Page 147 - Multifunctional Photocatalytic Materials for Energy

Graphene-based nanomaterials for solar cells 133

Nevertheless, because of the processing advantages and unique properties, such as
mechanical stability and tunable electrical and optical properties, GO and HRG ob-
tained from top-down approaches are attractive for flexible and bendable thin-film
electronics and optoelectronics, including solar cells. Moreover, HRG sheets can be
assembled into layered network structures that can be viewed as “polycrystalline”
films, where the single “crystals” are the individual sheets of HRG or GRO [51]. In the
form of thin films, HRG and GRO sheets can be easily deposited on various substrates
using different techniques, including dip-coating, drop-casting, spraying, and so on
[52]. Notably, by proper tuning the deposition and reduction parameters, the films can
become insulating, semiconducting, and semimetallic, while maintaining the optical
transparency required for photovoltaic applications [53]. Indeed, transparent and con-
ducting electrodes made from HRG offer more advantages in cost and processability
than ITO, which is made from increasingly expensive and scarce indium and requires
costly vacuum equipment for deposition. Graphene inorganic nanoparticle-based nano-
composites have attracted particular attention as a new class of graphene-based hybrid
materials [54]. Technologists’ interest in these materials continues to grow because of
their ability to combine the desirable properties of building blocks for a given appli-
cation [55]. The novel catalytic, magnetic, and optoelectronic properties of graphene
nanocomposites based on hybridization with nanoparticles (NPs) have been exploited
in various applications, including energy storage and energy conversion. Therefore
graphene has been combined with various metal and metal oxide NPs for photovol-
taic applications, such as ultra-thin film of HRG/platinum (Pt) nanocomposites were
prepared via a layer-by-layer self-assembly method GO and respective metal salt as
precursor [56]. When applied as counter electrodes, the nanocomposite demonstrated
an excellent power conversion efficiency of 7.66%, which is comparable to pristine Pt
NPs as counter electrodes. Therefore graphene has reduced the amount of Pt consider-
ably and has lowered the cost of counter electrodes without compromising the activity
of devices. In another example, HRG/TiO 2 photoanode-based dye-sensitized solar cells
(DSSCs) were fabricated and demonstrated a power conversion efficiency of 4.28%,
which is 59% higher than occurs without graphene. Here HRG not only enhanced dye
adsorption efficiency of the photoanode but also increased the electron’s lifespan.
A power conversion efficiency of 7.25% was achieved in another study in which the
morphology of TiO 2 NPs on graphene nanosheets was carefully controlled [57].
Thus the application of graphene inorganic NP-based composites as photoelectrode
materials have great potential in photovoltaic cells due to ease of processing and flex-
ible substrate compatibility. Graphene also offers a broad solar spectrum; therefore,
as transparent electrodes, graphene-based materials may further improve the quantum
efficiency of solar cells, which has boosted research that has led to the development
of various graphene-based materials with unique properties. These graphene mate-
rials have been incorporated into various solar cell technologies, enhancing device
performance and reducing cost. Graphene-based materials are employed mostly in
DSSCs, heterojunction solar cells, OVCs, and PSCs, which can be used as transparent
electrodes, nontransparent electrodes, catalytic counter electrodes, sensitizers, elec-
trolytes, light harvesting materials, electron transport layers, hole transport layers, and
so on [32,58–66]. The upcoming section reviews the incorporation of graphene-based