Multi-junction cells, also known as multi-layer cells or stack cells, are an advanced photovoltaic cell technology designed to improve the efficiency of converting sunlight into electricity. They are composed of several layers of semiconductor material, each with a different bandgap, enabling them to absorb and convert different wavelengths of sunlight.
The main idea behind multi-junction cells is to make the most of the solar spectrum by optimizing light absorption in each layer. As a result, they can convert more solar energy into electricity than single-junction photovoltaic cells, such as monocrystalline or polycrystalline silicon cells. Because of their high efficiency, multi-junction cells are mainly used in high-performance applications, such as aerospace, where space and weight are limited. However, their manufacturing cost is generally higher than that of single-junction cells, which limits their adoption in residential and commercial photovoltaic applications.
Composition of a multi-junction cell
A multi-junction cell is composed of several layers of semiconductor material stacked one on top of the other, each forming a p-n junction for the conversion of sunlight into electricity. The structure and composition of these cells are designed to optimize the absorption of different wavelengths of the solar spectrum, thus improving overall efficiency.
Here’s the composition of a multi-junction cell, presented logically:
- Top layer (substrate): The top layer is usually made of a wide-bandgap semiconductor material, such as gallium indium phosphide (GaInP). This layer is designed to absorb high-energy photons (short wavelengths) from the solar spectrum.
- Intermediate layer : The intermediate layer consists of a semiconductor material with an intermediate bandgap, such as gallium arsenide (GaAs). This layer absorbs the medium-wavelength photons that pass through the first layer without being absorbed.
- Bottom layer: The bottom layer consists of a low-bandgap semiconductor material, such as germanium (Ge) or indium gallium arsenide (InGaAs). This layer captures low-energy photons (long wavelengths) that have passed through the upper layers without being absorbed.
The layers are connected in series by tunnel junctions, enabling efficient current flow between them. Electrons and holes generated by photon absorption in each layer are separated by p-n junctions, creating voltage and current. This multi-layer structure enables multi-junction cells to convert a greater proportion of the solar spectrum into electricity, increasing their efficiency compared with single-junction cells.
Materials used in multi-junction cells
Multi-junction cells use a combination of semiconductor materials to optimize absorption of the different wavelengths of the solar spectrum. The materials commonly used in the manufacture of these cells are mainly III-V compounds, which take their names from groups III and V of the periodic table of elements. Here are some of the materials frequently used
- Gallium indium phosphide (GaInP): This wide-bandgap material is generally used in the top layer of multi-junction cells to absorb high-energy photons (short wavelengths).
- Gallium arsenide (GaAs): This semiconductor material with an intermediate bandgap is often used in the intermediate layer to absorb medium-wavelength photons.
- Indium gallium arsenide (InGaAs) or germanium (Ge): These low-bandgap materials are commonly used in the bottom layer of multi-junction cells to capture low-energy (long-wavelength) photons.
Multi-junction cells can also include other semiconductor materials, such as indium gallium nitride (InGaN) or aluminum gallium arsenide (AlGaAs), depending on specific application needs and efficiency targets. Recent advances in materials and nanotechnology also allow us to explore new combinations of materials to further improve the efficiency and performance of multi-junction cells.
Advantages and disadvantages of multi-junction cells
Advantages
- High efficiency: multi-junction cells have a higher photovoltaic conversion efficiency than crystalline silicon cells, often reaching over 40%. This is due to their ability to absorb and convert a greater proportion of the solar spectrum into electricity.
- Performance under variable light conditions: These cells generally perform better under variable light conditions, such as concentrated or indirect lighting, thanks to their ability to efficiently capture and convert light over a wide range of wavelengths.
- Temperature resistance: Multi-junction cells have better resistance to temperature-related degradation, which can make them more suitable for high-temperature environments.
Disadvantages
- High cost: Multi-junction cells are more expensive to produce due to the complexity of their manufacture and the materials used, such as III-V compounds (e.g. GaAs, InP). This can limit their adoption to specific, high-efficiency applications, such as aerospace or concentrated photovoltaics (CPV).
- Manufacturing complexity: The manufacture of multi-junction cells requires epitaxial growth processes and specialized equipment, which increases their production cost and makes them more difficult to produce in large quantities.
- Limited compatibility: multi-junction cells may not be as compatible with certain innovations, such as bifacial panels, cut cells or Tiling Ribbon Technology, due to their structure and high cost.
Innovations compatible with multi-junction cells
Bifacial panels, cut cells and Tiling Ribbon Technology are mainly developed and optimized for crystalline silicon cells, such as monocrystalline and polycrystalline cells. However, it is possible to envisage their application with multi-junction cells, although this may present technical and economic challenges.
Multi-junction cells could theoretically be used in bifacial panels, but their high cost and complex structure could make this combination less economically viable than using crystalline silicon cells. In addition, the performance gains offered by bifaciality could be limited by the absorption characteristics of multi-junction cells.
Multi-junction cells could also be cut to reduce internal resistance and shading losses. However, given the high cost of these cells and the complexity of their manufacture, this approach may not be cost-effective.
Tiling Ribbon Technology could be adapted to connect multi-junction cells, reducing efficiency losses due to soldering and increasing the cells’ active surface area. However, as with cut cells, the high cost and complexity of manufacturing multi-junction cells could make this combination less economically viable.