JP2017092253A - Conductive composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a conductive composition which enables the formation of an electrode allowed to make a good ohmic contact even with an LDE (Lightly Doped Emitter), and enables the materialization of a solar battery having an improved conversion efficiency.SOLUTION: A conductive composition for forming electrodes 12, 22 of a solar battery 10 comprises: conductive powder; glass frit; lead-containing compound powder; a silicone resin; an organic binder; and a dispersant. The glass frit is preferably included at a rate of 0.1-12 mass% to 100 mass% of the conductive powder. The lead-containing compound powder is preferably included by an amount which makes a content of PbO 30 mol% or less according to a composition in terms of oxide in virtual glass produced by mixing the glass frit with the lead-containing compound powder.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a conductive composition. More particularly, the present invention relates to a conductive composition that can be used to form a solar cell electrode.

  From the viewpoint of environmental awareness and energy saving in recent years, the spread of solar cells that convert light energy into electric power is rapidly progressing. In connection with this, the solar cell with favorable photoelectric conversion efficiency is calculated | required. As a measure for realizing this requirement, in a solar cell, a passivation film that suppresses recombination and an antireflection film that increases the light receiving efficiency are provided, and the electric power generated at the pn junction in the substrate is highly efficient. An attempt is being made to take it out of.

  In the production of this type of solar cell, a conductive composition for electrode formation is supplied in a desired electrode pattern onto an antireflection film provided on the light receiving surface of a silicon substrate and fired. This conductive composition typically includes a conductive powder such as silver, glass frit, a binder component, and a dispersion medium, and is prepared in a paste form (including a slurry form and an ink form). ing. The conductive composition is supplied to the light receiving surface of the solar cell with a predetermined electrode pattern by a method such as screen printing. Then, during the firing of the electrode, the glass frit in the conductive composition reacts with the antireflection film, and 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 ( Ohmic contact). Thereby, the current formed at the pn junction of the silicon substrate can be taken out to the electrode. Patent document 1 is mentioned as an electroconductive composition for electrode formation of the solar cell containing such a glass frit, for example.

JP 2010-087251 A JP 2012-023095 A Japanese Patent Laying-Open No. 2015-122177

  By the way, in recent years, it has been attempted to suppress the surface recombination by thinning the emitter layer of the solar cell, thereby improving the conversion efficiency. In a substrate having a thin emitter layer (Lightly Doped Emitter (LDE)), sheet resistance is increased, and it is required to achieve ohmic contact between the electrode and the substrate by fire-through in the thin emitter layer. It is done. However, it is difficult to achieve 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 be lowered. Accordingly, there is a demand for improvement in conversion efficiency particularly in LDE type solar cells.

  The present invention has been made in view of such a situation, and its main purpose is to provide a solar cell that can form an electrode that can provide a good ohmic contact with LDE, for example, and has improved conversion efficiency. It is to provide a conductive composition that can be realized. Another object of the present invention is to provide a solar cell element with improved power generation performance realized by the use of this conductive composition.

The present inventors have so far conducted a conductive composition that can suitably form a high aspect ratio electrode in order to reduce the line resistance of the light-receiving surface electrode of the solar cell and realize a solar cell with good output characteristics. (For example, Japanese Patent Application No. 2015-001855). This conductive composition contains glass frit and silicone resin (also simply referred to as silicone) in addition to the conductive powder, and the contact (contact) by adjusting the amount of SiO ₂ component in the composition. It is possible to suppress a rise in resistance and to stably form a fine and high aspect ratio electrode. However, for 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 electrode to be formed, from the electrochemical aspect such as realization of better ohmic contact There was room for improvement.

  Therefore, as a result of further diligent research by the present inventors, by reconsidering the composition of the conductive composition, a good ohmic contact is formed, for example, for LDE, and unprecedented high conversion efficiency is realized. It came to discover the conductive composition which can do. The present invention has been completed based on such findings. That is, the conductive composition which can be used suitably in order to form the electrode of a solar cell is provided by the technique disclosed here. This conductive composition is characterized by containing glass frit, silicone resin, and lead-containing compound powder in addition to conductive powder, organic binder, and dispersion medium.

Patent Document 1 is a technique relating to a conductive composition using a lead-free glass frit as a glass frit. In this Patent Document 1, when using a lead-free glass frit instead of a lead-containing glass frit capable of realizing a high conversion efficiency, the conversion efficiency is ensured by adding a Si-based compound to the conductive composition. Is disclosed.
Patent Document 2 is a technique relating to a conductive composition that does not contain glass frit that can be a resistance component after firing as a binder component. 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. In addition, it is disclosed that it is preferable to use a thermosetting epoxy resin, a polyester resin, a silicone resin, a urethane resin, or the like in order to improve the adhesion to the silicon substrate.
However, according to the tests of the present inventors, since the conductive compositions of Patent Documents 1 and 2 contain a silicone resin, the conversion efficiency of a solar cell using an LDE substrate produced using this composition It has been confirmed that there is room for improvement. Further, according to the study by the present inventors, the conductive composition containing a silicone resin has the knowledge 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 conductive composition using a glass frit containing tellurium as the glass frit (see Patent Document 3). In Patent Document 3, in order to improve the erosion-inhibiting action of the lead-free tellurium glass frit and to avoid the decrease in adhesive strength seen in the lead-tellurium glass frit, the lead-free tellurium glass frit and And lead-containing compound powder. According to the tests of the present inventors, the conductive composition of Patent Document 3 includes tellurium-containing glass frit, so that it is compared with the solar cell manufactured using the conductive compositions disclosed in Patent Documents 1 and 2. Thus, high conversion efficiency can be realized. However, there is still room for further improvement in this conversion efficiency.

  That is, the electroconductive composition disclosed here contains a silicone resin and a lead-containing compound powder. By producing a solar cell using such a conductive composition, it is possible to achieve an unprecedented high conversion efficiency without being greatly affected by the composition of the glass frit. Thereby, the solar cell which implement | achieves high conversion efficiency can be produced simply and stably.

  In a preferred embodiment of the conductive composition disclosed herein, the glass frit is included in a proportion of 0.1 to 12% by mass when the conductive powder is 100% by mass. The lead-containing compound powder is an amount such that when the virtual glass is prepared by mixing the glass frit and the lead-containing compound powder, the proportion of PbO in the oxide equivalent 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 using this electroconductive composition can be improved further stably.

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

  In a preferred embodiment of the conductive composition disclosed herein, the lead-containing compound powder is supported on the glass frit. Such a configuration can also stably improve the conversion efficiency of a solar cell manufactured using this conductive composition.

  In a preferred embodiment of the conductive composition disclosed herein, the silicone resin is included 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 a solar cell manufactured using this conductive composition can be stably increased.

  In a preferred embodiment of the conductive composition disclosed herein, the silicone resin has a weight average molecular weight of 1,000 to 150,000. 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 electrically conductive composition disclosed herein, the silicone resin contains at least one of polydimethylsiloxane and polyether-modified siloxane. With such a configuration, an electrode having good adhesion to the substrate can be formed.

  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.

The conductive composition of the present invention can realize good ohmic contact even with a thin solar cell substrate having an n ⁺ layer, for example. Thereby, the solar cell which suppresses surface recombination and implement | achieves the high conversion efficiency which has not existed before can be manufactured. Further, the adhesion between the electrode and the silicon substrate can be maintained high.

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 side view which showed typically the intensity | strength measuring apparatus which measures the adhesive strength of the board | substrate and electrode of a solar cell. It is the graph which showed the relationship between the silicone content of the electroconductive composition which concerns on a reference example, and the open circuit voltage (Voc) of a solar cell. It is the graph which showed the relationship between the silicone content of the electrically conductive composition which concerns on a reference example, and the fill 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 the implementation of the present invention are the technical content taught by the present specification and those of ordinary skill in the art. Based on common technical common sense. In the present specification, the notation “A to B” indicating a range means A or more and B or less.

  The conductive composition disclosed herein can typically form an electrode of a solar cell by firing. That is, the fired product of the conductive composition can constitute a solar cell electrode. 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 dispersion medium), and a silicone resin and a lead-containing compound powder as essential constituent elements.

  In the technique disclosed in Patent Document 1, lead-free glass frit is used instead of leaded glass frit, and the conversion efficiency is increased by adding a Si-based compound to the conductive composition. However, as the amount of silicone resin added increases, the power generation efficiency decreases significantly. In the technique disclosed in Patent Document 3, lead-free tellurium glass frit is used instead of lead-tellurium glass frit. In order to improve the substrate erosion inhibiting action of the lead-free tellurium glass frit, lead content is contained in the conductive composition. Additives were included. At this time, it is described that the lead-containing additive is present in a state of being combined with the lead-free tellurium glass frit in the conductive composition, so that an electrode having more excellent electrical characteristics can be obtained.

  However, in the technique disclosed herein, the conductive composition contains the silicone resin and the lead-containing compound powder at the same time. In this case, an electrode having higher electrical characteristics than the effect of improving electrical characteristics when a silicone resin or a lead-containing compound powder is added alone in the conductive composition can be obtained. Furthermore, it is possible to obtain an electrode having even higher electrical characteristics than the addition of the effect 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 including a glass frit of any composition disclosed herein, a silicone resin, and a lead-containing compound powder can form an electrode having excellent electrical characteristics. 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, and even if it is an LDE type substrate. However, it is considered that the erosion of the glass frit and the lead-containing compound powder with respect to the substrate is prevented from being excessive, and works to suitably proceed. This is considered to greatly relieve the rapid decrease in power generation performance due to the addition of silicone resin. In addition, while the erosion interface of the substrate with the lead-containing compound powder is roughened, it is considered that the erosion action between the glass frit and the lead-containing compound powder is homogenized to achieve smooth substrate erosion as a whole. .

Hereinafter, each component of the electroconductive composition disclosed here is demonstrated.
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 are those composed of a simple substance of a noble metal such as platinum, palladium, silver, and alloys thereof (for example, Ag—Pd alloy, Pt—Pd alloy, etc.), nickel, copper, aluminum, and alloys thereof. Can be cited as a material constituting the conductive powder. 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, although the case where Ag powder is used as a conductive powder is described as an example for the conductive composition of the present invention, the present invention is not limited thereto.

There is no restriction | limiting in particular about the particle size of electroconductive powder, The thing of the various particle size according to a use can be used. Typically, those having an average particle size of 5 μm or less are suitable, and those having an average particle size of 3 μm or less (typically 1 μm to 3 μm, for example, 1.5 μm to 2.5 μm) are preferably used. As 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. The average particle diameter of the conductive powder in this specification employs a value measured using a laser diffraction / scattering particle size distribution measuring device (LA-920, manufactured by Horiba, Ltd.).

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 (D ₁₀ ) of the particle size (D ₁₀ ) when the cumulative volume is 10% and the particle size (D ₉₀ ) when the cumulative volume is 90% from the small particle size side in the particle size distribution based on the laser diffraction / scattering method (D ₉₀ ) ₁₀ / _D90 ) can be employed. When all the particle sizes constituting the powder are equal, the value of D ₁₀ / D ₉₀ is 1, and conversely, the value of D ₁₀ / D ₉₀ 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 D ₁₀ / D ₉₀ is 0.2 or more (for example, 0.2 or more and 0.5 or less).

In another aspect, the conductive powder can be used by mixing two particle groups having different average particle diameters. In this case, for example, the average particle diameter (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 diameter (D ₅₀ ) of the second particle group is 2 μm to A preferable example is a 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 at a ratio of 80 to 95% by volume (for example, 90% by volume), and the second particle group is mixed at a ratio of 20 to 5% by volume (for example, 10% by volume). To do. Thereby, the electroconductive powder with favorable filling property can be prepared.
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 is not specifically limited by the manufacturing method etc. For example, conductive 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, for example, the formation of a light-receiving surface electrode of a solar cell, the conductive composition penetrates an antireflection film as a lower layer during firing due to the presence of the glass frit. Thus, good adhesion and electrical contact between the conductive particles (that is, the electrode) and the substrate can be realized.

  Such a glass frit is preferably 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 typically be 0.5 μm or more, and more preferably 1 μm or more. As the average particle size of the glass frit in this 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.

  In addition, it does not restrict | limit especially about the glass frit in this embodiment, The various glass used for this kind of electrically conductive composition can be used. As an approximate glass composition, for example, a so-called lead glass, a lead lithium glass, a zinc glass, a borate glass, a borosilicate glass, an alkali system, which is referred to as a name commonly expressed by those skilled in the art. It may be glass, tellurium-based glass, lead-tellurium-based glass, glass containing barium oxide, bismuth oxide, or the like. Needless to say, these glasses can contain any other element in addition to the main glass constituent elements appearing in the above designation. 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 and Nb. 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 crystallized glass partially containing crystals in addition to a general amorphous glass. As the glass frit, one kind of glass frit may be used alone, or two or more kinds of glass frit 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 (eg, 300 ° C. or higher and 500 ° C. or lower). Specific examples of the glass whose softening point can be adjusted in the range of 250 ° C. or higher and 600 ° C. or lower 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 glass, Pb-B-Mo 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 Glass, Pb-B-Si-Al-Li-Ti-P-Te glass, b-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 Glass, P—B—Al—Si—Pb—Li glass, PB—Al—Mg—F—K glass, Te—Pb glass, Te—Pb—Li glass, VP—Ba—Zn glass glass, V-P-Na-Zn-based glass, Pb-V-P based _{glass, AgI-Ag 2 O-B} -P based glass, Zn-B-Si-Li based glass, Si-Li-Zn-Bi- Examples thereof include Mg—W—Te glass, Si—Li—Zn—Bi—Mg—Mo—Te glass, and Si—Li—Zn—Bi—Mg—Cr—Te glass. The above glass contains at least a plurality of elements connected by hyphens (−), and the total of oxides of the elements based on the composition (oxide conversion composition) when these elements are converted into oxides. Means 70% by mol or more (typically 80% or more, more preferably 90% or more) of the whole. A conductive composition containing a glass frit having such a softening point, for example, when forming a light-receiving surface electrode of a solar cell element, exhibits good fire-through characteristics at a relatively low firing temperature. This is preferable because it contributes to high-performance electrode formation.

  The glass frit may be a leaded glass frit containing lead or a lead-free glass frit containing no lead. However, when using a leaded glass frit, it is preferable to make the ratio of lead in the glass frit 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 a ratio of 30 mol% or less of PbO in the oxide equivalent composition of the glass frit. It is because the knowledge that it is. However, (2) when a leaded glass frit is used, the eroded surface of the substrate by the conductive composition at the time of firing is uniformly formed and becomes smooth, so that although good electrical characteristics can be obtained, The adhesive strength tends to be low. In an electrode of a solar cell, in order to take out generated electric power to the outside or modularize, a lead wire for current extraction 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. Accordingly, it may be important that the lead component in the conductive composition disclosed herein includes at least a portion thereof as a lead-containing compound powder. In addition, when the adhesive strength of a board | substrate and an electrode is required, it is preferable to use a glass frit with little lead content as a glass frit, and it is especially preferable to use a lead-free glass frit. The lead-free glass frit here means that the ratio of lead (Pb) in the entire conductive composition is 1000 ppm (mass basis; the same shall apply hereinafter) or less. For example, lead in the entire glass frit It means that the ratio of (Pb) is 10000 ppm or less. More preferably, the lead-free glass frit is substantially free of lead.

The glass frit preferably has a low Si content 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 reduced, and the glass spreads onto the conductive powder and substrate at a lower temperature during firing. preferable. From this viewpoint, the Si component is preferably 1000 ppm or less with respect to the conductive composition before firing and 10,000 ppm or less with respect to the glass frit as SiO ₂ .
Further, the glass frit preferably has a low content of Cu as a glass constituent element. A reduction in the proportion of Cu (for example, as CuO) in the glass frit is preferable because the electrical characteristics of the formed electrode are improved. From this point of view, the Cu component is preferably 1000 ppm or less with respect to the conductive composition before firing and 10,000 ppm or less with respect to the glass frit as CuO.
In addition, when these elements are contained in the glass frit, since the below-described lead-containing compound powder is supported integrally on the glass frit, an inconvenient tendency due to these Si and Cu can be clearly suppressed. preferable.

Silicone resin is an essential constituent component contained in the conductive composition disclosed herein. By containing this silicone resin, this conductive composition acts on the above lead-containing glass frit at the time of firing, improves fire-through characteristics, suppresses excessive erosion of the Si substrate, and provides good contact. It is thought that it can be formed. Moreover, the silicone resin can maintain the shape of the coating film (unfired electrode) of the conductive composition formed on the Si substrate, for example, from printing to firing stably, and has a finer and higher aspect ratio. An electrode can be formed stably. Moreover, the silicone resin may generate SiO ₂ component in the electrode by firing. This SiO ₂ component does not directly increase the softening point of the leaded glass frit, but is incorporated into the glass component during the firing process, thereby improving the stability of the system and the adhesion between the electrode and the substrate. It is thought that it contributes to.

As this silicone resin, an organic compound containing silicon (Si) can be used without any particular limitation. For example, a high molecular organic compound having a main skeleton based on a siloxane bond (Si—O—Si) is preferably used. it can. Examples of the siloxane compound (polymer) forming the main skeleton portion include, for example, a polysiloxane 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 this. Specifically, examples of the silicone resin include polydialkylsiloxanes such as polydimethylsiloxane, polydiethylsiloxane, and polymethylethylsiloxane, polyalkylarylsiloxanes, and poly (dimethylsiloxane-methylsiloxane). A particularly suitable polymer constituting the main skeleton can be, for example, polydimethylsiloxane. 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, terminal, or both of the main skeleton. May be. Of these, polydimethylsiloxane and polyether-modified siloxane (polyether-modified silicone) in which a polyether group is introduced into the side chain, terminal, or both of polydimethylsiloxane can be preferably used. Alternatively, the polyether-modified siloxane may be a linear block copolymer (linear polyether-modified silicone) in which polyether and silicone are alternately bonded.

  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, if Mw exceeds about 150,000, defects such as disconnection of the obtained electrode are caused and resistance is increased, which is not preferable. 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 that contains a lead (Pb) component and can exist stably in the conductive composition can be used without particular limitation. Examples of such lead-containing materials include metal lead (Pb), alloys containing lead, lead monoxide (PbO), lead dioxide (PbO ₂ ), red lead (Pb ₃ O ₄ ), lead White (2PbCO ₃ · Pb (OH) ₂ ), lead nitrate (Pb (NO ₃ ) ₂ ), lead chloride (PbCl ₂ ), lead sulfide (PbS), yellow lead (PbCrO ₄ , Pb (SCr) O ₄ , PbO · PbCrO _4), lead carbonate (PbCO _3), lead sulfate (PbSO _4), lead fluoride _(PbF 2), 4 lead fluoride _(PbF 4), lead bromide (PbBr _2), lead iodide (PbI ₂ ) And other organic lead compounds such as lead acetate (Pb (CH ₃ COO) ₂ ), lead 4-carboxylate (Pb (OCOCH ₃ ) ₄ ), and tetraethyl lead (Pb (CH ₃ CH ₂ ) ₄ ) Illustrated. Among these, metallic lead, lead monoxide, red lead, lead nitrate and lead carbonate are relatively easy to handle and are not affected by other elements, and are particularly preferable as materials constituting the conductive powder.

There is no restriction | limiting in particular about the particle size of lead-containing compound powder, For example, it is preferable to adjust to the magnitude | size equivalent to or less than electroconductive powder. The average particle size of the lead-containing compound powder is, for example, preferably 4 μm or less, 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 typically be 0.1 μm or more, and more preferably 0.3 μm or more. It is preferable to adjust to such a particle size because the dispersibility with the glass frit becomes good. The average particle size about lead-containing compound powder, as well as the conductive powder, can be employed cumulative 50% particle diameter based on a laser diffraction and light scattering method (D _50).

  The shape of the particles constituting the lead-containing compound 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. Moreover, the thing of the form of the secondary particle which the primary particle aggregated may be sufficient. As such a lead-containing compound powder, for example, a powder produced by a known purification method, wet method, gas phase reaction 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 lead-containing compound powder may be mixed as it is in the state of powder in the conductive composition, but may be mixed in a state where a part or all of the lead-containing compound powder is supported on a glass frit. The means for supporting the lead-containing compound powder on the glass frit is not particularly limited, and examples thereof include use of a supporting method by calcination and a supporting method by a mechanochemical method.
In carrying by calcination, glass frit and lead-containing compound powder may be mixed, placed on a setter or the like, and calcinated at a temperature of about 300 to 500 ° C. in an oxidizing atmosphere. The calcining 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. Thereby, the glass frit and the lead-containing compound powder can be integrated without significantly affecting the properties and composition of the glass frit.

  In addition, for example, a dry particle compounding apparatus (for example, Nobilta NOB-130 manufactured by Hosokawa Micron Corporation) can be suitably used for carrying by the mechanochemical method. By treating the mixed powder of glass frit and lead-containing compound powder with such an apparatus, mechanical force such as shearing or compression acts between the particles to integrate the glass frit and lead-containing compound powder. Can do. Thereby, for example, as one example, it is possible to obtain composite particles in which the lead-containing compound powder is firmly fixed to the surface of one particle of the glass frit with a thickness of approximately one particle layer. Such composite particles can be used in place of glass frit and are simple.

  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 an organic binder having various compositions and an organic solvent as a dispersion medium. An organic binder means that it consists of an organic compound which has a binder function with respect to the binder effect by the glass frit which can also be called an inorganic binder. 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).

  As the organic binder, an organic compound having a binder function can be used without particular limitation. For example, organic polymers based on 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. A binder 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 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.). An organic solvent having a boiling point of about 230 ° C. or higher (typically about 230 ° C. 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 comprise based on 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 88% 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.

  The glass frit may be contained in the conductive composition at a ratio that is essentially required as an inorganic binder of the conductive powder. 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 preferably 0.5% by mass or more, more preferably 1% by mass or more. Excessive addition is not preferable because it erodes the silicon substrate and increases the resistance of the formed electrode. 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.

  The silicone resin can be added even in a very small amount with respect to the conductive powder, so that the electrical characteristics of the formed electrode can be improved and the printability can be improved. That is, the addition amount of silicone resin should just exceed 0 mass%. In order to clearly obtain the effect of addition of the silicone resin, the addition amount of the silicone resin can be typically 0.005% by mass or more when the conductive powder is 100% by mass, for example. The content 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 silicone resin can increase the resistance of the formed electrode. Moreover, it may act excessively on the erosion of the substrate by the glass frit and the lead-containing additive (suppress erosion excessively). Therefore, the addition amount of the silicone resin is typically about 1.0% by mass or less when the conductive powder is 100% by mass, and is 0.9% by mass or less. Preferably, it is 0.8 mass% or less, more preferably 0.6 mass% or less.

  By adding even a very small amount of the lead-containing compound powder to the conductive powder, it is possible to enhance the electrical characteristics of the formed electrode and also improve the adhesion of the electrode. That is, the content of the lead-containing compound powder only needs to exceed 0% by mass. In addition, it is more appropriate to consider the content of the lead-containing compound powder as the amount of the lead component in the entire conductive composition, and for example, it can be considered as the total amount with the PbO component contained in the glass frit. More specifically, for example, when it is assumed that a virtual glass is prepared by mixing glass frit and a lead-containing compound powder, the amount of PbO in the oxide-converted composition of the virtual glass is taken into account. 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 at which the proportion of the PbO amount in the virtual glass is 1 mol% or more, and 2 mol% or more. Of these, more preferably 5 mol% or more, and particularly preferably 10 mol% or more. The effect of adding the lead-containing compound powder may tend to saturate to some extent, and excessive addition is not preferable because it leads to a reduction in the proportion of the conductive powder. Therefore, the addition amount of the lead-containing compound powder can be added with the amount of the PbO amount in the virtual glass being typically about 40 mol% or less, preferably 35 mol% or less, preferably 30 mol % Or less, more preferably 25 mol% or less.

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 (10% to 10%) 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.

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

  Below, while showing the example which forms the comb-shaped electrode pattern containing a fine finger electrode by screen-printing this electroconductive composition on the light-receiving surface among various electrodes provided in a solar cell element, here The solar cell element disclosed in 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 (cell) 10 that can be preferably manufactured by implementing the present invention, and is made of single crystal, polycrystalline, or amorphous silicon (Si). This is a so-called silicon solar cell 10 in which a wafer is used as the semiconductor substrate 11. A cell 10 shown in FIG. 1 is a general single-sided light receiving solar cell 10. Specifically, this type of solar cell 10 includes an n-Si layer 16 formed by impurity doping on the light receiving surface 11A side of a p-Si layer (p-type crystalline silicon) 18 of a silicon substrate (Si wafer) 11. Pn junction is formed. On the surface of the silicon substrate 11, if necessary, a light-receiving surface formed of an antireflection film 14 made of titanium oxide, silicon nitride or the like formed by CVD or the like, and a conductive composition mainly containing Ag powder or the like. Electrodes 12 and 13 are provided. 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, the back surface 11B side of the p-Si layer 18 includes a back surface aluminum electrode 20 that exhibits a so-called back surface field (BSF) effect, and an external connection electrode 22 that extracts current from the aluminum electrode 20. . 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 is diffused 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. The external connection electrode 22 is typically formed by printing and baking a conductive paste whose conductive powder is Ag powder.

  As shown in FIG. 2, on the light receiving surface 11 </ b> A side of the silicon substrate 11 of the solar cell 10, several light receiving surface electrodes 12 and 13 (for example, about 1 to 3) are arranged in parallel with each other. A bus bar (for connection) electrode 12 and a plurality of (for example, about 60 to 90) striped finger (for current collection) electrodes 13 connected to cross the bus bar electrode 12 are formed. Has been. 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. 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 10.

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. At this time, the n-Si layer 16 can be formed so as to have a high sheet resistance (for example, 80 to 120Ω / □). Next, an antireflection film 14 made of silicon nitride or the like is formed by a technique such as plasma CVD.

  After that, 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-side conductor coated product 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 printed on the antireflection film 14 formed on the surface side of the silicon substrate 11 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. ) Finger electrode coating film (printed body) having a finger electrode can be 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. Baking is performed 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 side external connection electrodes (typically Ag electrodes) 22. At the same time, an Al—Si alloy layer (not shown) is formed, and aluminum is diffused into the p-Si layer 18 to form the p ⁺ layer (BSF layer) 24 described above, 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 crystal silicon is used here, phosphorus (P) having a different conductivity type is diffused to form a pn junction. However, when an n-type silicon substrate is used, p-type impurities (for example, B ) 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 also be provided on the back surface 11B side.
Further, instead of the 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.

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.
(Reference example)
[Preparation of conductive composition]
A conductive composition for electrode formation was prepared by the following procedure. First, as the conductive powder, spherical silver (Ag) powder having an average particle diameter of 2 μm was used. As the glass frit, the following lead-free and leaded glass powder (average particle diameter: 2 μm) was used. As the silicone resin, polydimethylsiloxane having a weight average molecular weight Mw of 50,000 was used.

[Lead-free glass frit]
_{1~25mol% Bi 2 O 3 -25~80mol%} TeO 2 -0.1~30mol% ZnO
[Leaded glass frit]
_{1~25mol% PbO-25~80mol% TeO 2} -0.1~30mol% ZnO
The leaded glass frit is a glass containing 25 mol% of PbO in an oxide equivalent 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), uses bismuth (Bi) that exhibits a function similar to 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 more and 600 ° C. or less.

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

[Production of solar cell element for evaluation]
The solar cell element of Examples 1-120 was produced by forming a light-receiving surface electrode (namely, the comb electrode which consists of a finger electrode and a bus-bar electrode) using the electrically conductive composition obtained above.
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 texture structure surface, and heat treatment was performed 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, 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 was screen-printed on the entire back surface and dried to form an aluminum film.

  Thereafter, 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 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. For screen printing, a screen plate having a 360 mesh (wire diameter: 16 μm, emulsion thickness: 15 μm) was used. The finger electrode pattern was adjusted such that the dimensions after firing were about 45 μm in line width and 15 μm to 25 μm in film thickness. 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 solar cell produced as mentioned above, an IV characteristic is measured using a solar simulator (PSS10, manufactured by Beger Co., Ltd.), and it is opened based on a “crystalline solar cell output measuring method” defined in JIS C8913. The voltage (Voc) and fill factor (FF) were calculated. The results are shown in FIG. 4 for the open circuit voltage (Voc) and in FIG. 5 for the fill 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 a solar cell including an electrode formed from this conductive composition tends to increase. I understood it. This tendency was confirmed when either a lead-free glass frit or a leaded glass frit was used as the glass frit. However, it has been confirmed that the increase in the open circuit voltage becomes more remarkable when a leaded glass frit is used. With respect to the conductive composition using the leaded glass frit, even when the silicone resin is not contained (0% by mass), a higher open-circuit voltage than that of the conductive composition using the lead-free glass frit can be obtained. It was found that the effect was greatly improved by adding resin. In addition, although no specific data is shown, such a tendency is greatly different depending on whether the glass frit is leaded or lead-free, but the leaded glass frit is not significantly affected by the ratio of glass components other than lead. It could be confirmed.

  On the other hand, as shown in FIG. 5, a tendency different from the open circuit voltage was observed for the fill factor. That is, it has been found that, for a conductive composition using a lead-free glass frit as a glass frit, the addition of a silicone resin significantly reduces the fill factor. The decrease in the fill factor is about 10% when 0.6 mass% of the silicone resin is added, indicating that it is not at a practical level. On the other hand, for conductive compositions using leaded glass frit, when silicone resin is added up to about 0.6% by mass, there is no significant effect on the fill factor, but more silicone resin can be added. It was found that the curve factor rapidly decreased by the addition. Although specific data are not shown, this trend is largely unaffected by the proportion of glass components other than lead in both leaded and leadless glass frit, although the glass frit differs greatly depending on whether it is leaded or leadless. I was able to confirm.

(Embodiment 1)
[Preparation of conductive composition]
Subsequently, the lead-containing compound powder was added to the conductive composition, and the conductive composition for electrode formation of Examples 1 to 120 was prepared in the same manner as in the above Reference Example under other conditions. That is, as the conductive powder, spherical silver (Ag) powder having an average particle diameter of 2 μm was used. As the glass frit, glass powders (average particle diameter: 2 μm) having three kinds of composition systems of lead-free and leaded materials shown in Table 1 below were 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 (Pb ₃ O ₄ ), lead monoxide (PbO), and lead nitrate (Pb (NO ₃ ) ₂ ) having an average particle size of about 2 μm were prepared from commercially available products.

  In Table 1, the lead-free glass frit does not contain lead (Pb), and includes (1) Si and B, (2) Te, (3) Te and W, respectively, as network-forming elements, and as network-modifying elements. Glass containing Zn or 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. About the leaded glass frit, as shown in Tables 2-4, the PbO amount in an oxide conversion composition is changed in the range of 5-30 mol%. These glass frits are all adjusted to have a softening point in the range of 250 ° C. or more and 600 ° C. or less.

Silver powder, silicone resin, glass frit, and lead-containing compound powder are mixed in the following proportions, 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 In an amount of 0.80% by mass and about 1% by mass of texanol as a dispersion medium, and well kneaded using a three-roll mill, the conductive compositions of Examples 1 to 120 were prepared. 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 mass% with respect to 100 mass% of the silver powder.

  Assuming that the lead-containing compound powder was prepared by adding each lead-containing compound to the glass frit to produce a virtual glass, the lead (PbO) amount in the oxide-converted composition of the virtual glass is shown in Tables 2-4. The ratio was adjusted so that it might become 0-30 mol%. In addition, the addition amount (mass%) of the lead-containing compound powder per 100 mass% of silver powder at that time is shown in the lower parenthesis.

For example, as shown in Table 2, a case where B ₂ O ₃ —SiO ₂ —ZnO-based glass frit (lead-free) is used as the glass frit will be described. At this time, when lead is used as the lead-containing compound powder, the amount of lead that the PbO amount of the virtual glass is 10 mol% is about 0.26% by mass (Example 11), and the amount of PbO is 20 mol%. The amount of red lead is about 0.51% by mass (Examples 12 to 16, 18), and the amount of lead reddish that makes the amount of PbO 30 mol% is about 0.77% by mass (Example 13). For example, when lead monoxide is used as the lead-containing compound powder, the amount of PbO in the virtual glass is 20 mol% is about 0.53 mass% (Examples 19 and 20), and lead nitrate is used. Is about 0.76% by mass (Examples 21, 22).

  The lead-containing compound powder was prepared in two ways: when blended in the form of a powder, and when blended in a state of being supported on a glass frit. The glass frit carrying the lead-containing compound powder is supplied with a predetermined amount of the lead-containing compound powder and the glass frit using a dry particle compounding apparatus (NOB-130), the blade rotation speed is 2500 rpm, and the power load is 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. And about this solar cell, an IV characteristic is measured using a solar simulator (the Beger company make, PSS10), and conversion efficiency (Eff) based on the "crystal system photovoltaic cell output measuring method" prescribed | regulated to JISC8913 Was calculated. The results are shown in Tables 2-4. In addition, the “Reff” column of “Eff” in the table shows the relative value of each Eff when the actual Eff value of Examples 1, 41, and 81 serving as a reference in each table is 1. In the “evaluation” column, “X” (bad) when the Reff 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. (Excellent).

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

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

From the results of Examples 2, 42, and 82, it was found that adding silicone resin to the conductive composition increased Reff by 0.001 to 0.002 and increased power generation efficiency.
However, from the results of Examples 23 to 25, 63 to 65, and 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 lead one. It has been found that the increase rate of is significantly increased, for example, from 0.167 to 0.204. In these examples, the increase in Reff peaked when the amount of PbO was 20 mol% (Examples 24, 64, and 104) regardless of the composition system of the leaded glass frit. Accordingly, it has been found that the increase in Reff does not depend on the type of glass frit, but simply does not increase with the amount of PbO.

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

  In contrast to the above examples, from the results of Examples 11-22, 51-62, 91-102, the ref is 0.159-0 by adding both the silicone resin and the lead-containing compound powder to the conductive composition. It was found that the rate of increase in power generation efficiency was greater than when the silicone resin and the lead-containing compound powder were added individually. The increase rate of Reff is much larger than the sum of the increase rate of Reff when the silicone resin is added alone and the increase rate of Reff 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 and contribute to an increase in power generation efficiency.

In these examples, the increase in Reff is when the addition amount of the silicone resin is 0.20 mass% and the amount of the lead-containing compound powder (converted value when contained as a glass frit) is 20 mol% (for example, 14, 54, 94). Note that there is a slight difference in power generation efficiency depending on the type of the 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).
In addition, 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 a leaded glass frit is used instead of a lead-free glass frit as the glass frit. It was. In this case, since the lead component is contained in the leaded glass frit, it has been found that the proportion of the lead-containing compound powder can be reduced.

Here, the solar cells of Examples 82, 84, and 91 to 120 manufactured using the conductive composition containing (3) a TeO ₂ —WO ₃ lead-free glass frit and a leaded glass frit shown in Table 4 A peel test of the light-receiving surface electrode was performed to measure the adhesive strength of the electrode to the substrate.

  In addition, evaluation of adhesive strength (peeling strength) was performed using the strength measuring apparatus 300 as shown in FIG. Specifically, first, an epoxy adhesive 42 was applied to the back surface 11B side of the solar cell 10 for evaluation, and was fixed on the glass substrate 41 with the light receiving surface 11A side facing up. The tab wire 35 for peeling was soldered to the light-receiving surface electrode 12 of the solar cell 10 for evaluation from one end through 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 fixed base 40 is 135 °, and the extension 35 e formed in advance on the tab wire 35 is pulled vertically upward (arrow). 45), and the adhesive strength (N / mm) at the electrode 12 / substrate 11 interface was measured. The measurement results of the adhesive strength are shown in the “ADH” column of Table 5. The “Radh” column of “ADH” in Table 5 shows the relative value of each adhesive strength when the actual 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 indicated as “◯”, and the case where it is less than 0.85 is indicated 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. And the electrode formed from the electroconductive composition of Examples 91-102 and 106-120 which added both the silicone resin and the lead-containing compound powder turned out that adhesive strength falls further. Radh was 0.908-0.947 for Examples 91-102 using lead-free glass frit, whereas it was 0.867-0.927 for Examples 106-120 using leaded glass frit. It was. In Examples 91-102 and Examples 106-120, the Eff values were almost the same level, but it was found that the adhesive strength tends to decrease by about 5% by using leaded glass frit. However, it was found that for Examples 103 to 105 using the leaded glass frit without adding the lead-containing compound powder, the adhesive strength was significantly reduced. From the above, for example, the conductive composition for realizing the Eff values (17.23 to 17.23) of Examples 103 to 105 includes a silicone resin and a lead component, and the lead component is leaded. It can be seen that it is better to use lead-containing compound powder instead of using glass frit.

(Embodiment 2)
[Preparation of conductive composition]
Next, two types of lead-free glass frits (A) and (B) are prepared. 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 set. By changing the other conditions in accordance with the first embodiment, eight conductive compositions for electrode formation were prepared. That is, (A) the lead-free glass frit and (B) the leaded glass frit were changed so as to be 0.5% by mass to 10% by mass with respect to 100% by mass of the silver powder. In addition, as a lead-containing compound powder, lead (Pb ₃ O ₄ ) is used, and this lead-containing compound is contained in an amount such that the PbO concentration in the oxide-converted composition of the glass frit is 20 mol% when it is contained in glass. did. In the parentheses in the column of the amount of Pb ₃ O ₄ in Table 6, the ratio (mass%) of Pb ₃ O ₄ actually added to 100 mass% of silver powder is shown.

  Using the conductive composition prepared as described above, a solar cell was produced in the same manner as in the reference example. And about this solar cell, an IV characteristic is measured using a solar simulator (the Beger company make, PSS10), and conversion efficiency (Eff) based on the "crystal system photovoltaic cell output measuring method" prescribed | regulated to JISC8913 Was calculated. The results are also shown in Table 6. The “Reff” column in the table shows the relative value of each Eff when the measured value of Eff in Examples 121 and 125 is 1. In the “evaluation” column, “X” (bad) when the Reff 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. (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 was changed from 0.5 mass% to 10 mass%, the conductive composition was composed of lead-containing compound powder and silicone. It was found that the conversion efficiency can be greatly improved by including both resins. Moreover, it turned out that the improvement of conversion efficiency is significantly improved compared with the case where an electroconductive composition contains only any one of a lead-containing compound powder and a silicone resin. In addition, although not specifically shown, such a tendency could be confirmed similarly for the leaded glass frit.

  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 (10)

A conductive composition for forming an electrode of a solar cell,
Conductive powder;
Glass frit,
Lead-containing compound powder;
Silicone resin,
An organic binder,
A dispersion medium;
A conductive composition comprising:   The said glass frit is an electrically conductive composition of Claim 1 contained in the ratio of 0.1-12 mass%, when electroconductive powder is 100 mass%.   The lead-containing compound powder is included in such an amount that when the glass frit and the lead-containing compound powder are mixed to produce a virtual glass, the proportion of PbO in the oxide-converted composition of the virtual glass is 30 mol% or less. The conductive composition according to claim 2.   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, red lead, lead nitrate and lead carbonate. The conductive composition according to item.   The conductive composition according to claim 1, 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 is contained in a proportion of 0.9% by mass or less with respect to 100% by mass of the conductive powder.   The conductive composition according to claim 1, wherein the silicone resin has a weight average molecular weight of 1,000 to 150,000.   The conductive composition according to claim 1, wherein the silicone resin contains at least one of polydimethylsiloxane and polyether-modified siloxane.   The metal species constituting the conductive powder includes any one or two 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 solar cell provided with the baked product of the electroconductive composition of any one of Claims 1-9 as an electrode.

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