阅读说明：本技术 用于局部背面场太阳能电池的铝浆及应用该铝浆的局部背面场太阳能电池 (Aluminum paste for local back surface field solar cell and local back surface field solar cell using same ) 是由 黄滢华 张弘樱 白友钦 于 2018-05-29 设计创作，主要内容包括：本发明为提供一种用于局部背面场太阳能电池的铝浆及应用该铝浆的局部背面场太阳能电池,所述铝浆包含：大颗铝粉；有机载体,其包括溶剂及树脂或纤维素；其中,所述大颗铝粉的中位粒径(μm)与含氧量(％)的比值(中位粒径(μm)/含氧量(％))为10～15。本发明的一种用于局部背面场太阳能电池的铝浆及应用该铝浆的局部背面场太阳能电池,可以减少因铝粉的出粉、铝珠、铝层对SiN<Sub>x</Sub>保护层的附着及LBSF位置的空孔等问题的产生,进而提高局部背面场太阳能电池的光电转换效率。(The invention provides an aluminum paste for a local back surface field solar cell and the local back surface field solar cell using the aluminum paste, wherein the aluminum paste comprises: large aluminum powder particles; an organic vehicle comprising a solvent and a resin or cellulose; wherein the ratio of the median particle diameter (mum) to the oxygen content (%) of the large aluminum powder (the median particle diameter (mum)/the oxygen content (%)) is 10-15. The aluminum paste for the local back surface field solar cell and the local back surface field solar cell using the aluminum paste can reduce the problems of powder discharge of aluminum powder, adhesion of aluminum beads and an aluminum layer to a SiNx protective layer, holes at LBSF (location based on fiber) positions and the like, and further improve the photoelectric conversion efficiency of the local back surface field solar cell.)

1. An aluminum paste for a local back surface field solar cell, comprising:

Large aluminum powder particles;

An organic vehicle comprising a solvent, and a resin or cellulose; wherein the content of the first and second substances,

The ratio of the median particle diameter (mum) to the oxygen content (%) of the large aluminum powder is 10-15, and the large aluminum powder accounts for 60-80 wt% of the aluminum paste.

2. The aluminum paste according to claim 1, wherein the ratio of the median particle diameter (μm) to the oxygen content (%) of the large aluminum powder is 11 to 13.

3. The aluminum paste according to claim 1, wherein the oxygen content of the large aluminum powder is 0.1-2.0 wt.%.

4. The aluminum paste according to claim 3, wherein the oxygen content of the large aluminum powder is 0.3 to 1.0 wt.%.

5. The aluminum paste according to claim 1, further comprising small aluminum powder, wherein the small aluminum powder accounts for 0.1-10 wt% of the aluminum paste.

6. The aluminum pastes of claim 5, wherein the large and small aluminum powder together constitute 65-85 wt.% of the aluminum paste.

7. The aluminum paste according to claim 1, wherein the organic vehicle has a viscosity of 1 to 15 Kcps.

8. The aluminum paste according to any one of claims 1 to 7, further comprising a glass frit.

9. The aluminum pastes of any one of claims 1 to 7, wherein the organic vehicle further comprises an additive selected from at least one of the group consisting of dispersants, leveling agents, defoaming agents, anti-settling agents, thixotropic aids and coupling agents.

10. A local back surface field solar cell comprising the aluminum paste according to any one of claims 1 to 9.

Technical Field

The present invention relates to an aluminum paste, and more particularly to an aluminum paste containing large aluminum powder having a specific ratio of particle size to oxygen content. The invention also relates to a local back surface field solar cell applying the aluminum paste.

background

In order to improve the optimal efficiency performance, the solar cell factory has gradually introduced a Local Back Surface Field (LBSF) technology from 2013. This technique operates by Emitter passivation and a back electrode (PERC). The method is to deposit SiOx, TiOx and AlOx on a silicon chip cell as a back passivation Layer by using an Atomic Layer Deposition (ALD) method or a Chemical Vapor Deposition (CVD) method, and then deposit SiNx to form a protective Layer (capping Layer) by using a CVD process. Passivation layer the main function of a local back surface field solar cell is to repair defects on the surface of the silicon chip. Because amorphous silicon (amorphous silicon) is generated during the silicon chip cutting and processing process, and more dangling bonds (dangling bonds) exist in amorphous silicon. These dangling bonds located at the edge of the silicon chip can neutralize the carriers (carriers) generated after the silicon chip is exposed to light, thereby reducing the carrier life time and reducing the electrical property. The local back surface field solar cell can improve the open circuit voltage (Voc) and the short circuit current (Isc) due to the function of the passivation layer, and can significantly increase the photoelectric conversion efficiency. However, since the passivation layer provides only limited al-si contact, the series resistance (Rs) is high and the Fill Factor (FF) is reduced.

The development of PERC aluminum paste was derived for silicon-based solar cells with a back passivation layer. Compared with the conventional solar cell or the traditional solar cell, the local back surface field solar cell has the main difference that the aluminum paste used by the traditional silicon-based solar cell is printed on the back surface of a silicon chip in a screen printing mode, and an aluminum layer of the local back surface field solar cell is directly contacted with the silicon chip and sintered to form a comprehensive Back Surface Field (BSF); however, in local back surface field solar cells, the aluminum layer mostly (> 95%) covers the SiNx protective layer, leaving only a limited laser open area to allow aluminum to directly contact silicon, which after sintering, forms a Local Back Surface Field (LBSF).

The lbs f technology faces one major technical problem: the aluminum layer may damage the passivation layer by adhering to the SiNx passivation layer. If the passivation layer is damaged, the local back surface field solar cell becomes unable to maintain high open circuit voltage (Voc), short circuit current (Isc), electrical performance and conversion efficiency. Secondly, because the contact between the aluminum layer and the silicon chip is only limited by the laser opening area, under the action of a proper amount of glass powder (1.0-5.0 wt% of the total weight of the aluminum paste), in the formula of the aluminum paste, the particle size of the aluminum powder and the thickness of the aluminum oxide layer on the surface of the aluminum powder become the key to influence whether the problem of void (void) is caused by the positions of powder discharge (powder issue), aluminum beads (beads) and Local Back Surface Field (LBSF) which damage the quality of the local back surface field solar cell is generated in the co-firing process of aluminum-silicon.

However, in the case of current silicon-based solar cells with a back passivation layer, attention is focused on the application of glass frit to the aluminum paste formulation to control the erosion and adhesion of the aluminum layer to the back passivation layer (SiOx, TiOx, or AlOx/SiNx) of the local back surface field solar cell. Therefore, in the prior art, the influence of the aluminum powder content in the aluminum paste exceeding 60 wt% on the overall quality characteristics and reliability of the local back surface field solar cell during the sintering process is ignored, and no teaching or suggestion is provided for the above technical problems. For example, an aluminum paste is proposed, which comprises 60 to 87 wt% of aluminum powder and glass powder with high lead content, and utilizes the characteristic that lead oxide (PbO) is easy to melt and react to enhance the adhesion between the aluminum paste and the protective layer; however, in the prior art, the influence of the problems of the powder generated during the sintering process of the aluminum powder with silicon, the adhesion of aluminum beads and aluminum layers to the SiNx protective layer, the holes caused by the local back surface field position, and the like in the aluminum paste with a content of more than 60 wt% on the overall quality characteristics and reliability of the local back surface field solar cell is not discussed.

Therefore, how to develop an aluminum paste for the LBSF and reduce the problem that the photoelectric conversion efficiency of the local back surface field solar cell is affected by the generation of phenomena such as the powder discharge of aluminum powder, the adhesion of aluminum beads and aluminum layers to the SiNx protective layer, the holes at the LBSF position, and the like is a technical focus that is desired by all LBSF research and development personnel.

Disclosure of Invention

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

More specifically, the present inventors have completed the present invention based on the following theory.

(discharging powder)

theoretically, in a conventional or conventional solar cell, because the maximum sintering temperature of metal paste (front silver paste, back silver paste, and back aluminum paste) of a general P-type silicon-based solar cell after printing and drying is 720 to 820 ℃, and the aluminum layer is in full-face contact with the silicon chip, the aluminum powder and the silicon chip start to react and fuse when the minimum temperature of the aluminum-silicon Eutectic is above about 577 ℃ (Eutectic point). Or, in the conventional solar cell, because the melting point of pure aluminum is 660.32 ℃, the aluminum powder can break the aluminum oxide shell layer on the surface of the aluminum powder after reaching the melting point, so the aluminum powder flows out and has a chance to form aluminum-silicon alloy with the silicon chip.

In contrast, in the silicon chip with local back surface field, due to the existence of the passivation layer, although the aluminum layer in the aluminum powder has the chance to contact with the silicon due to the local laser opening, the ratio of the aluminum and the silicon capable of reacting and melting together is far lower than that of the conventional or conventional silicon-based solar cell. Further, once the aluminum flows between porous alumina shells after melting, the molten aluminum, which is not in time co-melted with the silicon, may flow to the surface of the aluminum layer due to the high thermal expansion coefficient. If the molten aluminum is cooled, irregular small particle powder is formed on the surface of the aluminum layer, which is the phenomenon of aluminum powder discharge. Such generation of the outgas affects the adhesion strength of the Ethylene Vinyl Acetate (EVA) lamination when forming a module, and thus deteriorates reliability and reduces the service life of the solar cell module.

(aluminum bead)

When the composition of the aluminum beads is analyzed by an electron microscope (SEM/EDX) with an energy dispersive X-ray spectrometer, it can be known that the composition of the aluminum beads is mainly composed of aluminum elements and the silicon content is 5.0 to 30.0 wt%. Therefore, the reason for the generation of the aluminum bead (bead) is mainly because the aluminum-silicon eutectic is hindered by the presence of the back passivation layer of the local back surface field solar cell. Although the diffusion speed of silicon in the aluminum layer is known to be fast, in order to avoid the sintering temperature peak of the local back surface field solar cell from generating large damage to the passivation layer, the sintering temperature peak is 20-40 ℃ lower than that of the conventional solar cell, so the distribution of the aluminum-silicon alloy in the sintering process is not uniform. Moreover, during the cooling process, silicon in the aluminum-silicon alloy reflows to the silicon chip as the temperature decreases, which further causes the aluminum-silicon alloy and pure aluminum to be distributed more unevenly in the aluminum layer. Therefore, under the condition of obvious difference of thermal expansion coefficients, in the process of cooling, the contraction rate of the aluminum-silicon alloy with the smaller thermal expansion coefficient is slower, and the pure aluminum is influenced by the fast contraction rate in the process of cooling, so that part of the aluminum-silicon alloy is extruded to the surface of the film and aluminum beads are formed.

Aiming at the problem that aluminum beads are easily generated in the aluminum paste sintering process of a local back surface field, the key point is to effectively control the reaction time and reaction rate of aluminum and silicon. When the aluminum powder participates in the aluminum-silicon eutectic reaction, in addition to the fact that the glass powder is melted in advance in the sintering process to corrode aluminum oxide on the surface of the aluminum powder, so that pure aluminum can flow out of an aluminum oxide shell layer after being melted to be in contact reaction with silicon, the particle size of the aluminum powder and the thickness of the aluminum oxide shell layer become main influence factors on the time and the speed of the aluminum-silicon reaction. When the aluminum powder is small in particle size, the melting rate of the small-particle-size aluminum powder (D50: 1.0-3.0 microns) is far faster than that of the large-particle-size aluminum powder (D50: 6.0-9.0 microns) due to the large specific surface area under the same heating condition, and the probability of generating aluminum beads in the sintering process is obviously increased.

Besides, the particle size affects the melting speed of the aluminum powder, and the thickness of the aluminum oxide shell layer on the surface of the aluminum powder is also a main factor affecting the outflow speed of the molten aluminum. The thickness of the aluminum oxide shell layer on the surface of the aluminum powder is positively correlated with the oxygen content in the aluminum powder. The thicker the alumina shell layer is, the stronger the capability of the alumina shell layer for resisting the erosion action of the glass powder is, namely the time for the aluminum melt to flow out can be delayed; however, an excessively high degree of oxidation (an excessive amount of oxygen) means that the aluminum paste has a high content of alumina, which adversely affects the electrical conductivity and the structural strength between the entire aluminum layers, and weakens the adhesion strength after EVA lamination. Although the improvement can be achieved by increasing the addition amount of the glass frit, the glass frit is not a conductive material, and if the structural strength of the aluminum layer is enhanced by using more glass frit, the problem of the electrical property being affected by the increase of the resistance (resistance) is required. Therefore, the proper particle size of the aluminum powder and the proper thickness of the aluminum oxide shell layer on the surface of the aluminum powder (oxygen content in the aluminum powder) are key factors for effectively inhibiting the generation of aluminum beads and maintaining the ideal conductivity.

(holes)

In order to allow the aluminum paste to directly contact the silicon chip and provide space for the aluminum-silicon to generate good Ohmic contact (Ohmic contact) and facilitate carrier transfer after sintering, it is known to use laser openings to generate patterns (patterns) with different characteristics on the back passivation layer of the back passivated silicon-based P-type solar cell. Since the thickness of the back passivation layer is only 80-150 nm and the laser with proper power can melt and damage (ablation) the thickness of the back passivation layer to 1.0-3.0 μm, the pure silicon of the silicon chip can be completely exposed at the laser opening position after the laser action, and further has an opportunity to contact with the aluminum layer. During the Al-Si eutectic process in the sintering process, because the diffusion rate of Si in the Al layer is very fast, Si at the laser hole opening position will be quickly eutectic with Al and dispersed into the Al-Si eutectic alloy solution. Although silicon still flows towards the surface of the silicon chip in the cooling process, once the generation speed of the molten aluminum is high, so that the aluminum-silicon eutectic continues to act, the silicon which is diffused far away is not as far as the position of the laser hole opening in the cooling process, and the silicon at the position of the laser hole opening is in a similar hollow state, namely the generation of a hollow (void). If the aluminum paste is applied, the size of the aluminum powder particles and the thickness of the aluminum oxide shell on the surface of the aluminum powder are controlled, so that the aluminum-silicon eutectic is not excessive, and the problem of voids caused by excessive diffusion of silicon and too late return to the laser hole-opening position during cooling can be avoided.

therefore, after repeated research and study, 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 critical to the generation of the powder, the generation of the aluminum beads, and the generation of the voids due to the local back surface field position, and that the relationship between the particle size of the aluminum powder and the oxygen content of the aluminum powder is critical to the generation of the powder, the generation of the aluminum beads, and the generation of the voids due to the local back surface field position, because the thickness of the aluminum oxide layer depends on the oxygen content in the aluminum powder. Through proper proportion control of the grain diameter/the alumina layer, the problems of powder generation and the like in the sintering process can be effectively avoided. Meanwhile, the particle size of the aluminum powder and the oxygen content of the aluminum powder (the thickness of the surface alumina layer) are in a specific ratio, so that the generation of phenomena such as powder discharge of the aluminum powder, aluminum beads, and voids at local back surface field positions can be reduced, and the invention is completed.

To achieve the above and other objects, the present invention provides an aluminum paste for a local back surface field solar cell, comprising: large aluminum powder particles; an organic vehicle comprising a solvent and a resin or cellulose; wherein the ratio of the median particle diameter (mum) to the oxygen content (%) of the large aluminum powder (the median particle diameter (mum)/the oxygen content (%)) is 10-15.

the aluminum paste is characterized in that the ratio (median particle diameter (mum)/oxygen content (%)) is 11 to 13.

the aluminum paste is characterized in that the oxygen content of the large aluminum powder is 0.1-2.0 wt%.

The aluminum paste is characterized in that the oxygen content of the large aluminum powder is 0.3-1.0 wt%.

The aluminum paste is characterized by further comprising small aluminum powder, wherein the small aluminum powder accounts for 0.1-10 wt% of the aluminum paste.

the aluminum paste is characterized in that the large aluminum powder and the small aluminum powder together account for 60-85 wt% of the aluminum paste.

The aluminum paste is characterized in that the viscosity of the organic carrier is 1-15 Kcps.

the aluminum paste is characterized by further comprising glass powder.

The aluminum paste is characterized in that the organic vehicle further comprises an additive selected from at least one of the group consisting of a dispersant, a leveling agent, a defoaming agent, an anti-settling agent, a thixotropic aid and a coupling agent.

To achieve the above and other objects, the present invention further provides a local back surface field solar cell, which is characterized by comprising the above aluminum paste.

The aluminum paste for the local back surface field solar cell and the local back surface field solar cell applying the conductive aluminum paste can reduce the generation of phenomena such as powder discharge, aluminum beads, holes at the local back surface field position and the like, and further improve the photoelectric conversion efficiency and the tensile force of the local back surface field solar cell.

Drawings

FIG. 1A is a view showing a hole observed by an electroluminescence defect inspection apparatus in comparative example 1;

FIG. 1B is a view showing a hole observed by an electroluminescence defect inspection apparatus in example 1; and

FIG. 1C is a diagram showing observation of a hole in example 2 using an electroluminescence defect inspection apparatus.

Detailed Description

For a fuller understanding of the objects, features and effects of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings in which:

The aluminum paste provided by the invention mainly comprises large aluminum powder, an organic carrier and glass powder.

In the present specification, the large-particle powdery aluminum has a median particle diameter (D50) of 6.0 to 9.0. mu.m. The small aluminum powder has a median particle diameter (D50) of 1.0 to 3.0 μm with respect to the large aluminum powder. In the present specification, the "%" refers to "% by weight" unless otherwise specified.

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

The organic carrier comprises an organic solvent and a resin or cellulose, and may further comprise an additive. The organic carrier is 10-30 wt%, preferably 20-28 wt% of the total weight of the aluminum paste. Meanwhile, the viscosity of the organic vehicle is about 1 to 15Kcps, preferably 10 to 15 Kcps. The viscosity of the organic carrier is controlled to make the aluminum paste have the optimal viscosity.

The cellulose (or resin) is present in an amount of about 1 to 4 wt%, preferably 2 to 3 wt%, based on the total weight of the aluminum paste. Meanwhile, as for the selection of the resin, wood rosin, polyacrylonitrile or the like can be included, but not limited thereto; as the cellulose, ethyl cellulose, propyl cellulose, or the like can be included, but not limited thereto.

The organic solvent is present in an amount of about 10 to 25 wt%, preferably about 18 to 20 wt%, based on the total weight of the aluminum paste. Meanwhile, as for the selection of the organic solvent, an alcohol ether organic solvent, an ester alcohol film former (EASTMAN CHEMICAL COMPANY), terpineol or diethylene glycol butyl ether, etc. may be included, but not limited thereto.

The content of the additive is about 0.2 to 2.5 wt%, preferably 1.5 to 2 wt% of the total weight of the aluminum paste. Meanwhile, as for the selection of the additives, a dispersing agent, a leveling agent, a defoaming agent, an anti-settling agent, a thixotropic aid, a coupling agent, and the like may be included, but not limited thereto.

In a preferred embodiment, the aluminum paste comprises glass frit. The glass powder may account for 0.1-5 wt%, preferably 3-4 wt% of the total weight of the aluminum paste. As for the selection of the glass frit, vanadium-based, bismuth-based or other glass frits may be used, and preferably, the glass frits shown in the following table one are used, but not limited thereto. One kind of glass powder may be used alone or several kinds may be used together.

[ Table 1]

Glass powder 1	PbO-ZnO-B2O3-SiO2
		Glass powder 2	SiO2-PbO-B2O3-Al2O3-ZrO2
Glass powder 3	Bi2O3-ZnO-SiO2-B2O3-Al2O3
		Glass powder 4	Bi2O3-B2O3-Al2O3-BaO-ZnO
Glass powder 5	SiO2-PbO-ZnO-B2O3-A12O3
		glass powder 6	V2O5-B2O3-Al2O3-BaO-ZnO

(measurement of particle diameter and oxygen content of aluminum powder)

The particle size of the aluminum powder is measured by a laser scattering particle size analyzer-HORIBA LA 950. When measuring the aluminum powder, isopropyl alcohol (IPA) is used as the dispersion medium (medium), and before measuring, ultrasonic vibration is performed for the same time and circulation is performed at the same speed. Each aluminum powder was measured 3 times to confirm reproducibility of particle size measurement, and the measurement results are shown in table 2. In addition, the oxygen content of the aluminum powder was measured by a HORIBA EMGA-820 nitrogen/oxygen detector, and the measurement results are shown in Table 2.

[ Table 2]

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

[ Table 3]

(formation of passivation layer of local Back surface field solar cell)

The passivation layer of the silicon chip with the local back surface field can be formed by coating SiOx, TiOx and AlOx on the silicon chip by an ALD method or a CVD method, and then depositing a protective layer with the thickness of 70-120 nm on the passivation layer by using SiNx by the CVD method.

The silicon chip with the back passivation layer can use laser, and the passivation layer is removed in advance by different patterns, so that better contact and reaction of aluminum and silicon can be realized during co-firing after the conductive paste is printed, and the formation of a local back surface field is facilitated. The pattern of the laser holes may be dots (dot) having a diameter of 30 to 150 μm, dashes (dash) having a line width of 30 to 100 μm, or lines (line) having a line width of 30 to 100 μm.

Synthesis example

The aluminum paste is manufactured by the conventional aluminum paste manufacturing step of the local back surface field solar cell.

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

The method comprises the following steps: resin or cellulose (ethylcellulose polymer, ETHOCEL Standard 20, du pont, dow)/additive (thixotropic aid, castor oil modified derivative, Thiaxatrol ST, hai name steud ltd)/organic solvent (terpineol/diethylene glycol butyl ether, mixed in a volume ratio of 1: 3) was added to the reaction tank, stirred and mixed to a homogeneous organic vehicle.

Step two: and (4) adding other components such as glass powder, aluminum powder and the like into the organic carrier prepared in the step one to form mixed aluminum paste.

Step three: and (3) stirring the mixed aluminum paste prepared in the step two by using a high-speed stirrer, fully mixing the mixed aluminum paste, and then grinding the mixed aluminum paste by using a three-roll grinder (brand name: Exakt 80E), thereby obtaining the aluminum paste.

The composition ratios of the aluminum pastes produced by the above synthesis examples (comparative examples 1 to 4 and examples 1 to 5) are shown in table 4.

[ Table 4]

Test example

Using the aluminum pastes prepared in examples 1 to 5 and comparative examples 1 to 4, a local back surface field solar cell was fabricated according to the following steps:

Step one (printing): and respectively printing the back silver paste and the front silver paste on the back surface and the front surface of the silicon substrate of the LBSF semi-finished product (the front surface is SiNx, and the back surface is a 6nm Al2O3 bottom oxide layer and an 80nm SiNx top protection layer) in a coating or screen printing mode. Then, the silicon substrate was dried in an oven at 200 ℃ and the aluminum paste prepared in examples 1 to 5 and comparative examples 1 to 4 was printed on the back surface of the silicon substrate where silver was not coated. The silicon substrates printed with the aluminum pastes obtained in examples 1 to 5 and comparative examples 1 to 4 were dried to obtain printed silicon substrates to be sintered.

Step two (sintering): and after the drying step is finished, carrying out a sintering process by using crawler conveying (the crawler speed is 180-280 inch/nin), sintering the to-be-sintered printing silicon substrate prepared in the step one at the sintering temperature of 720-820 ℃ to prepare the local back surface field solar cell, wherein the thickness of the formed conductive electrode is about 20-30 microns. Through the sintering process, the organic matter and other media contained in the conductive paste of the front and back electrodes of the battery piece can be burnt out, and aluminum atoms of the back electrode are diffused into the silicon semiconductor substrate from the laser opening position 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 local back surface field solar cells, respectively, and the following properties were tested:

Measuring photoelectric conversion efficiency: the aluminum pastes prepared in the examples 1 to 5 and the comparative examples 1 to 4 were printed on blank local back surface field cells by the same screen printing and printing conditions in a printing machine, dried at 200 ℃, and then sent to a continuous sintering furnace for burning out organic matters and sintering conductive aluminum pastes. The electrical property of the sintered cell piece is measured by a voltage current test (IV test), and the photoelectric conversion efficiency (Eff) (%), the open-circuit voltage (voc) (mv)) and the fill factor (FF (%)) of the local back surface field solar cell are tested, and the model of the testing machine is QuickSun 120CA manufactured by Endeas corporation in finland. The results are shown in Table 5.

[ Table 5]

Powder discharge test: the results of comparative examples 1 to 4 and examples 1 to 5 were observed from the surface of the aluminum layer of the cell sheet, and the relationship between the powder discharge state of the aluminum layer and the sintering furnace temperature was recorded as shown in Table 6.

[ Table 6]

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

And (3) testing aluminum beads: the results of comparative examples 1 to 4 and examples 1 to 4 were observed from the surface of the aluminum layer of the cell sheet, and the relationship between the occurrence of aluminum beads and the temperature of the sintering furnace was recorded as shown in Table 7.

[ Table 7]

None: 0 particles/silicon chip; very slight: 1-5 silicon chips; slight: 5-10 silicon chips; severe: 10-15 pieces/silicon chip; extremely severe: >15 particles per silicon chip.

Please refer to tables 5 to 7. From the results of comparative examples 1-2, it can be seen that, while maintaining the same ratio of D50(μm)/oxygen content (%) of the large aluminum powder, the reduction of the ratio of D50(μm)/oxygen content (%) of the small aluminum powder can slightly improve the photoelectric conversion efficiency (Eff), but the powder discharge condition is severe or extremely severe at the sintering temperature (750-800 ℃), so that the adjustment of the ratio of D50(μm)/oxygen content (%) of the small aluminum powder does not substantially improve the powder discharge condition, and is not preferable in comparative examples 1-2.

Then, as compared with comparative examples 1 to 4 and examples 1 to 4, it can be seen from the results of example 5 in which small powdery aluminum is not used and the ratio of D50(μm)/oxygen content (%) of large powdery aluminum is within the range of the present invention that the effect of generating almost no (or only very slight) outgrowth and aluminum beads can be achieved. However, embodiment 5 has room for improvement in terms of photoelectric conversion efficiency (Eff).

Then, from the results of comparative examples 1 and 3 to 4 and examples 1 to 4, it can be seen that, when the same small aluminum powder is used, the photoelectric conversion efficiency of examples 1 to 4(20.59 to 20.65%) having the ratio of D50(μm)/oxygen content (%) between 10 to 15 is better than that of comparative examples 1 and 3 to 4(20.50 to 20.56) by adjusting the ratio of D50(μm)/oxygen content (%) of the large aluminum powder. Among them, the photoelectric conversion efficiency of examples 1 to 2 is preferably over 20.60%.

Furthermore, the comparison results of the powder discharge and the aluminum beads are not good because the comparative examples 1 to 3 generate serious powder discharge or aluminum beads. However, although the results of the powdering and the aluminum beads of comparative example 4 are almost the same as those of examples 1 to 4, and the degree of the powdering and the aluminum beads are slightly less, the photoelectric conversion efficiency of comparative example 4 is only 20.50%, which is inferior to those of comparative examples 1 to 3, and thus comparative example 4 is still not preferable. In addition, in the embodiments 1 to 2, not only the photoelectric conversion efficiency is the highest (more than 20.60%), but also there is almost no generation of powder and aluminum beads, so the embodiments 1 to 2 are preferred embodiments. Therefore, only the results of comparative example 1 and examples 1 to 2 were compared in the following void test.

And (3) testing a hollow hole: the results of comparative example 1 and examples 1 to 2 were observed using an electroluminescence defect detector. And is shown in FIGS. 1A to 1C. FIGS. 1A to 1C are views for observing a hole using an electroluminescence defect detector; fig. 1A shows the results of comparative example 1, fig. 1B shows the results of example 1, and fig. 1C shows the results of example 2.

As can be seen from fig. 1A to 1C, comparative example 1 is darker (i.e., indicates that many voids are generated), and comparative example 1 has a problem of generation of voids; on the other hand, the color of examples 1 to 2 is lighter (brighter) than that of comparative example 1, which means that the examples 1 to 2 hardly generate voids, so that examples 1 to 2 are preferable.

Therefore, in the present embodiment, the ratio of the median particle diameter (μm) to the oxygen content (%) of the large aluminum powder (median particle diameter (μm)/oxygen content (%)) is in the range of 10 to 15 (preferably 11 to 13), so that the effects of suppressing the generation of powder, aluminum beads and voids and maintaining the desired electrical properties can be achieved.

While the invention has been described in terms of preferred embodiments, it will be understood by those skilled in the art that the examples are intended in a descriptive sense only and not for purposes of limitation. It should be noted that equivalent variations and substitutions to those of the embodiments are intended to be included within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the claims.