JP5957546B2 - Conductive composition - Google Patents

Description

  The present invention relates to a conductive composition. In more detail, it is related with the electroconductive composition which can be used in order to form the electrode pattern of a solar cell.

  In recent years, the spread of solar cells has been rapidly progressing from the viewpoint of environmental awareness and energy saving, and accordingly, a cell structure having a higher performance than the conventional cell structure, that is, a cell structure having a high photoelectric conversion efficiency and a high output cell structure. It has been demanded. One measure for realizing this requirement is to increase the light receiving area per unit cell area of the solar cell. For example, as one means for expanding the light receiving area, it is desired to make a thin line (fine line) of a linear electrode formed on the light receiving surface.

  On the light-receiving surface of so-called crystalline silicon solar cells, which are currently mainstream, typically, a finger (collecting current) electrode made of a thin wire formed of a conductor such as silver, and the finer electrode A bus bar electrode to be connected is provided. Hereinafter, these electrodes are also collectively referred to as light receiving surface electrodes. Such a light receiving surface electrode includes a conductive powder such as silver as a conductor component, and an organic vehicle component composed of an organic binder and a solvent, and is prepared in a paste form (including a slurry form and an ink form). A material (hereinafter also referred to as “conductive composition”, simply “composition”) is printed on a light-receiving surface of a solar cell (cell) with a predetermined electrode pattern by a method such as a screen printing method, and fired. It is formed with. As a prior art regarding the electroconductive composition used in order to form the light-receiving surface electrode of such a solar cell, patent documents 1-3 are mentioned, for example.

JP 2010-087251 A JP 2012-023095 A Special table 2012-508812 gazette

  By the way, the conductive composition for forming an electrode of a solar cell may contain glass frit in addition to the above constituent materials. The glass frit can function as an inorganic binder that softens or melts during firing to achieve a good bond between the substrate and the electrode. And in manufacture of a solar cell, a favorable fire through characteristic is expressed because an electroconductive composition contains glass frit. That is, when manufacturing a solar cell, typically, an antireflection film is first formed on almost the entire light receiving surface of a silicon substrate, and a conductive composition for forming a light receiving surface electrode is formed on the antireflection film. Are supplied in a desired electrode pattern and fired. At this time, the glass frit in the conductive composition reacts with the antireflection film during firing and takes it into the glass. As a result, the conductive powder in the conductive composition passes through the antireflection film (fire-through), and realizes good electrical connection (ohmic contact) with the silicon substrate. When the fire-through characteristics of the conductive composition are used in this way, it is not necessary to remove the antireflection film partially when forming a fine light-receiving surface electrode, and it is easy to use. This eliminates the risk of gaps or overlaps with the surface electrode formation position, which is preferable.

  On the light receiving surface of the solar cell, the portion where the light receiving surface electrode is formed is a light shielding portion (non-light receiving portion). For this reason, if the light receiving surface electrode is made thinner (fine line) than before, the light receiving area per cell unit area can be expanded and the output per cell unit area can be improved. However, at this time, if the electrode is not bulky (thickened) by the amount of thinning, the line resistance of the electrode increases and the output characteristics of the solar cell deteriorate. Therefore, for the fine line formation of the light-receiving surface electrode, it is simultaneously required to improve the electrode thickness, that is, to have a high aspect ratio (ratio of electrode thickness to line width: large thickness / line width; the same shall apply hereinafter). It is done.

However, the conventional conductive composition is expected to be further improved from the viewpoint of achieving both good ohmic contact and fine line formation of the above-described electrode.
The present invention has been made in view of such a situation, and its main purpose is to realize thinning of the electrode pattern and high aspect ratio, and to satisfactorily form a contact point between the electrode and the substrate. It is providing the electroconductive composition for electrode formation. Another object of the present invention is to provide a semiconductor element with improved function or performance, for example, a solar cell element, that can be realized by adopting the conductive composition.

In order to achieve the above object, the present invention provides a conductive composition that can be suitably used for forming an electrode (electrode pattern). This conductive composition includes conductive powder, glass frit, silicone resin, organic binder, and dispersion medium, and the glass frit has a SiO ₂ component ratio of 0 mass when converted to oxide. % Or more and 5% by mass or less.

Since the conductive composition disclosed herein contains glass frit, it can form an electrode having good bonding properties with a substrate, and an antireflection film can be formed when forming a solar cell electrode. Even if it is a case where it supplies to the top, the contact of an electrode and a board | substrate can be suitably formed by a fire through. As a result, a good ohmic contact can be realized. Further, since the conductive composition contains a silicone resin, it is possible to stably form a fine and high aspect ratio electrode. Here, the SiO ₂ component derived from glass frit or silicone resin is not preferable in that it can increase the insulating resistance component in the electrode. Therefore, in the technology disclosed herein, the glass frit does not contain a SiO ₂ component, or the ratio of the SiO ₂ component in the glass frit is limited as described above, so that the electrode characteristics are not impaired. It achieves both high fire-through characteristics and electrode shape stability at a high level.

  In a preferred embodiment of the conductive composition disclosed herein, the ratio of the silicone resin to 100 parts by mass of the conductive powder is 0.005 parts by mass or more and 0.9 parts by mass or less. With such a configuration, an electrode with a high aspect ratio can be formed, and an electrode with better electrical characteristics can be formed.

  In a preferred embodiment of the conductive composition disclosed herein, the silicone resin has a weight average molecular weight of 3000 or more and 90000 or less. With such a configuration, electrical characteristics such as line resistance of the electrode can be further enhanced as compared with the case where no silicone resin is added.

  In a preferred embodiment of the conductive composition disclosed herein, the metal species constituting the conductive powder is any one or two selected from the group consisting of nickel, platinum, palladium, silver, copper, and aluminum. It is characterized by containing more than seed elements. With such a configuration, an electrode having excellent conductivity can be configured.

In another aspect for realizing the above object, the present invention also provides a semiconductor device including an electrode formed using any one of the conductive compositions described above. This semiconductor element can typically be a solar cell element including a light-receiving surface electrode formed using the conductive composition.
Specifically, when the conductive composition of the present invention is supplied, for example, on the light receiving surface of a semiconductor substrate by a screen printing method or the like, a finer electrode pattern and an electrode having a finer line width are bulky (higher (In aspect ratio). Therefore, for example, further fine lines can be realized in printing of electrode patterns of various semiconductor elements, and a high-performance semiconductor element in which the semiconductor element is further miniaturized and highly integrated is realized. Further, for example, it is particularly preferable to apply to the formation of the light receiving surface electrode of the solar cell element because the amount of light received per unit area of the light receiving surface can be increased and more electric power can be generated.

It is sectional drawing which shows an example of the structure of a solar cell typically. It is a top view which shows typically the pattern of the electrode formed in the light-receiving surface of a solar cell. It is the graph which showed the relationship between the weight average molecular weight of the silicone resin in the electrically conductive composition in one Embodiment, the number of disconnection of the formed electrode, and the aspect ratio. It is the graph which showed the relationship between the weight average molecular weight of the silicone resin in the electrically conductive composition in one Embodiment, and the line resistance of the formed electrode.

  Hereinafter, preferred embodiments of the present invention will be described. It should be noted that technical matters other than the contents particularly mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters for those skilled in the art based on the prior art. The present invention can be carried out based on the technical contents disclosed in the present specification and the common general technical knowledge in the field.

  The conductive composition disclosed herein is typically a conductive composition that can form an electrode by firing. The conductive composition is essentially the same as the conventional conductive composition of this type. The conductive powder, the glass frit, and the organic vehicle component for dispersing these components (described later, A mixture of an organic binder and a dispersant), and a silicone resin as an essential component. Hereinafter, each of these components will be described.

As the conductive powder that is the main component of the solid content of the paste, it is possible to consider powders made of various metals or their alloys having desired conductivity and other physical properties according to the application. Examples of the material constituting the conductive powder include gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), iridium ( Ir), metals such as osmium (Os), nickel (Ni) and aluminum (Al) and their alloys, carbonaceous materials such as carbon black, LaSrCoFeO ₃ -based oxides (for example, LaSrCoFeO ₃ ), LaMnO ₃ -based oxides ( Examples thereof include conductive ceramics represented by transition metal perovskite oxides represented by LaSrGaMgO ₃ ), LaFeO ₃ -based oxides (eg LaSrFeO ₃ ), LaCoO ₃ -based oxides (eg LaSrCoO ₃ ), and the like. Among these, particularly preferable conductive powders are composed of simple metals such as platinum, palladium and silver and alloys thereof (Ag—Pd alloy, Pt—Pd alloy, etc.), nickel, copper, aluminum and alloys thereof. It is mentioned as a material which comprises. From the viewpoint of relatively low cost and high electrical conductivity, a powder made of silver and its alloy (hereinafter also simply referred to as “Ag powder”) is particularly preferably used. Hereinafter, the conductive composition of the present invention will be described using an example in which Ag powder is used as the conductive powder.

  There is no restriction | limiting in particular about the particle size of Ag powder other conductive powder, The thing of the various particle size according to a use can be used. Typically, those having an average particle diameter of 5 μm or less based on the laser / scattering diffraction method are suitable, and those having an average particle diameter of 3 μm or less (typically 1 to 3 μm, for example 1 to 2 μm) are preferably used. It is done.

The shape of the particles constituting the conductive powder is not particularly limited. Typically, a spherical shape, a flake shape (flake shape), a conical shape, a rod shape, or the like can be preferably used. Spherical or scaly particles are preferably used for reasons such as easy formation of a fine light-receiving surface electrode with good filling properties. As the conductive powder to be used, those having a sharp (narrow) particle size distribution are preferable. For example, a conductive powder having a sharp particle size distribution that does not substantially contain particles having a particle diameter of 10 μm or more is preferably used. As this index, the ratio (D10 / D90) of the particle size (D10) when the cumulative volume is 10% and the particle size (D90) when the cumulative volume is 90% in the particle size distribution based on the laser scattering diffraction method can be adopted. When all the particle sizes constituting the powder are equal, the value of D10 / D90 is 1, and conversely, the value of D10 / D90 approaches 0 as the particle size distribution becomes wider. It is preferable to use a powder having a relatively narrow particle size distribution such that the value of D10 / D90 is 0.2 or more (for example, 0.2 or more and 0.5 or less).
A conductive composition using a conductive powder having such an average particle size and particle shape has a good filling property of the conductive powder and can form a dense electrode. This is advantageous in forming a fine electrode pattern with high shape accuracy.

  In addition, electroconductive powder, such as Ag powder, is not specifically limited by the manufacturing method. For example, conductive powder (typically Ag powder) produced by a known wet reduction method, gas phase reaction method, gas reduction method or the like can be classified and used as necessary. Such classification can be performed using, for example, a classification device using a centrifugal separation method.

The glass frit is a component that can function as an inorganic binder of the conductive powder, and improves the bonding between the conductive particles constituting the conductive powder and between the conductive particles and the substrate (object on which the electrode is formed). Work. In addition, when this conductive composition is used, for example, for forming a light-receiving surface electrode of a solar cell, the presence of the glass frit may cause the conductive composition to penetrate an antireflection film as a lower layer during firing. This makes it possible to achieve good adhesion and electrical contact with the substrate.
Such a glass frit is preferably adjusted to a size equal to or smaller than that of the conductive powder. For example, the average particle diameter based on the laser / scattering diffraction method is preferably 4 μm or less, more preferably 2 μm or less, and more preferably about 0.1 μm or more and 3 μm or less.

Note that the composition of the glass frit can be used as the proportion of SiO ₂ component when converted oxides 0 mass% to 5 mass% or less (e.g., less than 5 wt%). The SiO ₂ component is preferably contained in the glass frit in that the stability of the system is improved and the erosion property at the time of fire-through can be adjusted. However, in the technique disclosed herein, a silicone resin described later is included as an essential component, and this silicone resin forms a SiO ₂ component during firing. Excess SiO ₂ component can increase the softening point of the glass frit and reduce the erosion of the conductive composition during fire-through. In addition, if electrode formation by firing at a lower temperature becomes impossible, electrode performance may be adversely affected. Therefore, in the technique disclosed herein, the ratio of the SiO ₂ component of the glass frit is limited to an extremely small amount as described above. The SiO ₂ component in the glass frit is preferably 4% by mass or less, for example, 3% by mass or less. The SiO ₂ component in the glass frit may be 0% by mass (that is, not including the SiO ₂ component).

There is no restriction | limiting in particular about the other component contained in a glass frit, Glass of various compositions can be used. For example, as an approximate glass composition, a so-called lead-based glass, lead-lithium-based glass, zinc-based glass, borate-based glass, borosilicate-based glass (however, the amount of Si is a name that is conventionally expressed by those skilled in the art) The glass may be alkaline glass, lead-free glass, tellurium glass, glass containing barium oxide, bismuth oxide, or the like. Needless to say, these glasses include Si (but the amount of Si is limited), Pb, Zn, Ba, Bi, B, Al, Li, Na, K, in addition to the main glass constituent elements appearing in the above names. Rb, Te, Ag, Zr, Sn, Ti, W, Cs, Ge, Ga, In, Ni, Ca, Cu, Mg, Sr, Se, Mo, Y, As, La, Nd, C, Pr, Gd, Contains one or more elements selected from the group consisting of Sm, Dy, Eu, Ho, Yb, Lu, Ta, V, Fe, Hf, Cr, Cd, Sb, F, Mn, P, Ce and Nb You may go out. Such a glass frit may be, for example, a crystallized glass partially containing crystals in addition to a general amorphous glass. As for the glass frit, as long as the SiO ₂ component as a whole is adjusted as described above, one kind of glass frit may be used alone, or two or more kinds of glass frit may be mixed. It may be used.

Although the softening point of the glass which comprises a glass frit is not specifically limited, It is preferable that it is about 300-600 degreeC (for example, 400-500 degreeC). Specific examples of the glass whose softening point can be adjusted in the range of 300 ° C. or more and 600 ° C. or less include glass containing a combination of the following elements. B-Si-Al glass, Pb-B-Si glass, Si-Pb-Li glass, Si-Al-Mg glass, Ge-Zn-Li glass, B-Si-Zn-Sn glass, B-Si-Zn-Ta glass, B-Si-Zn-Ta-Ce glass, B-Zn-Pb glass, B-Si-Zn-Pb glass, B-Si-Zn-Pb-Cu glass Glass, B-Si-Zn-Al glass, Pb-B-Si-Ti-Bi glass, Pb-B-Si-Ti glass, Pb-B-Si-Al-Zn-P glass, Pb- Li-Bi-Te glass, Pb-Si-Al-Li-Zn-Te glass, Pb-B-Si-Al-Li-Ti-Zn glass, Pb-B-Si-Al-Li-Ti- P-Te glass, Pb-Si-Li-Bi-Te glass, Pb Si-Li-Bi-Te-W glass, P-Pb-Zn glass, P-Al-Zn glass, P-Si-Al-Zn glass, P-B-Al-Si-Pb-Li system Glass, P—B—Al—Mg—F—K glass, Te—Pb glass, Tr—Pb—Li glass, VP—Ba—Zn glass, VP—Na—Zn glass, _{AgI-Ag 2 O-B-} P based glass, Zn-B-Si-Li based glass, Si-Li-Zn-Bi -Mg-W-Te -based glass, Si-Li-Zn-Bi -Mg-Mo- Te glass, Si-Li-Zn-Bi-Mg-Cr-Te glass, and the like. When the conductive composition containing glass frit having such a softening point is used, for example, when forming a light-receiving surface electrode of a solar cell element, it exhibits good fire-through characteristics and forms a high-performance electrode. Preferred to contribute.

The silicone resin is characteristic as an essential constituent component contained in the conductive composition disclosed herein. By containing this silicone resin, such a conductive composition can maintain a stable shape from printing to firing, for example, and can stably form a finer and higher aspect ratio electrode. Become. Moreover, the silicone resin may generate SiO ₂ component in the electrode by firing. Such SiO ₂ component is preferable in that the stability of the system and the binding property between the electrode and the substrate can be improved without directly increasing the softening point of the glass frit.

  As the silicone resin (which may be simply referred to as silicone), an organic compound containing silicon (Si) can be used without particular limitation. For example, a main skeleton formed of siloxane bonds (Si—O—Si) can be used. The organic compound which has can be used preferably. For example, it may be a linear silicone in which an alkyl group or a phenyl group is introduced into a dangling bond (side chain, terminal) in the main skeleton. Further, it may be a linear modified silicone in which other substituents such as a polyether group, an epoxy group, an amine group, a carboxyl group, an aralkyl group, and a hydroxyl group are introduced into a side chain, a terminal, or both. It may be a linear block copolymer alternately bonded with silicone.

  Such a silicone resin is preferable because an electrode having a high aspect ratio can be formed as the weight average molecular weight (hereinafter sometimes simply referred to as “Mw”) increases. However, when Mw is about 110,000, it is not preferable because defects such as disconnection of the obtained electrode are caused and resistance is increased. From such a viewpoint, for example, Mw is preferably 90,000 or less, more preferably 70,000 or less, and particularly preferably 60,000 or less. The lower limit of Mw is not particularly limited, but can be, for example, 1,000 or more, preferably 3,000 or more, more preferably 5,000 or more, and particularly 10,000 or more, for example 20,000 or more. preferable.

As the organic vehicle component for dispersing the constituent elements such as the above conductive powder, various kinds of those conventionally used in this type of conductive composition are not particularly limited depending on the desired purpose. Can do. Typically, the vehicle is composed of organic binders and organic solvents of various compositions. In such an organic vehicle component, all of the organic binder may be dissolved in an organic solvent, or only a part thereof may be dissolved or dispersed (may be a so-called emulsion type organic vehicle).
Examples of the organic binder include cellulose polymers such as ethyl cellulose and hydroxyethyl cellulose, acrylic resins such as polybutyl methacrylate, polymethyl methacrylate, and polyethyl methacrylate, epoxy resins, phenol resins, alkyd resins, polyvinyl alcohol, polyvinyl butyral, etc. An organic binder based on is preferably used. In particular, a cellulosic polymer (for example, ethyl cellulose) is preferable, and a viscosity characteristic capable of performing particularly good screen printing can be realized.

  A preferable solvent constituting the organic vehicle is an organic solvent having a boiling point of about 200 ° C. or higher (typically about 200 to 260 ° C.). An organic solvent having a boiling point of about 230 ° C. or higher (typically about 230 to 260 ° C.) is more preferably used. Examples of such organic solvents include ester solvents such as butyl cellosolve acetate and butyl carbitol acetate (BCA: diethylene glycol monobutyl ether acetate), ether solvents such as butyl carbitol (BC: diethylene glycol monobutyl ether), ethylene glycol and diethylene glycol. An organic solvent such as a derivative, toluene, xylene, mineral spirit, terpineol, mentanol, or texanol can be preferably used. Particularly preferred solvent components include butyl carbitol (BC), butyl carbitol acetate (BCA), 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and the like.

The blending ratio of each constituent component contained in the conductive composition may vary depending on the electrode formation method, typically the printing method, etc., but generally conforms to the conductive composition of the composition conventionally employed. It can be set as a mixture ratio. As an example, for example, the ratio of each component can be determined using the following formulation as a guide.
That is, the content of the conductive powder in the conductive composition is suitably about 70% by mass or more (typically 70% to 95% by mass) when the entire paste is 100% by mass. More preferably, it is preferably about 80% by mass to 90% by mass, for example, about 85% by mass. Increasing the content of the conductive powder is preferable from the viewpoint of forming a dense electrode pattern with good shape accuracy. On the other hand, if the content is too high, the handleability of the paste and the suitability for various printability may be reduced.

  Silicone resin is preferable because the electrode can be formed with a higher aspect ratio by adding even a very small amount to the conductive powder. For example, when the conductive powder is 100 parts by mass, the addition amount of the silicone resin can typically be 0.005 parts by mass or more, preferably 0.01 parts by mass or more, and More preferably, it is 1 part by mass or more. Excessive addition is not preferable for increasing the resistance of the formed electrode. Therefore, the addition amount of the silicone resin can be typically 1.2 parts by mass or less, preferably 0.9 parts by mass or less when the conductive powder is 100 parts by mass, It is more preferable that the amount is not more than part by mass.

  The ratio of the glass frit to the conductive powder is unclear because of the relationship with the silicone resin, but in order to obtain good fire-through characteristics, typically when the conductive powder is 100 parts by mass, The amount can be 0.1 parts by mass or more, preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more. Excessive addition is not preferable in order to increase the resistance of the electrode to be formed, and can be typically 12 parts by mass or less, preferably 10 parts by mass or less, and 8 parts by mass or less. Is more preferable.

Note that the above-mentioned silicone resin and the glass frit is a source of SiO ₂ components contained in the electrode. And the content can be considered complementarily in that this SiO ₂ component can suppress the fire-through characteristic or can become an insulating resistance component in the electrode. More specifically, since the electroconductive composition disclosed here contains a silicone resin, the SiO ₂ component in the glass frit can be suppressed to a small amount. However, for example, when the amount of the silicone resin exceeds approximately 0.15 parts by mass (for example, 0.2 parts by mass or more), the conductive composition penetrates the antireflection film or forms a good contact with the substrate. It is preferable to secure a sufficient amount of glass frit corresponding to the amount of silicone resin. For example, the mass ratio of glass frit to silicone resin (mass of glass frit / mass of silicone resin) is preferably 7.5 or more, more preferably 8 or more, 8.3 or more, for example 10 or more Is particularly preferred. However, as described above, the silicone resin and the glass frit can themselves be the resistance component of the electrode. From this viewpoint, the mass ratio of the glass frit to the silicone resin is preferably about 18 or less, more preferably 16.5 or less, for example, 15 or less, and further preferably 12 or less. For example, as described above, the series resistance Rs can be effectively reduced by keeping the amount ratio of the glass frit to the silicone resin within a predetermined range.

Of the organic vehicle components, the organic binder may be contained in a proportion of about 15% by mass or less, typically about 1% by mass to 10% by mass, when the mass of the conductive powder is 100% by mass. preferable. Particularly preferably, it is contained at a ratio of 2% by mass to 6% by mass with respect to 100% by mass of the conductive powder. In addition, this organic binder may contain the organic binder component which is melt | dissolving in the organic solvent, and the organic binder component which is not melt | dissolving in the organic solvent, for example. When the organic binder component dissolved in the organic solvent and the organic binder component not dissolved are included, the ratio thereof is not particularly limited, but for example, the organic binder component dissolved in the organic solvent is (40% to 100%) can be occupied.
In addition, the content rate as a whole of the organic vehicle is variable in accordance with the properties of the obtained paste. As an approximate guide, when the entire conductive composition is 100% by mass, for example, 5% by mass to 30% by mass. %, Is preferably 5% by mass to 20% by mass, and more preferably 5% by mass to 15% by mass (particularly 7% by mass to 12% by mass).

In addition, the conductive composition disclosed herein can contain various inorganic additives and / or organic additives other than those described above without departing from the object of the present invention. Preferable examples of the inorganic additive, ceramic powder other than the above _(ZnO 2, _Al 2 _{O 3,} etc.), other various fillers and the like. Moreover, as a suitable example of an organic additive, additives, such as surfactant, an antifoamer, antioxidant, a dispersing agent, a viscosity modifier, are mentioned, for example.

  Since the above conductive composition has shape stability, for example, it may be a printing composition applied to screen printing, gravure printing, offset printing, ink jet printing, etc. Is suitable). And it can employ | adopt especially preferably, when using such a general purpose printing means in the case of formation of the electrode pattern for which a thin line and high aspect ratio are calculated | required. Therefore, for example, a solar cell element as an example of a semiconductor element is taken as an example, and a semiconductor element disclosed herein is shown while showing an example in which a comb-shaped electrode pattern including a fine finger electrode is formed on the light receiving surface by screen printing. The solar cell element will be described. The solar cell element may be the same as the conventional solar cell except for the configuration of the light-receiving surface electrode that characterizes the present invention. The detailed description is omitted.

  FIG. 1 and FIG. 2 schematically show an example of a solar cell element (cell) 10 that can be suitably manufactured by implementing the present invention, and is made of single crystal, polycrystalline, or amorphous silicon (Si). This is a so-called silicon-type solar cell element 10 that uses the wafer as the semiconductor substrate 11. A cell 10 shown in FIG. 1 is a general single-sided light receiving solar cell element 10. Specifically, this type of solar cell element 10 includes an n-Si layer 16 formed by forming a pn junction on the light-receiving surface side of a p-Si layer (p-type crystalline silicon) 18 of a silicon substrate (Si wafer) 11. On its surface, and an antireflection film 14 made of titanium oxide or silicon nitride formed by CVD or the like, and light receiving surface electrodes 12 and 13 made of a conductive composition mainly containing Ag powder or the like. .

On the other hand, on the back side of the p-Si layer 18, as with the light-receiving surface electrode 12, the back side outside formed by a predetermined conductive composition (typically a conductive paste whose conductive powder is Ag powder). A connection electrode 22 and a back surface aluminum electrode 20 having a so-called back surface field (BSF) effect are provided. The aluminum electrode 20 is formed on substantially the entire back surface by printing and baking a conductive composition mainly composed of aluminum powder. During this firing, an Al—Si alloy layer (not shown) is formed, and aluminum diffuses into the p-Si layer 18 to form the p ⁺ layer 24. By forming the p ⁺ layer 24, that is, the BSF layer, the photogenerated carriers are prevented from recombining in the vicinity of the back electrode, and for example, an improvement in short circuit current and open circuit voltage (Voc) is realized.

As shown in FIG. 2, on the light receiving surface 11A side of the silicon substrate 11 of the solar cell element 10, several (for example, about 1 to 3) straight lines parallel to each other are formed as the light receiving surface electrodes 12 and 13. Bus bar (connecting) electrode 12 and a plurality of parallel (for example, about 60 to 90) streaky finger (collecting) electrodes 13 connected so as to cross the bus bar electrode 12. Is formed.
A large number of finger electrodes 13 are formed to collect photogenerated carriers (holes and electrons) generated by light reception. The bus bar electrode 12 is a connection electrode for collecting carriers collected by the finger electrode 13. The portion where the light receiving surface electrodes 12 and 13 are formed forms a non-light receiving portion (light shielding portion) on the light receiving surface 11A of the solar cell element. Therefore, the bus bar electrode 12 and the finger electrode 13 (particularly the large number of finger electrodes 13) provided on the light receiving surface 11A side are made as fine lines as possible to reduce the corresponding non-light receiving portion (light shielding portion). Thus, the light receiving area per unit cell area is enlarged. This can extremely simply improve the output per unit area of the solar cell element 10.

  At this time, it is sufficient if the height of the thinned electrode is high and uniform, but for example, when a sagging or a dent occurs in a part of the electrode, the sagging or the dent causes an increase in resistance, thereby collecting current. Loin is produced. Moreover, if even a part of the thinned electrode breaks, it is impossible to collect the generated current through the broken part (current collection is generated as a current flowing through the high resistance substrate). Current will be collected). Therefore, the formation of the light-receiving surface electrode of the solar cell element requires a conductive composition having excellent electrical stability and excellent shape stability by printing.

Such a solar cell element 10 is generally manufactured through the following process.
That is, an appropriate silicon wafer is prepared, and the p-Si layer 18 and the n-Si layer 16 are formed by doping predetermined impurities by a general technique such as a thermal diffusion method or ion plantation. A substrate (semiconductor substrate) 11 is produced. Next, an antireflection film 14 made of silicon nitride or the like is formed by a technique such as plasma CVD.
Thereafter, on the back surface 11B side of the silicon substrate 11, first, a predetermined pattern is screen-printed using a predetermined conductive composition (typically a conductive composition in which the conductive powder is Ag powder) and dried. By doing so, a back side conductor coated material that becomes the back side external connection electrode 22 (see FIG. 1) after firing is formed. Next, a conductive composition containing aluminum powder as a conductor component is applied (supplied) by a screen printing method or the like on the entire back surface, and dried to form an aluminum film.

  Next, the conductive composition of the present invention is typically printed (supplied) on the antireflection film 14 formed on the surface side of the silicon substrate 11 with a wiring pattern as shown in FIG. 2 based on a screen printing method. ) The line width to be printed is not particularly limited, but by adopting the conductive composition of the present invention, the line width is about 70 μm or less (preferably in the range of about 50 μm to 60 μm, more preferably in the range of about 40 μm to 50 μm. ) Of the electrode pattern including the finger electrodes of () is formed. Next, the substrate is dried in an appropriate temperature range (typically 100 ° C. to 200 ° C., for example, about 120 ° C. to 150 ° C.). The contents of a suitable screen printing method will be described later.

In this way, the silicon substrate 11 on which the paste application (dried film-like application) is formed on both sides is subjected to an appropriate baking temperature (for example, using a baking furnace such as a near-infrared high-speed baking furnace) in an air atmosphere. 700-900 ° C).
By such firing, a fired aluminum electrode 20 is formed together with the light-receiving surface electrodes (typically Ag electrodes) 12 and 13 and the backside external connection electrode (typically Ag electrode) 22, and at the same time, Al (not shown) The -Si alloy layer is formed and aluminum is diffused into the p-Si layer 18 to form the p + layer (BSF layer) 24 described above, and the solar cell element 10 is manufactured.
Instead of simultaneous firing as described above, for example, firing for forming the light receiving surface electrodes (typically Ag electrodes) 12 and 13 on the light receiving surface 11A side, the aluminum electrode 20 on the back surface 11B side, and external connection The firing for forming the electrode 22 may be performed separately.

  According to the conductive composition disclosed herein, the conductive composition can be supplied (printed) on the silicon substrate 11 with a desired electrode pattern by, for example, screen printing. Since such a conductive composition is excellent in shape stability, for example, an electrode obtained after firing has a line width of 60 μm or less and a thickness of 20 μm or more (preferably a line width of 40 μm or more and 50 μm or less and a thickness of 20 μm or more. ) Of the finger electrode 13) can be formed with high quality in a state in which the occurrence of line thinning and disconnection is greatly reduced. The bus bar electrode is hardly affected by the thinning or disconnection of the line, so that it is not necessary to use such a conductive composition, but for example, a bus bar electrode having a line width of about 1000 to 3000 μm can be formed with high quality. Thus, when the thinning of the electrode line and the high aspect ratio are realized, for example, the output per unit area can be improved without increasing the resistance per finger electrode. Further, even when the resistance value of the electrode line is slightly increased, the line resistance value of the entire electrode pattern can be kept low. Therefore, a solar cell element with high photoelectric conversion efficiency is provided by designing the width and number of finger electrodes 13 as an optimal combination.

Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.
(Embodiment 1)
[Preparation of conductive composition]
A conductive composition for electrode formation was prepared by the following procedure. That is, as the conductive powder, silver (Ag) powder having an average particle diameter of 2 μm was used. As the glass frit, 12 kinds of glass powders (average particle diameter: 0.5 to 1.6 μm) shown in Table 1 below were used. As the silicone resin, polydimethylsiloxane having a weight average molecular weight Mw of 50,000 was used. Moreover, hydrogenated castor oil was used as the surfactant. As the organic vehicle component, a vehicle in which ethyl cellulose (EC) as an organic binder component was dispersed in texanol as a dispersion medium was used.
In Table 1, the symbol indicating the structure of the glass frit is “A” for leaded glass containing Pb, “B” for lead-free glass containing bismuth (Bi) or the like without containing Pb, and Pb. Other lead-free glass containing boron (B), silicon (Si), etc. without being included is represented by “C”, and a number indicating the content of the SiO ₂ component in each glass composition is added. In these glass frit, the softening point is changed in the range of 300 ° C. or more and 600 ° C. or less as shown in Table 1 by adjusting the composition.

  When these materials are 100 parts by mass of silver powder, the glass frit is 2.50 parts by mass, the silicone resin is 0 parts by mass, 0.0050 parts by mass, 0.30 parts by mass, or ethyl cellulose is 1 part by mass. 0.001 part by mass, blended with 0.80 parts by mass of hardened castor oil, and while kneading well using a three-roll mill, adjusting the viscosity to about 190 Pa · s with texanol, 21 conductive compositions were prepared. Table 2 below shows the types of glass frit used in the conductive compositions of the respective examples, the blending amount of the silicone resin, and the actually measured values of the viscosity of the obtained conductive compositions. In Table 2, the composition of the glass frit used in each example is represented by the symbols shown in Table 1. Moreover, "-" in the column of the amount of silicone resin indicates that no silicone resin is blended (0 part by mass). And the viscosity of the electrically conductive composition of each example is the value measured on condition of 20 rpm in 25 degreeC using the Brookfield type viscometer of HBT type.

[Production of test solar cell element (light-receiving surface electrode)]
Using the conductive compositions of Examples 1 to 21 obtained above, light-receiving surface electrodes (that is, comb-shaped electrodes composed of finger electrodes and bus bar electrodes) are formed, thereby producing solar cell elements of Examples 1 to 21. did.
Specifically, first, a commercially available p-type single crystal silicon substrate (plate thickness 180 μm) for 156 mm square (6 inch square) is prepared, and the surface (light receiving surface) is mixed acid of hydrofluoric acid and nitric acid. Etching was used to remove the damaged layer and form an uneven texture structure. Next, a phosphorus-containing solution was applied to the textured structure surface, and heat treatment was performed to form an n-Si layer (n ⁺ layer) having a thickness of about 0.5 μm on the light receiving surface of the silicon substrate. Next, a silicon nitride film having a thickness of about 80 nm was formed on the n-Si layer by a plasma CVD (PECVD) method to obtain an antireflection film.

  Next, the back side electrode pattern was formed on the back side of the silicon substrate by using a predetermined silver electrode forming paste, followed by screen printing with a predetermined pattern to be a back side external connection electrode and drying. . Then, an aluminum electrode forming paste is screen-printed on the entire back surface side and dried to form an aluminum film.

Thereafter, using the prepared conductive compositions of Examples 1 to 21, an electrode pattern for a light receiving surface electrode (Ag electrode) was printed on the antireflection film by a screen printing method in an air atmosphere at room temperature. And dried at 120 ° C. Specifically, as shown in FIG. 2, an electrode pattern comprising three mutually parallel linear busbar electrodes and 90 finger electrodes parallel to each other so as to be orthogonal to the busbar electrodes. It was formed by screen printing. The target finger electrode pattern has a range in which dimensions after firing are a line width of 45 μm to 55 μm and a film thickness of 15 μm to 25 μm. The bus bar electrode was set so that the line width after firing was approximately 1.5 mm.
Thus, the solar cell for evaluation was produced by baking the board | substrate which printed the electrode pattern on both surfaces in the air atmosphere at the baking temperature of 700-800 degreeC using the near-infrared high-speed baking furnace.

[Evaluation]
About the light-receiving surface electrode (finger electrode) of the solar cell produced as described above, the film thickness, the line width, the series resistance Rs, and the energy conversion efficiency Eff were measured by the following procedure.
The film thickness and the line width of the electrode were measured with a shape analysis laser microscope (manufactured by Keyence Corporation) for the thickness (height) and the line width at any position of the light-receiving surface electrode of the solar cell of each example. The results are shown in Table 2 as average values of values measured at 100 locations.

  The series resistance Rs and the energy conversion efficiency Eff of the electrode are defined in JIS C8913 from the IV curves obtained for the solar cells of each example using a solar simulator (Peg 10 manufactured by Beger). It was calculated based on the “output measurement method”. The result is shown in Table 2 as an average value of 100 data obtained by the solar simulator.

As shown in Table 2, Examples 5, 12 and 19 are examples in which a glass frit having a relatively high SiO ₂ content of 7% by mass was used for the conductive composition, but no silicone resin was blended. is there. It was confirmed that the electrodes formed using these conductive compositions were significantly thinner than the electrodes of other examples, regardless of the composition of the glass frit. That is, it turned out that the printed body (coating film) of the electroconductive composition which does not contain a silicone resin sags, and its shape stability is low.
On the other hand, the conductive compositions of Examples 1-4, 8-11, and 15-18, which use a glass frit with a SiO ₂ ratio of 0, 3 and 5 mass% and contain a silicone resin, are as described in Example 5 above. , 12 and 19 were found to be capable of forming an electrode having a larger film thickness. That is, it was confirmed that the shape stability during firing was improved and an electrode having a high aspect ratio could be formed regardless of the composition of the glass frit. Moreover, the series resistance Rs of the solar cell produced using this electroconductive composition was generally low, conversion efficiency Eff was high, and it has confirmed that electric power generation performance was improved with the improvement in aspect ratio.

Regarding the conductive compositions of Examples 6, 7, 13, 14, 20 and 21 in which the proportion of SiO ₂ in the glass frit is 7% by mass and contains a silicone resin, Example 5 does not contain a silicone resin. , 12 and 19, the thickness of the formed electrode is thicker, but on the other hand, it was confirmed that the series resistance and the conversion efficiency deteriorate. This is because a bulky electrode can be formed by the action of the silicone resin, but the glass frit and SiO ₂ derived from the silicone resin are present in the electrode after firing, and this relatively large amount of SiO ₂ serves as a resistance component. It is thought to be due to the action. Accordingly, it has been found that when a silicone resin is blended in the conductive composition, the proportion of SiO _{2 in} the glass frit is preferably less than 7% by mass, for example, 5% by mass or less.

Also, among the conductive composition containing a silicone resin, examples 1, 8 and 15 are examples using the glass frit that does not contain SiO ₂ component. According to conventional common sense, these cannot be sufficiently reacted with the Si substrate at the time of firing, and it is expected that a good contact at the Si substrate / electrode interface is not formed. However, the series resistance and conversion efficiency of the solar cells of Examples 1, 8 and 15 were all equal or better than those using the glass frit containing the SiO ₂ component. From this, it is considered that by adding a silicone resin to the conductive composition, this silicone resin exhibits the same action as SiO ₂ in the glass frit during firing and functions as a substitute for SiO ₂ . Then, by blending the silicone resin in the conductive composition, it was confirmed that it is possible to reduce or zero the proportion of SiO ₂ in the glass frit. Since the glass frit not containing SiO ₂ can greatly reduce the softening point, it is considered that the firing temperature during electrode formation can be lowered by using such a conductive composition containing glass frit.

Furthermore, Examples 3, 10 and 17 of the conductive composition containing a silicone resin are examples in which the silicone resin is suppressed to a very small amount. Even with the addition of such a very small amount of silicone resin, a thick electrode can be formed as compared with Examples 5, 12 and 19 using a glass frit having a SiO ₂ content of 7% by mass, The series resistance and conversion efficiency of solar cells were the same or better results. Therefore, by adding a silicone resin to the conductive composition even in a very small amount, the effect of improving the shape stability of the conductive composition (application) during printing or baking was obtained, and the aspect ratio was improved. It has been found that the electrode can be formed.

(Embodiment 2)
[Preparation of conductive composition]
The influence of the weight average molecular weight of the silicone resin on the characteristics of the conductive composition was evaluated by the following procedure. That is, A5 in Embodiment 1 was used as the glass frit. Moreover, as a silicone resin, the weight average molecular weight Mw is (S1) 3000, (S2) 10,000, (S3) 20,000, (S4) 50,000, (S5) 70,000, (S6) 90,000 and (S7) 11 Seven kinds of polydimethylsiloxane were used. Then, 0.3 parts by mass of any one of these silicone resins is blended with respect to 100 parts by mass of the silver powder, and the other conditions are the same as in the first embodiment to prepare the conductive compositions of S1 to S7. did. For comparison, an S0 conductive composition containing no silicone resin was also prepared.
Subsequently, the solar cell element of S0-S7 was formed using the electroconductive composition of S0-S7 prepared in this way using the screen printing method similarly to the said Embodiment 1. FIG.

[Evaluation]
About the light-receiving surface electrode (finger electrode) formed as described above, the number of disconnections, the aspect ratio, and the line resistance _RL were measured by the following procedure.
The number of electrode disconnections was determined by specifying the number of electrode disconnections (cracks) for 100 substrates using a solar cell electroluminescence (EL) inspection device. The result is shown in FIG. 3 as an average value of the number of disconnection points per substrate.

The aspect ratio of the electrode is calculated by measuring the width W and thickness (height) H of the light-receiving surface electrode of each example with a shape analysis laser microscope (manufactured by Keyence Corporation), and calculating the aspect ratio as (H / W). I asked for it. The results are shown in FIG. 3 as an average value of values measured for 100 light receiving surface electrodes.
The line resistance value of the electrode was measured as a resistance value (Ω) at an arbitrary interval (24 mm) on the finger electrode surface using a resistance meter (manufactured by Hioki Electric Co., Ltd., Digital HiTester). The results are shown in FIG. 4 as an average value of values measured for 100 light receiving surface electrodes.
The markers in the graphs of FIGS. 3 and 4 indicate the results of S0 to S7 in order from the left side according to the X axis (that is, the weight average molecular weight Mw).

As is apparent from FIG. 3, it was confirmed that the aspect ratio of the electrode can be significantly increased by adding a silicone resin to the conductive composition. Further, as the weight average molecular weight of the added silicone resin increases, it becomes possible to form an electrode with a high aspect ratio, and it is expected that Si contained in the silicone resin contributes to maintaining the shape of the electrode.
Moreover, it has confirmed from FIG. 3 that the number of disconnection of an electrode changes with addition of a silicone resin in an electroconductive composition, and the weight average molecular weight of the added silicone resin. That is, in this embodiment, it was confirmed that the number of disconnections of the electrode can be reduced by adding a silicone resin having a weight average molecular weight of, for example, 90,000 or less to the conductive composition as compared with the case where no silicone resin is added. . However, it was found that when the weight average molecular weight exceeds 90,000, for example, 110,000 silicone resin is used, the number of disconnections tends to increase.

  As is apparent from FIG. 4, it was found that the line resistance of the electrode was affected by the weight average molecular weight of the silicone resin in the conductive composition. This result shows a tendency similar to the result of the number of disconnections. That is, from the viewpoint of increasing the electrode shape to a high aspect ratio, it is preferable to add a silicone resin to the conductive composition. However, the inclusion of an excessive Si component can cause a disconnection of the electrode, and thus can be a resistance component. From these, it was found that it is preferable to use a silicone resin having an appropriate weight average molecular weight in order to achieve both high aspect ratio and low resistance characteristics of the electrode. In the present embodiment, it can be seen that the silicone resin added to the conductive composition is preferably, for example, one having a weight average molecular weight of less than 110,000, more preferably about 90,000 or less.

(Embodiment 3)
[Preparation of conductive composition]
A conductive composition was prepared by the following procedure, and the relationship between the silicone resin in the composition and the series resistance Rs was evaluated. Here, as the glass frit, the content of SiO ₂ component to a leaded glass is 5 mass% "A5", a lead-free glass 1 content of SiO ₂ component is 5 wt% "B5 Was used. As the silicone resin, polydimethylsiloxane having a weight average molecular weight Mw of 50,000 was used. And the ratio of these glass frit and silicone resin with respect to 100 mass parts of silver powder was changed by the combination shown in following Table 3, and other conditions were carried out similarly to the said Embodiment 1, and prepared the electrically conductive composition. Moreover, the solar cell element was produced by forming a light-receiving surface electrode using these electroconductive compositions.

The series resistance Rs of the solar cell element thus prepared was measured and shown in Table 3. In addition, it is known that the conductive composition using the leaded glass frit A5 can form electrodes having higher characteristics such as the series resistance Rs than the conductive composition using the lead-free glass frit B5. Yes. Accordingly, in Table 3 below, when glass frit A5 is used, it is determined that the resistance is low and good when Rs ≦ 3.73, and when glass frit B5 is used, Rs ≦ 3.91. The resistance was judged to be low and good. In Table 3, good results with low resistance are shown in bold.
Further, the ratio of the glass frit to the silicone resin was calculated from the blending amount of the glass frit and the silicone resin shown in Table 3 and shown in Table 4. In Table 4, the results are shown in bold for the combinations of the glass frit and the silicone resin with good resistance in Table 3.

[Evaluation]
As shown in Table 3, in the present embodiment, when the amount of the silicone resin contained in the conductive composition is approximately 0.2 parts by mass or less, the amount of glass frit added is within a predetermined range, and the series It can be seen that the resistance Rs can be effectively reduced. For example, it can be seen that the series resistance Rs can be effectively reduced by keeping the amount of glass frit added to 1.875 to 3.125 parts by mass regardless of the glass composition.
On the other hand, in the range where the amount of silicone resin contained in the conductive composition generally exceeds 0.15 parts by mass (for example, 0.2 parts by mass or more), the ratio of glass frit amount to silicone resin amount (glass frit mass / silicone) It can be seen that the series resistance Rs can be effectively reduced by keeping the resin mass) within a predetermined range. For example, regardless of the glass composition, the series resistance Rs can be effectively reduced by keeping the ratio of the glass frit amount in the range of about 7.5 to 18, preferably about 8.33 to 16.67. Recognize. This is considered to be because, for example, when the amount of the silicone resin contained in the conductive composition is increased, it is necessary to increase the amount of glass frit added in order to keep the series resistance Rs of the formed electrode low. That is, when a silicone resin is contained in the conductive composition, a SiO ₂ component can be generated from this silicone resin during electrode firing. This SiO ₂ component has an effect of suppressing erosion at the interface between the electrode and the anti-reflection film or the substrate, like the SiO ₂ component in the glass frit. Therefore, in order to achieve good contact between the electrode and the substrate while exhibiting good fire-through characteristics, the amount of glass frit added is adjusted to a value suitable for the amount of silicone resin added to the conductive composition. It is preferable to keep it.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible.

10 Solar cell element (cell)
11 Semiconductor substrate (silicon substrate)
11A Light-receiving surface 11B Back surface 12 Bus bar electrode (light-receiving surface electrode)
13 Finger electrode (light-receiving surface electrode)
14 Antireflection film 16 n-Si layer 18 p-Si layer 20 Back surface aluminum electrode 22 Back surface side external connection electrode 24 p ⁺ layer

Claims (9)

A conductive composition for forming an electrode, comprising:
Conductive powder;
Glass frit,
Silicone resin,
An organic binder,
A dispersion medium,
The glass frit has a SiO ₂ component ratio of 0% by mass or more and 5% by mass or less when converted to an oxide,
The electrically conductive composition whose weight average molecular weights of the said silicone resin are 90000 or less.   The conductive composition according to claim 1, wherein a ratio of the silicone resin to 100 parts by mass of the conductive powder is 0.005 parts by mass or more and 0.9 parts by mass or less.   The conductive composition according to claim 1 or 2, wherein the silicone resin has a weight average molecular weight of 3000 or more and 90000 or less.   The metal species constituting the conductive powder includes any one or more elements selected from the group consisting of nickel, platinum, palladium, silver, copper, and aluminum. 2. The conductive composition according to item 1.   The conductive composition according to any one of claims 1 to 4, wherein an average particle diameter of the conductive powder is 1 µm or more and 3 µm or less.   6. The conductive composition according to claim 1, wherein the organic binder is included in a ratio of 1% by mass to 10% by mass when the mass of the conductive powder is 100% by mass. .   The softening point of the glass frit is 300 ° C. or more and 600 ° C. or less,
  The conductive composition according to claim 1, which is used for forming the electrode by firing at a temperature equal to or higher than the softening point. To any one of claims 1 to 7 is provided with an electrode which is burned material of conductive composition according semiconductor device. The light receiving surface of the semiconductor substrate, and a light-receiving surface electrode is a fired product of the electrically conductive composition according to any one of claims 1 to 7 solar cell element.

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