Page 155 - Multifunctional Photocatalytic Materials for Energy

Graphene-based nanomaterials for solar cells 141

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Therefore, to replace I /I 3 -based electrolytes, work on other transition metals,
3+
3+
2+/
2+/
such as Co Co and Fe Fe , are being carried out. To date, graphene materials
are considered to be valuable candidates as additives in electrolytes to enhance device
performances [119]. In particular, the electron-accepting ability of HRG sheets is an
outstanding property that significantly enhances electron transport properties of re-
sulting materials when HRG is incorporated into inorganic materials, including NPs
to form nanocomposites. Use of 0.005% of graphene nanoribbons as an additive in an
electrolyte increases cell performance efficiency from 5.8% to 7%, which is mainly
due to the reduction of the visible light absorption by the electrolytes. This study
reports the early use of GRO as electrolytes as additives in small quantities, which
could yield a gel in a range of solvents containing an ionic liquid. Enhancement of
photovoltaic devices’ performance with the addition of GO was found to have a PCE
of 7.5%, compared with pure liquid electrolytes with a PCE of only 6.9%. In another
study, HRG was added to a gel electrolyte to improve the efficiency of DSSCs [120].
In this study, a gel polymer electrolyte consisting of poly(ethylene oxide) (PEO), γ-
butyrolactone (GBL), LiI, I 2 , and different concentrations of HRG was prepared. It
was found that the gel polymer electrolyte with 0.5 wt.% of HRG was the best solar
cell, delivering an efficiency of (5.07±0.97)% and the highest values of I sc and V OC .
HRG sheets acted as a multipurpose component in the electrolyte. The recovery of
the V OC values was attributed to the removal of the polyiodide species from the photo-
anode surface by interaction with the HRG sheets. It was suggested that the increase
in the I sc was due to an enhancement in the diffusion of I 3 -species and by the reduc-
tion of the electron transfer resistance in the counter electrode. In addition, because
it possesses the properties of a solid as well as a liquid, a quasi-solid state gel poly-
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mer electrolyte containing a solution of I /I 3 redox couple is also an effective route
toward improving stability. The unique hybrid network structure lowers leakage and
evaporation, thereby enhancing the stability of the cell. Many efforts have been made
to improve the ionic mobility, which in turn enhances the cell performance, and for
this purpose, different types of nanofillers have been introduced. Recently, NiO@TiO 2
nanocomposites have been applied as a working electrode in DSSCs (i.e., the applica-
bility of n-type TiO 2 and p-type NiO), whereas, gelatin hydrogel immobilized by GO
was used as the quasi-solid state gel electrolyte [121]. NiO was embedded into TiO 2
nanoparticles by a sol-gel approach to suppress the electron recombination in the dye
or electrolyte. The incorporation of GO in gelatin gel electrolytes greatly contributed
to reducing the charge transfer resistance as well as improving the current density.
Lorcan J. Bernnan and his coworkers reported the use of 2D graphene flakes as addi-
tives of various amounts in electrolytes to study the photovoltaic device performance
[122]. The addition of graphene nanosheets (1 wt.%) to pure electrolytes was found to
enhance the PCE 25-fold, because of the increase in the short-circuit current of DSSC
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devices. This facilitated the electrochemical reduction of I 3 in the electrolyte and thus
lowered the diffusion length of the redox couple through the electrolyte.
Electrolytes with high concentrations of graphene can be used as conductive fillers
and improve the charge transport. Ahmad et al. adapted a solution exfoliated method
to synthesize high-concentration graphene materials in order to create a gelled elec-
trolyte containing organic solvents and iodide [123]. The PCE was increased from