Document Type : Research Paper
Department of Chemistry, Faculty of Science, Yazd University, Yazd, Iran
Recently, titanium dioxide materials have been utilized in many fields such as dye sensitized solar cell [1,2], photocatalysis  and electrochemical measurements [4,5]. The main advantages of TiO2 nanomaterials are unique physical, chemical properties, the high surface to volume ratio and crystalline structure. TiO2 nanostructure preparation with unique properties such as hydrothermal synthesis , sol gel process , atomic layer deposition , reactive sputtering  has been expanded during past decay. However, extensive studies in this respect show that the nanostructures with one-dimensional shape (1D) have special features due to the axial ratio aspect . Among the 1D shape, nanofibers have obtained much interest due to long axial ratio, large surface area to volume and flexibility in surface functionalities. Recently, electrospinning process was used as a low cost and fast technology to product fibers with diameters in the range of 3 nm to greater than 10 μm . It proposes significant capabilities to synthesize nanofibers by controllable morphology with optimized conditions such as smaller diameter and higher surface area compared to the regular fibers. Nanofibers have been successfully applied in in a wide range of areas including filtration of air and water, tissue engineering scaffolds, biomedical and bio technology [12–14].
During the electrospinning, a polymer solution flows out from needle connected to a syringe, and a high voltage causes that the charge to be induced within the polymer and polymer pulled onto a collector by strong electric field and form nanofibers structure. Many parameters influence on final nanofibers structure, such as (1) solution conditions (2) operation conditions and (3) ambient conditions [15–17].
Dye sensitized solar cells DSSCs, a new type of photo-electrochemical solar cells, have been gained important alternative to the silicon based photovoltaic because they have significant benefits such as low cost, simple preparing processes and higher conversion efficiency in comparison of other types of solar cell. In recent years, one dimensional electrospun TiO2 nanofibers have been used as the DSSC photoanode to enhance the electron transport and the light harvest efficiency by scattering more light in the red part of the solar spectra.
Inthis study, we synthesized TiO2 nanofibers by optimization of electrospinning parameters such as polymer concentration, applied voltage, the distance between tip and collector, the calcinating temperature and the titanium isopropoxide concentrations to prepare TiO2 nanofibers with high roughness surface and utilize them in photoanode to investigate the effect of fibers morphology on dye sensitized solar cell performance.
Iodine (99.99%, Merck), lithium iodide (Merck), ethyl celluloses (30–50 mPas, Sigma Aldrich), terpineol (anhydrous, Sigma Aldrich), 4-tert-butylpyridine (4-tBP) (Sigma Aldrich), acetonitrile (Sigma Aldrich), valeronitrile (Sigma Aldrich), chloroplatinic acid hexahydrate (H2PtCl6) (Merck), and Ru complex dye [Dyesol, cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicar-boxylato)-ruthenium(II)bis-tetra butyl ammonium: (N719)] were used as received. TiCl4 (Sigma Aldrich) was diluted with water to 2.0 M at 0 oC which was used freshly and diluted to 40 mM with water for each treatment of the fluorine-doped tin oxide (FTO) (TEC-15, Dyesol) coated glass plates and surfaces of TiO2 mesoporous electrodes. Polyvinyl pyrrolidone (PVP, Mw =1300000) was dissolved in 4 mL ethanol (EtOH, Merck, 99.8%) and 1 mL N, N-dimethylformamide (DMF, Merck, 99.8%).
Preparation of Titanium Dioxide Nanofibers
A solution of titaniumisopropoxide (Ti(OCH (CH3)2)4, TiP, Aldrich,) with the metal contents of 25.35, 50.69, 76.05 wt% were prepared by stirring and 3 mL acetic acid was added to the solution until a transparent solution was obtained. Then, the titanium dioxide sol was added into polyvinyl pyrrolidone (PVP) solution while stirring for 6 h to obtain suitable viscosity. The obtained spin-able sol was injected by a 5 mL plastic syringe (diameter: 12.08 mm). A power supply with high voltage (EH-Series, Glassman High Voltage) influence on the prepared Titania sol that is flowed toward a cylindrical target with an aluminum (Al) foil. The distance between the Al foil and the spinneret and the applied voltage were 15 cm and 15 KV respectively. A flow rate of 0.5 mL/h was utilized with a syringe pump (FNM Co., Iran). The nanofibers were deposited on the Al foil and then dried at 70 oC for 6 h to evaporate their solvents. Then, the prepared Titania nanofibers were divided into three parts and calcined in different temperatures (450 oC, 650 oC, 850 oC) at the rate of 2 oC/min for 2 h and utilized as light harvesting layers in dye sensitized solar cells photoanode.
Preparation of Electrodes and DSSCs Assembly
A TiO2 nanoparticle paste was synthesized and covered on a FTO glass by the doctor Blade technique. Before using FTO as a substrate, it was first cleaned by distilled water, ethanol, and acetone for 20 min and then treated by 40 mM TiCl4 (30 min at 80 oC). The counter electrodes were cleaned by distilled water and HCl (0.1 M) in ethanol. Then, the FTO-TiO2 paste heated at 450 oC for 15 min to enhance electrical contact and adhesion (TiO2 electrode). The dye sensitized solar cells were prepared by immersing 0.09 cm2 calcined TiO2 electrode in a 0.5 mM Ruthenium-based N719 dye for 32 h at room temperature. The dye sensitized nanoparticle/nanofiber electrodes were dried and used immediately for measurement of their photovoltaic properties. Photovoltaic measurements were performed under 1 sun conditions (100 mWcm-2). The cells were sealed using a 0.5 mm wide strips of 60 mm thick surlyn and an iodide based electrolyte with 0.03 M Iodine (I2), 0.1 M LiI and 0.5 M 4-tBP in the 5 mL of acetonitrile: valeronitrile (85:15) was injected into the cell. After covering the holes with small cover glasses they were sealed and used to measure photovoltaic performance.
The structure of titanium dioxide nanofibers were characterized using X-ray diffractometer (Xpert Pro MPD model, wavelength of Cu K radiation in 2 range 10-80). Scanning electron microscopy (SEM) images of different nanofibers calcined in different temperatures, were taken out by TESCAN instrument model VEGA3. The infrared (IR) spectra of the electrospun nanofibers were done with a Bruker instrument model 55. UV-Vis of dye solutions and dyes adsorbed on TiO2 surface was performed by a spectrophotometer Optizen model 3220 from 200 to 900 nm wavelengths using a glass cuvette with 1 cm path length.
RESULTS AND DISCUSSION
XRD analysis was performed to find the crystalline phase of the prepared nanofibers in different calcinating temperatures. The results for the heat-treated samples are depicted in Fig. 1. As it is found from the XRD pattern, heated fibers at 450 oC for 1 h leads to formation of a white residue which is the crystalline TiO2 nanofibers in anatase phase. The obtained spectrum after heat treatment at 450 oC in air has the TiO2 anatase phase peaks at 2θ = 25.3, 37.7, 47.8, 54, 62.7, 68.6, 70.3, and 75.2. These results are in good agreement with previous reports . In the other words, when the heating rate was fast (5 oC/min), anatase titanium was formed after calcination at 450 oC Fig. (1A). It was found that all the sharp features observed at 2θ = 25.3, 37.0, 38.37, 47.9, 54, 55.21, 62.7, 68.6 and 70.08 in the XRD pattern are consistent with anatase (101), (004), (112), (200), (105), (211), (204), (220), (220) and (215) degrees with no impurity which is in agreement with 21-1272 standard cards from JCPDS. The prominent peaks representing anatase/rutile phase of nanocrystalline TiO2 powder at 650 oC used for this study can be seen at the 2θ values of 25.28, 27.48, 37.80, 48.05, 53.89, 55.06, 62.69, 68.76, 70.31 and 75.03 Fig. (1B). On the heating rate and temperature the crystal structures of the nanofibers can therefore be manipulated to yield distinct properties from the same starting materials. On the other hand, when the solution was slowly heated (0.5 oC/min) and calcined at 850 oC, rutile Titania resulted in Fig. (1C). The XRD features (101), (110), (111), (200), (211), (220), (002), (310) and (301) can be indexed to the rutile Titania crystal with the sharp features observed at 2θ = 25.09, 27.45, 39.5, 41.4, 55.21, 56.3, 64, 65.3 and 69.1. Fig. 2 (A) shows the FT-IRspectra of TiO2 nanofibers in a region of 400–4000 cm−1. The presence of a heteroatom and a carbonyl group in the pyrrolidone ring sufficiently reduces the symmetry  as shown in Fig. 2 (A). The sharp peak at 1632 cm−1 corresponds to the stretching vibration of carboxyl group, which is a typical mode of PVP, while the absorption band around 2923 cm−1 can be attributed to the C–H stretching vibrations . In addition, it can be clearly seen that the composite nanofibers contain a trace hydroxyl group due to the electrospun solution. A broad stretching band at about 3426 cm−1 appears in the composite nanofibers, which can be assigned to the vibration of the associated hydroxyl groups. As shown in Fig. 2 (B), the FT-IR spectra of nanofibers with 50.69 wt% of TiP is recorded in the range of 400-4000 cm−1 calcined at different temperature. In the spectra of the samples calcined at 450 oC, the bands associated with the vibration mode of the skeletal O-Ti-O bonds of anatase phase appeared at 404 cm−1 (Fig. 2B (a)), and several components not well resolved in the range of 550 to 900 cm−1 . After calcination at 650 oC for 2 h (Fig. 2B (b)), there was a shift of the main anatase band from 404 to 500 cm−1 and a new band appeared at around 700 cm−1. The shift of the main adsorption at 404 cm−1 has been generally attributed to variations in particle size and morphology. Within increasing the calcination temperature to 850 oC, the appearance of the band near 700 cm−1 with higher intensity (Fig. 2(c)) was associated with the TiO2 rutile phase . These results also showed the anatase to rutile transformation occurred at this temperature. Fig. 3 (A)-(F) shows the SEM images of electrospun nanofibers TiO2 nanofibers with different concentrations of Ti(OCH(CH3)2)4 (25.32, 50.69 and 76.05 wt%) and heat-treatment conditions. The images in Fig. 3 (A, B) indicate that the average diameter of the electrospun TiO2 nanofibers significantly decreases as the amount of TiO2 increases. As mentioned before, TiO2 nanofibers were generated after the calcination of electrospun Ti(IV) isopropoxide/PVP nanofibers. Low and high resolution SEM images of 50.69 wt% before and after calcination at different temperatures are shown in Fig. 3 (D-F). The as prepared composite nanofibers were straight with smooth surface, Fig. 3 (A). Increasing the amount of TiP resulted in decreasing the diameter of nanofibers, Fig. 3 (B). After calcinations at different temperatures also the nanofibers retained their fibrous structure. However, when the temperature was too high, the anatase phase transform to rutile phase.. The observation depicts that the morphology of samples is irregular and rough due to the crystal grains of the titanium dioxide and may be beneficial to enhance the dye adsorbing due to its great surface roughness and high surface area. The average diameter of calcined fibers was smaller than that of electrospun non-calcined fibers because of the decomposition of organic components by increasing the temperature. The schematic description of Scheme 1 seems to be consistent with the SEM images at different calcinating temperatures (Fig. 3).
Evaluation of DSSC performance
The photoanode electrodes were fabricated in four different manners and utilized as a scattering layer. (1) After blading the block layer on the surface of FTO glass, the TiO2 paste with particle size of 20-30 nm was bladed on the surface as the blocking layer (BNP. Standard sample), the TiO2 paste with particle size of 400 nm was bladed on the surface of the blocking layer (bladed nanoparticles paste scattering layer, BNS), (2) The prepared electrospun nanofiber was dissolved in 0.5 mM TiCl4, heated at 80 oC temperature for 30 min and exposed on the blocking layer (nanofibers TiCl4 post-treatment scattering layer, NTS), (3) The electrospun nanofiber was dissolved in 0.5 mM TiCl4 and exposed on the blocking layer by sputtering (sputtered nanofibers scattering layer, SNS), (4) TiO2 paste with particle size of 400 nm was mixed by TiO2 nanofibers and bladed on the surface of blocking layer (mixed nanoparticles and nanofibers scattering layer, NPFS). Fig. 4 displays the UV-Vis absorption spectra of the N719 dye and dye sensitized different photoanode electrodes in diluted ethanol solution. Metal-to-ligand charge-transfer is displayed with two broad visible bands at 540 and 395 nm in dye. The bands at 310 nm with a shoulder at 300 nm are attributed to intra ligand (π-π*) charge-transfer transitions . By adsorbing dye on TiO2 surface, the dye absorption spectra are broadened and blue-shifted more or less as compared to that in solutions because of H-type aggregation of the dye on the TiO2 surface . As obviously seen in this figure, the combination of TiO2 paste and TiO2 nanofibers with high surface area absorbed more dye than regular TiO2 nanoparticles sufficiently. In the other words, the concentration of dye molecules on the combined scattering layer is more than those on the other types of regular prepared layer, indicating improvement in the optical absorption efficiency of the dye molecules, resulting in increasing in the photocurrent density and carrier transport properties of the photoanode.
Photovoltaic Performance and EIS Analysis
Photocurrent density versus voltage features of DSSCs based on the nanoparticle electrode alone and the nanoparticle/nanofiber electrode are depicted in Fig. 5 and Table 1. For the TiO2 nanoparticle electrode without nanofibers, the short-circuit photocurrent density, Jsc, and the conversion efficiency η of the TiO2 nanoparticle cell were known to be decreased. The addition of large nanoparticles on the top of photoelectrode, acting as light-harvesting layers, was also reported previously . In comparison with the nanoparticle, the special properties of nanofibers such as well aligned and nanofibrils in one-dimension might resulted in enhancing the charge transfer because of fewer grain boundaries. While mixing nanoparticles and nanofibers to prepare scattering layer, significant penetration of the electrolyte into the TiO2 nanoparticle/nanofiber electrode occurs by increased porosity. Because of increasing the amount of adsorbed dye molecules on the electrode surface as compared with the only nanoparticle cell. The Jsc for nanofiber/nanoparticle cells enhanced significantly. The enhanced current density obtained with TiO2 nanoparticles/nanofibers was due to sufficient light harvesting by TiO2 films. Higher surface area of combined nanoparticles/nanofibers layer improved the anchoring of more dye molecules on the surface of TiO2 scattering layer, leading to a significant optical absorbance. The IPCE results (quantum efficiency) of device fabricated by bladed nanoparticles paste blotching layer, BNP, (nanofibers TiCl4 post-treatment scattering layer, NTS, and mixed nanoparticles and nanofibers scattering layer, NPFS, are shown in Fig. 5 (inset). The maximum efficiency for nanoparticles and nanofibers containing cell was over 68% at 550 nm that was increased from 41.4 % to 68 % at 550 nm by NPFS based cells. The enhancement of ~27% suggests that the mixed nanoparticles and nanofibers scattering layer based devices have more electron injection and better electron transportation treatment than those of the bladed nanoparticles paste blotching layer. The NPFS film having large surface area prepared by electrospinning technique could be one possible solution to the high efficiency electron conductor in the DSSCs.
The advantages of this work in comparison with others are optimizing the electrospinning parameters, the value of Tip and the efficient calcination temperature to obtain roughness and desirable diameters of nanofibers that are utilized for scattering layer, preparation different scattering layer by variable methods (BNP, BNS, NTS, SNS, and NPFS), the effect of different temperatures on phase changes (by increasing the temperature, the phase of nanofibers changes from anatase to rutile) and the study on their sufficient effect on the dye sensitized solar cell performance [24–29].
The conversion efficiency of DSSC using TiO2 nanoparticles/nanofibers was about 2 times higher than that of DSSC using regular TiO2 nanoparticles. The improved energy conversion efficiency was principally from the increase in Jsc compared to that for regular nanoparticles based DSSCs, indicating that nanofibrous structure makes the electron injection density easer through sufficient interface between dye molecules and nanoparticle/nanofiber scattering layers. The higher surface area is assigned to the larger amount of adsorbed dyes. Open circuit potential, Voc, is attributed to the energy gap between Fermi level of semiconductor and Nernst potential of I-/I3- couple. It is affected by the electron affinity of semiconductor and ionization potential of dye. The reduction in charge recombination and the negatively shift of semiconductor conduction bond resulting in higher open circuit voltage. The improved Voc for the combined TiO2 nanoparticles/nanofibers photoanode in DSSC is attributes to the facile penetration of electrolyte into TiO2 electrode surface, which retarded back electron transfer and enhanced the redox process between dye oxide cation and redox couple in electrolyte. The existence of mesoporous morphology in combination with nanofibrous web led to the better contact with electrolyte, and adsorption of dye molecules on the nanoparticles/nanofibers electrode surface were increased significantly. Undoubtedly, the increased efficiency aroused by the application of nanofibers as the scattering layer in TiO2 –electrode (photoanode) is attributable to the large surface area, which favors the adsorption of a large amount of dye molecules, as well as a high scattering effect for visible light. The results are summarized in Fig. 6.
Electrochemical impedance spectroscopy (EIS) was carried out at Voc to understand the improved transfer of electrons in solar cells with different TiO2 photoanodes under 1 sun illumination. The spectra are shown in Fig. 7 for different cells with nanoparticles and nanofibers scattering layers. Two semicircles, including a small one at high frequency and a large semicircle at low frequency, were observed in the Nyquist plots (Fig. 7). The small semicircle at high frequency corresponds to the I-/I3- reaction at the Pt-counter electrode, and the larger one at low frequency is assigned to the injection/recombination reactions and electron diffusion in the TiO2 electrode. With a fitting program using Z view software, these data were analyzed. The equivalent circuit used for fitting the impedance spectra of the DSSC is shown in the inset of Fig. 7. In a simple equivalent circuit , Rs, RCE, Rct and Rw are known as series resistance, charge transfer resistance at the counter electrode/electrolyte interface, electron recombination resistance at the dyed-TiO2/electrolyte interface and Nernst diffusion impedance, respectively. CP and Cμ are assigned to the chemical capacitance at the dyed-TiO2/electrolyte interface and the counter electrode/electrolyte interface, respectively. Due to the low frequency (fmin) characteristic of the diffusion coefficient, the electron life time (τr) can be calculated using τr = 1/ (2πfmin) . In the case of the nanoparticles/nanofiber cell, the electron life time was calculated to be 6.4 ms compared to 4.7 ms estimated for the BNS, 3.4 ms NTS and 1.5 ms SNS sample. The low value of electron life time depicts the increase in charge recombination between the electrons in conduction bond returned back to the electrolyte. Because of the existence of a higher transport resistance accompanied by a low electron life time, the dark current enhanced which leads to lower Voc and η. As it is found from the figure, at different prepared scattering layers the second semicircle in mixing nanoparticles and nanofibers is larger than only nanoparticles, resulting the improvement in charge recombination resistance and significant enhance in Voc and Jsc. When the charge recombination occurred, the electron density was reduced in the TiO2 layer and resulted in decreasing Rct. More electron accumulation in the TiO2 layer can cause to shift upward the electron quasi-Fermi level and to increase Voc consequently.
The TiO2 nanofibers with different diameters and calcination temperature and Ti(OCH(CH3)2)4 concentration were prepared using electrospun fibers of TiO2/PVP. The experimental results showed that uniform, rough and continuous fibers with decreased diameters are observed for the electrospun fibers containing a Ti(OCH(CH3)2)4 concentration of 50.69 wt% at a flow rate of 0.5 mL/h, and an electric field of 15 kV/cm and 850 oC. FT-IR results also showed the formation of anatase- rutile titanium dioxide at 650 oC and its transformation into rutile phase at 850 oC. The calcined and optimized nanofiber substrates were used in the fabrication of dye-sensitized solar cell as the different photoanodes. Among different prepared scattering layers, the combination of nanofibers and nanoparticles has the most values for Voc, Jsc and efficiency because of its high surface area, large amount of anchored dye and larger charge recombination resistance. These results suggest that light harvesting nanofibers-modified nanoparticles are very promising materials for the electrode structure of dye sensitized solar cell. The result clearly demonstrates that large surface area of semiconducting materials are necessary to prepare effective light and dye absorption, for better electrical connection with redox electrolytes, crystallinity and the photocurrent density, when TiO2 paste with particle size Titania is observed.
The authors wish to thank the Yazd University Research Council, IUT Research Council and Excellence in Sensors for financial support of this research.
CONFLICT OF INTERESTS
The authors declare that there is no conflict of interests regarding the publication of this paper.