JP7027025B2 - Conductive composition - Google Patents

Description

The present invention relates to a conductive composition. More specifically, it relates to a conductive composition that can be used to form an electrode of a solar cell.

From the viewpoint of increasing environmental awareness and energy saving in recent years, solar cells that convert light energy into electric power are rapidly becoming widespread. Along with this, there is a demand for solar cells having good photoelectric conversion efficiency. As one measure to realize this requirement, in the solar cell, a passivation film that suppresses recombination and an antireflection film that enhances the light receiving efficiency are provided, and the electric power generated by the pn junction in the substrate is electrode with high efficiency. Attempts have been made to remove from.

In the production of this type of solar cell, a conductive composition for forming an electrode is supplied in a desired electrode pattern on an antireflection film provided on a light receiving surface of a silicon substrate and fired. This conductive composition typically contains a conductive powder such as silver, a glass frit, a binder component, and a dispersion medium, and is prepared in the form of a paste (including a slurry and an ink). ing. The conductive composition is supplied to the light receiving surface of the solar cell in a predetermined electrode pattern by a method such as a screen printing method. Then, during the firing of the electrode, the glass frit in the conductive composition reacts with the antireflection film, the conductive powder passes through the antireflection film (fire-through), and is electrically connected to the emitter layer on the surface of the silicon substrate (fire-through). Ohmic contact) is realized. As a result, the current formed by the pn junction of the silicon substrate can be taken out to the electrode. As a conductive composition for forming an electrode of a solar cell containing such a glass frit, for example, Patent Document 1 can be mentioned.

Japanese Unexamined Patent Publication No. 2010-087251 Japanese Unexamined Patent Publication No. 2012-023095 Japanese Unexamined Patent Publication No. 2015-122177

By the way, in recent years, it has been attempted to suppress surface recombination by thinning the emitter layer of the solar cell, thereby improving the conversion efficiency. In a substrate having a thinned emitter layer (Lightly Doped Emitter (LDE)), it is required that the sheet resistance is increased and ohmic contact between the electrode and the substrate by fire-through is realized in the thin emitter layer. Be done. However, it is difficult to realize ohmic contact by fire-through in a thin emitter layer, and there is a problem that the conversion efficiency of the solar cell tends to decrease. Therefore, it is required to improve the conversion efficiency especially in the LDE type solar cell.

The present invention has been made in view of such a situation, and the main purpose thereof is to provide a solar cell capable of forming an electrode capable of obtaining good ohmic contact even with respect to LDE, for example, and having improved conversion efficiency. It is to provide a feasible conductive composition. Another object of the present invention is to provide a solar cell element having improved power generation performance, which is realized by adopting this conductive composition.

So far, the present inventors have made a conductive composition capable of preferably forming an electrode having a high aspect ratio in order to reduce the line resistance of the light receiving surface electrode of the solar cell and realize a solar cell having good output characteristics. (For example, Japanese Patent Application No. 2015-001855). This conductive composition contains glass frit and a silicone resin (also simply referred to as silicone) in addition to the conductive powder, and contacts (contacts) by adjusting the amount of SiO ₂ components in the composition. It is possible to suppress an increase in resistance and stably form a fine electrode having a high aspect ratio. However, with respect to the conductive composition containing such a silicone resin, apart from the viewpoint of improving the performance of the solar cell due to the shape of the formed electrodes, from the electrochemical aspect such as the realization of better ohmic contact. There was room for improvement.

Therefore, as a result of further diligent research by the present inventors, by reexamining the composition of the conductive composition, good ohmic contacts are formed even for LDE, for example, and unprecedented high conversion efficiency is realized. We have found a possible conductive composition. The present invention has been completed based on such findings. That is, the techniques disclosed herein provide a conductive composition that can be suitably used for forming the electrodes of a solar cell. This conductive composition is characterized by containing a glass frit, a silicone resin, and a lead-containing compound powder, in addition to a conductive powder, an organic binder, and a dispersion medium.

Patent Document 1 is a technique relating to a type of conductive composition using a lead-free glass frit as the glass frit. In Patent Document 1, when a lead-free glass frit is used instead of a lead-containing glass frit that can realize high conversion efficiency, the conversion efficiency is ensured by adding a Si-based compound to the conductive composition. Is disclosed.
Further, Patent Document 2 is a technique relating to a type of conductive composition that does not contain glass frit that can become a resistance component after firing as a binder component. This Patent Document 2 discloses that a fatty acid silver salt is used in order to realize good printability and good adhesion between a substrate and an electrode. Further, it is disclosed that it is preferable to use a thermosetting epoxy resin, polyester resin, silicone resin, urethane resin or the like in order to improve the adhesion to the silicon substrate.
However, according to the tests conducted by the present inventors, since the conductive compositions of Patent Documents 1 and 2 contain a silicone resin, the conversion efficiency of the solar cell using the LDE substrate produced by using this composition is high. Has been confirmed to have room for improvement. Further, according to the study by the present inventors, the conductive composition containing the silicone resin has a finding that the power generation efficiency is remarkably lowered when the addition amount of the silicone resin is increased.

On the other hand, the present inventors have proposed a type of conductive composition using a glass frit containing tellurium as the glass frit (see Patent Document 3). In this Patent Document 3, in order to improve the erosion suppressing effect of the lead-free tellurium glass frit and to avoid the decrease in the adhesive strength observed in the lead-tellurium glass frit, the lead-free tellurium glass frit is included in the conductive composition. It is designed to contain lead-containing compound powder. According to the tests by the present inventors, since the conductive composition of Patent Document 3 contains tellurium-containing glass frit, it is compared with the solar cell produced by using the conductive composition disclosed in Patent Documents 1 and 2. Therefore, high conversion efficiency can be realized. However, there is still room for further improvement in this conversion efficiency.

That is, the conductive composition disclosed herein contains a silicone resin and a lead-containing compound powder. By manufacturing a solar cell using such a conductive composition, it is possible to realize an unprecedentedly high conversion efficiency without being greatly affected by the composition of the glass frit. This makes it possible to easily and stably manufacture a solar cell that realizes high conversion efficiency.

In a preferred embodiment of the conductive composition disclosed herein, the glass frit is contained in a proportion of 0.1 to 12% by mass when the conductive powder is 100% by mass. Further, in the lead-containing compound powder, when the glass frit and the lead-containing compound powder are mixed to prepare a virtual glass, the ratio of PbO in the oxide conversion composition of the virtual glass is 30 mol% or less. It is characterized by being included. Thereby, the conversion efficiency of the solar cell produced by using this conductive composition can be further stably increased.

In a preferred embodiment of the conductive composition disclosed herein, the lead-containing compound powder contains at least one lead selected from the group consisting of metallic lead, lead monoxide, lead tan, lead nitrate and lead carbonate. It is characterized by being a powder of a compound. With such a configuration, the conversion efficiency of the solar cell produced by using this conductive composition can be stably increased. In addition, the adhesiveness between the electrode and the substrate after firing can be maintained high.

In a preferred embodiment of the conductive composition disclosed herein, the lead-containing compound powder is characterized in that it is supported on the glass frit. Even with such a configuration, the conversion efficiency of the solar cell produced by using this conductive composition can be stably improved.

In a preferred embodiment of the conductive composition disclosed herein, the silicone resin is characterized in that it is contained in a proportion of 0.9% by mass or less with respect to 100% by mass of the conductive powder. With such a configuration, the conversion efficiency of the solar cell produced by using this conductive composition can be stably increased.

In a preferred embodiment of the conductive composition disclosed herein, the silicone resin is characterized by having a weight average molecular weight of 1,000 or more and 150,000 or less. With such a configuration, the electrical characteristics such as the line resistance of the electrode can be further enhanced as compared with the case where the silicone resin is not added.

In a preferred embodiment of the conductive composition disclosed herein, the silicone resin is characterized by containing at least one of a polydimethylsiloxane and a polyether-modified siloxane. With such a configuration, it is possible to form an electrode having good adhesiveness to the substrate.

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 a species of element. With such a configuration, it is possible to construct an electrode having excellent conductivity.

The conductive composition of the present invention can realize good ohmic contact even for a solar cell substrate having a thin n ⁺ layer, for example. As a result, it is possible to manufacture a solar cell that suppresses surface recombination and realizes an unprecedentedly high conversion efficiency. In addition, the adhesiveness between the electrode and the silicon substrate can be maintained high.

It is sectional drawing which shows typically an example of the structure of a solar cell. It is a top view which shows typically the pattern of the electrode formed on the light receiving surface of a solar cell. It is a side view schematically showing the strength measuring apparatus which measures the adhesive strength between a substrate of a solar cell and an electrode. It is a graph which showed the relationship between the silicone content of the conductive composition which concerns on a reference example, and the open circuit voltage (Voc) of a solar cell. It is a graph which showed the relationship between the silicone content of the conductive composition which concerns on a reference example, and the curve factor (FF) of a solar cell.

Hereinafter, preferred embodiments of the present invention will be described. It should be noted that technical matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention are the technical contents taught in the present specification and general matters of those skilled in the art in the art. It can be carried out based on common technical knowledge. In this specification, the notation "A to B" indicating the range means A or more and B or less.

The conductive composition disclosed herein can typically be fired to form an electrode for a solar cell. That is, the fired product of this conductive composition can form an electrode of a solar cell. This conductive composition is essentially similar to conventional conductive compositions of this type, with conductive powders, glass frits, and organic vehicle components for dispersing these components (discussed below, but: It is composed of a mixture of an organic binder and a dispersion medium), and further contains a silicone resin and a lead-containing compound powder as essential components.

In the technique disclosed in Patent Document 1, a lead-free glass frit is used instead of the leaded glass frit, and the conversion efficiency is enhanced by adding a Si-based compound to the conductive composition. However, if the amount of the silicone resin added increases, the power generation efficiency will be significantly reduced. Further, in the technique disclosed in Patent Document 3, lead-free tellurium glass frit is used instead of lead-tellurium glass frit, and lead is contained in the conductive composition in order to improve the substrate erosion suppressing effect of the lead-free tellurium glass frit. It was designed to contain additives. At this time, it is described that the lead-containing additive is present in the conductive composition in a state of being bonded to the lead-free tellurium glass frit, so that an electrode having further excellent electrical characteristics can be obtained.

However, in the technique disclosed herein, the silicone resin and the lead-containing compound powder are simultaneously contained in the conductive composition. In this case, it is possible to obtain an electrode having higher electrical characteristics than the effect of improving the electrical characteristics when the silicone resin or the lead-containing compound powder is added alone to the conductive composition. Furthermore, it is possible to obtain an electrode having even higher electrical characteristics than the sum of the effects of improving the electrical characteristics when the silicone resin and the lead-containing compound powder are added alone. It has been confirmed that such a synergistic effect can be obtained when various glass frit and lead-containing compound powder are used in combination without depending on the composition of the glass frit. That is, it has been found that a conductive composition containing a glass frit having an arbitrary composition disclosed herein and a silicone resin and a lead-containing compound powder can form an electrode having an unprecedentedly excellent electrical property. Although the detailed mechanism is not clear, in the conductive composition disclosed herein, the silicone resin acts on each of the lead-containing additive, the glass frit, and the substrate, even if it is an LDE type substrate. Also, it is considered that the glass frit and the lead-containing compound powder are suppressed from being excessively eroded on the substrate and act to proceed favorably. It is considered that this greatly alleviates the sharp decrease in power generation performance due to the addition of the silicone resin. Further, it is considered that the erosion interface of the substrate by the lead-containing compound powder is roughened, but the erosion action of the glass frit and the lead-containing compound powder is homogenized to realize smooth erosion of the substrate as a whole. ..

Hereinafter, each component of the conductive composition disclosed herein will be described.
As the conductive powder that is the main component of the solid content of the paste, a powder made of various metals or alloys thereof having desired conductivity and other physical characteristics according to the intended use can be considered. As an example of the material constituting such a conductive powder, gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), iridium ( Metals such as Ir), osmium (Os), nickel (Ni) and aluminum (Al) and their alloys, carbonaceous materials such as carbon black, LaSrCoFeO ₃ oxides (eg LaSrCoFeO ₃ ), LaMnO ₃ oxides (eg LaSrCoFeO 3). For example, conductive ceramics typified by transition metal perovskite type oxides represented as LaSrGaMgO ₃ ), LaFeO ₃ series oxide (for example, LaSrFeO ₃ ), LaCoO ₃ series oxide (for example, LaSrCoO ₃ ) and the like are exemplified. Among them, simple substances of noble metals such as platinum, palladium and silver, alloys thereof (for example, Ag-Pd alloy, Pt-Pd alloy, etc.), nickel, copper, aluminum and alloys thereof are particularly preferable. It is mentioned as a material which constitutes a sex powder. From the viewpoints of relatively low cost and high electrical conductivity, a powder made of silver and an alloy thereof (hereinafter, also simply referred to as “Ag powder”) is particularly preferably used. Hereinafter, the conductive composition of the present invention may be described by taking the case where Ag powder is used as the conductive powder as an example, but the present invention is not limited thereto.

The particle size of the conductive powder is not particularly limited, and various particle sizes depending on the intended use can be used. Typically, one having an average particle diameter of 5 μm or less is suitable, and one having an average particle diameter of 3 μm or less (typically 1 μm to 3 μm, for example, 1.5 μm to 2.5 μm) is preferably used. For the average particle size of the conductive powder, an integrated 50% particle size (D ₅₀ ) in a volume-based particle size distribution measured by a particle size distribution measuring device based on a laser diffraction / scattering method can be adopted. As the average particle size of the conductive powder in the present specification, the value measured by using a laser diffraction / scattering type particle size distribution measuring device (LA-920, manufactured by HORIBA, Ltd.) is adopted.

The shape of the particles constituting the conductive powder is not particularly limited. Typically, a spherical shape, a flake shape, a conical shape, a rod shape, or the like can be preferably used. It is preferable to use spherical or scaly particles because the filling property is good and it is easy to form a dense light receiving surface electrode. As the conductive powder to be used, one having a sharp (narrow) particle size distribution is 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 (D) of the particle size (D ₁₀ ) at the cumulative volume of 10% and the particle size (D ₉₀ ) at the cumulative volume of 90% from the small particle size side in the particle size distribution based on the laser diffraction / scattering method. ₁₀ / D ₉₀ ) can be adopted. When all the particle sizes constituting the powder are equal, the value of D ₁₀ / D ₉₀ becomes 1, and conversely, the wider the particle size distribution, the closer the value of D ₁₀ / D ₉₀ approaches to 0. It is preferable to use a powder having a relatively narrow particle size distribution such that the value of D ₁₀ / D ₉₀ is 0.2 or more (for example, 0.2 or more and 0.5 or less).

On another aspect, the conductive powder can also be used as a mixture of two particle groups having different average particle diameters. In this case, for example, the average particle size (D ₅₀ ) of the first particle group is in the range of 1.5 μm to 2.5 μm (for example, 2 μm), and the average particle size (D ₅₀ ) of the second particle group is 2 μm to. A preferred example is the range of 3 μm (for example, 2.5 μm). At this time, the particle size distribution of each particle group is preferably sharp as described above. Then, for example, the first particle group is mixed so as to have a ratio of 80 to 95% by volume (for example, 90% by volume), and the second particle group has a ratio of 20 to 5% by volume (for example, 10% by volume). do. This makes it possible to prepare a conductive powder having good filling property.
A conductive composition using a conductive powder having such an average particle size and a particle shape has 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.

The conductive powder is not particularly limited depending on the production method and the like. For example, a conductive powder produced by a well-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 carried out, for example, by using a classification device or the like using a centrifugation method.

The glass frit is a component that can function as an inorganic binder of the conductive powder, and enhances the bondability between the conductive particles constituting the conductive powder and between the conductive particles and the substrate (the object on which the electrode is formed). To work. Further, when this conductive composition is used, for example, for forming a light receiving surface electrode of a solar cell, the presence of this glass frit causes the conductive composition to penetrate an antireflection film or the like as a lower layer during firing. It is possible to achieve good adhesion and electrical contact between the conductive particles (ie, the electrodes) and the substrate.

It is preferable that such a glass frit is adjusted to a size equal to or smaller than that of the conductive powder. The average particle size of the glass frit is, for example, preferably 4 μm or less, and more preferably about 3 μm or less. The lower limit of the average particle size of the glass frit is not particularly limited, but can be typically 0.5 μm or more, more preferably 1 μm or more. As the average particle size of the glass frit in the present specification, the integrated 50% particle size (D50) in the volume-based particle size distribution measured by the particle size distribution measuring device based on the laser diffraction / scattering method is adopted as in the case of the conductive powder. can do.

The glass frit in the present embodiment is not particularly limited, and various glasses used in this kind of conductive composition can be used. As an approximate glass composition, for example, in addition to the so-called lead-based glass commonly used by those skilled in the art, lead lithium-based glass, zinc-based glass, borate-based glass, borosilicate-based glass, and alkaline-based glass. It may be glass, tellurium-based glass, lead-tellu-based glass, or a glass-based glass containing barium oxide, bismuth oxide, or the like. Needless to say, these glasses may contain any other element in addition to the main glass constituent elements appearing in the above names. Such elements include Si, Zn, Ba, Bi, B, Pb, Al, Li, Na, K, Rb, Te, Ag, Zr, Sn, Ti, W, Cs, Ge, Ga, In, Ni. , Ca, Cu, Mg, Sr, Se, Mo, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Ta, V, Fe, Hf, Cr, Cd , Sb, F, Mn, P, Ce, Nb and the like. The glass frit can contain any one or more of these elements in any combination and proportion. Such a glass frit may be, for example, a general amorphous glass or a crystallized glass partially containing crystals. Further, as the glass frit, a glass frit having one kind of composition may be used alone, or a glass frit having two or more kinds of compositions may be mixed and used.

The softening point of the glass constituting the glass frit is not particularly limited, but is preferably about 250 ° C. or higher and 600 ° C. or lower (for example, 300 ° C. or higher and 500 ° C. or lower). Specific examples of the glass whose softening point can be adjusted within the range of 250 ° C. or higher and 600 ° C. or lower include glass containing a combination of the following elements. B-Si-Al-based glass, Pb-B-Si-based glass, Si-Pb-Li-based glass, Si-Al-Mg-based glass, Ge-Zn-Li-based glass, B-Si-Zn-Sn-based glass, B-Si-Zn-Ta-based glass, B-Si-Zn-Ta-Ce-based glass, B-Zn-Pb-based glass, B-Si-Zn-Pb-based glass, B-Si-Zn-Pb-Cu-based Glass, B-Si-Zn-Al-based glass, Pb-B-based glass, Pb-B-Mo-based glass, Pb-B-Si-Ti-Bi-based glass, Pb-B-Si-Ti-based glass, Pb- B-Si-Al-Zn-P-based glass, Pb-Li-Bi-Te-based glass, Pb-Si-Al-Li-Zn-Te-based glass, Pb-B-Si-Al-Li-Ti-Zn-based Glass, Pb-B-Si-Al-Li-Ti-P-Te-based glass, Pb-Si-Li-Bi-Te-based glass, Pb-Si-Li-Bi-Te-W-based glass, P-Pb- Zn-based glass, P-Al-Zn-based glass, P-Si-Al-Zn-based glass, P-B-Al-Si-Pb-Li-based glass, PB-Al-Mg-FK-based glass, Te- Pb-based glass, Te-Pb-Li-based glass, VP-Ba-Zn-based glass, VP-Na-Zn-based glass, Pb-VP-based glass, AgI-Ag ₂ O-BP-based Glass, Zn-B-Si-Li-based glass, Si-Li-Zn-Bi-Mg-W-Te-based glass, Si-Li-Zn-Bi-Mg-Mo-Te-based glass, Si-Li-Zn- Bi-Mg-Cr-Te glass and the like. The above glass contains at least a plurality of elements connected by a hyphen (-), and the total amount of oxides of the elements based on the composition when those elements are converted into oxides (oxide conversion composition). However, it means that it accounts for 70 mol% or more (typically 80% or more, more preferably 90% or more) of the whole. When a conductive composition containing a 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 at a relatively low firing temperature. It is preferable because it contributes to the formation of high-performance electrodes.

The glass frit may be a leaded glass frit containing lead or a lead-free glass frit containing no lead. However, when leaded glass frit is used, it is preferable that the ratio of lead in the glass frit is less than 30 mol% as PbO in the oxide conversion composition, as described later. This is because (1) in the conductive composition disclosed herein, it is most preferable that the lead component is essentially contained in the oxide equivalent composition of the glass frit in a proportion of PbO of 30 mol% or less. This is because we have obtained the knowledge that it is. However, when (2) leaded glass frit is used, the eroded surface of the substrate due to the conductive composition during firing is uniformly formed and smoothed, so that good electrical characteristics can be obtained, but the substrate and the substrate can be used. Adhesive strength tends to be low. In the electrode of a solar cell, in order to take out the generated electric power to the outside or modularize it, a lead wire for taking out a current may be soldered or a conductive film may be attached to the surface of the electrode. Since the solar cell is used outdoors for a long period of time, it is preferable that the physical adhesion between the substrate and the electrode is good even when a lead wire or a conductive film is connected. Therefore, it may be important that the lead component in the conductive composition disclosed herein contains at least a part as a lead-containing compound powder. When the adhesive strength between the substrate and the electrode is required, it is preferable to use a glass frit having a low lead content as the glass frit, and it is particularly preferable to use a lead-free glass frit. The lead-free glass frit referred to here means that the ratio of lead (Pb) in the entire conductive composition is 1000 ppm (mass standard; the same applies hereinafter) or less, and for example, lead in the entire glass frit. It means that the ratio of (Pb) is 10,000 ppm or less. More preferably, the lead-free glass frit is substantially free of lead.

Further, the glass frit preferably has a low content of Si as a glass constituent element. By reducing the proportion of Si (eg as SiO ₂ ) in the glass frit, the softening point of the glass frit is directly lowered and the glass wets and spreads over the conductive powder and substrate at lower temperatures during firing. preferable. From this point of view, the Si component as SiO ₂ is preferably 1000 ppm or less with respect to the conductive composition before firing and 10000 ppm or less with respect to the glass frit.
Further, the glass frit preferably has a low content of Cu as a glass constituent element. It is preferable that the ratio of Cu (for example, as CuO) in the glass frit is reduced because the electrical characteristics of the formed electrode are improved. From this point of view, the Cu component is preferably 1000 ppm or less for the conductive composition before firing and 10000 ppm or less for the glass frit as CuO.
When these elements are contained in the glass frit, the lead-containing compound powder described later is integrally supported on the glass frit, so that the inconvenient tendency due to these Si and Cu can be clearly suppressed. preferable.

The silicone resin is an essential component contained in the conductive composition disclosed herein. By containing this silicone resin, this conductive composition acts on the lead-containing glass frit described above during firing, improves fire-through characteristics, suppresses excessive erosion of the Si substrate, and makes good contacts. It is thought that it can be formed. Further, the silicone resin can maintain the shape of the coating film (unfired electrode) of the conductive composition formed on the Si substrate stably from printing to firing, and has a finer and higher aspect ratio. It enables stable formation of electrodes. Further, the silicone resin can generate a SiO ₂ component in the electrode by firing. This SiO ₂ component enhances the stability of the system and the bondability between the electrode and the substrate by being incorporated into the glass component during the firing process without directly increasing the softening point of the leaded glass frit. It is thought that it will contribute to.

As this silicone resin, an organic compound containing silicon (Si) can be used without particular limitation, and for example, a polymer organic compound having a main skeleton composed of a siloxane bond (Si—O—Si) can be preferably used. can. Examples of the siloxane compound (polymer) forming the main skeleton portion include poly containing a siloxane unit represented by the general formula: HO [-Si (R) ₂ O-] _n H, R is hydrogen or any functional group; It may be a siloxane, a polyalkylsiloxane in which R is an arbitrary alkyl group, or a polymer obtained by polymerizing a siloxane unit and a silicon-containing monomer different from the siloxane unit. Specific examples of the silicone resin include polydialkylsiloxanes such as polydimethylsiloxane, polydiethylsiloxane, and polymethylethylsiloxane, polyalkylarylsiloxane, and poly (dimethylsiloxane-methylsiloxane). The polymer constituting a particularly suitable main skeleton can be, for example, polydimethylsiloxane. Further, the silicone resin may be, for example, a linear silicone in which hydrogen, an alkyl group, a phenyl group or the like is introduced into an unbonded hand (side chain, terminal or both) in the main skeleton. Alternatively, the silicone resin is 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 the side chain, the terminal, or both of the main skeleton. May be. Among them, a polydimethylsiloxane or a polyether-modified siloxane (polyether-modified silicone) in which a polyether group is introduced into a side chain, a terminal, or both of the polydimethylsiloxane can be preferably used. Alternatively, the polyether-modified siloxane may be a linear block copolymer (straight-chain polyether-modified silicone) in which polyether and silicone are alternately bonded.

Such a silicone resin is preferable because an electrode having a higher aspect ratio can be formed as the weight average molecular weight (hereinafter, may be simply referred to as “Mw”) increases. However, if Mw exceeds about 150,000, it is not preferable because it causes defects such as disconnection of the obtained electrode and increases resistance. From such a viewpoint, the Mw of the silicone resin can be, for example, 150,000 or less, preferably 130,000 or less, more preferably 110,000 or less, and particularly preferably 90,000 or less. preferable. 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 lead-containing compound powder, a powdery material containing a lead (Pb) component and which can stably exist in the conductive composition can be used without particular limitation. Examples of such lead-containing materials include metallic lead (Pb), lead-containing alloys, lead monoxide (PbO), lead dioxide (PbO ₂ ), lead tan (Pb ₃ O ₄ ), and lead. White (2PbCO ₃ · Pb (OH) ₂ ), lead nitrate (Pb (NO ₃ ) ₂ ), lead chloride (PbCl ₂ ), lead sulfide (PbS), lead yellow lead (PbCrO ₄ , Pb (SCr) O ₄ , PbO -PbCrO ₄ ), lead carbonate (PbCO ₃ ), lead sulfate (PbSO ₄ ), lead fluoride (PbF ₂ ), lead tetrafluoride (PbF ₄ ), lead bromide (PbBr ₂ ), lead iodide (PbI ₂ ). ) And other inorganic lead compounds, lead acetate (Pb (CH ₃ COO) ₂ ), lead tetracarboxylate (Pb (OCOCH ₃ ) ₄ ), tetraethyl lead (Pb (CH ₃ CH ₂ ) ₄ ) and other organic lead compounds. Illustrated. Among them, metallic lead, lead monoxide, lead tan, lead nitrate and lead carbonate are mentioned as materials constituting a particularly preferable conductive powder because they are relatively easy to handle and are not affected by other elements.

The particle size of the lead-containing compound powder is not particularly limited, and is preferably adjusted to a size equal to or smaller than that of the conductive powder, for example. The average particle size of the lead-containing compound powder is, for example, preferably 4 μm or less, and more preferably about 3 μm or less. The lower limit of the average particle size of the lead-containing compound powder is not particularly limited, but can be typically 0.1 μm or more, more preferably 0.3 μm or more. Adjusting to such a particle size is preferable because the dispersibility with the glass frit is improved. As the average particle size of the lead-containing compound powder, an integrated 50% particle size (D ₅₀ ) based on the laser diffraction / light dispersion method can be adopted as in the case of the conductive powder.

The shape of the particles constituting the lead-containing compound powder is not particularly limited. Typically, a spherical shape, a flake shape, a conical shape, a rod shape, or the like can be preferably used. Further, it may be in the form of secondary particles in which primary particles are aggregated. As such a lead-containing compound powder, for example, those produced by a well-known purification method, wet method, gas phase reaction method or the like can be classified and used as necessary. Such classification can be carried out, for example, by using a classification device or the like using a centrifugation method.

The lead-containing compound powder may be mixed in the conductive composition as it is in the powder state, or may be mixed in a state where a part or the whole is supported on the glass frit. The means for supporting the lead-containing compound powder on the glass frit is not particularly limited, and examples thereof include the use of a carrier method by calcining and a carrier method by a mechanochemical method.
In the support by calcination, the glass frit and the lead-containing compound powder may be mixed, placed on a setter or the like, and calcinated in an oxidizing atmosphere at a temperature of about 300 to 500 ° C. The calcination temperature can be set sufficiently lower than the temperature at which the glass frit and the lead-containing compound powder react with each other by sintering. As a result, the glass frit and the lead-containing compound powder can be integrated without significantly affecting the properties and composition of the glass frit.

Further, in the support by the mechanochemical method, for example, a dry particle composite device (for example, Nobilta NOB-130 manufactured by Hosokawa Micron Co., Ltd.) can be preferably used. By processing a mixed powder of glass frit and lead-containing compound powder with such a device, mechanical forces such as shearing and compression are applied between the particles to integrate the glass frit and lead-containing compound powder. Can be done. Thereby, for example, it is possible to obtain composite particles in which lead-containing compound powder is firmly fixed to the surface of one particle of a glass frit with a thickness of about one particle layer. Such composite particles can be used in place of the glass frit and are convenient.

As the organic vehicle component for dispersing the above-mentioned components such as the conductive powder, various substances conventionally used in this kind of conductive composition shall be used without particular limitation according to the desired purpose. Can be done. Typically, the vehicle is composed of organic binders of various compositions and organic solvents as dispersion media. The organic binder means that it is composed of an organic compound having a binder function with respect to the binder effect due to the glass frit, which can be said to be an inorganic binder. In such an organic vehicle component, the organic binder may be completely dissolved in an organic solvent, or only a part thereof may be dissolved or dispersed (it may be a so-called emulsion type organic vehicle).

As the organic binder, an organic compound having a binder function can be used without particular limitation. For example, cellulosic polymers such as ethyl cellulose and hydroxyethyl cellulose, acrylic resins such as polybutyl methacrylate, polymethyl methacrylate and polyethyl methacrylate, and organics based on epoxy resins, phenol resins, alkyd resins, polyvinyl alcohol, polyvinyl butyral and the like. Binders are 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 organic solvent constituting the organic vehicle is an organic solvent having a boiling point of about 200 ° C. or higher (typically about 200 ° C. to 260 ° C.). More preferably, an organic solvent having a boiling point of about 230 ° C. or higher (typically about 230 ° C. to 260 ° C.) is 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. Organic solvents such as derivatives, toluene, xylene, mineral spirit, tarpineol, mentanol, and texanol can be preferably used. Particularly preferable 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 component contained in the conductive composition may differ depending on the electrode forming method, typically the printing method, etc., but is generally the same as that of the conductive composition having a conventionally adopted composition. It can be configured based on the blending ratio. As an example, for example, the ratio of each component can be determined with the following formulation as a guide.

That is, it is appropriate that the content ratio of the conductive powder in the conductive composition is about 70% by mass or more (typically 70% by mass to 95% by mass) when the whole paste is 100% by mass. It is more preferably about 80% by mass to 90% by mass, for example, about 88% by mass. Increasing the content ratio of the conductive powder is preferable from the viewpoint of forming a dense electrode pattern with good shape accuracy. On the other hand, if this content ratio is too high, the handleability of the paste, the suitability for various printability, and the like may deteriorate.

The glass frit may be contained in the conductive composition in a proportion essentially required as an inorganic binder of the conductive powder. Further, from the viewpoint of obtaining good fire-through characteristics, the ratio of the glass frit to the conductive powder is typically 0.1% by mass or more when the conductive powder is 100% by mass. It is possible, preferably 0.5% by mass or more, and more preferably 1% by mass or more. It should be noted that excessive addition is not preferable because it erodes the silicon substrate and increases the resistance of the formed electrodes. Therefore, the ratio of the leaded glass frit to the conductive powder can be typically 12% by mass or less, preferably 6% by mass or less, and more preferably 3% by mass or less.

By adding even a very small amount of the silicone resin to the conductive powder, the electrical characteristics of the formed electrode can be enhanced and the printability can also be enhanced. That is, the amount of the silicone resin added may be more than 0% by mass. In order to clearly obtain the effect of adding the silicone resin, the amount of the silicone resin added can be typically 0.005% by mass or more when the conductive powder is 100% by mass, and is 0. It is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and particularly preferably 0.1% by mass or more. It should be noted that excessive addition of the silicone resin can increase the resistance of the formed electrodes. In addition, it may act excessively on the erosion of the substrate by the glass frit and the lead-containing additive (suppress excessive erosion). Therefore, the amount of the silicone resin added can be typically about 1.0% by mass or less when the conductive powder is 100% by mass, and should be 0.9% by mass or less. It is preferably 0.8% by mass or less, and particularly preferably 0.6% by mass or less.

By adding even a very small amount of the lead-containing compound powder to the conductive powder, the electrical characteristics of the formed electrode can be enhanced and the adhesiveness of the electrode can be enhanced. That is, the content of the lead-containing compound powder may be more than 0% by mass. It is more appropriate to consider the content of the lead-containing compound powder as the amount of lead component in the entire conductive composition, and can be considered as, for example, the total amount with the PbO component contained in the glass frit. More specifically, for example, when it is assumed that a glass frit and a lead-containing compound powder are mixed to prepare a virtual glass, the lead-containing compound powder is prepared in consideration of the amount of PbO in the oxide conversion composition of the virtual glass. It is preferable to determine the proportion. In order to clearly obtain the effect of adding the lead-containing compound powder, the amount of the lead-containing compound powder added can be set to an amount such that the ratio of the amount of PbO in the virtual glass is 1 mol% or more, and is set to 2 mol% or more. Is preferable, 5 mol% or more is more preferable, and 10 mol% or more is particularly preferable. The effect of adding the lead-containing compound powder may tend to be saturated to some extent, and excessive addition is not preferable because it leads to a reduction in the proportion of the conductive powder. Therefore, the amount of the lead-containing compound powder added can be set so that the ratio of the amount of PbO in the virtual glass is typically about 40 mol% or less, and is preferably 35 mol% or less, preferably 30 mol. It is more preferably% or less, and particularly preferably 25 mol% or less.

Then, among 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 in a ratio of 2% by mass to 6% by mass with respect to 100% by mass of the conductive powder. The organic binder may contain, for example, an organic binder component dissolved in an organic solvent and an organic binder component not dissolved in the organic solvent. When the organic binder component dissolved in the organic solvent and the organic binder component not dissolved are contained, the ratio thereof is not particularly limited, but for example, the organic binder component dissolved in the organic solvent may be used. It can be made to occupy (10% to 100%).
The content ratio of the organic vehicle as a whole is variable according to the properties of the obtained paste, and as a rough guide, when the whole conductive composition is 100% by mass, for example, 5% by mass to 30% by mass. The amount to be% is appropriate, 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).

Further, the conductive composition disclosed herein may contain various inorganic additives and / or organic additives other than the above, as long as it does not deviate from the object of the present invention. Suitable examples of the inorganic additive include ceramic powders (ZnO ₂ , Al ₂ O ₃ , etc.) other than the above, and various other fillers. Further, as a preferable example of the organic additive, an additive such as a surfactant, a defoaming agent, an antioxidant, a dispersant, and a viscosity modifier can be mentioned.

In the above conductive composition, in addition to the conductive powder, a glass frit, a silicone resin, and a lead-containing compound powder are used in combination. The silicone resin may remain in the electrode as a Si component (eg, SiO ₂ ) after firing the conductive composition. Therefore, when this Si component acts as a resistance component, it becomes a factor that deteriorates the electrode characteristics. However, with respect to the conductive composition disclosed herein, the deterioration of the electrode characteristics (for example, the reduction of the conversion efficiency in the solar cell) is not observed by appropriately using the silicone resin and the lead-containing compound powder in combination. From this, it is considered that the Pb component in the lead-containing compound powder appropriately suppresses the action of reducing FF due to the Si component. On the other hand, when the conductive composition contains a silicone resin, shape stability during printing by, for example, screen printing, gravure printing, offset printing, inkjet printing, or the like is enhanced. Therefore, this conductive composition is particularly suitable as a printing composition (sometimes referred to as a paste, slurry, ink, etc.) for an electrode formed by printing. In particular, it can be particularly preferably adopted when such a general-purpose printing means is used in forming an electrode pattern that requires fine lines and a high aspect ratio.

Hereinafter, among various electrodes provided in the solar cell element, for example, a comb-shaped electrode pattern including a fine finger electrode is formed on a light receiving surface by screen printing this conductive composition. The solar cell element disclosed in the above 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, and the present invention relates to the same configuration as the conventional one and the part related to the use of the same material as the conventional one. Since it does not characterize, detailed description is omitted.

1 and 2 schematically show an example of a solar cell (cell) 10 that can be suitably manufactured by carrying out the present invention, and is made of single crystal, polycrystalline or amorphous silicon (Si). It is a so-called silicon type solar cell 10 that uses a wafer as a semiconductor substrate 11. The cell 10 shown in FIG. 1 is a general single-sided light receiving type solar cell 10. Specifically, in this type of solar cell 10, the n—Si layer 16 formed by impurity doping on the light receiving surface 11A side of the p—Si layer (p-type crystalline silicon) 18 of the silicon substrate (Si wafer) 11. By providing the above, a pn junction is formed. On the surface of the silicon substrate 11, if necessary, an antireflection film 14 made of titanium oxide, silicon nitride, etc. formed by CVD or the like, and a light receiving surface formed of a conductive composition mainly containing Ag powder or the like. It includes electrodes 12 and 13. The antireflection film 14 may also serve as a passivation film, or a passivation film may be provided separately from the antireflection film 14.

On the other hand, on the back surface 11B side of the p—Si layer 18, a back surface aluminum electrode 20 that exhibits a so-called back surface electric field (BSF) effect and an external connection electrode 22 that draws a current from the aluminum electrode 20 are provided. .. The aluminum electrode 20 is formed on substantially the entire surface of the back surface by printing and firing a conductive composition mainly composed of aluminum powder. During this firing, an Al—Si alloy layer (not shown) is formed, and aluminum is diffused into the p—Si layer 18 to form a p ⁺ layer 24. By forming such a p ⁺ layer 24, that is, a BSF layer, recombination of photogenerated carriers in the vicinity of the back surface electrode is prevented, and for example, an improvement in short-circuit current and open circuit voltage (Voc) is realized. Further, the external connection electrode 22 is typically formed by printing and firing a conductive paste in which the conductive powder is Ag powder.

As shown in FIG. 2, on the light receiving surface 11A side of the silicon substrate 11 of the solar cell 10, several (for example, about 1 to 3) light receiving surface electrodes 12 and 13 are linearly parallel to each other. A bus bar (connection) electrode 12 and a large number of mutually parallel (for example, about 60 to 90) streaky finger (collection) electrodes 13 connected so as to intersect the bus bar electrode 12 are formed. Has been done. A large number of finger electrodes 13 are formed to collect light-generating carriers (holes and electrons) generated by light reception. The bus bar electrode 12 is a connection electrode for collecting current from the carrier 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. Therefore, by making the bus bar electrode 12 and the finger electrode 13 (particularly a large number of finger electrodes 13) provided on the light receiving surface 11A side as fine as possible, the non-light receiving portion (light shielding portion) corresponding to this is reduced. The light receiving area per cell unit area is expanded. This can very simply improve the output per unit area of the solar cell 10.

Generally, such a solar cell element 10 is manufactured through the following process.
That is, the silicon is prepared by preparing an appropriate silicon wafer and doping it with a predetermined impurity by a general technique such as a thermal diffusion method or an ion plantation to form the p—Si layer 18 and the n—Si layer 16. A substrate (semiconductor substrate) 11 is manufactured. At this time, the n—Si layer 16 can be formed so that the sheet resistance is high (for example, 80 to 120 Ω / □). Next, the antireflection film 14 made of silicon nitride or the like is formed by a technique such as plasma CVD.

Then, on the back surface 11B side of the silicon substrate 11, a predetermined conductive composition (typically, a conductive composition in which the conductive powder is Ag powder) is screen-printed in a predetermined pattern and dried. As a result, a back surface side conductor coating material that becomes the back surface 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) on the entire surface of the back surface side by a screen printing method or the like, and dried to form an aluminum film.

Next, the conductive composition of the present invention is printed on the antireflection film 14 formed on the surface side of the silicon substrate 11 by a screen printing method, typically by a predetermined wiring pattern as shown in FIG. Supply). 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). ) Can form a coating film (printed body) of an electrode pattern including the finger electrodes. Then, 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.

The silicon substrate 11 on which the paste coating material (dry film-like coating material) is formed on both sides in this way is used in an air atmosphere using a firing furnace such as a near-infrared high-speed firing furnace, and has an appropriate firing temperature (for example,). Bake at a peak temperature of 700 ° C to 900 ° C). By this firing, a fired aluminum electrode 20 is formed together with the light receiving surface electrodes (typically Ag electrodes) 12 and 13 and the back surface side external connection electrode (typically Ag electrodes) 22. At the same time, an Al—Si alloy layer (not shown) is formed, and aluminum diffuses into the p—Si layer 18 to form the above-mentioned p ⁺ layer (BSF layer) 24, whereby the solar cell 10 is manufactured.

In the solar cell 10 shown in FIG. 1, an example in which a p-type crystalline silicon substrate is used as the substrate 11 is shown, but a substrate made of n-type crystalline silicon can also be used. Since p-type crystalline silicon was used here, different conductive type phosphorus (P) was diffused in order to form a pn junction, but when an n-type silicon substrate was used, p-type impurities (for example, B) were diffused. ) May be diffused. Further, although the aluminum electrode 20 is formed on the back surface 11B side of the p—Si layer 18 without forming a passivation film or the like, a passivation film may be provided on the back surface 11B side as well.
Further, 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, and firing for forming the light receiving surface electrodes 12 and 13, and the aluminum electrode 20 on the back surface 11B side and the external connection. The firing for forming the electrode 22 may be performed separately.

Hereinafter, some examples of the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.
(Reference example)
[Preparation of conductive composition]
A conductive composition for forming an electrode was prepared by the procedure shown below. First, as the conductive powder, a spherical silver (Ag) powder having an average particle diameter of 2 μm was used. As the glass frit, lead-free and leaded glass powder (average particle size: 2 μm) shown below was used. As the silicone resin, polydimethylsiloxane having a weight average molecular weight Mw of 50,000 was used.

[Lead-free glass frit]
1-25 mol% Bi ₂ O _3-25-80 mol% TeO _2-0.1-30 mol% ZnO
[Leaded glass frit]
1-25 mol% PbO-25-80 mol% TeO _2-0.1-30 mol% ZnO
The leaded glass frit is a glass containing 25 mol% of PbO in terms of oxide composition and containing tellurium (Te) and zinc (Zn) as network-forming elements. The lead-free glass frit is a glass that does not contain lead (Pb), instead uses bismuth (Bi) that exhibits a function similar to that of lead, and contains tellurium (Te) and zinc (Zn) as network-forming elements. All of these glass frits have a softening point in the range of 250 ° C. or higher and 600 ° C. or lower.

When the silver powder is 100% by mass, the glass frit is constant at 2% by mass, and the silicone resin is 0% by mass, 0.1% by mass, 0.2% by mass, and 0.3% by mass. , 0.6% by mass, 0.9% by mass, 1.2% by mass, organic vehicle 5% by mass, hardened castor oil as a surfactant 0.80% by mass, texanol as a dispersion medium It was blended so as to be about 1% by mass. As the organic vehicle, a mixture of ethyl cellulose and texanol in a mass ratio of 15:85 was used. Then, these materials were kneaded well using a three-roll mill to obtain the conductive composition of each example. In this embodiment, the viscosity of the conductive composition is adjusted to 180 to 200 Pa · s (20 rpm, 25 ° C.) in order to make the printability of the conductive composition of each example uniform.

[Manufacturing of solar cell elements for evaluation]
The solar cell elements of Examples 1 to 120 were manufactured by forming a light receiving surface electrode (that is, a comb-shaped electrode composed of a finger electrode and a bus bar electrode) using the conductive composition obtained above.
Specifically, first, a commercially available p-type single crystal silicon substrate (plate thickness 180 μm) for a solar cell having a size of 156 mm square (6 inch square) is prepared, and the surface (light receiving surface) thereof is a mixed acid of hydrofluoric acid and nitric acid. By etching with, the damaged layer was removed and the texture structure of unevenness was formed. Next, a phosphorus-containing solution was applied to the textured structural surface and heat-treated to form an n—Si layer (n ⁺ layer) having a sheet resistance of 90 ± 10 Ω / □ 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, on the back surface side of the silicon substrate, a predetermined silver electrode forming paste was used, screen-printed with a predetermined pattern so as to become an electrode for external connection on the back surface side, and dried to form a back surface side electrode pattern. .. Then, an aluminum electrode forming paste was screen-printed on the entire surface on the back surface side and dried to form an aluminum film.

Then, using the conductive composition prepared above, an electrode pattern for a light receiving surface electrode (Ag electrode) was printed on the antireflection film by a screen printing method, and dried at 120 ° C. Specifically, as shown in FIG. 2, an electrode pattern consisting of three linear bus bar electrodes parallel to each other and 90 finger electrodes parallel to each other so as to be orthogonal to the bus bar electrodes is formed. Formed by screen printing. For screen printing, a screen plate making of 360 mesh (wire diameter 16 μm, emulsion thickness 15 μm) was used. The finger electrode pattern was adjusted so that the dimensions after firing had a line width of about 45 μm and a film thickness of 15 μm to 25 μm. Further, the bus bar electrode was set so that the line width after firing was about 1.5 mm. A solar cell for evaluation was produced by firing the substrate on which the electrode patterns were printed on both sides in this way in an atmospheric atmosphere at a firing temperature of 700 to 800 ° C. using a near-infrared high-speed firing furnace.

[evaluation]
For the solar cell manufactured as described above, the IV characteristics are measured using a solar simulator (PSS10 manufactured by Beger), and the solar cell is opened based on the "crystal-based solar cell output measurement method" specified in JIS C8913. The voltage (Voc) and curve factor (FF) were calculated. The results are shown in FIG. 4 for the open circuit voltage (Voc) and in FIG. 5 for the curve factor (FF).

As shown in FIG. 4, by adding a silicone resin to the conductive composition and increasing its content, the open circuit voltage of the solar cell provided with the electrode formed by the conductive composition tends to increase. I understand. This tendency could be confirmed regardless of whether the lead-free glass frit or the leaded glass frit was used as the glass frit. However, it was confirmed that the increase in the open circuit voltage became more remarkable when the leaded glass frit was used. Regarding the conductive composition using the leaded glass frit, even when the silicone resin is not contained (0% by mass), a higher open circuit voltage can be obtained as compared with the conductive composition using the lead-free glass frit, but silicone. It was found that the effect was greatly improved by adding the resin. In addition, although specific data are not shown, this tendency differs greatly depending on whether the glass frit is leaded or unleaded, but the leaded glass frit is not significantly affected by the proportion of glass components other than lead. It could be confirmed.

On the other hand, as shown in FIG. 5, the curve factor tended to be different from the open circuit voltage. That is, it was found that for a conductive composition using a lead-free glass frit as the glass frit, the curve factor is significantly reduced by adding the silicone resin. This decrease in the curve factor is about 10% when 0.6% by mass of the silicone resin is added, and it can be seen that it is not at a practical level. On the other hand, for the conductive composition using the leaded glass frit, when the silicone resin is added up to about 0.6% by mass, the curve factor is not significantly affected, but more silicone resin is used. It was found that the curve factor decreased sharply with the addition. Although no specific data are given, this tendency is largely unaffected by the proportion of non-lead glass components in both leaded and unleaded glass frits, although this trend varies greatly depending on whether the glass frit is leaded or unleaded. I was able to confirm that.

(Embodiment 1)
[Preparation of conductive composition]
Next, a lead-containing compound powder was added to the conductive composition, and the conductive compositions for forming the electrodes of Examples 1 to 120 were prepared in the same manner as in the above-mentioned reference examples under other conditions. That is, as the conductive powder, a spherical silver (Ag) powder having an average particle diameter of 2 μm was used. As the glass frit, glass powder (average particle size: 2 μm) having three composition systems, lead-free and leaded, shown in Table 1 below was used. As the silicone resin, polydimethylsiloxane having a weight average molecular weight Mw of 50,000 was used. As the lead-containing compound powder, lead tan (Pb _{3O 4 )} , lead monoxide (PbO), and lead nitrate (Pb (NO ₃ ) ₂ ) having an average particle diameter of about 2 μm were all prepared from commercially available ones.

In Table 1, the lead-free glass frit does not contain lead (Pb), but contains (1) Si and B, (2) Te, (3) Te and W as network forming elements, respectively, and is used as a network modifying element. It is a glass containing Zn and the like. The leaded glass frit is a glass containing (1) Si, (2) Te, (3) Te and W as network forming elements, and Pb and the like as network modifying elements. As for the leaded glass frit, as shown in Tables 2 to 4, the amount of PbO in the oxide equivalent composition is changed in the range of 5 to 30 mol%. All of these glass frits are adjusted so that the softening point is in the range of 250 ° C. or higher and 600 ° C. or lower.

Silver powder, silicone resin, glass frit and lead-containing compound powder are mixed in the following proportions, and ethyl cellulose as an organic binder component is 6.7% by mass with respect to 100% by mass of silver powder, and hardened castor oil as a surfactant. Was added to 0.80% by mass and about 1% by mass of texanol as a dispersion medium, and kneaded well using a three-roll mill to prepare the conductive compositions of Examples 1 to 120. The viscosity of the conductive composition was adjusted to 180 to 200 Pa · s (20 rpm, 25 ° C.).
The glass frit was constant at 2% by mass when the silver powder was 100% by mass. As shown in Tables 2 to 4, the silicone resin was changed in the range of 0 to 0.9% by mass with respect to 100% by mass of the silver powder.

As for the lead-containing compound powder, assuming that virtual glass is prepared by adding each lead-containing compound to a glass frit, the amount of lead (PbO) in the oxide equivalent composition of the virtual glass is shown in Tables 2 to 4. The ratio was adjusted so as to be 0 to 30 mol%. The amount (% by mass) of the lead-containing compound powder added per 100% by mass of the silver powder at that time is shown in parentheses in the lower row.

For example, as shown in Table 2, a case where a B2O3 _{- SiO2 -} ZNO-based glass frit (lead-free) is used as the glass frit will be described. At this time, when lead tan is used as the lead-containing compound powder, the amount of lead tan in which the PbO amount of the virtual glass is 10 mol% is about 0.26% by mass (Example 11), and the lead in which the PbO amount is 20 mol%. The amount of lead tan is about 0.51% by mass (Examples 12 to 16, 18), and the amount of lead tan with a PbO amount of 30 mol% is about 0.77% by mass (Example 13). Further, for example, when lead monoxide is used as the lead-containing compound powder, the amount of PbO in the virtual glass is about 0.53% by mass (Examples 19 and 20), and when lead nitrate is used. Is about 0.76% by mass (Examples 21 and 22).

Further, the lead-containing compound powder was prepared in two ways, one in the form of a powder and the other in the state of being supported on a glass frit. For the glass frit carrying the lead-containing compound powder, a predetermined amount of the lead-containing compound powder and the glass frit were charged using a dry particle compounding device (NOB-130), the blade rotation speed was 2500 rpm, and the power load was about 4. It was prepared by processing at 7 kW and a processing time of about 10 minutes.

[evaluation]
Using the conductive composition prepared as described above, a solar cell was produced in the same manner as in the reference example. Then, the IV characteristics of this solar cell are measured using a solar simulator (PSS10 manufactured by Beger), and the conversion efficiency (Eff) is based on the "crystal-based solar cell output measurement method" specified in JIS C8913. Was calculated. The results are shown in Tables 2-4. In the column of "Ref" of "Eff" in the table, the relative value of each Eff is shown when the measured Eff value of Examples 1, 41, and 81 as the reference in each table is set to 1. In the "evaluation" column, "x" (defective) when the Ref value is less than 1, "△" (good) when it is 1 or more and less than 1.15, and "○" when it is 1.15 or more. Shown as (excellent).

As shown in Tables 2 to 4, it was confirmed that the conversion efficiency was significantly increased by adding the silicone resin and the lead-containing compound powder to the conductive composition.
When confirmed in detail, the conductive composition of Examples 1, 41, 81 is a conductive composition using lead-free glass frit and containing neither a silicone resin nor a lead-containing compound powder.
The conductive composition of Examples 2, 42, and 82 uses lead-free glass frit and contains a silicone resin but does not contain a lead-containing compound powder.
The conductive compositions of Examples 3 to 10, 43 to 50, and 83 to 90 are lead-free glass frits, do not contain a silicone resin, and contain a lead-containing compound powder.
The conductive compositions of Examples 11 to 22, 51 to 62, and 91 to 102 use lead-free glass frit and contain both a silicone resin and a lead-containing compound powder, and the blending amount and the type of the lead-containing compound powder are changed. It is a thing.

Examples 23 to 25, 63 to 65, and 103 to 105 are conductive compositions using leaded glass frit and containing a silicone resin but not a lead-containing compound powder.
The conductive compositions of Examples 26-40, 66-80, 106-120 use leaded glass frit and contain both a silicone resin and a lead-containing compound powder, and the blending amount and the type of the lead-containing compound powder are changed. It was made to.

From the results of Examples 2, 42 and 82, it was found that the addition of the silicone resin to the conductive composition increased Ref by 0.001 to 0.002 and increased the power generation efficiency.
However, from the results of Examples 23 to 25, 63 to 65, 103 to 105, when the silicone resin is added, the glass frit in the conductive composition is changed from a lead-free one to a leaded one, thereby ref. It was found that the rate of increase of was significantly large, for example, 0.167 to 0.204. In these examples, the increase in Ref peaked when the amount of PbO was 20 mol% (Examples 24, 64, 104) regardless of the composition system of the leaded glass frit. Therefore, it was found that the increase in Ref does not depend on the type of glass frit, but does not simply increase according to the amount of PbO.

On the other hand, from the results of Examples 3 to 10, 43 to 50, and 83 to 90, by adding the lead-containing compound powder to the conductive composition, Ref increased by 0.095 to 0.119, and the silicone resin was added. It was found that the rate of increase in power generation efficiency was greater than in the case of. In these examples, the increase in Ref peaked when the amount of lead-containing compound powder was equivalent to 20 mol% (Examples 4, 44, 84) when it was contained as a glass frit. Therefore, it was found that the increase in Ref does not simply increase with the amount of lead-containing compound powder. It was also found that there was no significant change in power generation efficiency depending on whether the lead-containing compound powder was added as it was or supported on a glass frit.

With respect to the above examples, from the results of Examples 11 to 22, 51 to 62, and 91 to 102, by adding the silicone resin and the lead-containing compound powder together to the conductive composition, Ref is 0.159 to 0. It increased by 205, and it was found that the rate of increase in power generation efficiency was larger than that when the silicone resin and the lead-containing compound powder were added alone. This increase rate of Ref is much larger than the sum of the increase rate of Ref when the silicone resin is added alone and the increase rate of Ref when the lead-containing compound powder is added alone (for example,). 1.5 times or more). Therefore, when forming an electrode using this conductive composition, it is clear that the silicone resin and the lead-containing compound powder act synergistically to contribute to the increase in power generation efficiency.

In these examples, the increase in Ref is when the amount of the silicone resin added is 0.20% by mass and the amount of the lead-containing compound powder (converted value when contained as glass frit) is 20 mol% (for example, eg). It peaked at 14, 54, 94). There is a slight difference in power generation efficiency depending on the type of lead-containing compound powder, and it can be said that it is preferable to use lead nitrate as the lead-containing compound powder (Examples 22, 62, 102).
From the results of Examples 26 to 40, 66 to 80, and 106 to 120, it can be confirmed that the effect of increasing the power generation efficiency can be obtained even when leaded glass frit is used instead of lead-free glass frit as the glass frit. rice field. In this case, it was found that the proportion of the lead-containing compound powder can be reduced because the lead component is contained in the leaded glass frit.

Here, the solar cells of Examples 82, 84, 91 to 120 produced using the conductive composition containing the ( ₃ ) TeO2 _- WO3 system lead-free glass frit and leaded glass frit shown in Table 4 are used. A peeling test of the light receiving surface electrode was performed, and the adhesive strength of the electrode to the substrate was measured.

The adhesive strength (peeling strength) was evaluated using the strength measuring device 300 as shown in FIG. Specifically, first, the epoxy adhesive 42 was applied to the back surface 11B side of the solar cell 10 for evaluation, and fixed on the glass substrate 41 with the light receiving surface 11A side facing up. A tab wire 35 for peeling was soldered to the light receiving surface electrode 12 of the solar cell 10 for evaluation from one end via the solder layer 30. Then, the solar cell 10 for evaluation was placed on the fixing base 40 of the strength measuring device 300 together with the glass substrate 41, and the glass substrate 41 portion was fixed to the fixing base 40 with the fixing screw 43 and the locking plate 44. Next, as shown in FIG. 3, the strength measuring device 300 is tilted so that the bottom surface of the fixing base 40 is 135 °, and the extension portion 35e previously formed on the tab line 35 is pulled vertically upward (arrow). 45), the adhesive strength (N / mm) at the interface between the electrode 12 and the substrate 11 was measured. The measurement results of the adhesive strength are shown in the "ADH" column of Table 5. In the "Radh" column of "ADH" in Table 5, the relative value of each adhesive strength is shown when the measured value of the adhesive strength of Example 42 is 1 (reference). In the "evaluation" column, the case where "Radh" is 0.85 or more is shown as "◯", and the case where "Radh" is less than 0.85 is shown as "Δ".

As shown in Table 5, it was found that the electrode formed from the conductive composition of Example 82 to which only the silicone resin was added showed the strongest adhesive strength to the substrate. Further, it was found that the electrode formed from the conductive composition of Example 84 to which only the lead-containing compound powder was added had a slightly lower adhesive strength than that of Example 82. Then, it was found that the electrodes formed from the conductive compositions of Examples 91 to 102 and 106 to 120 to which both the silicone resin and the lead-containing compound powder were added had further reduced adhesive strength. Radh is 0.908 to 0.947 for examples 91 to 102 using leaded glass frit, while it is 0.867 to 0.927 for examples 106 to 120 using leaded glass frit. rice field. In Examples 91 to 102 and Examples 106 to 120, the Eff values were almost the same level, but it was found that the adhesive strength tended to decrease by about 5% by using the leaded glass frit. However, it was found that the adhesive strength of Examples 103 to 105 using the leaded glass frit without adding the lead-containing compound powder was significantly reduced. From the above, for example, the conductive composition for realizing the Eff value (17.23 to 17.23) of Examples 103 to 105 contains a silicone resin and a lead component, and the lead component is lead-free. It turns out that it is better to include lead-containing compound powder rather than using glass frit.

(Embodiment 2)
[Preparation of conductive composition]
Next, two types of lead-free glass frit (A) and (B) are prepared, and as shown in Table 6 below, the blending amounts of the lead-free glass frit (A) or (B), the lead-containing compound powder, and the silicone resin are mixed. Eight kinds of conductive compositions for forming electrodes were prepared by changing and other conditions according to the above-mentioned first embodiment. That is, the (A) lead-free glass frit and (B) leaded glass frit were changed to 0.5% by mass to 10% by mass with respect to 100% by mass of the silver powder. In addition, lead tan (Pb ₃ O ₄ ) is used as the lead-containing compound powder, and the lead-containing compound is blended in an amount such that the PbO concentration in the oxide equivalent composition of the glass frit when it is contained in the glass is 20 mol%. did. In addition, the ratio (mass%) of Pb ₃ O ₄ actually added to 100 mass% of silver powder is shown in parentheses in the column of the compounding amount of Pb ₃ O ₄ in Table 6.

Using the conductive composition prepared as described above, a solar cell was produced in the same manner as in the reference example. Then, the IV characteristics of this solar cell are measured using a solar simulator (PSS10 manufactured by Beger), and the conversion efficiency (Eff) is based on the "crystal-based solar cell output measurement method" specified in JIS C8913. Was calculated. The results are also shown in Table 6. In the column of "Ref" in the table, the relative value of each Eff is shown when the measured value of Eff in Examples 121 and 125 is 1. In the "evaluation" column, "x" (defective) when the Ref value is less than 1, "△" (good) when it is 1 or more and less than 1.15, and "○" when it is 1.15 or more. Shown as (excellent). In this embodiment, there was no result of "x".

[evaluation]
As shown in Table 6, even when the blending amount of the glass frit in the conductive composition is changed from 0.5% by mass to 10% by mass, the conductive composition contains the lead-containing compound powder and silicone. It was found that the conversion efficiency can be greatly improved by including both the resin and the resin. It was also found that the improvement in conversion efficiency was significantly improved as compared with the case where the conductive composition contained only one of the lead-containing compound powder and the silicone resin. Although not specifically shown, such a tendency could be confirmed for leaded glass frit as well.

Although the present invention has been described above in terms of preferred embodiments, such a description is not a limitation and, of course, various modifications can be made.

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 side aluminum electrode 22 Back side side external connection electrode 24 p ⁺ layer

Claims (7)

A conductive composition for forming a light receiving surface electrode of a solar cell having an n ⁺ layer on the light receiving surface.
With conductive powder
With glass frit,
Lead-containing compound powder and
Silicone resin and
With an organic binder,
Dispersion medium and
Including
The silicone resin contains polydimethylsiloxane and contains
The metal species constituting the conductive powder is silver.
The silicone resin is a conductive composition contained in a proportion of 0.6% by mass or less with respect to 100% by mass of the conductive powder. The conductive composition according to claim 1, wherein the glass frit is contained in a proportion of 0.1 to 12% by mass when the conductive powder is 100% by mass. The lead-containing compound powder is contained in an amount such that the ratio of PbO in the oxide conversion composition of the virtual glass is 30 mol% or less when the glass frit and the lead-containing compound powder are mixed to prepare a virtual glass. , The conductive composition according to claim 2. The lead-containing compound powder is any one of claims 1 to 3, wherein the lead-containing compound powder is a powder of at least one lead-containing compound selected from the group consisting of metallic lead, lead monoxide, lead tan, lead nitrate and lead carbonate. The conductive composition according to the section. The conductive composition according to any one of claims 1 to 4, wherein the lead-containing compound powder is supported on the glass frit. The conductive composition according to any one of claims 1 to 5 , wherein the silicone resin has a weight average molecular weight of 1000 or more and 150,000 or less. A solar cell comprising a fired product of the conductive composition according to any one of claims 1 to 6 as an electrode.

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