Solar cells are the core element of photovoltaic panels: this is where electricity is generated by the photovoltaic effect. Thin-film cells are characterized by reduced thickness and better performance in low-light conditions. Manufactured from semiconductor materials such as cadmium telluride (CdTe), copper-indium-gallium diselenide (CIGS) or amorphous silicon (a-Si), they offer flexibility, lightness and aesthetics. They enable the development of flexible solar modules.
However, their energy efficiency is generally lower than that of crystalline silicon cells. What’s more, although they are less prone to light-induced degradation, their lifespan may also be slightly shorter. Finally, thin-film cells are less compatible with certain innovations, such as bifacial panels, cut cells and Tiling Ribbon Technology, due to their structural and manufacturing differences.
Structure of thin-film solar cells
Thin-film solar cells are composed of several layers of semiconductor material deposited on a substrate. Here’s a logical presentation of the composition of a thin-film cell:
- Substrate : The substrate is the base on which the other layers are deposited. It can be made of a variety of materials, such as glass, plastic or metal, depending on the application and desired properties.
- Insulation barrier layer: This layer is deposited on the substrate to prevent unwanted interactions between the substrate and the semiconductor layers, and to ensure good adhesion of the deposited materials.
- Back contact layer : The back contact layer is deposited on the insulation barrier and serves as the electrical connection for the negative side of the solar cell. It usually consists of a conductive material such as aluminum or molybdenum.
- Semiconductor layers: Semiconductor layers are deposited on the back contact layer and form the active part of the solar cell. They are generally made of thin-film materials such as cadmium telluride (CdTe), copper-indium-gallium diselenide (CIGS) or amorphous silicon (a-Si). These materials have different light absorption properties, enabling them to convert sunlight into electricity.
- Front contact layer: The front contact layer is deposited on top of the semiconductor layers and serves as the electrical connection for the positive side of the solar cell. It is usually transparent and conductive, based on indium tin oxide (ITO) or other transparent conductive materials.
- Anti-reflective coating: An anti-reflective coating is applied to the front contact layer to reduce sunlight reflections and improve light absorption by the semiconductor layers. This layer optimizes solar cell performance.
In summary, the composition of a thin-film cell follows a logical structure, starting with the substrate, the insulation barrier, the back and front contacts, the semiconductor layers and finally the anti-reflective coating.
The materials
The materials used for thin-film solar cells vary according to cell type. Here are some of the most commonly used materials:
- Cadmium telluride (CdTe): CdTe is a semiconductor material used to manufacture thin-film solar cells. It has an excellent light absorption coefficient and a relatively low production cost.
- Copper indium gallium diselenide (CIGS) : CIGS solar cells are manufactured by depositing a thin layer of semiconductor materials composed of copper, indium, gallium and selenium. CIGS cells offer good energy efficiency and better tolerance of variable environmental conditions.
- Amorphous silicon (a-Si): Amorphous silicon is a non-crystalline form of silicon used to manufacture thin-film solar cells. A-Si cells are less energy-efficient than crystalline silicon cells, but are less expensive to produce and more flexible.
- Indium tin oxide (ITO): ITO is a transparent conductive material used for the front contact layer in thin-film solar cells. It transmits light while providing an electrical connection to the positive side of the cell.
Other materials are also used in the manufacture of thin-film cells, such as glass, plastic or metal for the substrate, as well as conductive materials for the contact layers, such as aluminum or molybdenum.
Advantages and disadvantages of thin-film cells
Thin-film cells offer several advantages and disadvantages over other photovoltaic cell technologies, such as crystalline silicon cells.
Advantages
- Flexibility : thin-film cells can be manufactured on flexible substrates, enabling the creation of foldable, flexible solar panels.
- Reduced weight: Thin-film cells are generally lighter than crystalline silicon cells, making them easier to transport and install.
- Aesthetics : Thin-film cells have a more homogeneous appearance and can be produced in different colors, making them more aesthetically appealing.
- Low-light performance: Thin-film cells outperform crystalline silicon cells in low-light conditions and partial shading.
- Less light-induced degradation: Thin-film cells are generally less prone to light-induced degradation, which can extend their lifespan.
Disadvantages
- Lower efficiency: Thin-film cells generally have lower energy efficiency than crystalline silicon cells, which means they produce less electricity per unit area exposed to the sun.
- Larger surface area : Due to their lower efficiency, thin-film cells generally require a larger surface area to produce the same amount of energy as crystalline silicon cells, which can be a problem in confined spaces.
- Lifespan : Thin-film cells may have a slightly shorter lifespan than crystalline silicon cells, due to their sensitivity to certain degradation factors.
- System cost: Although thin-film cells can be cheaper to produce, the overall cost of a photovoltaic system based on thin-film cells can be higher due to the need to use more surface area and other components to achieve equivalent energy output.
- Compatibility limitations : Thin-film cells are less compatible with certain innovations such as bifacial panels, cut cells and Tiling Ribbon Technology due to their structural and manufacturing differences.
Compatibility with other innovations
Thin-film cells have structural and manufacturing differences from crystalline silicon cells, which may limit their compatibility with certain innovations such as bifacial panels, cut cells and Tiling Ribbon Technology.
- Bifacial panels: thin-film cells absorb light more uniformly across their entire surface, making the bifacial effect less beneficial. In addition, their structure and manufacturing method are not ideal for allowing light transmission through the back face of the panel.
- Cut cells: Thin-film cells are generally less prone to internal resistance problems and shading losses than crystalline silicon cells, making the benefits of cut cells less relevant to this technology.
- Tiling Ribbon Technology: Thin-film cells are manufactured by depositing semiconductor materials directly onto a substrate, making them less compatible with Tiling Ribbon Technology. This technology is mainly used for crystalline silicon cells, which are manufactured by cutting silicon wafers.
However, this doesn’t mean that innovations couldn’t be developed specifically for thin-film cells in the future.