What Is The Basic Construction Of Pv Cell?

A photovoltaic (PV) cell is a specialized semiconductor device that converts sunlight directly into electricity. Often referred to as solar cells, PV cells are the building blocks of photovoltaic solar panels. When sunlight (photons) strikes the cell, electrons are knocked loose from the atoms in the semiconductor material, creating electricity.

The first silicon PV cell was built in 1954 by scientists at Bell Labs. Since then, the technology has improved dramatically, with solar cells becoming steadily more efficient and cost-effective. Today, PV cells are used in a wide variety of applications, from small consumer electronics to utility-scale solar farms providing electricity to the grid.

PV cells provide a clean, renewable source of electricity from an abundant resource – the sun. When incorporated into solar panels and solar energy systems, PV cells are an important technology for generating sustainable power and reducing dependence on fossil fuels.

Semiconductor Materials

The most common semiconductor material used in PV cells is silicon. Silicon is abundant, non-toxic, and can be highly purified into a crystalline structure optimal for converting sunlight into electricity through the photovoltaic effect. The PV industry uses two main types of silicon – monocrystalline and polycrystalline.

Other semiconductor materials used in PV cells include:

• Gallium arsenide – Has a higher efficiency rate than silicon, but is more expensive. Mainly used for space applications.
• Cadmium telluride – Less expensive material that is easier to manufacture than silicon, but has a lower efficiency. Mainly used for thin-film PV panels.
• Copper indium gallium selenide – Can be fabricated into thin, flexible PV films with decent efficiency. Still an emerging PV technology.

Silicon remains the dominant material for commercial PV panels due to its abundant availability, optimized fabrication processes, and balance of efficiency and cost-effectiveness.

p-n Junction

The p-n junction is the fundamental building block of a photovoltaic (PV) cell. It consists of two layers of doped silicon, one layer doped with boron to create a p-type semiconductor, and another layer doped with phosphorus to create an n-type semiconductor. When these two layers are brought into contact, electrons diffuse from the n-type side to the p-type side, and holes diffuse from the p-type side to the n-type side. This diffusion of charge carriers creates a region at the junction called the depletion region where no free electrons or holes exist. The depletion region generates an electric field that causes electrons and holes to drift in opposite directions when energy is added to the system, creating a voltage difference between the two sides that can generate electricity.

The key aspects of the p-n junction are:

• Doped Silicon – Crystalline silicon is doped with specific impurities to produce p-type and n-type semiconductors with free charge carriers.
• p-type Silicon – Doped with boron, resulting in free hole charge carriers.
• n-type Silicon – Doped with phosphorus, resulting in free electron charge carriers.
• Electric Field – The diffusion of carriers across the junction creates a depletion region with a strong electric field.
• Depletion Region – The area near the junction where free carriers have diffused across, leaving behind ions that generate the electric field.

When light hits the PV cell, photons with sufficient energy can knock electrons loose in the depletion region, creating mobile electron-hole pairs that are swept by the electric field to generate electricity. The p-n junction design and properties are critical to enabling this photovoltaic effect in PV cells.

Front Electrical Contact

The front side of a PV cell has an electrical contact that collects the current generated by the solar cell. This contact is known as the front metal grid or front contact.

The front contact is made up of thin grid lines or fingers that run parallel to each other across the front surface of the solar cell. These grid lines are connected to larger busbars that consolidate all the current. The busbars run perpendicular to the fingers and allow the current to be collected from the entire surface.

The front contact grid must cover as little of the solar cell surface as possible, usually around 5-10%, so that light can enter the cell. At the same time, the grid must be thick and conductive enough to minimize electrical resistance. Finding the optimal balance between shade and resistance is key.

The front contact is typically made of a metal such as silver, aluminum or copper. Silver offers the highest conductivity but is expensive, while aluminum and copper provide a good balance of cost and performance.

By using a thin front contact grid with thick busbars, the electrical resistance is minimized. This improves the electrical efficiency of the solar module and prevents excessive power loss due to resistance heating.

Anti-Reflective Coating

The surface of silicon cells reflects a portion of the sunlight hitting it, so an anti-reflective coating is applied to the cells. This coating increases the amount of light absorbed and improves energy conversion efficiency.

Silicon nitride is commonly used as the anti-reflective coating for solar cells. It is a dielectric material that can reduce reflection of sunlight from 35% to 5%. The silicon nitride layer thickness is optimized to interfere destructively with the reflected sunlight rays, allowing more light to transmit into the cell.

By greatly reducing surface reflection, the silicon nitride coating enables the cell to absorb and convert more of the sun’s energy into electricity. It plays a key role in increasing the efficiency and performance of PV modules.

Back Electrical Contact

The back electrical contact in a photovoltaic (PV) cell is typically made from aluminum, molybdenum, or silver.

Aluminum is commonly used for its high conductivity and low cost. It can be easily applied through screen printing or physical vapor deposition.

Molybdenum offers improved electrical performance compared to aluminum. It has excellent chemical and thermal stability. However, molybdenum is more expensive.

Silver provides the highest conductivity. But it is rarely used due to its prohibitively high cost. Silver also tends to oxidize and diffuse into the silicon, degrading performance.

The back contact material is deposited as a thin layer over the entire back surface of the cell. It serves as a reflector to bounce unabsorbed light back into the cell for another pass through the semiconductor material. This helps increase efficiency.

The back contact metallization pattern is also designed to minimize resistive power losses and shadowing. Fine “fingers” transport current, while larger “busbars” consolidate it.

Encapsulant

The encapsulant is a thin layer of material that protects the solar cells. The most common encapsulant materials used in PV modules are ethylene vinyl acetate (EVA) and polyvinyl butyral (PVB).

The main purpose of the encapsulant is to provide protection against environmental factors like moisture, UV radiation, and mechanical stress. It fills the gaps between solar cells and helps hold them in place. The encapsulant wraps around the edges of the solar cells to prevent moisture ingress and corrosion.

Key properties that encapsulants must have include:

• Adhesion – Bonds to glass and cells
• Transparency – Allows light to pass through to cells
• Electrical isolation – Prevents shorts between cells
• Thermal conduction – Transfers heat away from cells
• Flexibility – Accommodates thermal expansion differences
• Durability – Withstands harsh environmental conditions

EVA is the dominant encapsulant due to its optimal combination of cost, performance, and reliability. PVB offers higher moisture resistance but is more expensive. The encapsulant plays a critical role in protecting the solar cells and ensuring the module’s long-term durability.

Backsheet

The backsheet is a protective covering that forms the back surface of a solar panel. It serves several important functions:

Weather protection – The backsheet protects the interior components of the solar panel from moisture, UV radiation, and physical impacts. It helps prevent corrosion and degradation over the panel’s lifetime.

Electrical isolation – The backsheet electrically isolates the backside of the solar cells and other internal components. This prevents electrical shorts and shock hazards.

Materials – Backsheets are commonly made from polymer films like Tedlar, Kynar, and other fluoropolymers. These materials are chosen for their durability, weather resistance, and insulating properties. Tedlar and Kynar films are thermoplastic PVDFs that provide excellent moisture and UV protection.

The backsheet may be a multilayer laminate with different functional layers. For example, a highly waterproof thermoplastic inner layer is sometimes combined with a protective fluoropolymer outer layer like Tedlar or Kynar.

Frame

The frame provides structural support and protection for the solar PV module. Most PV module frames are made from aluminum alloy, which offers high strength at low weight. Aluminum frames are also corrosion resistant.

The aluminum is typically anodized, which thickens the natural oxide layer on the surface of the metal. Anodizing improves corrosion and abrasion resistance. It also allows the metal to be dyed different colors if desired.

The dimensions and thickness of the frame depend on the size of the solar panel. Frames are designed to withstand wind, snow, and other structural loads over the lifetime of the solar module. Most PV module frames have a channel or lip for mounting the module to rails or racks.

Overall, the aluminum frame plays a key role in structurally supporting the solar cells, wiring, and other components. It protects the module from weathering and mechanical damage. And it provides a way to securely install the PV module for electricity generation.

Wiring

The wiring in a photovoltaic module connects the individual solar cells into a working unit. This wiring is typically made from copper, which is an excellent conductor of electricity and can handle the amperages produced by the PV module without overheating. Thin ribbons of copper are soldered onto the front and back contacts of the solar cells, interconnecting them in series and/or parallel depending on the module design and total number of cells.

The copper interconnects have a few important roles. They conduct the electricity produced by the solar cells out of the module. They also provide a path for electrical current to flow between the cells when they are connected together. Proper wiring is critical for extracting maximum power from the PV module and transmitting it to be used or stored. Careful wiring design minimizes resistive losses and cell mismatch within the module.

In addition to the thin interconnect ribbons between cells, larger copper wiring collects the current and carries it out of the module to the external electrical connections. This wiring is carefully engineered to minimize resistive losses and allow smooth flow of current at typical operating temperatures. The right balance must be struck between the cost of copper and its electrical performance.