Solar energy is a popular candidate for a sustainable alternative to fossil fuels. A solar cell, or photovoltaic (PV) cell, converts sunlight directly into electricity. However, the conversion efficiency has not been sufficient to allow widespread applications of solar cells.
A fundamental limit to the maximum efficiency of PV devices is given by the thermodynamic characteristics, namely temperature and entropy (a measure of disorder in a system). Specifically, this limit, known as the Landsberg limit, is imposed by the entropy of blackbody radiation that is often attributed to sunlight. Landsberg’s limit is widely considered to be the most general limit for the efficiency of any sunlight converter.
Another limit, called the Shockley-Queisser (SQ) limit, comes from Kirchhoff’s law, which states that absorptivity and emissivity must be equal for any photon energy and for any direction of propagation. It is essentially the principle of “detailed balance” that has governed the operation of solar cells for decades. Kirchhoff’s law is, in fact, a consequence of what is called “time reversal symmetry”. One way to circumvent the SQ limit is therefore to break this symmetry by allowing light to propagate only in one direction. Simply put, the SQ limit can be exceeded if the PV converter absorbs more and emits less radiation.
In a new study published in the Journal of Photonics for Energy (JPE), researchers Andrei Sergeev of the U.S. Army Research Laboratory and Kimberly Sablon of Army Futures Command and Texas A&M University propose a way to break the SQ limit using “non-reciprocal photonic structures” that can significantly reduce emissions of a PV converter without affecting its total light absorption.
The research explores a single-cell PV design integrated with non-reciprocal optical components to provide 100% reuse of radiation emitted from the same cell due to non-reciprocal recycling of photons. This contrasts with previous designs, which considered a PV converter with multiple multi-junction cells, arranged in such a way that light emitted by one cell was absorbed by another.
Following seminal work by Lorentz, von Laue, Einstein, Landau, Brillouin and Schrödinger, Sergeev and Sablon also discuss solar entropy in terms of coherence, relativity, non-equilibrium distributions, disorder, information and of negentropy. The authors observe that unlike the highly disordered radiation inside the sun, photons of sunlight travel along straight lines at a narrow solid angle. For Sergeev and Sablon, this observation suggests that sunlight provides us with true green energy and that its conversion efficiency only depends on how we are going to convert it.
The authors showed that for quasi-monochromatic radiation, the non-reciprocal single-cell PV converter achieves the theoretical maximum “Carnot efficiency”, the efficiency of an ideal heat engine, which exceeds the Landsberg limit. This was also the case for multicolored radiation (characteristic of sunlight).
Interestingly, this solved a thermodynamic paradox related to an optical diode. The paradox indicated that an optical diode could raise the temperature of the absorber above the temperature of the sun by allowing only unidirectional propagation of light. This would violate the second law of thermodynamics. The study showed that an infinite number of photon recycling would be required to achieve Carnot efficiency, and therefore, violate the law.
Additionally, researchers generalized thermodynamic considerations to out-of-equilibrium photon distributions with light-induced non-zero chemical potential and derived the limiting efficiency of a non-reciprocal single-cell PV converter.
“This research was motivated by the rapid advances in non-reciprocal optics and the development of low-cost photovoltaic materials with high quantum efficiency,” says Sergeev, citing perovskite materials in particular and noting, “Weak non-recombination radiation in these materials would enable advanced enhancement of PV Conversion via the management of radiative processes.”
With the increase in non-reciprocal photonic structures, the development of high-efficiency PV converters can be expected in the near future. As the search for sustainable solutions to the global energy crisis continues, this study offers great hope for solar cell technology.
Novel solar cell architecture performs well under real-world stresses
Andrei Sergeev et al, Nonreciprocal photonic management for photovoltaic conversion: design and fundamental efficiency limits, Journal of Photonics for Energy (2022). DOI: 10.1117/1.JPE.12.032207
Provided by SPIE – International Society for Optics and Photonics
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