TWI738996B - Aluminum paste for partial back field solar cells and partial back field solar cells using the aluminum paste - Google Patents

Abstract

The present invention provides an aluminum paste for partial back field solar cells, which contains: large aluminum powder; an organic carrier, which includes a solvent and resin or cellulose; wherein the median size of the large aluminum powder is The ratio of (μm) to oxygen content (%) (median particle size (μm)/oxygen content (%)) is 10-15. The aluminum paste for partial back field solar cells and the partial back field solar cell using the aluminum paste of the present invention can reduce _{the adhesion of aluminum powder, aluminum beads, aluminum layer to SiN x} protective layer and LBSF position The occurrence of problems such as pores, which in turn improves the photoelectric conversion efficiency of the local back-field solar cell.

Description

Aluminum paste for partial back field solar cells and partial back field solar cells using the aluminum paste

The present invention relates to an aluminum paste, in particular to an aluminum paste containing large aluminum powder with a specific ratio of particle size to oxygen content. The present invention also relates to a partial back field solar cell using the aluminum paste.

In order to improve the best efficiency performance, solar cell factories have gradually introduced local back surface field (LBSF) technology since 2013. The operation method of this technology is Passivated Emitter and Rear Contact (PERC). The method is to use the atomic layer deposition (ALD, Atomic Layer Deposition) method or the chemical vapor deposition (CVD, Chemical Vapor Deposition) method to _{deposit SiO x} , TiO _x , AlO _x on the silicon wafer cell as the back passivation layer, and then use The CVD process _{deposits SiN x to} form a capping layer. The main function of the passivation layer in the partial back field solar cell is to repair the defects on the surface of the silicon wafer. This is because during the cutting and processing of silicon wafers, amorphous silicon is produced, and amorphous silicon has more dangling bonds. These dangling bonds located on the edge of the silicon chip will neutralize the carriers generated after the silicon chip is exposed to light, thereby reducing carrier life time and reducing electrical properties. The partial back field solar cell will increase the open circuit voltage (Voc) and short circuit current (Isc) due to the function of the passivation layer, and can significantly increase the photoelectric conversion efficiency. However, because the passivation layer only provides limited aluminum-silicon contacts, the series resistance (Rs) is higher and the fill factor (FF) is lowered.

The development of PERC aluminum paste is derived from silicon-based solar cells with a back passivation layer. Compared with conventional or traditional solar cells in the past, the main difference between partial back-field solar cells is that the aluminum paste used in traditional silicon-based solar cells is printed on the back of the silicon wafer by screen printing, and the aluminum layer is directly in contact with the silicon wafer and sintered. After that, a full backside field (BSF) is formed; however, in the partial backside field solar cell, most of the aluminum layer (>95%) is covered on _{the protective layer of SiN x} , leaving only a limited laser opening area for The aluminum can be in direct contact with the silicon, and after sintering, a local back surface field (LBSF) is formed.

The LBSF technology faces a major technical problem: the aluminum layer will _{adhere to the protective layer of SiN x} , thereby causing damage to the passivation layer. If the passivation layer is damaged, the local back field solar cell becomes unable to maintain high open circuit voltage (Voc), short circuit current (Isc), electrical properties and conversion efficiency. Secondly, because the contact between the aluminum layer and the silicon wafer is only through the limited area of the laser opening, under the action of an appropriate amount of glass powder (1.0~5.0% by weight, accounting for the total weight of the aluminum paste), in the aluminum paste formulation, the aluminum paste The particle size of the powder itself and the thickness of the aluminum oxide layer on the surface of the aluminum powder will affect whether the aluminum-silicon co-firing process will damage the quality of the local back-field solar cell. Powder issue and aluminum beads (bead) And the key to the void problem occurs at the location of the local backside field (LBSF).

However, as far as the current silicon-based solar cells with back passivation layer are concerned, they focus on the application of glass powder in the aluminum paste formulation, and then control the effect of aluminum layer on the back passivation layer (SiO _x ,TiO _x ,or AlO _x /SiN _x ) erosion and adhesion ability. It can be seen from this that the conventional technology ignores the influence of aluminum powder in the aluminum paste with a content of more than 60% by weight on the overall quality characteristics and reliability of the partial back field solar cell during the sintering process, and does not address the above technical problems. Teaching or suggestion. For example, patent document CN103545013A discloses the use of an aluminum paste, which contains 60 to 87% by weight of aluminum powder and glass powder with high lead content, and uses the characteristics of lead oxide (PbO) to melt and react to strengthen the aluminum The adhesion between the paste and the top oxide layer; however, in the patent document CN103545013A, there is no discussion on the powder, aluminum beads, and aluminum layer produced during the sintering process of aluminum powder with a content of more than 60% by weight in aluminum paste. The adhesion to the SiN _x protective layer and the occurrence of voids in the local back field position affect the overall quality characteristics and reliability of the local back field solar cell.

Therefore, how to develop an aluminum paste that can be used in LBSF and reduce the occurrence of phenomena such as aluminum powder extraction, aluminum beads, aluminum layer _{adhesion to the SiN x} protective layer, and voids in the LBSF position, which affect the local back-field solar energy The issue of battery photoelectric conversion efficiency is a technical focus that all LBSF developers are looking forward to.

In order to solve the above-mentioned shortcomings of the prior art, the present invention provides an aluminum paste for a local back field solar cell and a local back field solar cell using the aluminum paste.

In more detail, the present inventors completed the present invention based on the following theory.

(Powder out)

Because the powder produced by the sintering process of the aluminum paste in the local back field solar cell is mainly related to the melting rate of the aluminum powder, theoretically, in traditional or conventional solar cells, because the general P-type silicon-based solar cell or local The maximum sintering temperature of the metal paste (front silver paste, back silver paste, back aluminum paste) of the back field solar cell after printing and drying is 720~820℃, and the aluminum layer is in full contact with the silicon wafer, so the aluminum powder is in contact with the silicon wafer. The silicon wafer starts to react when the minimum temperature of the aluminum-silicon eutectic is above 577℃ (Eutectic point). Or, in traditional or conventional solar cells, because the melting point of pure aluminum is 660.32℃, so the aluminum powder will break the aluminum oxide shell layer in the aluminum powder after it reaches the melting point, so the aluminum powder will flow out and have the opportunity to form an aluminum-silicon alloy with the silicon wafer.

In contrast, in the silicon wafer of the local backside field, because there is a passivation layer, although the aluminum layer in the aluminum powder has the opportunity to contact with the silicon due to the local laser opening, the aluminum and silicon can react and eutectic The ratio is much lower than that of traditional or conventional silicon-based solar cells. Furthermore, once aluminum flows between the porous alumina shell layers after melting, the molten aluminum that fails to react and eutectic with silicon in time may flow to the surface of the aluminum layer due to the high thermal expansion coefficient. If the molten aluminum is cooled again, irregular small particles of powder will be formed on the surface of the aluminum layer, and this phenomenon is the emergence of aluminum powder. The generation of this kind of powder will affect the bonding strength of EVA (Ethylene Vinyl Acetate) lamination when forming the module, thereby deteriorating the reliability and reducing the service life of the solar cell module.

(Aluminum beads)

If an electron microscope with energy dispersive X-ray spectroscopy (Scanning electron microscopy with energy dispersive X-ray spectroscopy, SEM/EDX) is used to analyze the composition of aluminum beads, it can be known that the composition of aluminum beads is mainly aluminum and the content of silicon At 5.0~30.0% by weight. Therefore, if we explore the cause of aluminum beads (beads), it is mainly because the existence of the back passivation layer of the local back field solar cell hinders the aluminum-silicon eutectic. Although it is known that the diffusion rate of silicon in the aluminum layer is very fast, in order to prevent the sintering temperature peak of the local back field solar cell from causing greater damage to the passivation layer, the above-mentioned sintering temperature peak is lower than that of traditional or conventional solar cells. 20~40℃, so the distribution of the aluminum-silicon alloy during the sintering process will not be very uniform. Furthermore, during the cooling process, the silicon in the aluminum-silicon alloy reflows to the silicon wafer as the temperature decreases, causing the distribution of the aluminum-silicon alloy and pure aluminum in the aluminum layer to be more uneven. Therefore, in the case of a significant difference in the thermal expansion coefficient, during the cooling process, the shrinkage rate of the aluminum-silicon alloy with a smaller thermal expansion coefficient With a slower rate, pure aluminum will be affected by the rapid shrinkage rate during cooling, and will be discharged to the film-forming surface and form aluminum beads.

Aiming at the problem that aluminum beads are easily generated during the aluminum paste sintering process in the local back field, the key is to effectively control the timing and reaction rate of the reaction between aluminum and silicon. Because the aluminum powder participates in the aluminum-silicon eutectic reaction, in addition to the pre-melting of the glass powder during the sintering process, the aluminum oxide on the surface of the aluminum powder is corroded, so that the pure aluminum can flow out of the aluminum oxide shell and react with the silicon after melting. In addition, the particle size of the aluminum powder itself and the thickness of the aluminum oxide shell have become the main factors affecting the timing and speed of the aluminum-silicon reaction. Aluminum powder particles are small, because of the large specific surface area, under the same heating and heating conditions, the melting rate of small particle size aluminum powder (D50: 1.0~3.0μm) will be much faster than that of large particle size (D50: 6.0~9.0μm) aluminum powder. Much faster, the chance of producing aluminum beads in the sintering process will be significantly higher.

In addition, in addition to the particle size that affects the melting rate of the aluminum powder, the thickness of the aluminum oxide shell on the surface of the aluminum powder is also the main factor that affects the flow rate of the molten aluminum. The thickness of the aluminum oxide shell on the surface of the aluminum powder is positively related to the oxygen content in the aluminum powder. The thicker the alumina shell layer, the stronger the ability to withstand the corrosive effect of glass powder, which can delay the flow of molten aluminum; however, excessive oxidation (excessive oxygen content) means that the aluminum paste The higher the alumina content, in addition to the impact on the conductivity, it also has a negative impact on the structural strength of the entire aluminum layer, which will weaken the adhesive strength after EVA lamination. Although it can be improved by increasing the amount of glass powder added, after all, glass powder is not a conductive material. If more glass powder is used to improve the structural strength of the aluminum layer, it must be affected by the increase in resistance. Electrical problems. Therefore, the appropriate size of the aluminum powder particle size and the appropriate thickness of the aluminum oxide shell on the surface of the aluminum powder (the oxygen content in the aluminum powder) can effectively inhibit the generation of aluminum beads while maintaining ideal electrical conductivity. The key factor.

(Empty hole)

In order to allow the aluminum paste to directly contact the silicon wafer, and provide a space for the aluminum-silicon to produce a good ohmic contact and facilitate carrier transfer after sintering, the conventional method is to passivate the silicon with a laser opening on the back. The back passivation layer of the P-type solar cell produces patterns with different characteristics. Since the thickness of the back passivation layer is only 80~150nm, and the action of the appropriate power laser can ablate the thickness of the back passivation layer 1.0~3.0μm, so after the laser action, the pure silicon of the silicon wafer can be used in the laser The position of the opening is completely exposed, and there is a chance to contact the aluminum layer. In the aluminum-silicon eutectic process of the sintering process, because the diffusion rate of silicon in the aluminum layer is extremely fast, the silicon at the position of the laser opening will quickly eutectic with aluminum and be dispersed into the aluminum-silicon eutectic alloy solution middle. Although the silicon still flows toward the surface of the silicon wafer during the cooling process, once the molten aluminum is produced at a high speed and the aluminum-silicon eutectic continues to work, the silicon that has spread farther will not be able to return to the laser hole during the cooling process. The position, in turn, makes the silicon at the original laser opening position resemble a hollowed out form, that is, the so-called void is generated. If it is possible to control the size of the aluminum powder particles and the thickness of the aluminum oxide shell on the surface of the aluminum powder at the beginning of the application of aluminum paste, so that the aluminum-silicon eutectic will not be excessively affected, that is, it can prevent the excessive diffusion of silicon and it will not be too late to return during cooling. The hole problem occurs due to the position of the laser hole.

Therefore, after repeated research and discussion, the inventors found that the relationship between the particle size of the aluminum powder and the thickness of the aluminum oxide layer on the surface of the aluminum powder is whether there will be powder, aluminum beads, and local backside field positions. The key to problems such as pores, and because the thickness of the aluminum oxide layer depends on the oxygen content in the aluminum powder, the relationship between the particle size of the aluminum powder and the oxygen content of the aluminum powder is whether there will be powder, aluminum The key to problems such as the occurrence of voids at the position of the beads and the local back field. Through proper particle size/alumina layer ratio control, it can effectively avoid the generation of powder during the sintering process. At the same time, by making the particle size of the aluminum powder and the oxygen content of the aluminum powder (the thickness of the surface alumina layer) into a specific ratio, it is possible to reduce the phenomenon of aluminum powder extraction, aluminum beads, and local back field positions. The emergence of, thus completed the present invention.

In order to achieve the above and other objectives, the present invention provides an aluminum paste for partial back field solar cells, which contains: large aluminum powder; an organic carrier, which includes solvent and resin or cellulose; wherein, the aforementioned large The ratio of the median particle size (μm) of the aluminum powder to the oxygen content (%) (median particle size (μm)/oxygen content (%)) is 10-15.

In the above aluminum paste, the aforementioned ratio (median particle size (μm)/oxygen content (%)) is 11-13.

In the above aluminum paste, the oxygen content of the large aluminum powder is 0.1 to 2.0% by weight.

In the above aluminum paste, the oxygen content of the large aluminum powder is 0.3 to 1.0% by weight.

The above-mentioned aluminum paste further includes small aluminum powder, and the small aluminum powder accounts for 0.1-10% by weight of the aluminum paste.

In the above aluminum paste, the large aluminum powder and the small aluminum powder together account for 60 to 85% by weight of the aluminum paste.

The aforementioned aluminum paste, wherein the viscosity of the aforementioned organic vehicle is 1-15Kcps.

The above-mentioned aluminum paste further contains glass powder.

The above-mentioned aluminum paste, wherein the organic carrier system further comprises an additive, which is selected from at least any one of the group consisting of a dispersant, a leveling agent, a defoaming agent, an anti-settling agent, a thixotropic additive, and a coupling agent By.

In order to achieve the above and other objectives, the present invention also provides a partial back field solar cell, which contains the above-mentioned aluminum paste.

The aluminum paste for partial back field solar cells and the partial back field solar cell using the conductive aluminum paste of the present invention can reduce the occurrence of powder, aluminum beads and voids in the local back field position, thereby improving the local The photoelectric conversion efficiency and tensile strength of back-field solar cells.

Fig. 1 is a view of observing holes using an electroluminescence defect detector; (a) is the result of Comparative Example 1, (b) is the result of Example 1, and (c) is the result of Example 2.

In order to fully understand the purpose, features and effects of the present invention, the following specific examples are used to illustrate the present invention in detail. The carrier may further include glass powder.

In this specification, large aluminum powder refers to aluminum powder with a median particle size (D50) of 6.0~9.0μm. Compared with large aluminum powder, small aluminum powder refers to aluminum powder with a median particle size (D50) of 1.0~3.0μm. In addition, in this specification, if the percentage "%" is not particularly limited, it means "% by weight".

In the aluminum paste of a preferred embodiment, large aluminum powder and small aluminum powder are mixed. The large aluminum powder and the small aluminum powder can account for 65% to 85% of the total weight of the aluminum paste, preferably 70% to 76%. The aforementioned large aluminum powder preferably accounts for 60 to 80% by weight of the total weight of the aluminum paste, more preferably 60 to 70% by weight. The aforementioned small aluminum powder preferably accounts for 0.1-10% by weight of the total weight of the aluminum paste.

The organic carrier system includes an organic solvent and a resin or a cellulose, and may further include additives. The organic carrier system accounts for 10-30% by weight of the total weight of the aluminum paste, preferably 20-28% by weight. At the same time, the viscosity of the organic vehicle is about 1-15Kcps, preferably 10-15Kcps. By controlling the viscosity of the organic carrier, the aluminum paste has an optimal viscosity.

The content of cellulose (or resin) is about 1 to 4% by weight of the total weight of the aluminum paste, preferably 2 to 3% by weight. At the same time, as far as the choice of resin is concerned, it can include wood rosin or polyacrylonitrile, but not This is a limitation; in terms of the selection of cellulose, ethyl cellulose or propyl cellulose can be included, but it is not limited thereto.

The content of the organic solvent is about 10-25% by weight of the total weight of the aluminum paste, preferably 18-20% by weight. At the same time, the choice of organic solvents can include alcohol ether organic solvents, ester alcohol film formers (TEXANOL®, EASTMAN CHEMICAL COMPANY), terpineol or diethylene glycol butyl ether, etc., but not limit.

The content of the additive accounts for about 0.2 to 2.5% by weight of the total weight of the aluminum paste, preferably 1.5 to 2% by weight. At the same time, as far as the choice of additives is concerned, it may include dispersing agents, leveling agents, defoaming agents, anti-settling agents, thixotropic additives, coupling agents, etc., but not limited to this.

In a preferred embodiment of the aluminum paste, it contains glass powder. The glass powder can account for 0.1 to 5 wt% of the total weight of the aluminum paste, preferably 3 to 4 wt%. As for the selection of glass powder, vanadium-based, bismuth-based glass powder or other glass powders can be selected, and it is preferable to use the glass powders shown in Table 1 below, but it is not limited to this. One kind of glass powder can be used alone, or multiple glass powders can be used in combination.

(Measurement of the particle size and oxygen content of aluminum powder)

The particle size of the aluminum powder is measured by the laser scattering particle size analyzer-HORIBA LA950. The aluminum powder is measured with isopropanol (IPA) as the dispersion medium (medium), and ultrasonic waves are used to oscillate for the same time before the measurement and circulate at the same speed. Each aluminum powder is measured 3 times to confirm the reproducibility of the particle size measurement. The measurement results are as follows Table 2 shows. In addition, the oxygen content of aluminum powder is measured with HORIBA EMGA-820 nitrogen/oxygen detector, and the measurement results are also shown in Table 2.

Therefore, the ratio of the median diameter D50 (μm)/oxygen content (%) of the small aluminum powders 1 to 2 and the large aluminum powders 1 to 7 can be calculated from the above table 2, as shown in Table 3.

(Formation of the passivation layer of partial back field solar cells)

The passivation layer of the silicon wafer with the local backside field can be _{coated on the silicon wafer with SiO x} , TiO _x , AlO _x by the ALD method or CVD method, and then deposited with a thickness of 70~120nm _{using SiN x by the CVD method.} The protective layer is on the silicon wafer.

The silicon wafer that has completed the partial back field of the back passivation layer can use a laser and remove the passivation layer in different patterns in advance to help aluminum and silicon have better contact and reaction when they are co-fired after the conductive paste is printed. Conducive to the formation of the local back surface field. The pattern of the laser opening can be a dot with a diameter of 60~150μm, a dash with a line width of 30~100μm, or a solid line with a line width of 30~100μm.

Synthesis example

The aluminum paste is made by the conventional aluminum paste manufacturing process of partial back field solar cells, for example, refer to Patent Document Taiwan No. I541833.

The aluminum pastes of Comparative Examples 1 to 4 and Examples 1 to 5 were prepared by the following steps:

Step 1: Combine resin or cellulose (ethyl cellulose polymer, ETHOCEL Standard 20, Dow DuPont Co., Ltd.)/additives (thixotropic additives, modified castor oil derivatives, Thiaxatrol ST, sea name Stechen Co., Ltd.)/organic solvent (terpineol/diethylene glycol butyl ether, volume ratio 1:3 mixing) is added to the reaction tank, stirred and mixed to form a uniform organic carrier.

Step 2: Add glass powder, aluminum powder and other ingredients to the organic vehicle prepared in Step 1 to form a mixed aluminum paste.

Step 3: Stir the mixed aluminum slurry prepared in Step 2 with a high-speed mixer, make it fully mixed, and grind with a three-roll mill (brand model: Exakt 80E) to obtain aluminum slurry.

The composition ratio (Comparative Examples 1 to 4 and Examples 1 to 5) of the aluminum paste manufactured through the above synthesis example is shown in Table 4.

[Table 4]

Test case

Using the aluminum pastes prepared in the foregoing Examples 1 to 5 and Comparative Examples 1 to 4, a partial back-field solar cell was fabricated according to the following steps:

Step 1 (Printing): Print the back silver glue and the front silver glue on the LBSF semi-finished product by coating or screen printing (SiN _x on the front, bottom oxide layer of 6nm Al ₂ O _{3 + 80nm SiN x} top oxide layer on the back) Silicon substrate back and front. Afterwards, it was dried in an oven at 200°C, and the aluminum paste prepared in Examples 1 to 5 and Comparative Examples 1 to 4 was printed on the place where the back of the silicon substrate was not covered with silver. The silicon substrate printed with the aluminum paste prepared in Examples 1 to 5 and Comparative Examples 1 to 4 was dried to prepare a printed silicon substrate to be sintered.

Step two (sintering): After the drying step is completed, the sintering process is carried out by the crawler belt (the crawler speed is 180 to 280 inches/rnin), and the printed silicon substrate to be sintered in step one is sintered at a sintering temperature of 720°C to 820°C Material is sintered to produce a partial back-field solar cell, and the formed conductive electric The thickness of the pole is about 20 to 30 μm. After the sintering process, the organic matter and other media contained in the conductive paste of the front and back electrodes of the battery can be burned out, and the aluminum atoms of the back electrode can diffuse into the silicon semiconductor substrate from the position of the laser opening to generate a local back surface field.

According to the above steps, the conductive aluminum pastes of Examples 1 to 5 and Comparative Examples 1 to 4 were used to fabricate partial back field solar cells, and the following properties were tested: Photoelectric conversion efficiency measurement: compare the foregoing Examples 1 to 5 and the comparison The aluminum prepared in Examples 1 to 4 is printed on the blank partial back field cell sheet on a printing machine with the same screen and printing conditions. After being dried at 150-250°C, it is sent to a high-temperature sintering furnace for organic matter burning and aluminum layer. sintering. The electrical properties of the sintered cell are measured by the voltage and current test (IV test) to test the photoelectric conversion efficiency (Eff) (%), open circuit voltage (Voc (mv)) and fill factor (FF (%) of the partial back field solar cell )), the test machine model is the QuickSun 120CA produced by Finland Endeas. The results are shown in Table 5.

Powder extraction test: Observe Comparative Examples 1 to 4 and Examples 1 to 5 from the surface of the aluminum layer of the battery, and record the relationship between the powder extraction of the aluminum layer and the temperature of the sintering furnace. The results are shown in Table 6.

None: 0 particles/cm ² ; very slight: 1~5 particles/cm ² ; slight: 5-10 particles/cm ² ; severe: 10-15 particles/cm ² ; extremely severe: >15 particles/cm ² .

Aluminum bead test: Observe Comparative Examples 1 to 4 and Examples 1 to 4 from the surface of the aluminum layer of the battery, and record the relationship between the occurrence of aluminum beads and the temperature of the sintering furnace. The results are shown in Table 7.

None: 0 pcs/silicon chip; very slight: 1~5 pcs/silicon chip; slight: 5~10 pcs/silicon chip; severe: 10~15 pcs/silicon chip; extremely serious: >15 pcs/silicon chip.

Please refer to Table 5 to Table 7. From the results of Comparative Examples 1~2, it can be known that while maintaining the same ratio of D50(μm)/oxygen content (%) of large aluminum powder, although the D50(μm)/ The ratio of oxygen content (%) can slightly increase the photoelectric conversion efficiency (Eff), but the powder output is severe or extremely serious at the sintering temperature (750~800℃), so adjust the D50 (μm) of the small aluminum powder The ratio of /oxygen content (%) did not significantly improve the powder extraction status, and Comparative Examples 1 and 2 were all poor.

Then, compared to Comparative Examples 1 to 4 and Examples 1 to 4, small aluminum powders are never used and the ratio of D50 (μm)/oxygen content (%) of large aluminum powders is within the scope of the present invention From the result of Example 5, it can be known that it can achieve the effect of almost no (or only very slight generation) of powder and aluminum beads. However, in terms of photoelectric conversion efficiency (Eff), Example 5 still has room for improvement.

Then, from the results of Comparative Examples 1, 3 to 4 and Examples 1 to 4, it can be known that when the same small aluminum powder is used, by adjusting the D50 (μm)/oxygen content of the large aluminum powder (%), the photoelectric conversion efficiency of Examples 1 to 4 (20.59 to 20.65%) with the above ratio between 10 and 15 is better than that of Comparative Examples 1 to 3 to 4 (20.50 to 20.56). Among them, the photoelectric conversion efficiency of Examples 1 to 2 exceeds 20.60%, so it is better.

Furthermore, in terms of the comparison results of powder extraction and aluminum beads, Comparative Examples 1 to 3 all have severe powder extraction or aluminum beads, which is not good. However, although the results of the powder and aluminum beads of Comparative Example 4 are almost the same as those of Examples 1 to 4, and the degree of powder and aluminum beads are all less than slight, the photoelectric conversion efficiency of Comparative Example 4 is only 20.50%, which is more comparable. Examples 1 to 3 are still poor, so Comparative Example 4 is still not good. In addition, Embodiments 1 and 2 not only have the highest photoelectric conversion efficiency (over 20.60%), but also almost no powder and aluminum beads are produced. Therefore, Embodiments 1 and 2 are preferred implementation aspects. Therefore, in the following void test, only the results of Comparative Example 1 and Examples 1 to 2 are compared.

Hole test: Use an electroluminescence defect detector to observe the results of Comparative Example 1 and Examples 1-2. And shown in Figure 1 (a) ~ (c). Fig. 1 is a view of observing holes using an electroluminescence defect detector; (a) is the result of Comparative Example 1, (b) is the result of Example 1, and (c) is the result of Example 2.

From (a) to (c) of Figure 1, it can be found that the color of Comparative Example 1 is darker (darker), which means that more holes are generated. Comparative Example 1 has the problem of generating holes; The colors of 1 to 2 are all lighter (brighter) than Comparative Example 1, which means that there is almost no voids in Examples 1 to 2, so Examples 1 to 2 are better.

Therefore, in this embodiment, the ratio of the median diameter (μm) of the large aluminum powder to the oxygen content (%) (median particle diameter (μm)/oxygen content (%)) becomes 10-15 The range (preferably 11~13) can achieve the effect of suppressing the generation of powder, aluminum beads and voids while maintaining the ideal electrical properties.

The present invention has been disclosed in a preferred embodiment above, but those skilled in the art should understand that the embodiment is only used to describe the present invention and should not be construed as limiting the scope of the present invention. It should be noted that all changes and substitutions equivalent to this embodiment should be included in the scope of the present invention. Therefore, the protection scope of the present invention should be defined by the scope of the patent application.

Claims (10)

An aluminum paste for partial back field solar cells, which contains: large aluminum powder with a median particle size of 6.0~9.0μm; an organic carrier, which includes solvent and resin or cellulose; wherein, the aforementioned large aluminum powder The ratio of the median particle size (μm) of the powder to the oxygen content (%) (median particle size (μm)/oxygen content (%)) is 10-13.188, and the aforementioned large aluminum powder accounts for the proportion of the aforementioned aluminum paste 60~80% by weight. The aluminum paste according to claim 1, wherein the aforementioned ratio (median particle size (μm)/oxygen content (%)) is 11-13. The aluminum paste according to claim 1, wherein the oxygen content of the large aluminum powder is 0.1 to 2.0% by weight. The aluminum paste according to claim 3, wherein the oxygen content of the large aluminum powder is 0.3 to 1.0% by weight. The aluminum paste according to claim 1, wherein the aluminum paste further comprises small aluminum powder, and the small aluminum powder accounts for 0.1-10% by weight of the aluminum paste. The aluminum paste according to claim 5, wherein the large aluminum powder and the small aluminum powder together account for 65 to 85% by weight of the aluminum paste. The aluminum paste according to claim 1, wherein the viscosity of the aforementioned organic vehicle is 1-15Kcps. The aluminum paste according to any one of claims 1 to 7, which further contains glass powder. The aluminum paste according to any one of claims 1 to 7, wherein the organic carrier system further comprises an additive, which is selected from the group consisting of dispersing agents, leveling agents, defoaming agents, anti-settling agents, thixotropic additives and At least any one of the group consisting of coupling agents. A partial back field solar cell, which contains the aluminum paste according to any one of claims 1-9.

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