Time-reversal Asymmetry Surpasses Conversion Efficiency Limit for Solar Cells

Solar energy is a favoured contender for a long-term replacement for fossil fuels. A Solar Cells, often known as a photovoltaic (PV) cell, transforms sunlight into energy directly. However, the conversion efficiency has not been high enough to allow Solar Cells to be widely used.

Thermodynamic parameters, such as temperature and entropy, set a basic constraint on the maximum efficiency of PV devices (a measure of disorder in a system). The entropy of the blackbody radiation, which is commonly attributed to sunlight, imposes this restriction, known as the Landsberg limit. The Landsberg limit is commonly regarded as the most universal efficiency limit for any solar converter.

The Shockley-Queisser (SQ) limit is derived from Kirchhoff’s law, which stipulates that the absorptivity and emissivity for every photon energy and propagation direction should be identical. This is precisely the “detailed balancing” idea that has regulated the performance of Solar Cells for decades. Kirchhoff’s law is a result of a phenomenon known as “time reversal symmetry.” Breaking this symmetry by allowing light to travel just in one direction is one technique to get around the SQ limit. The SQ limit can be exceeded if the PV converter absorbs more radiation while emitting less.

Researchers Andrei Sergeev of the US Army Research Laboratory and Kimberly Sablon of Army Futures Command and Texas A&M University propose a way to break the SQ limit by utilising “nonreciprocal photonic structures” that can drastically reduce emission from a PV converter without affecting its total light absorption in a new study published in the Journal of Photonics for Energy (JPE).