Use of Silver and Aluminum paste in Solar Panels
Silver and aluminum pastes are essential components in the manufacture of solar panels.
They are highly conductive pastes used to create ohmic contact between the upper and lower parts of the solar cells. The silver paste is often used on the front side facing the sun, while the aluminum paste is used on the rear side.
What does silver and aluminum paste contain?
Silver paste often constitutes glass frit, organic carriers, and metal particles such as silver nanoparticles. The glass frit mainly consists of silicon dioxide, lead oxide, titanium dioxide, boron oxide, and zinc oxide.
On the other hand, aluminum paste usually contains metal powders (aluminum), glasses, and additives mixed in an organic medium.
To understand the use of silver and aluminum paste in solar panels, you need to understand the manufacturing process. Let’s have a brief look at the process.
Understand Solar Panel Manufacturing
1. Acquiring the raw material – Most solar panels use silicon. Quartz sand, which contains silicon, is collected and processed in a furnace to produce high-purity silicon.
2. Solid silicon rocks are melted at high temperatures to produce ingots. Boron or phosphorus is added during the process to create p-type or n-type silicon cell types, respectively.
3. One silicon fragment forms a monocrystalline solar cell. On the other hand, melting many silicon fragments together produces polycrystalline cells.
4. The silicon ingot is sliced into thin and often disk-shaped wafers. Most manufacturers add an anti-reflective coating to improve uptake of sunlight. Metallic conductors are added to the surface of each wafer. Doping with phosphorous and boron helps create a p-n junction.
5. A grid-like metal contact, usually made of silver paste, is screen-printed onto the solar cells’ front surface. Additionally, the tabs of the cells are also screen-printed with silver paste for the interconnection to other cells through soldering.
A whole area or grid pattern metal contact is printed on the back surface of the solar cells using aluminum paste. The metal contact forms the aluminum back surface field (BSF). The second print of Aluminum/silver paste is fired for “solder-able contact” in some cases.
6. The cells are soldered together to form a solar panel.
7. A glass is placed on top to protect the front side of solar cells. A durable back-sheet, often made of polymer material, is also fixed to cover/protect the bottom side of the solar cells.
8. The junction box is attached at the backside to enclose and protect wire connections.
9. Finally, the frame is attached, and tests are done for functionality and efficiency.
Why Silver and Aluminum paste used in Solar Panels?
When producing silicon solar cells, it is essential to create good ohmic contact (non-rectifying electrical junction) between the silicon metal and the emitter. However, the contact material used must have a minimum contact resistance to attain the best electrical performance.
In the commercial production of solar cells, silver paste forms the contact that creates the collector gridlines and silver busbars. The front-side silver paste acts as an anode. An electrode through which a positive-to-negative current enters into an electrical device that is polarized (the p–n junction).
On the other hand, the rear-side aluminum paste acts as a cathode – an electrode from which a conventional (positive-to-negative) positive current leaves a polarized electrical device.
Is this getting too technical? Let’s simplify it. All we mean is, the front-side silver paste takes in power generated by the solar cell. On the other hand, the rear-side aluminum paste transfers the collected energy to the outlet system for storage or injection to the grid.
What is screen printing technology?
One of the most critical steps for fabricating solar panels is developing crystalline silicon solar cells and forming their contacts. The silicon solar cells are produced by creating a grid of fine conductive circuit lines on the front and rear sides of the wafer.
The circuit lines conduct the light-generated electrons away from the solar cell. Commercial-scale manufacturers of solar panels achieve this metallization process through screen printing technology. So, what is screen printing?
A conductive paste passes through the openings of a screen onto a wafer. This process forms the contacts or circuits on solar cells.
Usually, there are two separate screen printing steps for the front side (contact lines and busbars) and the backside (contact and busbars) of the solar cell. The process is straightforward but done at high precision.
1. The silicon wafer is placed on the printing table.
2. A print screen with a fine mesh, supported by a frame, is placed on the silicon wafer. The screen covers some areas and leaves other areas open where the paste will go through. The screen used to print the front side has a finer mesh than the screen for printing the backside. The front side requires fine metal lines to enhance receptivity.
3. After a controlled amount of paste is placed on the screen, a squeegee distributes it on the print screen to fill the open spaces uniformly. Silver paste is the most commonly used conductive paste for the front-side contact.
4. The squeegee pushes the paste through the open screen spaces until they reach the wafer surface.
Most manufacturers screen print the backside first and then flip the wafers to place the front side circuits. This method minimizes the potential damage that may occur during handling.
For this process to be successful, the operator must tightly control several variables, including temperature, machine speed, pressure, size of the squeegee, and the mesh, among others.
Why is silver paste and aluminum paste used in Solar Panels?
Silver is used in solar panels because it has a very high thermal and electrical conductivity than most metals.
On the other hand, an aluminum paste is used in solar panels due to its conductivity, strength (corrosion resistivity), and machinability. However, more characteristics are associated with these metals that make them a favorite in the solar industry. They include:
|Characteristics of front-side Silver paste||Characteristics of rear-side Aluminum paste|
|It has low contact resistance when used on a lightly doped emitter.||Aluminum powder can be mixed with silver paste to reduce contact resistance.|
|High capacity of adhesion which is greater than 3N/mm||It has excellent adhesion quality|
|High compatibility with Silicon wafer.||It is compatible with backside silver paste and mono and multi-crystalline wafers. Compatible with front side and backside silver pastes|
|High compatible with aluminum paste||Compatible with front side and backside silver pastes|
|It has higher efficiency and a wider processing window||It has a wide process window when used with front-side silver paste|
|Great bending resistance||Forms an excellent back surface field|
How is silver paste manufactured?
You may be wondering, how is low temperature curing silver paste with high conductivity and adhesion prepared? Well, there are many methods for preparing silver paste for use in solar panels. They include but are not limited to:
The process includes a conductive phase, the resin binder phase, the addition of a solvent, and an auxiliary agent. Silver paste is commonly prepared using conductive silver powder in the conductive phase, m-phenylenediamine, epoxy or polymer resin, polyethylene glycol as a surfactant, and Terpineol as the adhesive.
Using silver nanoparticles synthesized by e-beam irradiation – The silver nanoparticles are mixed with glass frit and sintered at 5000c for one hour to produce a silver paste. However, one must check and tightly control some variables to get the best paste.
How is Aluminum paste manufactured?
There are several types of aluminum pastes produced for different applications. For this case, we will cover the production of aluminum paste with or without boron for silicon solar cells.
Aluminum powder and aluminum oxide are poured into a mortar and mixed using ethanol.
More ethanol is added, and the solution is transferred to a beaker, and sonication is applied. Terpineol and ethyl cellulose are added to the solution and stirred using an ultrasound homogenizer.
Finally, ethanol is evaporated using an evaporator to obtain a boron-free aluminum paste. To produce an aluminum paste with boron; boric acid is added to the solution and sonicated using a homogenizer before the introduction of Terpineol.
What amount of aluminum and silver paste is used in solar panels?
The amount of silver and aluminum paste used in solar cells differs and is determined by:
- The metal gridline design
- The metallization technology
- Mesh and grid properties
- Whether the aluminum paste forms a grid or whole area contact
- The structure of the solar cells
- The size of the solar cells
- The required thickness of the paste
Typical screen-printed contacts are usually 125-150 µm wide.
Are there any other materials that do the same function?
Yes, other materials can serve the same purpose as silver and aluminum paste.
However, most of those alternatives are not as effective as the silver and aluminum paste in their application. Some of the other options to silver and aluminum pastes include but are not limited to:
Aluminum-added silver paste
This paste has aluminum and silver paste. Some manufacturers use aluminum/silver paste as metallization for p+ emitter of n-type solar cells. In most cases, aluminum-added silver paste is used on the backside of the solar cells. Let’s expound on this paste.
The silver pastes needed to provide electrical contact on boron-doped emitters on p-type cells are different from those used for the emitters on n-type cells. This difference is due to the varying doping elements in the emitters and the various transport processes for current collection at the emitter.
The addition of aluminum material in silver paste enhances the functionality of silver paste in n-type cells. Studies show that adding aluminum powders to silver pastes for n-type cells can help lower the contact resistivity.
Experiments indicate that the contact resistivity for the frit-less silver pastes contacting the polished Si wafers and the PV wafers are higher than the corresponding frit-less silver pastes with aluminum.
However, adding the aluminum powder leads to higher surface recombination and increased line resistivity of the silver electrode patterns. The line resistivity may limit the efficiency of the solar cells.
Other studies show that the amount and particle size of the aluminum powder significantly affects the contact resistance and the electrical characteristics.
The details show that contact resistance decreases with an increase in the content of the aluminum powder in the silver/aluminum paste. It also decreases when the particle size of the aluminum powder increases.
While attempts have been made to replace silver paste with other metals, studies show that it is hard to achieve similar functionality without fundamentally changing the formulation of the paste or the firing conditions.
In what type of solar systems are silver and aluminum paste used?
Silver and aluminum paste is used in the manufacture of various types of solar panels commonly:
- Monocrystalline solar panel systems
- Polycrystalline solar panel systems
- Thin-film solar panel systems
Other Application of Silver Paste
Silver paste is used in other applications that:
- Require low electrical resistance
- Need a conductive adhesive
- Have temperature-sensitive substrates
- Require low sintering temperatures
- Where soldering temperature can be harmful to substrates
- Require conductive material with rapid curing
Manufacturers also use silver paste in the fabrication of:
- Printable circuit boards (PCB) – Can also be used to fix defective tiny conductors in the PCB
- Flexible Printable Circuit boards (FPCB)
- Light-emitting Diode LED and Organic LED (OLED)
- Radio Frequency Identification (RFID) systems
- Switchable smart film – to make busbars
- The inner coating of storage boxes of electronic devices
Can I extract aluminum and silver paste from solar panels?
It is possible to retrieve aluminum and silver paste. However, a home-based extraction may not make economic sense. Some solar panel manufacturing companies request customers to return solar panels that malfunction or exhausted their useful life.
This recycling is influenced by a potential shortage of raw materials and the need to adhere to laws and national and regional directives. They then use an industry-scale recovery process of the metals in the solar panels.
Manufacturers and recycling firms use recovered metals such as silicon, silver, and aluminum to manufacture other solar panels. The following are some of the methods manufactures, researchers, organized groups and consortiums, scientists, and professional and government institutions use to retrieve silver from solar panels:
1. Recovery of silver using solvent extraction with Trioctylphosphine Oxide (TOPO)
2. Electrochemical recovery of silver using an organic acid solution which acts as an electrolyte. It involves electrowinning of silver from crystalline silicon solar cells using methanesulfonic acid solution.
3. Recovery of silver using a mechanical/physical separation, leaching, and precipitation
4. Recovery of silver using a three-step chemical process constituting sulphuric acid, hydrochloric acid, nitric acid, or caustic soda.
5. Experimental recovery of pure silicon and aluminum using a chemical treatment
Silver and aluminum paste are vital components in the manufacture of solar panels. They provide the necessary conductive contacts on the front side and backside of the solar cell, respectively.
Solar panel manufacturers prefer silver paste due to its high thermal and electrical conductivity, good adhesion, and low contact resistivity.
On the other hand, aluminum is relatively cheaper, has high electrical conductivity, and has strong corrosion resistance. You can retrieve from broken or end-of-life solar panels, but it’s often done at an industrial scale to make economic sense.