WO2011055995A2

WO2011055995A2 - Solar cell and paste composition for electrode of solar cell

Info

Publication number: WO2011055995A2
Application number: PCT/KR2010/007745
Authority: WO
Inventors: Soon Gil Kim; In Jae Lee; Sang Gon Kim; Jun Phil Eom; Jin Gyeong Park
Original assignee: Lg Innotek Co., Ltd.
Priority date: 2009-11-04
Filing date: 2010-11-04
Publication date: 2011-05-12
Also published as: KR20110049222A; WO2011055995A3; TW201123464A

Abstract

A paste composition for manufacturing an electrode of a solar cell of the embodiment includes a metal powder, an organic vehicle, and silicon oil.

Description

SOLAR CELL AND PASTE COMPOSITION FOR ELECTRODE OF SOLAR CELL

The disclosure relates to a solar cell and a paste composition for manufacturing an electrode of the solar cell.

Recently, the development of next generation clean energy has become more important due to the lack of fossil fuel. Among the next generation clean energy, a solar cell is spotlighted as an energy source for solving the future energy problem because it rarely causes environmental pollution and has the semi-permanent life span and there exists infinite resources for the solar cell.

Such a solar cell may include electrodes formed on a silicon substrate having an N type semiconductor and a P type semiconductor. In order to reinforce the bonding strength between the electrodes and the silicon substrate, PbO is added to a glass frit of the paste composition used to form the electrodes. The PbO may lower the calcination temperature of the paste composition and form a low resistance contact with respect to silicon during the calcination process.

Regarding the environmental problem, lead (Pb) is classified as environmental pollutant, so demands for developing the paste composition for the electrode of the solar cell by using a lead-free glass frit as an inorganic binder has been increased. However, if the lead-free glass frit is used in the paste composition, the mobility of the paste composition may be lowered and the wetting property of conductive powder is degraded, thereby lowering the bonding strength between the paste composition and the silicon substrate. As a result, the efficiency of the solar cell may be lowered.

The disclosure provides a paste composition for manufacturing an electrode of a solar cell, capable of reinforcing the bonding strength with respect to a silicon substrate and improving bowing characteristics while reducing Pb contents, and a solar cell including the electrode manufactured by using the paste composition.

A paste composition for manufacturing an electrode of a solar cell according to the embodiment may include a metal powder, an organic vehicle, and silicon oil.

The thermal decomposition temperature for the silicon oil is in the range of about 100 to 300℃.

The amount of the silicon oil is about 0.1 to 10wt% based on the total amount of the paste composition.

The metal powder may include an aluminum powder.

The aluminum powder comprises one type of powder or at least two types of powders having grain sizes different from each other.

The mean grain size of the aluminum powder is about 1 to 10.

The amount of the aluminum powder is about 50 to 80wt% based on the total amount of the paste composition.

The organic vehicle may include a polymer and a solvent.

The polymer may include one selected from the group consisting of acrylate resin, ethylcellulous, nitrocellulous, a polymer of ethylcellulous and phenol resin, wood rosin, and polymethacrylate of alcohol. The solvent may include one or at least two selected from the group consisting of butylcarbitolacetate, butylcarbitol, butylcellosolve, butylcellosolveacetate, propyleneglycolmonomethylether, dipropyleneglycolmonomethylether, propyleneglycolmonomethylpropionate, ethyletherpropionate, terpineol, propyleneglycolmonomethyletheracetate, dimethylamino formaldehyde, methylethylketone, gamma-butyrolactone, ethyllactate, and texanol.

The organic vehicle may include a thixotropic agent, a leveling agent, and an anti-foaming agent.

The amount of the organic vehicle is about 10 to 40wt% based on the total amount of the paste composition.

The paste composition may further include a glass frit.

The glass frit may include at least one selected from the group consisting of PbO-SiO₂based material, PbO-SiO₂-B₂O₃based material, ZnO-SiO₂based material, ZnO-B₂O₃-SiO₂ based material and Bi₂O₃-B₂O₃-ZnO-SiO₂based material_.

The amount of the glass frit is about 1 to 3wt% based on the total amount of the paste composition.

The solar cell according to the embodiment may include an electrode manufactured by using the paste composition.

The electrode may include a rear electrode.

According to the embodiment, by using silicon oil, the mobility of the paste composition and the wetting property of aluminum particles can be improved. Thus, the bowing characteristic of a wafer and the printing property can be enhanced, and the fine cracks in the electrode of aluminum can decrease. Accordingly, the paste composition can be utilized in practice even if the wafer has the thickness equal to or less than 200㎛. In addition, the bonding strength between the paste composition and the silicon substrate can be improved by the silicon oil.

FIG. 1 is a sectional view of a solar cell according to the embodiment.

Hereinafter, the disclosure will be explained in detail.

The paste composition for the electrode of the solar cell according to the disclosure includes a metal powder, an organic vehicle, and silicon oil. The paste composition may further include a little amount of glass frit in addition to the metal powder, the organic vehicle, and the silicon oil.

According to the disclosure, the metal powder may include an aluminum powder. The aluminum powder may include two types of powders having grain sizes identical to or different from each other. The amount of the aluminum powder is about 50 to 80wt% based on the total amount of the paste composition.

If the aluminum powder having various grain sizes is used, the contact area between the paste composition and the silicon substrate can be enlarged. In this case, the diffusion area of aluminum may be increased, so that the back surface field (BSF) can be effectively formed. In addition, the fill factor of the aluminum powder can be increased, so that the electric characteristics can be improved, and the thermal expansion of the metal components can be minimized during the heat treatment process, so that the compression rate of the grains can be diminished. Further, the small-the-better characteristic is ensured in the solar cell, so that the surface resistance value and the bowing characteristic can be minimized, thereby improving the efficiency of the solar cell. To this end, the aluminum powder has the mean grain size in the range of 1 to 10㎛.

The organic vehicle is mechanically mixed with inorganic components of the paste composition, so that the paste composition may have viscosity and rheological property adapted to be printed. The organic vehicle is typically used in the paste composition. For instance, the organic vehicle may include a mixture of a solvent and a polymer.

The polymer may include acrylate resin, ethylcellulous, nitrocellulous, polymer of ethylcellulous and phenol resin, wood rosin, or polymethacrylate of alcohol. Preferably, ethylcellulous is used as the polymer.

The solvent may include at least one or two selected from the group consisting of butylcarbitolacetate, butylcarbitol, butylcellosolve, butylcellosolveacetate, propyleneglycolmonomethylether, dipropyleneglycolmonomethylether, propyleneglycolmonomethylpropionate, ethyletherpropionate, terpineol, propyleneglycolmonomethyletheracetate, dimethylamino formaldehyde, methylethylketone, gamma-butyrolactone, ethyllactate, and texanol. Preferably, butylcarbitolacetate is used as the solvent.

The organic vehicle may include a thixotropic agent, a leveling agent, and an anti-foaming agent. The thixotropic agent may include urea type, amide type or urethane type polymer/organic substance or inorganic silica. The amount of the organic vehicle is about 10 to 40wt% based on the total amount of the paste composition. If the amount of the organic vehicle is less than 10wt%, the amount of the organic substance so small that the printing characteristic is deteriorated. If the amount of the organic paste exceeds 40wt%, viscosity is lowered, so that layers may be broken after the printing process has been performed.

The thermal decomposition temperature of the silicon oil is about 100 ~ 300℃. Thus, according to the disclosure, the bonding strength between the composition paste and the silicon substrate can be improved through the chemical reaction under the low temperature. In detail, silicon oxide, which is generated as the silicon oil is thermally decomposed, is densely formed between the silicon substrate and an aluminum electrode layer, which is formed when the silicon oil-containing paste composition coated on the silicon substrate is cooled after the calcination process, so that the bonding strength between the aluminum electrode layer and the silicon substrate can be improved.

According to the related art employing the lead-free glass frit, various metal oxides must be melted under the high temperature to control the composition of the lead-free glass frit. However, according to the disclosure, the bonding strength between the paste composition and the silicon substrate can be improved by performing the chemical reaction under the low temperature using the silicon oil. That is, the diffusion degree of the aluminum with respect to the silicon substrate can be controlled by adjusting the contents of the silicon oil serving as a silicon binder. Thus, the diffusion degree of the aluminum can be easily controlled so that the characteristics of the solar cell can be easily controlled.

In addition, when comparing with the related art employing the lead-free glass frit using bismuth, the disclosure can improve the wetting property of aluminum particles so that the efficiency and the printing property can be enhanced.

In the case of the paste composition according to the related art, the glass frit is necessarily required after the calcination process to reinforce the bonding strength between the aluminum electrode layer and the silicon substrate. In contrast, according to the disclosure, the bonding strength between the paste composition and the silicon substrate can be improved by the silicon oil, so that the glass frit can be omitted or only a little amount of the glass frit is required.

Preferably, the amount of the silicon oil is about 0.1 to 10wt% based on the total amount of the paste composition. If the amount of the silicon oil is less than 0.1wt%, the bonding strength may be rarely improved. If the amount of the silicon oil exceeds 10wt%, the silicon oil excessively surrounds the aluminum powder due to the mobility of the silicon oil, so that oxide may be formed around the aluminum powder particles. In this case, the surface resistance and the resistance of the BSF layer may be raised.

As mentioned above, the paste composition may not include the glass frit or may include a little amount of the glass frit. If the paste composition includes a little amount of the glass frit, the characteristics of the solar cell can be improved using a silicon binder obtained by mixing the silicon oil with the glass frit.

When the glass frit is added to the composition paste, the amount of the glass frit is 1 to 3wt% based on the total amount of the paste composition. If the amount of the glass frit is not less than 1wt%, the effect of the glass frit can be realized. However, if the amount of the glass frit exceeds 3wt%, the environmental problem may be caused.

The glass frit has the softening point of 300 to 600℃, and the mean grain size of 0.5 to 10㎛. In this case, the fill factor and sintering density can be maximized. In particular, if the mean grain size of the glass frit is less than 0.5㎛ when the paste composition is used as the front electrode, the softening point may lowered so that the anti-reflective layer may be excessively etched. In contrast, if the mean grain size of the glass frit exceeds 10㎛, the softening point is so high that the anti-reflective layer may be rarely etched.

The glass frit may include at least one selected from the group consisting of PbO-SiO₂ based material, PbO-SiO₂-B₂O₃ based material, ZnO-SiO₂ based material, ZnO-B₂O₃-SiO₂ based material, and Bi₂O₃-B₂O₃-ZnO-SiO₂based material.

Hereinafter, the method of preparing the paste composition for the electrode of the solar cell will be described.

First, the polymer resin, such as acrylate resin, ethylcellulous, or nitrocellulous is dissolved in the solvent, such as butylcarbitolacetate, and then premixed to prepare the organic vehicle.

Then, the organic vehicle, aluminum powders having grain sizes identical to or different from each other, and the silicon oil or the mixture of the silicon oil and the glass frit are premixed. After that, an amine-based, an acid-based or a bipolar dispersing additive is added to the mixture to improve the dispersing property of the particles. Preferably, the mixture is maturated for 1 to 12 hours for the purpose of desired dispersion of the mixture.

The matured mixture is secondarily mixed and dispersed through a paste mixer, a planetary mill or a 3-roll mill. Then, the filtering and defoaming processes are performed to provide the aluminum paste.

Hereinafter, an example of the solar cell employing the paste composition according to the embodiment will be described with reference to FIG. 1. FIG. 1 is a sectional view showing the solar cell.

Referring to FIG. 1, the solar cell includes a P type silicon substrate 10 provided on the top surface thereof with an N type semiconductor 11, a front electrode 12 electrically connected to the N type semiconductor 11 and a rear electrode 13 electrically connected to the P type silicon substrate 10. An anti-reflective layer 14 can be formed on the top surface of the N type semiconductor 11 except for an area where the front electrode 12 is formed. In addition, a BSF layer 15 is formed on the rear electrode 13 of the silicon substrate 10.

After the paste composition for the electrode of the solar cell is printed on the substrate, the drying and calcination processes are performed to manufacture the electrode of the solar cell. The substrate and the printing, drying and calcination processes used for manufacturing the solar cell according to the related art can be adopted in the method for manufacturing the electrode of the solar cell according to the embodiment, except for the paste composition for the electrode of the solar cell.

According to the embodiment, the electrode includes the rear electrode 13 of the silicon solar cell, the printing process includes the screen printing process, and the drying process is performed for 1 to 30 minutes under the temperature of 80 to 200℃. The high-temperature/high-speed calcination process is performed for 5 seconds to 1 minute at the temperature of about 700 to 900℃. In addition, the printing process is performed by using a screen printer, which can print the paste composition onto the epitaxial silicon substrate having the thickness of about 200㎛ under constant pressure and speed.

The rear electrode 13 manufactured through the above method may represent superior bonding strength with respect to the silicon substrate 10 due to the silicon oil.

Hereinafter, the experimental examples and the comparative example of the embodiment will be described in detail. These examples are illustrative purposes only, and the embodiment is not limited thereto.

Example 1

The paste composition was prepared by using 65g (65wt%) of aluminum powder, 1g (1wt%) of silicon oil, and the remainder of organic vehicle.

Example 2

Example 2 is similar to Example 1 except that the amount of the silicon oil was 2g (2wt%).

Example 3

Example 3 is similar to Example 1 except that the amount of the silicon oil was 3g (3wt%).

Example 4

Example 4 is similar to Example 1 except that the amount of the silicon oil was 4g (4wt%).

Example 5

Example 5 is similar to Example 1 except that the amount of the silicon oil was 10g (10wt%).

Example 6

Example 6 is similar to Example 1 except that the amount of the silicon oil was 1g (1wt%) and the amount of the glass frit was 1g (1wt%).

Example 7

Example 7 is similar to Example 1 except that the amount of the silicon oil was 4g (4wt%) and the amount of the glass frit was 1g (1wt%).

Comparative Example

Comparative Example is similar to Example 1 except that the silicon oil was not used and the amount of the glass frit was 3g (3wt%).

Experimental Example

The paste composition of Examples 1 to 7 and Comparative Example was printed on the silicon substrate through the screen printing process and then drying process was performed for 20 minutes at the temperature of 160℃. In addition, the rapid heat treatment process was performed for 30 seconds at the temperature of 850℃ to manufacture the rear electrode of the solar cell.

The performance and efficiency of the rear electrode of the solar cell manufactured through the above method are shown in Tables 1 and 2.

Table 1

	Surface resistacnce of rear electrode (mΩ)	Bonding Strength to silicon substrate	Bowing charactristic(mm)
Example 1	17.9	×	0.665
Example 2	16.7	△	0.735
Example 3	17.15	△	0.862
Example 4	17.54	○	0.74
Example 5	17.9	○	0.891
Example 6	15.2	○	0.852
Example 7	16.3	○	0.936
Comparative Example	14.2	○	1.219

Table 2

	Fill Factor (FF)	Efficiency (ratio)
Example 4	0.741	0.98
Example 6	0.742	0.96
Comparative Example	0.74	1.00

In Table 2, the efficiency is the photoelectronic transformation efficiency measured by a solar simulator after the solar cell has been manufactured and FF is the fill factor which is the ratio of the actual output to the maximum output of the solar cell. In addition, the surface resistance and the BSF resistance were measured by using a 4-point probe and the bowing characteristic was measured at the center of the rear electrode by using a dial gauge.

As shown in Table 1, the bowing characteristic is improved and the bonding strength between the wafer aluminum thick layers is enhanced as the content of the silicon oil is increased. In the examples 1 to 7, the BSF resistance has a value equal to or less than 50Ω, which means that superior BSF characteristic can be realized.

The bowing characteristic was equal to or less than 1mm in most wafers, so the rear electrode can be utilized in practice even if the wafer has the thickness equal to or less than 200㎛.

The efficiency of the solar cell according to the disclosure is approximately similar to the efficiency of the solar cell employing only the glass frit.

Therefore, according to the disclosure, the lead-free inorganic binder capable of improving the bowing characteristic of the wafer can be developed so that the environmental-friendly solar cell having the superior efficiency can be provided.

Any reference in this specification to one embodiment, an embodiment, example embodiment, etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments.

Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.

Claims

A paste composition for an electrode of a solar cell, the paste composition comprising:

a metal powder;

an organic vehicle; and

silicon oil.
The paste composition of claim 1, wherein a thermal decomposition temperature for the silicon oil is in a range of about 100 to 300℃.
The paste composition of claim 1, wherein an amount of the silicon oil is about 0.1 to 10wt% based on a total amount of the paste composition.
The paste composition of claim 1, wherein the metal powder includes an aluminum powder.
The paste composition of claim 4, wherein the aluminum powder comprises one type of powder or at least two types of powders having grain sizes different from each other.
The paste composition of claim 4, wherein a mean grain size of the aluminum powder is about 1 to 10㎛.
The paste composition of claim 4, wherein an amount of the aluminum powder is about 50 to 80wt% based on a total amount of the paste composition.
The paste composition of claim 1, wherein the organic vehicle includes a polymer and a solvent.
The paste composition of claim 8, wherein the polymer includes one selected from the group consisting of acrylate resin, ethylcellulous, nitrocellulous, a polymer of ethylcellulous and phenol resin, wood rosin, and polymethacrylate of alcohol; and

the solvent includes one or at least two selected from the group consisting of butylcarbitolacetate, butylcarbitol, butylcellosolve, butylcellosolveacetate, propyleneglycolmonomethylether, dipropyleneglycolmonomethylether, propyleneglycolmonomethylpropionate, ethyletherpropionate, terpineol, propyleneglycolmonomethyletheracetate, dimethylamino formaldehyde, methylethylketone, gamma-butyrolactone, ethyllactate, and texanol.
The paste composition of claim 8, wherein the organic vehicle further includes a thixotropic agent, a leveling agent, and an anti-foaming agent.
The paste composition of claim 1, wherein an amount of the organic vehicle is about 10 to 40wt% based on a total amount of the paste composition.
The paste composition of claim 1, further comprising a glass frit.
The paste composition of claim 12, wherein the glass frit includes at least one selected from the group consisting of PbO-SiO₂based material, PbO-SiO₂-B₂O₃based material, ZnO-SiO₂based material, ZnO-B₂O₃-SiO₂ based material and Bi₂O₃-B₂O₃-ZnO-SiO₂based material_.
The paste composition of claim 12, wherein an amount of the glass frit is about 1 to 3wt% based on a total amount of the paste composition.
A solar cell including an electrode manufactured by using the paste composition claimed in claim 1.
The solar cell of claim 15, wherein the electrode includes a rear electrode.