CNTs & photonics to break solar conversion theoretical limits

May 24, 2016 // By Julien Happich
In their Nature Energy paper "Enhanced photovoltaic energy conversion using thermally based spectral shaping", MIT researchers demonstrate how they were able to boost a traditional photovoltaic cell conversion efficiency by converting the broadband sunlight to a narrow-band thermal radiation precisely tuned for the photovoltaic cell at hand.

Relying on the broadband absorption of vertically grown carbon nanotubes (CNTs) integrated with a one-dimensional photonic crystal selective emitter and paired with a tandem plasma–interference optical filter, the researchers managed to suppress 80% of unconvertible photons, achieving a solar-to-electrical conversion rate that exceeded the performance of the photovoltaic cell alone.

Their research also established that the resulting device could operate more efficiently while reducing the heat generation rates in the photovoltaic cell by a factor of two for a given output power density.

With solar thermophotovoltaics, lead author MIT doctoral student David Bierman sees a new energy harvesting route that could help traditional solar cells break their energy conversion theoretical limits.

Instead of dissipating unusable solar energy as heat in the solar cell, thermophotovoltaics devices first absorb all of the energy and heat through an intermediate component (the ultra-black CNT layer on top of a one-dimensional photonic crystal seen on the top of the thermophotovoltaic assembly).

MIT's thermophotovoltaic assembly showing a circular layer of ultra-black CNTs covering a one-dimensional photonic crystal, over an optical filter (green) cover the standard solar cell. Courtesy, MIT's researchers.

Reaching high temperatures (1,000 degrees Celsius in their experiment), these added layers are tuned to only emit thermal radiation at the optimal wavelengths of light for the solar cell to operate at peak efficiency.

By pairing conventional solar cells with these custom-designed emissive layers, one could more than double the theoretical limit of efficiency, potentially making it possible to deliver twice as much power from a given area of panels, expects Bierman.