time when in contact with the tri-Iodide/Iodide
(I3–/I–) liquid electrolyte, reducing the efficiency.
Thus, the replacement of such elements with
lower cost and/or more reliable materials (
leading to high-efficiency devices) is needed. Graphene
can satisfy all the counter-electrode requirements
because of its high SSA ( 59), which is essential
to help the I3– reduction, high electric conductivity ( 4), low charge-transfer resistance ( 25),
and lower cost than platinum.
Graphene oxide ( 43) and hybrid structures of
RGO-CNTs ( 64) have been used as counter electrodes in DSSCs, with results close to the state of
the art with platinum (Fig. 2). Graphene nanoplatelets (sheets of functionalized graphene with
an overall thickness ranging from ~2 to 15 nm) are
now emerging as the best performing counter-electrodes in DSSCs, with ( 27) reporting the
highest h to date of 13%.
The need to develop a platinum-free counter-electrode has seen a rising interest also in inorganic LMs such as transition metal oxide (TMO)
and metal carbides, nitrides, and sulfides ( 65).
Thin flakes of MoS2 and WS2 counter-electrodes
were used in ( 65), with the I3–/I– redox couple,
achieving h = 7. 59 and 7.73%, respectively, which
is close to that of platinum counter-electrodes.
In particular, platinum was outperformed as a
counter-electrode by MoS2 ( 4.97%) and WS2
( 5.24%) in DSSCs using an organic disulfide/
thiolate (T2/T–) redox couple ( 65). Hybrid systems,
such as graphene-MoS2, were also used as counter-electrodes in DSSCs, achieving h = 5.81% ( 66).
Thus, although to date h is lower than the best
reported for platinum ( 12.3%) and graphene
nanoplatelets (13%) ( 27), with further optimization 2D crystals ( 65) and hybrids ( 66) could play
a key role as counter-electrodes in DSSCs.
GRMs exploited as counter-electrodes in DSSCs
( 65, 66) or in Cd Te ( 67) solar cells show encouraging results. The efficiency of PV devices based
on GRMs is progressing at a pace superior to
those based on conventional materials ( 32). The
highest h = 13% to date for DSSCs was recently
achieved by using graphene nanoplatelets as a
counter-electrode. Graphene/silicon hybrid solar
cells, although first reported in 2010 ( 46), already have h = 14.5% ( 68), whereas graphene-based perovskite solar cells have h = 15.6% ( 60)
for low-temperature (<150○C) processing, matching that reported for high-temperature (>500○C)
cells ( 38), thus with an advantage in processing
and cost reduction. In Fig. 2, we compare h of
GRM and conventional non-GRM–based PV devices. The results to date could enable integration
in existing devices with higher h and the development of new-concept devices, such as graphene/
silicon solar cells.
About half of the energy generated worldwide
is lost as waste heat ( 69, 70). Thermoelectrics,
solid-state devices (Fig. 1C) that generate electricity
from a temperature gradient, are ideal to recover
waste thermal energy ( 69, 70). Thermoelectric
devices can also convert heat produced by concen-
trated or unconcentrated sunlight, into electricity
( 69, 70). This is important because infrared ra-
diation with photon energies below the band gap
of the photosensitizers is not absorbed in con-
ventional PV cells and generates only waste heat
( 69, 70).
In a typical thermoelectric device, a junction
is formed between two different n- and p-doped
conducting materials (Fig. 1C). A heat source at
the junction causes carriers to flow away from
it, resulting in a “thermo-electric” generator (ex-
ploiting the Seebeck effect, resulting in a voltage
induced by a temperature gradient). In a thermo-
electric device, many of these junctions are con-
nected electrically in series and thermally in
parallel. They can also work inversely, using elec-
tricity to generate or remove heat. When a cur-
rent is passed in the appropriate direction through
a junction, both types of charge carriers move
from the junction and transport heat away, thus
cooling the junction (Peltier effect). Thermoelec-
tric devices are appealing, but their low effi-
ciencies limit their widespread use.
The effectiveness of a thermoelectric device is
assessed in two ways: by its Carnot efficiency (the
fraction of absorbed heat that is converted into
work) and by a material-dependent figure of
merit, known as z T; z T = TS2s/k ( 69), where S is
the Seebeck coefficient, T is the temperature, s is
the electric conductivity, k is the thermal conduc-
tivity, and z = S2s/k ( 69). Thus, thermoelectric
materials require high S and s and low k ( 69).
In order to optimize z T, phonons must experi-
ence a high scattering rate, thus lowering thermal
conductivity [like in a glass ( 69, 70)], whereas
electrons must experience very little scattering,
maintaining high electric conductivity (as in a
crystal) ( 70).
The majority of explored materials in thermo-
electric devices have z T 1 ( 69). LMs such as
Bi2Te3, Pb Te, and their alloys ( 29) and, in par-
ticular, the (Bi1–xSbx)2(Se1–y Tey) 3 alloy family have
been in commercial use for several decades be-
cause of their room-temperature zT 1 and
Carnot conversion efficiencies 5 to 6% ( 69).
State-of-the-art thermoelectric materials design
relies on engineering of the scattering mecha-
nisms for phonons ( 70) and charge carriers ( 70).
Currently, superlattices of Bi2Te3/Sb2Te3 ( 71) and
quantum dots fabricated by means of atomic
layer deposition designed to disrupt the pho-
non mean free path, while still allowing good
electron mobilities, have the highest z T 2.4 to
2.9 at 300 to 400 K ( 71) and 3. 5 at 575 K, re-
spectively ( 72).
Graphene has both high electric ( 4) and ther-
mal ( 73) conductivity, a combination not ideal
for thermoelectric devices. However, it is possible
to tailor the thermal transport properties of
graphene by nano-structuring techniques, such
as defect ( 74) and isotope ( 75) engineering or
edge roughness ( 74), or by introducing periodic
nano-holes ( 76). The combination of geometrical
structuring, GNRs with predefined geometries
( 19), and isotopic enrichment with 13C ( 75) can
reduce thermal conductivity by up to two or-
ders of magnitude with respect to pristine graphene
( 77). It has been estimated that z T up to 3. 25 can
be achieved by exploiting GNRs that have a
chevron-like geometry ( 77). However, scaling
up of GNRs via chemical synthesis ( 19) still
poses a challenge. Nevertheless, the modula-
tion of geometric factors determining electric
and thermal conductivity might be achieved via
LPE. This technique also allows the blending of
LMs with CNTs in order to increase the electrical
conductivity while not reducing the Seebeck
Advances in nanostructuring ( 74–76) to create
hybrid structures on demand, with high electrical
conductivity and low thermal conductivity, could
accelerate the development of high-performance
GRMs for thermoelectric devices.
1246501-4 2 JANUARY 2015 • VOL 347 ISSUE 6217 sciencemag.org SCIENCE
Fig. 3. Schematic of GRMs-based battery electrodes. In this example, the anode is composed of
graphene flakes, but other 2D crystals can also be used, alone or in hybrid structures, as detailed in the
text. The cathode is a hybrid graphene-lithium compound (such as LiCoO2 or LiFePO4), designed to
enhance electron transport kinetics compared with graphene-free lithium compounds.