What Is The Material Need To Prepare Solar Cell?

Solar cells, also known as photovoltaic cells, are devices that convert sunlight into electricity. They have become an increasingly important renewable energy source in recent years as the world looks to reduce reliance on fossil fuels. Understanding the key materials that go into solar cell production is important for improving efficiency and lowering costs.

Silicon

Silicon is the most common semiconductor material used in solar cells. It is abundant in the Earth’s crust as silicon dioxide or silica, making up about 28% of the crust’s mass. Silicon can be extracted and purified into the crystalline form needed for solar cells.

The use of silicon offers some key advantages for photovoltaics. It has a band gap of 1.1 eV, which is well-suited for absorbing visible light from the sun. Silicon solar cells typically have efficiencies around 15-20%. Silicon also has a high melting point (1414°C), giving it desirable stability and durability in solar modules. In addition, the technology for manufacturing silicon devices is highly advanced due to the extensive use of silicon in microelectronics.

Silicon has an indirect band gap, meaning it is a relatively poor absorber of light. This requires silicon solar cells to be manufactured in the form of thin wafers to allow sunlight to be absorbed before escaping out the sides. Nonetheless, silicon remains the dominant material for solar photovoltaics today.

Purification

diagram showing silicon purification methods
Silicon used in solar cells needs to be extremely pure, otherwise impurities will negatively impact the cell’s efficiency. There are several methods used for purifying metallurgical grade silicon (98-99% pure) into solar grade silicon (>99.999% pure):

Chemical Vapor Deposition (CVD) – CVD is one of the most common methods. Silane gas (SiH4) and hydrogen are heated to 1100°C, causing the silane to decompose and deposit pure silicon onto silicon rods. This produces polysilicon with 99.999% purity.

Czochralski Process – In this process, polycrystalline silicon is melted in a quartz crucible at just above its melting point of 1410°C. A tiny single crystal silicon seed is dipped into the melt and slowly extracted while being rotated. As it is extracted, liquid silicon solidifies onto the seed creating a large single crystal ingot up to 2 meters in length. This produces incredibly pure monocrystalline silicon.

Float Zone Refining – A polycrystalline silicon rod is passed through a heated coil causing a small section to melt. As the rod moves through the coil, impurities migrate to the molten zone. The impurities can then be removed by slicing off the ends of the rod, leaving ultra-pure silicon in the middle. This is repeated to achieve higher purities.

Doping

Doping is a critical step in creating solar cells. Silicon in its pure form is not very conductive. In order to make it conductive enough for use in solar cells, other atoms need to be intentionally added through a process called doping. The two most common dopants used in silicon solar cells are boron and phosphorus.

Boron atoms have one less electron than silicon. When boron is introduced into the silicon lattice, the missing electrons create “holes” that allow electricity to flow more freely through the material, making it positively charged. This is known as a p-type semiconductor.

Phosphorus atoms have one more electron than silicon. When phosphorus is added, the extra free electrons allow electricity to flow, making the silicon negatively charged, or n-type.

By carefully controlling the level of doping, both p-type and n-type silicon can be tailored to the optimal properties needed for converting sunlight into electricity in solar cells. The p-n junction created by joining positively doped and negatively doped silicon is crucial to the photovoltaic effect that generates electrical current when exposed to sunlight. Proper doping levels allows for free flow of electrons and holes while minimizing energy loss.

Wafer fabrication

Once the silicon has been purified and doped, it must be turned into wafers to make solar cells. Wafer fabrication is an important step in creating the base structure of the solar cell.

In wafer fabrication, the purified and doped silicon material is first melted down and formed into cylindrical ingots. These cylindrical silicon ingots are then sliced into very thin wafers using wire saws. The thickness of the wafers is typically between 150-300 micrometers.

It’s crucial that the wafer slices are cut as thin as possible while maintaining structural integrity. Thinner wafers help minimize costs by enabling more wafers to be cut from an ingot. Thinner wafers also require less silicon material and allow more sunlight to pass through to the interior layers of the cell.

However, thinner wafers can be more fragile and prone to cracking or breaking. There is a balance between making the wafers as thin as possible for efficiency and cost purposes while still being thick enough to maintain strength.

The wafer slices are then polished through mechanical grinding and chemical etching to achieve a mirror-smooth surface. This polishing ensures optimal performance by enabling maximum sunlight absorption and efficient electrical flow once the metal contacts are added in later steps.

The end result of wafer fabrication is a key building block of the solar cell – a highly pure, doped silicon wafer ready for further processing.

Surface Texturing

An important step in solar cell fabrication is texturing the surface of the silicon wafer to reduce light reflection. Smooth silicon wafers can reflect over 30% of incoming sunlight, reducing the amount of light absorbed and converted into electricity.

Texturing refers to creating tiny pyramid-shaped structures on the wafer surface using chemical etching. When light strikes these microscopically small pyramids, it gets refracted at different angles. This increases the likelihood of the sunlight entering the wafer rather than bouncing off the surface.

Reducing reflected light helps more photons enter the solar cell material, where they can excite electrons and generate current. Texturing enables solar cells to absorb close to 96% of incident sunlight versus 70% for an untextured cell. It provides one of the simplest and most effective ways to boost efficiency.

Common texturing techniques involve using alkaline solutions like potassium hydroxide or sodium hydroxide to anisotropically etch the silicon wafer. The etching rate depends on crystal orientation, creating angled surfaces and pyramids on the micron scale. Texturing adds minimal cost while providing substantial efficiency gains, making it a standard process in solar cell manufacturing.

Emitter Diffusion

Emitter diffusion is a crucial step in creating the p-n junction in a solar cell. This process involves depositing phosphorus onto the front surface of the silicon wafer to create an n-type layer. The phosphorus is applied by exposing the wafer to phosphine gas (PH3) at high temperatures of 800-1000°C. The high heat causes the phosphorus atoms to diffuse into the wafer up to a depth of 0.3-0.5 μm.

The n-type emitter layer serves several important functions:

  • Forms a p-n junction with the p-type silicon base.
  • Creates an electric field across the junction that sweeps photon-generated electrons to the front contacts.
  • Reduces recombination at the front surface.
  • Improves electrical contact with metal contacts.

Precise control over the temperature, gas concentration and exposure time is necessary to achieve the optimal emitter sheet resistance of 60-100 Ω/sq. Optical reflectance measurements and 4-point probe measurements are used to monitor and determine when the diffusion process is complete. The result is a thin but highly doped n-type emitter layer on the front surface.

Antireflective coating

Solar cells can lose a significant amount of light energy due to the reflective nature of the semiconductor surface. When light hits a reflective surface like silicon, some of it bounces back off instead of entering the cell. This reflective loss can account for over 30% of potential energy generation. To address this, solar cell manufacturers apply an antireflective coating to the cells.

Antireflective coatings are made from materials that have an index of refraction between air and silicon. This allows more light to transmit through the coating and into the cell, rather than reflecting off the surface. Common antireflective coatings for silicon solar cells include silicon nitride, titanium dioxide, silicon oxide, and aluminum oxide. These coatings can reduce reflection down to around 1-2%, dramatically increasing the amount of light entering the cell.

Applying just a single layer antireflective coating can cut reflective losses in half. Using multiple layers or a “stack” of coatings can minimize reflection even further. The coatings are deposited using chemical vapor deposition, sputtering, or other techniques. The thickness and composition of the coating layers are precision-engineered to maximize transmission across the solar spectrum.

Metallization

Metallization is the process of adding metal contacts to the solar cell. This allows current generated by the solar cell to flow out of the cell and be utilized. The metallization pattern on the front of the solar cell includes thin “fingers” to collect current and busbars to aggregate current from the fingers. On the rear of the cell, full area metallization is typically applied.

Screen printing is commonly used to apply metallization paste containing silver, aluminum or copper in precise patterns onto the solar cell wafer. The paste is then dried and fired at high temperature to sinter the metal particles, forming conductive metal electrodes. A plating step may follow to build up and improve conductivity. Photolithography is another metallization technique that can produce finer line resolution.

The front contact metallization must cover as little of the cell surface as possible to minimize shading, while still effectively collecting current. The rear contact metallization must provide low resistance over the entire back surface. Careful design of the metallization pattern is needed to optimize efficiency and reliability.

Conclusion

In summary, solar cells are made from purified and doped silicon that is processed into wafers. The wafers are textured, coated, and metallized before being interconnected into modules. The key materials needed are high purity silicon, dopant atoms, anti-reflective coatings, and conductive metals for contacts. Manufacturing solar cells is a complex process requiring substantial investments in facilities and equipment.

Looking ahead, the solar industry aims to lower costs and increase efficiencies further. This could be achieved through new materials like perovskites, improved manufacturing techniques like thin film deposition, and novel solar cell designs. With the push for renewable energy to combat climate change, solar power will likely continue growing its share of electricity generation around the world.

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