TWI541829B - Conductive composition and method for manufacturing - Google Patents

Description

Conductive composition and method of producing the same

The present invention relates to a conductive composition, and more particularly to a conductive composition for a solar cell and a method of manufacturing the same.

Solar cells convert solar radiant energy into electrical energy by means of semiconductor materials. The structure of the solar cell mainly includes a photoelectric conversion layer composed of a PN junction formed of a P-type semiconductor material and an N-type semiconductor material. When the sunlight is irradiated onto the photoelectric conversion layer, the photoelectric conversion layer absorbs light in a wavelength band corresponding to the semiconductor material in the sunlight, so that the light energy is converted into electric energy in the form of an electron-hole pair, thereby realizing photoelectric conversion. And externally connected to the load of the metal lead of the P-type semiconductor material layer and the N-type semiconductor material layer.

A solar cell is a semiconductor component that utilizes the photovoltaic effect to convert solar energy into electrical energy. Basically, any semiconductor diode can convert light energy into electrical energy. Solar cells generate electricity based on two factors: the light guide effect and the internal electric field. Therefore, when selecting the material of a solar cell, it is necessary to consider the light guiding effect of the material and how to generate an internal electric field.

The performance of solar cells is mainly judged by the conversion efficiency between photovoltaics. The factors affecting the conversion efficiency include sunlight intensity and temperature; material resistance and matrix quality, defect density; concentration and depth of PN junction; surface reflectance of light; metal electrode line width, line height, contact resistance . Therefore, it is necessary to closely control various influence factors to produce a solar cell with high conversion efficiency.

Conversion efficiency and manufacturing cost are the main considerations for manufacturing solar cells today. At present, solar cell products on the market have a large market share of solar cells using bismuth as raw materials. According to the crystal structure classification, there are three kinds of single crystal solar cells, polycrystalline solar cells, and amorphous solar cells. In terms of conversion efficiency, the single crystal germanium solar cell is still higher, with a conversion efficiency of about 24%, the polycrystalline germanium is approximately 19%, and the amorphous germanium is about 11%. Using other compound semiconductors as photoelectric conversion substrates, such as III-V gallium arsenide (GaAs), the conversion efficiency can be as high as 26 %the above.

How to improve its energy conversion efficiency and reduce the thickness of germanium wafers is also the main axis of solar cell technology development. Regarding the wafer thickness problem, a laser-fired contact (LFC) technique can be used in the prior art, except that the battery thickness can be reduced to less than 37 μm, and the efficiency can reach 20%. The steps are roughly as follows: on the back surface of the solar cell, an aluminum layer is formed by evaporation and a passivation layer is formed, and the aluminum layer is penetrated by laser light to form a conductive contact. The laser sintering method can effectively solve the problem of the original power loss, and the laser sintered joint technology does not require the use of conventional expensive lithography and etching techniques to form a hole pattern in the passivation layer on the back side of the crystal plate to accommodate the aluminum. Quality electrode.

In addition, in order to direct the current generated by the solar cell into usable electrical energy, a metal electrode must be formed at both ends of the semiconductor substrate to conduct current to the external current load terminal. However, the metal electrode on the light-receiving surface (ie, the front surface) of the substrate blocks the light-receiving surface and hinders the absorption of sunlight. Therefore, the smaller the metal electrode area of the front surface of the solar cell, the better, so as to increase the light-receiving area of the solar cell. Therefore, the current metal electrodes are mainly printed with a mesh electrode structure on the front and back sides of the solar cell by screen printing technology. The so-called screen printing electrode preparation, that is, using the screen printing method, the conductive metal paste (such as silver glue) is printed on the already doped germanium substrate according to the designed pattern, and under appropriate sintering conditions. The organic solvent in the conductive metal paste is volatilized, and the metal particles form a bismuth alloy with the ruthenium on the surface to form a good ohmic contact between the bismuth materials, thereby becoming a front and back metal electrode of the solar cell. However, the excessively thin electrode wire tends to cause wire breakage, or the resistance thereof is increased to lower the conversion efficiency of the solar cell, so how to achieve thinning without reducing the overall power generation efficiency of the battery is a technical focus in this field. In general, the metal electrode has a film thickness of about 10 to 25 micrometers (μm), and the front metal has a finger line width of about 120 to 200 μm. The use of such technology to fabricate solar cell electrodes has the advantages of automation, high throughput, and low cost. However, the composition of the prior art conductive adhesive tends to form a large agglomerate, which is not easily broken through the screen printing mesh or the screen-forming version.

Further, in the case of a general solar cell substrate (i.e., a non-light-receiving surface), the back electrode structure includes a silver electrode portion (wire electrode portion) and an aluminum electrode portion (and the back electric field portion described above). At present, in the industry, the silver electrode 11 pattern is first printed on the back surface of the ruthenium substrate 10 by screen printing, and then the aluminum electrode 12 layer is formed thereon, as shown in FIG. Since the solderability of aluminum is very poor, it is impossible to connect the solar cell modules by direct soldering. Therefore, a general welding strip 20 is used to solder the silver electrode 11 on the back portion of the solar cell to make each power generating module. They are electrically connected to each other. In the structure of FIG. 1, the silver electrode-germanium substrate interface 30 and the aluminum electrode-germanium substrate interface 50 are in the sintering process. A eutectic layer is formed to make it tightly bonded. However, it is difficult to form a eutectic structure between silver and aluminum, and a peeling phenomenon is likely to occur at the silver electrode-aluminum electrode interface 40, so that a crack occurs between the silver electrode and the aluminum electrode to degrade the overall performance of the solar cell. Therefore, in addition to the conversion efficiency test, the solar cell module must be subjected to a tensile test of the solder strip 10 and a peeling test of the silver electrode-aluminum electrode interface 40 on the back to ensure the stability of the back structure of the module. .

In summary, it can be seen that in addition to the semiconductor substrate forming the PN junction, the most important material for fabricating a solar cell is the portion of the conductive composition. The conductive compositions in the prior art are all composed of metal powders (especially silver powder), glass frits, organic carriers, and additives. Its composition, content, ratio, process parameters, etc. will affect the performance of the final electrode product. Taking the back metal electrode as an example, in addition to the above-mentioned relationship between the tensile strength of the soldering strip and the degree of peeling of the silver-aluminum electrode interface, the advantages and disadvantages of the conductive silver composition and the aluminum composition formed directly affect the conversion efficiency of the solar cell performance. The open circuit voltage Voc, the short circuit current Isc, the fill factor, the series resistance Rs, and the shunt resistance Rsh (shunt resistance), etc., also determine the effective sintering temperature range Ts and the adhesion force. Therefore, how to formulate a conductive composition that can improve the performance of the above solar cells is the focus of current research and development in the industry.

Generally, the silver-aluminum paste contains a mixture of silver powder and aluminum powder. However, since the eutectic structure is not easily formed between silver and aluminum, the conventional silver-aluminum paste conductive adhesive has insufficient tensile force, and the silver and the glass frit are easily peeled off; and if the conductive particles are All silver materials will cause cost increases. Accordingly, the present invention provides a method of manufacturing a conductive composition that is superior to conventional ones to overcome the above disadvantages.

In view of this, the main object of the present invention is to provide a conductive composition comprising: a conductive functional mixture composed of a metal and a metal oxide, a metal oxide as a filler material, and a metal as a main body to lift a tensile force; a metal oxide The metal is a 2-4 valence metal. Optionally, an outer layer portion is disposed overlying a portion of the surface of the filler material, wherein the outer layer portion comprises at least a metal or alloy to enhance electrical conductivity. Wherein the melting point of the above metal oxide is greater than the sintering temperature.

The metal oxide comprises a metal or an alloy among the metal oxides; for example, aluminum oxide, copper oxide, zinc oxide, zirconium oxide, cerium oxide or any combination thereof.

The conductive composition further comprises glass, an additive or any combination of the above; and further comprises an organic vehicle, wherein the metal oxide, the glass and the additive are mixed in the organic vehicle.

Another object of the present invention is to provide a conductive composition comprising a conductive functional mixture The composition comprises a metal and a metal oxide, a part of the metal oxide as a filling material, and a metal as a conductive body; and an outer layer portion selectively covering substantially a part of the surface of the filling material, wherein the material cost of the filling material Lower than the cost of the outer layer.

10‧‧‧矽 substrate

11‧‧‧ Silver electrode

12‧‧‧Aluminum electrode

20‧‧‧welding tape

30‧‧‧Silver electrode-tantalum substrate interface

40‧‧‧Silver electrode-aluminum electrode interface

50‧‧‧Aluminum electrode-矽 substrate interface

100‧‧‧矽 Wafer Solar Cells

101‧‧‧First electrode

102‧‧‧P-N semiconductor layer

103‧‧‧second electrode

110, 111, 112‧ ‧ steps

The above elements, as well as other features and advantages of the present invention, will become more apparent after reading the contents of the embodiments and the drawings thereof:

The first figure shows a cross-sectional view of a solar cell substrate structure.

The second figure shows a cross-sectional view of a germanium wafer solar cell structure.

The third figure is a flow chart for the fabrication of a conductive composition in accordance with the present invention.

The fourth figure shows a pull test chart.

The fifth to sixth figures are the microstructure diagrams of the alumina powder observed by a scanning electron microscope.

The seventh to ninth figures are microscopic structures of silver/alumina (Ag/alumina) particles observed by a scanning electron microscope.

The tenth to twelfth drawings are microscopic structures of alumina particles observed by a scanning electron microscope.

The thirteenth to eighteenth figures are the pulling forces that face up or down when sintered.

The invention will be described in detail below in conjunction with its preferred embodiments and the accompanying drawings. It should be understood that all of the preferred embodiments of the invention are intended to be illustrative only and not limiting. Therefore, the invention may be applied to other embodiments in addition to the preferred embodiments. The invention is not limited to any embodiment, but should be determined by the scope of the appended claims and their equivalents.

Hereinafter, preferred embodiments in accordance with the present invention will be described in detail with reference to the accompanying drawings. Further views and advantages of the novel inventive concept will be set forth in the description which follows, and the <RTIgt;

As shown in the first figure, it shows a cross-sectional view of a germanium wafer solar cell structure. The germanium wafer solar cell structure is only one embodiment of the present invention, and is not intended to limit the germanium wafer solar cell structure of the present invention and a method of forming the same. As shown in FIG. 2, the silicon wafer solar cell 100 includes a first electrode 101, a second electrode 103, and a P-N semiconductor layer 102. Both electrodes are electrically conductive, and at least one of the electrodes is a transparent material. The first surface of the first electrode 101 includes a P-N semiconductor layer (102).

The first electrode 101 (which may be referred to as a working electrode or a semiconductor electrode) may comprise any electrically conductive material. For example, the first electrode 101 may be glass or PET, PEN plastic plated with indium tin oxide (ITO) or fluoro tin oxide (FTO); or a conductive polymer may be used. The second electrode 103 (which may be referred to as a counter electrode) may include any material having electrically conductive properties. The second electrode comprises a conductive substrate comprising at least one selected from the group consisting of indium tin oxide (ITO), fluorine tin oxide (FTO), titanium-plated metal foil, zinc oxide, gallium trioxide or aluminum oxide, and tin-based oxide. And combinations of materials. In one embodiment, the material of the first electrode 101 and the second electrode 103 is any combination of a transparent material and a non-transparent material.

It is to be understood that the conductive composition of the present invention can be applied to the front or back side of any type of tantalum wafer solar cell, that is, the conductive composition disclosed herein can be applied to the front or back electrode.

In any case, the back electrode is described as an embodiment, and the present invention discloses a conductive composition, which can be applied to the material and manufacturing method of the above back electrode. It comprises: a conductive functional mixture composed of a metal and a metal oxide, a metal oxide as a filler, a metal as a main body to lift a tensile force; and a metal oxide metal as a 2-4 valent metal. The outer layer portion may selectively cover substantially a portion of the surface of the filling material, wherein the outer layer portion contains at least a metal or an alloy to enhance electrical conductivity. Wherein the melting point of the above metal oxide is greater than the sintering temperature. The weight percentage of the filler material is 3~5. When the metal oxide conductive particles coated with the outer layer are subjected to a process warming process, the outer layer portion of the surface will flow between the metal oxides to fill the voids, which can enhance the bonding force between the conductive compositions; Conductivity reduces impedance. Furthermore, the material of the filling material and the outer layer portion can be selected at a lower cost than the main body cost, so that the high-cost core can be replaced by a low-cost material, and the tensile force and the electrical conductivity can be increased.

The method for producing the conductive composition of the present invention will be further described in detail below with reference to the accompanying drawings and examples.

As shown in the third figure, it shows a manufacturing flow chart of the conductive composition for use in a solar cell of the present invention. First, in step 110, a surface conductive coating filler material, silver particles, glass frits, and additives are added to an organic vehicle. The particle size shape includes a sheet shape, a spherical shape, a column shape, a block shape, or a specific shape that conforms to the size. The particle size ranges from 0.1 to 10 microns (um). The above organic carrier may be selected from: hydroxypropyl cellulose (HPC), polyethylene glycol (PEG), polyethylene oxide (PEO), polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP) or other Polymer resin, etc. The organic vehicle can improve the dispersibility of the filler material, the silver particles, and further increase the adhesion to the substrate.

Then, in step 111, premixing is performed by using a mixer, for example, by using strong stirring, ultrasonic vibration (about 5 to 10 minutes) or a homogenizer to mix the predispersed solution with the organic carrier, that is, mixing the filling material and silver. Granules, glass melts and additives and organic carriers. Finally, in step 112, dispersion milling is carried out using a three-roller to prepare a silver paste, which forms a conductive composition.

The formed alumina, as shown in the fifth to sixth figures, is a particle microstructure diagram under a scanning electron microscope (SEM). The seventh to ninth figures show the microstructure of silver/alumina (Ag/alumina) particles observed by a scanning electron microscope. The tenth to twelfth figures show the microstructure of alumina particles observed by a scanning electron microscope.

The seventh figure shows the microstructure of the particles of silver/alumina powder in different spectra.

The conductive composition prepared by the invention is made by adding a metal oxide as a filling material, and the surface of the filling material is preferably coated with a conductive layer, such as a metal, an alloy or a combination of the above, and the above filling material is, for example, alumina, oxidized. Zirconium, cerium oxide, zinc oxide, copper oxide or any combination of the above; the surface of the material is modified, and the surface is coated with a layer of silver or copper metal; the result is that the tensile force can be increased, thereby increasing the silver-silver Peeling strength, increased peel strength between silver and glass; and the metal oxide filled material can achieve the purpose of reducing costs. In one embodiment, the electrically conductive composition of the present invention can be used on the front or back side of a solar cell.

The formed conductive composition can be passed through a screen printing program to form a conductive film, such as a mesh of stainless steel mesh 250, a mesh wiring diameter of 35 micrometers (μm), an emulsion thickness of 5 μm, and a printing pattern of 153 mm (cm). ) *4.4mm*2Line. The backside silver paste was screen printed onto a ruthenium substrate at a drying temperature of 200-300 ° C for a period of 0.5-1 minute (min). Thereafter, it is sintered by a belt moving method using an infrared sintering furnace, and the peak temperature is, for example, 700 to 900 °C.

Next, the measurement program, in the welding of the Solder Ribbon, first cuts the welding tape by about 25 cm (cm) with a cutter, and applies the flux on the welding tape to remove the oxide layer. The welding tape specifications are as follows:

Place the test component (solar cell: Cell) on the platform of the machine with an infrared stringer, set the temperature of the platform to 140 °C, and place the soldering strip on the busbar of the solar cell. Time and temperature for welding. The welding conditions are as follows:

In addition, in the tensile test, the solar cell sheet was fixed on the platform of the tension machine, and one end of the welding belt was fixed by a clamp, and the welding belt was pulled at an angle of 180°, and the tensile force value was measured and measured at a speed of 120 mm/s. The result can be referred to the fourth figure.

Embodiment 1

In the first embodiment, the effect of silver/alumina (Ag/Alumina) and alumina content on the tensile force is shown; the addition of alumina powder (Alumina powder) has difficulty in dispersing, and is not easily combined with silver in the sintering process. Crack. Refer to the thirteenth and fourteenth figures for the face up and down when sintering.

Embodiment 2

In Example 2, the effect of the silver/alumina (Ag/Alumina) content on the tensile force is shown: an appropriate amount of silver/alumina is added and a stable and high tensile force is achieved at different sintering temperatures. Refer to the fifteenth and sixteenth figures for the face up and down when sintering.

Embodiment 3

In the third embodiment, the effect of silver/alumina (Ag/Alumina) and alumina content on the tensile force is shown; when the solid content (silver content) is lowered, the weak silver layer cannot have strong structural support under the reduced printing amount. Adding silver/alumina can increase the bond strength between silver and silver; 2. silver-glass. Refer to the seventeenth and eighteenth figures for the face up and down when sintering.

From the above, it can be seen that in the present invention, an appropriate amount of filler material, for example, silver/alumina (zirconia, yttria, zinc oxide) is added to the conductive composition to increase the tensile force and avoid the cross section of the original silver layer. The conductive composition has excellent electrical conductivity and reduced resistance.

Embodiment 4

In this embodiment, alumina is mainly added to a conductive mixture mainly composed of silver. It can be seen from the fourth embodiment that the control group contains only silver (Ag), the control group does not add any alumina, and the welding force of the face-up welding force and the face-down welding force are 1.73 and 1.51, respectively; experiment and observation according to the present invention As a result, it is helpful to improve the tensile force by adding a trace amount of alumina, the alumina content is about 0.5-5% by weight, and the preferred range is about 2-4% by weight of the alumina, see the above table. It is shown that the tensile force of the experimental group (K, L, M, N) is greater than that of the control group; therefore, similarly, in the fourth embodiment, it is shown that the solid content (silver content) is lowered, and the weak silver layer cannot be obtained under the reduced printing amount. Strong structural support. Adding silver/alumina can increase the bond strength between silver and silver; 2. silver-glass. In addition, the addition of alumina has the same effect and can be filled with voids (a weaker silver layer structure) caused by a decrease in silver content. The present invention provides a conductive composition comprising: a conductive functional mixture composed of a metal and a metal oxide as a filler material, the metal being used as a main body to lift a tensile force; wherein the metal contains silver, wherein the metal is oxidized The aluminum weight percentage is about 0.5 to 5%. The metal oxide comprises aluminum oxide, copper oxide, zinc oxide, zirconium oxide, cerium oxide or any combination of the above, and the metal of the metal oxide is a 2-4 valent metal.

The present invention has been described above by way of a preferred example, and is not intended to limit the spirit of the invention. Modifications and similar configurations made within the spirit and scope of the invention are intended to be included within the scope of the appended claims.

110, 111, 112‧ ‧ steps

Claims (10)

A conductive composition comprising: a conductive functional mixture consisting of a metal and a metal oxide, the metal oxide being used as a filler material, the metal being used as a main body to lift a tensile force; and a conductive outer layer substantially covering the filling a portion of the surface of the material; wherein the metal oxide comprises aluminum oxide, copper oxide, zinc oxide, zirconium oxide, hafnium oxide, or any combination thereof; wherein the metal may comprise silver. The conductive composition of claim 1, wherein the silver weight percentage is less than or equal to 60%. The conductive composition of claim 1, wherein the metal oxide is from about 0.5 to 5% by weight. The electrically conductive composition of claim 1 wherein the alumina is present in an amount of from about 2% to about 4% by weight. The conductive composition of claim 1, further comprising glass, an additive, and the filler and the glass, the additive being mixed in the organic vehicle. A conductive composition for a solar cell sheet, characterized in that the conductive functional mixture is composed of a metal and a metal oxide, and the metal oxide is used as a filling material, and the metal is used as a main body to lift the pulling force; the conductive outer layer is substantially Overlying a portion of the surface of the fill material; wherein the metal oxide comprises aluminum oxide, copper oxide, zinc oxide, zirconium oxide, hafnium oxide, or any combination thereof; wherein the metal may comprise silver. The conductive composition for solar cell sheets of claim 6, wherein the silver weight percentage is less than or equal to 60%. The conductive composition for solar cell sheets of claim 6, wherein the metal oxide has a weight percentage of about 0.5 to 5%. The conductive composition for solar cell sheets of claim 6, wherein the alumina weight percentage is about 2 to 4%. The conductive composition for solar cell sheets of claim 6, further comprising glass, an additive, and the filler and the glass, the additive being mixed in the organic vehicle.