TW201428771A - Electro conductive paste for solar cell which can enhance adhesion strength, conversion efficiency and open-circuit voltage - Google Patents

Abstract

The present invention discloses an electro conductive paste for solar cell, including the following composition: aluminum powder; glass frit; and boron, the boron is about 0.01-1 weight percent of the electro conductive paste for the solar cell to enhance adhesion strength, conversion efficiency and open-circuit voltage for the solar cell; wherein the electro conductive paste for the solar cell is utilized to form the back electrode of the substrate for the solar cell.

Description

Conductive adhesive for solar cells that can increase tensile force, conversion efficiency and open circuit voltage

The present invention relates to a conductive paste, and more particularly to a conductive paste for a solar cell which can increase the tensile force, conversion efficiency and open circuit voltage 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 or GaP. A base material of a group III or group V compound such as InP or AlGaAs. In the case of a ruthenium substrate, it is common practice for a light-receiving region on the front side of a P-type substrate to be doped with a phosphorus atom to form a negative electrode region, and a region on the back side where no light is received is corresponding. The positive electrode end. The PN junction formed in the semiconductor substrate converts incident light energy of a specific wavelength (λ) into an electron hole pair (e ^- -h ⁺ pair) to move in opposite directions of the two electrodes to generate a current. This current is the source of electrical energy for the solar cell. In general, the light-receiving surface of the ruthenium substrate is also coated with an anti-reflection film (ARC, such as tantalum nitride SiN _x film) to avoid energy loss caused by light reflection to increase the conversion efficiency of the solar cell (Eff%). (efficiency). In addition to the semiconductor substrate of the above solar cell, the industry generally forms an aluminum back surface field (BSF) on the back surface of the germanium substrate after the PN interface is completed. The electric field on the back side of the aluminum can reduce the probability of a few carriers (and electrons e ^- , holes h ⁺ ) 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 aluminum doping concentration (Al-dopant) 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 an external current load. 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 general metal electrodes mainly use screen printing to print a mesh electrode structure on both front and back sides of the solar cell. 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.

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, the general practice in the industry is to first print a silver electrode pattern on the back side of the base material by screen printing, and then form an aluminum electrode layer thereon. Since the solderability of aluminum is poor and the solar cell modules cannot be joined by direct soldering, a general manufacturer uses a soldering ribbon to solder the silver electrode region on the back portion of the solar cell. Each power generation module is electrically connected to each other. The silver electrode-germanium substrate interface (ie, the fire through layer) and the aluminum electrode-germanium substrate interface (ie, the BSF layer) form a eutectic layer during the sintering process to make it tightly bonded, but the silver and aluminum are not easily formed. Eutectic structure Peeling tends to occur at the surface, causing cracks 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 ribbon and a peel test of the silver electrode and the aluminum electrode interface 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 paste. The conductive adhesives in the prior art are all composed of metal powder (especially silver powder), glass frit, organic vehicle, and additive. 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 peeling degree of the silver-aluminum electrode interface, the advantages and disadvantages of the conductive silver paste and the aluminum adhesive formed by the conductive strip directly affect the conversion efficiency of the solar cell performance η, open circuit Voltage V _oc (open circuit voltage), short circuit current I _sc (short circuit current), fill factor FF (fill factor), series resistance R _s (series resistance), and shunt resistance R _sh (shunt resistance), etc. The effective sintering temperature range T _s and the strength of the adhesion strength. Therefore, how to deploy a conductive adhesive that can improve the performance of the above solar cells is the focus of current research and development in the industry.

In one aspect, the present invention discloses a conductive paste for a solar cell comprising the following components: aluminum powder; glass frit; and boron element, wherein the boron element accounts for about 0.01 to 1 weight percent of the conductive paste for the solar cell to enhance solar energy. The tensile force, the conversion efficiency and the open circuit voltage of the battery; wherein the conductive paste for the solar cell is used to form the back electrode of the solar cell substrate.

One of the advantages of the present invention is that the welding tension of the conductive aluminum rubber can be improved, and the reliability between the welding electrode and the back aluminum electrode, between the back aluminum electrode and the ruthenium substrate, and between the back aluminum electrode and the back silver electrode can be improved. (or stability).

Another advantage of the present invention is that the conductive paste for a solar cell provided by the present invention can increase the open circuit voltage (V _oc ) of the entire solar cell.

Another advantage of the present invention is that the conductive paste for a solar cell provided by the present invention can improve the conversion efficiency of the overall solar cell.

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

The invention is described in detail herein with reference to the particular embodiments of the invention, and the description 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 aluminum glue which can be applied to the back electrode of a solar cell. The invention mainly aims to improve the welding tension (or adhesion) of the conductive aluminum glue, so that the application of the conductive aluminum glue can have good reliability when soldering and bonding other metal materials. In particular, when the back electrodes of the solar cell modules in the solar cell are connected in series or in parallel through the solder strip, the reliability between the solder strip and the back electrode and between the back electrode and the germanium substrate can be improved (or stability). In addition, the conductive aluminum paste of the present invention can increase the open circuit voltage (V _oc ) and conversion efficiency of the overall solar cell.

In one embodiment of the present invention, the conductive aluminum paste provided by the present invention generally comprises aluminum powder (Al), glass frit, boron element, organic vehicle, and various additives. . In the case of glass frits, in one embodiment, the glass frit may comprise cerium oxide, boron oxide, and cerium oxide. In one embodiment, the above cerium oxide includes, but is not limited to, antimony trioxide (Bi ₂ O ₃ ). In one embodiment, the boron oxides described above include, but are not limited to, boron oxide (B ₂ O ₃ ). In one embodiment, the above cerium oxide includes, but is not limited to, cerium oxide (SiO ₂ ). 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 weight of the conductive paste.

In one embodiment, the glass frit may further optionally contain one or more elements selected from the group consisting of zinc (Zn), aluminum (Al), tellurium (Te), and indium (In ), lithium (Li), antimony (Sb), barium (Ba), magnesium (Mg), calcium (Ca), silver (Ag), antimony (Sr), selenium (Se), antimony (Cs), titanium (Ti ), vanadium (V), molybdenum (Mo), tungsten (W), manganese (Mn), nickel (Ni), germanium (Ge), arsenic (As), and lanthanum (La).

In another embodiment, the glass frit may further optionally comprise one or more additions The above additives may be selected from the group consisting of zinc oxide, aluminum oxide, cerium oxide, indium oxide, lithium oxide, cerium oxide, cerium oxide, magnesium oxide, calcium oxide. , silver oxide, antimony oxide, selenium oxide, antimony oxide, titanium oxide, vanadium oxide, molybdenum oxide, tungsten oxide, manganese oxide, nickel oxide, antimony oxide, arsenic oxide, antimony Oxides and their derivatives.

In one 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 ). In one embodiment, the above cerium oxide includes, but is not limited to, cerium oxide (TeO ₂ ). In one embodiment, the indium oxide described above includes, but is not limited to, indium trioxide (In ₂ O ₃ ). In one embodiment, the lithium oxide includes, but is not limited to, lithium dioxide (Li ₂ O ₃ ). In one embodiment, the above cerium oxide includes, but is not limited to, antimony trioxide (Sb ₂ O ₃ ). In one embodiment, the above cerium oxide includes, but is not limited to, cerium oxide (BaO). In one embodiment, the magnesium oxide described above includes, but is not limited to, magnesium oxide (MgO). In one embodiment, the calcium oxide described above includes, but is not limited to, calcium oxide (CaO). In one embodiment, the silver oxide includes, but is not limited to, silver oxide (Ag ₂ O). In one embodiment, the above cerium oxide includes, but is not limited to, cerium oxide (SrO). In one embodiment, the selenium oxide described above includes, but is not limited to, selenium dioxide (SeO ₂ ). In one embodiment, the above cerium oxide includes, but is not limited to, antimony trioxide (Cs ₂ O ₃ ). In one embodiment, the titanium oxide includes, but is not limited to, titanium dioxide (TiO ₂ ). In one embodiment, the vanadium oxide described above includes, but is not limited to, vanadium pentoxide (V ₂ O ₅ ). In one embodiment, the molybdenum oxide includes, but is not limited to, molybdenum trioxide (MoO ₃ ). In one embodiment, the tungsten oxide described above includes, but is not limited to, tungsten trioxide (WO ₃ ). In one embodiment, the manganese oxide includes, but is not limited to, manganese dioxide (MnO ₂ ). In one embodiment, the nickel oxide described above includes, but is not limited to, nickel oxide (NiO). In one embodiment, the above cerium oxide includes, but is not limited to, germanium dioxide (GeO ₂ ). In one embodiment, the arsenic oxide includes, but is not limited to, arsenic trioxide (As ₂ O ₃ ). In one embodiment, the above cerium oxide includes, but is not limited to, antimony trioxide (La ₂ O ₃ ). Here, it should be noted that, for those having ordinary knowledge in the art, different valence oxides or non-oxides may be used for each of the above metal compounds or non-metal compounds, and therefore should not be used only in the specification. The embodiments are disclosed to define the scope of the invention.

In one embodiment, the boron element may be in the form of a powder, that is, boron powder. In one embodiment, the boron powder may have a particle size of from about 0.1 to 10 micrometers (μm) (D ₅₀ ). In one embodiment, the boron powder may be present in an amount of from about 0.01 to about 1 weight percent (wt%) of the total conductive paste. D ₅₀ represents the median particle diameter, i.e., the particle size is greater than 50% of the particles, and the particles smaller than it also account for 50%.

In an embodiment of the present invention, the organic vehicle may be formulated with an organic solvent and a resin. to make. In one embodiment, the organic solvent may be prepared by mixing two or more kinds of solvents, for example, an alcohol ether such as Butyl Carbitol (DB), α-terpineol (Al-Terpineol), and Texanol. A film or the like; and the resin may preferably be mixed with two or more kinds of celluloses having different molecular weights, such as ethyl cellulose (EC), wood rosin, polyacrylonitrile or a mixture thereof. However, it should be noted that, for those having ordinary knowledge in the art, other organic solvents having similar characteristics and resins may be applied to process the organic carrier according to actual use requirements, and therefore, should not be specifically determined by the above enumeration. Materials are intended to define 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 organic vehicle can attain the object of the present invention. However, in the present embodiment, the proportion of the conductive aluminum rubber is preferably 10 to 30% by weight.

In addition, the conductive aluminum paste provided by the present invention may also add a certain amount of additives according to various needs, such as: dispersant, leveling agent, thixotropic agent, stabilizer ( Auxiliary agents such as stabilizers, viscosity modifiers, and surfactants are not limited thereto, and the content thereof is preferably between 0 and 5 wt% of the total of the conductive aluminum paste. The above additives are added after the organic carrier is formed, and the main function is to adjust the viscosity of the colloid and have the function of uniformly wetting and dispersing the particles, and also improve the tightness and conductivity of the metal particles after sintering, so two or more kinds are added. The additive can achieve better results.

Here, it should be noted that the ratio of the organic carrier and the additive contained in the conductive aluminum paste provided by the present invention can be adjusted according to actual use requirements, and should not be limited.

In order to make the conductive aluminum glue provided by the present invention clearer and clearer. The present invention provides eight sets of examples. The following eight sets of examples have different levels of boron elements, and each set of conductive aluminum adhesives is measured for open circuit voltage (V _oc ), conversion efficiency, and tensile force (see Table 1 below). Since the conductive aluminum paste is subjected to sintering when it is joined to other metal materials, the sintering operation is also performed before the tensile measurement of each embodiment in the present invention.

Conversion efficiency (Eff%) 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 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 open circuit voltage (V _oc ) value is related to the maximum power point (MPP) of the solar cell. In general, the greater the open circuit voltage value (V _oc ) that a solar cell can produce, the greater the power output it can produce. For a good solar cell, a high open circuit voltage (V _oc ) is required.

Referring to Table 1, it can be seen that conductive aluminum paste with boron added (for example, experimental groups 1 to 8) generally has a large tensile force (about 7.23 Newtons to 10.46 Newtons) compared with the control group without added boron. When applied to other metals as described above, it is possible to maintain good reliability with other metals by such high tensile force, that is, to have high solderability. Referring to Table 1, it can be seen that conductive aluminum paste with boron added (such as experimental groups 1 to 6 and 8) generally has a higher open circuit voltage (about 0.6178 volts to 0.6213 volts) than a control group without added boron. between). In addition, conductive aluminum pastes with boron added (for example, experimental groups 2 to 4, 6 and 8) generally have a higher conversion efficiency (approximately 16.025% to 16.173%) relative to a control group without added boron. ).

The method of formulating the conductive paste will be described in the following examples. It should be noted that the steps of the configuration method, the component ratios, and the experimental parameters are merely illustrative of the embodiments of the present invention, and are not intended to limit the claims of the present invention.

Formulation procedure

First, a certain proportion of aluminum powder, glass frit, and boron powder are first added to an organic vehicle, and premixed by a mixer. The organic carrier may be a mixture of ethyl cellulose resin (EC, ethyl cellulose) and DB, butyl carbitol, but is not limited to use only Organic solvent. In other embodiments, other alcohol ether solvents or organic solvents of soluble cellulose resins such as wood rosin, polyacrylonitrile, terpineol, and the like may also be used. A certain proportion of additives or additives may also be added to the organic carrier. These additives may be functional additives such as viscosity modifiers, dispersing assistants, thixotropic additives, and wetting assistants. In this embodiment, the glass frit may contain a plurality of oxide glass components, the composition and ratio of which are determined by the experimenter. After the premixing is completed, the mixture containing aluminum powder, glass frit, boron powder, organic carrier and additives will be dispersed and evenly mixed using a 3-roll mill, and the paste or paste obtained after mixing will be obtained. It is a conductive aluminum glue.

Printing procedure

After the conductive aluminum glue is mixed, the conductive silver paste is then printed on the back side of the base material by screen printing, which is the silver electrode portion on the back side of the base material. After the back silver paste is screen printed, the front silver electrode and the back aluminum electrode are printed on the front and back sides of the substrate. In the embodiment of the invention, the front silver electrode is made of DuPont PV-145 silver glue, and the back aluminum electrode is made of the conductive aluminum glue of the invention. After the electrode is printed, the entire substrate is dried in an oven. For conductive pastes of different properties, the drying temperature and time required after screen printing will vary depending on the organic solvent used for the organic vehicle and the printing weight. In the present embodiment, the drying temperature is set at 150 to 200 ° C, and the drying time is 5 to 15 minutes.

Sintering procedure

After the drying step is completed, the entire tantalum substrate is placed in an infrared conveyor belt sintering furnace to perform a sintering process on the conductive paste. In this embodiment, the peak temperature of the sintering can be adjusted between 600 and 800 ° C. The sintered conductive paste forms a solid electrode on the front and back sides of the substrate and bonds with the tantalum substrate.

Measuring procedure Tensile test (adhesion test):

After the electrode is fabricated, a tin bar (in the present invention, the composition of the tin-lead alloy is coated on the outer surface of the copper sheet, the cross-sectional width is about 1.8 to 2 mm, and the ratio of tin to lead is 60:40) is soldered to the base by a soldering iron. On the aluminum electrode on the back of the material, the soldering iron tip adopts the shape of the oblique knife edge, and the welding temperature is 320~350 °C, and the moving speed is 1~5 cm/s. After the welding is completed, the tensile strength of the back aluminum electrode is measured by using a tensile machine to set a test angle of 180° and a speed of 120 to 360 mm/s.

Peeling test:

Using 3M 600 Scotch scotch tape as the test strip, the tape was adhered in a forward direction to the surface area containing the back silver electrode and the back aluminum electrode, after which the tape was torn at a 90° angle. After tearing up, observe the peeling condition. If the aluminum layer overlapping the back silver electrode and the back silver electrode is peeled off and remains on the tape, that is, the peeling test is not passed; otherwise, the test is passed.

Conversion efficiency Eff (%) test:

The solar stencil produced was electrically tested using a solar tester under the conditions of a light source illumination intensity of 1.5 1.5 G. The solar test machine model used was Berger's PSL-SCD.

While the preferred embodiment of the invention has been described, it will be understood by those of ordinary skill in the art On the other hand, those skilled in the art can make a number of changes and refinements within the spirit and scope of the invention as defined by the following claims.

Claims (10)

A conductive adhesive for solar cells, comprising the following components: aluminum powder; glass frit; and boron element, which accounts for 0.01-1% by weight of the conductive adhesive for the solar cell, so as to improve the pulling force, conversion efficiency and open circuit of the solar cell a voltage; wherein the solar cell uses a conductive paste to form a back electrode of the solar cell substrate. The conductive paste for a solar cell according to claim 1, wherein the boron element is boron powder. The conductive paste for a solar cell according to claim 2, wherein the boron powder has a median diameter of about 0.1 to 10 μm. The conductive paste for a solar cell according to claim 1, wherein the glass frit comprises cerium oxide. The conductive paste for a solar cell according to claim 4, wherein the glass frit further comprises boron oxide. The conductive paste for a solar cell according to claim 5, wherein the glass frit further comprises a cerium oxide. The conductive paste for a solar cell according to claim 6, wherein the cerium oxide comprises bismuth trioxide (Bi ₂ O ₃ ), the boron oxide comprises boron oxide (B ₂ O ₃ ), and the cerium oxide comprises two Cerium oxide (SiO ₂ ). The conductive paste for a solar cell according to claim 6, wherein the glass frit further comprises one or more elements selected from the group consisting of zinc (Zn), aluminum (Al), and antimony ( Te), indium (In), lithium (Li), antimony (Sb), barium (Ba), magnesium (Mg), calcium (Ca), silver (Ag), antimony (Sr), selenium (Se), antimony ( Cs), titanium (Ti), vanadium (V), molybdenum (Mo), tungsten (W), manganese (Mn), nickel (Ni), germanium (Ge), arsenic (As), and lanthanum (La). The conductive paste for a solar cell according to claim 6, wherein the glass frit further comprises one or more additives selected from the group consisting of zinc oxide, aluminum oxide, and antimony oxide. , indium oxide, lithium oxide, cerium oxide, cerium oxide, magnesium oxide, calcium oxide, silver oxide, cerium oxide, selenium oxide, cerium oxide, titanium oxide, vanadium oxide, Molybdenum oxide, tungsten oxide, manganese oxide, nickel oxide, niobium oxide, arsenic oxide, niobium oxide and its derivatives Things. The conductive paste for a solar cell according to claim 1, further comprising the following components: an organic vehicle; and an additive.