Optimum strategy for designing a graphene-based counter electrode for dye-sensitized solar cells

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Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Optimum strategy for designing a graphene-based counter electrode for dye-sensitized solar cells Van-Duong Dao a, Liudmila L. Larina Ho-Suk Choi a,*

a,b

, Hoyong Suh c, Kimin Hong c, Joong-Kee Lee d,

a

Department of Chemical Engineering, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, Republic of Korea Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St. 4, 119334 Moscow, Russia c Department of Physics, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, Republic of Korea d Energy Storage Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea b

A R T I C L E I N F O

A B S T R A C T

Article history:

In this study, we developed a cost-effective method to enhance the electro-catalytic activity

Received 25 March 2014

and conductivity of graphene-based counter electrodes (CEs) by diminishing the oxygen

Accepted 9 June 2014

content in graphene nanoplatelets (GNPs) through dry plasma reduction (DPR). As a result,

Available online 14 June 2014

the efficiency of dye-sensitized solar cells (DSCs) based on GNPs with an average surface area of 750 m2/g treated by DPR was enhanced by 15% over that of devices with pristine graphene oxide electrodes. The next step in the design strategy for improving DSC performance suggested the immobilization of platinum nanoparticles (PtNPs) on the electrode surface through DPR. DSCs based on the newly developed CEs had an increase in efficiency by 50.3% over that of the Pt-free device, and by 10.4% over the efficiency of state-of-the-art DSCs. The introduction of PtNPs on the surface of a CE through DPR along with removal of the oxygen functional groups from the surface of the GNP electrode reduces the chargetransfer resistance at the electrolyte/CE interface and the diffusion impedance of triiodide ions. PtNPs hybridization on the surface of CE facilitates the electron conductivity in the PtNPs/CE structure by creating a conductive network in the sp3 phase for charge carriers delocalized in a sp2 matrix.  2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Among renewable energy sources, photovoltaic (PV) elements possess great potential to become the universal environmentally friendly energy source. In the last decades, the progress in developing the new generation of PV cells was marked by the emergence of competitive dye-sensitized solar cells (DSCs) designed by Gratzel and coworkers, which exploited principles and materials different from conventional solid-state PV devices. From the scientific point of * Corresponding author. E-mail address: [email protected] (H.-S. Choi). http://dx.doi.org/10.1016/j.carbon.2014.06.015 0008-6223/ 2014 Elsevier Ltd. All rights reserved.

view, the DSC efficiency can reach 20% and higher, which allows this type of PV device to be highly competitive with the other types of conventional thin film PV cells [1]. While the efficiency of DSCs is still lower than that of conventional solid-state solar cells, these solution-processable devices have a great potential for increasing efficiency at the lowest possible cost. The further improvement of DSCs also includes such factors as cost and long-term stability. In this regard, the development of low-cost, scalable counter electrodes (CEs) remains one of the major challenges of

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improving DSC technologies. The advantages of the conventional Pt CE for DSCs include high catalytic activity, high conductivity and chemical stability in an acid-base environment [2–4]. To introduce an alternative type of efficient electrode, a material with high electrical conductivity and excellent electro-catalytic activity is required [2–4]. In recent years, graphene materials have attracted significant interest in the PV community because of their high electron mobility of 250,000 cm2/Vs, large specific area of the planar sheet of sp2-bonded carbon atoms, and excellent electrochemical stability under prolonged potential cycling [5,6]. Such materials can be considered as a promising alternative to replace the expensive Pt CE in DSCs. Graphene CEs are fabricated by coating with graphene paste using a doctor blade and drop-casting the graphene solution, or by transferring graphene from a copper foil onto a fluorinedoped tin oxide (FTO) glass substrate [5–7]. However, there is still a problem with integrating graphene into DSCs with single sheets of graphene through exfoliation of graphite oxide [8]. An alternative way to fabricate CEs is based on the reduction of graphene oxide (GO) film. Graphite oxide is highly hydrophilic and readily exfoliates in water, yielding GO. The film can be grown on FTO substrates using the layer-by-layer technique and dip-coating from a GO solution [9,10]. For this purpose, hydrazine and sodium borohydride are used as chemical reagents for reduction [8,11]. To complete the reduction process, GO is thermally treated under protection of Ar and/or H2 gas flow [10,12]. However, these methods have some drawbacks, such as chemical toxicity, a long reduction time, and high temperature processing [13]. The high

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temperature process is detrimental for flexible DSCs. Alternatively, the plasma reduction of GO to reduced graphene oxide (RGO) has been used. At the moment, the plasma reduction under vacuum conditions appears to be the most promising method because it can potentially overcome the drawbacks [13,14]. However, such a process requires hydrogen and expensive vacuum equipment. Despite many studies on the subject, the development of low-cost, scalable CEs remains one of the major challenges for DSC technology to date. Recently, we developed an alternative approach based on DPR as a novel process for synthesis of supported PtNPs and NiO-NPs without a toxic liquid environment at nearly room temperature and under atmospheric pressure [4,6,15–17]. We showed that the DPR process can also reduce oxygen functional groups on the surface of GO due to the presence of reducing agents like hydrogen radicals and electrons [16–18]. In this article, we introduce an environmentally friendly strategy for designing graphene-based materials to replace traditional Pt-based CEs in DSCs using commercially available low-cost graphene nanoplatelets (GNPs), as shown in Fig. 1. For this purpose, we conducted systematic studies on how the graphene flake size (designated by their surface area) influences the catalytic activity of the CE and how it influences PV parameters of the completed solar cell. Moreover, we optimized the tradeoff between the resistivity value and catalytic activity of GNP electrodes to find the best raw material for the CE. As a result, we succeeded in developing a CE with high conductivity along with ultrahigh catalytic activity for reduction of triiodide. We demonstrate a low-cost simultaneous way of removing oxygen functional groups and

Fig. 1 – A cost-effective strategy for designing graphene-based counter electrode for dye-sensitized solar cells. (A color version of this figure can be viewed online.)

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immobilizing PtNPs on the surface of GNPs through DPR. Furthermore, the introduction of PtNPs on the surface of the CE through DPR of Pt precursors along with removal of the oxygen functional groups from the surface of the GNP electrode resulted in improvement of the energy conversion efficiency of the DSC as compared with DSCs based on either metal-free or Pt-sputtered CEs. These findings are of great importance because they prove the advantages of our strategy to design graphene-based hybrid materials from low-cost commercially available GNPs for high-efficiency DSCs. Here, we give a detailed description of the mechanisms that impact the decrease of charge-transfer resistance at the electrolyte/electrode interface and facilitate the electron conductivity in the PtNPs/CE structure.

2.

Experimental section

2.1.

Materials

First we prepared Pt precursor solution containing 10 mM H2PtCl6ÆxH2O (P37.5% Pt basic, Sigma–Aldrich) in iso-propyl alcohol (IPA) (99.5%, Sigma–Aldrich). FTO glass for use as a transparent conducting electrode was purchased from Pilkington, USA (8 X/h). These substrates were used after cleaning by sonic treatment in acetone (Fluka). The GNPs Grade H (GH) had an average thickness of approximately 15 nm, an average particle diameter of 5–25 lm and a typical surface area of 50–80 m2/g; Grade M (GM) had an average thickness of approximately 6–8 nm, an average particle diameter of 5–25 lm and a typical surface area of 120–150 m2/g; Grade C (GC) had a particle diameter of less than 2 lm and a typical particle thickness of a few nanometers, depending on the surface area. Grade C particles that can be ordered with average surface areas of 300, 500 and 750 m2/g were obtained from XG Sciences. TiO2 paste and ruthenium based-dye (N719) were purchased from Solaronix, Switzerland. The dye was adsorbed from a 0.3 mM solution in a mixed solvent of acetonitrile (Sigma–Aldrich) and tert-butylalcohol (Aldrich) with a volume ratio of 1:1. The electrolyte was a solution of 0.60 M 1-methyl-3-butylimidazolium iodide (Sigma–Aldrich), 0.03 M I2 (Sigma–Aldrich), 0.10 M guanidiniumthiocyanate (Sigma–Aldrich), and 0.50 M 4-tertbutylpyridine (Aldrich) in a mixed solvent of acetonitrile (Sigma–Aldrich), and valeronitrile, with a volume ratio of 85:15.

2.2.

Preparation of GNP counter electrodes

The GNPs powder was mixed with a 1 wt.% solution of ethyl cellulose in terpineol by a three-roll miller. To obtain the GNP CEs, the mixture was coated on the FTO-glass substrate by a doctor blade and dried at 300 C for 30 min [18]. The Pt electrode was obtained by DC-sputtering of Pt at 10 mA and 2 · 103 torr for 5 min [2].

2.3.

Preparation of GC-750-DPR counter electrodes

The GC-750 CEs were treated by DPR using Ar plasma under atmospheric pressure at a power of 150 W and a gas flow rate

of 5 lpm, for a reduction time of 15 min, and a substrate moving speed of 5 mm/s [4,6].

2.4.

Synthesis of PtNPs hybridized on GC-750

3 ll of Pt precursor solution was dropped on the GC-750 CE and the solvent was allowed to evaporate at 70 C for 10 min. The specimens were then reduced using Ar plasma under atmospheric pressure at a power of 150 W and a gas flow rate of 5 lpm, for a reduction time of 15 min, and a substrate moving speed of 5 mm/s [4,6].

2.5.

Characterization

The morphology of the GNP electrodes was characterized using field emission scanning electron microscopy (FESEM) (Jeol JSM 7000F). The structure of the PtNPs/GC-750 hybrid was analyzed by transmission electron microscopy (TEM) (JEM-2100F, Joel, Japan). For TEM analysis of the PtNPs/GC750 nanohybrid, we scratched part of the coated layer, dispersed it in an ethanol suspension, and transferred it to a holey carbon grid [4]. The chemical state of the GC-750 and GC-750-DPR films were analyzed by X-ray photoelectron spectroscopy (XPS) using a Sigma Probe Thermo Fisher VG Scientific spectrometer equipped with a monochromatic Al Ka Xray source. Thermo-gravimetric analysis (TGA) was performed on a TGA/DSC 1 (Mettler-Toledo Inc.) at a heating rate of 10 C min1 in an N2 atmosphere. The Raman spectra were recorded using the 532 nm DPSS laser and with a UniRAM high resolution dispersive Raman spectrograph. The assembly of the DSCs was carried out as described in a previous study [4]. The photocurrent–voltage (J–V) characteristics of the DSCs were measured under one sun illumination from a Sun 3000 solar simulator composed of a 1000 W mercury-based Xe arc lamp and AM 1.5 G filters. A cyclic voltammogram (CV) was used to measure the electrochemical catalysis of the electrodes. The experiments were performed in a three-electrode cell with a potentiostat (IVIUMSTAT) as in a previous study [2]. The electrochemical impedance spectroscopy (EIS) of the DSCs was measured with constant light illumination (100 mW cm2) under open-circuit conditions. The measured frequency range was 100 kHz to 100 mHz with a perturbation amplitude of 10 mV. The obtained spectra were fitted using Z-view software (v3.2c, Scribner Associates, Inc.) with reference to the proposed equivalent circuit. The EIS of symmetric dummy cells was performed under open-circuit conditions. The measured frequency range was 100 kHz to 100 mHz with a perturbation amplitude of 5 mV [3].

3.

Results and discussion

3.1. Optimization of graphene electrodes for highly efficient DSCs

nanoplatelet

counter

The set of CEs based on GNPs with different grades (GH, GM, GC-300, GC-500 and GC-750) was fabricated. We then studied the effect of flake size on the surface morphologies of the CEs and their catalytic activity. Fig. 2 illustrates the comparative SEM plane images of the surface morphologies of the

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Fig. 2 – (a) FESEM image of Pt-sputtered CE; (b)–(f) FESEM images of GNP CEs prepared from GH, GM, GC-300, GC-500 and GC750, respectively.

Pt-sputtered CE and CEs fabricated with different grades of GNPs: GH, GM, GC-300, GC-500 and GC-750. Pt sputtered film uniformly covered the substrate and it well adhered to the substrate surface. A uniform covering along the grain boundaries of the SnO2:F substrate (grain size varied in the range of 100–300 nm) is clearly visible in the plane image (Fig. 2a). The GNP films coated by a doctor blade were not uniform with partial coverage (Fig. 2b–f). The pure coverage can be ascribed to a low content of GNPs (1 wt.%) in the paste. Graphene flakes are clearly visible in the micrographs. The flakes perpendicularly stacked form a 3D network structure. As can be seen in Fig. 2b–f, the graphene flakes were not well connected to the FTO surface, suggesting a lower conductivity of GNP CEs compared to that of the Pt-sputtered CE. The SEM results point out that the grade of the GNPs affects the morphology of the film. Indeed, the GNPs of grade H and grade M show a 3D network structure, which was formed by flakes with a large size in range of 5–25 lm (Fig. 2b and c). However, the films grown from the GNPs of grades GC-300, GC-500, and GC-750 exhibit more fine structures, where the flake size varied in a much smaller dimension range of 0.5–2 lm and underlying substrate is clearly visible in micrographs (Fig. 2d–f).

The catalytic performances of the CEs were evaluated through comparative analysis of the CV. The six electrodes were tested as working electrodes. The current density can be estimated from the large peak area in the CV curve, suggesting a largest electrode active surface [19]. As can be seen in Fig. 3a, the GC-500 electrode exhibited the highest current density among all the GNP electrodes. However, the catalytic activities of the GNP electrodes were less than the activity of the Pt-sputtered electrode. The highest electro-catalytic activity of the GC-500 electrode may be related to a small graphene flake size and a low amount of oxygen content on the GC-500 electrode surface [12]. Indeed, the flake size in the GC-500 electrode varied in the range of 0.5–2 lm (Fig. 2e) and the typical thickness was a few nanometers. The flakes with such dimensions can be ordered with average surface areas of 500 m2/g. The oxygen content on the GNP was around 5 atomic wt.% for the GC-500 and around 8 atomic wt.% for GC-750 (XG Sciences). The increase of the electro-catalytic activity of the CE suggests a decrease of the charge transfer resistance (Rct1) at the CE/electrolyte interface. Fig. 3b compares the J–V curves of the DSCs employing the Pt-sputtered CE and GNP CEs prepared with GNPs of different flake sizes. The corresponding PV parameters are listed in

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Fig. 3 – (a) Cyclic voltammogram curves of Pt-sputtered and GNP CEs; (b) The J–V curves of DSCs fabricated with Ptsputtered and GNP CEs; (c) The change in charge transfer resistance and constant phase element with respect to the different GNP electrodes. (A color version of this figure can be viewed online.)

Table 1. It can be seen that the g increases with a decrease in the flake size of GNPs in sequence of GH to GC-500 Grades. However, the efficiency of a DSC based on the GC-750 electrode was lower than that of the GC-500 device. Thus, the DSC with GC-500 CE had the highest g of 5.67(±0.14)%. The efficiency of the reference solar cell based on the Pt-sputtered CE (8.18(±0.08)%) was still higher than the efficiency of the GC500 cell. Two factors may impact to the increase of the Jsc value. The first is the progress of the catalytic behavior in reduction of triiodide to iodide, which determines the actual driving force for dye regeneration. The second factor is the facilitation of electron transport within the GC-500 electrode due to the improvement of the electron-transport network formed by GNPs [19], which is supported by the morphology analysis of the CEs (Fig. 2). However, the Jsc and g of the GC-750 cell were smaller than those of the cell with the GC-500 CE. Given that particles of Grade GC-750 can be ordered with average surface areas of 750 m2/g, which is larger than that of Grade GC-500, the difference in efficiency value can be related to the difference in the O/C stoichiometry in the films. Indeed, the values of carrier mobility and electrical conductivity in GO films are closely related to the existence of the oxygen functional groups, and strongly depend on the oxygen content [12]. A lowering of the charge mobility and electrical conductivity of GNPs films leads to a decrease of the Jsc [2–4]. Given that the oxygen content on the surface is around 5 atomic wt.% for GC-500 and around 8 atomic wt.% for GC-750 (XG Sciences), we can suggest that the amount of the oxygen functional groups in the GC-750 CE is higher than that in the GC-500 CE. The increase of the O/C atomic ratio can result in a decrease of the Jsc in the GC-750 cell. EIS analysis was provided for the set of CEs under study. Fig. 3c shows the changes in values of Rct1 and constant phase element-T (CPE1-T) with respect to the different type of GNP electrodes. The experimental data were obtained from the Nyquist plots (Supporting Information, Fig. S1 and Table S1). As can be seen from Fig. 3c, the value of Rct1 decreased as the GNP size decreased. However, the Rct1 was larger at the GC-750 CE/electrolyte interface than at the GC-500 CE/electrolyte interface, even though the GC-750 flake size was smaller than the size of GC-500. The minimum Rct1 value of 0.45 X cm2 was obtained for the GC-500 electrode. A smaller Rct1 value indicates an increase in the exchange current density (Jo = RT/nFRct1) [3] which corresponds to the cathodic peak current density in the CV (Fig. 3a). The largest peak area in the CV

Table 1 – Photovoltaic parameters of cells shown in Fig. 3b. CE

Jsc (mA cm2)

Voc (mV)

FF (%)

g (%)

Pt-sputtered GH GM GC-300 GC-500 GC-750

16.80 ± 0.05 14.10 ± 0.83 14.12 ± 0.81 14.77 ± 0.58 14.64 ± 0.22 14.31 ± 0.27

763.33 ± 2.36 772.50 ± 3.53 788.75 ± 2.50 761.67 ± 5.77 755.00 ± 7.07 765.00 ± 5.00

63.84 ± 0.09 24.11 ± 0.46 25.46 ± 2.07 43.8 ± 3.07 51.53 ± 0.04 46.24 ± 0.94

8.18 ± 0.08 2.63 ± 0.12 2.83 ± 0.13 4.93 ± 0.19 5.67 ± 0.14 5.06 ± 0.13

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curve was found for the GC-500 electrode (Fig. 3a). Therefore, the results of CV measurements agree well with results of the EIS analysis. Thus, the improvement of Jsc and FF can be attributed to a higher exchange current density [3]. The values of the constant phase element (CPE1 = (CPE1-T)1(jw)(CPE1-P), where j2 = 1, w is the frequency, CPE1-T and CPE1-P are the frequency-independent parameters of the CPE) of the CE also confirm that the active surface areas of the CEs became larger with a decrease in flake size in the sequence of GH to GC-750. The increase in the CPE1-T value indicates an increase in the active surface area of the electrode [2]. As can be seen in Fig. 3c, the active surface area of the GC-500 CE is similar to that of the GC-750 CE. Given that the oxygen content on the surface of the GC-500 film is less than the oxygen content on the surface of the GC-750 electrode (XG Sciences), we can expect structural damage on the basal plane of the GC-750. We can suggest that the loss of oxygen functional groups in the GC-750 film will lead to repair of the structural damage on the basal plane of the graphene and, in turn, a significant increase in both the electrical conductivity and electro-catalytic activity. Thus, the GC-750 CE can be considered as the optimal electrode among all the CEs under study for a further increase in the efficiency of the DSC. For that purpose, reduction of oxygen-containing functional groups on the surface of GC-750 CE was provided using DPR.

3.2. Effect of the loss of oxygen functional groups on the electro-catalytic activity of the GC-750 CE and the PV parameters of the DSC 3.2.1.

X-ray photoelectron spectroscopy analysis

GC-750-DPR film was prepared by DPR of GC-750 film. The loss of oxygen functional groups in the GC-750 film after DPR was studied by XPS. The XPS spectra of the C 1s core level of the GC-750 film and the GC-750-DPR films are presented in Fig. 4. Note that the chemical shift information is a very powerful tool for identification of functional groups. The peak fitting and deconvolution were performed for C 1s core-level emissions for both C 1s spectra, and the parameters are listed in Table 2. The fitting was done after a Shirley+Liner background subtraction on the fitting interval. The C 1s spectrum of the GC-750 film was decomposed to five components: C@C in aromatic rings (284.4 eV); C–C in aliphatic (284.9 eV); C–O in the hydroxyl and epoxy groups (286.1 eV); C@O in carbonyl groups (287.5 eV); and C(O)–(OH) in the carboxyl group (289.2 eV). The peak positions are in agreement with data reported elsewhere [11]. The changes in the spectral weight distribution in the C 1s core level emission after DPR can be seen in Fig. 4. Indeed, the carboxyl and carbonyl groups were almost entirely removed from the spectra of the GC-750-DPR film while some of the aliphatic C–C and C–O groups were also decreased and transformed into C@C groups (Fig. 4b and Table 2). Note that the shake-up satellite (p–p*) peak is below the detection limit in both spectra. We found that DPR leads to a significant increase in the C/O atomic ratio from 4.82 to 6.50. The XPS results can be explained as follows: the Ar+, electrons and hydrogen radicals, which were

Fig. 4 – (a) C1s core level XPS spectrum of GC-750 film; (b) C1s core level XPS spectrum of GC-750-DPR film prepared by Ar plasma reduction of GC-750 film. The fitting was done after Shirley+Liner background subtraction on the fitting intervals. The spectra are fitted by Lorentzian–Gaussian functions. (A color version of this figure can be viewed online.)

generated during plasma performance, have relatively high kinetic energy that can cut out the bonds between C and O [16,17]. Additionally, electrons can reduce oxygen functional groups during plasma reduction [16]. As a result, the percentage of C@C bonding increases, which leads to an increase in the aromatic domain size. It suggests a repair of structural damage on the basal plane of the graphene [16,17]. It is well known that the removal of oxygen from GC-750 leads to a significant increase in conductivity [8]. Thus, our XPS results suggest that the application of DPR to the GC-750 film results in enhanced conductivity in the GC-750-DPR film. We also found that DPR yields an increase in the thermal stability of the GC-750-DPR film with respect to that of the pristine GC-750 film (Supporting Information, Fig. S2) [20], suggesting the structural changes during DPR. The change in structure of the pristine GC-750 film induced by DPR was confirmed by Raman spectroscopy analysis.

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Table 2 – Results of the fitting of C1s core level spectra of GC750 film and GC-750-DPR film prepared by Ar plasma reduction of GC-750 film. Binding energy (eV)

C@C 284.4

C–C 284.9

Samples

Atomic percentage

GC-750 GC-750-DPR

55.40 71.59

18.98 15.35

C–O 286.1

16.22 13.06

C@O 287.5

C(O)OH 289.2

9.39 0.00

0.01 0.00

3.2.2.

3.2.3. CEs

Fig. 5 – Raman spectra of the GC-750 and GC-750-DPR films (a). D and G bands Raman spectra shown in enlarge scale for GC-750 (b) and GC-750-DPR film (c). The bands were fitted with Gaussian curves. (A color version of this figure can be viewed online.)

Raman spectroscopy analysis

Fig. 5 shows the Raman spectra for the GC-750 and GC-750DPR films. The spectra reveal the first-order allowed tangential G band, the disorder-induced D 0 mode, the disorderinduced D mode and its G 0 overtone the second-order spectra. D, D 0 and G bands are shown in enlarged scale in Fig. 5b and c. The bands were fitted with Gaussian curves. The D band spectrum can be fit using only a single peak centered at 1357 cm1. G and D 0 bands are centered at 1587 cm1 and 1620 cm1. Note that the G 0 band is always observed even in a single graphene sheet. The G mode is related to the in-plane motion of C sp2 atoms in both aromatic and olefinic molecules. The D peak intensity is related to the presence of six fold aromatic rings. Now, we consider the structural change in the Raman profile of the GC-750 film induced by DPR. Note that the change in the GC-750 film structure reflects on the Raman spectra. We found that the intensity of both D and D 0 modes decreased after DPR treatment. The relative intensity of the D 0 line with respect to the G line decreased from 0.30 to 0.18. It is known that the area ratio of the D and G bands varies with the size of sp2 cluster size [21]. In case of the GC-750-DPR film, the ID/IG area ratio was as small as 0.60, while that for the GC-750 was 1.11. The lowering of the ID/IG ratio points out the increase in the average size of the sp2 clusters upon reduction of the GC-750. We determined the sp2 cluster diameter (in-plane correlation length) La using the Tuinstra and Koenig empirical expression [21]. We found that the DPR treatment of the film results in an increase in the sp2 cluster size from 4.0 to 7.3 nm. Therefore, the DPR process results in the increase of the dimensions of sp2 clusters in a network of sp3 and sp2 bonded carbon. This result is related to the reduced structural damage on the basal plane of the graphene [16] due to the loss of oxygen upon reduction of the GC-750 [12]. Indeed, the oxygen functional groups form sp3 bonds with carbon atoms in the basal plane. Thus, the degree of clustering of the sp2 is increased along with sp2 bond restoration during the DPR. As we noted earlier, the results are in line with our XPS analyses.

PV parameters of DSCs with GC-750 and GC-750-DPR

We made a comparative study of DSCs fabricated with GC-750 and GC-750-DPR CEs. The conventional DSC with Pt-sputtered CE was also fabricated as a reference. Fig. 6a shows the J–V characteristics of the DSCs. The device PV parameters are summarized in Table 3. We found that the efficiency of the DSC based on the GC-750-DPR CE was enhanced by 15% over that of the device with a pristine GC-750 electrode. The effects can be ascribed to the high specific surface area of exfoliated GC-750-DPR. The increase in the electrical conductivity of the developed electrode can also produce an increase of Jsc. It is well known that the sp3 structure negatively affects the electrical conductivity and mobility in GO, thus hindering the electron transport. XPS results show that the DPR led to the partial restoration of the sp2 carbon–carbon bonds in the graphene lattice of the GC-750 film due to the loss of oxygen functional groups. Furthermore, the increase in the sp2 cluster size from 4.0 to 7.3 nm was revealed by Raman spectroscopy. The p states form pairs of aligned p states, or six fold

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restoration during the DPR. Therefore, the concentration of p increased, which led to an increase in the electrical conductivity of the GC-750-DPR electrode.

3.2.4. The effect of DPR of the GC-750 film on its electrocatalytic activity

Fig. 6 – (a) The J–V characteristics of DSCs fabricated with different CEs; (b) Cyclic voltammograms of different electrodes; (c) Equivalent circuit and Nyquist plots of the symmetric cells with identical CEs of GC-750, GC-750-DPR and Pt-sputtered. The cells were measured with the frequency range 100 kHz–100 mHz. Rct: charge-transfer resistance, Zw: diffusion impedance, Rh: ohmic internal resistance, CPE: constant phase element. (A color version of this figure can be viewed online.)

aromatic rings, and p bonding is maximized [22]. The degree of clustering of the sp2 is increased along with sp2 bond

The influence of oxygen content in the GNP CE on DSC performance was mainly derived from the study of the electrical conductivity of the CE and its electro-catalytic activity in the reduction of triiodide to iodide [23]. Therefore, to evaluate the effect of the structural and chemical change in GC-750 film induced by DPR on the electro-catalytic activity of the CE, we made a comparative study of CVs for GC-750 and GC-750-DPR electrodes and the Pt-sputtered CE. We found that, in the case of the GC-750 and GC-750-DPR CEs (Fig. 6b),  the peak positions of the I 3 /I redox energy level were positively shifted compared to the peak position in the case of the Pt-sputtered CE. Therefore, the realization of maximizing the value of Voc in the DSC based on GC-750-DPR can be understandable from our early study [2]. The DSC based on the GC-750-DPR CE exhibited a higher Jsc than that in the device based on the GC-750 CE. However, both values are still less than that in the reference device. Thus, we can conclude that the loss of oxygen functional groups on the surface of the GC-750 CE, induced by DPR, leads to an obvious improvement of its electro-catalytic activity. Our results confirmed that the high electro-catalytic activity agrees well with the charge transfer resistance at the CE/electrolyte interface. The high electro-catalytic activity not only reduces internal resistances, but also attenuates the recombination rates and concentration gradients in the electrolyte, which in turn strongly affects the values of Jsc and FF [24]. To clarify the correlation of the electro-catalytic activities of CEs with other electrochemical characteristics, EIS spectra were recorded. Nyquist plots of the symmetric configurations for GC-750, GC-750-DPR, and Pt-sputtered electrodes are presented in Fig. 6c. The simulated data from the EIS spectra are listed in Table 3. As mentioned above, the decrease of oxygen content on the surface of the GC-750 CE leads to improvement in the electro-catalytic activity [9]. We found that the Rct of the GC-750-DPR CE was 20.09 O cm2, while that of the GC750 CE was 81.14 O cm2. We can conclude that the Rct of the GC-750-DPR CE was reduced due to the decrease of oxygen content on its surface compared to that of the GC-750 CE. However, it was still larger than the Rct of the Pt-sputtered electrode (4.21 O cm2). The Nernst diffusion process of triiodide ions is shown at the low-frequency region in Fig. 6c. The diffusion impedance of the GC-750-DPR CE is only 2.55 O cm2, which is substantially lower than that of the GC750 CE (11.80 O cm2). The large diffusion impedance of the GC-750 CE can be explained by the existence of high-density defects on its surface. The decrease in the value of the diffusion impedance indicates the repair of the structural damage of the GC-750 film during DPR, which leads to a higher rate of reduction of triiodide ions. However, the diffusion impedance of the Pt-sputtered electrode is still smaller than those of developed CEs. Consequently, the DPR treatment of the GC750 CE results in a decrease in its overall impedance, which improves the PV performance of the DSC [19].

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Table 3 – Photovoltaic parameters of the DSCs with different CEs and simulated data from EIS spectra.a CE

Jsc (mA cm2)

Voc (mV)

FF (%)

g (%)

Rct (O cm2)

Zw (O cm2)

Rh (O cm2)

Pt-sputtered GC-750 GC-750-DPR

16.80 ± 0.05 14.31 ± 0.27 16.25 ± 0.77

763.33 ± 2.36 765.00 ± 5.00 771.67 ± 2.80

63.84 ± 0.09 46.24 ± 0.94 47.86 ± 1.07

8.18 ± 0.08 5.06 ± 0.13 6.01 ± 0.17

4.21 81.14 20.09

0.75 11.80 2.55

2.66 2.88 2.71

a

For spectra, see Fig. 6c. Rct: charge-transfer resistance; Zw: diffusion impedance; Rh ohmic internal resistance.

3.3. DSCs

Graphene-based hybrid materials for high efficiency

The decrease in the oxygen content in the GC-750 film, and in turn, the structural repair of GC-750, is an ideal strategy for developing cost-effective and highly efficient CE materials for DSCs. We have shown that the loss of oxygen functional groups in GC-750 film results in not only the decrease of the Rct at the electrolyte/CE interface, but also in the decrease of the diffusion impedance of triiodide ions, which leads to a high Jsc and FF for the DSC (Fig. 6a, Table 3). Our results are in good agreement with the literature [19,24]. Another reason for the high Jsc is an increase in the electrical conductivity of the CE due to the expansion in size of the continuous sp2 phase (from 4.0 to 7.3 nm) in a network of sp3 bonded carbons induced by DPR.

3.3.1. film

Characterization of nanohybrid PtNPs/GC-750-DPR

Based on principle mentioned above, we developed GC-750 supported PtNPs CE to obtain high PV performance of DSCs. The electrode was synthesized by DPR. Fig. 7 shows the TEM image of the PtNPs immobilized on the GC-750-DPR film (PtNPs/GC-750-DPR). The spherical PtNPs with a size of 2 nm were highly mono-dispersed on the GC-750-DPR surface. The PtNPs size was slightly larger than 1.5 nm, which was found for PtNPs immobilized on CVD-grown graphene in our earlier study [6]. The average sizes of 2.5 and 3 nm for PtNPs formed on the surface of the Cu grid and FTO glass, respectively, were reported elsewhere [4]. Thus, our experimental results show the dependence of the NP size on the nature of the substrates. This finding can be explained by the difference in the electrostatic energy of the substrates

Fig. 7 – (a, b) TEM image showing the PtNPs immobilized on GC-750 with scale bars: 5 nm (a), and 2 nm (b). (c) High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image, and (d) Element distribution of GC-750PtNPs. (A color version of this figure can be viewed online.)

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7 7 (2 0 1 4) 9 8 0–99 2

989

[6]. It should be noted that nanoparticle size was also strongly affected by the nature of the reducing agents. Indeed, the size of PtNPs grown on a graphene surface was larger than 10 nm due to the high reduction rate of the reducing agent (NaBH4), while the PtNPs size was around 5 nm in case of vitamin C reduction [15]. The reduction of the Pt precursor by the ethylene glycol with ultrafast microwave-assistance yields the PtNPs with a size of around 3 nm [16]. A heavy element, an ultrafine crystalline PtNP is distinctly observed in the TEM image in Fig. 7b, where the lattice spacing is shown in an ˚ , which enlarged scale. The estimated lattice spacing is 2.21 A coincides well with Pt {1 1 1} as a FFT inserted in Fig. 7b [6]. EDS, measured during the TEM observation, identified the PtNPs immobilized on the surface of the GC-750-DPR film. Furthermore, the immobilization of PtNPs on the GC-750DPR film was confirmed by element distribution as shown in Fig. 7c and d. It is known that the oxygen functional groups play the role of nucleation centers on the surface of RGO for metal NPs [25]. The network of sp3 (the oxygen functional groups) and sp2 bonded carbons in GC-750-DPR provide the possibility for uniform distribution of NPs on its surface. Indeed, PtNPs are not only uniform with a spherical shape, but also well distributed on the surface of the GC-750-DPR without any aggregation (Fig. 7a).

3.3.2. Electro-catalytic activity of a PtNPs/GC-750-DPR CE and the PV parameters of a DSC based on a nanohybrid When the PtNPs are conjugated with GC-750-DPR, a higher catalytic activity of the nanohybrid can be expected compared to that of the thick sputtered polycrystalline Pt film. It  is known that the limitation step of the I 3 /I redox process  is, particularly, the desorption rate of I ions [26]. The desorption rate of reduced I ions depends on various factors, such as the desorption energy, CE surface structure, surface morphology, CE roughness factor or the double layer thickness [27–30]. As can be seen in Fig. 2f, the GNP CE has a high roughness because the GNPs are not closely connected with the substrate. Note that the roughness of the PtNPs/GC-750-DPR CE should be higher than that of GC-750-DPR CE due to the hybridization of PtNPs on its surface. That suggests a thinner double layer as opposed to the thick double layer existing in the CE in a smooth surface of Pt-sputtered film (Fig. 2a) [30]. The double layer can block the diffusion of iodide and triiodide ions near the electrode surface [29]. Therefore, an increase in the CE roughness  ions, and in turn, may facilitate the diffusion of I 3 and I   increase the rate of the I3 /I redox process. The fast desorption of I ions from the PtNPs surface caused by a stable spherical surface, which with the minimal surface energy, also impacts the higher catalytic activity of GC-750-DPR supported PtNPs [27]. Furthermore, the spherical form of PtNPs provides an effective charge access. The lack of the recombination loss in the developed CE, compared to the loss induced by the grain boundary in the Pt-sputtered CE, can be considered as an advantage of the GC-750/PtNPs composite structure. Taking into account all these factors, we expect a higher electro-catalytic activity in the PtNPs/GC-750-DPR CE for regeneration of iodide compared to those in the GC-750-DPR and Pt-sputtered CEs. Furthermore, a higher electro-catalysis

Fig. 8 – (a) Cyclic voltammograms of different electrodes; (b) Equivalent circuit and Nyquist plots of the symmetric cells with identical CEs of GC-750-DPR, Pt-sputtered, and GC-750/ PtNPs. The cells were measured with the frequency range 100 kHz–100 mHz. Rct: charge-transfer resistance, Zw: diffusion impedance, Rh: ohmic internal resistance, CPE: constant phase element; (c) The J–V characteristics of DSCs fabricated with different CEs. (A color version of this figure can be viewed online.)

is suggested at the PtNPs/GC-750-DPR surface over those at the PtNPs/graphene-CVD [6] and PtNPs/RGO [16]. The effect is related to higher surface roughness of the developed nanohybrid and ultrafine crystalline structure of spherical PtNPs

990

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Table 4 – Photovoltaic parameters of the DSCs with different CEs and simulated data from EIS spectra.a CE

Jsc (mAcm2)

Voc (mV)

FF (%)

g (%)

Rct (O cm2)

Zw (O cm2)

Rh (O cm2)

Pt-sputtered GC-750-DPR GC-750/PtNPs

16.80 ± 0.05 16.25 ± 0.77 17.50 ± 0.43

763.33 ± 2.36 771.67 ± 2.80 770.00 ± 4.08

63.84 ± 0.09 47.86 ± 1.07 67.03 ± 0.61

8.18 ± 0.08 6.01 ± 0.17 9.03 ± 0.20

4.21 20.09 0.37

0.75 2.55 0.52

2.66 2.71 2.63

a

For spectra, see Fig. 8c. Rct: charge-transfer resistance; Zw: diffusion impedance; Rh ohmic internal resistance.

Table 5 – Comparison of photovoltaic performances of various graphene-based counter electrodes and charge-transfer resistances (Rct) measured from dummy cell with various graphene-based electrodes. Counter electrode

Preparation method

Layer thickness Jsc

Voc

FF g

Rct (O cm2) Ref.

Graphene GNS Graphene GNS N-graphene N-graphene foams PtNP/graphene Pt GNS/Pt Pt GNS/Pt nanohybrid GNS/Pt nanohybrid

Thermal exfoliation /drop casting Screen-printing Solvent-casting Dipping /thermal annealing Pyrolysis/drop-casting Freeze drying /doctor blade coating Ethylene glycol /solvent-casting E-gun evaporator Dipping /thermal annealing Sputtering Vitamin C /electrospraying Vitamin C /electrospraying Thermal annealing Pulsed laser ablation Sputtering Ethylene glycol /hydrothermal / doctor blade coating Thermal decomposition Dry plasma reduction

– – – – – 25 lm – 50 nm – 200 nm – –

7.70 16.99 11.59 10.64 10.50 15.84 12.06 10.64 11.42 11.22 15.88 15.56

680 747 780 710 729 770 790 730 730 740 760 830

54 54 32 25 71 58 67 68 73 75 66 69

11.70 1.20 14.23 1212a 8.75 5.60a 0.67 33.12 7.70a 5.80a 27.00 –

[31] [7] [32] [33] [34] [20] [32] [32] [33] [33] [35] [35]

– 20–25 nm 16.5 lm

6.67 740 59 2.90 2.36 5.05 680 58 2.00 7.73 12.86 744 61 5.70 4.93a

[36] [36] [37]

48 nm <2 lm

13.12 727 62 6.08 11.01a 17.50 770 67 9.03 0.37

[37] Present study

Graphene-Pt Pt Graphene-Ni12P5 Pt GC-750/PtNPs a

2.82 6.81 2.89 1.85 5.40 7.07 6.35 5.27 6.09 6.23 7.97 8.91

These values are presented in unit of O.

(Fig. 7a,b). It should be noted that the PtNPs hybridized on the surface of the GC-750-DPR CE and anchored to the oxygen functional groups can facilitate the electron conductivity in the PtNPs/GC-750-DPR structure by providing a conductive metallic network in the sp3 phase for charge carriers delocalized in the sp2 matrix. This factor along with the increase of the degree of clustering of the sp2 phase, induced by DPR, can impact a considerable increase of Jsc. To support our expectations, we conducted a comparative study of electro-catalytic activity of a PtNPs/GC-750-DPR CE, GC-750-DPR CE and Pt-sputtered CE by CV and EIS. The experimental results are presented in Fig. 8a andb. The simulated data from EIS are listed in Table 4. We found that both the values of the Zw and the Rct of triiodide ions significantly decreased at the electrolyte/PtNPs/GC-750-DPR CE interface as compared to those for the GC-750-DPR CE. Rct dropped down to 0.37 O cm2, which was the smallest or minimum value among all Rct given in Table 5 from dummy cells with the same system, while the Zw significantly decreased from 2.55 to 0.52. Such a dramatic drop in the main electrochemical parameters of a CE should lead to the remarkable increase of the Jsc and FF. To prove our strategy, the developed PtNPs/GC-750-DPR electrode was applied as a CE for DSCs. We conducted a comparative study of devices based on PtNPs/GC-750-DPR, GC-750-DPR, and Pt-sputtered CEs. Fig. 8c shows the J–V

characteristics of the DSCs. The device PV parameters are summarized in Table 4. We found that both the Jsc and FF were remarkably increased in the device based on the nanohybrid CE compared with those in the solar cell based on the GC750-DPR CE, reaching the values of 17.50 mA cm2 and 67.03%, respectively. Furthermore, the values of the Jsc and FF were higher than those in the state-of-the-art DSC. The increase of Jsc and FF resulted in a high device efficiency of 9.03%, which was higher by 50.3% over that of the Pt-free device, and by 10.4% over that of the state-of-the-art DSC. Moreover, until now, the highest efficiency obtained for graphene-supported PtNPs was 8.91%, which was less than that of the value in this study, as shown in Table 5. The combination of superior electro-catalytic activity of the PtNPs/GC750-DPR CE with its high electrical conductivity can explain this phenomenon. Indeed, the DPR resulted in the expansion of the sp2 clustering along with anchoring of PtNPs to the sp3 (creating a conductive network in the sp3 phase). Such reconstruction led to the creation of structure with excellent electro-catalytic activity, where a highly conductive graphene network for electron supply was combined with excellent access to the spherical PtNPs for triiodide ions. Based on experimental data, we can conclude that the immobilization of PtNPs on the surface of GC-750 though DPR of Pt precursors along with removal of the oxygen

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functional groups on its surface dramatically decrease the charge-transfer resistance at the electrolyte/CE interface and the diffusion impedance of triiodide ions, leading to a high Jsc and FF. The obtained PV parameters of the DSCs and EIS data (Table 4) are in a good agreement with our interpretation.

4.

Conclusions

We developed a cost-effective way to enhance the electro-catalytic activity and conductivity of graphene-based CEs by reducing of the oxygen functional groups and the immobilization of PtNPs on the surface of GNPs through DPR. CV and EIS results revealed that the effect is closely related to a decrease in the charge-transfer resistance at the interface of the electrolyte/PtNPs/GNP-DPR CE and a decrease in the diffusion impedance of triiodide ions. PtNPs hybridized on the surface of the PtNPs/GNP-DPR CE can facilitate the electron conductivity in the PtNPs/GC-750-DPR structure by providing a conductive metallic network in the sp3 phase for charge carriers delocalized in the sp2 matrix. As a result, the PtNPs/GNP based CEs with a high conductivity along with ultrahigh catalytic activity for reduction of triiodide was developed. The DSC based on the developed CE had an increase in the energy conversion efficiency by 50.3% as compared to that of the Pt-free device, and by 10.4% as compared to the efficiency of the state-of-the-art DSC. Our study clearly demonstrates that designing the PtNPs/GC-750 nanohybrid electrode through DPR is a cost-effective scientific approach to replace conventional Pt-based CEs in DSCs using low-cost commercially available GNPs. We also expect that the strategy suggested in this study could also be applied to the development of highly efficient electrodes for energy storage/conversion devices like supercapacitors, lithium ion batteries and fuel cells.

Acknowledgments This research was supported by the NRF-RFBR Joint Research Program (2013K2A1A7076282) through the National Research Foundation (NRF) of South Korea. This work was also supported by the Korean Brain Pool Program (131S-6-3-0538) and by the Future Core Technology Project (2E23964-13-045) of the Korea Institute of Science and Technology (KIST).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2014.06.015.

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