JP5816738B1 - Conductive composition - Google Patents

Abstract

Provided is a conductive composition capable of forming an electrode having good fire-through properties and good adhesion and bonding to a substrate. The present invention provides a conductive composition for forming an electrode for a solar cell. The conductive composition includes a conductive powder, a glass frit, and a vehicle. The glass frit has the following composition in terms of oxide: PbO 1 mol% or more and 20 mol% or less; TeO 235 mol% or more and 90 mol% or less; Bi2O 3 0.1 mol% or more and 10 mol% or less; Li2O 0.1 mol% or more The total of 30 mol% or less; ZnO 0 mol% or more and 30 mol% or less; Mg 0 mol% or more and 20 mol% or less; and WO 30 mol% or more and 30 mol% or less; is 95 mol% or more of the whole glass frit. [Selection] 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 increasing environmental awareness and energy saving in recent years, solar cells are rapidly spreading. Along with this, there is a demand for solar cells with good photoelectric conversion efficiency and high output. One measure for realizing this requirement is to provide an antireflection layer on the light receiving surface of the solar cell that can increase the light receiving efficiency, and to take out the power generated at the pn junction in the substrate from the electrode with high efficiency. It is done.

In the production of this solar cell, typically, after a phosphorus-containing solution is first applied to the entire light-receiving surface of a silicon substrate to construct an n-Si layer (hereinafter also referred to as an n ⁺ layer) on the substrate surface, An antireflection film is formed. Then, a conductive composition for forming an electrode is supplied in a desired electrode pattern on the antireflection film and fired. The conductive composition for forming an electrode typically includes a conductive powder, a glass frit, and an organic vehicle. During firing, the glass frit contained in the conductive composition reacts with the antireflection film, and the constituent components of the antireflection film are taken into the glass. As a result, the conductive powder passes through the antireflection film (fire through), and electrical connection (ohmic contact) with the n ⁺ layer of the silicon substrate is realized. As a prior art regarding such a conductive composition for forming an electrode of a solar cell, for example, Patent Documents 1 to 9 can be cited.

JP 2014-049743 A JP 2014-028740 A JP 2013-254726 A Patent No. 5480448 Special table 2013-533188 gazette Special table 2013-533187 gazette Special table 2013-534023 gazette Japanese Patent No. 5559509 Japanese Patent No. 5559510

Here, it is known that the surface recombination rate can be lowered by thinning the n ⁺ layer of the silicon substrate. However, in a substrate having a thin n ⁺ layer (Lightly Doped Emitter; LDE), sheet resistance is increased, and ohmic contact with the light-receiving surface electrode by fire-through is realized in the thin n ⁺ layer. It was difficult. Further, if the erosion of the silicon substrate by the light receiving surface electrode is suppressed, there is a problem that sufficient adhesion between the electrode and the substrate cannot be obtained.
The present invention has been made in view of such circumstances, and its main purpose is to provide a conductive composition that can form an electrode having good fire-through properties and good adhesion and bonding properties with a substrate. Is to provide. Another object of the present invention is to provide a solar cell element with improved functions or performance realized by the use of the conductive composition.

In order to achieve the above object, the present invention provides a conductive composition for forming a solar cell electrode. The conductive composition includes a conductive powder, glass frit, and an organic vehicle. The glass frit has the following basic components: PbO 1 mol% or more and 20 mol% or less; TeO ₂ 35 mol% or more and 90 mol% or less; Bi ₂ O ₃ 0.1 mol% or more and 10 mol% or less. The total of Li ₂ O 0.1 mol% or more and 30 mol% or less; ZnO 0 mol% or more and 30 mol% or less; Mg 0 mol% or more and 20 mol% or less; and WO ₃ 0 mol% or more and 30 mol% or less; It is.
With such a configuration, there is provided a conductive composition that can form an electrode that has good fire-through properties and good adhesion and bonding properties to the substrate.

In a preferred embodiment of the conductive composition disclosed herein, the basic component contains the ZnO, the MgO, and the WO ₃ in a ratio of 5 mol% to 40 mol% in total. With this configuration, a conductive composition capable of forming an electrode having better characteristics is provided.

In a preferred aspect of the conductive composition disclosed herein, the basic component includes all of the ZnO, the MgO, and the WO ₃ . With this configuration, the characteristics of the formed electrode can be further enhanced.

  In a preferred embodiment of the conductive composition disclosed herein, the metal species constituting the conductive powder is any one selected from the group consisting of silver, copper, gold, palladium, platinum, tin, aluminum, and nickel. It is characterized by containing one or more elements. Such a configuration provides a conductive composition capable of forming an electrode with higher conversion efficiency.

  In another aspect, the technology disclosed herein provides a solar cell element. This solar cell element is characterized in that a light-receiving surface electrode formed by using any one of the conductive compositions described above is provided on a light-receiving surface of a substrate. According to such a configuration, the solar cell element can be formed with a light-receiving surface electrode having good adhesion and bonding properties to the substrate. Thereby, for example, a solar cell element including an electrode having thermoelectric conversion performance represented by a fill factor and good bonding strength is realized.

  In a preferred aspect of the solar cell element disclosed herein, an antireflection film is provided in a region of the light receiving surface of the substrate where the light receiving surface electrode is not formed. Thereby, a solar cell element with more excellent thermoelectric conversion performance is provided.

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 a figure explaining a mode that the joining strength of an electrode is measured.

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

  The conductive composition disclosed herein is typically a conductive composition for forming a solar cell electrode by firing. The conductive composition is essentially the same as the conventional conductive composition of this type. The conductive powder, the glass frit, and the organic vehicle component for dispersing these components (described later, A mixture of an organic binder and a dispersant). And this glass frit is characterized by the total amount of the following basic components being 95 mol% or more of the whole glass frit in the composition when converted into an oxide.

PbO 1 mol% to 20 mol% TeO ₂ 35 mol% to 90 mol% Bi ₂ O ₃ 0.1 mol% to 10 mol% Li ₂ O 0.1 mol% to 30 mol% ZnO 0 mol% to 30 mol% MgO 0 mol% to 20 mol % WO ₃ 0 mol% or more and 30 mol% or less

That is, this glass frit contains four components of PbO, TeO ₂ , Bi ₂ O ₃ and Li ₂ O as essential constituent components. Here, these components are contained in amounts of Bi ₂ O ₃ and Li ₂ O so that the proportion of TeO ₂ is large and the proportion of PbO is small in the range where the softening point of the glass frit is 250 ° C. or more and 600 ° C. or less. Has been adjusted.
And if necessary, three components of ZnO, MgO and WO ₃ are included.
The glass frit does not prevent other components from being contained, but the content of such components is limited to 5 mol% or less. That is, the glass frit disclosed here can be understood as being essentially composed of the above seven basic components.

TeO ₂ functions as a network former and is an indispensable component for realizing good ohmic contact in the glass frit contained in the conductive composition for forming a solar cell electrode. For example, when the conductive powder contains silver (Ag), the amount of Ag solid solution in the glass phase (glass frit) is increased in order to achieve good contact at the interface between the electrode being fired and the silicon substrate of the solar cell. It is hoped that Here, the presence of Te in the glass phase can greatly increase the amount of Ag solid solution. Further, when the temperature is lowered during firing, Ag dissolved in the glass phase can be precipitated as Ag fine particles. Here, Te is present in the glass phase, so that the precipitation of Ag is moderate with respect to the change in the firing temperature, and the control range (margin) of the firing temperature can be widened. Such an effect is sufficiently expressed when TeO ₂ is present at a ratio of 35 mol% or more, and the effect can be enhanced as the amount of TeO ₂ increases. Therefore, TeO ₂ is preferably a larger amount. For example, TeO ₂ is preferably the main component (maximum component) of this glass frit. Specifically, the content of TeO ₂ is preferably 40 mol% or more, more preferably 45 mol% or more, and further preferably 50 mol% or more. However, if the content of TeO ₂ is too large, the erosion action of the silicon substrate is suppressed, the fire-through characteristics are lowered, the electrical characteristics of the formed electrode are lowered, and the firing margin is conversely narrowed. It is not preferable. From this viewpoint, the content of TeO ₂ is limited to 90 mol% or less.

PbO is a preferable component in that it functions as a network former in the glass frit disclosed herein, exhibits good fire-through characteristics, and can improve the electrical characteristics of the formed electrode. In the technique disclosed herein, PbO is blended at a ratio of 1 mol% or more and 20 mol% or less in order to compensate for the erosion property of the silicon substrate which is reduced as a result of containing a large amount of TeO ₂ . PbO is preferably 2 mol% or more, and more preferably 3 mol% or more. On the other hand, PbO is also a component for which it is preferable to reduce the content as much as possible from the viewpoint of environmental load in recent years. From this viewpoint, the content of PbO is preferably 15 mol% or less, more preferably 13 mol% or less, still more preferably 10 mol% or less, and particularly preferably 8 mol% or less, for example, 5 mol% % Or less.

Bi ₂ O ₃ is a component included for the purpose of exhibiting good fire-through characteristics together with the PbO, although it does not form glass in a binary system with TeO ₂ . Bi ₂ O ₃ is also preferable in that it has an effect of suppressing an increase in viscosity of the glass frit when the glass frit is melted by firing. If the content of Bi ₂ O ₃ is less than 0.1 mol%, it may be difficult to develop sufficient fire-through characteristics, which is not preferable. Therefore, the content of Bi ₂ O ₃ is preferably 0.1 mol% or more, more preferably 0.5 mol% or more, and particularly preferably 1 mol% or more. Bi ₂ O ₃ is not preferable if the content exceeds 10 mol% because the silicon substrate tends to be excessively eroded and the electrical characteristics can be adversely affected. The content of Bi ₂ O ₃ is preferably 8 mol% or less, more preferably 7 mol% or less, and particularly preferably 5 mol% or less.

Li ₂ O is a component that can serve as a dopant for the n ⁺ layer of the silicon substrate. When the glass frit contains Li ₂ O, the conductive composition can have a donor compensation function for the n ⁺ layer. In the formation application of the electrode for solar cells, the effect | action of the fall of an electrical property which is seen by other alkali components is not seen, but since a softening point can be lowered | hung, it is made to contain as an essential component. From such a point of view, the conductive composition disclosed herein may be particularly suitable for use in electrode formation for solar cells employing a shallow emitter structure that has a low donor element concentration and is likely to have a high sheet resistance. If the content of Li ₂ O is less than 0.1 mol%, the effect as a dopant and the effect of reducing the softening point are not sufficiently exhibited, which is not preferable. Therefore, the content of Li ₂ O is preferably 0.5 mol% or more, more preferably 1 mol% or more, and particularly preferably 5 mol% or more. On the other hand, if the content of Li ₂ O exceeds 30 mol%, the content of TeO ₂ becomes relatively low, which is not preferable. The content of Li ₂ O is preferably 28 mol% or less, more preferably 25 mol% or less, and particularly preferably 22 mol% or less.

ZnO is not an essential component, but can be preferably included because it has an effect of improving the electrical characteristics of the formed electrode. For example, the open circuit voltage and the short circuit current can be improved. It also has the effect of increasing the stability of the glass to widen the vitrification range and making it difficult to crystallize the glass frit after firing. When the content of ZnO exceeds 30 mol%, the content of TeO ₂ is relatively low, which may cause a decrease in electrical characteristics.

MgO is not an essential component, but it improves the solubility of glass to suppress the occurrence of bubble defects, lowers the softening point of glass, increases the stability of glass, widens the vitrification range, and fires Since it has the effect of making it difficult to crystallize the subsequent glass frit, it can be preferably contained. When the content of ZnO exceeds 20 mol%, the content of TeO ₂ becomes relatively low, which is not preferable because the electrical characteristics may be deteriorated.

WO ₃ is not an essential component, but functions as a network former in Te-based glass, and has the effect of stably expanding the vitrification range and stabilizing Te in the glass phase. Therefore, it can be preferably included. Moreover, in the formulation with a small amount of PbO, WO ₃ may be a preferable component from the viewpoint that the effect of enhancing the adhesiveness can be expressed. If the content of WO ₃ exceeds 30 mol%, the content of TeO ₂ becomes relatively low, which may cause a decrease in electrical characteristics.

The _three basic components ZnO, MgO and WO ₃ are not necessarily limited to this, but in order to avoid a relatively low TeO ₂ content, a total of 40 mol % Or less (preferably 36 mol% or less). Further, these three components are preferably added in a total amount of 5 mol% or more (preferably 7 mol% or more, for example, 10 mol% or more) in order to improve the stability of the glass phase.

  The glass frit can contain various other glass constituent components and additive components as long as the characteristics are not impaired. For example, Si, Al, Ba, B, Na, K, Rb, Ag, Zr, Sn, Ti, Fe, Co, Cs, Ge, Ga, In, Ni, Ca, S, Cu, Sr, Se, Mo, Y, As, La, Nd, C, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Ta, V, Fe, Hf, Cr, Cd, Sb, F, I, Mn, P, Ce and One element selected from the group consisting of Nb may be contained alone or in combination of two or more elements. However, inclusion of unnecessary elements can impair the electrical characteristics of the formed electrode. Therefore, in the technology disclosed herein, the components other than the above seven basic components are 5 mol% or less in total, preferably 4 mol% or less, more preferably 3 mol% or less, particularly preferably 2 mol% or less, for example, 1 mol%. It can be as follows. Or it is good also as 0 mol% substantially except the component mixed unavoidable. In other words, the above seven basic components may be substantially 100 mol%.

Such a glass frit is a component that can function as an inorganic binder in addition to the function of exhibiting the fire characteristic as described above in the conductive composition. It also has a function of enhancing 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).
Such a glass frit is preferably adjusted to a size equal to or smaller than that of the conductive powder described below. For example, the average particle size is preferably 3 μm or less, more preferably 2 μm or less, and more preferably about 0.1 μm or more and 2 μm or less. The average particle diameter D related to the glass frit is equivalent to a sphere calculated by the following formula: D = 6 / (ρS); based on the specific surface area S measured based on the air permeation method and the true density ρ of the glass frit. It means the diameter.

  Although the softening point of the glass which comprises a glass frit is not specifically limited, It is preferable that it is about 250-600 degreeC (for example, 300-400 degreeC). As an example, the glass frit specifically exemplified in the following examples has a softening point adjusted within a range of 300 ° C. or more and 600 ° C. or less. When the conductive composition containing glass frit having such a softening point is used, for example, when forming a light-receiving surface electrode of a solar cell element, it exhibits good fire-through characteristics and forms a high-performance electrode. Preferred to contribute.

Hereinafter, components other than the glass frit will be described.
As the conductive powder forming the main component of the solid content of the conductive composition disclosed herein, powders made of various conductive materials having desired conductivity and other physical properties according to the application can be used. . Examples of the material constituting the conductive powder include, for example, gold (Au), silver (Ag), copper (Cu), palladium (Pd), platinum (Pt), tin (Sn), ruthenium (Ru), Powders made of metals such as rhodium (Rh), iridium (Ir), osmium (Os), nickel (Ni) and aluminum (Al), and alloys thereof can be considered. Among these, in the formation of solar cell electrodes, simple metals such as gold, silver, platinum, palladium and alloys thereof (Ag—Pd alloy, Pt—Pd alloy, etc.), nickel, copper, tin, aluminum, In addition, materials made of these alloys and the like are particularly preferable as materials constituting the conductive powder. From the standpoint of relatively low cost and excellent electrical conductivity, a powder made of silver and its alloy (hereinafter sometimes simply referred to as “Ag powder”) is particularly preferably used. Hereinafter, the conductive composition of the present invention will be described using an example in which Ag powder is used as the conductive powder.

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

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

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

As the vehicle for dispersing the above conductive powder and glass frit, various types conventionally used in this type of conductive composition can be used without particular limitation depending on the desired purpose. Typically, the vehicle can be composed of organic binders and organic solvents of various compositions. In such an organic vehicle component, all of the organic binder may be dissolved in an organic solvent, or only a part thereof may be dissolved or dispersed (may be a so-called emulsion type organic vehicle).
Examples of the organic binder include cellulose polymers such as ethyl cellulose and hydroxyethyl cellulose, acrylic resins such as polybutyl methacrylate, polymethyl methacrylate, and polyethyl methacrylate, epoxy resins, phenol resins, alkyd resins, polyvinyl alcohol, and polyvinyl butyral. An organic binder based on is preferably used. In particular, a cellulosic polymer (for example, ethyl cellulose) is preferable, and a viscosity characteristic capable of performing particularly good screen printing can be realized.

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

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

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

Of the organic vehicle components, the organic binder is preferably 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. . Particularly preferred is a ratio of 2% by mass to 6% by mass with respect to 100% by mass of the conductive powder. In addition, this organic binder may contain the organic binder component which is melt | dissolving in the organic solvent, and the organic binder component which is not melt | dissolving in the organic solvent, for example. When the organic binder component dissolved in the organic solvent and the organic binder component not dissolved are included, the ratio thereof is not particularly limited, but for example, the organic binder component dissolved in the organic solvent is 40% to 10%).
Note that the content ratio of the organic vehicle as a whole is variable in accordance with the properties (typically, viscosity, concentration, etc.) required for the conductive composition. As a rough guide, when the total amount of the conductive composition is 100% by mass, for example, an amount of 5% by mass to 30% by mass is appropriate, and preferably 5% by mass to 20% by mass. An amount of -15% by mass (particularly 7% by mass to 12% by mass) is more preferable.

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 include metal oxide powders other than those described above (for example, NiO, ZnO ₂ , Al ₂ O _3, etc.) and other various fillers. 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.

  The conductive composition described above is suitable as a printing composition (sometimes referred to as paste, slurry, or ink) applied to screen printing, gravure printing, offset printing, inkjet printing, and the like. For example, it can be particularly preferably used when such a general-purpose printing means is employed in forming an electrode pattern that requires thinning and a high aspect ratio. Therefore, a description will be given of the solar cell element disclosed herein, showing an example in which a comb-shaped electrode pattern including a fine finger electrode is formed on the light receiving surface by screen printing, taking a crystalline silicon type solar cell element as an example. Do. The solar cell element may be the same as the conventional solar cell except for the configuration of the light-receiving surface electrode that characterizes the present invention. The detailed description is omitted.

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

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

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

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

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

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

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

Hereinafter, some examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in these examples.
[Production of glass frit]
Glass frit having the composition of Examples 1 to 50 shown in Table 1 below was prepared by the following procedure. First, lead Pb ₃ O ₄ as the Pb source, TeO ₂ as the Te source, Bi ₂ O ₃ as the Bi source, lithium carbonate Li ₂ CO ₃ as the Li source, and ZnO as the Zn source. MgO was used as the Mg source, WO ₃ was used as the W source, SiO ₂ was used as the Si source, MoO ₃ was used as the Mo source, and sodium carbonate Na ₂ CO ₃ was used as the Na source. And these raw materials were mix | blended by stoichiometric composition so that it might become the target glass composition, After throwing into a crucible, it heated at 900-1100 degreeC, fuse | melted, and obtained the glass composition by quenching.

Next, the glass composition was pulverized using a planetary mill and classified as necessary to obtain a glass frit having an average particle diameter in the range of 0.3 to 3 μm. Here, the average particle diameter of the glass frit is a sphere equivalent diameter calculated based on the specific surface area measured based on the air permeation method and the true density of the glass frit.
In addition, in the column “total of basic components” in Table 1, seven oxide components that are basic components of the glass frit disclosed herein; PbO, TeO ₂ , Bi ₂ O ₃ , Li ₂ O, ZnO, The total ratio of MgO and WO ₃ ; is shown.

  As the conductive powder, a substantially spherical silver (Ag) powder having an average particle diameter of 2 μm was used. As the organic vehicle, ethyl cellulose (EC) dissolved in terpineol was used. Texanol was used as the solvent. Then, the silver powder: glass frit: organic vehicle: solvent ratio is weighed so that the mass ratio is 89: 2: 6: 3, mixed using a stirrer or the like, and then dispersed with, for example, a three roll mill. The conductive compositions of Examples 1-50 were prepared by carrying out. In this example, the conductive compositions of Examples 1 to 50 were adjusted so as to have a viscosity of 180 to 200 Pa · s (20 rpm, 25 ° C.) in order to make printability described later substantially equal.

[Production of test solar cell element (light-receiving surface electrode)]
Using the conductive compositions of Examples 1 to 50 obtained above, light-receiving surface electrodes (that is, comb-shaped electrodes composed of finger electrodes and bus bar electrodes) are formed, thereby producing solar cell elements of Examples 1 to 50. did. Specifically, first, a commercially available p-type single crystal silicon substrate (plate thickness 180 μm) for 156 mm square (6 inch square) is prepared, and the surface (light receiving surface) is mixed acid of hydrofluoric acid and nitric acid. Etching was used to remove the damage layer and to form a textured textured surface. 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 thickness of about 0.5 μm on the light receiving surface of the silicon substrate. Then, a silicon nitride film having a thickness of about 80 nm was formed on the n-Si layer by plasma CVD (PECVD) to form an antireflection film.

Next, the back side electrode pattern was formed on the back side of the silicon substrate by screen printing using a predetermined silver electrode forming paste and drying. This back surface side electrode pattern becomes a back surface side external connection electrode by baking of a post process. Then, an aluminum electrode forming paste was screen-printed on the entire back surface side and dried to form an aluminum film.
Thereafter, the conductive compositions of Examples 1 to 50 prepared above were screen-printed on the antireflection film and dried at 120 ° C. to form an electrode pattern for the light-receiving surface electrode (silver electrode). A screen mesh (manufactured by SUS400, wire diameter 18 μm, emulsion thickness 15 μm) was used for printing plate making, and the printing conditions were set so that the width of the grid lines was 45 μm.
Thus, the solar cell for evaluation of Examples 1-50 was produced by baking the board | substrate which printed the electrode pattern at 700-800 degreeC of baking temperature in the atmospheric condition using a near-infrared high-speed baking furnace.

[Curve factor (FF)]
The IV characteristics of the solar cells of Examples 1 to 50 were measured using a solar simulator (manufactured by Beger, PSS10), and the fill factor (FF) was calculated from the obtained IV curves. The FF was calculated based on the “crystalline solar cell output measurement method” defined in JIS C-8913. The calculation result of FF was expressed in the form of percentage and shown in the column of “FF” in Table 1. Further, in the column of “Output characteristics” in Table 1, “◯” when FF was 76% or more, “△” when FF was 75% or more and less than 76%, and FF was less than 75%. X in the case.

[Adhesive strength]
Next, the adhesive strength of the silver electrode in the solar cells of Examples 1 to 50 produced as described above was evaluated. The adhesion strength (peel strength) of the silver electrode was evaluated using a peel tester 300 as shown in FIG.
Specifically, the glass substrate 41 is fixed on the fixing jig 40 of the peeling tester 300 via the fixing screw 43 and the locking plate 44, and the evaluation is performed on the glass substrate 41 via the epoxy adhesive 42. The solar cell 10 was placed with the light-receiving surface side up and the back surface side down, and was fixed.
The tab wire 35 was soldered via the solder layer 30 on the silver electrode 12 positioned on the upper surface side of the solar cell for evaluation fixed on the glass substrate 41 in this way.
Then, the peeling tester 300 is tilted so that the bottom surface of the fixing jig 40 has an angle of 180 °, and the extension portion 35e formed in advance on the tab wire 35 is pulled vertically upward (see arrow 45). The adhesion strength of the wire 35 / solder layer 30 / silver electrode 12 was measured. In the column of “Adhesive strength” in Table 1, ◯ when the measurement result of the adhesive strength is 3 N / mm or more, Δ when the measurement result is 2 N / mm or more and less than 3 N / mm, and × when it is less than 2 N / mm. Filled in.

[Evaluation]
Each of the conductive compositions of Examples 1 to 50 in the present embodiment includes a glass frit containing TeO ₂ as a main glass constituent.
Examples 14 to 23 show cases where the content of TeO ₂ is greatly changed. As shown in Examples 16 to 22, when the amount of TeO ₂ is in the range of 35 to 90 mol%, the Ag component is taken in (solid solution) from the silver powder to the glass phase at the time of firing and precipitated as Ag fine particles at the time of cooling. Thus, it is considered that good ohmic contact between the electrode and the substrate was realized. On the other hand, when the amount of TeO ₂ was less than 35 mol% as in Example 14 and Example 15, it was confirmed that these effects were not sufficiently exhibited and the output characteristics and adhesive strength were not sufficiently obtained. Moreover, when the amount of TeO ₂ exceeded 90 mol% as in Example 23, it was found that the fire-through characteristics were not sufficiently obtained and the ohmic contact was hindered.

  Examples 1 to 13 show cases where the content of PbO is greatly changed. As can be seen from the prior art, PbO is a component that can be contained in a large amount exceeding 20 mol% in relation to other components. In the technique disclosed here, as shown in Examples 3 to 12, the amount of PbO is set within a range of about 1 to 20 mol%. It was confirmed that sufficient output characteristics and adhesive strength were obtained even when the PbO amount was 1 mol%. On the other hand, as shown in Examples 1 and 2 (particularly as can be seen from the comparison between Example 2 and Example 3), if the amount of PbO is less than 1 mol%, the effect of improving the output by PbO cannot be sufficiently obtained. Therefore, it was confirmed that sufficient output characteristics could not be obtained. Further, as can be seen from the comparison between Example 12 and Example 13, in this system, when the PbO amount exceeds 20 mol%, the output characteristics are sufficient, but it is difficult to obtain sufficient adhesion to the substrate. It was.

Examples 24 to 30 show cases where the content of Bi ₂ O ₃ is greatly changed. In the technology disclosed herein, for example, as shown in Examples 25 to 29, the amount of Bi ₂ O ₃ is in the range of 0.1 to 10 mol%. As shown in Example 24, when the amount of Bi ₂ O ₃ is not included below 0.1 mol%, the softening temperature of the glass frit is not sufficiently lowered, and the output characteristics are rapidly deteriorated. It was. On the other hand, as shown in Example 30, when the amount of Bi ₂ O ₃ exceeds 10 mol%, it is difficult to obtain sufficient electrical characteristics.

Examples 31 to 35 show cases where the content of Li ₂ O is greatly changed. In the technique disclosed herein, for example, as shown in Examples 32 to 34, the amount of Li ₂ O is in the range of 0.1 to 30 mol%. As shown in Example 31, when the amount of Li ₂ O is not included, the effect of Li ₂ O as a dopant and the effect of reducing the softening point cannot be obtained, and the output characteristics are rapidly deteriorated. On the other hand, as shown in Example 35, it was confirmed that when the amount of Li ₂ O exceeded 30 mol%, it was difficult to obtain sufficient output characteristics and adhesive strength.

Examples 36 to 39 show cases where the content of ZnO is changed. In the technique disclosed herein, as shown in Example 39, it was confirmed that when the amount of ZnO exceeds 30 mol%, it is difficult to sufficiently obtain output characteristics and adhesive strength.
Examples 40 to 43 show cases where the content of MnO is changed. In the technique disclosed herein, as shown in Example 43, it was confirmed that when the MgO amount exceeds 20 mol%, it is difficult to sufficiently obtain output characteristics and adhesive strength.
Examples 44 to 47 show cases where the content of WO ₃ is changed. In the technique disclosed herein, as shown in Example 47, it was confirmed that when the amount of WO ₃ exceeds 20 mol%, it is difficult to obtain sufficient output characteristics.

The example 48 to example 50, (in this _{embodiment, SiO 2, MoO 3, Na} 2 O) components other than the basic components of the shows a case including. In the technology disclosed herein, as shown in these examples, even if it is a component other than the above, if the total amount is small (for example, a range of 5 mol% or less), it is contained without impairing output characteristics and adhesive strength. It was shown that it can be done.
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 (5)

A conductive composition for forming a solar cell electrode,
Conductive powder;
Glass frit,
An organic vehicle,
Including
The glass frit has the following basic components in the composition when converted to an oxide:
PbO 1 mol% or more and 20 mol% or less;
TeO ₂ 35 mol% or more and 90 mol% or less;
Bi ₂ O ₃ 0.1 mol% or more and 10 mol% or less;
Li ₂ O 0.1 mol% or more and 30 mol% or less;
ZnO exceeding 0 mol% and not more than 30 mol%;
20 mol% or less beyond the MgO 0 mol%; and WO ₃ 30 mol% or less exceed 0 mol%;
Total, der least 95 mol% of the total glass frit is, the ZnO, the all MgO and the WO ₃ is that contains a conductive composition. In the basic component,
2. The conductive composition according to claim 1, wherein the ZnO, the MgO, and the WO ₃ are included in a ratio of 5 mol% to 40 mol% in total. The metal species constituting the conductive powder includes any one or more elements selected from the group consisting of silver, copper, gold, palladium, platinum, tin, aluminum, and nickel. 2. The conductive composition according to 2. The light receiving surface of the substrate, the claims 1-3 light-receiving surface electrode formed using the conductive composition according to any one of are provided, the solar cell element. The solar cell element according to claim 4 , wherein an antireflection film is provided in a region of the light receiving surface of the substrate where the light receiving surface electrode is not formed.

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