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Researchers creating electricity through
photovoltaics want to convert as many of the sun’s wavelengths as possible to
achieve maximum efficiency. Otherwise, they’re eating only a small part of a
shot duck: wasting time and money by using only a tiny bit of the sun’s
incoming energies.
For this reason, they see indium gallium
nitride as a valuable future material for photovoltaic systems. Changing the
concentration of indium allows researchers to tune the material’s response so
it collects solar energy from a variety of wavelengths. The more variations
designed into the system, the more of the solar spectrum can be absorbed,
leading to increased solar cell efficiencies. Silicon, today’s photovoltaic
industry standard, is limited in the wavelength range it can ‘see’ and absorb.
But there is a problem: Indium gallium
nitride, part of a family of materials called III-nitrides, is typically grown
on thin films of gallium nitride. Because gallium nitride atomic layers have
different crystal lattice spacings from indium gallium nitride atomic layers,
the mismatch leads to structural strain that limits both the layer thickness
and percentage of indium that can be added. Thus, increasing the percentage of
indium added broadens the solar spectrum that can be collected, but reduces the
material’s ability to tolerate the strain.
Sandia National Laboratories scientists
Jonathan Wierer Jr. and George Wang reported in Nanotechnology that if the
indium mixture is grown on a phalanx of nanowires rather than on a flat
surface, the small surface areas of the nanowires allow the indium shell layer
to partially “relax” along each wire, easing strain. This relaxation allowed
the team to create a nanowire solar cell with indium percentages of roughly 33%,
higher than any other reported attempt at creating III-nitride solar cells.
This initial attempt also lowered the
absorption base energy from 2.4 eV to 2.1 eV, the lowest of any III-nitride
solar cell to date, and made a wider range of wavelengths available for power
conversion. Power conversion efficiencies were low—only 0.3% compared to a
standard commercial cell that hums along at about 15%—but the demonstration
took place on imperfect nanowire-array templates. Refinements should lead to
higher efficiencies and even lower energies.
Several unique techniques were used to
create the III-nitride nanowire array solar cell. A top-down fabrication
process was used to create the nanowire array by masking a gallium nitride
(GaN) layer with a colloidal silica mask, followed by dry and wet etching. The
resulting array consisted of nanowires with vertical sidewalls and of uniform
height.
Next, shell layers containing the higher
indium percentage of indium gallium nitride (InGaN) were formed on the GaN
nanowire template via metal organic chemical vapor deposition. Lastly, In0.02Ga0.98N was grown, in such a way that caused the
nanowires to coalescence. This process produced a canopy layer at the top,
facilitating simple planar processing and making the technology manufacturable.
The results, says Wierer, although modest,
represent a promising path forward for III-nitride solar cell research. The
nanoarchitecture not only enables higher indium proportion in the InGaN layers
but also increased absorption via light scattering in the faceted InGaN canopy
layer, as well as air voids that guide light within the nanowire array.
Source: Sandia National Laboratories
