Graphene application in solar cell

1. Promoting Effect of Graphene on Dye-Sensitized Solar Cells
Hui Wang, Samantha L Leonard, and Yun Hang Hu*
Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan
49931-1295, United States
ABSTRACT: In this paper, a simple approach without a prereduction of graphene oxide was exploited to prepare a graphene-
doped TiO2 film for dye-sensitized solar cells (DSSCs). The performance measurement of the DSSCs showed that the
incorporation of graphene could increase the short-circuit current density and power conversion efficiency by 52.4 and 55.3%,
respectively. Furthermore, it was demonstrated that the performance enhancement was due to the promoting effect of graphene
on electron transfer instead of the increase of dye loading in TiO2/graphene composite films. However, graphene can also absorb
solar light, which could lead to the decrease of light harvest of dye molecules and thus a negative effect on the power conversion
efficiency of DSSCs. Furthermore, graphene might decrease the actual dye loading on TiO2 in a TiO2/graphene film, which can
also make a negative contribution to the conversion efficiency. As a result, the promoting effect of graphene is strongly dependent
on its content; namely, the efficiency of DSSCs increases to the maximum value and then decreases with increasing graphene
content in TiO2/graphene composites.
1. INTRODUCTION
Since its experimental demonstration in 2004,1
graphenea
two-dimensional carbon nanomaterialhas been a rising star in
material science due to its unique properties, such as high
carrier mobility,2
excellent transmittance,3
large surface area,4
and high breaking strength and Young's modulus.5
Graphene can be synthesized via various approaches,
including mechanical exfoliation,1,6−9
epitaxial growth on SiC
substrate,10−14
chemical vapor deposition (CVD),15−18
and
chemical exfoliation method (also called oxidation−reduction
of graphite).19−23
The chemical exfoliation method, which
produces graphene sheets via three steps (oxidation of graphite,
exfoliation of graphite oxide to graphene oxide, and reduction
of graphene oxide), is attracting much attention due to its low
cost, its high yield to synthesize large area graphene films, and
its ability to achieve chemical functionalization. Furthermore,
this method is widely employed to prepare graphene-based
composites.24,25
This happened because graphene oxide can be
well dispersed in water or other polar solvents to form a
homogeneous solution.26,27
The graphene oxide solution can
allow one to mix graphene with a matrix homogenously,
producing graphene-based composites with promising mechan-
ical and thermal properties.25
Very recently, it was reported that
graphene sheets can provide pathways for electron transfer in
graphene-based composites.28−33
For example, transistors
fabricated with graphene−polystyrene composite could exhibit
ambipolar field effect characteristics.28
Recently, considerable
efforts have been made to fabricate TiO2/graphene compo-
sites,29−33
which showed improved photocatalytic activities
through the tests of dye photodegradation30−32
and the
enhanced performance in Li-based batteries.33
As a novel two-dimensional carbon nanomaterial, graphene is
being explored for its application in photovoltaic devices.34
The
dye-sensitized solar cell (DSSC) is a promising photovoltaic
device due to its low production cost, ease of manufacturing,
and acceptable power conversion efficiency.35−39
In principle,
photoexcitation of the dye in a DSSC leads to a rapid electron
injection to the conduction band of the semiconductor (i.e.,
TiO2), followed by electron transfer to the photoelectrode,
where the original state of the dye is restored by electron
donation from the electrolyte. The collected electrons in the
photoelectrode are transported through an external circuit to
the counter electrode, and the circuit is completed through
regenerating the electrolyte at the counter electrode.40,41
The
application of graphene sheets in DSSCs is very promising.
Graphene films have been explored as both the transparent
electrode and the counter electrode for DSSCs due to their
high transparency and electron transfer mobility as well as their
electrochemical activities.42−44
Furthermore, graphene sheets
can also mix with TiO2 semiconductor to form a composite film
and to enhance the electron transfer from TiO2 to a
photoelectrode. The performance enhancement of DSSCs as
a result of the graphene incorporation was demonstrated by
several groups.45−48
In previous works, three steps were employed to fabricate
graphene/TiO2 composite films for DSSCs:45−48
(1) mixing
graphene oxide with TiO2, (2) prereduction of graphene oxide
to graphene by UV radiation or chemical reductants (such as
hydrazine), and (3) coating graphene/TiO2 as a film on the
electrode, followed by calcination at 450 °C. However, it was
known that graphene oxide can be reduced to graphene by
annealing at temperatures above 150 °C.49−51
It would be
reasonable for us to have a hypothesis: the prereduction of
graphene oxide is not necessary in the fabrication of graphene/
TiO2-based DSSCs, because graphene oxide in TiO2 can be
reduced to graphene during the heat treatment of the
photoelectrode at 450 °C. In this work, experiments were
carried out to test this hypothesis. Furthermore, the effect of
graphene content on the performance of graphene/TiO2-based
Received: March 1, 2012
Revised: July 16, 2012
Accepted: July 20, 2012
Published: July 20, 2012
Article
pubs.acs.org/IECR
© 2012 American Chemical Society 10613 dx.doi.org/10.1021/ie300563h | Ind. Eng. Chem. Res. 2012, 51, 10613−10620

2. DSSCs was investigated. In addition, the promoting mechanism
of graphene was evaluated.
2. EXPERIMENT
Graphene oxide (GO) was obtained from graphite powder
(Aldrich) using the modified Hummer’s method reported in
our previous paper.52
It can be briefly described as follows:
graphite, sodium nitrate (NaNO3), and concentrated sulfuric
acid (H2SO4) were mixed in a beaker in an ice−water bath,
followed by gradual addition of KMnO4. The obtained mixture
in the beaker was moved to a 35 °C water bath and stirred for 5
h. After 40 mL of di-ionized (DI) water was added to the
mixture, the temperature of the water bath was increased to 90
°C and the mixture was stirred for 20 min, followed by the
addition of 100 mL of DI water and 3 mL of H2O2 (30%). The
obtained product was washed five times using DI water and
separated by centrifugation. Finally, the sediment was subjected
to drying treatment in a vacuum furnace to yield a yellow-
brown graphite oxide powder.
A 0.5 mg sample of graphite oxide powder was well dispersed
in 2 mL of ethanol in a screw-top vial, followed by addition of
240 mg of nanocrystalline TiO2 powder (P25, Degussa). The
obtained mixture was ultrasonically treated for 2 h to exfoliate
graphite oxide to graphene oxide (GO). As a result, GO sheets
were decorated by TiO2 nanoparticles to form a TiO2/GO
paste with 0.21 wt % GO. By using the same procedure, TiO2/
GO pastes with different GO concentrations (0.42, 0.83, 1.23,
1.64, 2.04 wt %) were prepared. TiO2/GO composite films
Figure 1. Performance of TiO2/graphene-based DSSCs vs GO content: (a) short-circuit current density (Jsc), (b) open-circuit voltage (Voc), (c)
power conversion efficiency (η), (d) fill factor (FF), and (e) I−V curves of DSSCs without and with 0.83 wt % GO.
Industrial & Engineering Chemistry Research Article
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3. were fabricated on fluorine-doped tin oxide (FTO) glass
substrates using a doctor blade printing method, forming TiO2/
GO-based photoelectrodes. The photoelectrodes were heated
at 150 °C for 30 min and then calcined at 450 °C for 30 min.
During the heat treatment, GO in composite films was
thermally reduced to graphene, forming TiO2/graphene-based
photoelectrodes. For comparison, TiO2-only paste was also
deposited on FTO glass to prepare a TiO2-based photo-
electrode without graphene. The average thickness of TiO2/
graphene films and TiO2 film was around 6−8 μm.
To fabricate DSSCs, the photoelectrodes were sensitized
with an ethanol solution of 0.3 mM N719 dye (Aldrich) for 24
h. Platinum coated FTO glass was used as a counter electrode.
The electrolyte in the DSSCs was composed of 0.5 M LiI, 0.05
M I2, and 0.5 M tert-butylpyridine (TBP) in 3-methoxypro-
pionitrile (MPN). The active area of a DSSC was 0.5 × 1.0 cm2
.
Photocurrent−voltage (I−V) measurements were performed
using a Keithley Model 2400 measurement unit. The light
source (AM 1.5 solar illumination, 100 mW/cm2
) was
generated by a Newport solar simulator equipped with a
1.5G air mass filter. Electrochemical impedance spectra (EIS)
of DSSCs were obtained in the dark at −0.7 V applied bias by a
CHI660 electrochemical workstation. The frequency was varied
from 0.1 Hz to 100 kHz for the EIS measurements.
Elemental analyses were recorded using a Control Equip-
ment Corp. Model 240XA analyzer. The X-ray diffraction
(XRD) measurements were carried out by a Scintag XDS-2000
powder diffractometer with Cu Kα (λ = 1.5406 Å) radiation.
Fourier transform infrared (FTIR) spectra were obtained for
samples of TiO2 and TiO2/graphene using a FTIR
spectrometer (FTIR Spectrum One, Perkin-Elmer). The
morphology of TiO2 on graphene sheets was evaluated by
transmittance electron microscopy (TEM, JEOL4000FX). To
determine the absorbed amount of dye in the TiO2 film and
TiO2/graphene films, the dye in the films was dissolved in 0.1
M NaOH aqueous solution and then measured by a UV−vis
spectrometer (Shimadzu UV-2400) in transmittance mode.
Furthermore, the transmittance mode of the UV−vis
spectrometer was also employed to evaluate the light
absorbance of the TiO2 film and TiO2/graphene films before
dye loading, whereas its reflectance mode using a detector with
integrating sphere was exploited to examine the light
absorbance of the TiO2 film and TiO2/graphene films after
dye loading.
3. RESULTS AND DISCUSSION
So far, the prereduction of graphene oxide has been required to
fabricate TiO2/graphene-based DSSCs.45−48
In this work,
however, TiO2/graphene-based DSSCs were fabricated without
the prereduction treatment. The effects of graphene on the
photocurrent, voltage, and power conversion efficiency of the
DSSCs were evaluated. As shown in Figure 1, one can see that,
for a TiO2-based DSSC without graphene, its short-circuit
current density (Jsc), open-circuit voltage (Voc), fill factor (FF),
and power conversion efficiency (η) were 4.96 mA/cm2
, 0.66 V,
0.545, and 1.79%, respectively. Furthermore, the best perform-
ance of TiO2/graphene-based DSSCs exhibited a Jsc of 7.6 mA/
cm2
, Voc of 0.67 V, FF of 0.54, and η of 2.78% (Table 1). In
addition, current densities at all voltages are higher for this best
TiO2/graphene-based DSSC than for TiO2-based DSSC
(Figure 1e). These indicate that the incorporation of graphene
can increase Jsc and η by 52.4 and 55.3%, respectively. It was
reported that the power conversion efficiency (η) of TiO2/
graphene-based DSSCs, which were fabricated with a
prereduction step, was 7.6−58.5% higher than that of a TiO2-
based DSSC without graphene.45−47
This indicates that the
performance enhancement of our TiO2/graphene-based DSSCs
without prereduction is comparable to those with prereduction.
Therefore, the prereduction of graphene oxide is not necessary
to fabricate TiO2/graphene-based DSSCs. This occurs because
the heat treatment of the photoelectrode at 450 °C could
reduce graphene oxide to graphene. The reduction of graphene
oxide was confirmed by FTIR spectra, in which the IR band at
1450 cm−1
(corresponding to a carboxyl functional group53
)
disappeared after the heat treatment (Figure 2). The GO
reduction can be further supported by XRD patterns (Figure
3). As shown in Figure 3, one can see that the diffraction peak
shifts from 2θ = 10.9° to 2θ = 26.1° after the thermal treatment
of GO powder (without TiO2) at 450 °C. This indicates the
reduction of GO to graphite. However, one cannot expect to
see the diffraction peak of the graphite structure for the TiO2/
graphene composite, because the graphene sheets were highly
isolated by TiO2 particles (Figure 3).
Figure 1 also shows that the enhancements of Jsc and η are
dependent on the content of GO. As the graphene content
increased, Jsc increased from 4.96 to 7.56 mA/cm2
and then
decreased. The concentration of original graphene oxide in the
TiO2/graphene composite, which is associated with the
maximum Jsc (7.56 mA/cm2
), is 0.83 wt %. The power
conversion efficiency (η) of DSSCs exhibited a change similar
to that of Jsc with increasing the content of graphene. In
contrast to Jsc, the open-circuit voltage (Voc) and fill factor (FF)
Table 1. Short-Circuit Current Density (Jsc), Open-Circuit
Voltage (Voc), Fill Factor (FF), Power Conversion Efficiency
(η), Series Resistance (Rs), Charge Transfer Resistance (Rct),
and Electron Lifetime (τ) of DSSCs without and with
Graphene
electrode
Jsc (mA
cm−2
)
Voc
(V) FF η (%)
Rs
(Ω)
Rct
(Ω)
τ
(ms)
TiO2 4.96 0.66 0.545 1.79 19.8 142.3 17.6
TiO2/
graphene
7.6 0.67 0.54 2.78 16.3 115.2 6.4
Figure 2. FTIR spectra of (a) TiO2/GO without heat treatment and
(b) TiO2/GO after heat treatment in air at 450 °C for 30 min.
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4. just showed a slight fluctuation by adding graphene into TiO2.
This indicates that the introduction of graphene into TiO2 film
caused a negligible effect on Voc and FF. In other words, the
enhancement in power conversion efficiency (η) of a DSSC by
adding graphene into TiO2 film is due to its effect on Jsc. It is
worth noting that the actual content of graphene associated
with the maximum Jsc and η, which was obtained from the
element analysis, is 0.26 wt %. This happened because the
TiO2/graphene film was subjected to the heat treatment in air,
resulting in burning off part of the graphene. Furthermore, the
obtained maximum power conversion efficiency of TiO2/
graphene-based DSSCs (2.78%) can be further improved by
replacing the P25 powder with a mixture of anatase and rutile,
because P25 possesses a nonoptimized surface area and particle
morphology.53c
It is well-known that Jsc is associated with the number of
ejected electrons through the external circuit,40
while Voc
corresponds to the difference between the Fermi level in the
semiconductor (TiO2) under illumination and the Nernst
potential of the I−
/I3
−
redox couple in the electrolyte.35
The
almost identical Voc values of the TiO2-based cell and the TiO2/
graphene-based cells reveal that the incorporation of graphene
does not influence the Fermi level of the composite
semiconductor (TiO2/graphene), which is consistent with
another report.46
It is widely accepted that the FF is sensitive to
the series resistance (Rs).54,55
Rs in DSSCs mainly comes from
three parts: the sheet resistance of the transparent conducting
oxide, the resistance at the counter electrode, and the resistance
in the electrolyte.54
Each of the three resistances is the same for
DSSCs both with and without graphene in this study, because
graphene was introduced only into TiO2 films. For this reason,
the incorporation of graphene into TiO2 films could not affect
the FF. In contrast, graphene in DSSCs has a significant
influence on the number of ejected electrons transferred from
the photoelectrode to the counter electrode, which was
reflected by the increase of Jsc. Two possible factors may
contribute to the increase in the number of electrons by doping
graphene into TiO2:46−48
(1) increasing the amount of dye
sensitizer absorbed in TiO2 and thus increasing excited
electrons from the dye sensitizer to the conduction band of
TiO2, and (2) enhancing electron transfer from TiO2 to a
photoelectrode, which can reduce the possibility of electron
recombination. To clarify these two factors, we evaluated the
effects of graphene on dye loading in TiO2 by UV−visible
spectra and on electron transfer in DSSCs by electrochemical
impedance spectra (EIS). It was reported that graphene could
increase the dye loading in TiO2 film.48
However, as shown in
Figure 4, one can see a negligible effect of graphene on dye
loading in TiO2 films. This indicates that graphene does not
increase the number of photoinduced electrons from the dye
sensitizer to the conduction band of TiO2. In other words, the
effect of graphene on the performance of DSSCs may be due to
the enhancement of electron transfer from TiO2 to a
photoelectrode. To confirm this, electrochemical impedance
spectra (EIS) were measured for DSSCs fabricated with TiO2
and TiO2/graphene as photoelectrodes at an applied bias of
−0.7 V in the frequency range 0.1 Hz−100 kHz. As shown in
Figure 5, one can observe a well-defined semicircle in the
middle-frequency region for Nyquist plots of DSSCs both with
and without graphene. The arc in the middle-frequency range
between 1 and 1000 Hz describes the charge transport process
at the TiO2/dye/electrolyte interface.56−58
The Nyquist plots
were analyzed by an equivalent circuit containing a constant
phase element (CPE), a series resistance (Rs), and a charge
transfer resistance (Rct). As expected above, the series
resistances (Rs) of TiO2-based and TiO2/graphene-based
DSSCs are comparable (Table 1). However, a large decrease
(from 142.3 to 115.2 Ω) in the charge transfer resistance (Rct)
was caused by introducing graphene into the TiO2 film in the
DSSC. Because the charge transfer resistance (Rct) is inversely
Figure 3. XRD patterns of (a) GO, (b) thermally reduced GO, and (c)
TiO2/graphene composite.
Figure 4. Dye loading of TiO2/graphene-based photoelectrodes.
Figure 5. EIS spectra of (a) TiO2-based and (b) TiO2/graphene-based
DSSCs.
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5. proportional to the transfer rate of electrons, the large Rct
decrease indicates that graphene in TiO2 accelerated electron
transfer in the DSSC. Furthermore, EIS spectra reveal that
electron lifetime in the TiO2/graphene-based DSSC is shorter
than that in the TiO2-based cell (Table 1). This further
confirms that graphene accelerated electron transfer from TiO2
to the photoelectrode, leading to the reduction of electron−
hole recombination and thus the increase of DSSC power
conversion efficiency. This can also be supported by a TEM
image. As shown in Figure 6, one can see that the sizes of TiO2
nanoparticles dispersed on graphene sheets are 20−40 nm.
Furthermore, the freestanding graphene sheets are not perfectly
flat but display intrinsic wrinkles, which are formed to keep
thermodynamically stable two-dimensional structures.59,60
As
reported about the TiO2/graphene composite,30
TiO2 nano-
particles can be easily attached on the graphene sheet. This
happens probably because GO sheets dispersed in ethanol
occupy negative charges on their surfaces, which could create
an attractive electrostatic interaction with ionic TiO2 particles
and thus generate an excellent contact between them.47
The
excellent contact, which was demonstrated by ζ-potential
measurements,47
ensured the electron transfer from TiO2 to
graphene sheets.
If graphene plays only the positive role of accelerating
electron transfer in a DSSC device, Jsc should always increase
with increasing graphene content in the TiO2/graphene
composite films. However, the variation of Jsc and η showed a
volcano shape with increasing graphene content, namely,
increasing to a maximum value and then decreasing (Figure
1a,c). This indicates that the introduction of graphene to DSSC
devices must also have a negative effect on Jsc and η. It is widely
recognized that graphene can absorb light in a large wavelength
range of 200−800 nm. Therefore, it is reasonable for one to
propose that the light absorbance of graphene decreases the
real light harvest of dye molecules, which is a negative effect on
the DSSC. To confirm this, the light absorbance of dye-
sensitized TiO2 and TiO2/graphene samples was evaluated by
UV−vis spectra in reflectance mode. As shown in Figure 7, one
can see that the absorbance intensity of TiO2/graphene
samples is higher than that of TiO2 in the whole range.
Because the dye loading for the TiO2/graphene composite film
is the same as that for the TiO2 film (Figure 4), the increased
absorbance intensity comes from graphene in the composites.
Furthermore, the absorbance intensity of the TiO2/graphene
composite increased with increasing content of graphene,
confirming the light absorbance of graphene. The light
absorbance of graphene in the composite causes a negative
effect on the performance of resulting DSSCs. This happens
because the total light energy input to a solar cell is a certain
amount, so that the absorbance of some light by graphene can
lead to the decrease of light harvest for dye molecules. The
higher the content of graphene in composites, the less the
harvest of dye molecules and thus the worse the performance of
DSSCs is. In addition, as shown in Figure 8, one can see that
the absorbance of the TiO2/graphene composite film without
Figure 6. Morphology of 0.83 wt % GO/TiO2 composite.
Figure 7. UV/vis reflectance spectra of dye-sensitized TiO2 and TiO2/
graphene-based photoelectrodes.
Figure 8. UV/vis transmittance spectra of TiO2 and TiO2/GO
composite films.
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6. dye sensitization increases with increasing content of graphene.
This further demonstrates that graphene can play a role of
absorbing light in the TiO2/graphene composite film. In
addition, because the total dye loading in the TiO2/graphene
film remained unchanged with increasing graphene content, the
actual dye loading on TiO2 in the film would decrease with
increasing graphene content. This can also contribute to the
decrease in efficiency.
Based on the above results and discussion, the roles of
graphene in the performance of a TiO2/graphene-based DSSC
can be illustrated in Figure 9 and described as follows: the
electron transfer in a DSSC photoelectrode can be divided into
two steps, which are (1) excited electrons from the dye
molecules to the conduction band of TiO2 and (2) electrons
from TiO2 to a photoelectrode. Due to the excellent electrical
conductivity of graphene sheets, they can act as bridges to
accelerate electron transfer from TiO2 to the photoelectrode,
which reduces the possibility of electron−hole recombination.
As a result, the power conversion efficiency of DSSCs is
enhanced. In contrast, the light absorbance of graphene can
lead to the decrease of light harvest of dye molecules and thus
decrease the number of excited electrons from dye molecules to
TiO2, which has a negative influence on the efficiency of the
DSSC. Therefore, the incorporation of graphene is beneficial to
step 2, but harmful to step 1, resulting in the volcano shapes of
Jsc and η with increasing graphene contents in TiO2/graphene
composites.
4. CONCLUSION
In conclusion, a simple approach without a prereduction of GO
was demonstrated to be effective for the fabrication of
graphene-based DSSC devices. Furthermore, it was shown
that the incorporation of graphene into the TiO2-based DSSC
increased its short-circuit current density (Jsc) and the power
conversion efficiency (η) by 52.4 and 55.3%, respectively. The
increases of Jsc and η were due to the enhancement of electron
transfer from TiO2 to a photoelectrode by graphene. However,
the increase of graphene content beyond the optimal
concentration can cause the decrease of the efficiency due to
the light absorbance of graphene. Furthermore, graphene might
decrease the actual dye loading on TiO2 in a TiO/graphene
film, which is also a negative effect on the conversion efficiency.
Therefore, an optimum content of graphene associated with the
maximum conversion efficiency was observed.
■ AUTHOR INFORMATION
Corresponding Author
*Tel.: (906) 487-2261. E-mail: yunhangh@mtu.edu.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the ACS Petroleum Research
Fund (PRF-51799-ND10) and the U.S. National Science
Foundation (NSF-CBET-0931587).
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