The time reversal asymmetry exceeds the conversion efficiency limit for solar cells

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 was not sufficient to allow the widespread use of solar cells.

The fundamental limit of the maximum efficiency of a PV plant is given by thermodynamic characteristics, namely temperature and entropy (degree of disorder in the system). More specifically, this limit, known as the Landsberg limit, is determined by the entropy of the black body’s radiation, which is often attributed to sunlight. The Landsberg limit is widely considered to be the most general limit to 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 should be the same for any photon energy and for any direction of propagation. This is basically the principle of “detailed balance” that has governed the operation of solar cells for decades. Kirchhoff’s law is, in fact, the result of what is called the “symmetry of the reversal of time.” Therefore, one way to bypass the SQ limit is to break this symmetry by allowing light to propagate in only one direction. Simply put, the SQ limit can be exceeded if the PV inverter absorbs more and emits less radiation.

In a new study published in Journal of Photonics for Energy (JPE), researchers Andrei Sergeev of the US Army Research Laboratory and Kimberly Sablon of the Army Futures Command and Texas A&M University are proposing a way to break the SQ limit by using ner non-reciprocal photonic structures ’that can drastically reduce PV converter emissions without affecting its overall performance. light absorption.

The research examines single-cell PV design integrated with non-reciprocal optical components to ensure 100% reuse of emitted radiation by the same cell through non-reciprocal photon recycling. This is in contrast to previous proposals that considered a PV inverter with several multi-transient cells arranged in such a way that the light emitted by one cell was absorbed by another.

Following the key works of Lorentz, von Laue, Einstein, Landau, Brillouin and Schrödinger, Sergeev and Sablon also discuss the entropy of sunlight in terms of coherence, relativity, nonequilibrium distributions, disorder, information and negentropia. The authors observe that, unlike strongly disordered radiation inside the sun, photons in sunlight move in straight lines at a narrow spatial angle. For Sergeev and Sablon, this observation suggests that sunlight provides us with real green energy, and its efficiency of transformation depends only on how we transform it.

The authors showed that for quasi-monochromatic radiation, a non-reciprocal single-cell PV converter theoretically reached the 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 helped solve the thermodynamic paradox associated with the optical diode. The paradox was that the optical diode can raise the temperature of the absorber above the temperature of the sun by allowing only one-way propagation of light. That would violate the second law of thermodynamics. The study showed that in order to achieve Carnot efficiency, an infinite number of photon recycling would be needed, and thus a violation of the law.

In addition, the researchers generalized thermodynamic considerations for the nonequilibrium distribution of photons 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 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, noting: “Weak non-radiation recombination in these materials would allow advanced improvement of PV conversion through radiation process control.”

Due to the growth of non-reciprocal photonic structures, the development of highly efficient 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.

Provides SPIE – International Society for Optics and Photonics