Polycrystalline cells are a silicon-based photovoltaic cell technology. They are made from blocks of molten silicon that are then slowly cooled, forming several crystals. The cells have a bluish appearance and a snowflake-like pattern due to the multiple crystals. Polycrystalline cells offer a compromise between cost and efficiency, generally costing less than monocrystalline cells but with slightly lower efficiency. Photovoltaic panels using them are generally between 15% and 18% efficient, although technological advances have made it possible to achieve higher values.
The structure of a polycrystalline photovoltaic cell
The structure of a polycrystalline photovoltaic cell is similar to that of a monocrystalline cell, with a few minor differences due to the nature of the base material. Here’s the structure of a polycrystalline cell, from front to back:
- Anti-reflective layer: A thin anti-reflective layer, usually made of silicon oxide or silicon nitride, is deposited on the front surface of the cell. This layer reduces the reflection of sunlight and increases light absorption in the cell.
- Metal grid: Metal contacts in the form of a grid are deposited on the front of the cell to collect electrons generated by sunlight. The grid is designed to minimize the surface area covered, allowing more light to reach the cell’s active surface.
- Emitter layer (P zone): The emitter layer is made of P-doped polycrystalline silicon, which generates holes when photons strike the cell surface.
- P-N junction: The P-N junction is where the P-doped emitter layer meets the N-doped base layer. This junction creates an electric field that separates electrons and holes, promoting current flow.
- Base layer (N zone) : The base layer is made of N-doped polycrystalline silicon, and generates electrons when exposed to sunlight. This layer contains the polycrystalline silicon wafers.
- Rear contacts: Metallic contacts are deposited at the rear of the cell to collect electrons and holes and route them to the external electrical circuit.
- Back encapsulation: A protective layer, usually made of glass, plastic or a composite material, is applied to the back of the cell to protect it from mechanical stress and environmental conditions.
Polycrystalline photovoltaic cell materials
Polycrystalline cells, also known as multicrystalline cells, are mainly composed of polycrystalline silicon. Silicon is an abundant element, widely used in the photovoltaic industry for its semiconducting properties.
Polycrystalline silicon is obtained by melting pure silicon in a crucible and allowing it to cool slowly to form a solid block of silicon with several crystals. This block is then cut into thin wafers, which serve as the basis for polycrystalline solar cells.
Polycrystalline silicon wafers are distinguished by their blue-gray appearance and granular structure, resulting from the presence of multiple crystals. Unlike monocrystalline silicon, polycrystalline silicon has a less ordered crystalline structure, resulting in a slightly lower energy efficiency for polycrystalline solar cells compared to their monocrystalline counterparts. However, polycrystalline cells are generally less costly to produce, making them an economical option for photovoltaic applications.
Advantages and disadvantages compared with monocrystalline cells
Polycrystalline cells offer lower cost and good durability, but have lower energy efficiency, greater temperature sensitivity and a less aesthetic appearance than monocrystalline cells.
Advantages :
- Cost: Polycrystalline cells are generally cheaper to produce than monocrystalline cells, due to a simpler manufacturing process and less material waste.
- Durability: Polycrystalline cells have a robust structure that makes them resistant to mechanical stress and harsh environmental conditions.
Disadvantages :
- Efficiency: Polycrystalline cells have a lower energy efficiency than monocrystalline cells, typically between 15% and 18%. This is due to the less ordered crystalline structure and grain boundaries in the material.
- Temperature sensitivity: Polycrystalline cells are more sensitive to temperature than monocrystalline cells, which can lead to a reduction in efficiency in high-temperature conditions.
- Aesthetics: Polycrystalline cells have a less uniform appearance than monocrystalline cells, with a light blue color and visible crystalline structure on the surface.
Comparison with other photovoltaic cell technologies
Polycrystalline cells have a number of distinctive features compared with their competitors: heterojunction, thin-film and multi-junction.
- Heterojunction cells combine two different semiconductor materials to improve performance and efficiency. They are more efficient than polycrystalline cells, but can be more expensive to manufacture.
- Thin-film cells are lighter and more flexible than polycrystalline cells, but generally have lower efficiency. They are also easier to integrate into architectural applications.
- Multi-junction cells employ multiple junctions to capture different wavelengths of light, offering very high efficiency. However, they are much more expensive to produce, and are mainly used in high-efficiency applications such as aerospace.
Technical innovations
Bifacial panels, cut cells, PERC technology and Tiling Ribbon Technology can be combined with polycrystalline cells to improve performance and efficiency. Here’s what each of these technologies brings to polycrystalline cells:
- Bifacial panels enable polycrystalline cells to capture sunlight from both sides of the panel, increasing energy production and overall system efficiency.
- Cut cells: Polycrystalline cells can be split into half or quarter cells, reducing internal resistance and shading losses. This translates into higher efficiency and greater tolerance to shading.
- PERC (Passivated Emitter Rear Contact)technology: The addition of a passivating layer to the rear of polycrystalline cells allows unabsorbed photons to be reflected back into the cell for another chance at absorption. This increases the efficiency of polycrystalline cells.
- Tiling Ribbon Technology: This technology uses flat, solderless conductive ribbons to connect polycrystalline cells together, reducing efficiency losses due to soldering and increasing the active surface area of the cells. Panels with this technology can benefit from increased efficiency and improved aesthetics.
By combining these technologies with polycrystalline cells, it is possible to optimize their performance, improve energy efficiency and reduce losses. This leads to higher energy production and economic benefits for polycrystalline solar panel users.