Georgia Tech
has developed a surface treatment that boosts light absorption in
silicon photovoltaic cells.
"Our simulations show that we can potentially increase the final
efficiency of the cells by as much as 2% with this surface
structure," said Professor CP Wong.
He etches the silicon to create both micrometre and nanometre
features.
"The surface treatment increases absorption both by trapping
light in three-dimensional structures and by making the surfaces
self-cleaning - allowing rain or dew to wash away the dust and dirt
that can accumulate on photovoltaic arrays," said the
university.
The surface is modelled on that of the lotus leaf which is not
wetted by water, but causes the water to form beads -
'superhydrophobic' behaviour.
The lotus leaf has roughness at two different size scales to
create high contact angles that encourage water from rain or
condensation to bead up and run off. As it runs off, it carries
away surface dust which also cannot get a grip.
Wong claims his surface treatment makes silicon work in the same
way.
"When a water droplet reaches the surface, it sits on top of
this two-tier roughness and only about 3% of it is in contact with
the silicon," he said.
Preparation of the superhydrophobic surface begins a potassium
hydroxide etch which preferentially removes silicon along
crystalline planes, creating micron-scale pyramid structures.
An electron-beam process is then used to apply nanometre-scale
gold particles to the pyramids.
The gold catalyses a metal-assisted etching process using a
solution of hydrogen fluoride and hydrogen peroxide which produces
the nanometre features - whose size is controlled by the diameter
of the gold particles and the etch time.
Finally, the gold is removed with a potassium iodide solution,
and the surface coated with the fluorocarbon perfluorooctyl
tricholosilane.
"A normal silicon surface reflects a lot of the light that comes
in, but by doing this texturing the reflection is reduced to less
than 5%," said Professor Dennis Hess.
"As much as 10% of the light that hits the cells is scattered
because of dust and dirt of the surface. If you can keep the cells
clean, in principle you can increase the efficiency. Even if you
only improve this by a few per cent, that could make a big
difference."
The university claims that even in deserts there is enough night
time dew to keep treated cells clean.
There are drawbacks.
"Because the structures are so small, they are fairly fragile.
Mechanical abrasion to the surface can destroy the
superhydrophobicity," said Hess. "We have tried to address that
here by creating a large superhydrophobic surface area so that
small amounts of damage won't affect the overall surface."
Production cost estimates haven't yet been done, but Hess said
the additional etching and vacuum deposition steps shouldn't add
dramatically to existing silicon solar cell manufacturing
processes.
The surface treatment may also create anti-bacterial coatings on
medical equipment, stop MEMS sticking together, and improved
microfluidic devices, said the university.
Micrometer silicon pyramids etched for two minutes using
hydrogen fluoride/hydrogen peroxide/water solution to add nanometre
roughness.
And the same thing, etched for only one
minute.