You may have heard the terms multi-junction, single-junction, dual-junction or triple-junction used when discussing solar. In this blog post we outline of some of the key concepts, advantages, disadvantages, and applications for these types of solar.
Multi-junction solar cells are made of multiple individual solar cells grown on top of each other. Due to the different materials in each layer, these cells can absorb different wavelengths of incoming light, leading to increased efficiencies.
A junction is where two different types of semiconductor material meet (p-type and n-type). The n-type has a high concentration of electrons. The p-type has a relatively low concentration of electrons. When light hits the n-type layer, loose electrons flow through the p-n junction and create an electrical current.
A Single junction cell means there is one p-n junction in the solar cell.
Dual junction cells have two p-n junctions.
Triple junction cells have three p-n junctions.
Multi-junction (MJ) cells typically refer to any cells with two or more p-n junctions.
Multi-Junction Cell Structure
MJ cells are made by growing thin layers of different photovoltaic materials on top of each other. The p-n junctions are often created within a single crystal to improve electrical properties. Materials such as gallium indium phosphide (GaInP), indium gallium arsenide (InGaAs), and germanium (Ge) may be used. The sub-cells are connected electrically in series.
Typically the shortest wavelength (largest bandgap) material is placed on top (first to be reached by sunlight) and will absorb blue light. Then materials are layered below (in order of the wavelengths they will respond to) and will each absorb narrow segments of the solar spectrum. The bottom cell will absorb infrared light.
Higher solar efficiency can be achieved with multiple junctions. Sunlight is a spectrum of different wavelengths of light. With more junctions, you can cover and absorb more of the solar spectrum, leading to higher efficiencies (sunlight to electrical energy conversion). Photons that can’t excite electrons in one layer of material will travel through to the next layer where they may be absorbed and generate a current. Higher solar efficiency is important when surface area is limited, such as on an aircraft or satellite.
A single junction cell has a theoretical maximum efficiency of approximately 33%, while a cell with an infinite number of junctions (multi junction cell), has a theoretical maximum efficiency of approximately 80%. In theory you can have infinite junctions. In practice, only up to 4 junctions has been achieved, and in laboratories up to 6 junctions. Alta Devices currently holds the world record for single junction solar at 29.1% efficiency.
A MJ cell can be tuned to specific applications where the solar spectrum is different. The solar spectrum will vary in intensity depending on the time of year, day, and angle of incidence of the light to the solar material. In space, the solar spectrum is different as there is no atmosphere. A lot of the solar spectrum doesn’t reach earth as it is absorbed by ozone and oxygen or reflected back into space by clouds and other parts of the atmosphere.
There are some trade-offs and disadvantages of MJ cells, especially in terms of cost. The manufacturing process for MJ cells is more complex and time-consuming with more steps, resulting in overall significantly higher cost, per watt of power than single junction cells.
Yields are typically lower for MJ solar manufacturing, due to the complex process which further increases costs. Generally, MJ materials are made in MOCVD (metal organic chemical vapor deposition) reactors and require highly specialized equipment and expensive materials.
The high cost of MJ cells has been prohibitive to their application outside of very specialized use cases such as on the Mars Rover. A high efficiency single-junction solar cell is a better solution in terms of price-to-performance for most applications including aerospace and LEO satellite applications.
Recent High Altitude Long Endurance UAV (HALE) and High Altitude Pseudo Satellites (HAPS) projects have chosen single junction cells for their stratospheric aircraft. These aircraft need a very high-density power source to power their multi-month unmanned missions and have relatively limited surface area, but also need to take cost into account.
Likewise for constellations of LEO satellites, launch weight and efficiency are important but with 10’s, 100s or 1000’s of satellites in a constellation, single junction solar is used for its excellent price-performance characteristics.
Please contact us to learn more about Alta Devices solar products.