A simple tweak in the design of solar cells could make them more efficient than ever

We can now reach the Landsberg limit in a solar cell.
Ameya Paleja
Blue photovoltaic solar panels.
Blue photovoltaic solar panels.


A team of researchers at the University of Houston has invented a new type of solar energy harvesting system that has broken the efficiency record of all existing technologies and could one day be deployed to use solar power 24/7, according to a press release by the institution.

Solar energy is the most abundant form of renewable energy available to us. Advances in solar energy harvesting systems mean that we can harvest a fair percentage of sunlight that is received upon Earth and efforts are ongoing to increase this number every day.

Last month, Interesting Engineering reported on how improvements using perovskites to make solar cells can help us break the 30 percent energy conversion barrier of solar cells. Now, the research conducted at the University of Houston opens up the use of solar cells around the clock.

Pushing solar cells to their thermodynamic limits

Conventionally used solar thermophotovoltaics (STPV) are designed with certain tweaks to improve their efficiency. One among them is the use of an intermediate layer that tailors the sunlight onto the solar cell.

The front side of this layer is designed to absorb all the photons coming from the sun, which converts it into thermal energy and elevates the temperature of the intermediate layer. The limit for such a conversion has been considered to be 85.4 percent for STPVs. However, this is still a fair distance away from the Landsberg limit of 93.3 percent, which is considered the absolute limit of solar energy harvesting.

Researcher Bo Zhao, an assistant professor of mechanical engineering at the university, suggested that inevitable back emission from the intermediate layer was responsible for the efficiency deficit, which could be improved by using a nonreciprocal STPV system.

A simple tweak in the design of solar cells could make them more efficient than ever
Illustration of (a) traditional STPV and (b) nonreciprocal STPV

Nonreciprocal STPVs

The researchers showed that such a system could be made by making the intermediate layer with material that has nonreciprocal radiative properties. Such as, STPV would funnel more photons towards the cell and substantially reduce the back emission to the sun.

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The result was an STPV that was closer to achieving the Landsberg limit and increasing the chances of making a single junction photovoltaic cells, thereby further increasing the efficiency of the system.

Such STPVs would be more compact and could also be combined with a thermal energy storage unit to generate electricity 24/7. This would be a much easier system to build up and use when compared to plans of putting mirrors in space to access solar energy at night to transition to a carbon-free electric grid.

According to estimates by the Solar Energy Technologies Office at the U.S. Department of Energy, solar energy could account for as much as 45 percent of the U.S. electric supply by 2050. A system of nonreciprocal STPVs could drastically increase this number, and the severe limitation associated with solar energy would go away.

The research findings were published last month in the journal Physical Review Applied.


Traditional solar thermophotovoltaics (STPVs) rely on an intermediate layer to tailor sunlight for better efficiencies. However, the thermodynamic efficiency limit of STPVs, which has long been understood to be the blackbody limit (85.4%), is still far lower than the Landsberg limit (93.3%), the ultimate efficiency limit for solar energy harvesting. In this work, we show that the efficiency deficit is caused by the inevitable back emission of the intermediate layer towards the sun resulting from the reciprocity of the system. We hereby propose nonreciprocal solar theromophotovoltaics (NSTPV) that utilize an intermediate layer with nonreciprocal radiative properties. Such a nonreciprocal intermediate layer can substantially suppress its back emission to the sun and funnel more photon flux towards the cell. We show that, with such improvement, the NSTPV system can reach the Landsberg limit, and practical NSTPV systems with single-junction photovoltaic cells can also experience a significant efficiency boost.

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