Researchers Push Upper Limit Of Solar Cell Potential

solar thermophotovoltaic cell

A group of researchers from Massachusetts Institute of Technology (MIT) have successfully demonstrated a method for exceeding the theoretical limit for solar cell conversion of sunlight to energy.

The development could effectively double the amount of power produced by a given area of solar panels.

In a paper published in Nature Energy, the team describe their building of a solar thermophotovoltaic cell, or STPV, a hybrid cell that converts the sun’s heat into usable wavelengths of light by pairing conventional PV cells with special nanomaterials.

STPV cells are part of a special family of solar devices designed to exceed the so-called Shockley-Queisser Limit, the absolute theoretical limit for how efficient conventional solar cells can be in their energy conversion. For a single-layer cell made of silicon — the type used for the vast majority of today’s solar panels — that upper limit is about 32 percent.

“With solar thermophotovoltaics you have the possibility to exceed that,” says MIT doctoral student David Bierman.

The device works by using unique nanomaterials called nanophotonic crystals, which can be tuned to emit specific wavelengths of light when heated. The crystals are aligned with a system of carbon nanotubes that absorb and transmit the light to an adjacent photovoltaic cell for conversion to electricity.

“The carbon nanotubes are virtually a perfect absorber over the entire colour spectrum,” Bierman says, allowing it to capture the full solar spectrum. “All of the energy of the photons gets converted to heat.”

This heat, which would otherwise be wasted or even cause damage to standard solar power systems, is then re-emitted as usable light that ideally matches the PV cell’s peak efficiency.

“We believe that this new work is an exciting advancement in the field,” says MIT professor Evelyn Wang. “As we have demonstrated, for the first time, an STPV device that has a higher solar-to-electrical conversion efficiency compared to that of the underlying PV cell.”

To demonstrate their method, the team paired a relatively low-efficiency 6.8 percent solar cell with STPV components and measured the device’s power output first in direct sunlight, then with the sunlight blocked so only light resulting from the nanophotonic crystals reached the PV cell.

The results matched the improved efficiencies predicted.

“A lot of the work thus far in this field has been proof-of-concept demonstrations,” Bierman says. “This is the first time we’ve actually put something between the sun and the PV cell to prove the efficiency of the thermal system. Even with this relatively simple early-stage demonstration we showed that just with our own unoptimised geometry, we in fact could break the Shockley-Queisser limit.”

Other recent thermophotovoltaic cell research we’ve covered includes a device developed by Australian National University and University of California scientists that not only utilises sunlight, but also harvests heat in the dark and convert it to electricity.

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