Aluminum active material for silicon solar cells

FIELD: materials for producing electricity-conducting layers by stenciling method.

SUBSTANCE: aluminum active material used for producing silicon solar cells has aluminum powder, glass frit, and organic binder; active-material aluminum powder has spherical or flaked particles measuring maximum 20 μm in proportion of 3 : 1 to 100 : 1, respectively, and lead-boron-silicate glass based frit whose softening temperature is 280 to 400 °C.

EFFECT: improved solar-cell electrophysical characteristics, such as their efficiency.

1 cl, 2 tbl, 9 ex

The invention relates to thick film microelectronics, namely, materials for the manufacture of electrically conductive layers by screen printing, and can be used in the production of silicon solar cells for forming a back electrode on a silicon substrate of p-type.

Known conductive paste (European patent No. 1119007, CL NW 1/22; H01L 31/00; published 25.07.2001,), for use in forming back electrodes of silicon solar cells. The paste consists of conductive powder and stellarity, homogenised in an organic binder. As the main component of the conductive phase is aluminum powder with an average particle diameter of from 2 to 10 μm. Its content in the paste 70-80% by weight or more. The paste contains steklovata based lead-borosilicate glass in the amount of 2.5% by weight. Steklovata has an average particle diameter of 1 μm and a softening point 585°C. the Organic binder is a solution of nitrocellulose and alkyd resin, alpha-terpineol and contained in the paste in the amount of 27.5% by mass. All components are mixed on a three-roll pasterka.

For forming the rear electrode paste is applied on the back surface of a silicon solar cell by screen printing. Drying of the paste is carried out at 150°, Veiga the s - in an infrared furnace at a maximum temperature of 750°C. the Coefficient of performance (COP) of solar cells of 14.5 to 15.0%.

Barriers to obtaining a technical result, which is provided by the claimed invention when using this similar to the following:

the softening temperature of stellarity used in this pasta, high - 585°C. Require a longer heat treatment for melting stellarity when receiving a back electrode on the basis of this paste.

- not enough low resistance electrode - 22-48 m·Ohm/cm;

- the level of efficiency of 14.5 to 15.0%

Signs similar to patent No. 1119007, coinciding with the essential features of the claimed invention:

paste of aluminum for silicon solar cells

contains:

powder of aluminium;

- steklovata based lead-borosilicate glass;

organic binder.

Known conductive paste (U.S. patent No. 5151386, CL H01L 21/44, 21/48; published 29.09.1992 g) for forming back electrodes of silicon solar cells containing 60-80% by weight of metal type aluminum or silver. In the case of silver pastes, the pastes of this type contain steklovata based lead-borosilicate glass in the amount of 4-10% by weight. These pastes have a viscosity of 1000 poises at 25°C. the Paste is diluted with carbitol the La reduce the viscosity of 50 poises. Diluted so pasta has a concentration of metal 50-75% by weight. The paste should be applied on the back surface of the silicon solar cell. Drying is carried out at 150°s With burn - at a maximum temperature of 850°C.

Barriers to obtaining a technical result, which is provided by the claimed invention when using this similar to the following:

the necessity of introducing an additional quantity of solvent to reduce the viscosity of the paste and improve its rheological characteristics and printing properties;

- dilute paste leads to a decrease in the concentration of the metal and, consequently, to increase the surface resistance of the layer, deterioration of the characteristics of solar cells fabricated using this paste;

paste wigout at a temperature of 850°With similar temperature regime does not ensure the formation of the contact system within a single cycle viginia front and back contact, since the temperature of viginia 850°is too high to qualify for facial contact to the element with shallow p-n-junction.

Signs similar to U.S. patent No. 5151386, coinciding with the essential features of the claimed invention:

paste of aluminum for silicon solar cells, contains:

powder of aluminium;

- steklovata;

organic binder.

As the closest to the invention analogue, which has a collection of characteristics that are closest to the essential features of the invention, selected U.S. patent No. 2004003836, CL H02N 6/00; H01L 31/00; H01L 25/00, published 08.01.2004, which describes the composition of the paste for forming an electric conductive layer on the p-type silicon semiconductor basis, i.e. for the formation of the back electrode of a silicon solar cell. Pasta contains aluminum powder in the amount of 60-75% by weight, an organic binder 20-35% by weight and steklovata not more than 5 mass%.

In the composition add a powder of inorganic compound from 0.3 to 10.0% by weight to reduce deflection (distortion) si substrates after annealing. Aluminum powder consists of spherical particles ranging in size from 2 to 20 μm. The organic binder is a polymer solution (nitrocellulose, polyvinyl butyral, ethylcellulose, acrylic resin, alkyd resin) in monobutyl ether of diethylene glycol. Steklovata in the paste is used on the basis of glasses systems SiO₂-Bi₂O₃-PbO,₂About₃-SiO₂-Bi₂O₃Bi₂O₃-SiO₂-ZnO, PbO-In₂O₃-SiO₂. The paste is applied on the back surface of a silicon solar cell by the method of the stencil is the second printing, followed by drying and burn at a temperature of from 710 to 720° C.

Barriers to obtaining a technical result, which is provided by the claimed invention, when using the above-described paste the following:

powder aluminum paste is composed only of spherical particles, therefore, the wiring layers are not dense and have low electrical conductivity, the contact area of the metal-semiconductor insufficient for the formation of an effective field near the back surface;

- this form of particles of aluminum powder gives a high specific resistance (12,2-15,2 IOM/□) and negatively affects the resistance R⁺layer on a silicon base (9,9-10.6 Om/□);

in this analogue is a wide range of compositions of stellarity, however, the use of stellarity with the softening temperature is outside the range of 280-400°To reduce the adhesion of the conductive layer and the increase in specific resistance, which impairs the main characteristics of the obtained solar cells.

Signs closest analogue for U.S. patent No. 2004003836, coinciding with the essential features of the claimed invention, the following:

paste of aluminum for silicon solar cells, contains:

powder of aluminium;

- steklovata;

organic binder.

The challenge aimed declare sabreena, is to create aluminum conductive paste to improve the electrophysical parameters, in particular, the efficiency of silicon solar cells.

The technical result that can be achieved by using the proposed technical solution is to increase the efficiency of silicon solar cells by reducing the layer resistance of the rear electrode and the formation of high-alloyed p⁺layer near the back surface of silicon solar cells with shallow p-n junction, increasing the yield of products as a result of reducing the number of mechanical breakdowns due to the decrease of the deflection of the silicon substrate and low education "microspheres" aluminum.

The technical result is achieved in that for the manufacture of silicon solar cells used aluminum paste containing powder of aluminum, steklovata and organic binder. The aluminium powder consists of spherical particles and scaly shape with a size of not more than 20 μm in a ratio of from 3:1 to 100:1, respectively. Pasta also contains steklovata based lead-borosilicate glass with a softening temperature 280-400°C.

The essential features of the claimed invention are as follows.

Pasta aluminum for silicon solar cells, contains Ashok aluminum, steklovata, organic binder.

Unlike most similar analogue claimed paste contains a powder of aluminum spherical particles and scaly shape no larger than 20 μm in a ratio of from 3:1 to 100:1, respectively, and steklovata based lead-borosilicate glass with a softening temperature 280-400°C.

The aluminum powder in the invention contains particles of spherical and flake forms. This ensures the formation of high-density layers with high electrical conductivity at the same thickness voiennogo layer and reduces the series resistance of silicon solar cell.

Layers containing a mixture of powders of spherical and flake forms also have a large contact surface with the semiconductor substrate. As a result, when the burn-in paste more aluminum diffuses into the silicon substrate, resulting in high-alloyed p⁺layer and provides the formation of an effective field near the back surface.

Using a paste containing aluminium powder of spherical particles and scaly form, is the formation of the back electrode of a silicon solar cell with a small layer resistance (less 11,3 IOM/□) and highly doped p⁺layer near the back surface, and low (less than 8.5 Ohm/sq) layer resistance, thanks to the efficiency of solar cells increased to 16.5%. In addition, decreases the formation of "microspheres" aluminum after viginia that leads to increased yield of products (see table 2).

The method of cooking pasta.

The pasta is made in the following way.

1. Manufacturer of aluminum powder flake form is produced on the planetary ball mill with the following materials:

powder aluminum spherical shape with a particle size of not more than 10 μm, 100 g;

- agate balls with a diameter of 5 mm - 200 g;

- isopropyl alcohol or ethyl - 100g;

- oleic acid 20g

Capacity with the obtained suspension is rotated for 60 minutes. Then, the suspension is unloaded on a filter funnel, filtered by vacuum and dried in a drying Cabinet at a temperature of 80-90°C for 2-3 hours. This produces a scaly powder form with a particle size of not more than 20 μm.

2. Dried to constant weight aluminum powders of spherical and flake form, taken in the ratio of from 3:1 to 100:1, mixed with steklovata and organic binder, which is a 3-7% solution of ethyl cellulose in terpineol or BUTYLCARBAMATE.

3. On the mixer in working capacity conduct mixing until a homogeneous mass. This is followed by a homogenization of the paste on a three-roll pass is Terce.

Measuring the degree of milling (grain) pastes carried out with the help of grindometer of Hagman (Germany) GOST 6589, method C.

Dynamic viscosity is measured by a rotational viscometer system plate-cone company "Haake (Germany) at the shear rate of 10^-1.

The prepared paste is applied on the silicon p-type semiconductor substrate through a mesh stencil with a cell size of 125 mesh. Drying is carried out at 150°C for 10-15 min, the burn - in a belt furnace at a peak temperature 710-730°C for 30 seconds. Then measure the resistivity of the aluminum layer.

The thickness voiennogo layer measured on the microscope OPTON" with precision ±2 microns.

Then the substrate is dipped in an aqueous solution of hydrochloric acid to remove the aluminum layer and carry out the measurement of the surface resistance R⁺-layer silicon substrate by four-probe method.

Examples.

1. Aluminum powders of spherical and flake form with a particle size of not more than 20 μm are mixed in a ratio of 3:1 in the amount of 60-80% by weight of the paste, add steklovata made on the basis of lead-borosilicate glass with a softening temperature of 300°in the amount of from 2.5 to 7.0% by weight of the paste, the rest is organic binder, usually 3-7% solution of ethyl cellulose in terpineol or butylacetoacetate.

2. Same as in example 1,except that the powders of spherical and flake forms taken in the ratio of 10:1.

3. Same as in example 1, except that the powders of spherical and flake forms taken in the ratio of 100:1.

4. Same as in example 2, but the softening temperature of the glass 280°C.

5. Same as in example 2, but the softening temperature of the glass 400°C.

6. Same as in example 1, except that the powders of spherical and flake shape taken in a 2:1 ratio.

7. Same as in example 1, except that the powders of spherical and flake shape taken in a ratio of 101:1.

8. Same as in example 2, but the softening temperature of the glass 450°C.

9. Same as in example 2, but the aluminium powder contains particles larger than 20 microns.

As can be seen from examples (see table 1) at a ratio of spherical and flake particles in the powder of aluminum from 3:1 to 100:1, respectively, optimally 10:1, the resulting paste has a viscosity, as satisfying the conditions of screen printing does not require dilution and has excellent printability. Grain size (degree of milling) this paste is 20 μm. This relation of spherical powders and flake form, the formation of high-density layers with a maximum area of contact metal-semiconductor, which in turn decreases the resistance of the back electrode of a silicon solar cell to 10.0 to 11.3 mω/□the resistance R⁺layer on a silicon base with therefore, its becomes of 7.3-8.5 Ohm/□ that leads to the reduction of the series resistance and increase the efficiency of the field near the back surface, the efficiency of the obtained silicon solar cells increases-16.0-16.5 per cent (see table 2).

According to table 1, when the increase in the paste scaly powder of more than 3:1 (example 6), the viscosity of the paste increases sharply, the grain becomes higher than normal. In the printing properties of the paste is reduced, the resulting layer has a higher thickness (50-60 µm), resulting in increased deflection of the silicon base after annealing (see table 2), increases the number of mechanical breakdown, which leads to a decrease of the yield of products in the manufacture of silicon solar cells.

When the content in the paste of flaky aluminium powder of less than 100:1 with respect to spherical (example 7) the viscosity of the paste decreases sharply, which also adversely affects the printing properties of the paste decreases the packing density of the layer and the area of contact metal-semiconductor forming the back electrode of a silicon solar cell. This is confirmed by the increase in the specific resistance of the rear electrode (see table 2). When the burn-in layers with the use of this paste on the surface of the back electrode increases the formation of "microspheres" aluminum that negative the effect on the percentage yield of products.

We offer pasta contains steklovata based lead-borosilicate glass with a softening temperature 280-400°C.

When heat-treated layers deposited on a semiconductor silicon substrate, the molten lead-borosilicate glass included in the paste dissolves the oxide film on the surface of aluminum particles, contributing to the formation of conductive chains, thereby lowering the electrical resistance of the conductive layer and improves the basic electrical characteristics of silicon solar cells.

Use the paste stellarity with a softening temperature in the range of 280-400°optimally 300°allows you to burn at a temperature 710-730°that enables the formation of the contact system of a solar cell within a single cycle viginia. Thus, in the manufacture of solar cells with shallow p-n junction of the inventive composition is optimal for joint viginia front and back contacts, which greatly reduces the labor costs in manufacturing and the cost of silicon solar cells. Using silver paste PPP-7-1 for the front contact and the silver-aluminum paste PAS-7-1 production of JSC "Similar", Stavropol.

When applying the paste stellarity based lead-Bo is silikatnogo glass with a softening temperature of more than 400° (Example 8), there has been a sharp increase in the resistance of the conductor layers (see table 2), which leads to deterioration of characteristics of the obtained solar cells, in particular to reduce efficiency. This is because steklovata with a softening temperature of more than 400°in the paste has not played a role steklovata to create sufficient contact in the conductive layer for forming the back electrode with the desired conductivity, while also not provided with the conductive adhesion layer to the silicon substrate and reduces the specific resistance R⁺layer.

Attempts to create glass system PbO-In₂About₃-SiO₂with a softening temperature less than 280°With no positive results.

In the present invention is used, the aluminum powder with a particle size of not more than 20 μm.

The aluminum powder with a particle size greater than 20 microns is interesting for its low cost, but the pasta based on it (example 9) has a large grain size, low viscosity and as a consequence, poor printing properties (see table 1). Also, when using a powder with a particle size greater than 20 microns, wojenny layers on the basis of such pastes have a greater thickness (see table 2), increases the deflection of the silicon substrate, reduced the percentage of yield of products as a result of increase to which icesta mechanical breakdown of silicon solar cells. The resistivity of the back electrode, is made on the basis of this paste has a higher value due to the decrease of the density of the conductive layer. The efficiency of solar cells falls.

When using aluminium powder with a particle size of not more than 20 μm, viscosity, grain size and printing properties of the paste fully comply with the terms of screen printing (see table 1). The thickness voiennogo layer is not more than 40 μm, decreases the deflection of the silicon substrate reduces the number of mechanical breakdowns, increases the percentage yield of products. The specific resistance of the rear electrode made pasta with aluminium powder of less than 20 microns, not greater than 11,3 IOM/□resulting in efficiency silicon solar cells reaches of 16.0 to 16.5%, (see table 2).

Pasta aluminum for silicon solar cells containing aluminium powder, steklovata, organic binder, characterized in that it contains aluminum powder of spherical particles and scaly shape no larger than 20 μm in a ratio of from 3:1 to 100:1, respectively, and steklovata based lead-borosilicate glass with a softening temperature 280-400°C.