Efficient highly flexible dye sensitized solar cells of three dimensional graphene decorated titanium dioxide nanoparticles on plastic substrate

Journal of Power Sources 281 (2015) 404e410

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Efficient highly flexible dye sensitized solar cells of three dimensional graphene decorated titanium dioxide nanoparticles on plastic substrate Jian Zhi a, Houlei Cui a, b, Angran Chen a, b, Yian Xie a, b, Fuqiang Huang a, b, * a

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


3D graphene-TiO2 on plastic substrate for DSSCs was fabricated at room temperature.
3D conductive graphene skeleton can significantly improve the surface area and charge transportation.
Highly flexible DSSCs assembled using 3D graphene-TiO2 film delivered a power conversion efficiency up to 6.41%.
3D graphene showed great potential for sinter-less flexible DSSCs.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 December 2014 Received in revised form 29 January 2015 Accepted 1 February 2015 Available online

Dye-sensitized solar cells (DSSCs) on flexible plastic substrates usually suffer from a slower electron diffusion rate and insufficient surface area due to no sintering process. Therefore, the conversion efficiency (ƞ) of such flexible DSSCs is normally below 6%. Here, the highly flexible DSSCs with enhanced performance are fabricated at room temperature, employing 3D graphene decorated nanocrystalline TiO2 films (3DGT) as anode on plastic substrates. Owing to the enhanced charge transportation and increased surface area from 3D conductive graphene skeleton, the 13 mm-thick 3DGT-0.85 (0.85 wt% 3D graphene plus TiO2 nanoparticles) anode achieves a power conversion efficiency of 6.41%, which is 56% higher than pristine TiO2 based anode. This efficiency is among the highest values for the previously reported TiO2 photoanodes on plastic substrates. © 2015 Elsevier B.V. All rights reserved.

Keywords: Three dimensional graphene Highly flexible Plastic substrate Titanium dioxide nanoparticles Dye sensitized solar cells

1. Introduction €tzel et al., dye-sensitized Since their initial introduction by Gra

* Corresponding author. CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China. E-mail address: [email protected] (F. Huang). http://dx.doi.org/10.1016/j.jpowsour.2015.02.001 0378-7753/© 2015 Elsevier B.V. All rights reserved.

solar cells (DSSCs) have received much attention as an alternative to silicon-based solar cells because of their potential low cost, easy fabrication procedures, and respectable energy conversion efficiencies [1e4]. Driven by the industrial requirements to produce DSSCs in a fast and low cost manner, the flexible DSSCs on conductive plastic substrates have attracted substantial research interests. The use of such plastic substrates requires that all the processes in fabrication of DSSCs need to be designed at a

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temperature lower than 150  C. So far, many alternative methods have been attempted to fabricate flexible photoanode at low temperature, such as mechanical compression [5], solegel [6], electrochemical anodization [7,8], microwave irradiance [9], electrophoretic deposition [10] and UV-ozone treatment [11]. However, owing to the sinter-less fabrication process, cells built on plastic substrates normally suffer from a slower electron diffusion rate and insufficient surface area. Therefore, the conversion efficiency (ƞ) of such flexible DSSCs is significantly lower (typically below 6%) than glass based DSSCs [12e14]. To increase the electron diffusion rate and particle interconnection of photoanode, carbon nanotubes (CNTs) have been employed along with TiO2 films fabricated at low temperature in the DSSCs [15,16]. After introduction of CNTs into TiO2 photoanode, significantly improvement of conversion efficiency was achieved [17,18]. However, due to low surface area and high inter-particle contact resistance between CNTs, the conversion efficiency of such plastic DSSCs are still unsatisfactory [19]. More recently, we and the other groups have produced freestanding three dimensional graphene (denoted as 3D graphene) foams by chemical vapor deposition (CVD) with a nickel foams template [20e22]. Such 3D graphene copies the porous structure of Ni template and possesses a great BET are. Furthermore, due to the absence of defects, 3D graphene displayed excellent electrical conductivity and intersheet contact resistance. Attributing its distinct physical properties, 3D graphene can be directly employed as flexible substrate for functional devices [23]. In this study, small amount (0.5e1.5 wt%) of 3D graphene was added into TiO2 nanoparticles (TiO2 NPs), and hybrid film (denoted as 3DGT-x, x is the weight amount of graphene) on plastic indiumetin oxide coated polyethylene terephthalate (ITO/PET) substrate was obtained after doctor blading process in room temperature, employing commercial available TiO2 powders (P25) with a little amount titanium isopropoxide (TTIP) as the TiO2 source. Such hybrid film exhibits not only high BET area but also outstanding electron conduction. Using 3DGT-0.85 with a thickness of 13 mm as anode materials, the fabricated flexible DSSCs showed a power conversion efficiency of up to 6.41%, 56% higher than pristine TiO2 DSSCs. This efficiency is among the highest value for the plastic based DSSCs. 2. Results and discussion The morphology of a 3D graphene foam was investigated by scanning electron microscopy (SEM), as shown in Fig. 1a. Obviously, after removal of the Ni template, the graphene replicates the 3D network and porous structure of the Ni foam. Macropores with the size varied from 100 to 500 m m are maintained, without collapsing and cracking. The quality and layer number of the 3D graphene foam were characterized by Raman spectra (Figure S1, see Supporting Information). Only G and 2D bands are observed with the absent of D band, suggesting a high quality of graphene [24]. The continuous 3D structure and high electric conductivity endow the graphene a great electron transport channel to bridge the TiO2 NPs. Fig. 1b shows the cross-sectional cut of 3DGT-0.85 on plastic ITO/PET substrate. The micrometer-sized 3D graphene flakes are well dispersed within the aggregation of TiO2 NPs, suggesting a good contact between graphene flakes and TiO2 NPs. Attributing to its distinct matrix (as shown in Fig. 1a), the 3DGT-0.85 composite film exhibits highly porous structure, which is favorable for dye absorption and electrolyte molecules transportation [25]. For comparison, electrode with pure TiO2 NPs was also prepared, and the morphology was examined using FESE, as shown in Figure S2. In general, without 3D graphene, the film exhibits less porous and rough surface. Subsidence damage can be clearly seen

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in the film, indicating the importance of 3D graphene to improve the anode film quality for DSSCs. The nanostructure of 3DGT-0.85 composite was further investigated by TEM (Fig. 1c). From Fig. 1c, the spherical TiO2 nanocrystals can be clearly distinguished with an average particle size of 25e30 nm. Due to the relatively smooth and planar structure of a 3D graphene (Figure S3), TiO2 nanocrystals were successfully anchored on the surface of graphene sheet. These homogenous and dense structures can increase the contact between graphene and TiO2, which will fully utilize the charge transportation of graphene between TiO2 NPs. Fig. 1d shows the lattice-resolved image of the TiO2 NPs in the composite, indicating its single crystalline anatase phase with the perfect lattice spacing of 0.35 nm of (101) from anatase [26]. Fig. 2a shows the XRD patterns of 3DGT-0.85, pure TiO2 NPs and 3D graphene samples. It can be seen that most diffraction peaks of 3DGT-0.85 and TiO2 NPs samples are very well indexed to an anatase structure of TiO2 with additional characteristic peaks of rutile TiO2 appeared at 2 h ¼ 24.8 , 36.1, which are typical for P25 nanoparticles. This is in agreement with the high resolution TEM image shown in Fig. 1d. However, ascribed to very low weight amount of graphene, no clear diffraction peaks of graphene are observed in 3DGT-0.85 composite films. The most intense diffraction peak (002) of graphene is largely overlapped by the anatase (101) peak [27,28]. Fig. 2b shows the nitrogen adsorptionedesorption isotherms and corresponding pore-size distribution curves (inset) of 3DGT-0.85 and pure TiO2 NPs. The samples show type IV isotherms according to BrunauereDemingeDemingeTeller classification [12], confirming the presence of mesopores (2e50 nm). The BET (BrunauereEmmetteTeller) surface area of 3DGT-0.85 is 78.72 m2 g1, higher than TiO2 NPs (53.24 m2 g1), indicating the enhanced porous structure after the decoration of 3D graphene, as was shown in Fig. 1b. The pore size distributions (inset of Fig. 2b) calculated from desorption branch of the nitrogen isotherm by the BJH method, are obviously broader than pristine TiO2 NPs, further proving the existence of mesopores and macropores in the composites. The corresponding physicochemical properties of the composite film electrodes with different 3D graphene loading are shown in Table S1. With increasing of graphene loading, both surface area (SBET) and pore volume of the TiO2 NPs/3D graphene composites increase, which is originated from the high surface area of 3D graphene incorporated [21]. Such organized porous structures with relatively high surface area could be extremely useful for solar cells, as they would provide efficient transport pathways for the electrolyte molecules. In order to develop DSSCs with high photoelectron conversion efficiency, a reduced e/h recombination rate and an enhancement of electron transport are highly desirable. The transient photocurrent of DSSCs with various anodes was investigated to determine the effects of graphene on the charge trapping and recombination of DSSCs. Fig. 3a shows the rise and fall of the ISC during several oneoff cycles upon illumination at 100 mW cm2. The ISC rise of 3DGT-x based film is faster than that of pristine TiO2 NPs film, indicating the more easily transportation of photogenerated electrons into the composite film thanks to 3D graphene [26]. It is noteworthy that the photocurrent of 3DGT-1.5 is clearly lower than 3DGT-0.85, which could be attributed to the higher charge recombination and shield-effect of illumination after further increasing the amount of 3D graphene [29]. In addition, the introduction of 3D graphene can significantly modify the optical properties of the TiO2 layers according to the percentage considered (see Supporting Information, Figure S4). The film transparency in the UVevis range remains unchanged for 3D graphene amount below 0.85 wt%. However, when the amount of 3D graphene was increased to 1.5 wt%, light absorption from 3D graphene starts to

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Fig. 1. (a) SEM image of a 3D graphene foam. (b) Cross-sectional FESEM of 3DGT-0.85 film. (c) TEM image of 3DGT-0.85 composites. (d) Lattice-resolved image of the TiO2 NPs.

Fig. 2. (a) XRD patterns of 3DGT-0.85, pure TiO2 NPs and 3D graphene samples. (b) Nitrogen adsorptionedesorption isotherms and BJH pore size distribution curves of 3DGT-0.85, pure TiO2 NPs respectively.

Fig. 3. (a) Transient photocurrent curves of DSSCs based on the 3DGT-0.85, 3DGT-1.5 and TiO2 NPs film electrodes. (b) EIS Nyquist plots of DSSCs based on the 3DGT-0.85, 3DGT-1.5 and TiO2 NPs film electrodes.

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play a role, resulting in the loss of transparency and loss of available solar radiation for current generation, which can negatively affect the photovoltaic properties of the device [30,31]. The electrochemical impedance spectroscopy (EIS) of the DSSCs with pure TiO2 NPs and 3DGT-x photoanodes at an applied bias of Voc are shown in Fig. 3b. The Z0 and Z00 are the real and imaginary parts of the impedance, respectively. In the high-frequency and middle-frequency regions, two semicircles can be seen in all curves. The first semicircle (10e14 U) over high-frequency region represents the Faraday resistance of the redox reaction between I/I3 at the electrolyte/Pt interface, while the second semicircle in the medium frequency region reflects the electron transfer resistance at the TiO2/electrolyte interface, which is the most important and decisive factor in DSSCs [32e34]. The comparison of the second semicircles indicates that the diameter increases this the order: 3DGT-0.85 < 3DGT-0.5 < 3DGT-1.5 < pure TiO2 NPs, suggesting that moderate graphene loading contributes to the reduction of charge transfer resistance at the TiO2-electrolyte interface. In particular, the 3DGT-0.85 DSSC has the smallest interfacial resistance, which implies the fastest electron transfer and highest energy conversion efficiency. However, with further increasing of graphene loading (from 0.85 to 1.5 wt%), the interfacial resistance increases. It is reported that higher graphene loading could increase the interfacial resistance and charge recombination [35]. Therefore, the electron diffusion and transport become poor, leading to low conversion efficiency [36]. In addition, higher graphene loading severely shields the light-harvesting of the dye-sensitizer and the number of photogenerated electrons decreases under illumination [37]. Overall, based on the transient photocurrent and EIS measurement, it can be concluded that moderate introduction of 3D graphene into TiO2 NPs electrodes can significantly reduce the recombination of charge carriers and enhance the performance of plastic based DSSCs. To investigate the photovoltaic performance of the DSSCs based on different photoanodes, JeV curves are shown in Fig. 4, and their corresponding photovoltaic characteristics are summarized in Table S2 (see supporting information). The overall efficiency h was evaluated using the equation h ¼ (FF  Jsc  Voc)/Pin, where FF is the fill factor, Jsc is the short circuit current density, Voc is the opencircuit voltage, and Pin is the incident light power density. Under optimal conditions, the highest efficiency value is 6.41% in the case of 3DGT-0.85, 56% higher than the pristine TiO2 NPs film. This improvement is due to the following reasons. Firstly, TiO2 NPs can

Fig. 4. Comparison of the IeV characteristics of DSSCs based on the 3DGT-0.5, 3DGT0.85, 3DGT-1.5 and TiO2 NPs film electrodes.

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be well anchored on the 3D graphene, and photoinduced electrons are easily transferred to the 3D graphene flakes from TiO2 (Scheme 1) [38]. The rapid transport of photogenerated electrons in 3D graphene not only reduces the recombination rate and resistance at the TiO2 NPs-electrolyte interface, but also bridges the electrons transportation from the films to the plastic ITO/PET substrates (Scheme 1). Secondly, the introduction of 3D porous graphene can increase the surface area of the photoanode film, of which the value is usually quite low owing to the low temperature fabrication process. As a result, 3DGT-x composite with a larger surface area provides more sites for the absorption of dye molecules, leading to more photoinduced electrons being injected from the excited state of the dye into the conduction band of TiO2-NPs [39]. More importantly, due to its bendable property, the 3D graphene can act as a flexible framework for the TiO2 NPs when employed as anode materials on plastic substrate, which is especially beneficial for the film stability of the devices [40]. However, at higher graphene content (3DGT-1.5), the conversion efficiency decreases. Higher graphene loading may cause light-harvesting competition between the dye and graphene and increased charge recombination [41], and thus the number of photogenerated electrons will decrease under illumination, resulting in to less photocurrent (Fig. 3a) and low conversion efficiency (Fig. 4). In order to further reveal the charge recombination with different 3D graphene concentrations, EIS measurement as a function of different applied voltages for all the devices was performed in dark (see Figure S5). The interfacial resistance (Rct) and capacitance (Cu) represented by the second semicircles were fitted with an equivalent circuit model (inset of Fig. S5(a)) [26,31]. Rct and Cu allows to estimate the electron lifetime according to te ¼ RctCm at each applied bias [31]. Figure S5(c-d) shows the dependence of Rct, Cm, and te as a function of different biases in the samples with different 3D graphene loading. Obviously, the film with moderate 3D graphene (0.85 wt%) has the largest Cu and longest electron lifetime, attributing to the improved charge transportation and increased surface area. However, in the case of the film with even larger graphene loading (1.5 wt%), although its surface area is high (85.02 m2 g1), an obvious decrease of the Rct along with an almost unaffected change in the Cu, resulting in a short electron lifetime. In this case, a large surface exposed to the electrolyte may create a preferential pathway for the recombination of photogenerated charges from the oxide to the electrolyte, which will reduce the functional performance of the cell. As discussed above, moderate amount of 3D graphene plays an important role in cell performance, too little or too high concentration will both have inferior effect on total conversion efficiency. The work function of graphene was reported to be 4.42 eV [42], which is slightly lower than the CB of TiO2 (4.4 eV) [43] and higher than that of the ITO (4.7 eV) [44]. As a result, graphene can act as a bridge between TiO2 and ITO substrate, which favors the transfer of photo-generated electrons stepwise from TiO2 to ITO without energy barrier (Figure S6). In traditional TiO2 photoanode, photoinduced electrons have to transfer through the film which is several micrometers thick before reaching the plastic substrate, and have more possibilities to be recombined by hole. In the hybrid 3DGT photoanode, as the graphene has a work function similar to that of ITO, it acts more likely as excellent current collectors penetrating into the TiO2 matrix, in which electrons are rapidly collected before being recombined. Table 1 is a summary for a number of DSSCs with different film remarks on flexible substrates. Interestingly, it is reported that due to the presence of SWCNT, the Fermi Level of TiO2 shifts to more positive potentials as it equilibrates with SWCNT [45]. As a result, the electrons transferred into the SWCNT network are quickly transported to the collecting electrode surface, minimizing the possibility of charge

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Scheme 1. Schematic illustration of the devices and electron transport path. Left: depicting the reference DSSC with only TiO2 NPs as anode semiconducting materials and N719 dye as sensitizer (the scheme is adapted from an SEM image, see Fig. S2 in the Supporting Information and right: illustrating DSSC with 3D graphene decorated TiO2 nanoparticle network. For purpose of illustration only, the TiO2 particle size is magnified and only one dye molecule is shown in each scheme.

Table 1 Comparison of the power conversion efficiency between the 3DGT based plastic DSSC and several reported flexible DSSCs. Film remarks

PCE (%)

Method of film fabrication

Refs.

Hierarchical TiO2 Nanostructured TiO2 arrays Ordered TiO2 nanotube

5.57 5.84 3.12

[46] [47] [48]

ZnO nanocactus array and top ZnO particle Porous ZnO TiO2 nanotube arrays 3D graphene decorated TiO2 NPs

5.24

Electrospray Hydrothermal process Anodization and ultrasonic vibration Chemical bath deposition

4.04 2.67 6.41

Electrophoresis deposition Laser-drilled technology Doctor blading process

[50] [51] This work

[49]

recombination at grain boundaries [45]. Analogically, the present of 3D graphene may also lead to the positive shift of Fermi level for TiO2, which is beneficial to increase the cell performance. Considering the power conversion efficiency, the money and time consumed in film fabrication, and the instrument employed, our devices exhibit more application potential in the field of flexible DSSCs [46e51].

The IPCE spectra in Fig. 5 offers detailed information in the ability of photo-current conversion of the DSSCs. The IPCE value depends on the light absorption of the dye, the electron injection quantum yield, and the electron collection efficiency. It is observed that the 3DGT-x (x ¼ 0.5, 0.85, 1.5) based DSSCs show much higher IPCE values than that of the pure TiO2 NPs DSSC, the trend of which is in good agreement with the Jsc for all electrodes. However, the IPCE values decrease at a higher graphene content (1.5 wt%), which may ascribe to the reduced light harvesting of dye molecules and increased charge recombination in the excess graphene decorated film [52]. 3. Conclusions In summary, small amount (0.5e1.5 wt%) of 3D graphene was added into TiO2 nanoparticles (TiO2 NPs), and hybrid composite film on plastic ITO/PET substrate was obtained after room temperature doctor blading process. Without high temperature calcination, such composite film exhibits not only improved BET area but also outstanding electron transport property. Using 3DGT-0.85 with a thickness of about 13 mm as anode materials, the fabricated flexible DSSCs show a power conversion efficiency of up to 6.41%, 56% higher than the pristine TiO2 NPs film, which is due to the improved BET area and outstanding electron transport property of the hybrid film. This efficiency is among the highest value for the plastic based DSSCs, indicating that 3D graphene material has good potential in the application of solar cells, especially for the flexible DSSCs. The PCE is expected to be further improved by engineering the morphology and the dye adsorption of 3DGT based films. 4. Experimental section 4.1. Materials

Fig. 5. IPCE curves of DSSCs based on the 3DGT-0.5, 3DGT-0.85, 3DGT-1.5 and TiO2 NPs film electrodes.

Titanium isopropoxide (TTIP) and absolute ethanol were all of A.R grade and were all purchased from Aladdin Chemical Reagent Co., Ltd, China. Sensitized dye N719 (RuL2 (NCS)2, L ¼ 4,40 -dicarboxylate-2,20 -bipyridine) was purchased from Solaronix SA (Switzerland). P123 (EO20PO70EO20) was purchased from SigmaeAldrich. All reagents were used without further treatment. Indiumetin Oxide/poly(ethylene terephthalate) (ITO/PET, 60 U, from IST/USA) was used as a substrate for DSSC test.

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4.2. 3D-graphene foam preparation Ni foams were used to catalyze the graphene growth. Firstly, Ni foams were immersed in a dilute solution of acetic acid for 30 min to remove the oxide layer on their surface, then washed in isopropanol and acetone for 10 min, respectively, and finally rinsed with deionized water and dried for use. Secondly, the Ni foams were heated to 1000  C in 40 min under H2, then a gas mixture flow of CH4, H2, and Ar was introduced to initiate graphene growth for 10e30 min. After growth, the samples were rapidly cooled to 500  C at a rate of 200  C min1 under Ar and H2. The Ni foams covered with graphene were drop-coated with a poly(methyl methacrylate) (PMMA) solution (4% in anisole), and then baked at 100  C for 2 h. The PMMA/graphene/Ni foam structure was obtained after solidification. Then the samples were put into a 5 M HCl solution for 5 h to completely dissolve the Ni foams to obtain the PMMA/graphene. Finally three-dimensional graphene networks were obtained after removing PMMA in acetone. 4.3. Preparation of 3D graphene decorated TiO2 NPs film on ITO/PET 1 g of P25 powder was added in 10 ml of ethanol followed by magnetic stirring to obtain a homogeneous dispersion. No surfactants were used as templates to the solution. TTIP was mixed in the previous dispersion in a concentration of 0.18 M. Then, the prepared 3D graphene foam (with the weight amount of 0.5e1.5wt% as P25, respectively) was added into the suspension, and slowly stirred (50 r min1) for 5 min. 3D graphene decorated TiO2 NPs Films with effective surface area of around 0.25 cm2 were formed with the help of adhesive paper on ITO/PET substrate by multi-step doctor blading method. The deposition of several layers (with the thickness up to 13 mm) is made by the intercalation of a room temperature stabilization step between each coating. Subsequently, a layer consist of pure P25 NPs was coated on the 3D graphene decorated TiO2 NPs films to improve the photocurrent and photovoltaic performances. All the films were left to dry in air for 5 min and then were thoroughly rinsed with distilled water several times to wash out any loose material that could be detached from the rest of the film (i.e., as prepared films). If necessary, to remove any humidity, the TiO2 films were left to dry in a 100  C oven for 30 min. The pristine TiO2 NPs film was also prepared through the same process above, without adding of 3D graphene. 4.4. Fabrication of flexible DSSCs The film was sensitized by a 2.5  104 M N719 absolute ethanol solution for 24 h to adsorb the dye adequately; thus, a dye sensitized TiO2 film electrode was obtained. Then, the electrode was sandwiched together with a platinized ITO/PET counter electrode. The redox electrolyte was injected into the aperture between the dye sensitized TiO2 film electrode and the counter electrode. 4.5. Measurements and characterization XRD measurements were carried out using powder X-ray diffraction (Bruker D8 Advance, with Cu-Ka radiation operating at 40 kV and 40 mA, scanning from 2q ¼ 20e80 ). Field emission scanning electron microscopy (FESEM; Hitachi S-4800) and transmission electron microscope (JEOL 2100 TEM, 200 kV) was used to characterize the morphology of the films. Nitrogen adsorption desorption isotherms for surface area and pore analyses were measured using a Nova 3200e (Quantachrome instruments). Raman spectra were recorded at room temperature using a microRaman spectrometer (InVia, Renishaw PLC, Gloucestershire, UK) with backscattering geometry, using a 514.5 nm Arþ laser as an

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excitation source. The amount of adsorbed dye was measured by desorbing the dye into 20 mM NaOH solution with equal amount of deionized water and ethanol and by absorption measurement of the solution using the absorption peak intensity of N719 at 511 nm. The current voltage test of DSSCs was performed under one sun condition using a solar light simulator (Oriel, 91160, AM 1.5 globe, 100 mW cm2). The incident-light intensity was calibrated using a radiant power/energy meter (Oriel, 70260) before each experiment. The incident photon to current conversion efficiency (IPCE) spectra was measured using a specially designed IPCE system (Newport Co., USA). Electrochemical impedance spectroscopy (EIS) measurements were performed on a computer-controlled electrochemical workstation with impedance analyzer (CHI660C Instruments, Shanghai Chenhua Instrument Corp., Shanghai, China). The measurements were carried out by various applied bias and were recorded over a frequency range of 0.005e105 Hz with AC amplitude of 10 mV. The transient photocurrent curves were measured using an electrochemical analyzer (CHI660C Instruments, Shanghai Chenhua Instrument Corp., Shanghai, China) controlled by a computer. The light was produced by a solar simulator (91160, Newport Corp., Irvine, CA, USA) at 100 mW cm2 (1 sun) intensity. Acknowledgments This work was financially supported by NSF of China (Grants 91122034, 51125006, 61376056), STC of Shanghai (Grants 13JC1405700, 14520722000), and CAS Project of Grant KGZD-EW303. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.02.001. References [1] B. Oregan, M. Gratzel, Nature 353 (1991) 737e740. [2] F. Zhu, D. Wu, Q. Li, H. Dong, J. Li, K. Jiang, D. Xu, RSC Adv. 2 (2012) 11629e11637. [3] Z. Xiang, X. Zhou, G. Wan, G. Zhang, D. Cao, ACS Sustain. Chem. Eng. (2014), 140403142952009. [4] H. Tüysüz, E.L. Salabas¸, E. Bill, H. Bongard, B. Spliethoff, C.W. Lehmann, F. Schüth, Chem. Mater. 24 (2012) 2493e2500. [5] T. Yamaguchi, N. Tobe, D. Matsumoto, H. Arakawa, Chem. Commun. (2007) 4767e4769. [6] F. Pichot, J.R. Pitts, B.A. Gregg, Langmuir 16 (2000) 5626e5630. [7] A. Vomiero, V. Galstyan, A. Braga, I. Concina, M. Brisotto, E. Bontempi, G. Sberveglieri, Energy Environ. Sci. 4 (2011) 3408e3413. [8] V. Galstyan, A. Vomiero, I. Concina, A. Braga, M. Brisotto, E. Bontempi, G. Faglia, G. Sberveglieri, Small 7 (2011) 2437e2442. [9] J.N. Hart, D. Menzies, Y.-B. Cheng, G.P. Simon, L. Spiccia, J. Sol Gel Sci. Technol. 40 (2006) 45e54. [10] T. Miyasaka, Y. Kijitori, J. Electrochem. Soc. 151 (2004) A1767eA1773. [11] D. Zhang, T. Yoshida, T. Oekermann, K. Furuta, H. Minoura, Adv. Funct. Mater. 16 (2006) 1228e1234. [12] E. Stathatos, Y. Chen, D.D. Dionysiou, Sol. Energy Mater. Sol. Cells 92 (2008) 1358e1365. [13] K.-M. Lee, Y.-C. Hsu, M. Ikegami, T. Miyasaka, K. Justin Thomas, J.T. Lin, K.C. Ho, J. Power Sources 196 (2011) 2416e2421. [14] H.-W. Chen, C.-P. Liang, H.-S. Huang, J.-G. Chen, R. Vittal, C.-Y. Lin, K.C.-W. Wu, K.-C. Ho, Chem. Commun. 47 (2011) 8346e8348. [15] H. Zhu, J. Wei, K. Wang, D. Wu, Sol. Energy Mater. Sol. Cells 93 (2009) 1461e1470. [16] M. Toivola, J. Halme, K. Miettunen, K. Aitola, P.D. Lund, Int. J. Energy Res. 33 (2009) 1145e1160. [17] K.-M. Lee, C.-W. Hu, H.-W. Chen, K.-C. Ho, Sol. Energy Mater. Sol. Cells 92 (2008) 1628e1633. [18] A. Rapsomanikis, D. Sygkridou, D. Karageorgopoulos, E. Stathatos, Mater. Sci. Semicond. Process. 27 (2014) 634e642. [19] K. Lee, C. Hu, H. Chen, K. Ho, Sol. Energy Mater. Sol. Cells 92 (2008) 1628e1633. [20] H. Bi, F. Huang, J. Liang, Y. Tang, X. Lü, X. Xie, M. Jiang, J. Mater. Chem. 21 (2011) 17366e17370. [21] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.-M. Cheng, Nat. Mater. 10 (2011)

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