JP4601318B2 - Method for manufacturing solar cell element - Google Patents

Description

  The present invention relates to a method for manufacturing a solar cell element, and more particularly to a method for manufacturing a solar cell element in which an electrode material is baked to form a front electrode and a back electrode.

A conventional solar cell element is shown in FIG. A reverse conductivity type diffusion region 2 exhibiting an n type is formed by diffusing an n type impurity (for example, P) of a reverse conductivity type to a certain depth in the vicinity of the light receiving surface side surface of a p type semiconductor substrate 1 such as polycrystalline silicon. A pn junction is formed between the p-type semiconductor substrate 1 and the p-type semiconductor substrate 1. An antireflection film 3 made of a silicon nitride film or the like is provided on the reverse conductivity type diffusion region 2. A surface electrode 4 is provided on the front surface on the light receiving surface side, and a back electrode 5 is provided on the back surface. The back electrode 5 includes a current collecting electrode 5a made of aluminum or the like and an output extraction electrode 5b made of silver or the like. . In order to improve the characteristics of the solar cell element, a high concentration p ⁺ region 6 may be formed on the back surface of the semiconductor substrate 1 in some cases.

  The surface electrode 4 and the back electrode 5 of these solar cell elements can be obtained by applying and firing an electrode material mainly composed of metal.

  For example, according to the method described in Patent Document 1, the back electrode 5 is formed as follows.

(1) In order to form the collector electrode 5a, an electrode material mainly composed of aluminum or the like is applied to most of the semiconductor substrate 1 except for a part of the back surface and dried. (2) The output extraction electrode 5b is formed. To form the surface electrode 4, the electrode material mainly composed of silver or the like is applied and dried so as to cover the portion where the electrode material is not applied in (1) and the peripheral portion thereof (3). (4) The electrode material applied on the front and back surfaces is simultaneously fired to obtain the front electrode 4 and the back electrode 5. Is a method in which the front electrode 4 and the back electrode 5 are fired simultaneously (simultaneous firing method), but may be fired separately. For example, Patent Document 2 describes the method shown below. Yes.

(A) In order to form the output extraction electrode 5b, an electrode material mainly composed of silver or the like is applied to a part of the back surface of the semiconductor substrate 1 and dried. (B) In order to form the collector electrode 5a, In (A), the electrode material mainly composed of aluminum is applied to most of the semiconductor substrate 1 except for a part of the back surface so as to cover the portion where the electrode material is not applied and the peripheral portion thereof, and is dried (C ) Firing back surface electrode 5 (collecting electrode 5a and output extraction electrode 5b) by firing (first time) (D) In order to form surface electrode 4, the surface of semiconductor substrate 1 is mainly composed of silver or the like. (E) Firing (second time) the surface electrode 4 is fired. (C) firing (first time) is performed immediately after (A) and output first. The extraction electrode 5b may be fired.

By the method as described above, the front electrode 4 and the back electrode 5 having good solder wettability mainly composed of silver (the current collecting electrode 5a mainly composed of aluminum and the good solder wettability mainly composed of silver). An output extraction electrode 5b) is formed. At this time, the collector electrode 5a formed on substantially the entire back surface is mainly composed of aluminum that acts as a p-type impurity element with respect to the silicon semiconductor substrate 1, and therefore is not in contact with the collector electrode 5a. A high concentration p ⁺ region 6 is formed.

  Thereafter, if necessary, the front electrode 4 and the output extraction electrode 5b of the back electrode are covered with solder (not shown). And in order to connect adjacent solar cell elements in series or in parallel, these solder-coated portions are connected by inner leads (not shown) having a copper foil as a base.

  In addition, as described above, the electrode material is applied to the semiconductor substrate 1 and then fired. In general, a continuous firing furnace is used for this firing. By using a continuous firing furnace, a large number of semiconductor substrates 1 can be continuously fired, and productivity is extremely high.

As an example of such a continuous firing furnace, a belt-type continuous firing furnace will be described. In this belt-type continuous firing furnace, a semiconductor substrate is placed on a conveyor belt that penetrates the heating section of the firing furnace, and the conveyor belt is rotated endlessly, thereby passing through the heating section of the firing furnace. , Firing. FIG. 3 is a view for explaining the structure of a belt-type continuous firing furnace, wherein 8 is a conveying belt, 9 is an upper heater, 10 is a lower heater, 11 is a cover, and 12 is a roller. In a firing furnace for processing the semiconductor substrate 1, an upper heater 9 and a lower heater 10 are mounted in a cover 11 provided to insulate heat and safety, and to cut off the atmosphere from outside air. The upper heater 9 and the lower heater 10 are heaters such as far infrared and near infrared, and are composed of resistance coils, lamps, and the like. A conveyor belt 8 is provided through the furnace in the longitudinal direction of the firing furnace, and this conveyor belt 8 is circulated endlessly by the roller 12 being rotationally driven, and thus is placed on the conveyor belt 8. The semiconductor substrate 1 spends a predetermined time and passes through the baking furnace, and baking is performed during that time.
JP 10-335267 A Japanese Patent Laid-Open No. 10-144543

  When the electrode is formed by firing as described above, the semiconductor substrate constituting the solar cell element is heated to cause defects, re-diffusion of impurities contained in the reverse conductivity type diffusion region 2, or reverse conductivity type by the electrode material. The output characteristics of the solar cell element are likely to be lowered or the substrate is likely to warp due to penetration of the diffusion region 2 or the like. These problems can be suppressed to some extent by firing the electrode at the lowest possible temperature and in the shortest time as long as the reliability of the electrode can be ensured.

  However, in recent years, a semiconductor substrate for forming a solar cell element has been increasing in size, and generally has a size of about 10 cm × 10 cm to 15 cm × 15 cm. Furthermore, from the viewpoint of cost reduction, the thickness of the substrate has been reduced in order to reduce the amount of semiconductor material used. Such a thin substrate having a large size is very likely to warp when the electrode is formed by firing, and cannot be avoided by the above measures. Such an enlarged thin semiconductor substrate is vulnerable to impacts and stresses, and therefore easily cracks when warped, which poses a serious problem.

  Further, in order to increase productivity, when a continuous firing furnace as described above (for example, a belt-type continuous firing furnace) is used, the problem of warpage and the problem that the output characteristics of the solar cell element deteriorate are remarkable. Met.

  The present invention has been made in view of the problems of these prior arts, and suppresses damage given to the solar cell element due to heating at the time of firing the electrode, reduces warpage, and has good output characteristics. An object is to provide a manufacturing method.

  In view of the above-mentioned problems, the present inventors have conducted extensive studies and found the following facts.

  The above-mentioned continuous firing furnace (for example, belt-type continuous firing furnace) suddenly increases the temperature from the vicinity of the inlet, and after reaching 650 to 800 ° C. necessary for sintering of the electrode material, the stress due to rapid cooling is increased. The temperature is gradually lowered for the purpose of preventing the cracks caused by the phenomenon. Here, when a large-sized semiconductor substrate (10 cm × 10 cm to 15 cm × 15 cm or more) is baked in a continuous baking furnace having such a temperature profile, there is a difference in thermal history in the substrate for the following reasons. Arise.

  First, the front area is heated by radiation or the like to rise in temperature with respect to the traveling direction of the substrate, but the temperature rises with a slight delay in the rear area due to heat conduction inside the substrate. And with the progress of the substrate in the firing furnace, the rear region eventually reaches the heating region of the firing furnace and is heated, so the rear with respect to the traveling direction of the substrate is heated twice by heat conduction and radiation, There will be a difference in the thermal history.

  Due to such a difference in thermal history, for example, the portion behind the semiconductor substrate with respect to the substrate traveling direction has a large amount of applied heat, so that the reverse conductivity type diffusion region of the solar cell element is damaged or the surface The electrode material of the electrode may enter the reverse conductivity type diffusion region and adversely affect the pn junction. As a result, it is presumed that the output characteristics of the solar cell element deteriorate, the characteristics vary depending on the location, or the internal stress accumulates and warps.

  Note that if the temperature distribution in the firing furnace is gradually increased and the temperature is gradually raised, the difference in the thermal history of the semiconductor substrate can be suppressed, but the entire semiconductor substrate is exposed to a high temperature for a long period of time. This is not preferable because the damage to the area increases.

  Based on the above findings, the present inventors have repeatedly studied to arrive at the present invention described below.

That is, the method for manufacturing a solar cell element according to claim 1 of the present invention includes a one-conductivity-type semiconductor substrate having a reverse-conductivity-type diffusion region on the surface side, a surface electrode formed on the surface side of the semiconductor substrate, A method of manufacturing a solar cell element comprising a back electrode comprising a current collecting electrode and an output extraction electrode formed on the back side of the semiconductor substrate, the electrode material applied to the semiconductor substrate being baked to The firing step includes forming a front surface electrode and / or a back electrode, and the firing step includes a continuous firing furnace having a temperature distribution in which the temperature rises from the vicinity of the inlet and reaches the maximum temperature and then decreases. A first baking step in which the semiconductor substrate is disposed substantially horizontally and is fired by passing through a heating section in the baking furnace, and the first baking step is performed with the semiconductor substrate as an axis about a vertical direction. Baked in Includes a second firing step of firing by passing through the heating section of the fired furnace while rotating et approximately 180 °, the.

  Moreover, the manufacturing method of the solar cell element which concerns on Claim 2 of this invention is a manufacturing method of the solar cell element of Claim 1, The said collector electrode performs said 1st baking process and said 2nd baking process. Formed through.

  And the manufacturing method of the solar cell element which concerns on Claim 3 of this invention is a manufacturing method of the solar cell element of Claim 1 or Claim 2, The said semiconductor substrate is a p-type silicon semiconductor, The said collection | recovery The electric electrode is formed of an electrode material mainly composed of aluminum.

  Furthermore, the manufacturing method of the solar cell element which concerns on Claim 4 of this invention is a manufacturing method of the solar cell element as described in any one of Claims 1-3. Said 1st baking process and said 2nd The firing step is performed using a belt-type continuous firing furnace having a transport belt that rotates endlessly, and the semiconductor substrate is disposed on the transport belt.

In the method for manufacturing a solar cell element according to claim 5 of the present invention, after the second firing step, the semiconductor substrate is heated in the firing furnace in a state where the semiconductor substrate is turned upside down from the state fired in the first firing step. A third firing step of passing through the part and firing ;

Moreover, the method for manufacturing a solar cell element according to claim 6 of the present invention is the method for manufacturing a solar cell element according to claim 4 , wherein the belt-type continuous firing furnace has an upper portion sandwiching the conveyor belt. An upper heater and a lower heater are provided below.

However, the method for manufacturing a solar cell element according to claim 7 of the present invention is the method for manufacturing a solar cell element according to claim 6 , wherein the semiconductor substrate is more than the amount of heat received from the lower heater in the firing step. The amount of heat received from the upper heater was made larger.

  As described above, the present invention is formed on one surface of a semiconductor substrate having a reverse conductivity type diffusion region on the front surface side, a surface electrode formed on the front surface side of the semiconductor substrate, and on the back surface side of the semiconductor substrate. The present invention relates to a method for manufacturing a solar cell element comprising a back electrode composed of a current collecting electrode and an output extraction electrode.

  Here, the method includes a firing step of firing the electrode material applied to the semiconductor substrate to form the front electrode and / or the back electrode, and the firing step includes firing the semiconductor substrate disposed substantially horizontally. Since the semiconductor substrate includes one firing step and a second firing step in which the semiconductor substrate is fired in a state where the semiconductor substrate is rotated by approximately 180 ° from the orientation fired in the first firing step with the vertical direction as an axis. As a result, it is possible to prevent a large amount of heat from being concentrated on a part of the substrate, so that the internal stress accumulated in the substrate is suppressed to prevent warpage and the damage given to the reverse conductivity type diffusion region by firing the electrode is reduced. A solar cell element with good output characteristics can be obtained.

  In addition, instead of the second baking step, it may include a third baking step of baking in a state where the front and back are reversed from the state of baking in the first baking step, and similarly concentrated on a part of the semiconductor substrate. A large amount of heat can be prevented, the internal stress accumulated on the substrate is suppressed to prevent warpage, and the damage given to the reverse conductivity type diffusion region due to the firing of the electrode is reduced, and the solar with good output characteristics A battery element can be obtained.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a cross-sectional view showing a general structure of a solar cell element, FIG. 2 (a) is a top view showing a surface electrode of the solar cell element shown in FIG. 1, and FIG. 2 (b) is a back electrode. FIG. In the figure, 1 is a semiconductor substrate, 2 is a reverse conductivity type diffusion region, 3 is an antireflection film, 4 is a front electrode, 5 is a back electrode, 5a is a collector electrode, 5b is an output extraction electrode, and 6 is p ⁺ Indicates the area.

  First, the operation of the solar cell element shown in FIG. 1 will be briefly described.

  When light is incident from the side of the antireflection film 3 that is the light receiving surface side of the solar cell element, it is absorbed and photoelectrically converted in the bulk region of the semiconductor substrate 1 that is mainly a p-type semiconductor, and electron-hole pairs (electrons). Carriers and hole carriers) are generated. Photoelectron-generated electron carriers and hole carriers (photogenerated carriers) generate a photovoltaic force between the front electrode 4 provided on the front side of the solar cell element and the back electrode 5 provided on the back side. The antireflection film 3 serves to increase the amount of photogenerated carriers by reducing the reflectance in a desired light wavelength region depending on the refractive index and film thickness of the film to be the antireflection film, and the photocurrent density Jsc is improves.

Further, the collecting electrode 5a which is the back electrode 5 is usually formed using aluminum which acts as a p-type impurity element with respect to silicon which is a semiconductor substrate, and ap ⁺ region 6 is formed on the surface layer portion on the back side of the silicon substrate. Form. This region is also called a BSF (Back Surface Field) region, and serves to reduce the rate at which photogenerated electron carriers reach the back electrode 5 and recombine loss, thereby improving the photocurrent density Jsc. Further, since the minority carrier (electron) density is reduced in this p ⁺ region 6, it functions to reduce the amount of diode current (dark current amount) in this region and the region in contact with the back electrode 5, and the open circuit voltage Voc is improved. To do.

A manufacturing process of the solar cell element having the above-described structure will be described. The semiconductor substrate 1 is made of single crystal or polycrystalline silicon. When semiconductor silicon is used as the semiconductor substrate 1, it contains about 1 × 10 ¹⁶ to 10 ¹⁸ atoms / cm ^{3 of} semiconductor impurities having p-type conductivity such as boron (B), and has a specific resistance of 1.0 to 2. A substrate of about 0 Ω · cm is preferably used. A single crystal silicon substrate is formed by a pulling method or the like, and a polycrystalline silicon substrate is formed by a casting method or the like. Since a polycrystalline silicon substrate can be mass-produced and is more advantageous than a single crystal silicon substrate in terms of manufacturing cost, an example using polycrystalline silicon will be described here.

  The polycrystalline silicon ingot is formed by, for example, a casting method, sliced to a thickness of about 300 μm, and cut into a size of about 10 cm × 10 cm or 15 cm × 15 cm to form the semiconductor substrate 1. In order to clean the cut surface of the substrate, it is desirable that the surface be etched by a very small amount with hydrofluoric acid or hydrofluoric acid.

Next, the semiconductor substrate 1 is placed in a diffusion furnace and heat-treated in a gas containing an impurity element such as phosphorus oxychloride (POCl ₃ ) to diffuse phosphorus atoms in the outer surface portion of the semiconductor substrate 1. A reverse conductivity type diffusion region 2 having an n type conductivity type with a sheet resistance of about 30 to 300 Ω / □ is formed.

  Then, after removing the remaining portion of the reverse conductivity type diffusion region 2 on the surface side of the semiconductor substrate 1, which is the light receiving surface side of the solar cell element, the substrate is washed with pure water. As this removal method, for example, a resist film resistant to hydrofluoric acid is applied to the surface side of the semiconductor substrate 1, and a reverse conductivity type diffusion other than the surface side of the semiconductor substrate 1 is performed using a mixed solution of hydrofluoric acid and nitric acid. After removing the region by etching, the resist film may be removed.

Next, an antireflection film 3 is formed on the surface side of the semiconductor substrate 1. The antireflection film 3 is made of, for example, a silicon nitride film, and is formed by, for example, a plasma CVD method in which a mixed gas of silane (SiH ₄ ) and ammonia (NH ₃ ) is plasmatized by glow discharge decomposition and deposited. The antireflection film 3 is formed so as to have a refractive index of about 1.8 to 2.3 in consideration of a refractive index difference with the semiconductor substrate 1 and is formed to a thickness of about 500 to 1000 mm. . When the silicon nitride film is formed in the presence of hydrogen plasma as described above, it has a passivation effect for terminating the dangling bonds of silicon of the semiconductor substrate 1 with hydrogen at the same time. There is an effect of improving the electrical characteristics of the solar cell.

  Next, the back electrode 5 including the front electrode 4, the current collecting electrode 5a, and the output extraction electrode 5b is formed as follows.

  The collector electrode 5a constituting the back electrode 5 is made by adding 10 to 30 parts by weight and 0.1 to 5 parts by weight, for example, of 100 parts by weight of aluminum, metal, organic vehicle, and glass frit made of aluminum powder. The first electrode material made into a paste is used. As a specific shape, for example, as shown in FIG. 2B, it is almost the entire back surface excluding a portion where an output extraction electrode 5b described later is formed. As a coating method, a known method such as a screen printing method can be used, and after coating, the solvent is evaporated at a predetermined temperature and dried.

  The output extraction electrode 5b and the front electrode 4 constituting the back electrode 5 are made of a metal material having better solder wettability than the first electrode material, such as silver powder, organic vehicle, and glass frit, 10 parts by weight per 100 parts by weight of silver. A second electrode material made into a paste by adding -30 parts by weight or 0.1-5 parts by weight is used. As a specific shape, for example, as shown in FIGS. 2A and 2B, the surface electrode 4 is formed in a lattice shape, and the output extraction electrode 5b is mainly composed of the above-described aluminum. It is provided at a site where the current collecting electrode 5a is not applied, and the peripheral edge is formed so as to overlap the current collecting electrode 5a. As a coating method, a known method such as a screen printing method can be used, and after coating, the solvent is evaporated at a predetermined temperature and dried.

  The surface electrode 4 and the back electrode 5 coated and dried as described above can be formed by baking the electrodes on the substrate by performing a baking process of baking at 600 to 800 ° C. for about 10 to 30 minutes. .

As this firing step, when using the sintering furnace of the continuous expression, it is possible to continuously process a large amount of the semiconductor substrate 1, because the productivity is high desirable. Among continuous firing furnaces, the semiconductor substrate 1 is placed on a conveyor belt that penetrates the heating section of the firing furnace, and this conveyor belt is rotated endlessly to pass through the heating section of the firing furnace. The belt-type continuous firing furnace is desirable because it is easy to adjust and provides high reproducibility, allows the semiconductor substrate to pass smoothly through the firing furnace, and further allows heating from below the conveyor belt.

  FIG. 3 is a diagram for explaining the structure of the belt-type continuous firing furnace. In the figure, 8 is a conveyor belt, 9 (9a, 9b, 9c) is an upper heater, 10 (10a, 10b, 10c) is a lower heater, 11 is a cover, and 12 is a roller.

  The belt-type continuous firing furnace for processing the semiconductor substrate 1 includes a plurality of upper heaters 9 (9a, 9b, 9c) in a cover 11 provided for heat insulation and safety, and for shielding the atmosphere from outside air. The lower heater 10 (10a, 10b, 10c) is mounted. The upper heater 9 and the lower heater 10 are heaters such as far infrared and near infrared, and are composed of resistance coils, lamps, and the like. In FIG. 3, each of the upper heater 9 and the lower heater 10 is divided into three parts. However, the upper heater 9 and the lower heater 10 are not limited to this and may be more or less than this. However, in order to freely set the heating profile of the semiconductor substrate, it is desirable to divide into three or more.

  Furthermore, although both the upper heater 9 and the lower heater 10 are described in FIG. 3, the present invention is not limited to this, and a structure having only the upper heater 9 or only the lower heater 10 may be used. However, when the electrode material is applied to both surfaces of the semiconductor substrate 1, it is more desirable that both the upper heater 9 and the lower heater 10 are provided, because using them can perform firing simultaneously.

  A conveyor belt 8 is provided through the furnace in the longitudinal direction of the belt-type continuous firing furnace, and this conveyor belt 8 circulates when the roller 12 is driven to rotate, and is thus placed on the conveyor belt 8. The semiconductor substrate 1 passes through the firing furnace. The conveyor belt 8 is made of, for example, a metal mesh having heat resistance. In addition, a loader (not shown) for continuously disposing the semiconductor substrate 1 before firing on the transport belt 8 is provided on the entrance side of the transport belt 8, and firing is performed on the exit side of the transport belt 8. You may provide the unloader (not shown) which can pick up the subsequent semiconductor substrate 1 from the conveyance belt 8 continuously.

  When the electrode applied to the semiconductor substrate 1 is baked by such a belt-type continuous baking furnace, the temperature usually rises suddenly from the vicinity of the entrance, rises to the maximum temperature of 650 to 800 ° C., and reaches the maximum temperature. After that, the temperature distribution gradually decreases. This is to prevent cracking due to stress due to rapid cooling after the temperature required for sintering of the electrode material is first reached. Specifically, the upper heater 9a and the lower heater 10a on the inlet side of the belt-type continuous firing furnace are set to a temperature higher than the maximum temperature in consideration of the outflow of heat, and the upper heaters 9b and 9c and the lower heaters 10b and 10c are set. May be set such that the temperature gradually decreases as the semiconductor substrate 1 moves downstream.

[First embodiment]
Hereinafter, 1st embodiment of the baking process which concerns on the manufacturing method of the solar cell of this invention is described.

  In the firing process according to the first embodiment of the present invention, when the semiconductor substrate 1 is fired, the first firing process in which the semiconductor substrate 1 is disposed substantially horizontally and fired, and the semiconductor substrate 1 that has undergone the first firing process are arranged in the vertical direction. And a second firing step in which the shaft is fired in a state rotated by approximately 180 ° from the orientation fired in the first firing step. As a result, warpage is reduced and damage to the reverse conductivity type diffusion region 2 is reduced, so that a solar cell element with good output characteristics can be obtained.

  The reason is estimated as follows. In addition, the following description is performed by the example which performed the baking process which concerns on this invention using the belt type continuous baking furnace demonstrated in the above-mentioned FIG.

  As described above, the belt-type continuous firing furnace shown in FIG. 3 suddenly increases in temperature from the vicinity of the inlet, rises to a maximum temperature of 650 to 800 ° C., and then gradually decreases after reaching the maximum temperature. The temperature distribution is set. When the semiconductor substrate 1 (size of about 10 cm × 10 cm to 15 cm × 15 cm) used for the solar cell element is baked by such a belt type continuous firing furnace, the substrate is warped and the output characteristics of the solar cell element are reduced or varied. Will occur. The reason is considered as follows. First, with respect to the traveling direction of the substrate, first, the temperature of the front region that has reached the heating region in the belt-type continuous firing furnace rises, and due to the heat conduction inside the substrate, the temperature of the rear region also rises with a slight delay. As the substrate progresses in the belt-type continuous firing furnace, the rear region also reaches the heating region of the firing furnace, so there is a difference in the heat history between the front and the rear with respect to the traveling direction of the substrate. It may be warped by internal stress due to a difference in history, the reverse conductivity type diffusion region 2 may be damaged, or the electrode material of the surface electrode 4 may enter the reverse conductivity type diffusion region 2 to adversely affect the pn junction.

  Here, in the first embodiment of the present invention, when the semiconductor substrate 1 is fired as the firing step, the A side of the semiconductor substrate 1 shown in FIG. In the second firing step, the semiconductor substrate 1 is rotated by about 180 ° from the orientation fired in the first firing step with the vertical direction as an axis. The semiconductor substrate 1 shown in FIG. 2A is baked with the B side facing the belt traveling direction of the belt-type continuous firing furnace. Accordingly, a heat history different from the heat history applied to the semiconductor substrate 1 in the first firing step is applied in the second firing step, so that heat is uniformly applied in a wider region. Further, since an unnecessarily large amount of heat is not applied to a part, the internal stress is alleviated, damage to the reverse conductivity type diffusion region 2 is reduced, and the electrode material of the surface electrode 4 enters the reverse conductivity type diffusion region 2. Thus, it is considered that a solar cell element with good output characteristics can be obtained without destroying the pn junction.

  By the way, the surface electrode 4 is obtained by etching a portion corresponding to the surface electrode 4 of the antireflection film 3 in advance, applying a second electrode material (silver paste or the like) to the portion, and baking it. The second electrode material (silver paste or the like) is applied directly on the antireflection film 3 and fired, and the antireflection film 3 is penetrated by a so-called fire-through method. You may make it conduct | electrically_connect with the reverse conductivity type diffusion region 2. FIG.

  Also, the firing order and combination for forming the electrodes may be other than the methods described above. For example, the back electrode 5 (the collector electrode 5a and the output extraction electrode 5b) may be formed in the first baking process, the surface electrode 4 may be formed in the second baking process, or the output extraction electrode 5b in the first baking process. The collector electrode 5a and the surface electrode 4 may be formed in the second firing step, but the collector electrode 5a may be formed through the first firing step and the second firing step. desirable.

  The reason is as follows. First, the current collecting electrode 5a has a large area and a large amount of electrode material is used. Therefore, since it is greatly affected by the difference in thermal expansion coefficient between the semiconductor substrate 1 and the electrode material, the influence on the warp of the semiconductor substrate 1 is large. According to the conventional firing method, the thermal history for the semiconductor substrate 1 differs depending on the traveling direction of the conveyor belt 8 for the reasons described above. There is a tendency to. Here, the firing process according to the present invention makes it possible to make the thermal history of the substrate more uniform for the reasons described above, so that the substrate is evenly warped to both sides without largely warping to one side. As a result, since large warpage can be suppressed, cracking of the semiconductor substrate 1 due to warpage can be reduced.

[Second Embodiment]
Next, a second embodiment of the firing step according to the method for manufacturing a solar cell of the present invention will be described.

  In the firing step according to the second embodiment of the present invention, instead of the second firing step according to the first embodiment of the present invention, the third firing is performed in a state where the front and back are reversed from the state fired in the first firing step. And a process. Specifically, the front surface side of the semiconductor substrate 1 is fired toward the upper heater 9 in the first firing step, the semiconductor substrate 1 is turned upside down in the third firing step, and the back surface side of the semiconductor substrate 1 is turned to the upper heater 9. It suffices to perform firing. Further, the back side of the semiconductor substrate 1 is fired toward the upper heater 9 in the first firing step, and the semiconductor substrate 1 is turned upside down in the third firing step, and the front side of the semiconductor substrate 1 is fired toward the upper heater 9. You may do.

  In this way, by performing the front and back inversion from the state fired in the first firing step in the third firing step, it is possible to prevent a large amount of heat from being concentrated on a part of the semiconductor substrate 1, In addition to suppressing the internal stress accumulated in the substrate, the warpage can be prevented, and the damage given to the reverse conductivity type diffusion region 2 due to the firing of the electrode can be reduced, so that a solar cell element with good output characteristics can be obtained. In the second embodiment according to the present invention, since the front and back are reversed from the first firing step in the third firing step, each electrode can be heated from both sides of the semiconductor substrate 1 and the heat history is made uniform. It can be done.

  Further, it is desirable that the current collecting electrode 5a is formed through the first firing process and the third firing process. The reason is that the current collecting electrode 5a has a large area and a large amount of electrode material to be used. Therefore, the current collector electrode 5a is greatly affected by the difference in thermal expansion coefficient between the semiconductor substrate and the electrode material. This is because the influence is great. Here, by the firing process of the present invention, the semiconductor substrate 1 is fired in the first firing process in a state of being disposed substantially horizontally on the conveyor belt of the belt-type continuous firing furnace, and the front and back are reversed in the third firing process. By baking, the thermal history of the substrate can be made more uniform, so that the warp of the semiconductor substrate 1 can be reduced and cracks can also be reduced.

  Further, by firing the back side toward the upper heater 9 in the first firing step, a stable ohmic contact is obtained between the electrode material of the current collecting electrode 5a and the semiconductor substrate 1, and the semiconductor in the third firing step. By reversing the front and back of the substrate 1, it is possible to suppress the warpage of the semiconductor substrate 1 compared to the conventional case, because the back surface does not take more heat than necessary. The same applies even if the first firing step and the third firing step are reversed.

Furthermore, when dividing the first firing step and the third firing step separately,
(A) During the first firing step, the semiconductor substrate 1 is placed on the conveyor belt 8 in a state where nothing is present on the light receiving surface side, and firing is performed with the back side facing the upper heater 9 (b) ) The surface electrode 4 is applied and dried on the light receiving surface side of the semiconductor substrate 1. (C) The surface electrode 4 side of the semiconductor substrate 1 is baked toward the upper heater 9 in the third baking step. If this is done, contact between the surface electrode 4 and the conveyor belt 8 can be avoided, contamination on the light receiving surface side of the semiconductor substrate 1 can be prevented, and reduction in the amount of received light can be prevented.

[Common items of each embodiment]
Next, matters common to both the first embodiment and the second embodiment will be described.

When a p-type silicon semiconductor is used as the semiconductor substrate 1, it is desirable to use a material containing aluminum as a main component as the electrode material of the current collecting electrode 5a. The reason is as follows. First, the collector electrode 5a is formed, and at the same time, the p ⁺ region 6 containing a large amount of aluminum which is a p-type impurity with respect to silicon is formed. This region serves to reduce the rate at which photogenerated electron carriers reach the back electrode 5 and recombine loss, and the photocurrent density Jsc is improved. Further, since the minority carrier (electron) density is reduced in the p ⁺ region 6, it serves to reduce the amount of diode current (dark current amount) in this region and the region in contact with the back electrode 5, and the open circuit voltage Voc is reduced. This is because the effect of improvement is obtained and the effect on the improvement of the characteristics of the solar cell element is great.

  Moreover, it is desirable to use a belt-type continuous firing furnace having an upper heater 9 and a lower heater 10 on the upper side and the lower side, respectively, with the conveying belt 8 in between. This is because the front electrode 4 and the back electrode 5 of the semiconductor substrate 1 can be heated and fired simultaneously.

  Further, when a belt-type continuous firing furnace using such an upper heater 9 and lower heater 10 is used, the amount of heat received by the semiconductor substrate 1 in the firing process is greater when received from the upper heater 9 than the lower heater 10. It is desirable to make it. With such a configuration, the heating state from above and below becomes asymmetric, and the symmetry of the temperature distribution inside the semiconductor substrate 1 in the baking process can be broken. As a result, the first embodiment of the present invention (the second baking step is performed in a state where the semiconductor substrate 1 is rotated by approximately 180 ° from the direction of baking the first baking step), the second embodiment (the semiconductor substrate 1 is The third firing step is performed in a state where the front and back are reversed from the state fired in the first firing step), and the thermal history inside the substrate can be made more random and uniform. is there.

  Particularly in the second embodiment, since the front and back of the semiconductor substrate 1 are reversed in the third firing step, a step of firing toward the lower heater 10 that always receives less heat on the front side of the semiconductor substrate 1 is incorporated. . Therefore, damage to the reverse conductivity type diffusion region 2 can be reduced, and the electrode material of the surface electrode 4 enters the reverse conductivity type diffusion region 2 to further prevent the pn junction from being broken, and the output characteristics are improved. It is possible to obtain a good solar cell element.

  The lower heater 10 is opposed to the lower surface of the semiconductor substrate 1 (the surface in contact with the conveying belt 8) with the conveying belt 8 in between, and compared to the upper heater 9 that directly faces the upper surface of the semiconductor substrate 1. It is difficult to supply a high amount of heat to the semiconductor substrate 1. Of course, it is possible by increasing the electric power, but from the viewpoint of efficient use of energy, it is natural and more preferable to increase the amount of heat from the upper heater 9.

  As described above, when the belt-type continuous firing furnace using the upper heater 9 and the lower heater 10 is used, whether or not the amount of heat received by the semiconductor substrate 1 in the firing process is greater from the upper heater 9 than the lower heater 10 is. Can be confirmed as follows.

(A) A thermocouple is attached to a plurality of locations (symmetric locations on the front and back sides) of the front and back surfaces of the dummy semiconductor substrate. (B) The upper heater 9 and the lower heater 10 are set to predetermined temperature conditions. (C) (a) A dummy semiconductor substrate is placed on the conveyor belt 8 and sent at a predetermined belt speed. (D) The temperature of the front and back surfaces of the dummy semiconductor substrate is measured by a thermocouple to obtain a profile of temperature change with time (e ) The temperature change profiles obtained in (d) are integrated over time and compared on the front and back surfaces, and the amount of heat applied is compared (takes an average value for multiple locations).
It should be noted that the embodiment of the present invention is not limited to the above-described example, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

  For example, in the second embodiment of the present invention, after the first baking step, the front and back of the semiconductor substrate are reversed and the third baking step is performed. At this time, the second baking step in the first embodiment (the vertical direction is changed). The third baking step may be performed after rotating about 180 ° from the direction of baking in the first baking step as an axis. As a result, the effects of the present invention can be obtained more reliably.

  Further, the first firing step and the second firing step in the first embodiment or the first firing step and the third firing step in the second embodiment are processed using different firing furnaces, preferably belt type continuous firing furnaces. This is desirable because a large amount of semiconductor substrates can be processed with high productivity. In this case, a mechanism may be provided for rotating the semiconductor substrate after the first baking step by approximately 180 ° about the vertical direction (first embodiment) or reversing the front and back (second embodiment). desirable. For example, after a semiconductor substrate is collected into a cassette by an unloader, a mechanism is provided for rotating the cassette itself about 180 ° about the vertical direction (first embodiment) or reversing the front and back (second embodiment). Then, if this cassette is set in a loader of a firing furnace used in the second firing step (first embodiment) or the third firing step (second embodiment), the semiconductor substrate is supplied in a predetermined state. Good. Needless to say, the loader is previously structured so that the cassette can be set in a predetermined state.

  In the above description, the semiconductor substrate 1 has been described as an example in which the upper heater 9 receives a larger amount of heat than the lower heater 10. Usually, the lower heater 10 is opposed to the semiconductor substrate 1 with the conveying belt 8 interposed therebetween as compared with the upper heater 9. Therefore, it is easier to increase the amount of heat from the upper heater 9. Needless to say, if the method for manufacturing a solar cell element according to the present invention is used, the thermal history of the substrate can be made uniform, the warpage of the substrate can be suppressed, and damage to the reverse conductivity type diffusion region 2 can be reduced.

  Furthermore, in the description of each of the above-described embodiments, two firings such as a first firing step and a second firing step (first embodiment) or a first firing step and a third firing step (second embodiment) are performed as firing steps. Although the example provided with the steps has been described, a plurality of baking steps may be further provided before, after, or between these two baking steps.

  Moreover, in each baking process including the first baking process, the semiconductor substrate is disposed substantially horizontally, but it is not necessarily required to be strictly horizontal. For example, in the case of a belt-type continuous firing furnace, the conveyor belt may have an inclination to improve exhaust, but such an inclination is sufficient if the semiconductor substrate does not slide off the conveyor belt. You may arrange | position in the state which has.

  Further, in FIG. 2A, in order to represent the direction of the semiconductor substrate in the firing step, the direction of the thin electrodes of the surface electrode 4 in the form of a lattice and the direction of the sides indicated by the symbols A and B coincide with each other. Although illustrated, the present invention is not limited to this.

And in the above description has been described by way of examples using a belt type continuous firing furnace as baking furnace, is not limited to this example, for example, it can have use of the continuous firing furnace of the walking beam type, the present invention By introducing the configuration, the thermal history to the substrate can be made uniform, the warpage of the substrate can be suppressed, and damage to the reverse conductivity type diffusion region 2 can be reduced.

  The material of the semiconductor substrate may be a known compound semiconductor other than silicon. Further, the electrode material to be fired, the electrode shape to be applied, or the application method are not limited to the above-described examples, and any combination of materials and shapes that can exhibit the characteristics as a solar cell element can be used according to the configuration of the present invention. Needless to say, the advantages of the present invention can be obtained.

It is a schematic sectional drawing which shows the structure of a solar cell element. (A) is the top view which looked at the solar cell element of FIG. 1 from the surface side, (b) is the bottom view which looked at the solar cell element of FIG. 1 from the back surface side. It is a schematic structure figure of a belt type continuous firing furnace.

Explanation of symbols

1: Semiconductor substrate 2: Reverse conductivity type diffusion region 3: Antireflection film 4: Front electrode 5: Back electrode 5a: Current collecting electrode 5b: Output extraction electrode 6: p ⁺ region 8: Conveying belt 9: Upper heater 9, 9a, 9b, 9c: Upper heater 10, 10a, 10b, 10c: Lower heater 11: Cover 12: Roller

Claims (7)

A semiconductor substrate of one conductivity type having a reverse conductivity type diffusion region on the front surface side, a surface electrode formed on the surface side of the semiconductor substrate, a collector electrode and an output extraction electrode formed on the back surface side of the semiconductor substrate; A method for producing a solar cell element comprising a back electrode comprising:
Including a firing step of firing the electrode material applied to the semiconductor substrate to form the front electrode and / or the back electrode;
The firing step is performed using a continuous firing furnace having a temperature distribution in which the temperature rises from the vicinity of the inlet, reaches the maximum temperature, and then decreases.
A first firing step in which the semiconductor substrate is disposed substantially horizontally and is fired by passing through a heating unit in the firing furnace ;
A second firing step in which the semiconductor substrate is fired by passing through a heating section in the firing furnace in a state where the semiconductor substrate is rotated approximately 180 ° from the orientation fired in the first firing step with the vertical direction as an axis. The manufacturing method of the solar cell element containing.   The said current collection electrode is a manufacturing method of the solar cell element of Claim 1 formed through said 1st baking process and said 2nd baking process.   3. The method for manufacturing a solar cell element according to claim 1, wherein the semiconductor substrate is a p-type silicon semiconductor, and the collecting electrode is formed of an electrode material mainly composed of aluminum. The first firing step and the second firing step are performed using a belt-type continuous firing furnace having a transport belt that rotates endlessly as the firing furnace, and the semiconductor substrate is disposed on the transport belt. The method for manufacturing a solar cell element according to any one of claims 1 to 3, wherein: After the second baking step, the method further includes a third baking step of baking the semiconductor substrate by passing through the heating section in the baking furnace in a state where the semiconductor substrate is turned upside down from the state of baking in the first baking step. The manufacturing method of the solar cell element as described in any one of Claims 1-4. The said belt-type continuous baking furnace is a manufacturing method of the solar cell element of Claim 4 provided with the upper heater and the lower heater in the upper direction and the downward direction on both sides of the said belt for conveyance. The method for manufacturing a solar cell element according to claim 6 , wherein the semiconductor substrate receives a larger amount of heat from the upper heater than to receive from the lower heater in the firing step.

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