Patent Application
20110101285
1. Field of the Invention
The present invention generally relates to a conductive paste. In particular, the present invention is directed to a conductive paste with a surfactant.
2. Description of the Prior Art
Because the internet-related and electronic-related techniques are developing more and more rapidly so that various older electronic products are being replaced by other newer ones and their size is shrinking quickly, the size of the elements for connecting the electric patterns, and for connecting the electric patterns and other elements is shrinking quickly, as well. Since conductive paste can be cured instantly or quickly, conveniently manipulated, and forms a layer which is usually smaller than conventional connecting elements such as pins, it is often used in electronic products.
Additionally, solar cells which use single-crystal Si or poly Si as their main semiconductor substrate have electrodes which use conductive pastes made of mixed metal powders and organic vehicles applied on Si wafers by screen printing and sintered in sintering furnaces. Accordingly, the composition and the sintering condition of the conductive paste are especially critical to the solar cells.
Generally speaking, a conductive paste includes an organic vehicle, conductive powders, glass frits and optional additives. The components in the conductive paste dominate the shape after printing, the conductivity of the electrodes, adherence to the semiconductor substrate and etching of the anti-reflective later.
The above-said glass frit usually includes some ingredients to etch the anti-reflective layer and to assist the sintering during the thermal sintering process. However, such ingredients and the glass itself are pollutants and harmful to the environment during the manufacture and usage.
As a result, a novel and environment-friendly conductive paste is still needed to keep the solar cell stable, to maintain high conversion yield and make it less harmful to the environment.
The present invention accordingly proposes a conductive paste for use in electronic products. In particular, the conductive paste of the present invention is particularly useful in the electrodes of solar cells.
The present invention proposes a conductive paste which includes at least one metal in powder form, an organic vehicle, glass frit and a surfactant. The surfactant has a representative formula as follows:
Mx(R)y(Q)z
wherein,
M is a metal element or a semiconductive element, such as Al, Ti, Zr or Si . . . etc.;
R is hydrophilic group which is directly connected to M, and R is capable of being hydrolyzed by water to form a corresponding hydrophilic functional group, such as alkyl oxide group, carbonyl group, carboxyl group, carbonyloxy group, or amido group; and
Q is a hydrophobic group with 1-24 carbon atoms, and Q may be a stable and less reactive alkyl group or alkoxy group, or includes a functional group which is capable of reacting with other polymeric units or moieties, such as vinyl group, aliphatic epoxy group, styryl group, methacryloxy group, acryloxy group, aliphatic amino group, chloropropyl group, aliphatic mercapto group, aliphatic sulfido group, isocyanato group, aliphatic ureido group, aliphatic carboxyl group, aliphatic hydroxyl group, cyclohexyl group, phenyl group, aliphatic formyl group, acetyl group, or benzoyl group. Due to the functional groups which are capable of reacting with other polymeric units, there may be one or more surfactants of the present invention which are linked to one polymeric chain such as Mx(R)y(Q)z. The surfactant which is attached to a polymeric chain may be one or more.
The surfactant of the present invention has a representative formula as follows:
Mx(R)y(Q)z
wherein,
X, Y and Z are all natural numbers.
Because the conductive paste of the present invention employs a specially designed surfactant, the hydrophilic group R in the surfactant may turn into another hydrophilic functional group after being hydrolyzed by water, but the hydrophobic group Q has greater affinity to organic solvents or is prone to react with polymers. The hydrophilic functional group in the surfactant is prone to attach to the surface of metals and the other hydrophobic group Q has greater affinity to organic solvents. In such a way, metal powders may be accordingly well dispersed. Furthermore, the glass content may become lower as compared to the conventional conductive paste when the surfactant is added into the conductive paste so that it introduces less contaminants into the environment.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
The present invention provides a conductive paste which includes at least one metal material in powder form, an organic vehicle, optional glass and a surfactant. The metal may be Al powder or Ag powder. The organic carrier is dissolved resins in an organic solvent with optional oil acid or other additives. The glass may be an amophorous compound containing metals in various oxide forms, mainly for etching the anti-reflective of solar cells and to assist the sintering during the thermal sintering process. To meet different demands, different glass may be added or no glass is added.
The surfactant which is provided by the present invention has a representative formula as follows:
Mx(R)y(Q)z representative formula 1
wherein X is between 1 and 6, Y is between 1 and 20 and Z is between 1 and 20, M is a metal element or a semiconductive element, such as Al, Ti, Zr, Si, Ge or other metal elements and semiconductive elements, R is hydrophilic group which is directly connected to M, and R is capable of being hydrolyzed by water to form a corresponding hydrophilic functional group. In accordance with the present invention, R may independently be an alkyl oxide group (—O—CmHn), carbonyl group (—(C═O)—CmHn), carboxyl group (—O—(C═)—CmHn), carbonyloxy group (—(C═O)O—CmHn), amido group (—(C═O)NH—CmHn), alkoxylene group (—O—CmHn—O—) and carboxylic ester group (—O—(C═O)—CmHn—O—). Generally speaking, m may be between 1 and 6, n may be between 3 and 13, CmHn may be a branched or unbranched alkyl or cycloalkyl group. The alkoxylene group may be ethoxylene (—O—CH2—CH2—O—), and the carboxylic ester group may be (—O—(C═O)—CH2—O—).
Q is a hydrophobic group with 1-24 carbon atoms. Q may be a stable and less reactive alkyl group or alkoxy group, or include a functional group which is capable of reacting with other polymeric units or moieties, such as a vinyl group, aliphatic epoxy group, styryl group, methacryloxy group, acryloxy group, aliphatic amino group, chloropropyl group, aliphatic mercapto group, aliphatic sulfido group, isocyanato group, aliphatic ureido group, aliphatic carboxyl group, aliphatic hydroxyl group, cyclohexyl group, phenyl group, aliphatic formyl group, acetyl group or benzoyl group. The aliphatic group may have 1-24 carbon atoms, such as a branched or unbranched alkyl group.
In addition to this, Q may also be a compound including N, O, P S, or the combination thereof. In some circumstances, Q may have at least one long pair which M may accept so that Q is a ligand when M is a metal. Furthermore, Q is capable of being chemically bonded to be a side chain or a terminal of a polymer chain by means of chemical reaction through the functional group, which means a polymeric chain may connect one or more functional groups of the surfactant Mx(R)y. In other words, the surfactant which is attached to a polymeric chain is not limited to just one type. As long as a polymeric chain is able to react with Q, it is a suitable polymeric chain. The polymeric chain may have 1 to 10000 polymeric units.
A preferred embodiment of the representative formula 1 of the present invention may be the compound of formula 1, formula 2, formula 3 or formula 4. However, these embodiments are for illustration purposes only and the scope of the present invention is not therefore limited.
In formula 1, M is illustrated as Al. As a result, X is 1. R is an isopropoxy group, i.e. (CH3)2CHO— which is attached to Al. There are two isopropoxy groups therein, so Y is 2. Since Q is (O—C(CH3)—CH—CO—O—C18H35), Z is 1. In formula 1, on one hand Q is bonded to M. And on the other hand, Q may also serve as a ligand, a bidentate for instance so that Q and M have coordination bonding to each other. In addition, in accordance with the demands of different products R may be selected from different hydrophilic groups. For example, one of R may be an isopropoxy group and the other R may be another group such as a carbonyl group or a carboxyl group.
In formula 2, X is 2 since M is illustrated as Si. R is an ethoxy group, i.e. —O—C2H5, so Y is 6. Q is C3H6S2 so Z is 2. In formula 2, Q is a divalent group which is capable of bonding to two independent Ms, and there are hetero-atoms in Q, S for example.
In formula 3, X is 1 because M is illustrated as Ti. R is an isopropoxy group, i.e. (CH3)2CHO—, so Y is 1. Q is —OC2H4NHC2H4NH2 so Z is 3.
In formula 4, X is 1 because M is illustrated as Ti. R is an isopropoxy group, i.e. (CH3)2CHO—, so Y is 1. Q is —P2O5H—(OC8H17)2 so Z is 3.
Table 1 to Table 3 in
As shown in
Accordingly, the surfactant of the present invention not only can effectively disperse the metal powders, but also 10% (wt. %) or less, preferably 0.1%-5% (wt. %), surfactant in the conductive paste is sufficiently effective. In the conventional methods, there is no other way to increase the connection between the electrodes of the solar cells and the semiconductor substrate in addition to the addition of glass into the conductive paste. However, in the presence of the conductive paste of the present invention, less glass is required. Less glass does less harm to the environment and makes no harm to the connection between the electrodes of the solar cells and the semiconductor substrate. Further, the efficiency of solar cells keeps substantially unchanged.
Moreover, the conductive paste of the present invention may not only be used in solar cells, but also may be used in other electronic elements such as ceramic capacitors, semiconductor packaging, or printed circuit boards.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 098136633 | Oct 2009 | TW | national |
