Page 149 - Multifunctional Photocatalytic Materials for Energy

Graphene-based nanomaterials for solar cells 135

several drawbacks: (i) limited presence of elemental indium, (ii) inability of the ma-
terial to sustain high temperature conditions, (iii) instability in the presence of an acid
or a base, (iv) limited transparency in the near-infrared region, and (v) current leakage
of FTO devices because of its structural defects [49,50]. In light of these drawbacks,
graphene-based materials are considered as prime candidates to replace the ITO sub-
strate and to provide an effective alternative TCE owing to its high electron mobil-
ity, transparency, and high flexibility. Several other studies have also reported that
graphene acted as an efficient transparent conductive electrode for DSSCs, organic
solar cells, and perovskite solar cells [19,69,70]. In general, ITO material possesses
a sheet resistance (R sh ) of 5 Ω/sq at 90% transmittance [71], which is comparable to a
graphene single-layer sheet of R sh 6.45 Ω/sq, suggesting that material with layers can
achieve a R sh of ~1 kΩ/sq for T = 90% [28]. The first report on graphene as a TCE in
DSSC applications was published by Wang et al. in 2008 [19]. They prepared 10-nm-
thick HRG films and achieved a R sh of 1.8 kΩ/sq and T of 62% at 550 nm that could be
used potentially as a window electrode. These HRGs were prepared by sequential dip
coating of GRO into silica glass under high temperatures. The HRG produced by this
method was found to have higher resistance and less transmittance to visible light than
commercially available TCE material. However, this study clearly opened the way
for graphene as TCEs in DSSC applications. It is evident from the literature that the
addition of CNTs, silver nanomaterials, or metal nanowires to the graphene skeleton
enhanced the performance of graphene-based TCE materials [57,72,73]. Recently, the
use of metal nanowires in graphene materials as TCEs showed a R sh of 3 Ω/sq at 80%
T or 20 Ω/sq at T = 90%, which represents properties that are very close to commer-
cially available TCE materials. These results led to the development of TCE materi-
als that successfully replaced existing TCE materials. For instance, copper nanowire
graphene-based TCE demonstrated better optical and electrical properties than a con-
ventional ITO TCE [74]. The core-shell nanostructured composite was prepared by
using a low-temperature, plasma-enhanced CVD process at 400°C. In addition, the
resulting composite demonstrated superb thermal oxidation and chemical stability be-
cause of the tight encapsulation of the CuNW with gas-impermeable graphene shells,
which was applied to fabricate bulk heterojunction polymer solar cells. In addition,
TCE based on a PEDOT:PSS/AgNW/graphene nanocomposite showed 216.67 Ω/sq
sheet resistance with ~83% transparency [75]. Additionally, after 100 cycles of bend-
ing, the sheet resistance of the PEDOT:PSS/AgNW/graphene electrode on the flexi-
ble polyethylene terephthalate (PET) substrate was found to be ~223 Ω/sq, whereas
conventional ITO-coated PET substrate exhibited 83,050 Ω/sq resistance, which was
~400 times more than the resistance before bending.

7.4.2 Graphene as semiconducting layer in DSSC
In DSSC applications, the photoanode material (semiconducting layer) is considered
to be the heart of the device and is deposited on a transparent conducting oxide on
a glass or plastic substrate. Most of the photoanode materials studied in DSSCs are
TiO 2 nanomaterials. However, various properties of TiO 2 that is to be modified to de-
velop the cell performance of the DSSC include (i) comparable energy levels between