Environmental Engineering Reference
In-Depth Information
and a corresponding transmission of 90 % has been observed in doped graphene
sheets, thus making graphene potentially suitable as transparent conductors [
57
].
Apart from R
sh
and transmission, several other properties such as the high
chemical and thermal stability, high charge carrier mobility (200 cm
2
V
-1
s
-1
),
high current carrying capacity (3 9 10
8
Acm
-2
), high stretchability, and low-
contact resistance with organic materials render graphene a very favorable alter-
native to ITO [
58
,
59
].
In principle, a monolayer of graphene possesses ballistic charge transport due to
delocalization of electrons over the complete sheet, however, in practice defects
are introduced during growth and processing of graphene. Such defects, for
example, lattice defects, grain boundaries, and oxidative traps due to functional-
ization result in high R
sh
of graphene [
54
,
60
]. Such a challenge is reflected in its
application as transparent conductors in PSCs. The earlier reports on graphene as
transparent conductors were adopted in dye sensitized and small molecule solar
cells, however, the performance of such devices were limited (PCEs of \1%)
largely due to the high R
sh
of the films which is often in the kX range [
60
,
61
]. The
doping of graphene with AuCl
3
is reported to reduce R
sh
of graphene by 77 % with
only 2 % decrease in transmission [
62
]. As such, an improvement of PCE of PSCs
from 1.36 % for undoped graphene to 1.63 % for AuCl
3
-doped graphene film has
been observed [
63
]. With high quality CVD-grown multilayer (15 layers) graph-
ene, a PCE of 2.60 % has been observed in a P3HT:PCBM-based PSCs. A strong
dependence of the photovoltaic properties of PSCs on the growth temperature of
high quality CVD-grown graphene has been observed [
64
]. In addition to the high
R
sh
, the poor wetting properties of graphene is yet another hindrance to their
application in PSCs. Graphene is hydrophobic and requires functionalization (e.g.,
UV/ozone treatment or acid treatment) to improve its wetting properties. Such
functionalization in turn creates defects in graphene sheets increasing its R
sh
and
therefore limiting the final performance of a solar cell. Noncovalent functionali-
zation improves wetting while maintaining the structural integrity of graphene.
Several routes for noncovalent functionalization have been proposed. For example,
a noncovalent functionalization of graphene with self-assembled pyrene butanoic
acid succidymidyl ester (PBASE) is observed to improve the wetting properties of
graphene toward PEDOT:PSS, thereby, resulting in greater than two-fold increase
in PCE [
65
]. Other methods include deposition of a thin (20 Å) layer of MoO
3
over
graphene which has been seen to improve the wetting properties of graphene
toward PEDOT:PSS as well as tune work function of graphene electrodes [
66
].
Using MoO
3
in PSC in normal architecture (doped graphene/MoO
3
/PEDOT:PSS/
P3HT:PCBM/LiF/Al), a significant improvement in the device was observed and
resulted in a PCE of 2.5 %. In comparison, ITO-based equivalent cells had a PCE
of 3 %.
These preliminary investigations of graphene transparent conductors in PSCs
are mere proof- of -concept studies. Most graphene transparent conductor film are
reported to have R
sh
values seldom lower than 100 X!
-1
and are often investi-
gated on very small area devices (often less than a cm
2
). Such high R
sh
may not be
critical in small area devices but will prove detrimental to photovoltaic properties
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