JP2011524636A - Method for forming silicon solar cell - Google Patents

Abstract

  An aluminum paste containing magnesium oxide and / or a magnesium compound capable of forming magnesium oxide upon firing is provided on the back side of a silicon wafer provided with a silicon nitride antireflection coating on the front side and contaminated with silicon nitride on the back side. A method for producing a silicon solar cell, comprising: applying and baking, and baking the aluminum paste after the application.

Description

  The present invention relates to a method for forming a p-type aluminum back electrode of a silicon solar cell, that is, a method for forming a silicon solar cell.

  Conventional solar cell structures having a p-type base typically have a negative electrode on the front side ("sun" side) of the battery and a positive electrode on the back side. It is well known that radiation of the appropriate wavelength incident on a semiconductor pn junction functions as an external energy source that generates electron-hole pairs. The potential difference present at the pn junction moves holes and electrons across the junction in the opposite direction, thereby generating a current that can power the external circuit. Most solar cells are in the form of a metallized silicon wafer, i.e. a silicon wafer provided with conductive metal contacts.

  In forming a silicon solar cell, generally, an aluminum paste is screen-printed on the back side of a silicon wafer and dried. Subsequently, the wafer is baked to a temperature exceeding the melting point of aluminum to form an aluminum-silicon melt. Then, in the subsequent cooling step, an epitaxially grown layer of silicon doped with aluminum is formed. This layer is commonly referred to as a back surface field (BSF) and assists in improving the energy conversion efficiency of the solar cell.

  Most power generating solar cells currently used are silicon solar cells. The process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. The electrodes are made in particular from a metal paste by a method such as screen printing.

  An example of this manufacturing method will be described below with reference to FIG. FIG. 1A shows a p-type silicon substrate 10.

In FIG. 1B, the reverse conductivity type n-type diffusion layer 20 is formed by thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl ₃ ) is usually used as a gaseous phosphorus diffusion source, and other liquid sources are phosphoric acid and the like. If there is no particular change, the diffusion layer 20 is formed on the entire surface of the silicon substrate 10. The pn junction is formed at a portion where the concentration of the p-type dopant is equal to the concentration of the n-type dopant. A conventional battery having a pn junction close to the sun side has a junction depth between 0.05 and 0.5 μm.

  Once this diffusion layer is formed, excess surface glass is removed from the remaining surface by etching with an acid such as hydrofluoric acid.

  Next, as shown in FIG. 1D, an antireflection coating (ARC) 30 is formed on the n-type diffusion layer 20 on the front side of the silicon wafer, for example, sputtering or CVD (chemical vapor deposition), for example, A thickness of 0.05 to 1 μm is formed by a method such as LPCVD (low pressure CVD) or PECVD (plasma enhanced CVD).

  As shown in FIG. 1E, a front-side silver paste (front-electrode-forming silver paste) 500 for a front electrode is screen-printed on the antireflection coating 30 and then dried. Subsequently, the back side silver or silver / aluminum paste 70 and the aluminum paste 60 are screen printed (or printed by some other coating method) on the back side of the substrate and subsequently dried. Typically, the back side silver or silver / aluminum paste is first used as two parallel strips (busbars) or to solder an interconnect string (pre-soldered copper ribbon) The resulting rectangle (tab) is screen printed onto the silicon and then the aluminum paste is printed on the exposed surface with slight overlay on the backside silver or silver / aluminum. In some cases, the silver or silver / aluminum paste is printed after printing the aluminum paste. Subsequently, firing is usually carried out in a belt furnace for 1-5 minutes with the wafer reaching a peak temperature in the range of 700-900 ° C. The front electrode and the back electrode can be fired one after the other or can be co-fired.

  As a result, as shown in FIG. 1F, the molten aluminum from the paste dissolves the silicon during the firing process, and subsequently forms a eutectic layer that grows epitaxially from the silicon base 10 upon cooling, resulting in a high concentration of aluminum dopant. A p + layer 40 containing is formed. This layer is commonly referred to as the back surface field (BSF) layer and assists in improving the energy conversion efficiency of the solar cell. A thin layer of aluminum is generally present on the surface of this epitaxial layer.

  The aluminum paste is converted from the dry state 60 to the aluminum back electrode 61 by firing. The silver or silver / aluminum paste 70 on the back side is also fired simultaneously to form a silver or silver / aluminum back electrode 71. During firing, the boundary between the backside aluminum and the backside silver or silver / aluminum assumes an alloy state and is also electrically connected. The aluminum electrode occupies most of the area of the back electrode, and only partially meets the need for formation of the p + layer 40. Since soldering to an aluminum electrode is not possible, a silver or silver / aluminum back electrode is used as an electrode for interconnecting solar cells, such as with a pre-soldered copper ribbon, and several subregions on the back side Form on top (often as a 2-6 mm wide busbar). Furthermore, the silver paste 500 on the front side is sintered during firing and penetrates the antireflection coating 30, thereby allowing electrical contact with the n-type layer 20. This type of manufacturing method is generally referred to as “fire through”. This fire-through condition is manifested in layer 501 in FIG. 1F.

  As described above, a silicon solar cell usually includes an antireflection coating formed by a CVD method, particularly LPCVD. Current antireflection coatings for silicon solar cells are typically in the form of a silicon nitride (SiNx) layer. In the formation of this silicon nitride antireflection coating on the front side of the silicon wafer by CVD, if no special measures are taken in that a special back side cover means is used, there will also be some on the back side of the silicon wafer. It is inevitable that undesirable silicon nitride is deposited. Such visible overflow of silicon nitride takes the form of a seam extending along the back side edge of the silicon wafer and covers, for example, 5-20% of the back side area of the silicon wafer. When such back side contamination by silicon nitride occurs, compared to the case of silicon solar cells manufactured using the above special back side cover measures, that is, silicon solar cells without back side contamination by silicon nitride Not only is the adhesion obtained after firing between the backside of the silicon wafer and the aluminum backside electrode weakened, but the electrical performance (power generation) of the finished silicon solar cell is also impaired. Good adhesion obtained after firing between the back surface of the silicon wafer and the aluminum back electrode is important from the viewpoint of the long operational life of the silicon solar cell.

  An aluminum paste containing magnesium oxide suitable for manufacturing a back electrode of a silicon solar cell is known from Japanese Patent Application Laid-Open No. 2004-152827.

  The electrical performance of a silicon solar cell produced from a silicon wafer with a silicon nitride antireflection coating on the front side and contaminated with silicon nitride overflow on the back side is for manufacturing the back electrode of a silicon solar cell. As a result, it was found that the improvement can be achieved by using an aluminum paste containing a specific Mg-containing additive. Furthermore, the adhesion obtained after firing between the back surface of the silicon wafer and the aluminum back electrode can also be improved.

Accordingly, the present invention relates to a method for manufacturing a silicon solar cell including the following steps. That is,
(I) A silicon wafer having a p-type region, an n-type region, and a pn junction, wherein a silicon nitride antireflection coating is provided on the front side, and the back side is contaminated with silicon nitride. Applying an aluminum paste to the back side of
(Ii) firing the surface to which the aluminum paste has been applied, whereby the wafer reaches a peak temperature of 700-900 ° C .;
A manufacturing method comprising:
The aluminum paste is particulate aluminum, magnesium oxide, at least one Mg-containing additive selected from the group consisting of magnesium compounds capable of forming magnesium oxide upon firing in step (ii) and any combination thereof; Including an organic vehicle (organic medium) containing an organic solvent,
Is the method.

  The silicon solar cell obtained by the present invention has an electrical performance (compared to a silicon solar cell manufactured using an aluminum paste manufactured under the same conditions but not including the Mg-containing additive as described above. (Power generation amount) is increased. Furthermore, the adhesion between the aluminum back electrode included in the silicon solar cell and the back surface of the silicon wafer can also be improved. When producing an antireflection coating of silicon nitride on the front side of a silicon wafer, it is advantageous that no special back surface cover measures need be used.

The present invention also relates to a method for improving the electrical performance (power generation amount) of a silicon solar cell including the following steps. That is,
(I ′) silicon wafer having a p-type region, an n-type region, and a pn junction, wherein a silicon nitride antireflection coating is provided on the front side and the back side is contaminated with silicon nitride Preparing a wafer;
(I) applying an aluminum paste to the back side of the silicon wafer;
(Ii) firing the surface to which the aluminum paste has been applied, whereby the wafer reaches a peak temperature of 700-900 ° C .;
An improvement method including
The aluminum paste is particulate aluminum, magnesium oxide, at least one Mg-containing additive selected from the group consisting of magnesium compounds capable of forming magnesium oxide upon firing in step (ii) and any combination thereof; Including organic vehicles including organic solvents,
Is the method.

A silicon wafer as a starting point is shown as an illustrative example of a manufacturing process of a silicon solar cell including a silicon wafer provided with a silicon nitride antireflection coating. The 1st process of the said manufacture process is shown. The 2nd process of the said manufacture process is shown. 3rd process of the said manufacture process is shown. 4th process of the said manufacture process is shown. The 5th process of the said manufacture process is shown. The first step is shown as an illustration for explaining the production process of the silicon solar cell of the present invention. The 2nd process of the said manufacture process is shown. 3rd process of the said manufacture process is shown. 4th process of the said manufacture process is shown.

In step (i ′) of the method of the present invention, a silicon wafer having a p-type region, an n-type region, and a pn junction is provided with an antireflection coating of silicon nitride on the front side thereof, and the back side thereof Prepare a silicon wafer contaminated with silicon nitride on the side. Such silicon wafers and their manufacture are well known to those skilled in the art and need not be described repeatedly. In other words, please refer to [Background Art]. The silicon wafer can include monocrystalline or polycrystalline silicon and can have an area in the range of, for example, 100-250 cm ² and a thickness of, for example, 180-300 μm.

  In step (i) of the method of the present invention, a silicon wafer having a p-type region, an n-type region, and a pn junction is provided with a silicon nitride antireflection coating on its front side, and its back side is An aluminum paste is applied to the back side of the silicon wafer contaminated by the overflow of silicon nitride.

  The aluminum paste used in step (i) of the method of the present invention is at least one Mg selected from particulate aluminum and magnesium oxide and / or a magnesium compound capable of forming magnesium oxide upon firing in step (ii). It includes an additive additive, an organic vehicle, and in one embodiment, one or more glass frit compositions.

  Particulate aluminum can be composed of aluminum or an alloy of aluminum with one or more other metals such as, for example, zinc, tin, silver and magnesium. In the case of an aluminum alloy, the aluminum content is, for example, 99.7 to less than 100% by mass. The particulate aluminum can include variously shaped aluminum particles, such as aluminum flakes, spherical aluminum powder, agglomerated (amorphous) particle aluminum powder, or any combination thereof. In one embodiment, the particulate aluminum is in the form of aluminum powder. This aluminium powder has an average particle size of, for example, 4 to 10 μm. The particulate aluminum can be present in the aluminum paste in a proportion of 50-80% by weight, or in one embodiment 70-75% by weight, based on the total aluminum paste composition.

  In the present specification and claims, the term “average particle size” is used, and this means the average particle size (average particle size, d50) measured by the laser scattering method.

  Any statement relating to the average particle size in the present description and claims relates to the average particle size of the material present in the aluminum paste composition.

  The particulate aluminum present in the aluminum paste may be accompanied by other particulate metals such as, for example, silver or silver alloy powder. The ratio of such other particulate metals is, for example, 0 to 10% by mass based on particulate aluminum + particulate metal.

  The aluminum paste includes magnesium oxide and / or at least one Mg-containing additive selected from magnesium compounds that can form magnesium oxide upon firing in step (ii). The one or more Mg-containing additives may be 0.1 to 5% by weight, or in one embodiment 0.2 to 1% by weight of total magnesium, based on the total aluminum paste composition. It can exist in a total ratio corresponding to the contribution.

  In one embodiment, magnesium oxide is included in the aluminum paste as an Mg-containing additive. In another embodiment, it is included as the only Mg-containing additive. The magnesium oxide can have an average particle size in the range of, for example, 10 nm to 10 μm, or in one embodiment, 40 nm to 5 μm.

  When magnesium oxide itself is included, it should not be confused with magnesium oxide, which may constitute one or more glass frit components that may optionally be included in the aluminum paste.

  The magnesium compound capable of forming magnesium oxide upon firing in step (ii) can be a solid compound present in particulate form in the aluminum paste if it is insoluble in the organic vehicle of the aluminum paste. . In the latter case, it can have an average particle size of, for example, 10 nm to 10 μm, or in one embodiment 40 nm to 5 μm.

  Examples of magnesium compounds that can form magnesium oxide upon firing in step (ii) and can be included in the aluminum paste include certain thermally decomposable inorganic magnesium compounds, ie, the action of heat. Inorganic magnesium compounds which decompose under magnesium oxide and gaseous decomposition products are included. Examples of such thermally decomposable inorganic magnesium compounds include magnesium hydroxide, magnesium carbonate, and magnesium nitrate. Another example of a magnesium compound that can form magnesium oxide during firing in step (ii) and can be contained in the aluminum paste includes a magnesium organic compound. The term “magnesium organic compound” means a magnesium compound containing at least one organic moiety in the molecule. The magnesium organic compound is stable or essentially stable, for example, in the presence of atmospheric oxygen or air moisture, under conditions that occur primarily during the preparation, storage and application of aluminum paste. The same is true under application conditions, particularly those that occur primarily when screen printing aluminum paste on the silicon nitride contaminated back side of a silicon wafer. However, during firing of the aluminum paste, the organic portion of the magnesium organic compound will be removed or essentially removed, eg, burned and / or carbonized. The magnesium organic compound can be added as such or as a solution in an organic solvent when preparing the aluminum paste. In one embodiment, the magnesium organic compound comprises a magnesium organic salt compound. Examples of suitable magnesium organic salt compounds include in particular magnesium resinates (acidic resins, in particular magnesium salts of resins having a carboxyl group) and magnesium carboxylates (magnesium carboxylates). Examples of the latter are, for example, magnesium acetate, magnesium octoate, magnesium neodecanoate, magnesium oleate and magnesium stearate.

  In one embodiment, the aluminum paste includes at least one glass frit composition as an inorganic binder. This glass frit composition may contain PbO, but in one embodiment may be lead-free. The glass frit composition may include a composition that recrystallizes or phase separates upon firing to release the frit with a separated phase having a softening point lower than the original softening point.

  The (initial) softening point (measured by differential thermal analysis DTA at a glass transition temperature, heating rate of 10 K / min) of the glass frit composition can be in the range of 325-600 ° C.

  This glass frit has a particle size of, for example, 2 to 20 μm as an average particle size (average particle size) measured by a laser scattering method. In the case of an aluminum paste containing glass frit, the glass frit content is 0.01 to 5% by weight, or in one embodiment 0.1 to 2% by weight, based on the total aluminum paste composition, In the embodiment, the content may be 0.2 to 1.25% by mass.

Some glass frits useful in aluminum pastes are those conventionally used in the art. Some examples include borosilicate glasses and aluminosilicate glasses. Yet another example includes combinations of oxides such as B ₂ O ₃ , SiO ₂ , Al ₂ O ₃ , CdO, CaO, BaO, ZnO, Na ₂ O, Li ₂ O, PbO, and ZrO _2. . These can be used independently or in combination to form a glass binder.

  The conventional glass frit may be a borosilicate frit. For example, it may be a lead borosilicate frit or a borosilicate frit of bismuth, cadmium, barium, calcium or other alkaline earth metals. The preparation of such glass frit is well known, for example, the steps of melting together the glass components in the form of component oxides, and pouring the molten composition into water to introduce the frit. Forming a step. The batch feedstock can of course be any compound that produces the desired oxide under normal frit manufacturing conditions. For example, boron oxide is obtained from boric acid, silicon dioxide is produced from flint, barium oxide is produced from barium carbonate, and so on.

  The glass can be ball milled with water or a low viscosity and low boiling inert organic liquid to reduce the frit particle size and to obtain a substantially uniform size frit. Subsequently, it can be settled in water or its organic liquid to separate the particulates and remove the supernatant fluid containing the particulates. Other classification methods can be used as well.

  The glass is prepared by conventional glass making techniques by mixing the required components in the required ratio and heating the mixture to a melt. As is well known in the art, heating can be performed at peak temperatures and times such that the melt is completely liquid and homogeneous.

  The aluminum paste contains an organic vehicle. Many types of inert viscous materials can be used as the organic vehicle. The organic vehicle should be such that the particulate constituents (particulate aluminum, particulate and insoluble Mg-containing additive, in some cases glass frit) can be dispersed sufficiently stably. The properties of the organic vehicle, particularly the rheological properties, should be such that they provide excellent coating properties to the aluminum paste composition. The characteristics include the following. That is, stable dispersion of undissolved solids, appropriate viscosity and thixotropy for coating, especially screen printing, appropriate wettability to silicon wafer substrate and paste solids, good drying speed, and good firing characteristics. The organic vehicle used for the aluminum paste can be a non-aqueous inert liquid. The organic vehicle can be a single organic solvent or a mixture of organic solvents. In one embodiment, the organic vehicle can be a solution of an organic polymer in an organic solvent. The polymer used for this may be ethylcellulose in one embodiment. Other examples of polymers that can be used alone or in combination include ethyl hydroxyethyl cellulose, wood rosin, phenolic resins, and poly (meth) acrylates of lower alcohols. Examples of suitable organic solvents include ester alcohols and terpenes such as α or β terpineol, or other solvents such as kerosene, dibutyl phthalate, diethylene glycol butyl ether, diethylene glycol butyl ether acetate, hexylene glycol and high Mixtures with boiling alcohols are included. Further, the organic vehicle can include a volatile organic solvent for accelerating rapid curing after the aluminum paste is applied to the back side of the silicon wafer. Various combinations of these and other solvents can be formulated to obtain the required viscosity and volatility requirements.

  The content of the organic solvent in the aluminum paste can be in the range of 5 to 25% by mass, or in one embodiment in the range of 10 to 20% by mass, based on the total aluminum paste composition.

  The organic polymer can be present in the organic vehicle in a proportion within the range of 0-20% by weight, or in one embodiment 5-10% by weight, based on the total aluminum paste composition.

  The aluminum paste can include one or more organic additives such as, for example, surfactants, thickeners, rheology modifiers and stabilizers. The organic additive may be part of the organic vehicle, but it is also possible to add the organic additive separately when preparing the aluminum paste. The organic additive can be present in the aluminum paste in a total proportion of, for example, 0 to 10% by weight, based on the total aluminum paste composition.

  The content of the organic vehicle in the aluminum paste depends on the paste application method and the type of organic vehicle used and can be varied. In one embodiment, it can be 20-45% by weight based on the total aluminum paste composition, and in another embodiment it can be in the range of 22-35% by weight. . The number 20-45% by weight includes organic solvents, possible organic polymers, and possible organic additives.

In one embodiment, the aluminum paste is
70-75% by weight of particulate aluminum;
A total proportion of Mg-containing additive corresponding to a concentration contribution of 0.2 to 1% by weight of total magnesium;
0.2 to 1.25% by weight of one or more glass frits;
10-20% by weight of one or more organic solvents;
5-10% by weight of one or more organic polymers;
0 to 5% by weight of one or more organic additives;
including.

  The aluminum paste is a viscous composition and can be prepared by mechanically mixing particulate aluminum, an Mg-containing additive, and optionally added glass frit composition together with an organic vehicle. In one embodiment, power mixing as a manufacturing method, that is, an equivalent dispersion technique can be used for traditional roll milling. Roll milling or other mixing techniques can also be used.

  The aluminum paste can be used as such, or it can be diluted, for example by adding an additional organic solvent. In this case, the weight percentage of all other components of the aluminum paste will be reduced.

  In step (i) of the method according to the invention, the aluminum paste is coated on the silicon nitride-contaminated back side of the silicon wafer, ie other back side metal pastes, in particular the back side silver or silver / aluminum paste Alternatively, it is applied to the surface portion on the back side that is not covered. The aluminum paste can be applied so as to have a dry film thickness of, for example, 15 to 60 μm. The method of applying the aluminum paste can be printing, for example silicone pad printing, or in one embodiment screen printing. The coating viscosity of the aluminum paste can be 20-200 Pa · s when measured with a utility cup using a Brookfield HBT viscometer and a # 14 spindle at a spindle speed of 10 rpm and 25 ° C.

  After applying the aluminum paste to the silicon nitride contaminated back side of the silicon wafer, it can be dried, for example, for 1-100 minutes with the wafer reaching a peak temperature in the range of 100-300 ° C. Drying can be performed using, for example, a belt dryer, a rotary or stationary dryer, particularly an IR (infrared) belt dryer.

  After its application, or in one embodiment, after its application and drying, in step (ii) of the method according to the invention, the aluminum paste is fired to form an aluminum back electrode. Baking can be performed, for example, for 1 to 5 minutes in a state where the silicon wafer reaches a peak temperature in the range of 700 to 900 ° C. Firing can be performed, for example, using a single zone or multi-zone belt furnace, particularly a multi-zone IR belt furnace. Calcination occurs in the presence of oxygen, especially air. In calcination it is possible to remove organic substances including non-volatile organic materials and optionally organic parts which have not evaporated during the drying step, ie combustion and / or carbonization, in particular combustion. Organic materials removed during calcination include organic solvents, optionally added organic polymers, optionally added organic additives, and optionally added organic portions of magnesium organic compounds. When the aluminum paste includes a magnesium compound that can form magnesium oxide upon firing in step (ii), the magnesium provided by the magnesium compound remains or substantially remains after the firing as magnesium oxide. If the aluminum paste includes glass frit, another process may occur during firing, i.e., sintering of the glass frit. Baking is performed with another metal paste applied to the silicon wafer, that is, with the front side and / or back side metal paste applied to form front and / or back side electrodes on the wafer surface in the baking step. The so-called co-firing can be performed. One embodiment includes a front side silver paste and a back side silver or silver / aluminum paste.

  A non-limiting example of preparing a silicon solar cell according to the process of the present invention will now be described with reference to FIG.

  First, a silicon wafer substrate 102 is prepared. A silicon wafer provided with a silicon nitride antireflection coating, and a front-side electrode (for example, mainly from silver) on a light-receiving side surface (front-side surface) of a silicon wafer having a pn junction close to the normal surface. A configured electrode) 104 is attached (FIG. 2A). A silver or silver / aluminum conductive paste (eg, PV202 or PV502 or PV583 or PV581 commercially available from EI DuPont de Nemours and Company) is applied to the silicon nitride contaminated back side of the silicon wafer. Thus forming either bus bars or tabs to allow interconnection in parallel electrical configurations with other solar cell sets. On the back side of the silicon wafer contaminated with silicon nitride, an aluminum paste 106 containing an Mg-containing additive and used as a back side (or p-type contact) electrode of the solar cell is silver or silver / aluminum mentioned above. Apply by screen printing using a pattern that allows some overlap with the paste, followed by drying (FIG. 2B). The paste is dried, for example, in an IR belt dryer for 1 to 10 minutes with the wafer reaching a peak temperature of 100 to 300 ° C. Also, the aluminum paste can have a dry film thickness of 15-60 μm, and the thickness of the silver or silver / aluminum paste can be 15-30 μm. Further, the overlapping portion of the aluminum paste and silver or silver / aluminum paste can be about 0.5-2.5 mm.

  Next, the substrate obtained above is baked in a belt furnace so that a desired silicon solar cell is obtained for 1 to 5 minutes in a state where the wafer reaches a peak temperature of 700 to 900 ° C., for example (FIG. 2D). ). The electrode 110 is formed from the aluminum paste. In this case, the paste is baked to remove organic substances, and when the aluminum paste includes glass frit, the glass frit is sintered. The electrode 110 is formed from the aluminum paste. In that case, the paste is baked to remove organic substances, and when the aluminum paste contains a magnesium compound that can form magnesium oxide when baked, magnesium oxide is formed. If the aluminum paste contains glass frit, the glass frit is sintered.

  As shown in FIG. 2D, the silicon solar cell obtained using the aluminum paste includes an electrode 104 on the light receiving surface (front surface) of the silicon substrate 102, and an aluminum electrode 110 mainly composed of aluminum on the back surface side. It has a silver or silver / aluminum electrode 112 mainly composed of silver or silver and aluminum (formed by baking of silver or silver / aluminum paste 108).

(1) Manufacture of solar cell A solar cell was formed as follows.
(I) p-type 200 μm thick area 243 cm ² with Si substrate (20 μm thick silver electrode (PV145, commercially available Ag composition from EI DuPont de Nemours and Company) on the front) Boron) A bulk silicon polycrystalline silicon wafer, having an n-type diffused POCl ₃ emitter, surface textured with acid, and includes a SiNx anti-reflective coating (ARC) deposited by CVD on the wafer emitter. 15% of the area of the back surface of the silicon wafer is on the back surface of a silicon wafer covered with a rectangular SiNx layer of about 50 nm thickness artificially deposited by CVD so as to simulate SiNx contamination. Ag / Al paste (from PV202, EI DuPont de Nemours and Company A commercially available Ag / Al composition) was printed as a 5 mm wide bus bar and dried. Subsequently, an aluminum paste for the back electrode of the solar cell was screen-printed so as to have a dry film thickness of 30 μm. In this case, the aluminum film was provided with a portion where the Ag / Al bus bar overlapped by 1 mm at both ends thereof to ensure electrical continuity. The screen printed aluminum paste was dried before firing.

  The aluminum paste used in this example consists of 72% by weight air-sprayed aluminum powder (average particle size 6 μm), 26% by weight polymer resin and organic solvent organic vehicle, 0.5% by weight glass frit, Was included. The aluminum pastes B and C as examples (according to the invention) contain magnesium oxide (average particle size 8 μm), while the aluminum paste as control example A (comparative example) does not contain magnesium oxide.

  (Ii) Subsequently, the printed wafer is placed in a Centrotherm furnace with a belt speed of 3000 mm / min, and each zone temperature is set to zone 1 = 450 ° C., zone 2 = 520 ° C., zone 3 = 570 ° C. The zone was set to 950 ° C. and fired. For this reason, the wafer has reached a peak temperature of 850 ° C. After firing, the metallized wafer became a functional photovoltaic device.

  Measurements were made regarding electrical performance and fired adhesion in the SiNx contaminated zone.

(2) Test procedure efficiency The solar cell formed by the above method was subjected to a commercially available IV tester (sold by EETS Ltd.) for the purpose of measuring the light conversion efficiency. The lamp of this IV tester simulates sunlight of known intensity (about 1000 W / m ² ) and illuminates the battery emitter. Subsequently, four electrical probes were brought into contact with the metallized part printed on the fired battery. The photocurrent generated by the solar cell (Voc, open circuit voltage; Isc, short circuit current) was measured for all resistance configurations and an IV response curve was calculated. Subsequently, fill factor (FF) and efficiency (Eff) values were calculated from the IV response curve.

Firing adhesion In order to measure the adhesion strength of the aluminum metallized part, the amount of material removed from the SiNx contaminated part on the backside of the fired wafer was measured by a tensile test. For this reason, a transparent adhesive tape layer was applied and subsequently peeled off. The adhesion numbers in Table 1 indicate that paste adhesion increases with increasing magnesium oxide content of the composition.

  Examples AC quoted in Table 1 show the electrical properties of the aluminum paste as a function of magnesium oxide content compared to a standard composition without the addition of magnesium oxide (control). From the data in Table 1, it can be seen that the electrical performance of the solar cells made using the aluminum pastes according to Examples B and C is significantly improved compared to the solar cells made with the pastes according to Control Example A. It is confirmed. It has been shown that adhesion of the Al-BSF thick film layer to the SiNx-contaminated region on the backside of the battery is also improved.

10 p-type silicon wafer 20 n-type diffusion layer 30 SiNx antireflection coating 40 p + layer (back surface electric field, BSF)
60 Aluminum paste molded on the back side 61 Aluminum back electrode (obtained by firing the aluminum paste on the back side)
70 Silver or silver / aluminum paste formed on the back side 71 Silver or silver / aluminum back electrode (obtained by firing the silver or silver / aluminum paste on the back side)
500 Silver paste molded on the front side 501 Silver front electrode (obtained by firing the silver paste on the front side)
102 Silicon substrate (a silicon wafer having a silicon nitride antireflection coating provided on the front side thereof and the back side contaminated with silicon nitride)
104 Photosensitive surface side electrode 106 Paste composition for first electrode 108 Conductive paste for second electrode 110 First electrode 112 Second electrode

Claims (10)

(I) A silicon wafer having a p-type region, an n-type region, and a pn junction, wherein a silicon nitride antireflection coating is provided on the front side, and the back side is contaminated with silicon nitride. Applying an aluminum paste to the back side of
(Ii) firing the surface coated with the aluminum paste to bring the wafer to a peak temperature of 700 to 900 ° C .;
A method for producing a silicon solar cell comprising:
The aluminum paste is particulate aluminum, magnesium oxide, at least one Mg-containing additive selected from the group consisting of magnesium compounds capable of forming magnesium oxide upon firing in step (ii), and any combination thereof; And an organic vehicle containing an organic solvent. (I ′) silicon wafer having a p-type region, an n-type region, and a pn junction, wherein a silicon nitride antireflection coating is provided on the front side and the back side is contaminated with silicon nitride Preparing a wafer;
(I) applying an aluminum paste to the back side of the silicon wafer;
(Ii) firing the surface coated with the aluminum paste to bring the wafer to a peak temperature of 700 to 900 ° C .;
A method for improving the electrical performance of a silicon solar cell comprising:
The aluminum paste is particulate aluminum, magnesium oxide, at least one Mg-containing additive selected from the group consisting of magnesium compounds capable of forming magnesium oxide upon firing in step (ii), and any combination thereof; Including organic vehicles including organic solvents,
A method characterized by that.   The method according to claim 1 or 2, wherein the aluminum paste contains one or more glass frit in a total ratio of 0.01 to 5 mass% based on the total aluminum paste composition.   The method according to claim 1 or 2, wherein the particulate aluminum is present in a ratio of 50 to 80 mass% based on the total aluminum paste composition.   The one or more Mg-containing additives are present in a total ratio corresponding to a concentration contribution of 0.1 to 5% by mass of total magnesium based on the total aluminum paste composition. Item 3. The method according to Item 1 or 2.   The method according to claim 1 or 2, wherein the aluminum paste contains only one Mg-containing additive, and the Mg-containing additive is magnesium oxide.   The magnesium compound capable of forming magnesium oxide at the time of firing in the step (ii) is selected from magnesium hydroxide, magnesium carbonate, magnesium nitrate, magnesium resinate, and magnesium carboxylate. the method of.   The method according to claim 1 or 2, wherein the organic vehicle further comprises an organic polymer and / or an organic additive.   3. The method according to claim 1, wherein the application of the aluminum paste is performed by printing.   The firing is performed as co-firing with other metal paste applied to the silicon wafer on the front side and / or back side, and the front electrode and / or the back electrode are formed during the firing. Or the method of 2.

Images