Solar cells are the core element of photovoltaic panels: they are where electricity is generated by the photovoltaic effect. Heterojunction cells are a type of solar cell that combines different layers of semiconductor materials to improve photovoltaic conversion efficiency. They take their name from the presence of a “heterojunction”, which is an interface between two semiconductor materials with different band gaps.
The structure of a heterojunction photovoltaic cell
A heterojunction cell consists of several layers of semiconductor materials, each with specific properties to optimize photovoltaic conversion efficiency. Here’s the composition of a heterojunction cell, presented logically from top to bottom:
- Anti-reflective layer: This layer, generally made of silicon oxide, is applied to the cell surface to minimize light loss through reflection and increase the amount of light absorbed by the cell.
- Emitter layer (n-type): A thin layer of n-type doped amorphous silicon is deposited on the crystalline silicon substrate. It promotes charge carrier separation and reduces recombination losses.
- Heterojunction : This is the special feature of these cells. The interface between amorphous and crystalline silicon is the heterojunction, where materials with different band gaps meet. This junction improves charge separation and reduces recombination losses.
- Crystalline silicon substrate : The heart of the heterojunction cell is the crystalline silicon substrate (mono or poly), which absorbs the majority of incident light and generates electron-hole pairs to create an electric current.
- Collector layer (p-type): Another layer of amorphous silicon, this time p-type doped, is deposited on the crystalline silicon substrate. It also facilitates charge carrier separation and reduces recombination.
- Metallic contacts: Metallic contacts are deposited on the emitter and collector layers to enable collection of the electric current generated by the cell. These contacts are generally made of silver or aluminum.
- Back layer: Finally, a back layer is applied to protect the cell from environmental influences, improve light reflection inside the cell and facilitate heat dissipation. This layer can be made of various materials, such as glass or polymer films.
Materials for heterojunction cells
Heterojunction cells generally consist of a thin layer of amorphous silicon (a-Si) deposited on a crystalline silicon substrate (mono or poly), creating a heterojunction. Amorphous silicon layers improve interface quality and reduce charge carrier recombination, thus increasing cell efficiency.
Heterojunction cells mainly use the following semiconductor materials:
- Crystalline silicon: The main substrate of the heterojunction cell is crystalline silicon, which can be monocrystalline or polycrystalline. Crystalline silicon absorbs most of the incident light and generates electron-hole pairs to create an electric current.
- Amorphous silicon: The emitter (n-type) and collector (p-type) layers are made of doped amorphous silicon. Amorphous silicon is a non-crystalline semiconductor with optical and electrical properties that differ from those of crystalline silicon. Amorphous silicon layers promote charge carrier separation and reduce recombination losses.
- Silicon oxide: The anti-reflective layer, usually made of silicon oxide, is applied to the cell surface to minimize light loss by reflection and increase the amount of light absorbed by the cell.
- Contacts : Metallic contacts are deposited on the emitter and collector layers to enable collection of the electric current generated by the cell. These contacts are generally made of silver or aluminum.
- Back layer materials: The back layer protects the cell from environmental influences and facilitates heat dissipation. This layer can be made of a variety of materials, such as glass or polymer films.
Advantages and disadvantages of heterojunction cells
Heterojunction cells offer a number of advantages and disadvantages:
Advantages
- High efficiency: heterojunction cells are more efficient than conventional monocrystalline or polycrystalline silicon photovoltaic cells. They can achieve energy conversion efficiencies of over 25%, making them particularly attractive for high-efficiency applications.
- Good high-temperature performance: heterojunction cells have a lower temperature coefficient than crystalline silicon cells, which means they retain better energy efficiency at higher temperatures. This makes them suitable for hot environments and applications where cell temperature is a critical factor.
- Low light-induced degradation (LID): Heterojunction cells are less sensitive to light-induced degradation than crystalline silicon cells, enabling them to maintain better performance over time.
Disadvantages
- Cost and manufacturing complexity: The manufacture of heterojunction cells can be more costly and complex than that of traditional crystalline silicon cells, due to thin-film deposition processes and the need to manage interfaces between different semiconductor materials.
- Sensitivity to impurities: Heterojunction cells are sensitive to impurities in semiconductor materials, which can affect their performance. It is therefore crucial to maintain a high level of quality control when manufacturing these cells.
- Limited availability: Heterojunction cells are not as widely available on the market as crystalline silicon cells, which can limit options for consumers and solar panel manufacturers.
Compatibility with technical innovations
Heterojunction cells are compatible with bifacial panels, half-cells and Tiling Ribbon Technology.
- Bifacial panels: Heterojunction cells can be used in bifacial solar panels thanks to their high efficiency and good defect management. The design of heterojunction cells favors charge carrier separation and reduces recombination losses, which is advantageous for bifacial panels.
- Cut cells (half-cells): Heterojunction cells can be cut into half-cells to reduce internal resistance and shading losses. This technique improves overall panel efficiency and offers greater tolerance to shading.
- Tiling Ribbon Technology, which uses flat, solderless conductive ribbons to connect photovoltaic cells together, can be used with heterojunction cells. This technology reduces efficiency losses due to soldering and increases the active surface area of the cells, which is beneficial for heterojunction cells due to their high efficiency.