A group of scientists from the Tampere University in Finland has developed a III-V multi-junction solar cell which is claimed to have the potential for reaching a power conversion efficiency of close to 50%.
Presented in the paper Wide spectral coverage (0.7–2.2 eV) lattice‐matched multijunction solar cells based on AlGaInP, AlGaAs and GaInNAsSb materials, published in Progress of Photovoltaics, the four-junction device was built with three different III-V materials: indium gallium phosphide (InGaP) for the first junction; gallium arsenide (GaAs) for the second element; and gallium-indium-nitride-arsenide-antimony (GaInNAsSb) for the other two junctions.
The four devices have bandgaps of 1.88, 1.42, 1.17, and 0.93 electronvolts (eV), respectively, and their combination, according to the researchers, achieves a wide spectral coverage. “This structure covers, efficiently, the spectral range of 350-1,310nm, in which intra‐band efficiency can theoretically reach over 52% efficiency under 1,000‐sun illumination,” the scientists explained. “Assuming the use of materials with optimal bandgaps of 2, 1.51, 1.16, and 0.79 eV, we estimate to be plausible achieving an efficiency of 49.8% at 1,500 suns, owing to reduced transmission and thermalization losses.”
The bandgap of a semiconductor device such as a solar cell determines how many photons are needed from the sun for conduction. Crystalline silicon (C-Si), for example, has a bandgap energy of 1.11 eV and cadmium telluride (CdTe), which is less efficient than C-Si for power generation, has a bandgap of 1.44 eV.
The cell was monolithically grown on GaAs by molecular beam epitaxy (MBE). The performance of the multi-junction device was measured through a commercial steady‐state OAI 7‐kW TriSOL solar simulator and external quantum efficiency (EQE) measurements were made through an in‐house-built system. The EQE is an indicator of how well the solar cell converts incident photons of a specific wavelength to electricity. “The 4J solar cell exhibited an efficiency of 39% at 560‐sun illumination while showing good electrical performance even up to 1,000 suns with open-circuit voltage (Voc) of over 4.1 V,” the researchers stated.
To further improve the performance of the solar cell, the Finnish group is investigating the potential of combining aluminum-gallium-indium-phosphide (AlGaInP) subcells, which have a bandgap of over 2 eV, with GaInNAsSb substrates with a bandgap lower than 0.8 eV.
The cost of producing solar cells based on compounds of III-V element materials–named according to the groups of the periodic table that they belong to–has confined such devices to niche applications including drones and satellites, where low weight and high efficiency are more pressing concerns than costs, in relation to the energy produced.
*The article was amended to reflect that the element used for two of the cell junctions is gallium-indium-nitride-arsenide-antimony (GaInNAsSb) and not gallium-indium-sodium sulfide–antimony, as we previously reported.
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