Multi-junction solar cells: The hunt for 50 percent
Lattice-matched solar cells with five or six junctions look to surpass the landmark conversion efficiency of 50 percent.
BY MIRCEA GUINA, RIKU ISOAHO, ARTO AHO AND Antti Tukiainen FROM TAMPERE UNIVERSITY
When it comes to photovoltaic conversion efficiencies, III−V multi-junction solar cells are the gold standard. For the past three decades this class of compound semiconductor devices has been breaking record efficiencies and pushing the limits of photovoltaic technologies.
There are opportunities for these multi-junction cells on earth and up in space. Their combination of high efficiencies, high power-to-weight ratios and excellent durability has ensured that these devices are the key technology for power generation in space. Meanwhile, in terrestrial applications, these cells can be deployed in concentrated photovoltaic systems, where focusing sunlight onto the solar cells by lenses or mirrors leads to even higher efficiencies.
Operation under very high levels of concentration, typically many hundreds of suns, is behind record results for efficiency. The current world record stands at 47.6 percent, realised by researchers at Fraunhofer ISE in 2022 with a wafer-bonded four-junction photovoltaic cell operating at a concentration of 665 suns. That’s tantalisingly close to the 50 percent efficiency milestone, which yet remains elusive, despite extensive efforts.
During the last decade or so, realising record efficiencies with multi-junction devices has involved at least four junctions. Now the focus is shifting to five or more junctions, a move that’s considered essential for realising practical candidates that will eclipse the 50 percent target (see Figure 1).
Figure 1. Conversion efficiencies for practical devices.
Unlocking higher efficiencies
Multi-junction solar cells excel in efficiency by dividing the incident solar spectrum between multiple subcells, each absorbing photons at different wavelength ranges. It is the distribution of the incident sunlight spectrum that ensures that radiation is utilised more efficiently, and in turn boosts the overall efficiency of the device.
In principle, the greater the number of subcells, the higher the maximum efficiency. For a purely hypothetical multi-junction solar cell that’s equipped with an infinite number of junctions, the maximum efficiency is 68.7 percent. That figure is calculated for illumination under one sun, with the theoretical maximum of this hypothetical device rising to 86.8 percent under concentration. For a two-junction solar cell the theoretical efficiency under concentration is as high as 50 percent, but it’s likely five or more junctions are needed to reach this value with a practical device.
There are a variety of approaches and materials that can be utilised for realising multi-junction solar cells (see Figure 2). The most widely adopted are stacked architectures based on III−V compounds. III−Vs are a prime choice because of their wide range of tunability, in terms of bandgap energies and lattice constants, assets that enable the monolithic stacking of multiple subcells from different III−V materials.