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allotropes, why not use
carbon in solar cells?
This is how carbon
nanotubes (a special form
of carbon) came to be
considered for tapping
solar energy.
Before studying about
carbon nanotubes, we
need to know what
n a n o t u b e s a r e .
Nanotubes are particles
that are on the nanoscale,
which is are about 1 to
100 nanometres (nm) in
size. For comparison, the
thickness of a sheet of
paper is about 100,000
nm and the width of a
hair, about 40,000 –
80,000 nm. Materials that
exhibit a dimension
below 100 nm have very
different and interesting
properties than the bulk
material. As an example,
gold is a very inert metal,
but below 100 nm,
nanoparticles of gold
possess properties that
m a k e t h e m g o o d
catalysts and sensors.
With this in mind, let us
s e e w h a t c a r b o n
nanotubes are. Carbon
nanotubes (CNTs) are
allotropes of carbon with
a c y l i n d r i c a l
nanostructure. These
c y l i n d r i c a l c a r b o n
molecules have unusual
properties, which are
v a l u a b l e f o r
n a n o t e c h n o l o g y ,
electronics, optics and
other fields of materials
science and technology.
So, Why use CNTs in
Solar Cells?
Any solar cell should
have good photovoltaic
properties to provide
maximum number of
electron-hole pairs for a
g i v e n i n t e n s i t y o f
incident light. The cell
should be as strong as
possible to improve its
life and should work at
maximum possible
efficiency. Above all, it
should be cheap. Well,
CNTs have many such
desirable properties that
make it a potential
photovoltaic material in
the coming years. When
light falls on a solar cell,
electrons are generated
and transferred to the
external circuit via
metallic conductors and
connectors. If the photoreception
part of the cell
contains CNTs, a lot of
positive differences can
be observed. This is due
t o t h e e l e c t r i c a l
properties of CNTs. They
are hollow cylinders and
hence electrons can only
flow over its surface
c o m p a r e d t o t h e
conventional wires. Due
to this reason, CNTs
offers lesser resistance to
the flow of current and
can help reduce losses.
As for light harvesting
p r o p e r t i e s , c a r b o n
nanotubes possess a wide
range of direct bandgaps
m a t c h i n g t h e s o l a r
spectrum and its strong
photoabsorption from
infrared to ultraviolet,
allows it to absorb light
belonging to different
f r e q u e n c i e s a n d
wavelength. It has high
carrier mobility and
reduced carrier transport
scattering which allows
C N T s t o t r a n s f e r
maximum current to the
external load as much as
possible with minimum
loss of electrons within
the bulk of the cell. What
good can a solar cell do
if it can’t protect itself
from mechanical
s t r e s s e s ? C a r b o n
nanotubes are the
strongest and stiffest
materials yet discovered
in terms of tensile
strength and elastic
modulus respectively.
Recently, SWNTs were
directly configured as
e n e r g y c o n v e r s i o n
materials to fabricate
thin-film solar cells, with
nanotubes serving as
both photogeneration
sites and a charge carriers
collecting/transport layer.
The solar cells consist of
a semitransparent thin
f i l m o f n a n o t u b e s
conformally coated on an
n-type crystalline silicon
substrate to create highd
e n s i t y
p - n
heterojunctions between
nanotubes and n-Si to
favor charge separation
and extract electrons
(through n-Si) and holes
(through nanotubes).
Initial tests have shown a
p o w e r c o n v e r s i o n
efficiency of >1\%,
proving that CNTs-on-Si
is a potentially suitable
configuration for making
solar cells. For the first
t i m e , Z h o n g r u i L i
demonstrated that SOCl2
treatment of SWNT
b o o s t s t h e p o w e r
conversion efficiency of
S W N T / n - S i
heterojunction solar cells
by more than 60%. Later
on the acid doping
a p p r o a c h i s w i d e l y
adopted in the later
published CNT/Si works.
Even higher efficiency
can be achieved if acid
liquid is kept inside the
void space of nanotube
network. Acid infiltration
of nanotube networks
significantly boosts the
cell efficiency to 13.8%,
as reported by Yi Jia, by
reducing the internal
resistance that improves
f i l l f a c t o r, a n d b y
f o r m i n g
photoelectrochemical
units that enhance charge
separation and transport.
The wet acid induced
problems can be avoided
by using aligned CNT