Photovoltaic cells are the basic element of photovoltaic panels. They are semiconductor devices that convert solar energy into electricity. They are generally made from materials such as crystalline silicon (monocrystalline or polycrystalline), amorphous silicon, cadmium telluride or copper indium gallium diselenide. The cells are assembled to form solar panels, which are then integrated into systems to generate renewable electricity.
At a glance: photovoltaic cell types and innovations
There are essentially 4 types of photovoltaic cells:
- Traditional” monocrystalline or polycrystalline silicon cells : These cells are made from silicon wafers cut from monocrystalline or polycrystalline silicon ingots.
- Thin-film cells: These are manufactured by depositing very thin layers of semiconductor materials, such as cadmium telluride (CdTe), copper indium diselenide (CIS) or copper indium gallium diselenide (CIGS), on a substrate. Thin-film cells generally have lower energy efficiency than silicon cells, but are more flexible, lighter and less expensive to produce.
- Heterojunction cells: These cells combine amorphous and crystalline silicon layers to create a solar cell with greater energy efficiency. They offer advantages such as higher open-circuit voltages and better temperature tolerance than traditional silicon cells.
- Multi-junction cells: These cells use several layers of semiconductor materials with different band gaps to capture a greater proportion of the solar spectrum. They are mainly used in space and concentrated photovoltaic (CPV) applications due to their high energy efficiency, but they are more expensive to produce.
Generic components
All photovoltaic cells are relatively similar in several respects:
- Anti-reflective top layer: This layer is applied to the surface of the cell to reduce light reflection and enable better absorption of solar energy.
- Front contact: This is a metal grid deposited on the surface of the cell. It collects the electrons generated by the photovoltaic effect and transfers them to the outside of the cell. The grid is designed to minimize the surface area covered in order to maximize light exposure.
- {This is generally where the “heart” of the cell is located. Example: the N and P semiconductor layers and their junction for traditional cells}
- Back contact : This is a metal layer on the back of the cell that collects electrons and transfers them to the outside of the cell. The back contact generally covers the entire rear surface of the cell to ensure good electrical conductivity.
- Back layer: This layer protects the cell from mechanical and environmental damage, and can also act as a moisture barrier to prevent corrosion.
- Encapsulation: Photovoltaic cells are generally encapsulated between layers of protective materials, such as EVA (ethylene vinyl acetate) or PVB (polyvinyl butyral), to protect the cells from moisture, dust and mechanical impact.
The composition of mono or polycrystalline photovoltaic cells
A photovoltaic cell is made up of several layers and elements that work together to convert sunlight into electricity. Here are the main components of a photovoltaic cell:
- N-type semiconductor: This is a layer of semiconductor material that has been doped with impurities to give an excess of electrons. In silicon cells, the silicon is usually doped with phosphorus to form the n-type layer.
- P-n junction: The p-n junction is formed at the interface between the n-type and p-type semiconductor layers. It is at this junction that the photovoltaic effect occurs, generating an electrical voltage when exposed to light.
- P-type semiconductor: This is a layer of semiconductor material that has been doped with impurities to create a deficit of electrons (or an excess of holes). In silicon cells, the silicon is usually doped with boron to form the p-type layer.
The difference between monocrystalline and polycrystalline cells lies in the design of the silicon wafer. In the former case, it’s designed from a silicon ingot using the Czochralski process, and in the latter case with the directed solidification method. In short, the second method is less wasteful and less costly, but delivers lower-quality results.
PERC technology, specific to traditional panels, adds a passivating dielectric layer to the back of the cell to reduce charge carrier recombination and increase energy efficiency.
Multi-junction cells
Multi-junction cells, also known as multi-layer cells or tandem cells, are advanced photovoltaic cells that use multiple junctions and semiconductor materials to capture and convert different parts of the solar spectrum. The basic structure of a multi-junction cell comprises several sub-cells stacked on top of each other, each sensitive to a specific wavelength range of sunlight. Here’s a detailed description of the key components of a multi-junction cell:
- Subcells : Subcells are the key elements of multi-function cells. Each sub-cell is made of a specific semiconductor material that absorbs a particular wavelength range of the solar spectrum. The sub-cells are stacked one on top of the other so that the top sub-cell captures the shorter, more energetic wavelengths, while the lower sub-cells progressively capture the longer, less energetic wavelengths.
- Tunnel junctions : Tunnel junctions are thin conductive layers that electrically connect adjacent sub-cells. They allow charge carriers (electrons and holes) to pass between sub-cells without significant energy loss, thus ensuring high conversion efficiency.
Multi-junction cells are often made from materials based on III-V compounds, such as gallium arsenide (GaAs), indium phosphide (InP) or gallium indium phosphide (GaInP). These materials have advantageous optical and electronic properties, enabling very high efficiencies to be achieved, sometimes in excess of 40% under laboratory conditions.
However, multi-junction cells are more complex and costly to produce than traditional silicon photovoltaic cells. As a result, their use is mainly limited to concentrated photovoltaic (CPV) applications, where sunlight is concentrated on the cells using lenses or mirrors to increase incident power and maximize conversion efficiency.
Heterojunction cells
Heterojunction cells are photovoltaic cells that combine different types of semiconductor materials to improve performance and energy efficiency. In the case of silicon heterojunction cells, a thin layer of amorphous silicon is deposited on a crystalline silicon substrate (monocrystalline or polycrystalline). Here’s a detailed description of the key components of a heterojunction cell:
- Crystalline silicon substrate: The crystalline silicon substrate is the basis of the cell, and is responsible for converting much of the sun’s energy into electricity. It can be monocrystalline or polycrystalline.
- Amorphous silicon layers (a-Si): Amorphous silicon layers are deposited on either side of the crystalline silicon substrate. They have several functions, including passivation of defects on the crystalline silicon surface, improved charge separation and heterojunction formation. Amorphous silicon has a wide bandgap, enabling better defect management and improved energy conversion efficiency.
- Doping layers : Doping layers, generally p-type (positive) and n-type (negative), are added to either side of the crystalline silicon substrate, beneath the amorphous silicon layers. They create an internal electric field that facilitates the separation of charge carriers and the flow of electric current.
Heterojunction cells offer several advantages over traditional silicon photovoltaic cells, including better defect management, higher energy conversion efficiency and lower temperature sensitivity. However, their manufacture can be more complex and costly due to the need to deposit layers of amorphous silicon and other materials on the crystalline silicon substrate.
Thin-film cells
Thin-film cells are photovoltaic cells composed of semiconductor materials deposited as ultra-thin layers on a substrate, often made of glass, plastic or metal. Here’s a detailed description of the key components of a thin-film cell:
- Substrate: The substrate is the support on which the thin films are deposited. It can be made of glass, plastic or metal, and serves as a solid base for the photovoltaic cell.
- Semiconductor layers: Semiconductor layers are deposited on the substrate. They are responsible for converting solar energy into electricity. Semiconductor materials commonly used for thin-film cells include cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) and amorphous silicon (a-Si).
- Doping layers: Doping layers, usually p-type (positive) and n-type (negative), are added to semiconductor layers to create an internal electric field that facilitates charge carrier separation and electric current flow.
Thin-film cells offer several advantages over traditional silicon photovoltaic cells, including lower manufacturing cost, better low-light performance and less performance degradation at high temperatures. However, the energy conversion efficiency of thin-film cells is generally lower than that of crystalline silicon cells.
Innovations in photovoltaic cells
In addition to PERC cells, which are specific to traditional cells, there are several innovations concerning photovoltaic cells in general:
- Half-cells (or half-cut cells)
- Bifacial panels
- Tiling Ribbon Technology
Photovoltaic half-cells
Half-cells are a recent innovation designed to improve the performance and efficiency of solar panels. This technology divides a standard photovoltaic cell into two equal parts, reducing internal resistance and power losses due to partial shading. The half-cells are then assembled into solar panels in a similar way to whole cells.
The appeal of half-cells lies in their ability to minimize energy losses and improve the overall performance of solar panels. By reducing internal resistance and shading losses, half-cells enable better energy production, especially in less-than-ideal lighting conditions. What’s more, solar panels equipped with half-cells offer greater temperature tolerance and reliability, making them attractive to homeowners and investors in solar energy.
Half-cell technology is mainly applied to crystalline silicon photovoltaic cells, such as monocrystalline and polycrystalline cells. However, it is not generally used for thin-film, heterojunction or multi-junction cells, due to differences in the structure and manufacture of these cell types.
Bifacial panels
Bifacial solar panels are capable of capturing and converting sunlight on both sides of the cell, thereby increasing their energy output. Bifacial cells are generally made of crystalline silicon and benefit from specific technologies, such as PERC or heterojunction cells, to optimize their efficiency. On the contrary, they are not very compatible with thin-film and multi-junction cells. Bifacial panels offer superior energy production and better performance in low-light or partially shaded conditions.
Tiling Ribbon Technology
Tiling Ribbon Technology aims to eliminate gaps between photovoltaic cells by using conductive ribbons soldered directly onto the cells, thus reducing power losses caused by electrical resistance. This technology is particularly suitable for crystalline silicon cells, such as monocrystalline and polycrystalline cells. It offers higher efficiency, reduced shading losses and improved aesthetics due to the absence of visible soldering. On the other hand, it is less advantageous for thin-film and multi-junction cells, due to differences in their structure and manufacture.
Back-contact cells
In front-contact cells, the electrical contacts are mainly located on the front of the cell (the side exposed to sunlight). These contacts, often in the form of fine grids or metal fingers, collect the electrons generated when sunlight is absorbed. However, this arrangement has one drawback: the metal contacts on the front side block part of the sunlight, reducing the cell’s overall efficiency.
To overcome this problem, cells with rear contacts were developed. In this design, all electrical contacts are placed on the rear face of the cell, thus eliminating efficiency losses due to shading by metal contacts on the front face. As a result, the front surface of the cell is completely dedicated to absorbing sunlight, improving energy efficiency. Rear-contact cells require more complex manufacturing techniques, but generally offer better performance than front-contact cells.