so-called solar thermophotovoltaics, the
MIT device is the first one to absorb more
energy than its photovoltaic cell alone,
demonstrating that the approach could
dramatically increase efficiency.
Standard silicon solar cells mainly
capture the visual light from violet to red.
That and other factors mean that they can
never turn more than around 32 percent
of the energy in sunlight into electricity.
The MIT device is still a crude prototype,
operating at just 6. 8 percent efficiency—
but with various enhancements it could
be roughly twice as efficient as conventional photovoltaics.
The key step in creating the device
was the development of something called
an absorber-emitter. It essentially acts
as a light funnel above the solar cells.
The absorbing layer is built from solid
black carbon nanotubes that capture all
the energy in sunlight and convert most
of it into heat. As temperatures reach
around 1,000 °C, the adjacent emitting
layer radiates that energy back out as
light, now mostly narrowed to bands that
the photovoltaic cells can absorb. The
emitter is made from a photonic crystal,
a structure that can be designed at the
nanoscale to control which wavelengths
of light flow through it. Another critical
advance was the addition of a highly spe-
cialized optical filter that transmits the
tailored light while reflecting nearly all
the unusable photons back. This “pho-
ton recycling” produces more heat, which
generates more of the light that the solar
cell can absorb, improving the efficiency
of the system.
There are some downsides to the MIT
team’s approach, including the relatively
high cost of certain components. It also
currently works only in a vacuum. But the
economics should improve as efficiency
levels climb, and the researchers now have
Above: Black carbon nanotubes sit on top
of the absorber-emitter layer, collecting
energy across the solar spectrum and
converting it to heat.
Facing page: The absorber-emitter
layer is situated above an optical filter
and photovoltaic cell, which is visible