Recent progress on reduced graphene oxide....

DOI: 10.1002/ijch.201400213
Recent Progress on Reduced Graphene Oxide-Based
Counter Electrodes for Cost-Effective Dye-Sensitized Solar
Cells
Suresh Kannan Balasingam[a]
and Yongseok Jun*[b]
1. Introduction
Since modern society relies entirely on an energy-based
economy, the existing fossil fuel reserves are expected to
run out in a few decades, causing future generations to
face a severe energy crisis. Moreover, the existing fossil
fuel-based energy technologies emit greenhouse gases,
which have a great impact on the environment, including
what is known as global warming.[1]
To solve these two
key issues, alternative energy conversion systems that
could solely depend on renewable energy resources (e.g.,
solar, wind, tidal, or geothermal energy) need to be de-
veloped. Of these, solar cells are one of the prominent
energy conversion devices, in which the direct conversion
of solar to electrical energy could be feasible. In addition,
the sun supplies an abundant amount of solar energy
throughout the day, indicating that this energy source is
unlimited. The solar cell technology currently available
on the market is based on first- and second-generation
solar cells (e.g., silicon, CdTe, and copper indium gallium
[di]selenide solar cells) that have the merits of high effi-
ciency and long-term stability. However, the cost of these
modules is very high, which prevents this technology
from reaching widespread clients. To reduce the cost of
solar cells, third-generation solar cells have been under
progressive research for the past few decades.[2]
Dye-sen-
sitized solar cells (DSSCs) are one type of third-genera-
tion solar cells that has attracted many researchers be-
cause of their numerous merits, including the following:
low cost, ease of fabrication, variety of colors, and high
power conversion efficiency (PCE).[3]
Recent literature
on DSSCs reported a maximum efficiency of around
13%, which revealed that the technology is commercially
viable if we could reduce the cost of the solar cell compo-
nents.[4]
DSSCs are composed of dye-coated mesoporous
TiO2 on fluorine-doped tin oxide (FTO)-coated glass as
a working electrode, an iodine-based electrolyte, and
a thin Pt layer coated on FTO-coated glass as a CE. The
two most expensive components are the FTO-coated
glass substrate and Pt-coated CE. Substitution of the
FTO-coated glass substrate with various metal substrates
either on the working or counter electrode part could cer-
tainly reduce the cost of the DSSCs. This possibility was
briefly discussed in our previous feature article.[5]
Anoth-
er approach is to replace the noble Pt metal-based CEs
with carbonaceous materials and/or non-noble metal
oxide/chalcogenide-based composite catalysts, which have
been briefly described in previous review articles.[6]
Of
Abstract: Dye-sensitized solar cells (DSSCs) are one type of
highly efficient low-cost solar cells among third-generation
photovoltaic devices. Replacing the expensive components
of DSSCs with alternative inexpensive and earth-abundant
materials would further reduce their price in the solar cell
market. Recently, graphene-based low-cost counter electro-
des (CEs) have been developed, which could serve as a po-
tential alternative to the expensive platinum-based CEs. In
this review article, we have summarized recent research on
various reduced graphene oxide (rGO)-based composite CE
materials, methods for their synthesis, their catalytic activity,
and the effective utilization of such CEs in DSSCs. The pho-
tovoltaic performance of DSSCs made of rGO-based compo-
site CEs were compared with the reference Pt-based cells,
and the photovoltaic parameters are summarized in tables.
Keywords: composites · counter electrodes · catalysis · reduced graphene oxide · dye-sensitized solar cells
[a] S. K. Balasingam
Department of Chemistry
School of Natural Science
Ulsan National Institute of Science and Technology (UNIST)
Ulsan 689-798 (Republic of Korea)
[b] Y. Jun
Department of Materials Chemistry and Engineering
Konkuk University
Seoul 143-701 (Republic of Korea)
Tel: (+82) 2-450-0440
e-mail: yjun@konkuk.ac.kr
Isr. J. Chem. 2015, 55, 955 – 965 2015 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim 955
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these, graphene, a new class of carbon allotrope, has been
investigated by many researchers because of its low cost,
high surface area, high electrical conductivity, and me-
chanical stability. Graphene materials have been inten-
sively studied by many researchers for the past few years,
and some review articles that generally focus on gra-
phene-based materials for solar cell applications were
published at least two years before.[7]
According to the
Web of Science database, research on alternative CEs for
DSSCs has dramatically increased in the past few years,
particularly since 2010. A couple of years ago, Wang et al.
published a mini review article, which focused particularly
on graphene-based CE materials for DSSCs.[8]
In the
present review article, we describe recent research trends
in the area of rGO-based alternative CE materials, the
method of reduction of GO into rGO, the catalytic activi-
ty of rGO-based composite materials and their PCE in
real-time DSSCs, with comparison to reference Pt-based
CEs.
2. Reduced Graphene Oxide-Based Composites
2.1 Metal Nanoparticle/rGO Composites
Basic trials of rGO for DSSC CEs have been performed.
Types of rGO used in recent representative trials include
rGO ribbons,[9]
jet-processed rGO,[10]
and rGO produced
by a green photothermal reduction process.[11]
In addition
to the standard method, rGO ribbons have also been pre-
pared by wet-spinning, a process in which an aqueous gra-
phene oxide (GO) solution is injected into a chitosan-
based solution. The method provides various types of rib-
bons such as oriented, randomly wrinkled, or spring-like
loops, by controlling drying conditions. Hydrochloric acid
was used to reduce the as-spun rGO ribbons, and their
conductivity reached approximately 100–150 ScmÀ1
. A
DSSC device with the rGO ribbon CE exhibited 2.80%
efficiency with short circuit current density (Jsc) of
8.77 mAcmÀ2
, open circuit voltage (Voc) of 0.61 V, and
quite a low fill factor (FF). This low performance was im-
proved to 3.87% efficiency once Pt nanoparticles were
embedded in the materials. Another recently reported
work, which applied a rapid atmospheric pressure plasma
jet process method, showed improved results. To prepare
a CE of DSSCs according to this method, rGO was first
prepared as a paste by mixing it with an organic vehicle.
The paste was then screen-printed on the FTO-coated
glass substrate, and then the electrode fabrication process
was completed by plasma jet flow. Because the tempera-
ture of the substrate reaches over 4008C, the organic ve-
hicles are burned-out, and rGO forms. A DSSC device
with this type of rGO exhibited efficiencies up to 5.19%,
whereas a reference cell with a conventional Pt CE exhib-
ited an efficiency of 5.65%.
In most cases, rGO itself was not a suitable candidate
for the CE of DSSCs. Therefore, many research groups
attempted to achieve improved performance by adding
metal nanoparticles to the rGO. The trials include silver,
nickel, tungsten, as well as platinum, nanoparticles. Em-
ploying metal nanoparticles provides a few advantages,
such as reduction of the required loading of metal cata-
lyst, improvement of the conductivity of the rGO, and an
increase in the transmittance of the CEs due to the re-
duced amount of metal loading. Gong et al. reported that
a Pt/graphene composite can reduce the amount of Pt
needed by approximately 1000-fold with comparable effi-
ciency.[12]
Guai et al. also utilized GO to prepare CEs
using an electrophoretic method. These researchers pre-
pared a GO solution and then reduced the solution into
graphene nanosheets by the chemical reduction with hy-
drazine.[13]
The dispersed graphene nanosheets were electrophoret-
ically deposited onto Pt-sputtered indium tin oxide
Suresh Kannan Balasingam is currently
a Ph.D. candidate at the Ulsan Nation-
al Institute of Science and Technology
(UNIST), Republic of Korea. He re-
ceived his B.Sc. degree in chemistry
from the Madura College (affiliated
with Madurai Kamaraj University),
India, in 2006. Balasingam completed
his M.Sc. degree in industrial chemistry
(specialization in electrochemistry)
with a gold medal from Alagappa Uni-
versity, India, in 2008. After that, he ob-
tained two years of research experience
at the Central Electrochemical Research Institute (CECRI-CSIR, Gov-
ernment of India) and one year of research experience at the Techni-
cal University of Denmark, prior to joining UNIST. His current re-
search interests mainly focus on supercapacitors and photoelectro-
chemical energy conversion and storage, encompassing DSSCs,
photoelectrochemical water splitting, and CO2 reduction.
Yongseok Jun is presently an associate
professor at Konkuk University, Repub-
lic of Korea. He received his B.Sc. and
M.Sc. degrees at Korea University
(Prof. Kang-Jin Kim’s group) and his
Ph.D. degree at the University of Min-
nesota at Minneapolis (Prof. Xiaoyang
Zhu’s group). He obtained three years
of research experience at the Electronic
and Telecommunications Research In-
stitute (2006–2009), followed by five
years of teaching and research experi-
ence, first as an assistant professor
(2009–2012) and then as an associate professor (2012–2013) at
UNIST, before starting his position at Konkuk University. His main
research interests include DSSCs, perovskite solar cells, and photo-
chemical reactions with TiO2 nanostructures. He has authored
more than seventy peer-reviewed journal articles, twenty-seven pat-
ents, and two book chapters, including those on flexible DSSCs
based on metal substrates.
Isr. J. Chem. 2015, 55, 955 – 965 2015 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim www.ijc.wiley-vch.de 956
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(ITO)-coated glass, or Pt was sputtered on graphene-
coated ITO. The highest temperature applied for the pro-
cesses was only 958C for the chemical reduction using the
hydrazine agent. When the researchers prepared the CEs
using this method, the amount of Pt loading varied from
1.9–5.6 mgcmÀ2
. Irrespective of the amount of Pt loading,
devices made of Pt/rGO CEs exhibited quite comparable
performance to devices made with only Pt/ITO. A refer-
ence device with a conventional Pt CE exhibited a power
conversion efficiency (PCE) value of 7.03%, whereas de-
vices with graphene-assisted Pt exhibited PCE values of
7.10–7.57%.
Tjoa et al. prepared a rGO/Pt composite using light-as-
sisted spontaneous co-reduction of GO and chloroplatinic
acid without any reducing chemicals.[14]
The illumination
transfers energy for the reduction, and the solvent (etha-
nol) works as a hole scavenger to form very small Pt
nanoparticles with simultaneous reduction of GO. This
method formed very uniform Pt nanoparticles that were
3 nm in size, whereas hydrazine reduction often yields Pt
particles with 20–200 nm diameters because of the vigo-
rous nature of the reduction. The light was from an incan-
descent bulb (100 W), and the local intensity was
50 WmÀ2
. Briefly, the sheet resistance values of the nano-
materials on the glass substrate using the two different re-
duction methods (the light-assisted method and the con-
ventional hydrazine method) are summarized in Table 1.
Dao et al. successfully applied a rGO/Pt nanohybrid as
a robust CE for DSSCs.[15]
To fabricate the CEs, they pre-
pared a GO paste by simply mixing GO powder in ethyl
cellulose and terpineol. The paste was applied on the
FTO-coated glass using doctor-blade method and dried
out at 3008C for 30 minutes. Finally, a small amount of Pt
precursor was dropped on the electrode, followed by Ar
plasma treatment for 15 minutes. Figure 1 shows the mor-
phology of the Pt nanoparticles/rGO hybrid. The images
show the general morphology of the Pt/rGO hybrid films,
including the uniform distribution of metal nanoparticles
on the film and the reduced nucleation of Pt nanoparti-
cles compared with the conventional Pt-only method. Ac-
cording to the Nyquist plot from electrochemical impe-
dance spectroscopy (EIS) analysis, the Pt/rGO CE exhib-
ited a charge transfer resistance (Rct) value of 0.52 Wcm2
,
which is only half of the Rct value of the Pt-sputtered CE.
The GO-only CE exhibited 20 times higher values for Rct
than that of the Pt/rGO CE. Finally, the PCE value ach-
ieved for the Pt/rGO CE was approximately 8.56%,
whereas the Pt-sputtered CE exhibited a PCE value of
8.18%.
Jang et al. reported a transparent rGO/Pt composite
CE for DSSCs, fabricated using a pulsed current electro-
deposition method.[16]
GO-dispersed deionized water/di-
methylformamide was spin coated onto FTO-coated glass,
followed by heating at 3508C for 10 minutes in air. For
the Pt addition, pulse current electrodeposition was per-
formed in a proper electrolyte solution with a Pt precur-
sor. Figure 2 presents the transmittance spectra of various
CEs.
Other types of metals, including Ni, Ag, and W, were
used to decorate the rGO surface.[17]
Similarly to Pt, the
other metal nanoparticles were evenly distributed on the
rGO surface. The distribution of Ni nanoparticles was
performed using a pulsed laser ablation process. The stan-
dard Pt CE exhibited Rct and PCE values of 7.73 Wcm2
and 2%, respectively, and the Ni/rGO CE exhibited Rct
and PCE values of 4.67 Wcm2
and 2.19%, respectively.
Table 1. Sheet resistance measurement of spray-coated Pt/rGO thin
film on glass (the thickness is approximately 0.8–1.0 mm)
Method of reduction Light assisted Hydrazine reduction
Pt/GO GO
Sheet resistance 0.3–3.2 kW 11.39 kW 15.52 kW
Figure 1. SEM images of (a) a GO CE and (b) Pt-distributed rGO. (c)
The TEM image shows that the Pt nanoparticles are evenly distrib-
uted. (d) Energy-dispersive X-ray spectroscopy (EDS) analysis and
(e) XRD pattern of the sample on a Si wafer substrate. Reprinted
with permission from reference [15].
Figure 2. Transmittance spectra of various CEs. Reprinted with per-
mission from reference [16].
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Silver nanowires and nanoparticles were embedded on
rGO either by drop casting or pulsed laser reduction. The
best cell performance was obtained when silver nanopar-
ticles were combined with rGO, which resulted in a PCE
value of 7.72%. However, the standard Pt electrode ex-
hibited a higher PCE value of 8.25%. Tungsten-decorated
rGO CE also exhibited comparable Rct and PCE values.
The best PCE value of 5.88% was obtained for W/rGO
CE, which is almost comparable to that of the Pt-based
cells (5.92%). The photovoltaic characteristics of the
above-mentioned DSSCs made of metal nanoparticles/
rGO composite CEs are summarized in Table 2.
2.2 Metal Compound/rGO Composites
Various types of metal compounds, such as transition
metal oxides (TMOs) and transition metal dichalcoge-
nides (TMDs), have been under consideration as alterna-
tive CE materials for DSSCs. These compounds can be
easily synthesized, are low in cost, and are abundant in
nature. Therefore, many researchers have performed in-
tense research on these materials, and their catalytic abili-
ties have been investigated in depth. Because of the pos-
sibility of increasing the conductivity and synergic catalyt-
ic ability, the mixing of rGO with these metal compounds
has recently received intense attention. Dou et al. synthe-
sized a Ni12P5/rGO composite using the simple thermal
hydrolysis of elemental P, nickel chloride, and GO in an
ethylene glycol-water mixture.[18]
When Ni12P5 was pre-
pared without GO, the morphology of the Ni12P5 was sim-
ilar to a honeycomb structure. When GO was introduced,
Ni12P5 formed in between the inter-layers of rGO, al-
though some nanoparticles were located on the outer sur-
face. Evaluation using EIS indicated that the Rct values at
the CE/electrolyte interface of individual Pt (as a refer-
ence), Ni12P5, Ni12P5/rGO, and rGO electrodes were
11.05 W, 6.22 W, 4.93 W, and 17.41 W in their system, re-
spectively.
The enhanced electrocatalytic performance of rGO/
SiO2 nanoparticles was introduced by Gong et al.[19]
A
schematic illustration of their preparation is presented in
Figure 3. The fabrication method is primarily drop cast-
ing, and the rGO/SiO2 composite was simply prepared by
mixing the two components with a hydrazine hydrate re-
ducing agent. SiO2 nanoparticles were used to increase
the volume for coating, as observed in Figure 3. The Bru-
nauer-Emmett-Teller (BET) surface area and pore size
distribution were investigated for the film. The theoretical
BET surface area of graphene is approximately
2620 m2
gÀ1
. The BET surface area of the as-prepared gra-
phene film in this experimental condition (without silica
nanoparticles) was only 8.6 m2
gÀ1
, with a pore size of
0.002 cm3
gÀ1
. When silica nanoparticles were embedded
into the rGO, the BET surface area was increased to
229.0 m2
gÀ1
, with a pore volume of 0.66 cm3
gÀ1
. The BET
surface area for silica nanoparticles used was 151.8 m2
gÀ1
.
The specific surface area of graphene in the CEs was cal-
culated to be 383.4 cm3
gÀ1
, after subtracting the contribu-
tion of the silica nanoparticles. A DSSC with the rGO/
SiO2 CEs exhibited a PCE value of 6.82%.
NiO is a common TMO semiconductor material.
Bajpai et al. demonstrated that NiO-decorated rGO ex-
hibits better electrocatalytic ability than pure NiO, and
the material could be applied as a CE for DSSCs, which
showed a PCE value of near 3.06%.[20]
Another NiO/
Figure 3. Schematic illustration of preparation and structure of
rGO and rGO/SiO2 CEs. Reprinted with permission from reference
[19].
Table 2. Photovoltaic parameters of DSSCs made of metal nanoparticle/rGO CEs and the reference DSSCs made of conventional Pt-based
CEs
Sl. No rGO-based CEs[a]
Pt-based CEs[b]
Ref.
Description Voc Jsc FF h Rct
[c]
Voc Jsc FF h Rct
[d]
(V) (mAcmÀ2
) (%) (Wcm2
) (V) (mAcmÀ2
) (%) (Wcm2
)
1 Pt/rGO 0.72 14.1 0.67 6.77 – 0.72 13.1 0.672 6.29 – [14]
2 Pt/rGO 0.771 16.66 0.667 8.56 0.62 0.763 16.80 0.638 8.18 1.22 [15]
3 Pt/rGO 0.71 14.98 0.663 7.05 ~5[e]
0.71 14.11 0.589 5.82 ~7[e]
[16]
6 Ag/rGO 0.732 14.67 0.72 7.72 1.02 0.733 16.24 0.69 8.25 0.76 [17a]
4 Ni/rGO 0.7 5.21 0.6 2.19 4.67 0.68 5.05 0.59 2.00 7.73 [17b]
5 W/rGO 0.72 10.02 0.63 4.55 – 0.69 13.62 0.63 5.92 – [17d]
6 Ag NW/rGO 0.55 6.45 0.52 1.61 11.57 0.58 6.47 0.44 1.87 16.47 [17c]
[a] DSSCs made of rGO-based CEs. [b] DSSCs made of Pt-based conventional reference electrodes. [c] Rct of rGO-based CEs from EIS analy-
sis. [d] Rct of Pt-based CE from EIS analysis. [e] No specific area, so the corresponding unit is W not WcmÀ2
.
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rGO CE was attempted by Dao et al.[21]
These researchers
prepared Ni-decorated rGO by drop casting of a Ni pre-
cursor, followed by Ar plasma reduction. The PCE value
for the Ni/rGO CE was 7.42%, whereas the Pt-sputtered
reference cell exhibited a PCE value of 8.18%.
A few other metal oxides have also been combined
with rGO. Hausmannite (Mn3O4) exhibits a distinctive
structure and unique physicochemical properties, such as
magnetic and catalytic effects. A Mn3O4/rGO suspension
was prepared by mixing GO and metal precursors, fol-
lowed by addition of hydrazine hydrate reducing agent.[22]
The morphology of the obtained product was aggregated
nanoparticles. The Rct (PCE) values of the CEs with
Mn3O4/rGO and Pt reference electrode were 5.24 W
(5.90%) and 1.43 W (6.84%), respectively. ZnO nanorods
were also used for the CE with rGO addition.[23]
The
ZnO nanorods were grown on FTO-coated glass, and
a laser pulse-reduced GO suspension was spin coated on
top. The Rct value of ZnO is greater than 4000 Wcm2
, and
once rGO was added, the Rct decreased to 4 Wcm2
, which
remains higher than the value for Pt CE of 0.5 Wcm2
. The
PCE value of the ZnO/rGO composite CE is 8.12%,
which is quite comparable to the Pt-based CEs (8.82%).
MoS2 has also received intensive attention, and MoS2/
rGO CE was introduced by Liu et al. for DSSC CEs.[24]
MoS2 has a hexagonal structure, where Mo metal atoms
are located in between sulfur layers. The structure is very
similar to the graphene structure. Therefore, it is consid-
ered as an inorganic graphene analogue. A solution of
GO and ammonium tetrathiomolybdate was prepared,
from which a MoS2/rGO composite was easily formed
under H2 flow at 6508C. The higher conductivity of rGO
helps electrons shuttle to MoS2 sites. In addition, the
MoS2 exhibited catalytic reduction of I3
À
ions. The Rct
value for MoS2/rGO CE is 0.57 Wcm2
, whereas the value
for the Pt-sputtered reference CE is 1.93 Wcm2
. However,
the PCE value for a device with MoS2/rGO CE (6.04%)
was lower than that for a device with the Pt-sputtered CE
(6.38%) by 0.34%. The main difference in performance
is due to the lower Jsc value. Similarly, Lin et al. reported
the electrophoretic deposition of transparent MoS2/rGO
composite films as the CE for DSSCs.[25]
These research-
ers synthesized the MoS2/rGO composite by thermal hy-
drolysis/dissociation of thiourea in the presence of GO.
The precursor of Mo was sodium molybdate. This CE ex-
hibited a high transmittance of approximately 70% near
the visible light region (400–800 nm). The Rct values for
MoS2, MoS2/rGO, and sputtered Pt CE are 3.65 Wcm2
,
2.34 Wcm2
, and 1.79 Wcm2
, respectively.
NiS/rGO CE was introduced by Li et al.[26]
The com-
pound was also synthesized by the hydrothermal reaction
of nickel ions and sulfur precursors with GO. These re-
searchers compared the peak-to-peak separation (Epp),
which is inversely correlated with the standard electro-
chemical rate constant of a redox reaction. The peak cur-
rent values are clearly important factors. The Epp values
for NiS2, NiS2/rGO, and Pt electrodes were observed to
be 0.65 V, 0.5 V, and 0.44 V, respectively. Note that Epp
for rGO was not obtained from the experimental window
of the cyclic voltammetry (CV) scanning. The PCE value
for a device with the NiS2/rGO composite was 8.55%,
whereas the devices made of Pt or NiS2 alone exhibited
Table 3. Photovoltaic parameters of DSSCs made of metal compounds/rGO CEs and the reference DSSCs made of conventional Pt-based
CEs
Pt-based CEs[b]
Ref.
[c]
Voc Jsc FF h Rct
[d]
(V) (mAcmÀ2
) (%) (Wcm2
) (V) (mAcmÀ2
) (%) (Wcm2
)
1 Ni12P5/rGO 0.727 12.86 0.61 5.70 4.93[e]
0.744 13.12 0.62 6.08 11.01[e]
[18]
2 SiO2/rGO 0.72 15.52 0.61 6.82 39.8[e]
0.72 15.79 0.64 7.28 2.6[e]
[19]
3 NiO/rGO 0.67 7.53 0.61 3.06 0.85 0.71 8.04 0.63 3.57 0.63 [20]
4 NiO/rGO 0.763 15.57 0.624 7.42 3.06 0.763 16.8 0.638 8.18 1.22 [21]
5 MoS2/rGO 0.73 12.51 0.66 6.04 0.57 0.72 13.42 0.66 6.38 1.93 [24]
6 MoS2/rGO 0.773 12.79 0.59 5.81 2.34 0.763 13.12 0.62 6.24 1.79 [25]
7 NiS2/rGO 0.749 16.55 0.69 8.55 2.9 0.739 15.75 0.70 8.15 0.5 [26]
8 Ta3N5/rGO 0.837 13.53 0.693 7.85 1.39 0.828 13.38 0.685 7.59 1.80 [27]
9 TaON/rGO 0.829 13.38 0.69 7.65 1.9 0.835 13.73 0.69 7.91 1.8 [28]
10 CZTS/rGO 0.71 16.77 0.656 7.81 13.33[e]
0.70 16.79 0.567 6.66 23.89[e]
[29]
11 CIS/rGO 0.728 16.61 0.576 6.96 0.98 0.751 14.77 0.624 6.92 1.73 [30]
12 CoS2/rGO 0.73 15.12 0.60 6.55 1.3 0.73 14.69 0.58 6.20 1.9 [31]
13 CoS/rGO 0.710 20.38 0.74 10.71 2.58 0.698 20.21 0.69 9.73 4.62 [32]
14 Bi2S3/rGO 0.74 15.33 0.608 6.91 4.98 0.73 16.47 0.618 7.44 1.70 [33]
15 Mn3O4/rGO 0.635 15.20 0.61 5.90 5.24 0.635 16.25 0.66 6.84 1.43 [22]
16 ZnO/rGO 0.765 21.7 0.671 8.12 4 0.777 24.2 0.708 8.82 0.50 [23]
17 CoS/rGO 0.718 14.95 0.66 7.08 1.01 0.711 15.53 0.65 7.18 0.85 [345]
18 NiSe/rGO 0.73 15.82 0.61 6.94 0.15 0.71 16.00 0.60 6.82 1.43 [35]
[a] DSSCs made of rGO-based CEs. [b] DSSCs made of Pt-based conventional reference electrodes. [c] Rct of rGO-based counter electrodes
from EIS analysis. [d] Rct of Pt-based CEs from EIS analysis. [e] No specific area, so the corresponding unit is W not WcmÀ2
.
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values of 8.15% and 7.02%, respectively. NiSe2/rGO
could be prepared by hydrothermal treatment with pre-
cursors such as NiCl2, selenium, ascorbic acid, and GO.[35]
Microsphere- and octahedron-structured NiSe2 are
formed in this reaction, the as-prepared materials exhibit-
ed quite low Rct values of 0.15 Wcm2
and 0.25 Wcm2
, re-
spectively, whereas Pt exhibited an Rct value of
1.43 Wcm2
.
Li et al. prepared Ta-containing metal compounds such
as Ta3N5 or TaON with rGO.[27,28]
The Rct values for the
CE of Pt, rGO, Ta3N5/rGO, and TaON/rGO are
1.80 Wcm2
, 7.0 Wcm2
, 1.39 Wcm2
, and 1.9 Wcm2
, respec-
tively. Ta3N5/rGO and TaON/rGO composite CEs exhibit-
ed quite comparable PCE values (7.85% and 7.65%, re-
spectively) to the Pt-based reference cells (7.6%).
Copper indium zinc can be used to prepare sulfur com-
pounds, which can also be applied as a CE material for
DSSCs as well as for thin-film solar cells starting from
copper indium gallium selenide (or sulfide). Bai et al. [29]
prepared a copper zinc tin sulfide (CZTS)/rGO compo-
site, which was successfully applied as a CE for DSSCs.
CZTS was successfully synthesized in an ethylene glycol
solvent, and it was then dispersed in ethanol and ethylene
glycol to make a paste. Similarly, Zhou et al. [30]
prepared
CuInS2/rGO. Although they did not mention the cell area
for the EIS experiment, a rough comparison might be
possible. For CZTS/rGO CE, the highest PCE value is
7.81%, with an Rct value of 13.33 W. CIS/rGO CE exhibit-
ed a PCE value of 6.96% with an Rct value of 0.99 W.
A few additional metal sulfide/rGO compounds have
also been investigated as CE materials. Hydrothermal
synthesis of a CoS/rGO composite at 1808C was per-
formed by two research groups, and the composite was
successfully applied as a CE for DSSCs.[31,34]
These re-
searchers achieved a PCE value of 7.08% with an Rct
value of 1.01 Wcm2
. Similarly, Bi et al. prepared CoS/rGO
(N [nitrogen doped]) and applied it as a CE for DSSCs.[32]
The CoS/rGO (N) CE exhibited Rct and Epp values of
2.58 W and 0.31 mV, respectively. The Epp value is quite
small even when compared with the Pt CEs. The CE of
Pt exhibited Rct and Epp values of 4.62 W and 0.4 mV, re-
spectively.
Bi2S3 has also been combined with rGO and exhibited
catalytic ability for the CE of DSSCs.[33,36]
Bi2S3 is a semi-
conductor with a direct bandgap of 1.7 eV. The composite
was synthesized by solvothermal treatment at 1508C, and
the shape of the product was a microsphere composed of
nanorods. The mechanism of formation was suggested to
be the nucleation of a GO sheet, as illustrated in
Figure 4. Mesoporous Bi2S3 prepared by Guo et al. exhib-
ited a similar nanostructure. The highest PCE and Rct
values of the Bi2S3-treated CE were 6.91% and 4.98 W,
respectively, when a 9 wt% rGO composite was used.
The reference Pt cell exhibited a PCE value of 7.44%
with an Rct value of 1.70 W. CdS particles were also treat-
ed with rGO, and they exhibited good catalytic effects
on the electrochemical analyses.[37]
The photovoltaic
characteristics of the above-mentioned DSSCs made of
metal compound/rGO composite CEs are summarized in
Table 3.
2.3 Polymer/rGO Composites
Various conducting polymer/graphene composites were
synthesized by different methods. Of these, polyaniline/
graphene nanocomposites were synthesized and applied
as a CE for DSSCs by Wang et al. in 2012.[38]
Initially, the
GO was reduced by a hydrothermal method in a Teflon-
lined autoclave at 1808C. Then, the reduced GO was
mixed with an aniline monomer, followed by an in situ
polymerization reaction. The as-prepared polyaniline
(PANI)/graphene composite was used as a CE material
for DSSCs, and a corresponding PCE value of 6.09% was
obtained, which is comparable to the reference Pt-based
cells. Liu and co-workers also synthesized graphene-modi-
fied polyaniline-based CE materials for DSSCs.[39]
They
reduced GO into rGO using thermal exfoliation and re-
duction method. The graphene obtained was mixed with
an aniline monomer and subjected to an electropolymeri-
zation reaction on an FTO substrate. The device made of
the graphene-modified polyaniline CE exhibited a PCE
value of 7.17%, which is nearly equal to that of the refer-
ence Pt-based DSSCs (7.24%) prepared under similar
conditions.
Niu and colleagues followed the in situ reduction of
GO into rGO during the polymerization of aniline into
polyaniline.[40]
These researchers prepared a polyaniline/
rGO composite via a one-step chemical synthesis. The re-
duction of GO into rGO was confirmed by Raman spec-
troscopy. The DSSC made of 1-Fe2O3 as the photoanode
material and PANI-rGO as the CE exhibited a PCE
value of 1.24%, which is much lower than those of the
other TiO2-based photoanode DSSCs.
Recently, Wang and co-workers directly deposited
a PANI/rGO composite onto an FTO-coated glass sub-
Figure 4. The formation process of the Bi2S3 microspheres. Most of
the nanostructure growing mechanisms on rGO are very similar.
Reprinted with permission from reference [36].
RReevviieeww

strate using electrostatic deposition.[41]
Initially, the PANI/
GO composite was deposited onto the FTO, which was
further subjected to chemical reduction with hydroiodic
acid to form the PANI/rGO CE. DSSCs made of the
PANI/rGO CE exhibited efficiency (7.84%), which is
comparable to that of the Pt-based reference cells
(8.19%) during front illumination by sunlight. For the
rear-side illumination process, DSSCs made of both CEs
exhibited similar PCE values (Pt—6.1%; PANI/rGO—
6.08%). Therefore, the authors claimed that the PANI-
rGO CE material could be a potential candidate for bifa-
cial DSSCs. Wan et al. also synthesized a polyaniline/gra-
phene nanocomposite via the facile in situ polymerization
of aniline in a graphene solution.[42]
Initially, GO was re-
duced to rGO by chemical reduction (hydrazine and am-
monia solution). The device made of this nanocomposite
CE exhibited a lower PCE value of 4.46%, compared
with the Pt-based cells (5.71%).
PANI-graphene/GO multilayer CEs were synthesized
by Wang and colleagues.[47]
Initially, positively charged
PANI-graphene complexes were prepared by reflux. The
positively charged PANI-graphene complex was multi-
layered with negatively charged GO using repeated alter-
nating immersion of an FTO-coated glass substrate in sol-
utions of PANI-graphene and GO. The device made from
the multi-layered CE exhibited the best PCE value of
7.88% with a Voc of 0.727 V, Jsc of 16.79 mAcmÀ2
, and FF
of 0.646. The authors did not compare their values with
those of reference Pt-based cells.
Polypyrrole (PPy)/rGO nanocomposite CEs were syn-
thesized by Lim et al., via in situ electrochemical polymer-
ization.[43]
The device made with the PPy/rGO nanocom-
posite CE exhibited a similar PCE value (2.21%) to the
reference Pt-based cells (2.19%). Liu and co-workers also
prepared a PPy/rGO composite CE on rigid FTO and
flexible plastic substrates via a low-temperature electro-
chemical deposition method.[44]
They followed a two-step
electrochemical process. In the first step, the PPy/GO
composite was electrochemically deposited onto the con-
ducting substrate. In the second step, the as-deposited
PPy/GO was electrochemically reduced into PPy/rGO.
DSSCs made of the PPy/rGO composite CE exhibited
a PCE of 6.45%, which is comparable to that of the Pt-
based cells. The authors also fabricated a PPy/rGO CE
on a conducting plastic substrate (polyethylene naphtha-
late-ITO). The efficiency of the DSSC made of the flexi-
ble substrate was lower than that using the rigid glass-
based substrate. However, the PCE value of PPy/rGO
(4.25%) was still comparable to that of the Pt-based ref-
erence cells (4.83%). Gong and colleagues synthesized
a rGO/PPy composite CE via in situ reduction of GO/
PPy to rGO/PPy.[45]
The composite rGO/PPy exhibited
a superior PCE value compared with that of the individu-
al components (PPy and rGO CEs). The Rct and PCE
values of rGO/PPy are almost equal to those of the refer-
ence Pt-based cells.
Xu et al. synthesized poly(diallyldimethylammonium
chloride) (PDDA)/rGO composite CEs for DSSCs.[46]
Ini-
tially, the PDDA/GO composite was formed by layer-by-
layer assembly of negatively charged GO and positively
charged PDDA. In the subsequent step, the as-deposited
PDDA/GO was electrochemically reduced to PDDA/
rGO. DSSCs made from PDDA/rGO CEs exhibited
higher PCE values than the Pt-based reference cells. The
photovoltaic characteristics of the above-mentioned
DSSCs made with polymer/rGO composite CEs are sum-
marized in Table 4.
2.4 Functionalized or Doped rGO Composites
Compared with other rGO-based materials, functional-
ized or doped rGO CEs exhibit superior electrocatalytic
activity. In 2012, Xu et al. functionalized rGO with an
iron-containing porphyrin, called hemin, using a micro-
wave-assisted chemical reduction method (hydrazine hy-
drate and ammonia).[48]
The as-prepared hemin-function-
alized rGO was coated on FTO by drop casting followed
by drying. The Rct value (at the CE/electrolyte interface)
Table 4. Photovoltaic parameters of DSSCs made of polymer/rGO composite CEs and reference DSSCs made of conventional Pt-based CEs
Pt-based CEs[b]
Ref.
[c]
Voc Jsc FF h Rct
[d]
(V) (mAcmÀ2
) (%) (Wcm2
) (V) (mAcmÀ2
) (%) (Wcm2
)
1 PANI/graphene nanocomposite 0.685 13.28 0.67 6.09 – 0.695 14.20 0.70 6.88 – [38b]
2 Graphene modified PANI 0.67 16.28 0.67 7.17 0.64 0.69 15.01 0.70 7.24 0.22 [39]
3 rGO/PANI composite 0.57 4.07 0.528 1.24 28[e]
0.55 2.94 0.580 0.94 30[e]
[40]
4 PANI/rGO 0.775 15.8 0.64 7.84 0.71 0.780 15.9 0.66 8.19 0.69 [41]
5 PANI/graphene nanocomposite 0.69 12.19 0.53 4.46 – 0.75 11.36 0.67 5.71 – [42]
6 PPy/rGO nanocomposite 0.70 7.49 0.42 2.21 14.8 0.71 5.12 0.60 2.19 14.3 [43]
7 PPy/rGO composite 0.695 15.48 0.60 6.45 6.59[e]
0.70 15.45 0.66 7.14 4.19[e]
[44]
8 rGO/PPy 0.725 15.81 0.71 8.14 5.0 0.724 16.00 0.72 8.34 1.1 [45]
9 PDDA/rGO composite 0.692 18.77 0.74 9.54 – 0.686 18.11 0.74 9.14 – [46]
sis. [d] Rct of Pt-based CEs from EIS analysis. [e] No specific area, so the corresponding unit is W not WcmÀ2
.
RReevviieeww

of the hemin-functionalized rGO was 9 W, which was
slightly higher than that of the reference Pt-based CEs
(7 W). It is apparent that the DSSCs made of hemin-func-
tionalized rGO CEs exhibited a lower PCE value
(2.45%) than that of the Pt-based reference cells. The de-
tailed I-V curve parameters are listed in Table 5.
Nitrogen-doped reduced graphene oxide (NG) has
been investigated as an efficient CE material by many re-
searchers. Yen and colleagues synthesized nitrogen-doped
graphene sheets by combined chemical (ammonia and hy-
drazine hydrate) and hydrothermal reduction of GO.[49]
The DSSCs made of NG film CEs exhibit a superior PCE
value (4.75%) compared with that of the undoped one
(1.92%), yet their efficiency is slightly lower than the Pt-
based reference cells (5.03%). Wang et al. also prepared
nitrogen-doped graphene via the hydrothermal reduction
of GO in the presence of ammonia.[50]
CV studies re-
vealed that the NG samples exhibited similar redox peaks
compared with those of the reference Pt electrodes,
whereas in the pristine graphene electrodes, no distinct
redox peaks were observed. Therefore, the authors claim-
ed that the higher electrocatalytic activity of the NG elec-
trode is predominantly due to the structural defects
caused by the doping with nitrogen atoms, which have
a lone pair of electrons and, in turn, bring negative
charge onto the delocalized p-electron system of gra-
phene. The PCE value of the nitrogen-doped graphene
was 6.12%, with a Jsc of 15.19 mAcmÀ2
, Voc of 0.683 V,
and FF of 0.59. The PCE value is comparable to that the
Pt-based reference cells (6.97%).
Xue et al. fabricated three-dimensional nitrogen-doped
graphene foams (3D-NGFs) by annealing freeze-dried
GO foams in ammonia.[51]
The three-dimensional NGFs
exhibited a superior PCE value (7.07%) compared with
that of the undoped rGO foams (rGOFs; 4.84%). More-
over, the PCE values of the NGFs and rGOFs were still
higher than those of the corresponding film structures.
Generally, nitrogen doping enhanced the electrical con-
ductivity and resulted in good catalytic activity. In addi-
tion, the 3D-foam structure with a large surface area and
well-defined pores enhanced the catalytic sites for I3
À
/IÀ
redox.
In 2013, Wang et al. also adopted the hydrothermal
route to prepare nitrogen-doped graphene sheets
(NGSs).[52]
The DSSC made of an NGS CE exhibited an
efficiency of 7.01%, which is comparable to that of the
Pt-based reference cells (7.34%). The NGS-based sym-
metrical cells exhibited an Rct value of 0.9 Wcm2
, which is
much smaller than that of the pristine graphene-based
symmetric cells (14.1 Wcm2
). Zhang and co-workers pre-
pared NGSs by heating GO at high temperatures (600–
9008C for 2 h) in the presence of ammonia.[53]
The NGS
sample was mixed with polytetrafluoroethylene, and the
resultant slurry was coated on a stainless steel substrate.
The device made of the NGS electrode heated at 6008C
exhibited a higher PCE value than those of Pt and sam-
ples treated at higher temperatures. Notably, the Voc
values of all the NGS-based DSSCs (0.858–0.868 V) are
higher than that of the reference Pt-based cells (0.796 V).
The authors suggested that the higher Voc values may be
due to the shift of the flat-band potential, which, in turn,
enhances the reactivity and electrocatalytic performance
of DSSCs.
Ju and colleagues synthesized N-doped graphene plate-
lets (NGPs) via a novel two-step sequential method.[54]
In
the first step, edge-aminobenzoyl functionalized graphite
(EFG) was attained using edge-selective functionaliza-
tion. In the second step, the EFG powder was heated to
high temperature (9008C for 2 h) under a nitrogen atmos-
phere to obtain the NGPs. The PCE of the NGP-based
DSSCs exhibited a superior value (9.05%), which is
slightly higher than the Pt-based reference cells (8.43%),
a Jsc of 13.83%, Voc of 0.883 V, and FF of 0.74. Notably,
the Rct value of the NGP electrodes (3.06 Wcm2
) is much
smaller than the Rct value of the Pt-based reference cells
(8.44 Wcm2
).
The same group also selectively doped nitrogen at the
edges of graphene nanoplatelets (NGnPs) via a simple
ball milling reaction in the presence of nitrogen gas.[60]
DSSCs made of an NGnP-based CE exhibited a lower Rct
value and a higher PCE value than those of the reference
Pt-based DSSCs. The authors also adopted a similar ball
milling process to selectively synthesize edge-carboxylat-
ed graphene nanoplatelets (ECGnPs) in the presence of
carbon dioxide.[61]
The device made of oxygen-rich
ECGnPs exhibited superior catalytic activity and a higher
PCE value than that of the chemically reduced GO and
Pt-based CEs.
Recently, Song and co-workers adopted the previously
described concept of Xue et al., three-dimensional porous
graphene foams, but performed the nitrogen doping by
hydrothermal treatment of a solution of GO and ammo-
nia.[55]
The N-doped porous graphene foams (NPGFs)
that formed exhibited a PCE value of 4.5%, which is
slightly lower than their Pt-based reference cells (4.9%).
Compared with the previous report by Xue et al., the re-
ported PCE value of the NPGFs is much smaller in both
iodine and sulfide-based electrolytes. Very recently, brick-
like N-doped graphene/carbon nanotube (NGC) compo-
site three-dimensional films were synthesized by Ma
et al.[56]
Initially, the carbon nanotubes (CNTs) were pre-
pared by chemical vapor deposition, and the GO solution
was prepared by ultrasonic exfoliation. The GO/CNTs
composite film was made by mixing together solutions of
each component, followed by vacuum filtration. The as-
prepared film was first ground, then mixed with mela-
mine, and finally subjected to high-temperature thermal
treatment under argon to dope with nitrogen. The Rct
value of the NGC CE is much smaller (1.78 Wcm2
) than
that of the Pt-based reference cells (8.97 Wcm2
). The
PCE value of the NGC-based DSSCs is similar to that of
the Pt-based DSSCs.
RReevviieeww

Luo and co-workers prepared nitrogen-doped rGO
(NRGO), sulfur-doped rGO (SRGO), and nitrogen and
sulfur dual-doped rGO (NSRGO) CEs via hydrothermal
reactions of the various precursors.[57]
DSSCs made of the
SRGO-based CEs exhibited a higher PCE value (4.73%)
than that of the device made of NRGO-based CEs
(3.85%). Of the above three types of CEs, the NSRGO-
based CE exhibited a higher catalytic activity and, in
turn, a higher PCE value for DSSCs made with a disul-
fide/thiolate redox shuttle.
Boron-doped graphene (BG) CEs were synthesized by
Fang and colleagues.[58]
B2O3 was used as a boron source,
and GO and B2O3 were ground together and then sub-
jected to high-temperature annealing. DSSCs made of
BG CEs exhibited a PCE value of 6.73%, which is higher
than that of the reference Pt-based CEs (6.34%). Very
recently, Jung et al. also prepared BG CEs using BBr3
precursors, via a modified Wurtz reaction in a high-pres-
sure reactor.[59]
The device made of the BG CEs exhibited
a higher PCE value (9.21%) than that of the reference
Pt-based cells (8.45%).
Wang et al. synthesized phosphorous-doped rGO
(PRGO) CEs using a facile high-temperature annealing
method.[62]
For the first time, a PRGO-based CE was
used in DSSCs. Although the Rct value of the PRGO CE
is lower than that of the reference Pt-based cells, the
overall PCE value of the Pt-based cells exhibited a higher
value (6.80%) than that of the PRGO-based cells
(6.25%). The photovoltaic characteristics of the above-
mentioned DSSCs, made of functionalized or doped/rGO
composite CEs, are summarized in Table 5.
3. Summary and Future Outlook
We have briefly summarized the recent progress in the
field of rGO-based CEs for DSSC applications. GO con-
taining several oxygen functional groups remains a good
catalyst for the I3
À
/IÀ
redox reaction. However, the elec-
trical conductivity of GO is significantly lower than that
of rGO. Therefore, several researchers focused on various
reduction methods (e.g., hydrothermal, heat treatment, or
chemical reduction) to obtain highly conducting rGO.
Defect sites in rGO act as a pathway for the transfer of
electrons at the CE/electrolyte interface, which enhances
the catalytic activity. In recent years, heteroatoms such as
nitrogen, sulfur, boron, and phosphorous were intention-
ally used to dope the rGO matrix to enrich the defect
sites on its surface. Adhesion of carbon-based materials
on the FTO substrate is one of the key issues in CEs of
DSSCs. To overcome these issues, rGO-based composite
materials have been developed by researchers. Metal
nanoparticles incorporated into rGO CEs have led to
lower Rct, much higher catalytic activity, and increased
PCE values. TMOs and TMDs also display good catalytic
activity. However, their conductivity needs improvement.
Composites made of rGO with TMOs and rGO with
TMDs showed higher conductivity and promising electro-
catalytic activity toward the I3
À
/IÀ
redox reaction. Con-
ducting polymer/rGO composites also showed high con-
ductivity and remarkable electrocatalytic properties.
Moreover, polymer acts like a binder, facilitating proper
adhesion of electroactive material on FTO substrates.
Thus far, various materials have been synthesized and uti-
lized in DSSC applications, but most of these composites
showed lower PCE values than those of the Pt-based ref-
erence cells. Only a few reports showed higher PCE
values than those of the reference cells. To achieve higher
PCE values, the electrocatalytic mechanism and kinetics
Table 5. Photovoltaic parameters of DSSCs made of functionalized or doped rGO composite CEs and of reference DSSCs made of conven-
tional Pt-based CEs
Pt-based CEs[b]
Ref.
[c]
Voc Jsc FF h Rct
[d]
(V) (mAcmÀ2
) (%) (Wcm2
) (V) (mAcmÀ2
) (%) (Wcm2
)
1 Hemin-rGO 0.65 5.75 0.31 2.45 9[e]
0.73 6.56 0.67 3.18 7[e]
[48]
2 N-doped graphene sheets 0.82 10.55 0.55 4.75 – 0.77 9.37 0.70 5.03 – [49]
3 N-doped graphene (NG) 0.683 15.19 0.59 6.12 – – – – 6.97 – [50]
4 3D-N-doped graphene Foams (NGFs) 0.77 15.84 0.58 7.07 5.6[e]
0.79 14.27 0.66 7.44 8.8[e]
[51]
5 N-doped graphene sheets 0.695 15.76 0.64 7.01 0.9 0.691 16.11 0.66 7.34 0.75 [52]
6 N-doped rGO 0.858 13.00 0.72 8.03 5.76 0.796 13.26 0.69 7.33 – [53]
7 N-doped graphene nanoplatelets (NGPs) 0.883 13.83 0.74 9.05 3.06 0.885 13.48 0.70 8.43 8.44 [54]
8 N-doped porous graphene foams (NPGFs) 0.708 13.14 0.48 4.5 15.2 0.756 14.64 0.45 4.9 9.8 [55]
9 N-doped graphene/CNTs composite (NGC) 0.766 16.23 0.54 6.74 1.78 0.768 16.66 0.53 6.89 8.97 [56]
10 N and S- dual doped rGO (NSRGO) 0.601 11.70 0.67 4.73 0.39 0.609 10.22 0.50 3.11 34.9 [57]
11 B-doped graphene (BG) 0.73 13.93 0.66 6.73 1.37[e]
0.73 13.28 0.65 6.34 8.40[e]
[58]
12 B-doped graphene (BG) 0.887 13.73 0.756 9.21 1.41 0.885 13.44 0.711 8.45 2.84 [59]
sis. [d] Rct of Pt-based CEs from EIS analysis. [e] No specific area, so the corresponding unit is W not WcmÀ2
.
RReevviieeww

of these composite materials must be further investigated
in detail. Although most of the rGO-based composite ma-
terials showed comparable efficiencies to those of the Pt-
based cells, their long-term stability must also be verified.
Future research should focus on measuring and, if neces-
sary, improving the long-term stability of these materials
and the devices they comprise, hopefully promoting them
toward commercial applications.
Acknowledgments
This research was supported by the National Research
Foundation of Korea (NRF), funded by the Korean gov-
ernment, MSIP/KETEP (2014060255, 2014060440,
20133030000140, and 20123010010070).
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Received: December 31, 2014
Accepted: February 24, 2015
Published online: May 15, 2015
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