New silicon allotrope could revolution is solar cells
Sodium atoms can be 'driven off' by heating to
leave the new orthorhombic silicon structure ©
NPG
The new Si24 allotrope has an open-framework
structure of 5-, 6- and 8-membered sp3-bonded
silicon rings © Duck Young Kim
New silicon allotrope
could revolutionise
solar cells
A new, direct band gap allotrope of silicon has
been synthesised by researchers in the US. It
could potentially revolutionise solar cells and
light-emitting devices by combining the light
absorbency of materials like gallium arsenide with
the processing advantages of traditional silicon.
The present synthesis is long and expensive, but
the researchers think it might be possible to get
around this.
Silicon is the mainstay of the electronics industry,
but the common cubic diamond-structured
allotrope has an indirect band gap, which means
electrons cannot travel between the valence and
conduction bands simply by absorbing or emitting
a photon: they also require a phonon to conserve
momentum. This makes silicon a relatively
inefficient absorber and emitter of light. Silicon
solar cells need a thick silicon wafer to absorb
enough light, while LEDs require more expensive
materials like gallium arsenide, which decomposes
easily and is toxic.
The tetrahedral bonding of silicon enables a wide
variety of hypothetical metastable structures,
many of which have only slightly higher energy
than the common ground state. Many of these
have been observed at high pressure and four are
dynamically stable under ambient conditions. In
2013, Timothy Strobel and colleagues at the
Carnegie Institute in Washington discovered
Na Si . Now they have shown that heating the
Na Si to 400K in a vacuum gradually drives off
the sodium atoms, leaving a new, orthorhombic
allotrope of silicon. Theoretical calculations and
experiments suggest the material is stable up to
750K and 10GPa, and has a direct band gap of
about 1.3eV – ideal for photovoltaic cells.
The material has only been produced in powder
samples and the tricky manufacturing process
would obviously limit its industrial potential.
Nevertheless, Strobel is optimistic that these
difficulties can be overcome. ‘Right now, we're
focusing on developing a recipe to grow very nice
quality single crystals,’ he says. ‘Once we do
that, we can really validate whether or not this
material has a chance to revolutionise
semiconductor technology. Furthermore, if we can
get a reasonably-sized substrate of this crystal,
it's entirely possible that we can produce this
allotrope completely free of any high pressure
whatsoever and epitaxially grow crystals on the
size that we can make now with diamond.’
Physicist George Nolas of the University of South
Florida, US, believes the most significant aspect
of the paper is the novel synthetic approach,
which, he says, ‘can be employed in the synthesis
of other open-framework material systems’.
Electronic structure theorist Artem Oganov of
Stony Brook University in New York, US, also
applauds the sophisticated methods used to
produce the material. ‘The question now is: can
this material beat silicon or not?’ he says. ‘If it
cannot, well, it was a good attempt, and these
attempts certainly should be repeated. If it can,
then it’s time to open the champagne!’
REFERENCES
1 O O Kurakevych et al, Cryst. Growth Des.,
2013, 13, 303 (DOI: 10.1021/cg3017084)
2 D Y Kim et al, Nat. Mater., 2014, DOI:
10.1038/nmat4140
leave the new orthorhombic silicon structure ©
NPG
The new Si24 allotrope has an open-framework
structure of 5-, 6- and 8-membered sp3-bonded
silicon rings © Duck Young Kim
New silicon allotrope
could revolutionise
solar cells
A new, direct band gap allotrope of silicon has
been synthesised by researchers in the US. It
could potentially revolutionise solar cells and
light-emitting devices by combining the light
absorbency of materials like gallium arsenide with
the processing advantages of traditional silicon.
The present synthesis is long and expensive, but
the researchers think it might be possible to get
around this.
Silicon is the mainstay of the electronics industry,
but the common cubic diamond-structured
allotrope has an indirect band gap, which means
electrons cannot travel between the valence and
conduction bands simply by absorbing or emitting
a photon: they also require a phonon to conserve
momentum. This makes silicon a relatively
inefficient absorber and emitter of light. Silicon
solar cells need a thick silicon wafer to absorb
enough light, while LEDs require more expensive
materials like gallium arsenide, which decomposes
easily and is toxic.
The tetrahedral bonding of silicon enables a wide
variety of hypothetical metastable structures,
many of which have only slightly higher energy
than the common ground state. Many of these
have been observed at high pressure and four are
dynamically stable under ambient conditions. In
2013, Timothy Strobel and colleagues at the
Carnegie Institute in Washington discovered
Na Si . Now they have shown that heating the
Na Si to 400K in a vacuum gradually drives off
the sodium atoms, leaving a new, orthorhombic
allotrope of silicon. Theoretical calculations and
experiments suggest the material is stable up to
750K and 10GPa, and has a direct band gap of
about 1.3eV – ideal for photovoltaic cells.
The material has only been produced in powder
samples and the tricky manufacturing process
would obviously limit its industrial potential.
Nevertheless, Strobel is optimistic that these
difficulties can be overcome. ‘Right now, we're
focusing on developing a recipe to grow very nice
quality single crystals,’ he says. ‘Once we do
that, we can really validate whether or not this
material has a chance to revolutionise
semiconductor technology. Furthermore, if we can
get a reasonably-sized substrate of this crystal,
it's entirely possible that we can produce this
allotrope completely free of any high pressure
whatsoever and epitaxially grow crystals on the
size that we can make now with diamond.’
Physicist George Nolas of the University of South
Florida, US, believes the most significant aspect
of the paper is the novel synthetic approach,
which, he says, ‘can be employed in the synthesis
of other open-framework material systems’.
Electronic structure theorist Artem Oganov of
Stony Brook University in New York, US, also
applauds the sophisticated methods used to
produce the material. ‘The question now is: can
this material beat silicon or not?’ he says. ‘If it
cannot, well, it was a good attempt, and these
attempts certainly should be repeated. If it can,
then it’s time to open the champagne!’
REFERENCES
1 O O Kurakevych et al, Cryst. Growth Des.,
2013, 13, 303 (DOI: 10.1021/cg3017084)
2 D Y Kim et al, Nat. Mater., 2014, DOI:
10.1038/nmat4140
