Scientists are working to make multi-layer solar cells more effective. Richard Stevenson investigates why they are more likely to be achieved in space than on Earth. When it comes to solar cells, reliability is crucial. Commercial solar panels, which are made of silicon, usually achieve a 20 percent yield, which defines how much electricity can be collected from rooftops and solar farms. In the case of satellites, performance determines the size as well as the weight of solar panels used to power the craft, which has a significant impact on production and launch costs.
It’s enticing to use a material that blocks all of the Sun’s light, from high-energy rays in ultraviolet, across the visible, as well as out to the very long wavelengths of the infrared, to create a truly effective unit. A cell made of mercury telluride, which transforms virtually all of the Sun’s incoming photons into the current-generating electrons, may be the result of this strategy. However, there is a significant cost: each photon consumed by this substance contains a negligible amount of electricity, implying that the device’s power output will be pitiful.
A safer strategy is to choose a semiconductor with an absorption profile that balances the energy produced by every captured photon with the proportion of the sunlight absorbed by the cell. Gallium arsenide is a substance that exists in this sweet place (GaAs). GaAs has long been one of go-to products for designing high-efficiency solar cells. It’s often used in smartphones to enhance radio-frequency signals and produce laser illumination for facial recognition. These cells aren’t flawless, though; even after removing material flaws that degrade efficiency, the best GaAs solar cells still fail to achieve efficiencies above 25%.
Stacking various semiconductors on top of the other and deliberately picking a mixture that effectively harvests the Sun’s production yields even more benefits. Over many decades, solar-cell efficiencies have risen, as has the number of light-absorbing layers, on this well-trodden course. Last year, a team from National Renewable Energy Laboratory (NREL) situated in Golden, Colorado, unveiled a system with a record-breaking performance of 47.1 percent, edging dangerously near the 50 percent mark (Nature Energy 5 326). Structures of four absorbing layers had previously kept the title, but the US researchers discovered that six is a “natural sweet spot,” as per team leader John Geisz.
It’s not been easy to get so far since creating layered frameworks from various materials is far from simple. Epitaxy, a method in that material is grown one atomic layer at the moment on the crystalline substrate, is used to make high-efficiency solar cells. Only if the atomic spacing of every material inside the stack is very close will epitaxial growth provide the high-quality crystal frameworks needed for an effective solar cell. This condition, identified as lattice matching, limits the range of products that can be utilized; silicon, for instance, cannot be used since it lacks a family of alloys of equal atomic spacing.