Enhanced efficiency of dye-sensitized solar cells with counter electrodes consisting of platinum nanoparticles and nanographites

Electrochimica Acta 59 (2012) 128–134

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Enhanced efficiency of dye-sensitized solar cells with counter electrodes consisting of platinum nanoparticles and nanographites Chen-Yu Liu a , Kuan-Chieh Huang a , Chun-Chieh Wang a , Kuo-Chuan Ho a,b,∗ a b

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

a r t i c l e

i n f o

Article history: Received 15 August 2011 Received in revised form 11 October 2011 Accepted 15 October 2011 Available online 22 October 2011 Keywords: Aniline Counter electrode Dye-sensitized solar cell Nanographite Platinum nanoparticle

a b s t r a c t The effect of a counter electrode (CE), fabricated by hybridizing the platinum nanoparticle (PtNP) and the nanographite (NG) on a dye-sensitized solar cell (DSSC), has been studied in this work. The catalytic PtNP/NG composite film for a CE is prepared using aniline (ANI) monomers as a dispersing medium, followed by spin-coating and annealing processes. The PtNP/NG composite film owns a high catalytic ability of converting tri-iodide to iodide due to the large surface roughness of the film. Thus, the DSSC assembled with the corresponding CE gives enhanced short-circuit current density (JSC ) and power-conversion efficiency () of 17.57 mA cm−2 and 7.07%, respectively, while the corresponding values are 14.57 mA cm−2 and 6.65% for a DSSC with a bare PtNP CE. Lower loading amounts of PtNPs for the PtNP/NG CE than those for the bare PtNP CE is demonstrated. Transmission electron microscopy (TEM) and UV/Vis absorption measurements are used to observe the dispersion of NGs in the solutions. X-ray diffraction (XRD) and Raman analyses are used to confirm the PtNP/NG composite film. The results are also substantiated by the characterizations of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electronic microscopy (SEM), and atomic force microscopy (AFM). © 2011 Elsevier Ltd. All rights reserved.

1. Introduction The major structure of a dye-sensitized solar cell (DSSC) consists of a dye-adsorbed nanocrystalline TiO2 photoanode, an electrolyte, and a counter electrode (CE) [1]. A high electrochemical activity of the CE is generally essential for a high performance DSSC. Conventionally, a platinum (Pt) layer was coated onto the conducting substrate, i.e., indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO), as the CE. The Pt film was used to offer not only a superior catalytic ability but also an efficient corrosion-resistance toward iodide species in the electrolyte [2–5]. The role of a catalytic layer on the CE for the DSSC is to promote the tri-iodide (I3 − ) reduction produced by the oxidation of the iodide (I− ) in the electrolyte by the oxidized dye. When the CE had no catalytic layer, the power-conversion efficiency of the cell was extremely low [6]. Several alternative materials used as the catalytic layers for the CEs in DSSCs have been proposed [7] because the application of the Pt was subject to its price variation in the past decade. Among them, the catalytic layers based on the carbon materials [8–14] and the conducting polymers are effective for the DSSCs [15–20].

∗ Corresponding author at: Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan. Tel.: +886 2 2366 0739; fax: +886 2 2362 3040. E-mail address: [email protected] (K.-C. Ho). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.10.050

Especially, the carbon material of nanographite (NG) was folded up as a spherical shape with an average of 30 nm in diameter, providing higher conductivity and larger surface area. In addition, the inherent property of high heat-resistance for the NGs is beneficial for the annealing process. Consequently, the NGs are potential alternatives to be introduced as CE layers in DSSCs, because of their advantages in terms of low-cost, superior conductivity, and large surface area. We have combined both NGs and platinum nanoparticles (PtNPs) to prepare a catalytic composite layer for the CE in a DSSC by using aniline (ANI) monomers as the media in this study. The ANI monomer was treated as a dispersant and a stabilizer [21] for the NGs in a hexachloroplatinate/isopropanol (H2 PtCl6 /IPA) solution. The composite PtNP/NG was synthesized via a noncovalent blending of PtNPs with the NGs. Recently, Han et al. employed a Pt/carbon composite consisting of Pt, graphite, and carbon black as the catalytic layer for the CE to fabricate an allsolid-state DSSC [22]. In another report, the DSSC assembled with a Pt/carbon black CE has also been proposed [23]. In this study, the power-conversion efficiency () of the DSSC with the CE coated with the composite of the PtNP/NG catalytic layer reached 7.07%, with reference to that of a DSSC ( = 6.65%) with the bare Pt CE. To the best of our knowledge, this is the first report on the use of a CE with the composite film of the PtNP/NG for a DSSC, especially employing the ANI monomers as media for preparing this catalytic film. Furthermore, the use of Pt was controlled within

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limits and optimized for the preparation of the PtNP/NG catalytic layer by using an UV/Visible (UV/Vis) absorption spectrometer. The X-ray diffraction (XRD) and Raman analyses were used to confirm this nanocomposite film. The results were also substantiated by the characterizations of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and atomic force microscopy (AFM). 2. Experimental 2.1. Materials Nanographites (NGs, carbon ECP600JD, 99.8%) were purchased from Ketjen Black International Company (Tokyo, Japan). The average dimension of the NGs was measured to be about 30 nm in diameter. Dihydrogen hexachloroplatinate (H2 PtCl6 , ACS, Premion, 99.95%) was obtained from Alfa Aesar Chemical Co. Isopropanol (IPA) was bought from TEDIA. Anhydrous LiI, I2 , acetonitrile (AN), aniline (ANI) and poly(ethylene glycol) (PEG, M.W. = 20,000) were purchased from Merck. Titanium(IV) isopropoxide (TTIP, +98%), 4-tert-butylpyridine (TBP, 96%), and tert-butanol (TBA, 99.5%) were obtained from Acros. 3-Methoxypropionitrile (MPN, 99%) and sulfuric acid (H2 SO4 , 95–97%) were products from Fluka. Nitric acid (HNO3 , 65%) was obtained from Cis-bis(isothio-cyanato)bis(2,2 -bipyridyl-4,4 Sigma–Aldrich. dicarboxylato)ruthenium(II)bis-tetrabutylammonium (so-called N719), 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and FTO (surface resistance = 15  sq−1 ) were purchased from Solaronix S.A., Aubonne, Switzerland. 2.2. Preparation of PtNP/NG composite counter electrodes The NGs (0.025 g) were incorporated and dispersed in the ANI medium (4.975 g) to obtain a concentration of 0.5 wt% NG/ANI solution at the beginning. Then the resulted solution was stirred continuously for 2 h. In the separate glass container, H2 PtCl6 (1.0 g, 0.0024 mol) were homogeneously dissolved in the IPA (96.54 g), followed by sonicating with a VCX 500 ultrasonicator at ambient temperature for 15 min to form a 20 mM H2 PtCl6 /IPA solution. Various concentrations (2, 5, and 10 mM) of H2 PtCl6 /IPA were also prepared. These different concentrations of H2 PtCl6 /IPA solutions (1.0 g) were then mixed together with the NG/ANI solutions (0.5 g). The fine dispersion of NGs in the mixture was further observed and verified using a transmission electron microscopy (TEM, JOEL JEM-1230 electron microscope with Gatan DualVision CCD Camera, JEOL Co. Ltd.) and an UV/Vis absorption spectrometer (V-570, Jasco). The obtained mixtures containing NGs and H2 PtCl6 were then spincoated onto the FTO substrate at 4000 rpm for 30 s to form the thin films. The thin films were annealed at 385 ◦ C for 15 min after spincoating. The PtNPs were converted completely from the H2 PtCl6 , and the ANI and the IPA were totally decomposed during annealing. Thus, the PtNP/NG-coated FTO substrates were obtained after annealing and used as the CEs for the DSSCs.

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the doctor-blade method. After the film was dried at room temperature, the TiO2 -coated FTO was annealed at 450 ◦ C for 30 min. By such coating and annealing twice, a third layer of TiO2 containing light scattering particles of TiO2 in an average diameter of 300 nm was further applied in the same way. The TiO2 film was immersed in an AN/TBA (v/v = 1/1) solution containing 0.3 mM of N719 dye for 12 h and its projective area of 0.16 cm2 was controlled by a mask. Thus, this dye-adsorbed TiO2 photoanode and the PtNP/NG CE were used to fabricate the DSSC. The gap of 25 ␮m between the two electrodes was controlled by using the Surlyn (Solaronix) as a spacer. Finally, the electrolyte containing 0.6 M DMPII, 0.1 M LiI, 0.05 M I2 , and 0.5 M TBP in MPN was slowly injected through a hole between the electrodes by the capillary effect. 2.4. Instrumentation The dispersing behavior of the NGs in different solutions (H2 PtCl6 /IPA and H2 PtCl6 /IPA/ANI) was observed using a TEM (same as the one mentioned before, operating at 100 kV and photographed with a Gatan DualVision CCD Camera, JEOL Co. Ltd.). The sample for TEM was prepared by dropping the sample solution on a copper grid coated with a carbon film. An UV/Vis absorption spectrophotometer was also used to analyze the dispersing behavior of the NGs. XRD (X’Pert, Philips) was utilized to verify the crystalline orientations of the PtNPs. The Raman spectra of the samples were recorded by a Dimension-P2 Raman system (Lamba Solution, Inc.). The catalytic abilities of the CEs were characterized using CV measurements, equipped with a three-electrode system (CH Instruments, Austin, TX) in the AN-based solution consisting of 0.01 M LiI, 1.0 mM I2 , and 0.1 M LiClO4 . The Pt wire and the Ag/Ag+ electrode were employed as the auxiliary electrode and the reference electrode, respectively. The specific surface area of the PtNPs was also obtained by the CH Instruments, performed in the 0.5 M H2 SO4 solution. The reference electrode of Ag/AgCl/saturated KCl was used. The photoelectrochemical characteristics of the DSSCs were recorded with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Netherlands). The EIS analyses of the samples were explored in a frequency range between 40 kHz and 1 Hz. Observations with cross-sectional views of the samples were performed using a scanning electronic microscopy (SEM, NovaTM NanoSEM 230) at 3 keV. AFM (NanoScope) was employed to estimate the surface roughness of the substrates using a probe with a tapping mode (NCH-10, resonance frequency = 320 kHz, force constant = 42 N m−1 , NanoWorld). The surface of the DSSC was illuminated by a class A quality solar simulator (PEC-L11, AM 1.5G, Peccell Technologies, Inc.) and the incident light intensity (100 mW cm−2 ) was calibrated with a standard silicon cell (PECSI01, Peccell Technologies, Inc.). 3. Results and discussion 3.1. Dispersing abilities of NGs in different solutions

2.3. Fabrication of DSSCs The TiO2 was formed by adding the TTIP (72 mL) into a 0.1 M HNO3 (430 mL) solution. The hydrolyzed TiO2 solution was then stirred and heated to 88 ◦ C for 8 h via the sol–gel process. After this solution was cooled down to the room temperature, the TiO2 solution was controlled at 240 ◦ C for 12 h in an autoclave via the hydrothermal process. By concentrating the autoclaved solution to 13 wt%, the paste consisting of nanocrystalline TiO2 was obtained. To prevent the paste from cracking during drying and to control the pore size of TiO2 , 30 wt% PEG with respect to the amount of TiO2 was incorporated into the TiO2 paste [24]. This TiO2 paste was further coated onto the FTO to form the TiO2 film by using

The main purpose of this study is to fabricate a catalytic layer of PtNP/NG composite in the CE for a DSSC. Thus, well-dispersed NGs in a solution consisting of H2 PtCl6 , the precursor of PtNP, was a concern for us at the beginning. A H2 PtCl6 -based solution containing well-dispersed NGs is required for preparing an uniform PtNP/NG composite film. We used ANI monomers for dispersing the NGs due to the ␲–␲ interactions resulted from the benzene rings of ANI monomers and the ␲-electrons of NGs. This interaction contributed to the van der Waals forces [21] which have to do with an enhanced dispersing ability of NGs in the ANI (Fig. 1). In the photograph of Fig. 1a, four vials arranged in order from the left side to the right side are the vials containing

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Fig. 1. (a) Photograph shows four vials consisting of different solutions. TEM images of the NGs in the solutions of (b) H2 PtCl6 /IPA and (c) H2 PtCl6 /IPA/ANI.

the solutions of H2 PtCl6 /IPA, NG/H2 PtCl6 /IPA, NG/H2 PtCl6 /IPA/ANI, and H2 PtCl6 /IPA/ANI. Compared with the solution of H2 PtCl6 /IPA, the solution of H2 PtCl6 /IPA/ANI containing ANI also exhibited a homogeneous state observed from its appearance. Hence, the homogeneous solution of H2 PtCl6 /IPA/ANI was beneficial for serving as a medium to disperse the NGs. When the NGs were incorporated into the solutions of H2 PtCl6 /IPA and H2 PtCl6 /IPA/ANI, a less aggregation of the NGs in the H2 PtCl6 /IPA/ANI solution was clearly observed in the TEM image of Fig. 1c, in comparison with the degree of aggregation of NGs for the NG/H2 PtCl6 /IPA (Fig. 1b). This is consistent with the above explanations for the effect of ␲–␲ interactions derived from NGs and ANI monomers on the enhanced dispersing ability of NGs in ANI. Moreover, these well-dispersed NGs in the solution of NG/H2 PtCl6 /IPA/ANI were also verified by using an UV/Vis absorption spectrometer [25]. In brief, the absorbance at 550 nm for the NG/H2 PtCl6 /IPA/ANI (Abs550 = 1.13) was higher than that for the NG/H2 PtCl6 /IPA (Abs550 = 0.28), indicating a better dispersing behavior of NGs in the H2 PtCl6 /IPA/ANI solution than in the H2 PtCl6 /IPA solution. 3.2. Preparation and characterization of PtNP/NG composite films The prepared solution of NG/H2 PtCl6 /IPA/ANI was then spincoated into a film on the FTO glass at 4000 rpm for 30 s. This film was further annealed at 385 ◦ C not only to remove the IPA and the ANI monomers but also to form the PtNPs. The PtNPs could be converted from the H2 PtCl6 by the thermal decomposition [2,26]. The PtNP/NG composite film was then characterized by XRD and Raman measurements (Fig. 2). In Fig. 2a, the XRD pattern of PtNP/NG composite film shows characteristic peaks of 2 at 39.4◦ and 46.0◦ corresponding to Pt(1 1 1) and Pt(2 0 0) orientations [23] of PtNP crystalline lattice, respectively. Raman spectrum of this composite film exhibits the characteristic signals at around 1350 and 1600 cm−1 correspond to D band and G band, respectively [27] (Fig. 2b). These observations confirmed the formation of PtNPs and the presence of NGs in the PtNP/NG composite film. Consequently, the composite film was successfully established.

3.3. Cyclic voltammetry (CV) of PtNP/NG composite films The CV analyses were further performed in the AN-based electrolytes containing 0.01 M LiI, 1.0 mM I2 , and 0.1 M LiClO4 to characterize the electrochemical reactions at the PtNP/NG composite films, prepared from different concentrations (2, 5, 10, and 20 mM) of H2 PtCl6 /IPA. These composite films were denoted as 2, 5, 10, and 20 mM PtNP/NG. In comparison, a bare 20 mM PtNP film was also prepared and characterized. In Fig. 3, each CV curve shows an anodic peak current (Ipa ) and a cathodic peak current (Ipc ) corresponding to the oxidation of I− ions and the reduction of I3 − ions, respectively. The electrochemical reaction at the interface of the composite film and the electrolyte is represented as follows: I3 − + 2e− → 3I−

(1)

Thus, the magnitude of Ipc can be regarded as the catalytic ability of this composite film for the I3 − reduction. The obtained values of Ipc are summarized in Table 1. In Table 1, the Ipc of 1.37 mA cm−2 for the 10 mM PtNP/NG is larger than that for other composite films, even the 20 mM PtNP (Ipc = 1.28 mA cm−2 ); implying that such a higher catalytic ability for the I3 − reduction could be achieved by the 10 mM PtNP/NG composite film. 3.4. Electrochemical impedance spectroscopy (EIS) of PtNP/NG composite films The catalytic properties of these films were also studied by using an EIS (Fig. 4). Each spectrum obtained from the EIS measurement was performed by analyzing a symmetric cell containing two identical FTO glasses, coated with the composite films. The geometrical surface areas of the films were selected to be 1.0 cm2 for the symmetric cell. The distance between them was controlled by the Surlyn of 25 ␮m in thickness. In this study, the electrolyte used for the symmetric cell was the same as that in a DSSC [28]. The symmetric cell was measured at the frequencies ranging from 40 kHz to 100 Hz to estimate the charge-transfer resistance (Rct ) of a cell. The value of Rct for a symmetric cell was obtained from its

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Table 1 Cathodic peak current (Ipc ), and surface area of PtNP (Ar /Ag ), charge-transfer resistance (Rct ), and Pt loading of bare PtNP film and various PtNP/NG composite films. Films

Ipc /mA cm−2

Rct / cm2

Ar /Ag

Pt loading/10−3 mg (piece FTO)−1

20 mM PtNP 20 mM PtNP/NG 10 mM PtNP/NG 5 mM PtNP/NG 2 mM PtNP/NG

1.28 1.32 1.37 0.96 0.74

1.42 0.71 0.48 1.88 3.17

2.49 3.54 3.74 1.52 1.01

1.53 1.48 1.44 1.07 0.28

corresponding semicircle in Fig. 4, followed by taking a half of the intercept of semicircle. The values of Rct for the PtNP film and various PtNP/NG composite films are summarized in Table 1. In Table 1, a lower value of Rct of 0.48  cm2 of the symmetric cell with the 10 mM PtNP/NG composite films is in accordance with the higher catalytic ability of this composite film for the I3 − reduction. The tendency of Rct values was consistent with the observation made with the CV (see Fig. 4 and Table 1). It was thus confirmed that the 10 mM PtNP/NG composite film has the highest catalytic ability for the I3 − reduction among the conditions of bare 20 mM PtNP film and other composite films (2, 5, and 20 mM PtNP/NG). On the other hand, low loadings of PtNPs of PtNP/NG composite films (2 and 5 mM PtNP/NG) give relatively lower catalytic abilities of these composite films. This is attributed to such a low loading of PtNPs that cannot effectively catalyze the I3 − reduction in the electrolyte.

Fig. 3. CV curves of bare PtNP film and various PtNP/NG composite films.

3.5. Surface areas and loadings of PtNP for the bare PtNPs and the PtNP/NG composite films Based on the same content (0.5 wt%) of NGs incorporated into each PtNP/NG composite film, the catalytic abilities of these composite films were affected by the loadings of PtNPs. Various loadings of PtNPs on the films would result in variation of surface area of Pt, which is an essential factor in determining the performances of DSSCs assembled with these CEs. In this point of view, the CV and UV/Vis absorption analyses were further applied to characterize the surface areas of PtNPs and their loadings, respectively. Fig. 5 presents the CV curves of the films performed in a 0.5 M H2 SO4

Fig. 2. (a) XRD patterns of PtNP/NG-FTO/glass and FTO/glass, and (b) Raman spectra of PtNP/NG composite film and pristine NGs.

Fig. 4. EIS spectra of bare PtNP film and various PtNP/NG composite films. The inset shows the relevant equivalent circuit model.

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C.-Y. Liu et al. / Electrochimica Acta 59 (2012) 128–134 Table 2 Photovoltaic parameters of the DSSCs with bare PtNP CE and various PtNP/NG CEs at 100 mW cm−2 illumination.

Fig. 5. CV curves of bare PtNP film and various PtNP/NG composite films in the deionized water containing 0.5 M H2 SO4 , at a scan rate of 50 mV s−1 .

CEs

VOC /V

JSC /mA cm−2

FF

 (%)

20 mM PtNP 20 mM PtNP/NG 10 mM PtNP/NG 5 mM PtNP/NG 2 mM PtNP/NG

0.68 0.62 0.63 0.59 0.62

14.57 16.51 17.57 16.21 16.06

0.68 0.67 0.64 0.43 0.36

6.65 6.80 7.07 4.14 3.61

This finding implies that troughs on the composite film were filled with the PtNPs at higher loading of PtNPs in the case of 20 mM PtNP/NG; thus decreasing its surface area for catalyzing the I3 − at the composite film in comparison with the case of 10 mM PtNP/NG. Compared with the cases of 10 and 20 mM PtNP/NG composite films, the bare 20 mM PtNP film possessed a higher loading of PtNPs (1.53 × 10−3 mg (piece FTO)−1 ) but a lower surface area of PtNPs (Ar /Ag = 2.49) (Table 1). This finding is probably related to the larger surface roughness of the PtNP/NG composite film, which could be attributed to the presence of NGs in these composite films. In fact, a larger surface roughness of the film has led to a higher catalytic ability of the composite films [31].

solution [29] at a scan rate of 50 mV s−1 with a reference electrode of Ag/AgCl/saturated KCl. The geometrical surface area (Ag ) of the bare PtNP film and the PtNP/NG composite films was controlled to be 1.0 cm2 . The real surface areas (Ar ) of them were estimated by integrating the currents under the curves of CVs during the forward and reverse scans within the range of −0.2 to 0.2 V (vs. Ag/AgCl/saturated KCl), followed by subtracting the charge associated with the double layers in the corresponding range of potential. Additionally, a value of 2.10 C m−2 [30] was the conversion factor used during the calculation. The estimated values of Ar /Ag are then summarized in Table 1. It was evidenced that the ratio Ar /Ag of 3.74 for the 10 mM PtNP/NG composite film delivered the largest surface area of PtNPs on this film among other composite films (2, 5, and 20 mM PtNP/NG) and the bare PtNP film (Table 1). A larger surface area of PtNPs contributed to a higher catalytic ability of the PtNP/NG composite film for the I3 − reduction. Moreover, the tendency for the values of Ar /Ag was also consistent with the values of Ipc and Rct , as shown in Table 1. In addition, the amounts of PtNPs loaded on the bare PtNP film and various PtNP/NG composite films were quantified by using an UV/Vis absorption spectrometer. These experiments were carried out separately. In brief, the films spin-coated with the H2 PtCl6 /IPA and various NG/H2 PtCl6 /IPA/ANI solutions on the FTO substrates were immersed in the IPA solvent to dissolve the Pt salt. The resultant solutions were measured at 263 nm for the maximum absorption corresponding to the absorption of Pt4+ by UV/Vis absorption analyses. Consequently, the loadings of PtNPs were calculated by the Beer’s Law and the corresponding chemical reaction for the thermal decomposition of H2 PtCl6 , which is:

3.6. Surface morphologies of the bare PtNP and the PtNP/NG composite films

2H2 PtCl6 + O2 → 2H2 O + 2Pt + 6Cl2 ↑

3.7. Photovoltaic performances of DSSCs with various CEs

(2)

The obtained values for PtNPs loadings are summarized in Table 1. For the cases of these PtNP/NG composite films, the estimated loadings of PtNPs increased with the increase in the amount of PtNPs in the composite films. However, this tendency is not consistent with the values of the catalytic abilities (Ar /Ag ) of these composite films, i.e., a higher catalytic ability (Ar /Ag = 3.74) of 10 mM PtNP/NG has lower PtNP loading of 1.44 × 10−3 mg (piece FTO)−1 in comparison with the case of 20 mM PtNP/NG (Ar /Ag = 3.54; Pt loading = 1.48 × 10−3 mg (piece FTO)−1 ) (Table 1).

To take insight into the surface morphologies of the 10 mM PtNP/NG composite and 20 mM bare PtNP films, SEM and AFM techniques were utilized for this investigation. The actual appearances of the roughness for the two different films were compared by the cross-sectional images of SEM (Fig. 6a and b). Fig. 6a shows many significant bulges affected by the NGs at the surface for the 10 mM PtNP/NG composite film. On the contrary, the surface morphology of the 20 mM bare PtNP film presents a few humps on a relatively flat surface (Fig. 6b). The rougher surface of PtNP/NG composite film thus renders a larger surface area for the I3 − reduction. In addition, an obvious difference in surface morphologies of AFM images for the PtNP/NG composite film and the bare PtNP film were shown in Fig. 6c and d, respectively. Consequently, a larger value of the rootmean-square surface roughness (Rms ) for the PtNP/NG composite film was obtained to be 31.48 nm, as judged by the corresponding AFM image (Fig. 6c), in contrast to the case of bare PtNP film (Rms = 22.49 nm) (Fig. 6d). The thicknesses of both 10 mM PtNP/NG composite film and 20 mM PtNP film give a very close value of about 588 nm (see Fig. 6a and b), indicating that the catalytic ability of these composite films (see Fig. 6c and d) is mainly determined by their surface roughness. The above observations confirmed that the 10 mM PtNP/NG composite film owned a larger surface roughness in comparison with that of 20 mM bare PtNP film. In other words, the high catalytic ability of PtNPs in the composite film is related to the high surface roughness observed previously by CV, EIS, SEM, and AFM.

Fig. 7 presents the photocurrent density–voltage (J–V) characteristics of DSSCs employing different CEs coated with the 20 mM PtNP and various composite films (2, 5, 10, and 20 mM PtNP/NG), under an illumination at 100 mW cm−2 . The corresponding photovoltaic parameters of these DSSCs are obtained from Fig. 7 and summarized in Table 2. In Table 2, the short-circuit current density (JSC ) of the DSSC with 20 mM PtNP CE is 14.57 mA cm−2 , and its power-conversion efficiency () is 6.65%. In comparison, the DSSC, made from 10 mM PtNP/NG CE, delivered a higher JSC and  of 17.57 mA cm−2 and 7.07%, respectively (Table 2). The higher JSC and  for the DSSC with 10 mM PtNP/NG CE is attributed to

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Fig. 6. Cross-sectional SEM image (a) and AFM image (c) of 10 mM PtNP/NG composite film. Cross-sectional SEM image (b) and AFM image (d) of 20 mM PtNP film.

higher catalytic ability for the I3 − reduction, owing to its favorable surface morphology in terms of larger values of Rms , as seen from both Fig. 6c and the value of Ar /Ag (see Table 1). Comparing the photovoltaic parameters (VOC = 0.68 V and FF = 0.68) of the DSSC with a 20 mM PtNP CE, those of DSSCs with the PtNP/NG (2, 5, 10, and 20 mM) CEs give lower values (Table 2). One possible reason for lower VOC and FF for the cells with various PtNP/NG CEs is attributed to the small amount of NGs attaching to the surfaces of TiO2 , thereby rendering the unwanted adsorption of NGs as a potential recombination center. This would generate the dark current in a DSSC. In fact, similar result was also reported by Hou et al. [32]. Namely, the decrease in both VOC and FF of the DSSC with a

counter electrode consisting of Pt and carbon fiber, with reference to those of the DSSC employing a Pt wire electrode, was observed. We have also prepared another mixture consisting of 0.5 g H2 PtCl6 /IPA and 1.0 g NG/ANI solutions, with reference to the original mixture containing 1.0 g H2 PtCl6 /IPA and 0.5 g NG/ANI. The loading of NG was relatively higher in this new mixture, which was further prepared for catalytic films on CEs in DSSCs. At 100 mW cm−2 illumination, the efficiencies of DSSCs with these CEs gave values of 2.22, 2.47, 3.29, and 2.64% for the cases of 2, 5, 10, and 20 mM PtNP/NG CE, respectively. Comparing with the efficiencies reported in Table 2, the lower performances of DSSCs with the PtNP/NG CEs containing higher loading of NG were observed. This may due to the fact that NGs do not provide as high catalytic ability as that of Pt for the I3 − reduction. 4. Conclusions

Fig. 7. J–V characteristics of the DSSCs with bare PtNP CE and various PtNP/NG composite CEs at 100 mW cm−2 illumination.

In this study, the well-dispersed NG/H2 PtCl6 /IPA/ANI solution was successfully prepared and confirmed by TEM and UV/Vis absorption measurements. The catalytic PtNP/NG CE for a DSSC was prepared by spin-coating and annealing processes from this solution. The presence and distribution of PtNP/NG on the CE were verified using the analyses of XRD and Raman. CV and EIS characterizations have demonstrated the higher catalytic ability of PtNP/NG CE than that of the bare PtNP CE. An enhancement in both JSC (from 14.57 to 17.57 mA cm−2 ) and  (from 6.65 to 7.07%) for the DSSC with a 10 mM PtNP/NG CE was observed, with reference to the values of a DSSC applying a 20 mM PtNP CE (1 sun). From the investigations (SEM and AFM images) on the surface morphologies of the CEs, it is concluded that 10 mM PtNP/NG CE possesses rougher surface and consequently higher catalytic ability than those of 20 mM PtNP CE; and this is the reason for the better performance of the DSSC with 10 mM PtNP/NG CE than that of the cell with the bare PtNP CE.

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