JP2012074669A - Manufacturing method of solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of an excellent solar cell which makes the characteristics less likely to be affected by eliminating a natural oxide film through a method which causes less impact on a surface of a semiconductor.SOLUTION: The invention relates to a manufacturing method of a solar cell which forms an anti-reflection film made of nitride on a surface of a semiconductor. The manufacturing method of the solar cell includes an etching process in which the surface of the semiconductor is etched with plasma using one or more kinds of gas, which are selected from fluorine, chlorine, fluorine compound, and chlorine compound, and a film formation process in which the anti-reflection film is formed on the surface of the semiconductor after the etching process.

Description

  The present invention relates to a method for manufacturing a solar cell in which a silicon nitride film is formed on the surface of a semiconductor.

  In order to efficiently absorb light incident on the solar cell, the light-receiving surface of a semiconductor generally constituting the solar cell is covered with an antireflection film.

  Conventionally, before forming such an antireflection film, the natural oxide film on the surface of the semiconductor has been removed to suppress the adverse effect of the solar cell characteristics due to the natural oxide film (for example, the following (See Patent Document 1).

JP 2010-56242 A

  However, when the natural oxide film formed on the semiconductor surface is physically removed by ion irradiation by low-frequency discharge using an inert gas or the like, the impact on the semiconductor surface due to ion irradiation occurs, causing damage to the semiconductor. It is possible to give.

  Accordingly, an object of the present invention is to provide an excellent method for manufacturing a solar cell with little influence on characteristics by removing a natural oxide film with a method having little impact on a semiconductor surface.

  A method for manufacturing a solar cell according to the present invention is a method for manufacturing a solar cell in which an antireflection film made of nitride is formed on the surface of a semiconductor, wherein the surface of the semiconductor is made of fluorine, chlorine, a fluorine compound, and a chlorine compound. And an etching process for etching with plasma using one or more gases selected from the above, and a film forming process for forming the antireflection film on the surface of the semiconductor after the etching process.

  According to the solar cell manufacturing method described above, the natural oxide film can be removed by a method with less impact on the semiconductor surface, and an excellent solar cell with little influence on the characteristics can be provided.

It is a figure which illustrates typically the solar cell obtained by the manufacturing method of the solar cell which concerns on this invention, (a) is a top view by the side of the light-receiving surface of a solar cell, (b) is a top view by the side of the back surface of a solar cell. It is. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates the manufacturing method of the solar cell which concerns on this invention, (a)-(e) is sectional drawing of a solar cell, respectively. It is the cross-sectional schematic diagram which showed the structural example of the in-line parallel plate type plasma CVD apparatus used for the manufacturing method of the solar cell which concerns on this invention. It is a top view which shows an example of the conveyance cart used for the in-line type plasma CVD apparatus used for the manufacturing method of the solar cell which concerns on this invention. It is a cross-sectional schematic diagram which shows an example of the in-line type CVD-sputtering apparatus used for the manufacturing method of the solar cell which concerns on this invention.

  Hereinafter, an embodiment of the present invention (hereinafter referred to as the present embodiment) will be described in detail with reference to the drawings.

<Solar cell>
As shown in FIG. 1, the solar cell 1 obtained in the present embodiment is a semiconductor substrate having a first surface 2a on the light incident side and a second surface 2b located on the back side of the first surface 2a. 2, a bus bar electrode 3 and a finger electrode 4 provided on the first surface 2 a of the substrate 2, and a collector electrode 5 and an output extraction electrode 6 provided on the second surface 2 b.

  The substrate 2 is made of a semiconductor substrate such as single crystal silicon or polycrystalline silicon, and has a rectangular flat plate shape with a side of, for example, about 150 to 160 mm and a thickness of about 150 to 250 μm in plan view. A junction (pn junction) between a p-type semiconductor layer and an n-type semiconductor layer, which will be described later, is formed inside the substrate 2.

  On the first surface 2a, about 2 to 4 wide bus bar electrodes 3 having a width of about 1 to 3 mm are provided. Further, a large number of finger electrodes 4 having a width of about 2 to 5 mm and a width of about 50 to 200 μm are provided on the first surface 2 a so as to intersect with the bus bar electrode 3 substantially perpendicularly. The thickness of the bus bar electrode 3 and the finger electrode 4 is about 10 to 20 μm. An antireflection film 8 is formed on the entire first surface 2a in order to improve light absorption.

  The electrode on the second surface 2 b side includes a collecting electrode 5 and an output extraction electrode 6. The collector electrode 5 is formed on substantially the entire surface on the second surface 2 b side excluding the outer peripheral edge of the substrate 2. The output extraction electrodes 6 have a width of about 2 mm to 5 mm, and are formed about 2 to 4 so that the collector electrode 5 and at least a part thereof are in contact with each other in the same direction as the bus bar electrode 3. The thickness of the output extraction electrode 6 is about 10 μm to 20 μm, and the thickness of the collecting electrode 5 is about 15 μm to 50 μm.

  The finger electrode 4 and the collector electrode 5 have a role of collecting the generated carriers, and the bus bar electrode 3 and the output extraction electrode 6 collect carriers (electric power) collected by the finger electrode 4 and the collector electrode 5. , Has the role of outputting to the outside.

  The solar cell 1 operates as follows. When light is incident from the first surface 2a side of the substrate 2, which is the light receiving surface side of the solar cell 1, absorption and photoelectric conversion are performed on the substrate 2 to generate electron-hole pairs (electron carriers and hole carriers). The electron carriers and hole carriers (photogenerated carriers) originating from the photoexcitation are collected by the above-described electrodes provided on the first surface 2a and the second surface 2b of the solar cell 1 by the above-described pn junction, A potential difference is generated between the electrodes.

<Method for manufacturing solar cell>
Next, an example of a method for manufacturing a solar cell according to the present invention will be described with reference to FIG.

  First, the board | substrate 2 which sliced the semiconductor block and gave the surface treatment is prepared. The substrate 2 is made of single conductivity type single crystal or polycrystalline silicon. By adding a small amount of impurities such as boron (B) to this silicon, a semiconductor substrate 2 having a p-type conductivity and a specific resistance of about 0.2 to 2.0 Ω · cm is obtained.

  The substrate 2 is produced by, for example, a pulling method such as the Czochralski method when a single crystal silicon wafer is used, and a block is produced by a casting method or the like when a polycrystalline silicon wafer is used.

  The block produced in this way is sliced to a thickness of 350 μm or less, more preferably about 150 to 250 μm using a wire saw or the like to obtain a wafer. The shape of the wafer is a circle, a square, or a rectangle. The size of the wafer is about 100 to 200 mm in diameter in the circle, and one side is about 100 to 200 mm in the square or rectangle.

  On the surface layer of the substrate 2 immediately after slicing, a damage layer due to slicing is formed to a thickness of about several μm to several tens μm, and fine contaminants at the time of slicing adhere to the surface.

  Therefore, the wafer is immersed in an alkaline aqueous solution such as a sodium hydroxide (NaOH) aqueous solution or a potassium hydroxide (KOH) aqueous solution in order to remove the damaged layer and clean the contaminants.

  In particular, when a wafer is used as a substrate of a solar cell, it is preferable to use a sodium hydroxide aqueous solution because the surface of the wafer can be roughened to a fine shape suitable for suppressing light reflection. In the immersion of the wafer in the alkaline aqueous solution, it is desirable to arrange the wafer in the cassette and immerse the entire cassette in order to increase the working efficiency.

  Thereafter, the wafer is cleaned, and the wafer is dried in a drier or a drying furnace to prepare the substrate 2 as shown in FIG. At this time, an uneven (roughened) structure having a light reflectance reduction function is formed on the first surface 2a side of the substrate 2 by using a dry etching method, a wet etching method, or an RIE (Reactive Ion Etching) apparatus. It is preferable to do this.

  Next, as shown in FIG. 2B, an n-type semiconductor layer 9 is formed on the entire surface of the substrate 2. Phosphorus (P) is preferably used as the n-type doping element, and an n-type semiconductor having a sheet resistance of about 30 to 150Ω / □ is used. As a result, a pn junction is formed between the n-type semiconductor layer 9 and the p-type semiconductor layer 10 which is a p-type bulk region.

The n-type semiconductor layer 9 is formed by, for example, processing for about 20 to 40 minutes in a phosphorus oxychloride (POCl ₃ ) atmosphere in a gas state as a diffusion source while keeping the substrate 2 heated to about 700 to 900 ° C. The n-type semiconductor layer 9 is formed to a depth of about 0.2 to 0.7 μm by a vapor phase thermal diffusion method or the like. Thereafter, in order to remove the phosphor glass layer formed on the surface, the substrate 2 is immersed in hydrofluoric acid and washed and dried.

  Thereafter, as shown in FIG. 2C, a part of the n-type semiconductor layer 9 formed on the outer peripheral portion of the end surface of the second surface 2b of the substrate 2 is removed, and the removed portion 7 generated by this removal Perform pn separation. Part of the n-type semiconductor layer 9 is removed by a sandblasting method in which particles of alumina or silicon oxide are sprayed onto the outer peripheral portion of the second surface 2b of the substrate 2 at a high pressure, or a YAG (yttrium, aluminum, garnet) laser. This is possible by forming a separation groove reaching the pn junction.

Next, as shown in FIG. 2D, an antireflection film 8 is formed on the first surface 2 a side of the substrate 2. In the formation of the antireflection film 8, a natural oxide film (having a thickness of about 0.5 to 2 nm) formed on the surface of the n-type semiconductor layer 9 on the first surface 2a side of the substrate 2 is made of fluorine, chlorine. An etching process for removing by plasma etching using one or more gases selected from fluorine compounds (for example, NF ₃ or SF ₆ ) and chlorine compounds (CF ₄ etc.), and an n-type after the etching process And a film forming step of forming an antireflection film 8 on the surface of the semiconductor layer 9.

  In order to describe the above process in detail, first, a plasma CVD apparatus and a transport cart used in the next process will be described with reference to FIGS.

  FIG. 3 shows a structural example of an inline parallel plate type plasma CVD (Chemical Vapor Deposition) apparatus 11. In FIG. 3, 12 is a transport cart for carrying the substrate 2, 13 is a load lock chamber, 14 is connected to the load lock chamber 13, a processing chamber for etching and film formation, and 15 is a processing chamber 14. The connected unload lock chamber, 16 is a transport mechanism for transporting the transport cart 12, 17 is an electrode plate provided in the processing chamber 14, 18 is a power supply means electrically connected to the electrode plate 17, 19 is a gas supply line including a mass flow controller connected to the processing chamber 14, 20 is a vacuum pump capable of evacuating the processing chamber 14, 21 is a ground connected to the processing chamber 14, 22 is a load lock chamber 13 and a processing chamber. The heater provided in the chamber 14, 23 is an entrance of the load lock chamber 13, between the load lock chamber 13 and the processing chamber 14, between the processing chamber 14 and the unload chamber 15, And shows a gate valve provided at the outlet of the unloading chamber 15.

  FIG. 4 shows an example of the transport cart 12 used for movement in the plasma CVD apparatus 11. In FIG. 4, 27 is a rectangular plate in plan view for arranging a large number of mounted objects such as the substrate 2, and 28 is provided so as to be positioned around the substrate 2 in order to accurately arrange the substrates 2. 4 is a plate fixing jig provided at four positions in the example of FIG. 4 to fix the plate 27 to the metal frame 29, and 31 is a transport cart. In order to fix 12 in an apparatus, the example of FIG. 4 shows the conveyance cart fixing jig provided in four places.

  First, the board | substrate 2 is mounted on the conveyance cart 12 so that the 1st surface 2a side may turn up. The transport cart 12 is fixed by a plate fixing jig 30 with a plate 27 fitted into a metal frame 29. The transport cart 12 is moved by a transport mechanism 16 in the plasma CVD apparatus 11 and is fixed by a transport cart fixing jig 31 at predetermined positions in the load lock chamber 13, the processing chamber 14, and the unload lock chamber 15. To be. The metal frame 29, the plate fixing jig 30, and the transport cart fixing jig 31 are made of stainless steel or the like.

  The substrate positioning pins 28 are provided in order to prevent the substrates 2 from moving due to vibration or the like during transportation to overlap each other, and the substrate 2 from falling from the placement plate 27. The substrate positioning pins 28 are implanted in the plate 27 so that the distance between them is slightly larger than the size of the substrate 2. Such a substrate positioning pin 28 is made of ceramics such as carbon or alumina, or stainless steel.

  The transfer cart 12 on which the substrate 2 is thus placed is carried into the load lock chamber 13 from the entrance side of the load lock chamber 13 by the transfer mechanism 16 of the plasma CVD apparatus 11. Here, the air is exhausted from the atmospheric state to a reduced pressure state, and the temperature is raised to a predetermined temperature (eg, about 400 to 500 ° C.) by the heater 22.

  After that, the transfer mechanism 16 opens the gate valve 23 between the load lock chamber 13 and the processing chamber 14, and the transfer cart 12 on which the substrate 2 is placed is transferred to the processing chamber 14. It is kept closed.

In the process chamber 14, with the substrate 2 and the transport cart 12 heated, first, as an etching process, one or more gases selected from fluorine, chlorine, a fluorine compound, and a chlorine compound are supplied from the gas supply line 19. Then, it is introduced into the processing chamber 14 while controlling the flow rate with a mass flow controller or the like. At this time, while introducing the gas, the power supply means 18 such as an RF power supply R
By applying F power, the introduced gas is excited into an active state, and plasma is generated between the electrode plate 17 and the transport cart 12 on the ground 21 side. Thus, the natural oxide film formed on the surface of the n-type semiconductor layer 9 on the first surface 2a side of the substrate 2 is etched and removed.

  Conditions such as the amount of gas, RF power output value, and time in plasma processing in this etching process are optimally determined as a result of measuring output characteristics such as output current, output voltage, and photoelectric conversion efficiency of the completed solar cell element. You just have to decide.

  Furthermore, it is desirable that the RF power supplied from the power supply means 18 in the etching process is a high frequency (especially microwave) of 1 MHz or more. When plasma is generated using a high frequency (especially microwaves) of 1 MHz or more, damage to the silicon wafer can be reduced, and removal of the natural oxide film damages the entire n-type semiconductor layer 9 and the substrate 2. Can be done without.

In addition, this etching process is optimal when it is performed by plasma using NF ₃ gas used for cleaning the inner wall of the chamber of the CVD apparatus, because the process can be simplified. That is, when CF ₄ gas or SF ₆ gas is used, a powdery product is generated, and this purified product adheres to the surface of the substrate 2 and the inner wall of the reaction chamber of the CVD apparatus and is removed. A removal step is required. However, by using NF ₃ gas, the generation of products can be suppressed, and such a product removal step becomes unnecessary.

In the etching step, when etching is performed with plasma using NF ₃ gas and Ar gas, the plasma is easily spread, and the etching can be performed uniformly on the semiconductor surface. Thereby, it is possible to further reduce damage to the n-type semiconductor layer 9 and the entire substrate 2 due to etching.

  Further, in the etching process, plasma is generated in another reaction chamber, and etching by remote plasma that guides the generated plasma to the processing chamber 14 is performed, so that the position of plasma generation is made another reaction chamber. Damage to the n-type semiconductor layer 9 and the entire substrate 2 can be reduced.

  After determining that the natural oxide film has been removed from the surface of the substrate 2, the supply of gas and RF power is stopped, and the gas in the processing chamber 14 is exhausted by the vacuum pump 20.

Next, as a film formation process after the etching process, an antireflection film 8 is formed on the first surface 2a as shown in FIG. 2D without being exposed to an oxidizing atmosphere such as air. Examples of the material of the antireflection film 8 include a SiNx film that is a nitride (the composition ratio (x has a width centered on Si ₃ N ₄ stoichiometry)), a TiO ₂ film, an SiO ₂ film, and an MgO film. An ITO film, a SnO ₂ film, a ZnO film, or the like can be used. The thickness is appropriately selected depending on the material so that low reflection conditions can be realized with respect to appropriate incident light. For example, in the case of the substrate 2, the refractive index may be about 1.8 to 2.3 and the thickness may be about 500 to 1200 mm.

For example, when a SiNx film is formed as the antireflection film 8, a predetermined amount of monosilane gas (SiH ₄ ) and ammonia gas (NH ₃ ) are supplied from the gas supply line 19, and RF power is applied from the power supply means 18. The introduced gas is excited into an active state, and plasma is generated between the electrode plate 17 and the transport cart 12 on the ground 21 side. Thereby, a thin film of a SiNx film is formed on the upper surface of the substrate 2 placed on the transport cart 12.

In this film formation step, it is desirable that the frequency of the RF power to be applied is a relatively low frequency of about 100 to 600 kHz because the passivation effect of hydrogen on the substrate 2 can be increased. For this reason, the power supply means 18 according to the present invention is preferably capable of outputting two types of frequencies, a frequency of 1 MHz or more and a frequency of about 100 to 600 kHz.

  Conditions such as the amount of gas, the output value of RF power, and the time in this film formation process are optimally determined while measuring the refractive index and film thickness of the formed reflective film 8 and the output characteristics of the completed solar cell element. That's fine.

  Further, it is desirable to perform the process from the end of the etching process to the start of the film formation process without being exposed to an oxidizing atmosphere such as air as described above in order to suppress the formation of a new oxide film. For example, it is desirable to perform the film forming process continuously in the chamber in which the etching process has been performed. Alternatively, it is desirable to move from the chamber in which the etching process is performed to the chamber in which the film forming process is performed in a vacuum state or in an inert gas atmosphere such as nitrogen.

  Thus, in the manufacturing method of the solar cell of this embodiment, the damage given to the board | substrate 2 can be reduced by removal of the natural oxide film in an etching process. In addition, since a film forming process is performed while maintaining a reduced pressure state from the etching process, generation of a new oxide film on the surface of the substrate 2 can be suppressed.

  Therefore, when an electrode is formed on the antireflection film 8 in the subsequent process, the ohmic contact between the substrate 2 and the electrode can be improved, and further, passivation to the substrate 2 by hydrogen is improved. It is possible to provide a solar cell with improved photoelectric conversion efficiency.

  Here, as a manufacturing method of the antireflection film 8, in addition to the above-mentioned plasma CVD method, it can be formed by using a vapor deposition method or a sputtering method. In particular, the anti-reflection film 8 can be firmly formed by performing the etching process of the previous process by the plasma CVD method and the film forming process of the subsequent process by the sputtering method, thereby improving the reliability of the completed solar cell. Can be increased. Next, this method will be described.

  FIG. 5 shows an example of an apparatus (hereinafter referred to as a CVD-sputter apparatus) that performs an etching process by a plasma CVD method and forms a film formation process by a sputtering method.

  In the in-line CVD-sputter apparatus 32 used in this case, a load lock chamber 33, an etching chamber 34, a film formation chamber 35, and an unload lock chamber 36 are connected in a line. And the gate valve (44a-44e) is provided in the entrance / exit of each chamber as shown in the figure, and when the gate valves 44a-44e are closed, airtightness is maintained for each chamber. The rooms are connected to each other. In addition, gas supply lines (39a to 39d) are connected to each chamber as shown in the figure. Each chamber is provided with a transport mechanism 42 as in the apparatus of FIG. The CVD-sputter device 32 is heat resistant, has sufficient strength to maintain a reduced pressure state, and is made of a material that is corrosion resistant to the gas used, for example, an iron alloy such as stainless steel. Have been made.

  In such a configuration, first, nitrogen gas or the like is supplied from the gas supply line 39a to bring the load lock chamber 33 to atmospheric pressure, and then the transfer cart 12 on which the substrate 2 is placed is moved by the transfer mechanism 42, and the inlet From the side, the gate valve 44a is opened, and it is carried into the load lock chamber 33. Thereafter, the gate valve 44 a is closed by the transport mechanism 42. In this state, supply of nitrogen gas or the like to the load lock chamber 33 is stopped, the load lock chamber 33 is exhausted to a vacuum state by the vacuum pump 40a, and the temperature is raised to a predetermined temperature by the heater 46a.

  Next, the transfer cart 12 is moved by the transfer mechanism 42, the gate valve 44b is opened, and the transfer cart 12 is transferred to the etching chamber 34, which is decompressed to a predetermined pressure by the vacuum pump 40b. Thereafter, the gate valve 44b is closed by the transport mechanism 42, and a predetermined temperature is maintained by the heater 46b. In this state, a predetermined amount of one or more gases selected from fluorine, chlorine, a fluorine compound, and a chlorine compound is supplied into the etching chamber 34 from a gas supply line 39b that communicates with the etching chamber 34. Further, a voltage is applied to the transport cart 12 by the power supply means 38a to generate plasma. That is, in the etching chamber 34, as shown in FIG. 5, the electrode plate 37 and the transport cart 12 serve as parallel plate electrodes, and plasma is generated in this gap. As a result, the surface of the substrate 2 is exposed to the generated plasma, and the natural oxide film on the surface of the substrate 2 is removed.

  Thereafter, while maintaining the reduced pressure state, the transfer mechanism 42 opens the gate valve 44c, the substrate 2 and the transfer cart 12 are transferred to the film forming chamber 35, and the gate valve 44c is maintained closed. Thereafter, a thin film to be the antireflection film 8 is formed on the surface of the substrate 2 by a sputtering method.

That is, the film formation chamber 35 is depressurized to a predetermined pressure by the vacuum pump 40b. The substrate 2 and the transport cart 12 are maintained at a predetermined temperature by the heater 46c. In this state, argon gas (Ar), ammonia gas (NH ₃ ), and nitrogen gas (N ₂ ) are supplied from the gas supply line 39c, and a voltage having a frequency of about 10 to 100 kHz is applied to the sputtering target 43 by the power supply means 38b. Applied. Thus, the antireflection film 8 is formed on the surface of the substrate 2 by ionizing argon gas or the like and causing the ion species to collide with the sputtering target 43 to fly the target material.

  Since the antireflection film 8 is formed by the sputtering method, it has low power consumption and less impurities than when the plasma CVD method is used. Therefore, the light transmittance is high, the density is high, and the uniformity is high. A film can be formed. In addition, it is not necessary to use difficult gas such as monosilane gas.

  After the antireflection film 8 is formed, the substrate 2 and the transfer cart 12 are carried to the unload lock chamber 36 by the transfer mechanism 42 in the same manner as described above. Thereafter, in the unload lock chamber 36, nitrogen gas or the like is supplied from the gas supply line 39d, the vacuum state is returned to the atmospheric state, the substrate 2 is unloaded from the gate valve 44e, and the substrate 2 is lowered to a predetermined temperature or lower. The substrate 2 is collected from the transport cart 12.

  Next, as shown in FIG. 2 (e), the collector electrode 5 is formed on the second surface 2 b side of the substrate 2. The collector electrode 5 is formed by applying a paste mainly composed of aluminum to substantially the entire second surface 2b except for the outer peripheral portion of about 1 to 5 mm on the second surface 2b. As the coating method, a screen printing method or the like can be used. The paste used for forming the collector electrode 5 is made of an aluminum powder and an organic vehicle. After applying the paste, heat treatment (firing) is performed at a temperature of about 700 to 850 ° C., and aluminum is baked onto the silicon wafer 2. By printing and baking this aluminum paste, aluminum, which is a p-type impurity, can be diffused at a high concentration in the coated portion of the substrate 2, and the n-type layer formed also on the second surface 2b side is p. Type heavily doped layer.

  Next, an electrode (bus bar electrode 3 and finger electrode 4) on the first surface 2a and an output extraction electrode 6 on the second surface are formed.

The output extraction electrode 6 on the second surface 2b is coated with a conductive paste mainly composed of silver so that the collector electrode 5 and a part thereof are in contact with each other. The conductive paste mainly composed of silver is, for example, a silver filler 1
A blend of 5 to 30 parts by weight of organic vehicle and 0.1 to 15 parts by weight of glass frit with respect to 00 parts by weight, kneaded and adjusted to a viscosity of about 50 to 200 Pa · s using a solvent. is there.

  As the coating method, a screen printing method or the like can be used, and it is preferable that the solvent is evaporated and dried at a predetermined temperature after coating.

  Next, electrodes (bus bar electrodes 3 and finger electrodes 4) on the first surface 2a of the substrate 2 are formed. Also in the formation of the bus bar electrode 3 and the finger electrode 4, as described above, the conductive paste mainly composed of silver is formed by coating, drying, and baking using a screen printing method or the like, thereby completing the solar cell 1. To do.

  In addition, the formation method of the antireflection film including the etching process and the film formation process according to the present embodiment is not limited to the above-described double-sided electrode type solar battery, but a solar battery using the substrate 2 made of a semiconductor. Then, for example, the present invention can also be applied to a back contact solar cell in which part or all of the light receiving surface side electrode is disposed on the second surface 2b side.

As described above, the natural oxide film on the semiconductor surface can be removed by a method with less impact on the semiconductor surface before forming the antireflection film 8, and this has excellent reliability with little influence on the characteristics. A solar cell can be provided.

  Hereinafter, examples in which the present embodiment is more specific will be described.

<Production of test product>
A polycrystalline silicon substrate prepared by casting and doped with boron was prepared as the substrate 2. The substrate 2 has a p-type with a specific resistance of about 0.5 to 1.5 Ω · cm, a size of about 156 mm square, and a thickness of about 200 μm.

  The substrate 2 was etched using a NaOH aqueous solution at a depth of several μm, and then washed and dried. Next, a concavo-convex surface was formed on the surface side of the substrate 2 serving as a light incident surface using an RIE apparatus.

Thereafter, an n-type semiconductor layer 9 having a sheet resistance of about 100 Ω / □ was formed on the surface of the substrate 2 by using phosphorus oxychloride (POCl ₃ ) by a thermal diffusion method. Thereafter, in order to remove the phosphorus glass on the surface, the substrate 2 was immersed in hydrofluoric acid and washed and dried.

  Thereafter, the n-type semiconductor layer 9 formed on the outer peripheral portion of the end surface of the second surface 2 b of the substrate 2 was removed, and pn separation was performed by the removal portion 7. The removal of the n-type semiconductor layer 9 was performed by a sandblasting method in which alumina particles were sprayed onto the outer peripheral portion of the end surface of the second surface 2b of the substrate 2 at a high pressure.

  Thereafter, a silicon nitride thin film was formed as the antireflection film 8 by using a parallel plate type plasma CVD apparatus 11 as shown in FIG.

The antireflection film 8 is formed by first removing the natural oxide film formed on the surface of the n-type semiconductor layer 9 on the first surface 2a side of the substrate 2 by plasma etching using NF ₃ gas. Was done. That is, the substrate 2 placed on the transfer cart 12 for the plasma CVD apparatus was carried into the load lock chamber 13 from the inlet side of the plasma CVD apparatus, and the temperature was raised to about 470 to 500 ° C. while reducing the pressure.

Thereafter, the transport cart 12 on which the substrate 2 was placed was transported to the processing chamber 14, and then NF ₃ gas and Ar gas were introduced from the gas supply line 19.

  While introducing these gases and maintaining the temperature, 13.56 MHz RF power was applied from the power supply means 18 for about 10 seconds. As a result, the introduced gas is excited into an active state, and plasma is generated between the electrode plate 17 and the transfer cart 2 on the earth 21 side, so that the surface of the n-type semiconductor layer 9 on the first surface 2a side of the substrate 2 is obtained. The natural oxide film formed on was etched.

Thereafter, the supply of gas and RF power was stopped, and the gas in the processing chamber 14 was exhausted by the vacuum pump 20. Thereafter, monosilane gas (SiH ₄ ) and ammonia gas (NH ₃ ) were continuously supplied from the gas supply line 19, and 400 kHz RF power was applied from the power supply means 18. As a result, the introduced gas is excited into an active state, and plasma is generated between the electrode plate 17 and the transfer cart 12 on the earth 21 side, and a refractive index of about 2.0 and a film thickness of about An 800 珪 素 silicon nitride (SiNx) thin film was formed.

  Next, a paste containing aluminum as a main component is applied to the second surface 2b side of the substrate 2 by screen printing, excluding the outer peripheral portion (width of about 2 mm) on the back surface, and then fired at a temperature of about 750 ° C. Then, aluminum was baked on the silicon wafer 2 to form the collector electrode 5.

  Thereafter, on the antireflection film 8 of the substrate 2, a conductive paste in which 10 parts by weight of silver powder and organic vehicle are added to 100 parts by weight of silver and 5 parts by weight of glass frit is added to 100 parts by weight of silver, Direct application using screen printing. And the bus-bar electrode 3 and the finger electrode 4 were formed by baking this about tens of seconds-several tens of minutes at the maximum temperature of 600-850 degreeC.

  Further, a paste mainly composed of silver was applied in a predetermined shape onto the collector electrode 5 on the second surface side of the substrate 2 by a screen printing method. afterwards. The output extraction electrode 6 was formed by baking this.

  Similarly, an electrode on the first surface 2a (front surface) side of the substrate 2 was formed. That is, the above-described paste containing silver as a main component was applied to a predetermined electrode shape by a screen printing method and baked at a maximum temperature of 600 to 850 ° C. for several tens of seconds to several tens of minutes to form an electrode.

<Production of comparison product>
A comparative product was produced using the same polycrystalline silicon substrate as the test product. The comparative product is the same as the test product except that there is no etching process for removing the natural oxide film by plasma etching using NF ₃ gas in the formation of the antireflection film 8. It was.

<About output characteristics of test and comparative products>
Each of 64 test products and comparative products was prepared and measured using a solar simulator under conditions of AM 1.5, 25 ° C., and light intensity of 1000 W / m ² , and the average was calculated.

  The results are shown in Table 1. In Table 1, each characteristic value of the test product is shown as an index when each measured value of the comparative product is 100.

  From this result, in the test product, damage to the substrate 2 was reduced when the natural oxide film before the antireflection film 8 was formed was removed. Furthermore, since an oxide film was not interposed at the interface between the substrate 2 and the antireflection film 8, the ohmic contact between the electrode and the surface of the substrate 2 and the hydrogen passivation to the substrate 2 were improved. As a result, values higher than the comparative product were confirmed in all the characteristic values of the short circuit current, the open circuit voltage, the fill factor, and the conversion efficiency.

1: solar cell 2: substrate 2a: first surface 2b: second surface 3: bus bar electrode 4: finger electrode 5: collector electrode 6: output extraction electrode 7: removal portion 8: antireflection film 9: n-type semiconductor layer 10 : P-type semiconductor layer 11: plasma CVD apparatus 12: transfer cart 13, 33: load lock chamber 14: treatment chamber 15, 36: unload lock chamber 16, 42: transfer mechanism 17: electrode plates 18, 38 a to 38 b: electric power Supply means 19: Gas supply line 20, 40a-40d: Vacuum pump 21, 41: Earth 22, 46a-40c: Heater 23, 44a-40e: Gate valve 27: Plate 28: Substrate positioning pin 29: Metal frame 30: Plate Fixing jig 31: Transport cart fixing jig 32: CVD-sputtering device 34: Etching chamber 35: Film forming chamber 43: Sputtering target

Claims (6)

A method for manufacturing a solar cell, wherein an antireflection film made of nitride is formed on the surface of a semiconductor,
An etching step of etching the surface of the semiconductor with plasma using one or more gases selected from fluorine, chlorine, a fluorine compound, and a chlorine compound;
And a film forming step of forming the antireflection film on the surface of the semiconductor after the etching step.   The method for manufacturing a solar cell according to claim 1, wherein the plasma is generated using a microwave of 1 MHz or more in the etching step. The method for manufacturing a solar cell according to claim 1, wherein the etching step is performed by the plasma using NF ₃ gas. The etching process, for producing a solar cell according to claim 3, characterized in that etching by the plasma using NF ₃ gas and Ar gas.   5. The method for manufacturing a solar cell according to claim 1, wherein both the etching step and the film forming step are performed by a plasma CVD method. 6.   The method for manufacturing a solar cell according to claim 1, wherein the etching step is performed using a plasma CVD method, and the film forming step is performed using a sputtering method.

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