Achieving Enhanced Dye-Sensitized Solar Cell Performance by TiCl 4 / Al 2 O 3 Doped TiO 2 Nanotube Array Photoelectrodes

For various reasons, low cost, easy fabrication, and so forth, dye-sensitized solar cells (DSSCs) have been consistently studied in many laboratories. To improve the DSSCs performance, using an aqueous solution of titanium tetrachloride (TiCl 4 ) treatment is one of many processes. Before the treatment of TiCl 4 , nanoporous TiO 2 nanotubes (TNTs) are fabricated through a secondary anodization process. TiCl 4 treatment on TNTs film enhanced short-circuit current density (JSC) and aluminum oxide (Al2O3) posttreatment enhanced open-circuit voltage (VOC). As a result, Al2O3 posttreatment on TNTs film conversion efficiency of 8.65% is realized, which is 7%higher thanTiCl 4 treatment onTNTs film. In thiswork, we investigated that double dip-coating of TiCl 4 /Al 2 O 3


Introduction
Since the first report by Grätzel in 1991, dye-sensitized solar cells (DSSCs) have been consistently researched due to their low manufacturing costs, simple structure, and wide range of applications [1][2][3].Generally, DSSCs are composed of as parts titanium oxide (TiO 2 ) which carries an anchored organic dye on working electrode layer, counter electrode layer made of Pt and an electrolyte containing a redox couple (I − /I 3 − ) between them [4][5][6].
Some researchers have suggested that the nanostructures such as nanowires, nanorods, nanofibers, or nanotubes need to avoid the electron transport in the nanocrystal boundaries of TiO 2 nanoparticles and the electron recombination with the electrolyte during the electron migration process [7][8][9].Recently, TiO 2 nanotubes layers (TNTs) have attracted much attention, because of their geometric shape and simple anodic process of fabrication, which improve the electron transport between electrode layers.One-dimensional TiO 2 nanostructures have function of light scattering, fast electron transport, and slower recombination rates but reduced dye adsorption by their surface areas [10][11][12].
To improve the DSSCs performance, well-known method is using an aqueous solution of titanium tetrachloride (TiCl 4 ) treatment [13][14][15].The TiCl 4 treatment effect is not quite clear but many studies explain that TiCl 4 treatment on the starting TiO 2 nanoparticle shifts downwards in the TiO 2 conduction band gap and decreases recombination rate in the electron/electrolyte.Although previous studies that treatment on TiO 2 nanoparticle, which are applied to TNTs because it depends on the starting materials of TiO 2 [16].Also method of improving the DSSCs efficiency is using incorporation of atomic impurities Zn, Wo, Al, and so forth [17][18][19][20][21].It results in reduction in the charge recombination rate and progressing of electron mobility at the interface between electrode layers.
The TiCl 4 /Al 2 O 3 posttreatment on TNT film were studied in terms of dye adsorption, charge transport, and electron lifetime.To enhance the short-circuit current density ( SC ) and open-circuit voltage ( OC ), we used the TiCl

Experimental
TiO 2 nanotubes were prepared by an optimized three-step anodization process.Ti foil (0.25 mm thickness, 99.7% purity, Sigma-Aldrich, St. Louis, MO, USA) with an area of 2 cm × 3 cm was degreased by ultrasonic agitation in acetone, isopropanol, and deionized water for 30 min each and then dried with N 2 gas.The ethylene glycol electrolyte contained 0.25 wt% NH 4 F (99%, Sigma-Aldrich) and 2 vol% deionized water.The anodization was performed in a two-electrode system where the Ti foil served as the working electrode and a Pt plate as the counter electrode.Anodization, using an electrolyte at 0-4 ∘ C, was conducted at room temperature at a constant voltage of 60 V for 30 min.Then, the as-prepared TNT array was removed by sonication for 5 min in DI water.The second-step anodization was carried out under the same conditions for 5 h.The as-prepared amorphous TNT array was crystallized into an anatase phase at 450 ∘ C for 2 h in air at a heating rate of 1 ∘ C/min.After another anodization under the same conditions for 20 min and then immersion into a H 2 O 2 solution (33%) for 10 min, the anatase TNT array was detached from the Ti substrate.After rinsing and drying, the self-standing TNT array was cut into 5 × 5 mm 2 squares for transfer.
A TiO 2 paste was prepared from TiO 2 powder (anatase, 99.9% purity, Sigma-Aldrich) and used as the reference [22].The TiO 2 paste was coated onto a fluorine-doped tin oxide (FTO) conductive glass substrate using the doctor blade method, and then the TNT array was transferred onto the TiO 2 paste.
TNTs film was dipped for 30, 60, 90, and 120 min in a 90 mM TiCl 4 aqueous solution at 70 ∘ C which was prepared by adding titanium tetrachloride (Sigma-Aldrich) to precooled distilled water in an ice bath.Following the posttreatment, the TiO 2 film was annealed at 450 ∘ C for 15 min.After TiCl 4 doped TNTs film was dipped in Al 2 O 3 aqueous solution at 90 ∘ C which was prepared by adding aluminum oxide (Sigma-Aldrich), made of different amount of aluminum oxide in ethanol, in different moles of 40, 80, 120, and 160 mM to preheated ethanol at 50 ∘ C bath.The substrate was sintered at a temperature of 450 ∘ C for 15 min.
A Pt catalyst electrode was prepared by mixing H 2 PtCl 6 (5 mM, Sigma-Aldrich) in isopropyl alcohol with an ultrasonic treatment.A counter electrode, which facilitates the redox reaction of the electrolyte, was fabricated by spin coating of the H 2 PtCl 6 solution at 1000 rpm for 30 s and annealed at 450 ∘ C for 30 min.The dye solution to be adsorbed on the electrode films was prepared by mixing 0.5 mM Ru-dye (N719, Solaronix) with ethanol.To facilitate the adsorption of the dye molecules, the prepared TiO 2 electrode films  were placed in the dye solution in darkness for 24 h.Finally, the DSSCs were fabricated by sandwiching the prepared electrode film and counter electrode at 120 ∘ C for 10 min using a hot melt sealant (60 ∘ C).The electrolyte (I − /I 3 − ) was injected between the two electrodes with the inlet and then sealed by a cover glass.
The phases of the TNT array prepared by anodization, as well as those of TNPs, were examined by X-ray diffraction (XRD) using a Rigaku D/MAX-2200 X-ray diffractometer with a Cu-K  radiation source.The morphology of the prepared TNT/TNP photoelectrode film was investigated by field emission-scanning electron microscopy (FE-SEM, S-4700, Hitachi).The absorbance of the TNT/TNP photoelectrode film was measured using a UV-VIS spectrometer (Lambda 750, PerkinElmer).The conversion efficiency and electrochemical impedance spectroscopy (EIS) of the fabricated DSSCs were measured using an I-V solar simulator (K3400, K3000, McScience) under a simulated solar light irradiation of 100 mW⋅cm 2 (AM 1.5).The active area of the cell exposed to light was approximately 0.25 cm 2 (0.5 cm × 0.5 cm).

Results and Discussion
The Energy Dispersive X-ray Spectroscopy (EDX) analysis of the TiCl 4 90 mM treatment for 90 min and Al 2 O 3 120 mM posttreatment for 60 min on TNTs film is shown in Figure 1, which indicated the presence of about 6.18 atomic % of Al and 22.28 atomic % of Ti.
Figure 2(a) shows the field emission-scanning electron microscopy (FE-SEM) image of the surface morphologies of bare TNTs array.TNTs array was processed by a secondary anodization.Then, diameter of uniform surface is about 100 nm and according to anodic oxidation at present conditions TNTs array can come to various lengths.We used 18 m of TNTs in study; the fast electron transportation in the film is helpful (Figure 2   Figure 3 shows UV-VIS absorption spectrum of N-719 dye in the 400-800 nm wavelength in the different TiCl 4 posttreatment for time from 30 to 120 min on TNTs films.It was found that absorbance increased with increasing TiCl 4 dip-coating times, excluding TiCl 4 treatment for 120 min.An appropriate amount of TiCl 4 in the TNTs films can have effect on providing a large surface area for dye adsorption reported [23][24][25].The number of dye molecules influences photocurrent of DSSCs indirectly; thus adsorption of dye molecules is connected to increasing of more light harvesting and  SC as expected.TiCl 4 treatment increased the amount of adsorbed dye because TiCl 4 treatment can decrease the TiO 2 particle size on the electrode surface, leading to the increased active surface area [15,26].In case of TiCl 4 treatment for 120 min, the absorbance value was lower than TiCl 4 treatment for 90 min.Reduction of the TNTs films porosity by increasing of the nucleation in the nanoparticle caused decrement of dye absorption in TNTs film with many chances over dip-coating time.Therefore, the increase of inefficient charge-transfer routes and recombination rate of electrons can decrease the photocurrent density and conversion efficiency, as discussed [16].
The I-V curves of TiCl 4 treatment on TNTs photoanodes with different dip-coating time conditions are shown in Figure 4, and the photovoltaic parameters including shortcircuit current density ( SC ), open-circuit voltage ( OC ), fill factor (FF), and photovoltaic efficiency () are listed in Table 1.The light-to-electric energy conversion efficiency was achieved under a simulated solar light irradiation of 100 mW⋅cm 2 (AM 1.  conversion efficiencies in cells due to increment of dye adsorption, improvement of charge transport, and reduction of electron recombination rate.When the TNTs film was electrochemically dip-coated, the values of  increased with TiCl 4 treatment time to reach a maximum of 8.08% at 90 min and thereafter decreased.It is well known that the  of DSSCs performance is significantly affected by the amount of dye loading to photoanodes [26][27][28]. The Nyquist plots of real axis residence (Z  ) and imaginary axis residence (Z  ) analyzed by the classical equivalent electrical circuit are shown in Figure 5.In the EIS, R  (ohmic series resistance),  CT 1 (3 charge-transfer resistance of the counter electrode),  PE 1 (constant phase element of the counter electrode),  CT 2 (4 charge-transfer resistance of the working electrode), and  PE 2 (constant phase element of the photoelectrode) are indicated in Figure 5.The small semicircle is fit to a charge-transfer resistance ( CT 1) and constant phase, while the large semicircle is fit to a transfer resistance ( CT 2) and constant phase.As  CT 2 is affected by the use of TiO 2 nanoparticles/TNTs, large one appearing at intermediate frequency (around 10 Hz) represents electron transfer resistance ( CT 2) at photoelectrode and electrolyte interface    (8.65%).Therefore, the optimum condition for high conversion efficiency was the treatment of Al 2 O 3 120 mM.

Conclusions
To improve the efficiency of DSSCs, we took a two-step treatment in the TiO 2 nanotubes film.In the first step, an aqueous solution of titanium tetrachloride (TiCl 4 ) was treated on TNTs film for different time from 30 to 120 min.The highest efficiency of DSSCs using TiCl 4 90 mM treatment for 90 min reached 8.08% and  SC of that condition was 20.72 mA/cm 2 .In the second step, an aqueous solution of aluminum oxide (Al 2 O 3 ) was treated on TiCl 4 treatment TNTs film with different concentration from 40 to 160 mM.Successfully, performance of DSSCs was improved that  OC was increased to 0.67 V and efficiency was 8.64%.The optimum condition for high conversion efficiency was the treatment of Al 2 O 3 120 mM.Al 2 O 3 influenced a higher electron lifetime due to the increased interfacial charge-transfer resistance.
In conclusion, using the TNTs and TiCl 4 /Al 2 O 3 posttreatment process was found to be an effective method to improve the efficiency of TiO 2 nanoparticle based DSSCs.

4
and aluminum oxide (Al 2 O 3 ).As a result, effect of double dipcoating with TiCl 4 /Al 2 O 3 posttreatment on TNTs film enhanced the overall energy conversion efficiency of DSSCs.In this work, we investigated the effects of double dip-coating with TiCl 4 treatment and after Al 2 O 3 posttreatment on TNTs based DSSCs.
Figure 2(c) shows surface of TiCl 4 treatment on TNTs film and Figure 2(d) shows surface of TiCl 4 /Al 2 O 3 posttreatment on TNTs film.The surface morphologies of TiCl 4 /Al 2 O 3 posttreatment on TNTs film are rougher than TiCl 4 treatment or bare TNTs.

Figure 2 :Figure 3 :
Figure 2: FE-SEM images of (a) the surface of TiO 2 nanotubes array; (b) the cross section of a TiO 2 nanotube array; (c) the surface of TiCl 4 treatment on TiO 2 nanotubes array; and (d) the surface of TiCl 4 /Al 2 O 3 posttreatment on TiO 2 nanotubes array.

2 )Figure 4 :
Figure 4: I-V curve of TiCl 4 treatment on TiO 2 nanotubes film for different time conditions.

Table 2 :
The integral photocurrent density ( SC ), open-circuit voltage ( OC ), fill factor (FF), and efficiency () of dye-sensitized solar cells fabricated using TiCl 4 /Al 2 O 3 posttreatment on TiO 2 nanotubes film.EIS), all TiCl 4 /Al 2 O 3 posttreatment on TNTs film indicated that impedance is smaller than bare condition of TiCl 4 treatment.The smallest semicircle is TiCl 4 treatment and Al 2 O 3 120 mM posttreatment on TNTs film.Small  CT 2 indicates much faster electron transport and it is closely connected to efficiency of DSSCs.Therefore, the TiCl 4 /Al 2 O 3 treatment of a less defective morphology effect significantly improved electron transport.Figure 6 shows I-V curve of Al 2 O 3 posttreatment of TiCl 4 treatment TNTs film with different concentration in 40 to 160 mM and the photovoltaic properties of TiCl 4 /Al 2 O 3 posttreatment TNTs film are summarized in Table 2. From

Table 2 ,
[29]s with TiCl 4 /Al 2 O 3 posttreatment on TNTs film presented increased  OC from 0.61 to 0.67 V.In reported studies, Al 2 O 3 influenced a higher electron lifetime and that contributed to increasing of  OC because Al 2 O 3 affects to suppress the charge recombination between the TiO 2 /TNTs/electrolyte interfaces[29].It was found that Al 2 O 3 120 mM posttreatment indicated  OC (0.67 V) and TiCl 4 -treated TNT film + 80 mM Al 2 O 3 TiCl 4 -treated TNT film + 120 mM Al 2 O 3 TiCl 4 -treated TNT film + 160 mM Al 2 O 3 I-V curve of TiCl 4 /Al 2 O 3 posttreatment on TiO 2 nanotubes film with different concentrations.