutes the inter-element connection body 130 together with the inter-element connection section 131.

  As shown in FIGS. 5 and 6, the above solar cell 100 is connected by using an inter-element connection body 130 and an inter-string connection body 136 each including an inter-element connection portion 131 and a main body portion 132. 5 shows the light receiving surface 100A side of the solar cell 100, and FIG. 6 shows the back surface 100B side. A string string in which a plurality of strings are connected in series can be created. Glass and ethylene vinyl acetate on the light receiving surface 100A side of the string string, ethylene vinyl acetate on the back surface 100B side, and a back surface such as polyvinyl fluoride or polyethylene terephthalate A solar cell module can be produced by bonding a protective material. A flat copper wire covered with solder can be used as the inter-element connection portion 131 and the inter-string connection body 136 of the inter-element connection body 130 and the conductive wire 137 at the end of the string.

  FIG. 7 is a schematic cross-sectional view corresponding to the line segment of AB in FIGS. 1, 4, 5, and 6. Also in FIG. 2, a line segment AB is shown in order to show the positional relationship between the back surface dotted electrode 122D, which is the device electrode pattern on the back surface, and FIG. 7, but in FIG. The main body 132 is not shown because it is shown through 132. As shown in FIG. 7, in the semiconductor substrate 110, except for the connection portion between the light receiving surface electrode 115 and the back surface electrode 125, the light receiving surface 100 </ b> A and the substrate surface on the back surface 100 </ b> B side are covered with the passivation films 111 and 121. The light receiving surface electrode 115 and the back surface electrode 125 which are element electrodes are in contact with the semiconductor substrate 110 through the openings h of the passivation films 111 and 121 to form contact portions between the semiconductor substrate 110 and the element electrodes.

  As the passivation films 111 and 121, silicon nitride, silicon oxide, aluminum oxide, or a laminate thereof can be used as described above, and different films are used on the light receiving surface 100A side and the back surface 100B side. Also good. The surface of the back surface dotted electrode 122D, which is an element electrode on the back surface 100B side, is directly connected to the main body portion 132 made of metal foil, and the back surface dotted electrode 122D and the main body portion 132 made of metal foil are electrically connected. While the semiconductor substrate 110 and the main body 132 of the electrical connection body are electrically connected via the back surface dotted electrode 122D portion, the semiconductor substrate 110 and the main body portion 132 of the electrical connection body are otherwise connected to each other. There is no direct contact. 7 that the main body portion 132 is shown separated from the semiconductor substrate 110 in FIG. 7, but is not necessarily insulated from the semiconductor substrate by the passivation film. Therefore, the main body portion 132 and the passivation film are not necessarily provided. It is not necessary to be spatially separated from the surface, and a part or the whole of the main body part 132 of the electrical connection body may be in contact with or bonded to the passivation film. As the first electrical connection body 141 as described above, when the inter-element connection portion 131 made of metal foil is made of a material mainly composed of copper, for example, a tin-silver solder containing about 3.5% silver is used. A conductive material can be used. The main body 132 made of metal foil may have wrinkles or slack as shown in FIG. As shown in FIG. 9, one end of the inter-element connection portion 131 of the inter-element connection body 130 is covered by the first electric connection body 141 and the other end is covered by the second electric connection body 133. 5 and 6, the description is omitted. In FIG. 1, the light receiving surface bus electrode 113 on the light receiving surface 100 </ b> A side is also covered with the first electrical connection body 141 formed on the surface of the inter-element connection portion 131 by modularization.

  5 and 6 are schematic views of a cross section of the string shown in FIG. 5 and FIG. 6 cut along a line segment of CD which is a direction orthogonal to the light-receiving surface bus electrode 113 through the inter-element connection portion 131 of the inter-element connection body 130. As shown in FIG. The CD section shows a section passing through the light-receiving surface bus electrode 113. On the light-receiving surface 100A side of the solar cell 100 in FIG. 8, the inter-element connection portion 131 of the inter-element connection body 130 is connected to the light-receiving surface electrode 115 by the first electric connection body 141. It is connected to the electrode 113. On the back surface 100B side of the solar cell 100, the inter-element connection portion 131 is connected to the main body portion 132 made of metal foil by the second electric connection body 133, and the main body portion 132, the second electric connection body 133, and the back surface point-like electrode 122D. It is electrically connected to the semiconductor substrate 110 through. Note that the electrical connection body does not necessarily have to be used for the connection between the main body portion 132 and the second electrical connection body 133. For example, the connection may be performed by a laser welding method or a spot welding method. However, only the local heat is applied to portions other than the semiconductor substrate, and the semiconductor substrate itself does not reach a high temperature, so that the electrical characteristics of the semiconductor substrate do not deteriorate.

  FIG. 9 shows a schematic cross-sectional view corresponding to the line segment part E-F in FIGS. 5 and 6, which is a cross-section passing through the light-receiving surface bus electrode 113 of the string shown in FIGS. 5 and 6. In the EF cross section, on the light receiving surface 100A side of the solar cell 100, the light receiving surface bus electrode 113 and one end of the inter-element connection portion 131 are connected by the first electrical connection body 141, and the other end of the inter-element connection portion 131 is adjacent. On the back surface 100B side of the solar cell 100, the body portion 132 made of metal foil and the second electrical connection body 133 are connected, and the elements are electrically connected. The main body 132 is directly connected to the back surface dotted electrode 122D on the back surface side. In the present embodiment, the first electrical connection body 141 and the second electrical connection body 133 are made of the same solder material.

  In the first embodiment, as shown in FIG. 10 (a), which is an enlarged cross-sectional view of a main part of the portion Q in FIG. 8, a back surface dotted electrode 122D made of a cured product of molten aluminum, and aluminum in the semiconductor substrate 110. A high-concentration p-type doped layer 123 formed by diffusion is formed. Here, the back surface dotted electrode 122D includes a region made of an aluminum silicon alloy, and constitutes a good contact with the semiconductor substrate 110. An aluminum oxide film 122S formed by being oxidized in the atmosphere exists on the outer surface of the aluminum layer constituting the back surface dotted electrode 122D, but the aluminum oxide film 122S is formed inside the back surface dotted electrode 122D. However, since it is formed only on the surface of the back surface point-like electrode 122D, it is easy to solder or braze by removing the oxide film on the surface.

  The first electrical connection body 141 and the second electrical connection body 133 may be formed between the back electrode 125 and the body portion 132 of the inter-element connection body, but in the first embodiment, the first electrical connection body 141 or the second electrical connection body 133 is not provided, and the back surface dotted electrode 122D and the main body 132 are directly connected.

  When the main body 132 made of metal foil is not connected to the element electrode by the first electrical connection body 141 as described above, the main body made of metal foil is formed by connecting the metal foil to the semiconductor substrate 110 with wrinkles. There is an advantage that warpage of the solar cell 100 due to a difference in thermal expansion coefficient between the n-type single crystal silicon substrate 132 and the n-type single crystal silicon substrate 110 can be suppressed. In addition, the main body part 132 made of metal foil and the inter-element connection part 131 are not necessarily configured separately, and the inter-element connection part 131 and the main body part 132 are formed of a single metal foil, An inter-element connection body may be configured.

  In the present embodiment, lead containing tin and silver is used as the same solder material for the first electrical connection body 141 and the second electrical connection body 133, and aluminum is used as the metal foil constituting the body portion 132 of the inter-element connection body. Although the molten aluminum hardened | cured material is used as foil and back surface dotted electrode 122D, it reacts with the semiconductor substrate 110 surface in a baking process, and becomes a silicon aluminum alloy in fact. In the solar cell 100 of the first embodiment, a silicon aluminum alloy containing about 12% silicon is formed.

  In the present embodiment, the same solder material is used for the first electrical connection body 141 and the second electrical connection body 133, but different solder materials or brazing materials may be used. In using such different materials, it is preferable that the melting point of the later soldering is lower than the melting point of the first soldering or brazing.

  As a connection method, for example, the semiconductor substrate 110 having an opening in the passivation film is preliminarily immersed in an aluminum melting bath to form the back surface dotted electrode 122D, and then the silver paste is shown in FIG. 1 on the light receiving surface 100A side by screen printing. The electrode is formed on the light receiving surface. Hereinafter, an aluminum foil is used as the metal foil constituting the main body 132. Then, with the main body portion 132 made of aluminum foil placed on the back side of the semiconductor substrate 110, heating is performed at 700 ° C. to 900 ° C., the screen-printed electrode is baked, and the back surface dotted electrode 122D is melted to form the aluminum foil. The main body part 132 made of aluminum foil and the inter-element connection part 131 are simultaneously connected via the second electric connection body 133 with the second electric connection body 133 made of aluminum and silicon. To do. In this embodiment, the main body 132 and the back electrode 125 of the element are connected by fusion bonding, but soldering may be used. In this case, it may be difficult to connect the back surface point-like electrode 122D of each point to the inter-element connector 132. In this case, the shape of the element electrode is linear as in the second embodiment. It is preferable that the inter-element connector can be connected to at least a part of each line. Thereafter, the second electrical connector 133 and the light-receiving surface bus electrode 113 can be connected with a solder having a relatively low melting point by using aluminum flux and solder.

  As a combination of the second electrical connection body 133 and the first electrical connection body 141, for example, a conductive material such as an alloy of aluminum and silver and lead containing tin and silver can be used. In these soldering, different fluxes may be used. Note that a resin such as ethylene vinyl acetate or silicone may be filled between the semiconductor substrate 110 of the solar cell 100 and the main body portion 132 made of metal foil.

  In general, when the surface of the semiconductor substrate as well as the interface between the electrode and the semiconductor is exposed to the atmosphere or the module sealing material, recombination loss occurs through defects present on the semiconductor surface. Covering with a passivation film that lowers the coupling speed is necessary to improve photoelectric conversion efficiency. Therefore, in order to increase the photoelectric conversion efficiency, it is generally performed to reduce the electrode area on the back surface side of the solar cell and cover the other region with a passivation film. The conventional solar cell has a structure in which electrodes are widely distributed over the entire semiconductor substrate surface not only on the light receiving surface side but also on the back surface side.

  The structure in which the electrodes are widely distributed throughout the surface of the semiconductor substrate as described above ensures the conductivity in the in-plane direction of the solar cell and reduces the recombination loss due to the contact portion between the electrode and the semiconductor substrate as much as possible. Therefore, it is necessary to separate the electrodes at a certain interval so as not to cover the entire substrate. Since the semiconductor substrate used for solar cells has a conductivity lower than that of metal, for example, by about 5 digits, in the above structure, in the part where there is no electrode, in addition to the thickness of the element until current flows through the element and reaches the electrode part Thus, the resistance of the semiconductor substrate itself up to the element electrode is added, and the resistance loss in the substrate increases. Therefore, it is preferable to reduce the distance between the device electrodes from the viewpoint of reducing the resistance. On the other hand, since the recombination rate of the carrier is large at the part where the semiconductor and the metal are in contact, the contact area between the device electrode and the semiconductor is preferably small. Therefore, in terms of the recombination rate in the semiconductor, the back surface side is also preferable. It is preferable that the distance between the element electrodes is narrow. In order to maximize the photoelectric conversion efficiency from the viewpoint of the resistance and the recombination speed, it is necessary to adjust the element electrode interval and the contact area between the electrode and the semiconductor so as to have appropriate values.

  Effective recombination velocity at or near the interface between the semiconductor substrate and the electrode by doping the contact area between the semiconductor and the electrode metal at a higher concentration than the semiconductor surface on the back electrode side where the metal is not in contact And the contact resistance between the electrode and the semiconductor substrate can be reduced. For this reason, by using aluminum which is a good conductor as an electrode while acting as a p-type dopant inside the semiconductor, recombination can be suppressed by making the inside of the semiconductor adjacent to the electrode p-type in a self-aligned manner. . Therefore, there is an advantage that the photoelectric conversion efficiency of the solar cell can be improved.

  However, since the surface of aluminum is easily oxidized, there is a problem that soldering is not easy. In particular, since the aluminum electrode formed by conventional screen printing and firing bakes fine agglomerates of aluminum particles by firing, the aluminum particles 122P can be taken in any cross section as shown in FIG. The aluminum oxide film 122S appeared on the surface with a period of the grain size, and soldering and brazing were very difficult.

  The cross section can be seen in Non-Patent Document 1, for example, and the aluminum particle shape is about several μm to 20 μm. Therefore, it has been difficult to use molten plating for connecting molten metal to metal for electrical connection. On the other hand, in the present embodiment, each back electrode electrically connected to the semiconductor substrate has at least an aluminum continuous body portion larger than the opening h of the passivation film 121. Therefore, FIG. As shown in the enlarged cross-sectional view of the main part, in the aluminum continuum portion, the aluminum oxide film 122S is formed only on the outer surface of the aluminum layer constituting the back surface dotted electrode 122D and is not formed inside the electrode. By removing the film, the structure can be easily soldered or brazed. For this reason, it is possible to connect the aluminum electrode and the metal foil or the inter-element connection body by using soldering or brazing which requires the wettability between the materials of the connection point, up to the inter-element connection body on the back side. This has the advantage that aluminum can be used for current collection. In conventional solar cells, even if aluminum is used as the electrode material on the back side, it is necessary to use silver on the back side as a device electrode for soldering at least partly on the back side, which causes little oxidation even after firing. There is. For this reason, silver has been the cause of increasing recombination of photogenerated carriers at the interface with the semiconductor. On the other hand, the solar cell of Embodiment 1 does not require the use of a silver electrode that has been indispensable for soldering the conventional inter-element connection lines, so that the solar cell is excellent in resource saving, cost, and photoelectric conversion efficiency. Can be obtained. Further, according to the method for manufacturing the solar cell of the first embodiment, since aluminum acting as a dopant can be used for brazing with a metal foil, the silver electrode forming step and the soldering step between the element electrode and the foil are omitted. As a result, the manufacturing process of the solar cell can be simplified, and resource saving and cost reduction can be achieved.

  The main body portion 132 made of a metal foil has a function of lowering the current collecting resistance in the element surface and reflects the light transmitted through the element without being absorbed because the light absorption rate of the semiconductor is low. It has a function of increasing the photoelectric conversion efficiency of the solar cell by increasing the light utilization efficiency by making light incident again. For example, when silicon having a thickness of about 200 μm is a semiconductor substrate of a solar cell, a part of light having a wavelength of 900 nm to 1300 nm is not completely absorbed and passes through the element. On the other hand, it is known that when copper or aluminum is used as the metal foil, light having a wavelength of 900 nm or more and 1300 nm or less is reflected, and the photoelectric conversion efficiency of the solar cell and thus the power generation output is improved.

(Production method)
Next, an example of a method for manufacturing solar cell 100 of Embodiment 1 will be described. FIG. 11A to FIG. 11I are process cross-sectional views schematically showing the method for manufacturing the solar cell of the first embodiment, and FIGS. 12 and 13 are flowcharts showing the process.

  First, an n-type single crystal silicon substrate 101 having a thickness of several hundred μm is prepared as a starting material for the semiconductor substrate 110, and the substrate is cleaned in step S10. Since the p-type single crystal silicon substrate is manufactured by slicing an ingot formed by cooling and solidifying molten silicon with a wire saw, damage at the time of slicing remains on the surface. Therefore, a damaged layer removing step is performed. The damaged layer removing step is a step of immersing the n-type single crystal silicon substrate in a sodium hydroxide (NaOH) aqueous solution at a temperature of about 70 ° C. to 90 ° C. for several minutes or tens of minutes to etch the surface of the silicon substrate. It is. The concentration of the sodium hydroxide aqueous solution is desirably 1 wt% or more and 50 wt% or less. The damaged layer removing step includes an aqueous solution of sodium hydroxide, an aqueous solution of potassium hydroxide (KOH), an aqueous solution of sodium hydroxide and an aqueous solution of potassium hydroxide, or an etching solution such as a mixture of hydrofluoric acid and nitric acid. May be used.

  In the etching step, the surface of the substrate is etched by about 5 μm to 20 μm in order to remove the mechanically altered layer and dirt on the surface of the n-type single crystal silicon substrate generated when slicing the single crystal ingot.

After the etching, a texture forming process is performed in step S20. The texture forming step is a step of forming an uneven portion called a texture structure on the surface of the n-type single crystal silicon substrate 101. The texture structure is a light confinement technique using multiple reflections of incident light, and is performed in order to improve the performance of the solar cell. In order to obtain the texture structure, a chemical removal method such as a wet etching method, a groove processing method using a mechanical method, or a dry etching method is performed. As a method by wet etching, for example, a process in which 1 wt% to 30 wt% isopropyl alcohol is added to the same alkaline aqueous solution used in the damaged layer removing process, or a sodium carbonate (Na ₂ CO ₃ ) aqueous solution is used. There is. Further, a texture having a depth of 1 μm or more and 3 μm or less may be formed on the surface of the n-type single crystal silicon substrate by a dry etching process such as reactive ion etching (RIE).

  By the above method, an n-type single crystal silicon substrate 101 having a texture structure as shown in FIG. 11A can be obtained. Here, pyramidal uneven portions are uniformly formed. In the drawing, the size of the unevenness is enlarged in order to reveal the features.

Next, as shown in FIG. 11B, in step S30, the n-type single crystal silicon substrate 101 having a textured structure is placed in a high-temperature thermal oxidation furnace to diffuse phosphorus oxychloride (POCl ₃ ). Do. In this diffusion step, for example, an n-type single crystal silicon substrate is subjected to a heat treatment of an n-type single crystal silicon substrate that has been processed for 10 minutes to 60 minutes at a temperature of 750 ° C. to 900 ° C. by a vapor phase diffusion method in phosphorus oxychloride gas By forming the n-type diffusion layer 102 on the surface, an internal electric field region called a front surface field (FSF) is formed.

  Next, Step S40 is performed, and the boron glass mainly composed of glass generated when the n-type diffusion layer 102 is formed is removed with a hydrofluoric acid aqueous solution. Specifically, the n-type single crystal silicon substrate 101 is horizontally moved while being held at the liquid surface portion of the etching solution, and the hydrofluoric acid solution is brought into contact with the lower surface of the n-type single crystal silicon substrate 101, thereby Of the n-type diffusion layers 102 formed on both sides in the diffusion step, the n-type diffusion layer 102 on one side is removed. Boron glass is an impurity-containing film that is generated in a diffusion process and contains boron as an impurity. Step S40 is an impurity-containing film removal step.

Next, as shown in FIG. 11C, in step S50, using the single-sided etcher, one side of the back surface 110B side of the n-type single crystal silicon substrate 101 is wet-etched with a mixed solution of hydrofluoric acid and nitric acid. The n-type diffusion layer 102 of 110B is removed, and the diffusion layer on the back surface is removed. Specifically, the phosphorus substrate is formed on both sides in the diffusion step by horizontally moving while holding the silicon substrate on the liquid surface portion of the etching solution and bringing the hydrofluoric acid solution into contact with the lower surface of the semiconductor substrate. The n-type diffusion layer 102 on one surface of the n-type diffusion layer 102 made of is removed. Furthermore, in the present embodiment, silicon isotropically etched using fluorinated nitric acid, so that the back surface has no texture and becomes flat. For such etching, an alkaline solution may be used, or a plasma gas generated by high-frequency discharge in a low-pressure CF ₄ gas atmosphere or the like may be used.

  Further, as shown in FIG. 11D, in step S60, a heavily doped layer 103 is formed on the light receiving surface 100A side. Phosphorus ions are ion-implanted onto the silicon substrate through a mask opened in the same pattern as the light-receiving surface side electrode, so that another portion of the phosphorus-doped surface is formed on the surface of the n-type single crystal silicon substrate 101 immediately below the light-receiving surface-side electrode formation region. Thus, a heavily doped layer 103 which is a phosphorous doped region having a higher concentration than the above is formed. In this region, the concentration of phosphorus, which is an n-type impurity, is high on the surface, and the contact resistance with the light receiving surface side electrode can be reduced. Such selective high-concentration doping to the portion in contact with the electrode can be applied to the electrode formation region on the non-light-receiving surface side to improve the photoelectric conversion efficiency, and can be applied to the low-concentration region and the high-concentration region. Boron and aluminum may be used for each. In the present invention, ion implantation is used for doping of phosphorus. However, as the impurity doping method, vapor phase diffusion such as a method of heating in a phosphorus source atmosphere can be used.

Next, the surface of the n-type single crystal silicon substrate 101 is cleaned. Cleaning can be performed using hydrochloric acid / hydrogen peroxide (HPM), sulfuric acid / hydrogen peroxide (SPM), sulfuric acid (H ₂ SO ₄ ), nitric acid, hydrogen peroxide (H ₂ O ₂ ), ozone (O ₃ ) water, hydrofluoric acid, or these It can be carried out by a mixture of acids or the like, or a combination or repetition of these acids. Next, boron ions accelerated in an electric field by an ion implanter are implanted into the n-type single crystal silicon substrate 101 on the entire back surface 100B from which the phosphorus-doped layer has been removed by etching to form a p-type diffusion layer 102p by boron doping to form a solar cell. A pn junction is formed on the back side of the battery element. In this case, the solar cell element is a back-emitter type element having an emitter on the back side, but the present invention is not necessarily limited to the back-emitter type solar cell, and a solar cell having an aluminum electrode on the p-type silicon side. It can be applied to battery elements.

  In step S70, the substrate is heated to about 800 ° C. to 1100 ° C. in an oxygen atmosphere to form a silicon oxide film having a thickness of about 2 to 30 nm on the entire surface of the substrate.

  Thereafter, a passivation film forming step S80 is performed. Using a chemical vapor deposition (CVD) method, a silicon nitride film is provided on the light receiving surface 101A and the back surface 101B of the n-type single crystal silicon substrate 101 to perform passivation and to form an antireflection film. As a result, the cross section of the element has the structure shown in FIG. In the drawing, the silicon oxide film and the silicon nitride film formed at this time are combined to form passivation films 111 and 121. As the passivation film, a laminated film of alumina and a silicon nitride film or a single silicon nitride film may be used in accordance with the conductivity type of silicon. A sufficient passivation effect is obtained by the silicon oxide film on the light-receiving surface side, and the silicon nitride film functions as a passivation film but effectively functions as an antireflection film. On the other hand, on the back side, the silicon oxide film and the silicon nitride film act as a passivation film 121. In the description of FIGS. 1 to 10A and 10B, all of the outer layer film of the n-type diffusion layer 102 on the light receiving surface side and the outer layer film on the back surface side are the passivation films 111 and 121, respectively.

  Next, in step S90, the surface of the outer edge portion of the back surface 101B of the n-type single crystal silicon substrate 101 is removed to perform pn separation. As a method for removing the wafer surface, although not shown in the drawing, the outer edge of the silicon substrate is irradiated with a laser to locally remove the n-type diffusion layer 102, which is a phosphorus-doped layer, to have a width of 10 μm and a depth of 2 μm to 40 μm. A separation groove of a degree is formed. As described above, a portion without the p-type diffusion layer 102p is formed along the outer edge of the silicon substrate, and a high resistance is formed between the heavily doped layer 103 which is the n-type diffusion layer on the light-receiving surface side and the surface on the back surface side. Therefore, a short circuit between the light receiving surface side electrode and the back surface side electrode to be formed later can be prevented. As such pn separation, in addition to the laser, the substrate is wound around the doped layer formed on the semiconductor substrate side end surface or the light receiving surface by using sand blasting to the end surface of the semiconductor substrate or plasma etching using a corrosive gas. As such, it may be partially removed.

  In the electrode forming step, an electrode is formed. Hereinafter, the electrode forming step will be described in detail with reference to the flowchart of FIG.

  As shown in FIG. 11 (f), an opening h is formed by laser irradiation in the passivation film opening step S101 on the n-type single crystal silicon substrate 101 in which the pn junction is formed in the steps up to step 90, that is, the semiconductor substrate 110. To do.

  Using a laser having a wavelength that is absorbed by the n-type single crystal silicon substrate 101 or the passivation film 121, as shown in FIG. 11G, the portion of the passivation film for forming the back surface dotted electrode 122D is removed. By irradiating the portion where the back surface point-like electrode 122D is formed with a laser, an opening h having no passivation film is formed locally. The semiconductor substrate 110 in which the passivation film 121 is locally opened is immersed in a diluted hydrofluoric acid aqueous solution to remove the oxide film in the portion where the passivation film 121 is opened.

  As shown in FIG. 14, step S <b> 102 for immersing semiconductor substrate 110 in molten aluminum bath 200 is performed. In step S102, only one side of the back surface 110B side of the semiconductor substrate 110 is immersed in a metal melting tank 201 containing molten aluminum 202 heated to 570 ° C. or more and 700 ° C. or less in a nitrogen atmosphere. At this time, as shown in FIG. 11 (h), the n-type single crystal silicon substrate 101 reacts with aluminum in the opening h where the passivation film 121 is not locally present, and the aluminum silicon alloy serves as the back surface point electrode 122D. It is formed consistently. At this time or during subsequent electrode firing, the aluminum layer diffuses into the silicon of the semiconductor substrate 110, and as shown in the enlarged view of the main part in FIG. 10A, the aluminum silicon alloy and the high-concentration p-type are formed in the semiconductor substrate. A doped layer 123 is formed. At this time, the damaged area formed by the laser is removed. In addition, when the space | interval of the laser opening h of a back surface is narrow, between aluminum alloy electrodes formed in a back surface may connect, and it may not become a dot shape. Also, the aluminum alloy electrode may be formed into a film shape depending on the temperature of the molten aluminum and the contact time between the substrate and the bath, and may not be in the form of a dot.

  When the reaction between silicon and aluminum in the n-type single crystal silicon substrate 101 proceeds excessively, the amount of silicon that escapes from the semiconductor substrate is large, and the alloy portion of the semiconductor substrate becomes too large, the molten aluminum in the metal molten bath May be an aluminum silicon alloy to increase the silicon concentration.

  Further, in order to reduce thermal damage to the semiconductor substrate, an aluminum jet part 203 as shown in FIG. 15 may be formed, and the back surface dotted electrode 122D may be formed so as not to immerse the entire semiconductor substrate in molten aluminum. . In addition to a method of moving the semiconductor substrate while fixing a metal melt such as molten aluminum, a method of supplying the metal melt onto the semiconductor substrate may be used.

  Next, as shown in FIG. 11 (i), a silver paste containing a commercially available glass frit is screen-printed on the surface of the high-concentration doped layer 103 formed by phosphorous diffusion in the light-receiving surface electrode printing step S103. The light receiving surface grid electrode 112 on the light receiving surface 100A side is formed. When the difference in height between the respective back electrodes becomes a problem during printing of the light receiving surface grid electrode 112, the back electrode 125 may be polished in advance so that the height of the back electrode becomes uniform. .

  Thereafter, the surface removal step S104 of the back electrode is performed. The entire surface of the back surface spot-like electrode 122D is polished with a polishing material such as a flat file, and the height of each spot-like electrode is made uniform and the oxide film formed on the outermost surface is removed.

  And step S105 which mounts aluminum foil on an element is implemented. In the above step, the body part 132 of the inter-element connection body made of aluminum foil is mounted on the back surface 110B side of the semiconductor substrate 110 with the back surface 110B side of the semiconductor substrate 110 on which the paste pattern of the light receiving surface grid electrode 112 is formed facing upward. Put.

  And electrode baking and aluminum foil connection step S106 are implemented. In the above step, the solvent is evaporated by rapid heating (RTA: Rapid Thermal Annealing) method using a flash lamp or the like simultaneously on the semiconductor substrate 110, the printed electrode paste, the back surface point electrode 122D, and the main body portion 132 made of aluminum foil. In addition to burning, heating is further performed from about 700 ° C. to about 900 ° C. At this time, in the light-receiving surface electrode portion, the passivation film 111 on the light-receiving surface side is eroded by glass frit, and as shown in FIG. 11 (j), the electric current between the n-type single crystal silicon substrate 101 and the light-receiving surface grid electrode 112 is obtained. At the same time, connection between the back surface point-like electrode 122D and the main body 132 is achieved. At this time, the aluminum layer is further diffused into the silicon of the semiconductor substrate 110 from the back surface dotted electrode 122D.

  Moreover, although it depends on the silver paste on the light receiving surface 100A side, an inert gas atmosphere may be used as the high-temperature atmosphere during firing. Further, the electrode firing temperature for firing the light receiving surface electrode needs to be higher than the melting temperature of the alloy of aluminum and silicon so that the back surface dotted electrode 122D is melted. By connecting the back surface dotted electrode 122D of the solar cell 100 and the main body 132 made of aluminum foil in this manner, the solar cell 100 shown in FIG. 1 on the light receiving surface 100A side and in FIG. 4 on the back surface 100B side is manufactured. Can do.

  Next, an aluminum foil surface cutting step S107 is performed. Since the surface of the main body portion 132 formed of aluminum foil is oxidized when the light-receiving surface grid electrode 112 is fired, the portion connected to the inter-element connection portion 131 of the inter-element connection body is surfaced with an abrasive such as a file. The surface oxide film is removed.

  The aluminum foil constituting the main body 132 of the inter-element connector of the present embodiment has a thickness of 50 μm. If the thickness is several tens of μm or more, the aluminum foil of the main body 132 having a size of about 150 mm can be handled as a single unit without being cut by its own weight. On the other hand, in the case of a metal film with a thickness of several tens of nanometers to 1 μm or less that can be realistically formed by vapor deposition or the like, since the strength is not sufficient, it is easily cut by its own weight, etc. There is a problem that an oxide film that can be formed cannot be mechanically removed. Moreover, since vapor deposition is a batch type process, there is a problem that it takes time and cost, but it is solved by using a foil.

  The thickness of the copper foil constituting the inter-element connection portion 131 of the inter-element connection body of the present embodiment is about 0.2 mm. Since the inter-element connection portion 131 of the inter-element connection body has a smaller area in the plane direction of the substrate than the main body portion 132, a thickness is required. On the other hand, if it is too thick, the silicon substrate is easily broken. On the surface of the inter-element connection portion 131, a first electrical connection body 141 is formed at one end and a second electrical connection body 133 is formed at the other end.

  The same material can be used for the first electrical connection body 141 and the second electrical connection body 133. In this embodiment, lead solder containing silver and tin is used. The semiconductor substrate 110 to which the main body part 132 made of aluminum foil is connected is placed on a hot plate and heated to a temperature at which the first electrical connection body 141 is melted, and the surface of the main body part 132 is applied while applying ultrasonic waves to the surface of the main body part 132. While destroying the oxide film, the part connecting the inter-element connection portion 131 of the inter-element connector is covered with solder. As for the removal of the oxide film, instead of applying ultrasonic waves to the surface of the main body 132, solder may be supplied while mechanically scraping the surface, or a flux containing halogen is partially applied to the aluminum foil. It can also be used.

  Finally, step S108 for soldering the inter-element connection body connected to the element and the inter-element connection portion 131 is performed. The portion of the main body portion 132 made of aluminum foil from which the oxide film has been removed and the flat copper wire covered with solder are heated and soldered to connect the main body portion 132 and the inter-element connection portion 131. By connecting another inter-element connection portion 131 to the light-receiving surface bus electrode 113 during the connection on the back surface side, the main body portion 132 and the light-receiving surface bus electrode 113 of the back-side inter-element connection body are simultaneously applied during heating. On the other hand, the inter-element connection part 131 can be connected.

  By repeating this between adjacent elements, a string in which the solar cells 100 shown in FIGS. 5 and 6 are connected in series can be created. Note that different metals may be used for the first electrical connection body 141 and the second electrical connection body 133.

  In this way, the solar cell string is completed. The solar cell string is sealed with a glass substrate and a resin sheet to complete a solar cell module.

  The solar cell formed by the above method is a lump of continuous aluminum or aluminum alloy film having a back electrode cured with molten aluminum and having no oxide film inside, and a high-density aluminum diffusion region immediately below the back electrode. Since the p-type doped layer has a concentration, the contact resistance is low, recombination at the back electrode portion is reduced, soldering is possible, and a solar cell excellent in productivity can be obtained.

Embodiment 2. FIG.
FIG. 16 is a plan view showing the shape of the element electrode on the back surface side of the solar cell according to the second embodiment of the present invention, and shows a single element excluding the metal foil on the element back surface. The solar cell 100S of the second embodiment is different from the solar cell of the first embodiment in the shape of the back surface element electrode. As shown in FIG. 16, instead of the back surface dotted electrode 122D, the back surface side element electrode The pattern is a back surface linear electrode 122L composed of a large number of thin lines.

  Note that, when the number of island-shaped electrodes on the back surface point-like electrode 122D is large, not all the island-shaped electrodes and the main body portion 132 made of aluminum foil may be connected to each other. Since the linear electrode 122L is configured, connection with the inter-element connector is facilitated. It is necessary to adjust the shape, size, interval, and height of the back surface linear electrode 122L in accordance with the main body 132.

Embodiment 3 FIG.
FIG. 17 is a plan view showing the shape of the element electrode on the back surface side of the solar cell according to the third embodiment of the present invention, and shows a single element excluding the metal foil on the element back surface. Solar cell 100P in the third embodiment is different from the solar cell in the first and second embodiments in the shape of the back surface element electrode, and instead of back surface dotted electrode 122D, as shown in FIG. The element electrode pattern is a back surface grid electrode 122G composed of a large number of thin lines.

  When the number of island-shaped electrodes on the back surface dotted electrode 122D is large, not all the island-shaped electrodes and the main body portion 132 made of aluminum foil may be connected to each other. However, in the configuration of the third embodiment, the back surface Since the grid electrode 122G is configured, the connection with the inter-element connector is facilitated. It is necessary to adjust the shape, size, interval, and height of the back surface grid electrode 122G according to the main body 132.

  A portion corresponding to the inter-element connection on the back side has a structure in which no element electrode is provided on the semiconductor substrate 110. The solar cell of Embodiment 1 and each component are the above-mentioned in that the light-receiving surface electrode 115 and the back electrode 125, which are element electrodes formed on the solar cell having the passivation films 111 and 121 formed on both surfaces, are formed. This is the same as the device structure.

  In the above embodiment, a method for forming an aluminum electrode on a solar cell using an n-type single crystal silicon substrate as a semiconductor substrate has been described. However, the method is not limited to silicon, but other than silicon, and thus other than solar cells. The electrode formation method of Embodiment 1 can also be applied to a semiconductor device or the like.

  In the above description, the case of using a single crystal silicon substrate has been described. However, the present invention can also be applied to a solar cell using a semiconductor substrate such as a polycrystalline silicon substrate.

  Further, any combination of the impurity diffusion layers on the light receiving surface and the back surface inside the semiconductor substrate and the semiconductor conductivity type of the semiconductor substrate itself may be used. Since the aluminum doped layer needs to be formed on the p-type surface, the electrode pattern on the surface without the metal foil needs to be connected as shown in FIG. . For example, an n-type substrate may be used to make the boron doped layer side, which is the emitter side, the light-receiving surface side, but in that case, the entire front and back of the string of Embodiment 1 is turned upside down. Specifically, for the device electrode, the side with the phosphorus doped layer is replaced with that of FIG. 2, and the boron doped surface side is replaced with that of FIG. 1, and the electrode formed of molten aluminum becomes the light receiving surface side. In this case, it is possible to use molten aluminum at least on the light receiving surface bus electrode 113 and to connect to the inter-element connector with solder. The main body 132 of the inter-element connection body may be omitted. In some cases, the light receiving surface bus electrodes 113 are connected to each light receiving surface bus electrode so as not to interrupt the light receiving surface of the solar cell. When the body part 132 of the inter-element connection body is not provided, the light-receiving surface bus electrode 113 is covered with a coating material such as tin and then connected to the inter-element connection part 131 of the inter-element connection body by a connection material such as solder. Other parts may be the same as those in the first and second embodiments. Further, in the case of an element having an emitter on the light receiving surface side with a p-type substrate, the configuration other than that may be the same as in the first and second embodiments only by changing the conductivity type in the diffusion layer.

  According to the above method, since the aluminum melt can be reacted with silicon only at the opening of the passivation film, the back electrode of the solar cell can be formed without alignment.

  In addition, since the aluminum melt is brought into contact with only one surface, an extra aluminum layer is not formed on the light receiving surface side, so that there is an advantage that the effective light receiving area of the solar cell is not reduced.

  In addition, since the metal aluminum can be disposed on the back electrode and the entire back surface, it is possible to maintain a high reflectance as compared with the conventional printed electrode containing a fired organic material component. This has the advantage that the power generation output can be increased by increasing the efficiency.

  In addition, by forming the back electrode of the solar cell in a self-aligned manner without performing alignment, the area on which the device electrode on the back surface is formed can be reduced. The amount of absorbed light can be reduced, reflected by the main body 132 made of metal foil, and incident again on the semiconductor substrate from the back side of the solar cell, which can contribute to power generation and improve photoelectric conversion efficiency. It has the advantage of being able to.

  In addition, according to the manufacturing method of the present application, aluminum acting as a dopant can be used for connection to the main body portion made of metal foil, and therefore, by omitting the silver electrode forming step for soldering the inter-element connection body, This has the advantage that the manufacturing process can be simplified.

  Further, when the element electrode is made of only aluminum, it is difficult to connect the inter-element connection body because soldering to aluminum is difficult. In particular, since an aluminum electrode formed by conventional screen printing is baked by firing a mass of fine aluminum particles, an aluminum oxide film appears on the surface of any cross section, and soldering is difficult. On the other hand, in the method according to the embodiment, aluminum oxide can be prevented from being included in the electrode, and aluminum can be used for connection to the semiconductor substrate. This has the advantage that the aluminum electrode and the inter-element connector can be connected by using soldering or brazing. In addition, since it takes a long time to increase the thickness of the deposited aluminum film, the film thickness is reduced, resulting in a large electric resistance. When the film thickness is 100 m or less, oxidation due to moisture in the atmosphere, etc. As a result, there is a problem that all of the oxide film becomes an aluminum oxide film. Further, since the amount of electrode aluminum formed is small, there is also a problem that the amount of aluminum diffused is small and the FSF in the electrode portion is not sufficiently formed.

  The manufacturing method of the solar cell of Embodiment 1 suppresses the reduction of current collecting resistance and the recombination of carriers, which are the problems of the conventional solar cell, by forming the back electrode using molten aluminum. However, the two points of improving the photoelectric conversion efficiency can be solved from the viewpoint of the back electrode. Conventionally, aluminum having a particle size smaller than that of a mesh opening for screen printing is often used for forming an element electrode, and aluminum oxide or resin is formed on the surface of aluminum particles by firing together with a binder such as resin. Residue is formed. For this reason, the solderability is poor due to the presence of an aluminum oxide film or resin residue. Even when aluminum is used as an element electrode, it is difficult to connect a plurality of elements to the aluminum electrode because it is difficult to solder, and silver that is not easily oxidized by baking is used as an element electrode together with the aluminum electrode. Needed to be used.

  Further, when the aluminum electrode erodes the passivation film and contacts the semiconductor substrate during firing, it is difficult to satisfy both the erosion of the passivation film and the action of aluminum as a dopant. In other words, in the baking process, the passivation film is eroded, and the aluminum obtained as a contact with the semiconductor layer is often not sufficiently formed with an aluminum doping layer in silicon. It was difficult to reduce recombination at the interface between the silicon substrate and the silicon substrate.

  Due to these factors, the conventional solar cell element has a problem that the lifetime power generation amount of the solar cell is reduced.

  The method of Patent Document 1 has a problem that the reactivity between the metal and the silicon substrate differs depending on the laser irradiation point, and there is a portion where local thermal damage remains in the semiconductor substrate and the metal foil. On the other hand, in the method of the embodiment, since aluminum dissolves the silicon in the damaged region after laser irradiation, the damaged region can be removed, and an electrode having excellent interface characteristics and low contact resistance can be formed. Further, by heating the entire semiconductor substrate with the RTA and connecting the inter-element connector, the entire metal foil constituting the main body is also heated, so that local distortion in the vicinity of the opening is unlikely to occur.

  In the structure of Patent Document 2, there is a problem that the contact resistance between the back-side element electrode and the metal foil cannot be stably kept low. In the case of using a conductive adhesive, pressure is required to establish conduction, and there is a possibility that peeling of the element electrode, reduction in photoelectric conversion efficiency, and damage to the element may occur.

  The configuration described in the above embodiment shows an example of the contents of the present invention, and can be combined with another known technique, and can be combined with other configurations without departing from the gist of the present invention. It is also possible to omit or change the part.

  100, 100S solar cell, 101 n-type single crystal silicon substrate, 110 semiconductor substrate, 111 passivation film, 112 light-receiving surface grid electrode, 113 light-receiving surface bus electrode, 115 light-receiving surface electrode, 121 passivation film, 122D back surface point electrode, 122L Back surface linear electrode, 122G Back surface grid electrode, 123 High-concentration p-type doped layer, 125 Back surface electrode, 141 First electrical connection body, 130 Inter-element connection body, 131 Inter-element connection section, 132 Main body section, 133 Second electrical connection Body, 136 inter-string connecting body, 137 wire at the end of the string, 200 molten aluminum bath, 201 metal melting tank, 202 molten aluminum, 203 aluminum jet, h opening.

Claims (10)

coating the surface of the light receiving surface side and the back surface side of the semiconductor substrate having a pn junction with a passivation film;
Forming an opening in the passivation film on the back side;
A step of bringing aluminum or an aluminum alloy into contact with the opening to form a back electrode made of an aluminum-containing film;
A step of printing a film for forming a light receiving surface electrode containing glass frit distributed over the entire light receiving surface on the light receiving surface;
Heating the semiconductor substrate on which the light-receiving surface electrode forming film is formed, firing the light-receiving surface electrode containing glass frit through the passivation film;
Heating the metal foil covering the back surface of the back electrode in a contacted state, diffusing aluminum from the back electrode directly under the back electrode;
A method for producing a solar cell, comprising:   The method for manufacturing a solar cell according to claim 1, wherein the firing step and the diffusing step are performed in the same heat treatment step.   3. The solar cell according to claim 1, wherein the step of forming the back electrode is a step of bringing a molten aluminum or aluminum alloy into contact with the opening to form a back electrode made of an aluminum-containing film. Manufacturing method.   The step of forming the back electrode is a step of forming the back electrode distributed over the entire back surface of the semiconductor substrate, and the back electrode is a continuum of aluminum that is at least larger than the minimum width of the opening of the passivation film. The method for manufacturing a solar cell according to any one of claims 1 to 3, wherein:   The method for manufacturing a solar cell according to claim 1, further comprising a step of forming a solder layer on the back electrode after the step of forming the back electrode.   The method for manufacturing a solar cell according to claim 3, wherein the metal foil is an aluminum foil.   7. The step of forming the back electrode includes a step of bringing a back surface of the semiconductor substrate into contact with a molten metal tank filled with a molten metal containing aluminum. The manufacturing method of the solar cell of description. After the step of forming the back electrode, removing the oxide film formed on the metal foil surface;
The method for manufacturing a solar cell according to claim 3, further comprising a step of connecting an inter-element connector to the back electrode from which the oxide film has been removed via a solder layer.   9. The method for manufacturing a solar cell according to claim 8, wherein the step of removing the oxide film on the surface of the metal foil is a step of mechanically removing the oxide film.   The method for manufacturing a solar cell according to claim 1, wherein the step of forming the opening is a step of removing the passivation film by laser irradiation.

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