Discover the groundbreaking research that could revolutionize the solar energy industry. Explore how harnessing the power of hot carrier solar cells can pave the way for a brighter, more efficient future.
Getting past those efficiency limits
It has been considered for hot carrier solar cells over the decades a holy grail of solar energy. These cells combine to create a multi-junction solar cell, which has the potential to surpass the Shockley–Queisser efficiency limit, a limit established in 1961 by William Shockley and Hans-Joachim Queisser that is well known among researchers as defining the maximum theoretical single-junction solar cell efficiency.
Yet, the realization of hot carrier solar cells in practice remains a very challenging scientific problem — especially when it comes to harvesting fast photoexcited hot electrons at material interfaces. In this case, researchers have been investigating different ways to solve the problem, such as relying on satellite valleys in the conduction band as moments where hot electrons are placed until being collected.
However, the transfer process has recently been complicated by a parasitic barrier at the heterostructure interface between absorber and extraction layers (30). One such barrier is created when the energy bands of two materials are mismatched, so electrons must instead tunnel around the obstruction, a process dictated by nuanced band structures.
The Mystery of Electron Tunneling Set to be Solved
Therefore, a new study published in the Journal of Photonics for Energy, has approached these evanescent states combined with their effect in electron tunneling using an empirical pseudopotential method [21].
This permits them to determine momentum-space energy bands and compare with experimental data on critical points, leading to an improved understanding of the relevant physics responsible for the extraction of hot carriers between carrier valley states as well as heterointerface charge transfer.
In addition, the results of this study give insights into the tunneling transport which is essential for an effective transfer of hot electrons. Theses researchers found the tunneling coefficient (the property that describes how difficult or easy it is for an electron to pass through the barrier) of structures composed of indium-aluminum-arsenide/indium-gallium-arsenide (InAlAs/InGaAs) can be exponentially high.
Electron transfer is then severely slowed down by even a small roughness at the interface (a few atoms thick) in this case. Such results are consistent with poor device-level performance when using this system of materials in experiments.
However, they have no doubt that it can be done much better in a system with AlGaAs and gallium-selenide (GaAs) materials. Here, the presence of aluminum in the barrier introduces a degeneracy in the lower-energy satellite valleys that allows us to achieve band-alignment and to grow with atomic dimension control. This indicates an order of magnitude higher transfer with the promise of valley photovoltaics where efficiencies can now be pursued high above existing single bandgap limits for solar cells.
Conclusion
Sandia has found, in researching the hot carrier solar cell concept, something that could totally revolutionize the world of solar. With so much more yet to discover—the complicated dance of energy bands, the science-going-on of satellite valleys, and electron tunneling—researchers are beginning work that could lead to a day when hot carrier solar cells meet or even exceed the efficiency limits inherent in today’s technology. This new work is bringing us close to cheaper and more effective solar power, it’s also a leap in an energy direction that makes sense for the future.
