Effect of Wormhole-like Mesoporous Anatase TiO2 Nanofiber

Article pubs.acs.org/JPCC

Effect of Wormhole-like Mesoporous Anatase TiO2 Nanofiber Prepared by Electrospinning with Ionic Liquid on Dye-Sensitized Solar Cells Yu Pin Lin,† Yu Ying Chen,† Yuan Ching Lee,† and Yui Whei Chen-Yang*,†,‡ †

Department of Chemistry and ‡Center for Nanotechnology, Chung Yuan Christian University, 200 Chung Pei Road, Chung Li, Taiwan 32023, R. O. C.

ABSTRACT: A series of 1-D mesoporous anatase TiO2 nanofibers (TNFx, x = 0−3 in wt %) were synthesized via a simple electrospinning technique combined with a sol−gel process which used 0−3 wt % of room-temperature ionic liquid (RTIL) as the mesopore formation template. TNF1 electrospun with 1 wt % RTIL was found to be the most favorable TNFx for application in a dye-sensitized solar cell (DSSC). The effects of TNFx as photoanode materials on the property of DSSC were further investigated using electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS) measurements. The results indicated that the unique morphology of TNF1, being a straight, large surface, wormhole-like mesoporous anatase nanofiber with the smallest average fiber diameter, was the main reason leading to the largest improvement in its light harvesting, electron transport, and charge collection efficiencies among the series of TNFx-based photoanodes. This study demonstrated TNF1 as a promising photoanode material for DSSC; specifically, the DSSC fabricated with TNF1 showed the largest improvement (∼50.4%) in energy conversion efficiency (5.64%) over that with TNF0 electrospun without RTIL (3.75%).

1. INTRODUCTION Dye-sensitized solar cells (DSSCs), developed by Grätzel and co-workers using a simple process, have been regarded as potential regenerative and low-cost energy conversion devices in recent decades.1,2 Typically, the working electrode of a DSSC is made of a TiO2 nanoparticle-based (∼20 nm diameter) mesoporous film adsorbed with a monolayer Ru-based complex dye. Although the certified energy conversion efficiency of DSSCs has exceeded 12%,3 the photoexcited electrons that pass through the grain boundaries and interparticle connections are strongly influenced by the surface trapping/detrapping effect, leading to slow electron transport and enhancement of the recombination probability.4 Therefore, several one-dimensional (1-D) nanostructured TiO2 materials such as nanorods and nanotubes have been synthesized by electrochemical anodization,5,6 the hydrothermal process,7,8 the template-modified sol− gel method,9,10 etc., and used as photoanode materials for DSSCs. In particular, these 1-D TiO2 nanostructure-based photoanodes are regarded as an effective pathway for facilitating electron transport and are able to minimize the recombination © 2012 American Chemical Society

rate, resulting in improvement of the charge collection efficiency. Nevertheless, drawbacks in the processes of preparing the 1-D TiO2, such as long growth times, low electrode thicknesses, and small working areas, have still limited their applications in DSSCs. Among the methods for preparing 1-D TiO2 nanostructures, electrospinning is a top-down approach that can be combined with the sol−gel method.11 The technique has attracted much attention because it provides a cost-effective, versatile, simple, and continuous process for fabricating 1-D nanostructured TiO2 materials that can be directly deposited on various substrates within a short period of process time. Because of its inherent adaptability and applicability, the electrospinning technique has been applied in developing energy conversion and storage devices.12 In practice, even though extremely fast electron transport was available in the 1-D nanostructures, Received: December 16, 2011 Revised: May 25, 2012 Published: June 1, 2012 13003

dx.doi.org/10.1021/jp212146p | J. Phys. Chem. C 2012, 116, 13003−13012

The Journal of Physical Chemistry C

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sol onto a cleaned FTO glass substrate (TCO22−15, 15 Ω/□, Solaronix) that was fixed on an aluminum-foil-covered rotator to ensure that the nanofibers were collected uniformly. The film thickness (∼20 μm) was controlled by the electrospinning time. The TNFC spun without an RTIL is abbreviated as TNFC0, and those spun with the RTIL, TiO2/PVP/RTIL, are abbreviated as TNFCx, in which x (= 0.5−3) represents the wt % of RTIL used. The RTIL entrapped in a TNFCx was subsequently removed by washing it with anhydrous ethanol several times, and the TNFCx nanofibers with the RTIL removed, abbreviated as TNFCxR, were then dried in the oven at 70 °C. The extracted RTIL was recycled by purification to reduce the cost and allow reuse. In order to prevent the TiO2 nanofibers from peeling off from the substrate, a suitable temperature and pressure were applied by a homemade hotpress machine to improve the adhesion of the TiO2 nanofibers to the FTO glass without destroying them.21 DSSC Fabrication. For application in DSSCs, the obtained TNFC0 and TNFCxR films were calcined at 450 °C for 3 h in air to remove the PVP and convert them into mesoporous anatase TiO2 nanofibers. These nanofibers are abbreviated as TNFx, accordingly. After they were cooled to 80 °C, the TNFx films were sensitized by immersing them in an acetonitrile/tertbutanol solution (1:1, v/v) containing a 0.3 mM concentration of Ru-based dye, cis-diisothiocyanatobis(2,2′-bipyridyl-4,4′dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (N719, Solaronix), at room temperature for 24 h. The nanofibers were then rinsed with anhydrous ethanol and dried in an oven at 100 °C to obtain the final TiO2 nanofiber film electrodes. The electrolyte solution was prepared by dissolving 0.1 M LiI, 0.03 M I2, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and 0.5 M 4-tert-butylpyridine (TBP) in 3methoxypropionitrile (MPN). The dye-adsorbed TNFx film electrode and the Pt-sputtered FTO glass counter electrode were assembled with a U-type hot-melt film (Surlyn SX-117025, Solaronix) to form a gap, and the electrolyte was filled into the gap to obtain the corresponding TNFx-based DSSC cell (TNFx-D). An opaque mask was placed on the cell to control the active area (∼0.25 cm2) during the photovoltaic measurement. Characterization and Measurements. The Fourier transform infrared (FT-IR) spectra of the TiO2 composite nanofibers were recorded on an infrared spectrometer (FTS-7, Bio-Rad) with a resolution of 2 cm−1. The samples were prepared as KBr pellets with 1/100 weight ratio of sample to KBr. The wide-angle powder X-ray diffraction (WXRD) patterns were collected using an X-ray diffractometer (X’Pert Pro, PW3040/60, Philips, Netherlands) with Cu Kα radiation (λ = 0.154 06 nm) at a generator voltage of 45 kV and generator current of 40 mA to investigate crystalline phase of the TiO2 nanofibers. The scanning range was from 15° to 70°. The Brunauer−Emmett−Teller (BET) specific surface area, Barrett−Joyner−Halenda (BJH) calculated average pore size, and pore volume of the TiO2 nanofibers were obtained from the N2 adsorption/desorption isotherms measured by the surface and porosity analyzer (Micromeritics Tristar 3000) at 77 K. The sample was degassed and dried under a vacuum system at 120 °C for 12 h prior to the measurement. The surface morphologies of the TiO2 nanofibers were investigated by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan). The TEM images were obtained using a transmission electron microscope (TECNAI 20, Philips, Netherlands) at an accelerating voltage of 200 kV.

these 1-D TiO2-based DSSCs usually showed relatively lower efficiencies than nanoparticle-based ones, mainly because of their smooth surfaces and low porosity, which led to low dye adsorption. Hence, it is worthwhile to pursue methods of preparing mesoporous 1-D TiO2 nanofibers with large surface areas. On the other hand, in the conventional sol−gel process for preparation of mesoporous TiO2, amphiphilic organic molecules such as block copolymers, surfactants, etc., are used as templates. However, these templates are usually difficult to remove completely at relatively low temperatures (∼450 °C) without collapsing the pores they formed. This results in poor crystallinity, which is detrimental to the electron transport, limiting this method’s utility for producing DSSCs.13,14 Room-temperature ionic liquids (RTILs) are a new type of organic salt composed of an organic cation and an inorganic anion with a melting point near room temperature, negligible vapor pressure, good compatibility, high thermal stability, high ionic conductivity, and a wide electrochemical window.15,16 Recently, various RTILs have been used as templates to prepare mesoporous TiO2 nanomaterials at room temperature using the conventional sol−gel method.17−20 The results indicated that with a proper RTIL mesoporous anatase TiO2 materials were thermally stable and could be obtained with/without posttreatment at high temperature. Although the effect of the RTIL on the properties of some electrospun polymer fibers has been studied,16 to our best knowledge, no ionic-liquid-assisted electrospun ceramic nanofibers (e.g., TiO2) have been reported. In this study, thermally stable, wormhole-like mesoporous anatase TiO2 nanofibers were prepared using the sol−gel process with an RTIL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM+][BF4−]), as a mesopore formation template via the electrospinning process. It was anticipated that the wormhole-like TiO2 nanofibers in DSSCs could not only provide fast photoelectron transport but also improve the energy conversion efficiency because of their large surface area. Hence, the morphology, mesoporosity, and crystallinity of the wormhole-like mesoporous TiO2 nanofibers and the photovoltaic and electron dynamic characteristics of the corresponding TiO2 nanofiber-based DSSCs were investigated.

2. EXPERIMENTAL SECTION Preparation of TiO2 Spinnable Sol. The precursor sol used for the electrospinning was prepared as follows: A solution was prepared by adding 1 mL of titanium tetraisopropoxide (97%, Aldrich) and various weights of the RTIL, 1-butyl-3methylimidazolium tetrafluoroborate ([BMIM+][BF4−], ≥99%, Merck), into 2 mL of 2-propanol (≥99.5%, Aldrich). Another solution was prepared by separately dissolving 0.43 g of poly(vinylpyrrolidone) (PVP, MW = 1 300 000 g/mol, Aldrich) in 5 mL of 2-propanol at room temperature. These two solutions were then mixed at room temperature for 1 h to produce a viscous transparent spinnable TiO2 sol. The RTIL content was adjusted to between 0 and 3 wt % of the spinnable sol. Preparation of TiO2 Nanofibers by Electrospinning. The TiO2 sol was loaded in a plastic syringe equipped with a 24-gauge stainless steel needle, and an electrical potential of 15 kV was applied by a high-voltage power supply (You-Shang Technical Corp., Taiwan). The fluid rate was kept at 0.5 mL h−1 using a syringe pump (Model 100, KD Scientific), and the working distance was fixed at 10 cm. The TiO2/PVP composite nanofibers (TNFC) were directly electrospun from the TiO2 13004



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characteristic C−H vibrations of the [BMIM+] ring.22 The spectrum of TNFC0, the TiO2/PVP composite nanofiber, in Figure 1b clearly has the characteristic peaks of CO, CC, and C−N stretching vibrations of PVP at 1661, 1466, and 1285 cm−1, respectively.23 Similarly, all of these peaks are also found in Figure 1c, confirming that the RTIL was entrapped in the TNFCx (x > 0), the as-spun TiO2/PVP/RTIL composite nanofiber. After the samples were washed with anhydrous ethanol, the corresponding characteristic peaks of the RTIL disappeared in the spectrum of TNFCxR (Figure 1d), clearly indicating that the IL had been removed. After the 450 °C calcination, all the peaks of the organic compounds disappeared in Figure 1e, confirming that PVP and the other organic residues were removed completely in TNFx. The remaining peak at ∼500 cm−1 was the vibration peak of the skeletal O− Ti−O bonds of the anatase phase.24 The new broad band at 3000−3500 cm−1 was attributed to the hydroxyl group of Ti− O−H, and that at ∼1640 cm−1 was attributed to the O−H vibration of the physisorbed water moisture from air.25 The results confirm that the TNFx, the fibers obtained from the process applied, were pure TiO2 nanofibers, and the IL was successfully recycled by the extraction method. Characterization of TiO2 Nanofibers. Figure 2 shows the FE-SEM images of the TNFx fibers. As can be seen from Figure 2a, the fibers of TNF0 were straight, the diameters were quite uniform (∼500 ± 15 nm), and the surfaces of the fibers, shown in the magnified image of Figure 2b, were smoothly formed by the closely packed nanoparticles. Compared to the image of TNF0, the images of TNFx in Figure 2c−f show that as the amount of RTIL used, represented by x, was increased from 0.5 to 1 wt %, the roughness of the fiber surface increased somewhat and the average particle size slightly decreased. In addition, the fibers’ average diameter was significantly reduced (from ∼383 ± 10 to ∼153 ± 8 nm). This result is similar to the results reported for polymer nanofibers; in that case, the reduction was ascribed mainly to the change in conductivity of the solution caused by inclusion of salts or ionic surfactants in the polymer solutions.26,27 However, Figure 2g−j shows that as the amount of RTIL added was further increased to 2 and 3 wt %, parts of the fibers in TNF2 and TNF3 were bent, and the average fiber diameter (∼225 ± 11 and 360 ± 16 nm, respectively), porosity, and fiber surface roughness all gradually increased. Furthermore, some fused fibers were obviously found in TNF3, indicating the formation of nonaxisymmetric instability. These results are ascribed to the higher ionic charge density due to the higher content of RTIL. When the high charge density sol fluid had just been ejected from the needle, the surface charges repelled each other, causing nonaxisymmetric conducting instabilities that made the fibers undergo bending or whipping motions. The fusion phenomena can be attributed to the fact that the charges on fibers’ surfaces attracted each other before solidification and deposition onto the collector.16 When the RTIL content was increased over 3 wt %, it was found that the charge density of the sol was so high that the electrospun fibers adhered to one another and severely fused together, which made it difficult for them to collect on the collector and form a good fibrous membrane. In summary, the above results reveal that TNF1, prepared with 1 wt % of RTIL and exhibiting straight fibers with the smallest average diameter, would be most suitable for application in DSSCs. Figure 3a shows the wide-angle X-ray powder diffraction (WXRD) patterns of the TiO2/PVP composite nanofibers, TNFC0, and those of the TiO2/PVP composite nanofibers with

Photovoltaic Characterization. Photocurrent density− voltage (J−V) curves were recorded by using a potentiostat (PGSTAT30, Metrohm Autolab B.V., Netherlands). The samples were illuminated under a solar simulator (XES-40S1, San Ei, Japan) at AM 1.5 G conditions. The standard incident irradiance intensity (100 mW cm−2) was adjusted with a calibrated monocrystalline Si reference solar cell equipped with a KG-5 filter. The amount of dye adsorbed on the TiO2 film was determined by desorbing the dye in a solution of 0.1 M NaOH in H2O/EtOH (1/1, v/v). The absorption spectrum of the solution was analyzed using a UV−vis spectrophotometer (Cary 100, Varian). The thicknesses of TiO2 films were measured by using a surface profilometer (Dektak 150, Veeco). The incident photon to current conversion efficiency (IPCE) spectra were collected over the wavelength rage of 350− 800 nm by a commercial IPCE system (QE-mini, Enli Technology Co., Ltd., Taiwan). Intensity-modulated photocurrent spectroscopy (IMPS) measurements, intensity-modulated photovoltage spectroscopy (IMVS) measurements, and electrochemical impedance spectra (EIS) were investigated to determine the electron transport characteristics. For IMPS/ IMVS, the solar cells were illuminated under a red LED (λ = 625 nm) as a light source, in which the amplitude of the ac light modulation was 10% of the dc light intensity and was adjusted with a calibrated Si photodiode. For EIS, the bias potential was set at 0.7 V with an ac potential amplitude of 10 mV under an AM 1.5 G solar illumination of 100 mW cm−2. The IMPS/ IMVS and EIS spectra were recorded by a potentiostat equipped with FRA2 (frequency response analyzer 2), and the frequencies ranged from 10k to 0.1 Hz and 100k to 0.05 Hz, respectively. All of the impedance spectra were fitted using appropriate equivalent circuit models built in the Z-View software.

3. RESULTS AND DISCUSSION As is known, the water-miscible RTIL, [BMIM+][BF4−], can be easily extracted by the proper organic solvent and water. Figure 1

Figure 1. FT-IR spectra of (a) RTIL, [BMIM+][BF4−], (b) TNFC0, (c) TNFC1, (d) TNFC1R, and (e) TNF1.

shows the FT-IR spectra of the RTIL, TNFC0, TNFC1, TNF1R, and TNF1 as examples. In Figure 1a, the peaks at 3172, 1571, and 1168 cm−1 denoted with asterisks are the 13005



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Figure 3. WXRD patterns of TiO2 nanofibers with various RTIL contents: (a) TNFCRx; (b) TNFx, x = 0, 0.5, 1, 2, and 3. A = anatase phase, R = rutile phase.

Table 1. Morphological Parameters of TNFx Figure 2. FE-SEM surface images and magnification of TNFx membranes: (a, b) TNF0, (c, d) TNF0.5, (e, f) TNF1, (g, h) TNF2, and (i, j) TNF3.

the RTIL removed, TNFCRx (x = 0.5, 1, 2, 3). As can be seen, TNFC0, TNFCR0.5, and TNFCR1 exhibited amorphous structures, while the peaks observed for TNFCR2 and TNFCR3 showed the anatase crystalline structure (JCPDS #89-4921). This reveals that the addition of the RTIL assisted the formation of the anatase structure.17−20 On the other hand, in Figure 3b the WXRD patterns of the 450 °C-calcined TiO2 nanofibers, TNFx (x = 0−3), show that a mixture of anatase phase, (101), peaked at 2θ = 25.3°, and rutile phase, (110), peaked at 2θ = 27.5° (JCPDS #89-4920), was formed in TNF0. This is attributed to a decrease in the phase transformation onset temperature caused by the presence of PVP, as has been reported.28 However, only the anatase characteristic peaks are observed for all TNFx (x > 0), indicating that after calcination all TNFCRx were well transformed into the anatase structure without any formation of the rutile phase. It was also found that as x was increased, the characteristic anatase peaks broadened, implying that the crystal grain size estimated by the Scherrer formula29 decreased accordingly (Table 1). This is ascribed to the different nucleation rates.18,19 This result reveals that the

samples

SBET (m2 g−1)

Vpore (cm3 g−1)

DBJH (nm)

CSXRD (nm)

TNF0 TNF0.5 TNF1 TNF2 TNF3

41.5 56.8 90.4 91.4 84.1

0.121 0.157 0.199 0.184 0.132

7.6 7.1 6.5 5.7 4.1

14.9 14.6 14.4 12.7 12.2

dTNFxa (nm) 500 383 153 225 360

± ± ± ± ±

15 10 8 11 16

a dTNFx: the average diameter of TNFx fiber estimated from the SEM images.

grain growth and nucleation rates varied with the amount of RTIL present. The fact that TNFx (x > 0) retained the anatase structure after calcination also suggests that during the calcination the nanofibers spun with RTIL were thermally more stable than those spun without RTIL, as reported previously for preparation of TiO2 particles.17−20 The morphologies of the calcined nanofibers, TNFx, were investigated through a combination of TEM and BET measurements. Typical TEM images are displayed in Figure 4. As shown in Figure 4a, after PVP was removed via the calcination process, TNF0 was composed of randomly agglomerated nanocrystals. The images in Figure 4b−e indicate that an interconnected framework with irregular “wormhole-like” 13006



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Figure 4. TEM images of (a) TNF0, (b) TNF0.5, (c) TNF1, (d) TNF2, and (e) TNF3. The insets are the corresponding SAED diffraction patterns.

In addition, among the TNFx nanofibers, TNF1 not only exhibited a pure anatase crystalline structure and straight fibers but also had a higher surface area and the smallest fiber diameter. Therefore, it is anticipated that it could adsorb a large amount of dye and that the electrons could be transported easily within the interconnected wormhole-like mesostructures in the TNFx-based electrode. Thus, its electron injection efficiency would be improved, leading the electrode to be of great benefit to DSSCs. Photovoltaic Performance of DSSCs. After being loaded with dye, the as-prepared TNFx fibers were used as electrode materials to form various photoanodes, and their performances were then investigated under illumination with AM 1.5 G sunlight at a standard light intensity of 100 mW cm−2. The photocurrent density−voltage (J−V) curves of the TNFx-based DSSCs (TNFx-D) are shown in Figure 6a, and the corresponding photovoltaic parameters are tabulated in Table 2. It is found that TNF0-D had the lowest performance with a Jsc of 8.6 mA cm−2 and a conversion efficiency (η) of 3.75%. These two values increased at first and then decreased as the x value was increased, and TNF1-D exhibited the highest η (5.64%) and Jsc (13.7 mA cm−2). These values were about 50.4% and 59.3% higher than those of TNF0-D, respectively. Except for TNF3-D, the fill factor (FF) and Voc values for the devices were similar (the differences are all less than 4%). The η variation trend is the same as the Jsc trend, implying that the major reason for the improvement in conversion efficiency for the TNFx-based devices is the increase in Jsc, which is mainly caused by the increase in the dye loading due to the increase in the surface area. As indicated in Table 2, the dye loading of TNFx increased from 3.81 × 10−7 to 9.51 × 10−7 mol cm−2 as the x value was increased from 0 to 2, and it then decreased to 8.35 × 10−7 mol cm−2 as the x value was further increased to 3. The dye loading obtained for TNF1-D (9.46 × 10−7 mol cm−2) was almost the same as that for TNF2-D and is about 2.5-fold higher than that for TNF0-D. The increasing trend of the dye loading is consistent with the trend observed for the TNFx surface area. Hence, the poor performance of the TNF0-D is mainly ascribed to its small surface area (41.5 m2 g−1), which led to low dye loading, and the existence of some rutile phase, which is not as favorable as the anatase phase for electron

mesopore structures was gradually formed in the TNFx (x > 0) as the RTIL content increased. The fact that the wormhole-like mesoporous structure did not collapse even though a long calcination at high temperature was performed is attributed to the fact that strong hydrogen bonds formed between the anions of the RTIL and the hydroxyl groups of the TiO2 gel during the sol−gel process. This caused the anions to be oriented along the pore walls and the cations to be aligned with the anions because of the Coulomb interaction, the π−π stacking interaction, or other noncovalent interactions between the neighboring imidazolium rings.17−20 The inset images show the corresponding selected area electron diffraction (SAED) patterns of the nanofibers. All the polycrystalline diffraction rings confirm that the TNFx nanofibers were in the anatase phase and are consistent with the XRD results. The N2 adsorption−desorption isotherms for the various TiO2 nanofibers are presented in Figure 5. According to the IUPAC classification,30 TNF0 exhibits a type IV isotherm with an H2 hysteresis loop (Figure 5a), indicating that TNF0 was a typical mesoporous material. On the other hand, Figure 5b−e also shows type IV isotherms, but with H3 hysteresis loops that occur at relatively high pressures for TNF0.5, TNF1, and TNF2 and with a combination of H3 and H2 hysteresis loops for TNF3. This implies that the mesopores of all the TNFx nanofibers were all less regular30 than those of TNF0, but the mesopores of TNF3 were a little bit more regular than those of the other TNFx. As listed in Table 1, the BET surface area of TNF0 was 41.5 m2 g−1, while those of TNFx increased to 56.8, 90.4, and 91.4 m2 g−1 for TNF0.5, TNF1, and TNF2, respectively. The BET then decreased to 84.1 m2 g−1 for TNF3. It is noteworthy that TNF1 and TNF2 had similar surface areas that are both more than 2 times higher than that of TNF0, implying that ∼2 wt % of RTIL content is a possible critical threshold for obtaining a high surface area in the electrospinning process applied in this study. In addition, Figure 5f shows that as the RTIL content was increased the average pore size decreased gradually and the size distribution narrowed, indicating good homogeneity of these mesopores.18−20 In summary, these isotherm plots show that the mesopores in TNFx (x > 0) are irregular and are consistent with the TEM images discussed above. 13007



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Figure 5. N2 adsorption−desorption isotherms of (a) TNF0, (b) TNF0.5, (c) TNF1, (d) TNF2, (e) TNF3, and (f) the corresponding pore size distribution curves.

transport in DSSCs.31 On the other hand, the higher conversion efficiencies for TNF1-D and TNF2-D are ascribed to their higher Jsc values, which are due to the larger amounts of dye molecules adsorbed on the surfaces of the TiO2 mesopores allowed by their larger surface areas (90.4 and 91.4 m2 g−1 for TNF1 and TNF2, respectively). Furthermore, the Jsc variation trend is also related to the corresponding IPCE spectra. As shown in Figure 6b, the IPCE variation trend is the same as that of the Jsc values. The maximum IPCE values obtained at ∼540 nm for the TNFx-based devices with x > 0 are all higher than

that for TNF0-D, especially that for TNF1-D, which is up to ∼68% higher than that for TNF0-D. This suggests that the light harvesting was significantly more efficient for the devices fabricated with the wormhole-like mesoporous TiO2 nanofibers than for that fabricated with the TiO2 nanofibers prepared without an RTIL template. The IPCE variation trend also corresponds to the trend of the conversion efficiencies, in which the highest value was obtained from TNF1-D. Electron Transport and Recombination Characteristics of the DSSCs. In order to better understand the effect of 13008



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Figure 7. (a) IMPS and (b) IMVS complex plane plots of TNFx-D under an incident dc light intensity of 3.2 mW cm−2. Figure 6. (a) Photocurrent−voltage (J−V) curves under simulated AM 1.5G illumination and (b) IPCE spectra of various DSSC devices, TNFx-D.

the TiO2.35 The fact that TNF1-D had the shortest τt,IMPS implies that its photoanode had the shortest electron transport time, resulting in the most charge collection. When the light intensity was increased, a higher amount of photoelectrons in the conduction band of TiO2 was generated, such that the deep traps were filled and the trapping/detrapping phenomenon occurred mainly at shallow energy levels, resulting in an increase in the electron transport rate and De,IMPS. From the morphological point of view, the increased τt,IMPS values for TNF2-D and TNF3-D compared to that for TNF1-D are ascribed to the significant distortion and larger diameters of the nanofibers, which result in a more tortuous pathway and a more pronounced electron scattering effect, respectively.36,37 The IMPS result confirms that the straightness, smaller diameter, and pure anatase phase of the wormhole-like mesoporous nanofibers made TNF1 the best photoanode material for the device in this study. In contrast with the De,IMPS values, all the τn,IMVS values decreased with increasing light intensity. This is because the higher light intensity caused the photoelectrons to be more easily trapped in the trapping sites or surface states of TiO2, thus increasing the back-reaction with I3− and decreasing τn,IMVS. In addition, a significant decrease was found in τn,IMVS as x was increased from 1 to 3, revealing an increase in electron recombination with I3−. This result is consistent with the decrease in Voc (Table 2) and is ascribed to the reduction in the pore size38 (Table 1). To further elucidate the electron transport characteristics, the Nyquist impedance spectra of the TNFx-D in open circuits under 100 mW cm−2 of solar illumination were measured and

Table 2. Photovoltaic Performance Data of TNFx-D TNFx-D

Jsc (mA cm−2)

Voc (V)

FF (%)

η (%)

dye loading (×10−7 mol cm−2)

TNF0-D TNF0.5-D TNF1-D TNF2-D TNF3-D

8.6 10.0 13.7 13.0 11.7

0.752 0.768 0.731 0.721 0.713

58.0 57.8 56.3 58.7 53.4

3.75 4.44 5.64 5.50 4.45

3.81 4.87 9.46 9.51 8.35

the morphology of the TiO2 nanofibers on the efficiency of the corresponding DSSC devices, IMPS and IMVS measurements, which provided valuable electron transport information for porous nanocrystalline films,32,33 were carried out for all TNFxD devices. The plots obtained at a light intensity of 3.2 mW cm−2 and the comparison of De,IMPS and τn,IMVS at various light intensities are shown in Figures 7 and 8, respectively. The transit time (τt,IMPS) and lifetime (τn,IMVS) of the photoelectrons that traveled through the TiO2 film were calculated using the equations τt,IMPS = 1/(2πf IMPS,min) and τn,IMVS = 1/(2πf IMVS,min), respectively, where f IMPS,min and f IMVS,min are the frequencies at the minimum imaginary component in the corresponding complex plots.32,33 The corresponding electron diffusion coefficient (De,IMPS) was estimated using the equation De,IMPS = d2/(2.35τt,IMPS), where d is the thickness of the TiO2 film.34 The smallest De,IMPS value was obtained for TNF0-D, which is mainly attributed to the presence of the mixture of phases of 13009



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are shown in Figure 9a. As is known, from high to low frequency, the three semicircles in the Nyquist impedance spectra of the DSSCs correspond to the charge transfer resistance at the Pt/electrolyte interface, electron transport resistance in the TiO2 mesoporous film, and diffusion resistance of I−/I3− in the electrolyte. To simulate and interpret the impedance spectra, an equivalent circuit of the corresponding transmission line model is suggested in Figure 9b.39−41 From the equivalent circuit, the lifetime (τn,EIS) and transit time (τt,EIS) of electrons in the TiO2 film were calculated from the equations τn,EIS = RctCμ and τt,EIS = RtCμ = L2/De,EIS, respectively, where Rt, Rct, Cμ, and L are the electron transport resistance, interfacial charge-transfer resistance related to recombination of the electrons, chemical capacitance produced by the accumulation of electrons in the TiO2 film, and the TiO2 film thickness, respectively.41,42 All of the simulated data are listed in Table 3. In addition, a parameter useful for estimating the competition between electron diffusion and recombination, the electron average diffusion length (Ln), was calculated from the equation Ln = (De,EISτn,EIS)1/2.41,42 As listed in Table 3, all the Ln values of TNFx-D are larger than the thickness of the photoanode (∼20 μm), and the Ln value of TNF1-D is the largest. This suggests that less electron loss and a higher conversion efficiency would be expected for TNF1-D than for the other TNFx-D. On the other hand, it is also known that Jsc can be approximated by the expression43

Jsc = qηlhηinjηcoll I0 where q is the elementary charge, ηlh is the light harvesting efficiency of a cell, which is commonly determined by the amount of adsorbed dye, ηinj is the charge injection efficiency, ηcoll is the charge collection efficiency, and I0 is the light intensity. Therefore, the charge collection efficiency (ηcoll) is also an important index for the performance of the DSSC and is

Figure 8. Variation of (a) electron diffusion coefficient (De,IMPS) and (b) electron lifetime (τn,IMVS) of TNFx-D versus the incident light density.

Figure 9. (a) Nyquist impedance plots of TNFx-D. The solid lines in the figure show the fitted results. (b) Generalized equivalent circuit model used to fit impedance spectra of TNFx-D. rt = Rt/L, rct = RctL, and cμ = Cμ/L, where Rs is sheet resistance of the FTO, Welectrolyte is diffusion impedance of redox species in the electrolyte, and RPt and CPt are charge transfer resistance and interfacial capacitance at the Pt/electrolyte interface, respectively. 13010



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Table 3. Electron Dynamic Parameters Estimated by the Nyquist Impedance Plots TNFx-D

Cμ (mF)

Rt (Ω)

Rct (Ω)

τn,EIS (ms)

τt,EIS (ms)

TNF0-D TNF0.5-D TNF1-D TNF2-D TNF3-D

0.65 0.80 0.73 0.67 0.54

17.6 13.8 5.0 7.0 11.2

67.0 55.3 44.0 44.7 52.9

43.6 44.2 32.1 30.0 28.6

11.4 11.0 3.7 4.7 6.0

3.5 3.6 1.1 8.5 6.7

× × × × ×

10−4 10−4 10−3 10−4 10−4

Ln (μm)

ηcoll (%)

39.1 39.9 59.4 50.5 43.8

73.9 75.1 88.5 84.3 79.0

University, Taiwan, R.O.C. (CYCU-98-CR-CH), for supporting the research work.

determined by the competition between recombination and transport of electrons within the photoanode according to the equation ηcoll = 1 − (τt,EIS/τn,EIS).42,44 Obviously, Table 3 indicates that TNF1-D exhibited the largest Ln and ηcoll, suggesting that, compared to the other TNFx-based devices, TNF1-D exhibited the highest Jsc and energy conversion efficiency. This result supports the conclusion that the TNF1 material, which has high dye loading and fast electron transport, was most beneficial to the conversion efficiency of the DSSC in this study and is consistent with the J−V performance and IPCE spectra shown above. In summary, the results of IPCE, IMPS/IMVS, and EIS measurements all support that the fiber morphology of TNF1, including a pure anatase phase, straight shape, wormhole-like mesoporous structure, and large surface area, is the main reason that it produced the largest improvement in its light harvesting, charge transport, and charge collection efficiencies and consequently produced the highest photocurrent and energy conversion efficiency from its corresponding DSSC, TNF1-D, in this study.

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REFERENCES

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4. CONCLUSION In summary, large surface area, wormhole-like mesoporous anatase TiO2 nanofibers, TNFx (x > 0), were prepared by electrospinning with various amounts of the RTIL, after which the RTIL was removed with solvent and the samples were calcined. The performance of the DSSCs fabricated with TNFx (x > 0) was significantly higher for all TNFx (x > 0) than for TNF0 electrospun without RTIL. The enhancement of the photocurrent and energy conversion efficiency was mainly attributed to the more efficient light harvesting caused by the larger amount of dye adsorbed and faster electron transport in the TNFx-based (x > 0) photoanodes. The most favorable TNFx for application in a DSSC was TNF1 electrospun in the presence of 1 wt % RTIL. TNF1 had straight, wormhole-like mesoporous anatase nanofibers with the smallest average fiber diameter and the larger surface area. In conclusion, the fact that TNF1-D had the largest improvement (∼50.4%) in energy conversion efficiency (5.64%) over that of TNF0-D (3.75%) is attributed to the unique morphology of TNF1, which led to more dye loading, the fastest electron transport rate, and the highest charge collection efficiency.

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De,EIS (cm2/s)

AUTHOR INFORMATION

Corresponding Author

*Tel +886-3-2653317; Fax +886-3-2653399; e-mail yuiwhei@ cycu.edu.tw. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge National Science Council, R.O.C. (97-2113-M-033-004-MY3), and Chung Yuan Christian 13011



Article

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Effect of Wormhole-like Mesoporous Anatase TiO2 Nanofiber

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