CN104810076A - Silver-aluminum paste used for positive electrode of high-performance N type solar cell - Google Patents

Abstract

In order to solve the problem that the elemental metal aluminum powder is used as an additive in the screen printing silver-aluminum paste of n-type crystalline silicon solar cells, and the aluminum powder is easily oxidized to prevent further diffusion and alloying of aluminum to the p+ emitter layer, the present invention proposes a high-performance N-type solar cell The silver-aluminum paste for the front electrode of the battery is composed of conductive silver powder, glass powder, organic carrier phase and additive doped with aluminum-silicon alloy powder, which is characterized in that the additive is aluminum-silicon alloy powder, and the content of silicon in the aluminum-silicon alloy powder is 1- 20wt%. Aluminum-silicon alloy powder forms heavy doping on the p+ emitter through diffusion, so that the silver-silicon emitter forms a high-conductivity path and reduces contact resistance. The presence of silicon in the aluminum-silicon alloy prevents the silicon in the p+ emitter from diffusing into the electrode due to the inter-expansion of aluminum and silicon at the interface of the silver-silicon emitter at high temperature, which can avoid surface defects of the emitter, reduce leakage, and increase the opening voltage.

Description

a high performance N Silver-aluminum paste for front electrode of solar cells

technical field

The invention belongs to the field of solar cells, in particular to a silver-aluminum paste material for front electrodes.

Background technique

Improving conversion efficiency and reducing costs are the goals that solar photovoltaics have been pursuing. Because common metal impurities such as iron have a larger capture cross section for electrons than for holes, n-type silicon has a higher minority carrier lifetime than p-type silicon under low injection conditions. Among solar-grade silicon materials, the effective minority carrier lifetime of n-type Cz silicon is above 1ms, and even the minority carrier lifetime of n-type polysilicon can reach 100µs, which is much higher than that of p-type crystalline silicon. Therefore, compared with p-type silicon, n-type crystalline silicon is more qualified to improve the conversion efficiency of solar cells, especially high-efficiency solar cells, such as IBC, HIT, double-sided cells, etc., have higher requirements for minority carrier life, and only n-type crystalline silicon can fulfil requirements. In addition, in recent years, the attenuation of battery components has attracted more and more attention, especially in windy, sandy, saline-alkali, and humid environments. The light-induced attenuation effect of p-type crystalline silicon cells is particularly serious. As early as 1973, H.Fischer and W.Pschunder (Fischer,H. and W.Pschunder. Investigation of photon and thermal induced changes in silicon solar cells. In 10 ^th IEEE Photovoltaic Specialists Conference. 1973. Palo Alto, CA, USA .) It was found that the newly fabricated boron-doped p-type Cz single crystal solar cells would have obvious attenuation under light. In 1997, J.Schmidt et al. (Schmidt, J., AG Aberle, and R. Hezel. Investigation of carrier lifetime instabilities in Cz-grown silicon. In 26 ^th IEEE PVSC.1997. New York, USA.) confirmed that boron-doped The light-induced attenuation phenomenon of doped Cz silicon is caused by the boron-oxygen pair. Since the boron content in phosphorus-doped n-type Cz silicon is extremely low, the light-induced attenuation caused by the boron-oxygen pair is not obvious. Therefore, from the aspects of further improving battery conversion efficiency and anti-induced degradation, n-type crystalline silicon batteries will be the main direction of future research and marketization.

Continuous cost reduction and continuous improvement of conversion efficiency are the inevitable requirements for the widespread use of solar photovoltaics. Screen printing is currently the most important metallization process for commercial silicon-based solar cells, and it is also the metallization method with the lowest cost. However, if the current commercialized p-type silver paste is directly used on the n-type battery, the contact resistance is too large, which greatly reduces the efficiency. Considering that the aluminum paste is successfully used on the backside of p-type silicon, it is believed that aluminum plays a key role in forming a good ohmic contact between the silver paste and the p+ emitter. In fact, experiments have shown that when aluminum is added to the silver paste as an additive, the contact resistance decreases significantly with the increase of aluminum content, but it also causes a significant increase in leakage and bulk resistance (H. Kerp et al. “Development of screen printable contacts for p+ emitters in bifacial solar cells” Proceedings 21st European Photovoltaic Solar Energy Conference, Dresden Germany, 2006). The resistivity of pure metal aluminum is 2.65X10 ^-8 Ωm, and the resistivity of pure metal silver is 1.586X10 ^-8 Ωm. Therefore, with the increase of aluminum content, the bulk resistance will also increase. Moreover, since the elemental aluminum is easily oxidized, an insulating aluminum oxide protective layer is formed on the surface of the ultrafine aluminum powder, which prevents the further diffusion and alloying process of the p+ layer by aluminum.

In order to solve the problem that the elemental metal aluminum powder is used as an additive in the screen printing silver-aluminum paste of n-type crystalline silicon solar cells, and the aluminum powder is easily oxidized to prevent further diffusion and alloying of aluminum to the p+ emitter layer, the present invention provides a front side of an N-type solar cell Silver-aluminum paste is used for electrodes, and pure metal aluminum powder is replaced by aluminum-silicon alloy powder, which is used for p+ contact of n-type crystalline silicon solar cells. The silver-aluminum paste provided by the invention is screen-printed on an n-type crystalline silicon battery, and sintered at a proper temperature in an infrared sintering furnace. The battery has low contact resistance, high opening voltage, small leakage and high conversion efficiency. The thick film paste consists of conductive silver powder, glass powder, organic carrier phase and aluminum-silicon alloy powder as an additive.

Contents of the invention

The composition of the silver-aluminum paste for the front electrode of the N-type solar cell includes: conductive silver powder, glass powder, organic carrier phase and additive-doped aluminum-silicon alloy powder. The additive aluminum-silicon alloy powder forms heavy doping on the p+ emitter through diffusion, so that the silver-silicon emitter forms a high-conduction path and reduces contact resistance. At the same time, the existence of silicon in the aluminum-silicon alloy prevents the silicon in the p+ emitter from diffusing into the electrode due to the inter-expansion of aluminum and silicon at the interface of the silver-silicon emitter at high temperature, so it can avoid surface defects of the emitter, reduce leakage, and increase the opening voltage. . The paste of the invention is suitable for screen printing electrodes of n-type polycrystalline and monocrystalline silicon solar cells.

1 conductive silver powder

Silver powder is the conductive phase in silver paste. Conductive silver powder can be simple silver or silver alloy, including Ag-Cu, Ag-Ni, Ag-Pd, Ag-Mg alloy, etc. Conductive silver powder can be spherical, flake, spherical, massive, dendritic, etc. The diameter of conductive silver powder is 0.2--10µm. If the silver powder particles are smaller than 0.2µm, the surface energy of the silver powder is large, the sintering temperature is low, the contact points at the silver-silicon interface are few, and the contact resistance is large. If the silver powder particles are larger than 10µm and the sintering temperature is high, the silver is easy to diffuse into the silicon emitter, forming a recombination center, causing leakage, and even breakdown of the emitter, resulting in battery failure. The suitable size of the conductive silver powder particles in the present invention is 1-5µm, the proportion of the conductive silver powder in the silver paste is 75-95wt%, and the suitable proportion is 80-90wt%.

2 Additives

Aluminum-silicon alloys have excellent chemical stability at room temperature and are not easily oxidized. When Al-Si alloy has a silicon content of 12.2% (molar content), the eutectic point temperature is 577°C. When it is sintered at high temperature as an n-type contact material, the Al-Si alloy melts, making it easier for the Al-Si alloy to flow to the silver-silicon contact interface, which is convenient for aluminum Diffusion into the p+ emitter increases the concentration of emitter carriers under the electrode, reduces the contact resistance of the silver-silicon interface, and also reduces the aluminum content in the bulk phase, reducing the bulk resistance. In addition, the presence of silicon in the aluminum-silicon alloy prevents the silicon in the p+ emitter from diffusing into the electrode due to the inter-expansion of aluminum and silicon at the interface of the silver-silicon emitter at high temperature, so that it can avoid surface defects of the emitter, reduce leakage, and improve the opening pressure.

The silicon content in the aluminum-silicon alloy is 1-20wt%, and the suitable content is in the range of 5-15wt%. If the silicon content is too low, the interdiffusion of aluminum and silicon at the silver-silicon interface cannot be effectively suppressed. If the silicon content is too high, the melting temperature of the aluminum-silicon alloy is high, and it is not easy to melt during sintering, so that the aluminum-silicon cannot play an effective role, and at the same time, the volume resistance increases. The particle size of Al-Si alloy powder is in the range of 0.5-10µm. The addition amount is between 1-10wt%. If the addition amount is less than 1wt%, effective aluminum heavy doping cannot be formed on the silver-silicon interface emitter; if the addition amount is greater than 10%, the electrode body resistance will increase and the conversion efficiency will decrease. At the same time, it will also reduce the welding performance of the battery. The suitable range of adding amount of Al-Si alloy powder is between 1-6wt%.

3 glass powder

Glass frit acts as a high temperature binder phase in the slurry. The glass used in the solar cell paste should have a low melting point and glass transition temperature, have good wettability to the silicon emitter surface and silver at high temperature, and can penetrate the SiNx anti-reflection layer to form a good ohmic contact. In addition, the composition of the glass frit and the sintering process will also affect the solderability and solderability of the electrode. Therefore, the glass involved in the present invention is PbO-B ₂ O ₃ -SiO ₂ , TeO ₂ -B ₂ O ₃ -PbO, Bi ₂ O ₃ -B ₂ O ₃ -SiO ₂ , TeO ₂ -PbO system glass. The softening temperature of the glass is 280-550°C, and the particle size of the glass powder is 1-10µm. The proportion of glass powder in the paste is 1-10wt%. If the glass content is less than 1wt%, the electrode bonding strength will be small, the ability to dissolve silver and open holes for SiNx will be poor, and the contact resistance will be large. If the glass content is higher than 10wt%, the glass layer at the silver-silicon contact interface will be thicker, which will affect the transmission and collection of photoelectrons. At the same time, more glass in the bulk phase will increase the bulk resistance and reduce the conversion efficiency.

4 Organic carrier phase

The role of the organic carrier is to mix and disperse conductive phase silver powder, glass powder, and additive aluminum-silicon alloy powder into a paste to form a slurry with special rheological and thixotropic properties. Make the slurry print out the designed electrode pattern accurately under the shearing force of screen printing, and make good physical contact between the electrode and the silicon, so that the electrode has a certain aspect ratio. The organic carrier phase is mainly composed of solvent, thickener, plasticizer, surfactant and thixotropic agent. The solvent is mainly composed of one or more of turpentine, terpineol, butyl carbitol, butyl carbitol acetate, and tributyl citrate. The thickener is mainly ethyl cellulose, butyl fiber The main plasticizers are phthalates, the main surfactants are caprylic acid, lecithin, and Span 85, and the main thixotropic agents are hydrogenated castor oil.

The content of the organic carrier phase in the silver paste is 3-10%. When the content of the organic carrier phase is less than 3%, it is difficult for the organic carrier phase to fully wet and disperse the silver powder, glass powder, and additive aluminum-silicon alloy powder; and when the content of the organic carrier phase is greater than 10%, the printed electrode is sintered The post-sintering density is too small, resulting in a large battery series resistance.

The preparation method of the slurry: accurately weigh silver powder (spherical silver powder with a particle size of 1-3µm), tellurate system glass powder (with a particle size of 3-5µm), aluminum-silicon alloy powder (with a silicon content of 15wt%, and a particle size of 1-3µm), the above three The total content of the powder in the slurry is 90wt%, the glass powder and the organic phase remain unchanged, and the ratio of the silver powder and the aluminum-silicon alloy powder is changed. After the three solid powders are accurately weighed according to the proportion, the three powders are fully mixed to make them uniformly dispersed, then the organic phase is added to fully stir for pre-dispersion, and then rolled by a three-roller until the fineness of the scraper is less than 14µm. The silver-aluminum paste for the front electrode of the n-type crystalline silicon solar cell is obtained.

The obtained n-type silver-aluminum paste is screen-printed on an n-type silicon chip, and is sintered in an infrared sintering furnace at a suitable temperature to obtain a solar cell, and its electrical properties are tested. The silicon wafers used are n-type monocrystalline silicon or polycrystalline silicon, which are chemically textured and boron-expanded to form a p-n junction with a square resistance of 80-90 Ω/□, after removal of borosilicate glass and edge cutting, PECVD deposits SiNx anti-reflection layer with a film thickness of 80-90nm.

Detailed ways

Example

Prepare silver pastes S1, S2, and S3 according to the formula in Table 1, and print them on single crystal (156mm×156mm, square resistance 80Ω/□) and polycrystalline (156mm×156mm, square resistance 90Ω/□) respectively. On the silicon wafer, after sintering, the solar cells were made, the electrical properties were tested, and the average data was taken. The results are listed in Table 3 and Table 4.

comparative example

The conductive silver paste with a solid content of 90wt% and the addition of 0, 5wt, and 10wt% of pure metal aluminum powder was prepared according to the same process as in the examples, which were denoted as A1, A2, and A3. Electrodes were printed on silicon wafers of two specifications: single crystal (156mm×156mm, square resistance 80Ω/□) and polycrystalline (156mm×156mm, square resistance 90Ω/□), and were sintered to form silicon solar cells. Performance, take the average data. The electrical properties of the solar cells prepared in Examples and Comparative Examples are based on the S1 paste cell, and the comparison results are listed in Table 3 and Table 4.

The formula composition table of table 1 embodiment

Silver paste number silver powder Al-Si Alloy Powder glass powder The organic phase Silicon substrate S1 86% 1% 3% 10% Monocrystalline silicon (80Ω/□) S2 82% 5% 3% 10% Monocrystalline silicon (80Ω/□) S3 77% 10% 3% 10% Monocrystalline silicon (80Ω/□) S1 86% 1% 3% 10% Polysilicon (90Ω/□) S2 82% 5% 3% 10% Polysilicon (90Ω/□) S3 77% 10% 3% 10% Polysilicon (90Ω/□)

The formula composition table of table 2 comparative example

Silver paste number silver powder Aluminum powder glass powder The organic phase Silicon substrate A1 87% 0 3% 10% Monocrystalline silicon (80Ω/□) A2 82% 5% 3% 10% Monocrystalline silicon (80Ω/□) A3 77% 10% 3% 10% Monocrystalline silicon (80Ω/□) A1 87% 0 3% 10% Polysilicon (90Ω/□) A2 82% 5% 3% 10% Polysilicon (90Ω/□) A3 77% 10% 3% 10% Polysilicon (90Ω/□)

Table 3 Comparison of electrical performance data of metallized n-type crystalline silicon solar cells screen-printed with different silver-aluminum pastes (single crystal 80Ω/□)

Table 4 Comparison of electrical performance data of screen-printed metallized n-type crystalline silicon solar cells with different silver-aluminum pastes (polycrystalline 90Ω/□)

Claims (6)

1. A silver-aluminum paste for the front electrode of a high-performance N-type solar cell is characterized in that the silver-aluminum paste is made up of conductive silver powder, additive aluminum-silicon alloy powder, glass powder and an organic carrier phase, wherein the content of the conductive silver powder is 75% -95wt%, the content of the additive aluminum-silicon alloy powder is 1-10wt%, the content of glass powder is 1-10wt%, and the organic carrier phase is 3-10wt%. 2. the silver-aluminum paste for n-type solar cell front electrode according to claim 1, is characterized in that described conductive silver powder is Ag, Ag-Cu alloy, Ag-Ni alloy, Ag-Pd alloy, Ag-Mg alloy One or more of them. 3. The silver-aluminum paste for front electrodes of n-type solar cells according to claims 1 and 2, characterized in that the average particle diameter of the conductive silver powder is 0.2-10 μm. 4. The silver-aluminum paste for front electrodes of n-type solar cells according to claim 1, characterized in that the content of silicon in the additive aluminum-silicon alloy powder is 1-20wt%. 5. The silver-aluminum paste for front electrodes of n-type solar cells according to claims 1 and 4, characterized in that the average particle size of the aluminum-silicon alloy powder is 0.5-10 μm. 6. The silver-aluminum paste for front electrodes of n-type solar cells according to claim 1, characterized in that the average particle diameter of the glass frit is 1-10 μm.