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Enhancement of Dye-Sensitized Solar Cells Efficiency Using Mixed-Phase Ti[O.sub.2] Nanoparticles as Photoanode.

1. Introduction

Significant demand for energy gives rise to the depletion of fossil resources, global warming, and climate change; thus energy substitutes are always urgent mission to protect living condition without losing life convenience and economic benefits. Among all alternatives, dye-sensitized solar cells (DSSCs) with simple structure and acceptable conversion of sunlight into electricity with low cost and high efficiency have attracted much attention. Traditionally, Ti[O.sub.2] semiconductor acting as anode electrode is attached to sensitizer ruthenium dye and is involved in volatile liquid electrolyte containing [I.sup.-]/[I.sup.3]-redox couple and counterelectrode to form a complete DSSC cell [1-3]. Therefore, the DSSC performance strongly depends on the Ti[O.sub.2] characteristics such as its morphology, phase compositions, and other properties [4, 5]. Recently, many studies have offered one-dimensional Ti[O.sub.2] nanomaterials such as nanowires and nanotubes which offer high surface-to-volume ratio in the structure of Ti[O.sub.2] photoanode because abundant dye can be loaded on the Ti[O.sub.2] photoanode surface to maximize the amount of photogeneration [6]. In efficient DSSC operation, one cycle of photon-to-electricity conversion is completed by fast electron injection from a photoexcited dye into the Ti[O.sub.2] conduction band and subsequently dye regeneration and holes transportation to the counterelectrode. Lower resistance of charge pathways and electron-hole recombination rate of photogenerated carriers are necessary for longer electron diffusion length and extending lifetime in Ti[O.sub.2] photoanode and to obtain good collection efficiency [7, 8]. Therefore, the Ti[O.sub.2] semiconductor with adequate band gap and pathway resistance of electron transport plays an important role in obtaining the most efficient DSSCs. Although 1D of nanocrystalline Ti[O.sub.2] is preferred to act as photoanode by proposing enough dye attaching, well order and good arrangement patterns are difficult to fabrication using low cost process. Besides, minimized interfacial charge recombination has to be considered during carrier transport and final conversion efficiency. To decrease interfacial recombination and increasing electron lifetime, metal oxides of core shell 1D configuration on top of the transparent Ti[O.sub.2] film have been investigated using Zr[O.sub.2] and ZnO. In order to adjust conduction band position, overcoat materials Mg(OH)2, Zn(OH)2 proposed more negative conduction band in comparison with Ti[O.sub.2]; thus these metal hydroxides were applied as a blocking Ti[O.sub.2] layer at the FTO/Ti[O.sub.2] interface to decrease electron leakage.

Due to the multiplicity of semiconductor Ti[O.sub.2], there have many fabrication methods to decorate the Ti[O.sub.2] photoelectrode by changing Ti[O.sub.2] characteristics in order to enhance cell conversion efficiency [9, 10]. For instance, a mixture of Ti[O.sub.2] nanoparticles with different size, phase composition, and morphology result in conversion efficiency because of the light scattering and the facile electron transport [11, 12]. Alternatively, metal decoration of Ti[O.sub.2] photoanodes by tailoring photoanode properties, redistributing defects, and trapping levels in the band gap enables changing the conduction band position [13-15]. However, metal-decorated Ti[O.sub.2] suffers from thermal stability issue which is worse for electron-hole recombination. Oppositely, nitrogen and carbon doping on Ti[O.sub.2] can eliminate oxygen vacancies and surface-deficiency-related defects and results in lower resistivity and contact resistance on the carrier transport pathway.

In previous works, we concentrated on the mixture of core shell structures of Ti[O.sub.2] nanotube with nanoparticles and then presented the electron transport behavior related to DSSC conversion efficiency. Different shapes of conduct carrier transport behaviors in Ti[O.sub.2] photoanode were discussed. Ti[O.sub.2] phase is controlled at around 500[degrees]Cby sintering process and the results demonstrate that sintering treatment can significantly affect crystal nanoporous Ti[O.sub.2] photoanode for DSSCs. In this study, we investigate the photovoltaic behavior using mixed-phase Ti[O.sub.2] structure as anode in DSSC cell. It is well known that Ti[O.sub.2] has three polymorphs phases in nature: rutile, anatase, and brookite; the band gap of rutile phase and anatase phase are 3.0 eV and 3.2 eV, respectively. By mixing the different contents of rutile phase into anatase matric and coating it on FTO-glass substrate to be photoanode, the cell employing the composited photoanode was investigated to find out the conversion efficiency of DSSC by material properties, carrier transport, and EIS measurement.

2. Materials and Methods

Prior to the fabrication of the Ti[O.sub.2] photoanodes, fluorine doped tin oxide (FTO) glass substrates were cleaned by the same volume ratio of acetone and isopropyl alcohol mixture in an ultrasonic water bath for 30 min. To prepare Ti[O.sub.2] nanoparticle paste, Degussa-P25 (anatase: 80%, rutile: 20%) powder and P90 (anatase: 90%, rutile: 10%) powder acting as Ti[O.sub.2] precursors were mixed with specific contents to meet experimental design. First, 1 g of polyethylene glycol (PEG) was hydrolyzed in 3 mL of deionized water under stirring at room temperature for 15 min. Second, separately P25 and P90 mixtures (w/w = 0%/100%, 30%/70%, 50%/50%, 70%/30%, 100%/0%) were add to PEG colloids, to obtain rutile weight percent from 5% to 15% which was denoted as R5, R9, R10, R14, and R15, respectively. Then, colloid was stirred at room temperature for 8 h. To prepare Ti[O.sub.2] photoanodes, first, spin coat FTO glass with TiCl4 and anneal substrate at 80[degrees]Cfor 50 minutes. Second, 10 um~14 um of synthesized mixed-phase Ti[O.sub.2] nanoparticle paste was formed on substrate using a glass rod. All as-prepared Ti[O.sub.2] photoanodes were calcined at 450[degrees]C for 30 min in air atmosphere to form microstructure.

An active area of 0.5 cm x 0.5 cm was selected from the Ti[O.sub.2] photoanode and immersed in a 3.0 x [10.sup.-3] M solution of the ruthenium based dye [[RuL.sub.2][(NCS).sub.2]] [TBA.sub.2] for overnight, where Ru is ruthenium, L represents 2,27 -bipyridyl-4,47 dicarboxylic acid, NCS stands for isothiocyanate, and TBA is tetrabutylammonium (N719 dye, Everlight Chemical, Taiwan). The specimens were washed with ethanol after immersing in N719 dye solution. A thin Pt sputtered on an FTO glass was used as the counterelectrode. The iodide/tri-iodide ([I.sub.-]/[I.sub.3.sup.-]) electrolyte (lodolyte R-150) was cast into the dye absorbed Ti[O.sub.2] electrodes. The Ti[O.sub.2] photoanode and the Pt coated cathode were clamped together in order to assemble the DSSC devices. The film morphology was observed by field-emission scanning electron microscope (FESEM). The crystalline phases of the obtained titanium electrodes were characterized by X-ray diffraction (XRD) radiation at scanning rate of 0.01 deg/min from 2[theta] = 5[degrees] to 60[degrees] and Raman spectroscopy from wavelength = 100 to 2000 cm- . The photovoltaic characteristics of DSSC devices were measured by an electrochemical analyzer under a standard AM 1.5 sunlight illumination with 100 mW/[cm.sup.2] light source. The electrical impedance spectra (EIS) were also measured in the range of 0.01 Hz to 100 kHz using the same equipment and setup. To obtain the information about band gap energy of the as-synthesized nanoparticle, spectrum has been recorded using UV/vis spectrophotometer. Photoluminescence (PL) spectroscopy was used to obtain the information about defects of the as-synthesized nanoparticle.

3. Results and Discussion

Characteristics of crystal Ti[O.sub.2] photoanode, resistance of carrier pathway, EIS curves, and photovoltaic results are overall discussed by the integrated DSSC cell.

3.1. Mixed Ti[O.sub.2] Photoanode Morphology and Properties. To investigate morphology of various rutile contents and ensure good adhesive interface between mixed Ti[O.sub.2] nanocomposite and FTO before applying to DSSC, the mixture was used to be verified by SEM. Figure 1 shows the top-view SEM images of Ti[O.sub.2] nanoparticles. It can be seen that Ti[O.sub.2] nanoparticles with approximate spherical shape has an average particle size around 20-30 nm of R5 and random size distribution from 30 nm to 70 nm in R9 to R15, respectively. The average particle aggregates as the rutile phase increases. Figure 2 shows XRD diffraction patterns of synthesized Ti[O.sub.2] after sintering at 450[degrees]C which were used to investigate the crystallization degree, compositions, and grain size. Moreover, peak patterns can be used to estimate the anatase content in the nanostructured using the following equation:

[C.sub.A] = [A.sub.A]/[A.sub.A]+1.265X[A.sub.R] x 100%, (1)

where [C.sub.A] is the anatase content in the Ti[O.sub.2] and AR and A A are the areas covered by rutile peak (110) and anatase peak (101) in the XRD pattern, respectively. In Figure 2, R15 peak simultaneously shows anatase (101) and rutile (110) planes that imply the anatase and rutile phase are well-crystallized due to sharp diffractions. In contrast, R5 only has the sharp diffraction pattern associated with anatase (101) crystal plane and broad diffraction from the rutile (110) plane illustrates a relatively low degree of crystal structure of the rutile Ti[O.sub.2]. Besides, the grain size was determined from the width at half maximum of the (101) anatase peak according to the Scherrer formula:

D = 0.9[lambda]/wxcos[theta], (2)

where [lambda] is the X-ray wavelength, w is width at half maximum peak, and [theta] is peak position. Grain size is gradually agglomeration from 11.8 nm to 19.5 nm by increasing rutile content. Synthesized Ti[O.sub.2] nanocomposite was coated on fluoride-doped tin oxide (FTO) conducting glasses acting as photoanode. Table 1 shows the results of different amount of the rutile phase and grain size in anatase matrix. The mixtures contain various ratios of rutile phase in anatase matrix which are denoted from R5 to R15, respectively, where the numbers represent the rutile content. Photoluminescence (PL) is a common method to analyze the characteristics of photogenerated charge trapping and carrier separation behavior, and the PL emissions result from the recombination of photogenerated charge carriers. In this work, the PL spectra excited by 325 nm light source was used to express the deficiency in Ti[O.sub.2] nanoparticles. As shown in Figure 3, three main signal peaks are found at 395 nm, 470 nm, and 545 nm. A peak centered at 395 nm is found among all samples which results from the transient of shallow donor level in forbidden gap. It is speculated that electron-trapped oxygen vacancy that formed shallow energy levels is located at forbidden gap and near conduct band, resulting in emission at near 395 nm wavelength [16]. These defect levels as well as surface states can act as chemisorptions of oxygen which control electron scavenge on the photoanode Ti[O.sub.2] surface [17]. Both R15 and R14 show lowest peak intensity at 395 nm, indicating lower electron-trapped oxygen vacancy of defect state level in band gap. Because the PL spectra of Ti[O.sub.2] are sensitive to the prepared conditions and crystal characteristic, sample R5 related to high lower crystalline could intrinsically chemosorb oxygen molecular and exhibit high intensity at peak 395 nm among all samples. The peak at 395 nm related to electron-trapped oxygen vacancy is the dominant factor to affect carrier transport in DSSC operation. A peak around 545 nm is attributed to the relaxation of self-trapped exciton which is thought to be electron-hole recombination rate in Ti[O.sub.2] structure. To be noted that this recombination within Ti[O.sub.2] photoanode is not major dominant rule in DSSC performance. A sharp peak at around 465 nm was found among all samples; the back-electron transfer at the interface of Ti[O.sub.2] photoanode electrolyte is illustrated to be the main recombination pathway in deteriorating the DSSC efficiency. Higher rutile content proposes lower defect state levels in band gap [18,19]. UV-vis spectra were measured to investigate the effect of rutile content on the optical properties in anatase Ti[O.sub.2] matrix. Different rutile contents in anatase matrix are denoted as R5, R9, R10, R14, and R15, respectively. Figure 4 indicates that the absorption edges of synthesized Ti[O.sub.2] materials were successfully extended to the visible region. All samples exhibit optical absorption below 400 nm which is attributed to the band-to-band electron transition in the Ti[O.sub.2] nanocrystals related to its band gap energy near 3.1 eV. It can be observed that the absorption thresholds at 400 nm for R5 samples are slightly blue-shifted compared to that of R15 one and the degree of blue shift slightly increases with the amount of rutile content. The integral band gap energy agrees with the full width at half maximum (FWHM) shown in Figure 2 XRD pattern, meaning the smaller FWHM has the better conductivity, and proposes narrowband gap energy [20, 21].

3.2. Photovoltaic Results of DSSC. The photovoltaic characteristics of DSSC were measured under the intensity of 100 mW/[cm.sup.2] simulated solar light. The overall solar conversion efficiency ([eta]) is a product of the short-circuit current density ([I.sub.SC]), the open-circuit photo voltage ([V.sub.OC]), and the fill factor (FF), according to

[eta] = [I.sub.SC] x [V.sub.OC] x FF/[P.sub.in], (3)

where [P.sub.in] is the total light incident on the cell (100 mW/[cm.sup.2]). Figure 5 shows the J-V curve; the values of the photovoltaic (PV) parameters including open-circuit voltage ([V.sub.OC]), photocurrent ([J.sub.SC]), fill factor (FF), and energy-conversion efficiency ([eta]) are summarized in Table 2. The fill factor is the ratio of the maximum cell power to the product of [I.sub.SC] and [V.sub.OC]. It can be seen from Table 2 that the highest overall efficiency (3.82%) was obtained at R14 and the cell performance was enhanced by rutile content (R14 > R15 > R10 > R9 > R5) until 14% of rutile content. Too much of rutile content is not good for energy-conversion efficiency. The synergistic effect between the anatase and rutile phases occurs in R14, suggesting that an optimal rutile percentage near 14wt% obtains the best performance by mixed-phase DSSCs. It highlights the existence of a synergistic effect between the mixture Ti[O.sub.2] photoanode in DSSCs. It is known that efficiency of electron transfer is determined from the degree of recombination rates and pure rutile displays photocatalytic inactive due to high recombination center compared to anatase. By decorating rutile content in anatase matrix, slight rutile crystallizes and the defect state relating to electron-trapped oxygen vacancy and electron scavenge decreases. Therefore, the present rutile content prompts electron transfer from rutile to anatase lattice trapping sites, future inhibiting electron/hole recombination occurrence [22, 23]. Figure 6 shows the schematic drawing of the energy diagram, illustrating the pathway of excited photoelectrons injection from the dye to the rutile conduction band, passing through anatase trapped level in the band gap and arriving at photoanode surface [24].

3.3. Parameters of Electron Transport Determined by EIS. There are three types of impedance and electron pathway in DSSC system including the recombination in Ti[O.sub.2]-electrolyte dye and proposed carriers transport pathway. The total impedance ([Z.sub.S]) of the DSSC is given by the sum of the summation of impedance of diffusion and recombination in Ti[O.sub.2] photoanode ([Z.sub.T]), impedance at Pt electrode/electrolyte interface ([Z.sub.P]), and impedance of tri-iodide diffusion in the electrolyte ([Z.sub.N]). The impedance of [Z.sub.T] consists of [R.sub.W] and [R.sub.K], which represents electron transport resistance in Ti[O.sub.2] photoanode and charge-transfer resistance related to electrons recombination with electrolyte, respectively. All electron transport parameters on DSSC including chargetransfer resistance ([R.sub.k]), electron density ([n.sub.s]), and electron life time ([tau]) on conduction band are evaluated by EIS measurement. Figure 7(a) shows the typical experimental spectra of Nyquist plot; each sample contains three arcs ([omega]1, [omega]2, [omega]3) corresponding to charge transport within Pt counterelectrode and electrolyte interface, Ti[O.sub.2] photoanodeelectrolyte- dye system, and diffusion of tri-iodide ions in the electrolyte, respectively [25]. The [R.sub.k] value is estimated from the diameter of central arc [omega]2. Moreover, at the steady state, electron density [n.sub.s] value in the conduction band is calculated as the following equations:

[mathematical expression not reproducible]. (4)

Here L, [K.sub.eff], q, A, [n.sub.s], [K.sub.B], and T represent photoanode Ti[O.sub.2] thickness, peak frequency of the central arc [[omega].sub.2], the charge of an electron, electrode area, electron density in the conduction band, Boltzmann constant, and absolute temperature. The life time ([tau]) of photoinjected electrons within Ti[O.sub.2] photoanode is calculated as 1/2[pi]f and the highest peak frequency f is obtained via the Bode plot shown in Figure 7(b) whose frequency ranges from 1 to 100 Hz. Table 3 summaries EIS parameters of all samples. Because higher [R.sub.k] value could suppress the back-electron recombination at the Ti[O.sub.2] photoanode/electrolyte interface and then increases [V.sub.OC], in contrast, surface state and oxygen vacancy acting as recombination pathway could simultaneously decrease [R.sub.k]. Although R5 is composed of high ns value on conduction band and longer [tau], lower [R.sub.k] value 34.9 Q deteriorates the efficiency of energy conversion due to lower [V.sub.OC] of backelectron recombination. It consists with photoluminescence result that lower surface state and oxygen vacancies contained in high rutile content samples, >14% rutile content in anatase Ti[O.sub.2], obtain high electron life time, smooth migration, and density in CB; simultaneously the higher [R.sub.k] facilitates the higher [V.sub.OC] and [J.sub.SC] values; thus the highest efficiency of energy conversion is achieved. Rutile content may affect relative location of rutile conduction band to anatase trapping site, resulting in increasing of electron density and efficiency of energy conversion.

4. Conclusion

The addition of rutile content in anatase Ti[O.sub.2] matrix prompts Ti[O.sub.2] crystalline and reduces the integral band gap and defect density states in forbidden gap. Moreover, light harvest on DSSC significantly enhanced the photocurrent and overall solar conversion efficiency by increasing rutile content into anatase photoanodes. The obvious increase in /SC on the 14% rutile Ti[O.sub.2] phase in photoanode has the highest energyconversion efficiency ([eta]) of 3.8% and is 58% higher than that on 5%. The addition of rutile is the major reason to reduce oxygen vacancies and less electron-back recombination. In appropriate rutile content on anatase Ti[O.sub.2] matrix, the pathway of excited photoelectrons injection from the dye to the rutile conduction band, passing through anatase trapped level in the band gap and finally arriving at photoanode surface, becomes smooth. An optimal rutile content around 14 wt% is found to be increase the DSSCs performance with regard to photoanode crystalline, defect density level, and electron transport.

https://doi.org/10.1155/2017/9152973

Received 7 April 2017; Revised 24 May 2017; Accepted 12 June 2017; Published 15 August 2017

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

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Yi-Hua Fan, Ching-Yuan Ho, and Yaw-Jen Chang

Department of Mechanical Engineering, Chung Yuan Christian University, Chung-Li, Taiwan Correspondence should be addressed to Ching-Yuan Ho; cyho@cycu.edu.tw

Academic Editor: Guosong Wu

Caption: FIGURE 1: SEM images of the photoelectrodes top view: (a) R5, (b) R9, (c) R14, and (d) R15.

Caption: FIGURE 2: XRD patterns of Ti[O.sub.2] nanoparticles.

Caption: FIGURE 3: PL spectra of Ti[O.sub.2] specimens.

Caption: FIGURE 4: UV-vis spectra of Ti[O.sub.2] photoanodes.

Caption: FIGURE 5: J-V plots measured by various Ti[O.sub.2] photoanodes of DSSC devices.

Caption: FIGURE 6: Carriers transport paths of anatase and rutile phases Ti[O.sub.2].

Caption: FIGURE 7: (a) Nyquist plots corresponding to specific resistance in photoanode/electrolyte/dye system. (b) Bode plots related to peak frequency

TABLE 1: Value of the grain size and the rutile
content of Ti[O.sub.2] photoanodes.

      Grain size (nm)   Rutile content

R5         11.78            5.29%
R9         14.89            8.97%
R10        16.47            10.34%
R14        18.12            13.89%
R15        19.46            15.04%

TABLE 2: Summary of photovoltaic parameters on DSSC under
stimulated sunlight.

        [J.sub.SC]      [V.sub.OC]      THK       FF    [eta] (%)
      (mA[cm.sup.-2])      (V)       ([micro]m)

R5         4.544          0.761         9.6       0.7      2.4
R9         4.812           0.84         10.2      0.7      2.7
R10        5.34           0.807          12       0.7     3.01
R14        6.776          0.821          14       0.7     3.82
R15        6.812          0.825         10.5      0.6     3.42

TABLE 3: EIS measured results of various Ti[O.sub.2]
photoanodes on DSSC devices.

      [R.sub.k]   [[tau].sub.eff]     [n.sub.s]
      ([OMEGA])        (ms)         ([cm.sup.-3])

R5      34.9            7.5          9.37 * 1017
R9      39.47          9.13          6.70 * 1017
R10     40.16          6.19          6.00 * 1017
R14     42.18          11.08         1.53 * 1018
R15     45.65          11.08         9.94 * 1017
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