# Achieving Enhanced Dye-Sensitized Solar Cell Performance by TiCl4/Al2O3 Doped TiO2 Nanotube Array Photoelectrodes

Authors: Jin Soo Lee; Kyung Hwan Kim; Chang Seob Kim; Hyung Wook Choi
Journal: Journal of Nanomaterials
 (2015)

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## Abstract

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 (TiCl4) treatment is one of many processes. Before the treatment of TiCl4, nanoporous TiO2 nanotubes (TNTs) are fabricated through a secondary anodization process. TiCl4 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 than TiCl4 treatment on TNTs film. In this work, we investigated that double dip-coating of TiCl4/Al2O3 treatment on TiO2 nanotubes film had an effect on enhancement of JSC and VOC due to improvement of electron transfer, increment of dye adsorption, and reduction of recombination rate of charge. Photoelectrode DSSCs with light-to-electric energy conversion efficiency were achieved under a simulated solar light irradiation of 100 mW·cm2 (AM 1.5).


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## Body

## 1. 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 [1–3]. Generally, DSSCs are composed of as parts titanium oxide (TiO2) which carries an anchored organic dye on working electrode layer, counter electrode layer made of Pt and an electrolyte containing a redox couple (I-/I3-) between them [4–6].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 TiO2 nanoparticles and the electron recombination with the electrolyte during the electron migration process [7–9]. Recently, TiO2 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 TiO2 nanostructures have function of light scattering, fast electron transport, and slower recombination rates but reduced dye adsorption by their surface areas [10–12].To improve the DSSCs performance, well-known method is using an aqueous solution of titanium tetrachloride (TiCl4) treatment [13–15]. The TiCl4 treatment effect is not quite clear but many studies explain that TiCl4 treatment on the starting TiO2 nanoparticle shifts downwards in the TiO2 conduction band gap and decreases recombination rate in the electron/electrolyte. Although previous studies that treatment on TiO2 nanoparticle, which are applied to TNTs because it depends on the starting materials of TiO2 [16]. Also method of improving the DSSCs efficiency is using incorporation of atomic impurities Zn, Wo, Al, and so forth [17–21]. It results in reduction in the charge recombination rate and progressing of electron mobility at the interface between electrode layers.The TiCl4/Al2O3 posttreatment on TNT film were studied in terms of dye adsorption, charge transport, and electron lifetime. To enhance the short-circuit current density (JSC) and open-circuit voltage (VOC), we used the TiCl4 and aluminum oxide (Al2O3). As a result, effect of double dip-coating with TiCl4/Al2O3 posttreatment on TNTs film enhanced the overall energy conversion efficiency of DSSCs. In this work, we investigated the effects of double dip-coating with TiCl4 treatment and after Al2O3 posttreatment on TNTs based DSSCs.


## 2. Experimental

TiO2 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 N2 gas. The ethylene glycol electrolyte contained 0.25 wt% NH4F (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 H2O2 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 mm2 squares for transfer.A TiO2 paste was prepared from TiO2 powder (anatase, 99.9% purity, Sigma-Aldrich) and used as the reference [22]. The TiO2 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 TiO2 paste.TNTs film was dipped for 30, 60, 90, and 120 min in a 90 mM TiCl4 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 TiO2 film was annealed at 450°C for 15 min. After TiCl4 doped TNTs film was dipped in Al2O3 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 H2PtCl6 (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 H2PtCl6 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 TiO2 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-/I3-) 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-Kα 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 anI-V solar simulator (K3400, K3000, McScience) under a simulated solar light irradiation of 100 mW·cm2 (AM 1.5). The active area of the cell exposed to light was approximately 0.25 cm2 (0.5 cm × 0.5 cm).


## 3. Results and Discussion

The Energy Dispersive X-ray Spectroscopy (EDX) analysis of the TiCl4 90 mM treatment for 90 min and Al2O3 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 1
EDX spectra of Al2O3 posttreatment on TiCl4 treatment on TNTs film.Figure2(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 μm of TNTs in study; the fast electron transportation in the film is helpful (Figure 2(b)). Figure 2(c) shows surface of TiCl4 treatment on TNTs film and Figure 2(d) shows surface of TiCl4/Al2O3 posttreatment on TNTs film. The surface morphologies of TiCl4/Al2O3 posttreatment on TNTs film are rougher than TiCl4 treatment or bare TNTs.Figure 2
FE-SEM images of (a) the surface of TiO2 nanotubes array; (b) the cross section of a TiO2 nanotube array; (c) the surface of TiCl4 treatment on TiO2 nanotubes array; and (d) the surface of TiCl4/Al2O3 posttreatment on TiO2 nanotubes array.
(a)
(b)
(c)
(d)Figure3 shows UV-VIS absorption spectrum of N-719 dye in the 400–800 nm wavelength in the different TiCl4 posttreatment for time from 30 to 120 min on TNTs films. It was found that absorbance increased with increasing TiCl4 dip-coating times, excluding TiCl4 treatment for 120 min. An appropriate amount of TiCl4 in the TNTs films can have effect on providing a large surface area for dye adsorption reported [23–25]. The number of dye molecules influences photocurrent of DSSCs indirectly; thus adsorption of dye molecules is connected to increasing of more light harvesting and JSC as expected. TiCl4 treatment increased the amount of adsorbed dye because TiCl4 treatment can decrease the TiO2 particle size on the electrode surface, leading to the increased active surface area [15, 26]. In case of TiCl4 treatment for 120 min, the absorbance value was lower than TiCl4 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].Figure 3
UV-VIS absorbance of TiCl4 treatment on TiO2 nanotubes film for different time conditions.TheI-V curves of TiCl4 treatment on TNTs photoanodes with different dip-coating time conditions are shown in Figure 4, and the photovoltaic parameters including short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and photovoltaic efficiency (η) are listed in Table 1. The light-to-electric energy conversion efficiency was achieved under a simulated solar light irradiation of 100 mW·cm2 (AM 1.5). The DSSCs with bare TNTs film exhibited a JSC of 12.53 mA/cm2 and yielded η of 4.69%. However, TiCl4 treatment on TNTs film enhanced energy 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 η increased with TiCl4 treatment time to reach a maximum of 8.08% at 90 min and thereafter decreased. It is well known that the η of DSSCs performance is significantly affected by the amount of dye loading to photoanodes [26–28].Table 1
The integral photocurrent density (JSC), open-circuit voltage (VOC), fill factor (FF), and efficiency (η) of dye-sensitized solar cells fabricated using TiCl4 treatment on TiO2 nanotubes film.
Sample
V
OC (V)
J
SC (A/cm2)
FF (%)
η (%)
Bare TNTs film
0.61
12.53
60.78
4.69
TiCl4 90 mM (30 min)
0.60
18.94
62.57
7.16
TiCl4 90 mM (60 min)
0.60
20.09
62.50
7.59
TiCl4 90 mM (90 min)
0.61
20.72
63.90
8.08
TiCl4 90 mM (120 min)
0.60
20.26
66.92
7.71Figure 4
I-V curve of TiCl4 treatment on TiO2 nanotubes film for different time conditions.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, RS (ohmic series resistance), RCT1 (3 charge-transfer resistance of the counter electrode), CPE1 (constant phase element of the counter electrode), RCT2 (4 charge-transfer resistance of the working electrode), and CPE2 (constant phase element of the photoelectrode) are indicated in Figure 5. The small semicircle is fit to a charge-transfer resistance (RCT1) and constant phase, while the large semicircle is fit to a transfer resistance (RCT2) and constant phase. As RCT2 is affected by the use of TiO2 nanoparticles/TNTs, large one appearing at intermediate frequency (around 10 Hz) represents electron transfer resistance (RCT2) at photoelectrode and electrolyte interface that is most relevant for TNTs film. In the electrochemical impedance spectroscopy (EIS), all TiCl4/Al2O3 posttreatment on TNTs film indicated that impedance is smaller than bare condition of TiCl4 treatment. The smallest semicircle is TiCl4 treatment and Al2O3 120 mM posttreatment on TNTs film. Small RCT2 indicates much faster electron transport and it is closely connected to efficiency of DSSCs. Therefore, the TiCl4/Al2O3 treatment of a less defective morphology effect significantly improved electron transport.Figure 5
Electrochemical impedance spectroscopy (EIS) Nyquist plots of TiCl4/Al2O3 posttreatment on TiO2 nanotubes film with different concentrations.Figure6 showsI-V curve of Al2O3 posttreatment of TiCl4 treatment TNTs film with different concentration in 40 to 160 mM and the photovoltaic properties of TiCl4/Al2O3 posttreatment TNTs film are summarized in Table 2. From Table 2, DSSCs with TiCl4/Al2O3 posttreatment on TNTs film presented increased VOC from 0.61 to 0.67 V. In reported studies, Al2O3 influenced a higher electron lifetime and that contributed to increasing of VOC because Al2O3 affects to suppress the charge recombination between the TiO2/TNTs/electrolyte interfaces [29]. It was found that Al2O3 120 mM posttreatment indicated VOC (0.67 V) and η (8.65%). Therefore, the optimum condition for high conversion efficiency was the treatment of Al2O3 120 mM.Table 2
The integral photocurrent density (JSC), open-circuit voltage (VOC), fill factor (FF), and efficiency (η) of dye-sensitized solar cells fabricated using TiCl4/Al2O3 posttreatment on TiO2 nanotubes film.
Sample
V
OC (V)
J
SC (A/cm2)
FF (%)
η (%)
Bare TiCl4 90 mM
0.61
20.72
63.90
8.08
TiCl4/Al2O3 40 mM
0.65
20.60
60.02
8.11
TiCl4/Al2O3 80 mM
0.66
20.68
60.80
8.40
TiCl4/Al2O3 120 mM
0.67
20.93
61.54
8.65
TiCl4/Al2O3 160 mM
0.67
21.10
55.49
7.93Figure 6
I-V curve of TiCl4/Al2O3 posttreatment on TiO2 nanotubes film with different concentrations.


## 4. Conclusions

To improve the efficiency of DSSCs, we took a two-step treatment in the TiO2 nanotubes film. In the first step, an aqueous solution of titanium tetrachloride (TiCl4) was treated on TNTs film for different time from 30 to 120 min. The highest efficiency of DSSCs using TiCl4 90 mM treatment for 90 min reached 8.08% and JSC of that condition was 20.72 mA/cm2. In the second step, an aqueous solution of aluminum oxide (Al2O3) was treated on TiCl4 treatment TNTs film with different concentration from 40 to 160 mM. Successfully, performance of DSSCs was improved that VOC was increased to 0.67 V and efficiency was 8.64%. The optimum condition for high conversion efficiency was the treatment of Al2O3 120 mM. Al2O3 influenced a higher electron lifetime due to the increased interfacial charge-transfer resistance.In conclusion, using the TNTs and TiCl4/Al2O3 posttreatment process was found to be an effective method to improve the efficiency of TiO2 nanoparticle based DSSCs.



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