Promoting Effect of Graphene on Dye-Sensitized Solar Cells

Article pubs.acs.org/IECR

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 graphenedoped 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 mechanical 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 composites,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 © 2012 American Chemical Society

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: Revised: Accepted: Published: 10613

March 1, 2012 July 16, 2012 July 20, 2012 July 20, 2012 dx.doi.org/10.1021/ie300563h | Ind. Eng. Chem. Res. 2012, 51, 10613−10620

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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.

DSSCs was investigated. In addition, the promoting mechanism of graphene was evaluated.

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 yellowbrown 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

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 10614

dx.doi.org/10.1021/ie300563h | Ind. Eng. Chem. Res. 2012, 51, 10613−10620


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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 photoelectrode 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-methoxypropionitrile (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 Equipment 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 TiO 2 and TiO 2 /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.

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 TiO2 TiO2/ graphene

Jsc (mA cm−2)

Voc (V)

FF

η (%)

Rs (Ω)

Rct (Ω)

τ (ms)

4.96 7.6

0.66 0.67

0.545 0.54

1.79 2.78

19.8 16.3

142.3 115.2

17.6 6.4

graphene-based DSSCs, which were fabricated with a prereduction step, was 7.6−58.5% higher than that of a TiO2based 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

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.

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 performance 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/

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) 10615



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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

Figure 3. XRD patterns of (a) GO, (b) thermally reduced GO, and (c) TiO2/graphene composite.

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

Figure 4. Dye loading of TiO2/graphene-based photoelectrodes.

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

Figure 5. EIS spectra of (a) TiO2-based and (b) TiO2/graphene-based DSSCs.

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 10616



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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

UV−vis spectra in reflectance mode. As shown in Figure 7, one can see that the absorbance intensity of TiO2/graphene

Figure 7. UV/vis reflectance spectra of dye-sensitized TiO2 and TiO2/ graphene-based photoelectrodes.

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.

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 nanoparticles 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 dyesensitized TiO2 and TiO2/graphene samples was evaluated by

Figure 8. UV/vis transmittance spectra of TiO2 and TiO2/GO composite films. 10617


Industrial & Engineering Chemistry Research 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

ACKNOWLEDGMENTS

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REFERENCES

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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Figure 9. Principle scheme of TiO2/graphene-based DSSC.

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.

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The authors declare no competing financial interest. 10618



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