TWI493729B - Conductive paste for front electrode of solar cell and the manufacturing method thereof - Google Patents

Description

Conductive paste for solar cell front electrode and method of manufacturing same

The present invention relates to a conductive paste, and more particularly to a conductive paste for improving the conversion efficiency of a solar cell.

The solar power industry is an emerging industry full of development prospects and has gradually emerged from the energy concept industry. At present, international oil prices are rising, the global oil resources are limited, and the Kyoto Protocol's awareness of environmental protection for waste gas reduction has limited traditional power generation methods such as burning oil and coal. As a result, major countries in the world have been actively researching and developing renewable energy sources to replace fossil fuel power generation in recent years to alleviate the pollution problems caused by traditional power generation methods. Among the alternative energy sources, whether it is solar energy, wind energy, geothermal energy, biomass energy, etc., are the goals of the advanced countries. Among them, the demand for solar energy is the strongest. According to Solarbuzz, a solar research institute, demand for solar photovoltaics has shown an upward trend in the past two decades. From the perspective of the installation of solar photovoltaic systems, the global installed capacity has climbed from 340 MW to 1,744 MW from 2001 to 2006, and has grown more than five times in five years, with an average annual increase of about 39%. It is foreseeable that the role of solar power in the future of human energy use is becoming more and more important.

A solar cell is a device that converts light energy into electrical energy. It is generally a semiconductor material, such as a germanium substrate containing single crystal germanium, polycrystalline germanium, and amorphous germanium, or a compound semiconductor such as GaAs, GaP, InP, AlGaAs. It is made of a compound of a group III or V element compound. In the case of a ruthenium substrate, it is common practice for the light-receiving region on the front side of the P-type ruthenium substrate to be doped with phosphorus atoms to form a negative electrode region, and the region on the back side where the light is not received is the corresponding positive electrode terminal. The PN junction formed in the semiconductor substrate converts the incident light energy of a specific wavelength (λ) into an electron hole to move in the opposite direction of the two electrodes to generate a current, which is a source of electric energy of the solar cell. In general, the light-receiving surface of the ruthenium substrate is also plated with an anti-reflection film (such as a tantalum nitride SiNx film) to avoid energy loss caused by light reflection to increase the conversion efficiency (Eff%) of the solar cell. In addition to the semiconductor substrate of the above solar cell, the industry generally A layer of aluminum back surface field (BSF) is formed on the back side of the tantalum substrate after the PN interface is completed. The aluminum back surface electric field can reduce the probability of a few carriers (electrons, holes) in the back recombination, and can also be used as the back electrode of the battery, thereby improving the conversion efficiency of the solar cell. The simplest way to fabricate the electric field on the back side of the aluminum is to apply an aluminum film on the back side of the tantalum substrate and sinter the aluminum atoms into the tantalum wafer to form a high-alloy (Al-dopant) concentration P+ layer.

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 (ie, conductive paste) 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 μm, and the front metal has a finger line width of about 120 to 150 μm. The use of such technology to fabricate solar cell electrodes has the advantages of automation, high throughput, and low cost.

In summary, 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 paste. The composition, content, ratio, process parameters, etc. of the conductive paste affect the performance of the final electrode product. Taking the front metal electrode as an example, the quality of the conductive paste used to directly affect the important solar cell performance, such as conversion efficiency (Eff%), open circuit voltage Voc (open circuit voltage), short circuit current Isc (short Circuit current), fill factor (FF, fill factor), series resistance Rs (series resistance), and shunt resistance Rsh (shunt resistance), etc., also determine the effective sintering temperature range Ts and adhesion. Therefore, how to deploy a conductive paste that can improve the performance of the above solar cells is the focus of current research and development in the industry.

Generally, the conductive paste in the prior art is added to a lead-containing material such as lead oxide (PbO). The addition of lead oxide can achieve many excellent effects, such as reducing the sintering temperature of the conductive paste and reducing it. Contact resistance between the conductive paste and the semiconductor substrate. However, under the current environmental awareness, pollution-containing materials such as lead or cadmium, such as cadmium oxide (CdO), have been banned from being used in industrial products to avoid harm to humans. Therefore, the industry is actively looking for or developing conductive paste containing other elemental components in order to produce a solar cell with superior performance.

The object of the present invention is that the front electrode of the solar cell has good ohmic contact, high fill factor and improved conversion efficiency after sintering. The front electrode of the solar cell substrate may be composed of a conductive paste.

In one aspect, the present invention discloses a conductive paste for a front electrode of a solar cell, comprising the following components: silver powder; and a glass frit, wherein the glass frit comprises 21.5 to 90 weight percent lead oxide, 0.5 to 20.0 weight. a percentage of cerium oxide, 3 to 70.0% by weight of cerium oxide, 0.1 to 30.0 weight percent of zinc oxide, and 0.5 to 15.0 weight percent of aluminum oxide to improve the conversion efficiency of the solar cell; It is used to form the front electrode of the solar cell substrate.

The lead oxide is lead oxide (PbO), the bismuth oxide is cerium oxide (SiO ₂ ), the cerium oxide is cerium oxide (TeO ₂ ), the zinc oxide is zinc oxide (ZnO), and the aluminum oxide is three. Aluminum oxide (Al ₂ O ₃ ).

The above conductive paste further includes an organic carrier and an additive.

An advantage of the present invention is that the conductive paste provided by the present invention can be sintered at a low sintering temperature, thereby enabling the front electrode of the solar cell to achieve high conversion efficiency at a low sintering temperature, thereby achieving energy saving effect.

Another advantage of the present invention is that the front electrode of the solar cell completed by the conductive paste for solar cells provided by the present invention can have high conversion efficiency.

These and other advantages are apparent from the following description of the preferred embodiments and claims.

The invention will be described in detail herein with respect to specific embodiments of the invention and aspects thereof The description is made to explain the structure or the steps of the present invention, which are intended to be illustrative and not to limit the scope of the invention. Therefore, the present invention may be widely practiced in other different embodiments in addition to the specific embodiments and preferred embodiments of the specification.

The invention provides a conductive paste which can improve the conversion efficiency of a solar cell, and is used for forming a front electrode (ie, a light receiving surface) of a solar cell substrate. The conductive paste proposed by the present invention can be sintered at a low sintering temperature, so that the completed solar cell has high conversion efficiency.

To achieve the above object, the conductive paste component of the present invention generally comprises silver powder (Ag), a glass frit, an organic vehicle, and various additives. Silver is a very conductive medium. In the embodiment of the invention, the silver powder particles may be flake, spherical or a mixture of the two. The particle size of the silver powder will affect the sintering speed of the conductive paste. Generally speaking, the larger the silver particle is, the slower the sintering speed is, and the phenomenon that the sintering is incomplete is easy to cause the conductivity and strength of the electrode to decrease; and the particle is too small and will cause the particle to be too small. Part of the area is sintered too fast, causing cracks at the interface between silver and other components. In the embodiment of the invention, the particle size distribution of the silver powder is between 0.1 and 10.0 μm. In the examples, the present invention is not particularly limited in its proportion as long as the silver content of the conductive paste can achieve the object of the present invention. However, in the present embodiment, the proportion of silver powder is preferably about 60 to 90% by weight (% by weight) based on the total weight of the conductive paste.

In the case of a glass frit, in one embodiment, the composition contains components of lead oxide, cerium oxide, cerium oxide, zinc oxide, and aluminum oxide. In one embodiment, the lead oxide includes, but is not limited to, lead oxide (PbO). In one embodiment, the above cerium oxide includes, but is not limited to, cerium oxide (SiO ₂ ). In one embodiment, the above cerium oxide includes, but is not limited to, cerium oxide (TeO ₂ ). In another embodiment, the zinc oxide includes, but is not limited to, zinc oxide (ZnO). In one embodiment, the aluminum oxide described above includes, but is not limited to, aluminum oxide (Al ₂ O ₃ ). Here, it should be noted that, for those having ordinary knowledge in the art, oxides of different valences may be used for each of the above metal compounds or non-metal compounds, and therefore, the embodiments disclosed in the specification should not be used only. To limit the scope of the invention. In the examples, the present invention is not particularly limited in its proportion as long as the content of the glass frit can achieve the object of the present invention. However, in the present embodiment, the proportion of the glass frit is preferably about 0 to 10 wt% of the total conductive paste weight. In one embodiment, the lead oxide (eg, lead oxide) is preferably present in an amount of from about 40% to about 90% by weight based on the total weight of the glass frit. In one embodiment, the cerium oxide (e.g., cerium oxide) is preferably present in an amount of from 0.1 to 20% by weight based on the total weight of the glass frit. In one embodiment, the proportion of the above-mentioned niobium oxide (for example, ceria) is preferably from about 15 to 70% by weight (% by weight) based on the total weight of the glass frit. In one embodiment, the zinc oxide (e.g., zinc oxide) is preferably present in an amount of from 0.1 to 30% by weight based on the total weight of the glass frit. In one embodiment, the aluminum oxide (e.g., aluminum oxide) is preferably present in an amount of from 0.5 to 15% by weight based on the total weight of the glass frit.

In one embodiment, the composition of the glass frit may further contain one or more elements or oxides thereof, and the element may be selected from the group consisting of tin (Sn), phosphorus (P), and bismuth (Bi). , fluorine (F), lithium (Li), barium (Ba), magnesium (Mg), calcium (Ca), strontium (Sr), titanium (Ti), zirconium (Zr), vanadium (V), selenium (Se) , molybdenum (Mo), tungsten (W), manganese (Mn), nickel (Ni), silver (Ag), boron (B), europium (Er), germanium (Ge), gallium (Ga), germanium (Ce) , 钕 (Nd), 钐 (Sm), 镧 (La).

In the embodiment of the present invention, the organic vehicle is prepared by mixing an organic solvent and a resin, and the organic solvent is preferably a mixture of two or more solvents, such as an alcohol ether of diethylene glycol monobutyl ether (Butyl Carbitol). , DB), α-terpineol (alpha-Terpineol), Texanol film-forming agent, etc.; the resin is also preferably mixed with two or more different molecular weight cellulose, such as ethyl cellulose (EC) or a mixture thereof . In the examples, the present invention is not particularly limited in its proportion as long as the content of the organic vehicle can attain the object of the present invention. However, in the present embodiment, the proportion of the conductive paste is preferably from 10 to 30% by weight based on the total weight of the conductive paste.

In addition, the conductive paste of the present invention may also add a certain amount of additives according to various requirements, such as a dispersant, a levelling agent, a thixotropic agent, a stabilizer, a viscosity modifier ( A auxiliaries such as a viscosity adjuster and a surfactant are present in an amount of about 0 to 5 wt% of the entire conductive paste. Its main function is to adjust the viscosity of the colloid and have the function of wetting and uniform dispersion of the particles, and can also improve the tightness and conductivity of the metal particles after sintering, so adding two or more kinds of additives can achieve better effects.

In one embodiment, the conductive paste of the present invention may further comprise one or more additives, and the additive may be selected from the group consisting of zirconium oxide (ZrO ₂ ), vanadium pentoxide (V ₂ O _{5 )} ), silver oxide (Ag ₂ O), antimony trioxide (Cs ₂ O ₃ ), antimony trioxide (Er ₂ O ₃ ), tin oxide (SnO), magnesium oxide (MgO), antimony trioxide (Nd) ₂ O ₃ ), calcium oxide (CaO), titanium dioxide (TiO ₂ ), selenium dioxide (SeO ₂ ), chromium oxide (Cr ₂ O ₃ ), tungsten trioxide (WO ₃ ), antimony trioxide (Bi ₂ O ₃ ), manganese dioxide (MnO ₂ ), nickel oxide (NiO), antimony trioxide (Sm ₂ O ₃ ), germanium dioxide (GeO ₂ ), zinc fluoride (ZnF ₂ ), indium trioxide (In ₂ O ₃ ), gallium trioxide (Ga ₂ O ₃ ) and derivatives thereof.

Conversion efficiency is one of the most important indicators of solar cell performance, and it is related to how much of the received light energy can be converted into usable energy. The fill factor is the maximum power point (MPP) (ie, the maximum power that can be output) divided by the ratio of open circuit voltage (Voc) to short circuit current (Isc). The so-called open circuit voltage is the voltage that the solar cell is in when the load is infinite, that is, the voltage is measured when the external current is disconnected, and the output current at this time is zero. The short-circuit current is the output current of the solar cell under no-load condition, that is, when the external circuit is short-circuited, and the voltage is 0. In an ideal state, the short-circuit current of the solar cell is equal to the current generated when the light is illuminated. The values of the open circuit voltage and the short circuit current are related to the maximum power point of the solar cell. In general, when the open circuit voltage value and the short circuit current value that the solar cell can generate are larger, the power output that can be generated is larger. For a good solar cell, in addition to high open circuit voltage and short circuit current, there should be a fill factor as close as possible to 1, because the fill factor means the maximum output power is close to the Voc, Isc product, the closer 1 The closer its maximum output power is to the theoretically open circuit voltage and the short-circuit current.

Tables 1 and 2 illustrate examples of 12 sets of glass frits in which the glass component mainly contains lead oxide (PbO), cerium oxide (SiO ₂ ), cerium oxide (TeO ₂ ), zinc oxide (ZnO) and three. Alaluminum oxide (Al ₂ O ₃ ), and one or two of adding underlying additives such as magnesium oxide (MgO), antimony trioxide (Bi ₂ O ₃ ), zirconium oxide (ZrO ₂ ) or selenium dioxide (SeO ₂ ) One. Tables 1 and 2 also show the series resistance (Rs), fill factor (FF), and conversion efficiency (Eff%) after sintering of the conductive paste containing the above glass frit. It can be seen from Tables 1 and 2 above that the series resistance (Rs) of the PbO-SiO ₂ -TeO ₂ -ZnO-Al ₂ O ₃ glass frit in the above 12 sets of examples after sintering of the conductive paste is 0.0028 to 0.0057. The fill factor (FF) is 75.97~79.08, and the conversion efficiency (Eff%) is 16.36~17.51. It can be seen from the test results recorded in Table 1 and Table 2 that the conductive paste composed of the above PbO-SiO ₂ -TeO ₂ -ZnO-Al ₂ O ₃ glass frit causes the front electrode of the solar cell to have good ohms after sintering. Contact, high fill factor and improved conversion efficiency.

The preparation method of the conductive paste will be described in the following examples. It should be noted that the steps of the configuration method, the component ratio, and the experimental parameters are only for explaining the embodiments of the present invention, and are not intended to limit the claims of the present invention. First, an organic carrier of a conductive paste is prepared by dissolving 5 to 25 grams of ethyl cellulose (EC) in an organic solvent of 5 to 75 grams by weight, and the organic solvent used in this embodiment. It is diethylene glycol butyl ether, but it is not limited to the use of such an organic solvent. In other examples, other alcohol ether solvents or organic solvents of soluble cellulose resins may be used. In this embodiment, the boiling point of the solvent is between about 120 and 300 °C. A small amount of additives may be added to the organic vehicle. These additives may be functional additives such as viscosity modifiers, dispersing aids, thixotropic agents, and wetting aids. Next, take 70~95g of common commercial silver powder and mix it with 10~30g organic carrier and 1~10g glass frit, and use the Three-roll mill to help mix and disperse evenly. The paste or paste obtained after mixing is a conductive paste.

Next, a conductive paste is applied to the front surface of the solar cell substrate using a screen printing technique, and the surface of the substrate is subjected to a tantalum nitride antireflection treatment in advance. The aluminum paste used to form the back electrode is GSMC A136 aluminum paste. For conductive pastes with different functions, the drying temperature and time after screen printing will vary with the organic solvent used in the organic carrier and the printing weight. The drying temperature is between 100 and 250 ° C, and dried. The time is about 5~30 minutes. After the screen printing operation of the front and back conductive paste is completed, the conductive paste is sintered by using an infrared conveyor belt sintering furnace, and the sintering temperature can be adjusted between 850 and 880 ° C. The sintered conductive paste is formed on the front and back sides of the substrate. Solid state electrode. After the electrode fabrication is completed, the solar cell is electrically tested using a solar tester. The electrical characteristics of the solar cell were measured under the solar state of AM 1.5G.

In addition, the various modifications that can be made by the embodiments and the embodiments described in the present invention are intended to be included within the scope of the present invention. Therefore, the drawings and the examples are intended to be illustrative and not to limit the invention, and the scope of the invention is intended to be limited only by the scope of the claims.

Claims (9)

A conductive paste for a front electrode of a solar cell, comprising: silver powder; a glass frit, wherein the glass frit comprises 21.5 to 90% by weight of lead oxide, 0.5 to 20.0% by weight of cerium oxide, and 3 to 70.0% by weight Thereafter, the oxide, 0.1 to 30.0% by weight of zinc oxide, and 0.5 to 15.0% by weight of aluminum oxide to enhance the conversion efficiency of the solar cell; and an organic vehicle. The conductive paste for a front electrode of a solar cell according to claim 1, wherein the glass frit further comprises one or more of the following elements or oxides thereof, including tin (Sn), phosphorus (P), bismuth (Bi), Fluorine (F), lithium (Li), barium (Ba), magnesium (Mg), calcium (Ca), strontium (Sr), titanium (Ti), zirconium (Zr), vanadium (V), selenium (Se), Molybdenum (Mo), tungsten (W), manganese (Mn), nickel (Ni), silver (Ag), boron (B), europium (Er), germanium (Ge), gallium (Ga), germanium (Ce),钕 (Nd), 钐 (Sm), 镧 (La). The conductive paste for a front electrode of a solar cell according to claim 1 or 2, wherein the lead oxide is lead oxide (PbO), and the cerium oxide is cerium oxide (SiO ₂ ). The conductive paste for a front electrode of a solar cell according to claim 1 or 2, wherein the cerium oxide is cerium oxide (TeO ₂ ), the zinc oxide is zinc oxide (ZnO), and the aluminum oxide is three Aluminum oxide (Al ₂ O ₃ ). The conductive paste for a front electrode of a solar cell according to claim 1 or 2, wherein the additive is selected from the group consisting of zirconia (ZrO ₂ ), vanadium pentoxide (V ₂ O ₅ ) , silver oxide (Ag ₂ O), antimony trioxide (Cs ₂ O ₃ ), antimony trioxide (Er ₂ O ₃ ), tin oxide (SnO), magnesium oxide (MgO), antimony trioxide (Nd _{2 )} O ₃ ), calcium oxide (CaO), titanium dioxide (TiO ₂ ), selenium dioxide (SeO ₂ ), chromium oxide (Cr ₂ O ₃ ), tungsten trioxide (WO ₃ ), antimony trioxide (Bi _{2 )} O ₃ ), manganese dioxide (MnO ₂ ), nickel oxide (NiO), antimony trioxide (Sm ₂ O ₃ ), germanium dioxide (GeO ₂ ), zinc fluoride (ZnF ₂ ), indium trioxide ( In ₂ O ₃ ), gallium trioxide (Ga ₂ O ₃ ) and derivatives thereof. A method of preparing a solar cell electrode, comprising: providing a solar cell substrate having a front surface and a back surface opposite to the front surface; forming an anti-reflection film on the front surface; forming a conductive paste on the anti-reflection film The conductive paste comprises: silver powder; a glass frit, wherein the glass frit comprises 21.5 to 90% by weight of lead oxide, 0.5 to 20.0% by weight of cerium oxide, and 3 to 70.0% by weight of cerium oxide. And 0.1 to 30.0% by weight of zinc oxide and 0.5 to 15.0% by weight of aluminum oxide; and an organic vehicle; and sintering the conductive paste. The method of preparing a solar cell electrode according to claim 6, wherein the temperature of the conductive paste is 850-880 degrees. A method of preparing a solar cell electrode according to claim 6, wherein the antireflection film is a tantalum nitride film. The method for preparing a solar cell electrode according to claim 6, further comprising drying the conductive paste, and the drying temperature is between 100 and 250 degrees.