Influence of Anodization Time on Photovoltaic Performance of DSSCs Based on TiO 2 Nanotube Array

Highly ordered TiO 2 nanotube arrays (TNT arrays) were fabricated by two-step anodization process. In order to further improve the performance of DSSCs, TNT arrays were optimized by changing the anodization conditions to meet the requirements of highperformance photoanode. The photoelectric conversion properties of DSSCs based on P25/TNT arrays double-layer film with different anodization time were investigated and compared. The results show that the conversion efficiency of 4.20% was achieved in double-layer photoanode at 18 h, with an open-circuit voltage (Voc) of 0.65V and short-circuit current density (Jsc) of 9.98mA cm.


Introduction
At present, energy problem becomes one of the major problems that human being must confront.Solar energy is abundant and pollution-free and is not limited by the geographical environment and gradually becomes the main research object to solve the energy problem.In all solar cells, dye-sensitized solar cells (DSSCs) have attracted extensive attentions from the researchers all over the world due to their low production cost, simple preparation process, high conversion efficiency, and environment friendly properties [1][2][3].
Different from the traditional semiconductor solar cell, DSSCs are a new type of special structure, which consist of dye-sensitized nanocrystalline TiO 2 photoanode, redox electrolyte, and counter electrode [4][5][6][7].TiO 2 photoanode is an important part of DSSCs and is closely related to light harvesting and electron injection and collection [8].The dye molecules were adsorbed on the surface of nanocrystalline TiO 2 , and the energy band structure of nanocrystalline TiO 2 is matched with dye molecule, so the electrons can be injected into the TiO 2 conduction band quickly and efficiently [9][10][11][12].In order to obtain high efficiency DSSCs, the preparation of TiO 2 photoanode with excellent performance is essential.
So far, the research of nanocrystalline TiO 2 photoanode is mainly focused on the design of the morphology of TiO 2 [13][14][15][16].A variety of one-dimensional (1D) TiO 2 structures with different micromorphology were prepared and applied to DSSCs, such as TiO 2 nanorods [17], TiO 2 nanowires [18], and TiO 2 nanotubes [19].Compared with other TiO 2 structures, TiO 2 nanotube arrays (TNT arrays) have the characteristics of the vertical hollow structure, which greatly promote the electronic transmission in photoanode [20,21].In 2001, Gong et al. first synthesized TNT arrays by anodic oxidation reactions, and well aligned and organized nanotube arrays were obtained [22].Lei et al. reported that the photovoltaic performance of DSSCs based on TNT arrays on FTO glass reached 8.07%, which is higher than that of a TiO 2 nanoparticle electrode [23].These reports all indicate that the fabrication of TNT arrays is a new potential approach for increasing the power conversion efficiency of DSSCs.In the study of TNT arrays, a variety of preparation methods are developed, such as hydrothermal method [24,25], sol-gel method [26,27], and electrochemical anodization [28][29][30].Among all the methods for preparation of TNT arrays, the electrochemical anodization has the advantages of low cost, simple equipment, and easy process [31,32].The preparation of TNT arrays by electrochemical anodization method is a typical two-electrode system; the length, diameter, and wall thickness of nanotubes are closely related to anodization voltage and time.In addition, reports have shown that DSSCs with composite photoanode showed the best conversion efficiency.Zheng et al. reported that a combined structure with a mixture of TiO 2 nanotubes and nanoparticles was fabricated, and a high efficiency of 5.75% was achieved [33].
Accordingly, the synthesis of the composite photoanodes which were composed of various TiO 2 structures is an effective way to enhance the efficiency of DSSCs.
In this work, we fabricated a TiO 2 double-layer photoanode consisting of TNT arrays and TiO 2 nanoparticle (P25) for application in dye-sensitized solar cells.The crystallized TNT arrays films were prepared by using a two-step anodization technique.And TNT arrays film as overlayer can promote the transmission of electrons and reduces the recombination of electrons and holes.P25 has excellent capacity of dye adsorption and offers good contact between TNT arrays and FTO glass.In order to study the impact of TNT arrays with different morphologies on the performance of DSSCs, the preparation conditions of electrochemical anodization method were optimized.As a result, the photoelectronic conversion efficiency (PCE) of 4.20% was achieved in doublelayer photoanode at 18 h, with an open-circuit voltage ( oc ) of 0.65 V and short-circuit current density ( sc ) of 9.98 mA cm −2 .

Experiment
2.1.Preparation of the TiO 2 Nanotube Arrays.TiO 2 nanotube arrays (TNT arrays) films were prepared by anodization of Ti foils (0.25 mm thickness, 99.7% purity) in an NH 4 F/ethylene glycol electrolyte solution by using a DC power source.There are a lot of impurities on the surface of titanium, so the Ti foils were first degreased by chemical polishing solution before anodization.The chemical polishing solution was composed of deionized water, hydrogen nitrate, and hydrofluoric acid in volume ratio of 5 : 4 : 1.Then, the first anodization was carried out in the ethylene glycol electrolyte solution containing 0.5 wt% NH 4 F and 3 vol% deionized water, while voltage of 50 V was applied versus a Pt counter electrode for different time (5 h, 12 h, 18 h, and 22 h).The as-anodized TNT arrays/Ti substrate was rinsed with DI water and ethanol and then annealed in an air furnace at 450 ∘ C for 30 min to form a crystalline structure.And the microstructure parameters of TNT arrays are slightly different due to the change of anodization time.Then, the as-anodized Ti foil was re anodized under the same conditions as the first anodization, and whether the TNT arrays film was separated from the Ti substrate carefully observed.After about an hour or so, the free-standing TNT arrays could be easily removed from the Ti substrate.The appearance of TiO 2 nanotube arrays in different stages of the preparation process is shown in Figure 1.

Fabrication of DSSCs Substance.
In order to prepare the P25/TNT arrays composite film of photoelectrode, firstly, the P25 nanoparticle paste was pasted onto the FTO glass by using doctor-blade technique.Then the TNT arrays films were divided into 4 mm × 4 mm thin films and transferred onto the P25 underlayer immediately.After being dried in air, the double-layer composite films (active area, 4 mm × 4 mm) were annealed at 450 ∘ C for 30 min in a furnace.
After that, the as-prepared photoelectrodes were immersed into a 0.5 mM ethanol solution of N719 at room temperature and avoided light for 24 h.After being dried naturally, the three sides of the sensitized photoanodes were pasted with scotch tape, and then the photoanode and Pt counter electrode were clamped together by a clamp.The electrolyte (0.5 M LiI, 0.05 M I 2 , 0.5 M 4-tertbutylpyridine, and 0.3 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII) in dry acetonitrile) was injected into the edge without scotch tape of cell and absorbed into the inner space of cell, and the assembled cell was tested immediately.

Characterization and
Measurements.X-ray powder diffraction (XRD) is one of the common test means in the analysis of material structure.The test instrument used in this paper is D/MAX-RB RU-200B target X-ray diffractometer produced by Rigaku.The morphology of the as-prepared samples was characterized by scanning electron microscopy (SEM, JSM-IT300, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan).The microscopic imaging of the material can be obtained by focusing the electron beam on the surface of the sample by pointby-point scanning in SEM.And the size and crystal state of the samples were observed by focusing the electron beam through the sample in TEM.The absorption spectra of the samples in a certain wavelength range can be tested by UV-vis spectrophotometer (UV-3600, Shimadzu, Japan).The photoelectric properties of the DSSCs are mainly analyzed by current-voltage (J-V) characteristics and electrochemical impedance spectra (EIS).The photocurrent-voltage (J-V) and EIS were studied by an electrochemical workstation (IM6, Germany) under AM 1.5 G solar simulator (100 mWcm −2 ) provided by a solar light simulator (Oriel Sol3A, Newport Corporation, USA).The impedance parameters were obtained by fitting the equivalent circuit of Nyquist graph using Z-view software.

Results and Discussion
3.1.Phase Structures. Figure 2 shows XRD patterns of TNT arrays before and after annealing.As exhibited in Figure 2(a), the XRD pattern of as-anodized TNT arrays only shows the peak of Ti.It suggests that the TNT arrays have an amorphous structure before annealing.After calcinations at 450 ∘ C for 30 min, the strongest diffraction peak 2 = 25.3 ∘ of TNT arrays is the characteristic diffraction peaks of anatase TiO 2 , and other diffraction peaks were indexed to the standard anatase phase of TiO 2 (JCPDS card number: 21-1272).Through the XRD analyses, we can see that the phase structure of TNT arrays changes from amorphous structure to anatase structure by annealing at 450 ∘ C. Research shows that the anatase structure of TiO 2 has better photocatalytic activity than other crystal structures [34][35][36].

Microstructure and Morphology. Figures 3(a)-3(c) show
typical SEM images of as-prepared TNT arrays film in a top view, bottom view, and cross-sectional view, respectively.It can be seen that the nanotubes opened on the top but closed on the bottom, and the distribution of nanotubes is highly ordered.The cross-sectional SEM image of doublelayer film was shown in Figure 3(d), from which the doublelayer structure of photoanode can be clearly seen.The fabrication procedure of the double-layer structure is shown in Figure 4.The bottom layer of the composite structure is P25 nanoparticles and the thickness is 9 m.As a compact layer between the TNT arrays film and the FTO glass, P25 nanoparticle underlayer will enlarge contact area between them and reduce the occurrence of dark current.Studies have shown that TiO 2 composite films combined the characteristics of different TiO 2 nanostructures and achieved the higher conversion efficiency [37,38].As the overlayer of composite film, the TNT arrays film greatly improves the photoelectric properties of DSSCs.First of all, the unique structure of TiO 2 nanotube arrays provides a good channel for electron transfer and facilitates electron transport in the TiO 2 photoelectrode.Second, the presence of channel accelerates the penetration of the dye molecules into the deep film electrode, which increases the adsorption amount of dye.Finally, the electrolyte solution has a high rate of diffusion and metastases, which facilitate the process of electronic cycle.
In order to study the influence of anodization time on the micrographs of TNT arrays, anodization experiments at different anodization time (5 h, 12 h, 18 h, and 22 h) were performed under the same external experimental conditions.The cross-sectional SEM images of double-layer film with different anodization time were shown in Figure 5.As shown in Figure 5, the length of nanotubes increased from 7.13 um to 30 m with increasing the anodization time from 5 h to 22 h.The influence of anodization time on TNT arrays is mainly reflected in the following two aspects.(1) When the anodization time is not enough, there is a part of the titanium substrate that has not yet formed a complete morphology of TiO 2 nanotubes.(2) The anodization time is the key factor to control the length of nanotubes.Therefore, it is very necessary to fabricate the high quality TNT arrays by controlling the anodization time reasonably.the photoelectric performance of DSSCs [39,40].The absorption spectra of composite films at different anodization time were shown in Figure 6.It can be seen that the absorption peak of TiO 2 photoelectrode is located in 370 nm, and there is a light-absorption edge at the wavelength of 200 nm-300 nm.

UV-Visible Absorbed
It is found that, with the increase of anodization time, the light-absorption edge of TiO 2 photoelectrode first increases and then decreases and reaches the peak at 18 h.The thickness of thin film increased with the increase of time, thereby promoting the adsorption of dye molecules.However, when the anodization time is too long, the covering on the surface of nanotubes will restrain the entry of sunlight.Therefore, the microstructure of TNT arrays with anodization time of 18 h is beneficial to the absorption of light.and  oc , which were summarized in Table 1.According to the curve in Figure 7 and the data in Table 1, the conversion efficiency () of DSSCs can reach the maximum value of 4.20% in 18 hours of anodization and reach the minimum value of 1.39% in 5 hours of anodization.The increase of the thickness of the film ensures that the photoelectrode has a  large enough surface to adsorb the dye molecules, and the photoelectric conversion efficiency of DSSCs is improved [41].However, when the thickness of the film is too thick, the transmission of electrons and the diffusion of electrolyte will slow down, which leads to the reduction of conversion efficiency.Therefore, there exists an anodization time to make the electrode in an optimum film thickness, and DSSCs have the best photoelectric performance.Hence, the photovoltaic performance of DSSCs firstly increases and then decreases with the increase of anodization time.Finally, a highest overall conversion efficiency of 4.20% is achieved for a TiO 2 composite film at 18 h, with short-circuit current density ( sc ) of 9.98 mA cm −2 , open-circuit voltage ( oc ) of 0.65 V, and fill factor (FF) of 0.65.

EIS Analysis.
In order to further study the electron transport characteristics of DSSCs based on TNT arrays film with different anodization time, the electrochemical impedance spectroscopy (EIS) was performed under onesun illumination and the resulting graphs were shown in Figure 8.As shown in Figure 8(a), two semicircles were shown in Nyquist plots at the frequency range of 10 −2 -10 5 Hz, which represent two different charge transport processes [42,43].The equivalent circuit diagram is shown in the inset of  shows the Bode phase plots of EIS spectra, which display the frequency peaks of different chargetransfer process.The electron lifetime (  ) of DSSCs at different anodization time can be calculated by using the following equation:   = 1/2 max [44], where  max is the maximum frequency of the low-frequency peak.As is shown in Table 2, the electron lifetime (  ) of DSSCs with anodization time of 18 h is longer than other films.Having a longer electron lifetime to ensure the occurrence of electronic cycle is another reason why TiO 2 composite film at 18 h has the highest conversion efficiency.

Conclusions
In conclusion, we have successfully fabricated TNT arrays films with different anodization time by two-step anodization process.And double-layer photoanodes with TiO 2 nanoparticles (P25) as the underlayer and TiO 2 nanotube arrays (TNT arrays) as overlayer film have been fabricated for application in DSSCs.The experiment results demonstrate that the length of TiO 2 nanotube arrays is closely related to anodization time and TiO 2 composite film DSSCs achieved the highest conversion efficiency of 4.20% when the anodization time is 18 h.Thus, higher-performance DSSCs were obtained by optimizing the preparation conditions.

Figure 1 :
Figure 1: The appearance of TiO 2 nanotube arrays in different stages of the preparation process: (a) Ti foil before treatment, (b) TNT arrays after the first anodization, (c) TNT arrays after second anodization, and (d) TNT arrays film eventually obtained.

Figure 2 :
Figure 2: XRD patterns of TNT arrays before and after annealing.

Figure 3 :
Figure 3: SEM images of TiO 2 nanotube arrays in (a) top view, (b) bottom view, and (c) cross-sectional view; (d) SEM image of double-layer composite film.

Figure 4 :
Figure 4: Schematic illustrating the fabrication process of P25/TNT arrays double-layer composite film.

Figure 6 :
Figure 6: Absorption spectra of TiO 2 photoelectrodes based on P25/TNT arrays double-layer films at different anodization time.

Figure 7
shows the photocurrent density-photovoltage (J-V) of DSSCs based on TNT arrays film with different anodization time under AM 1.5 illuminations.The short-circuit current density ( sc ) and open-circuit voltage ( oc ) can be obtained directly from the J-V characteristic curve.And we can further calculate the fill factor (FF) and conversion efficiency () according to  sc

Figure 7 :
Figure 7: I-V performance of the DSSCs based on P25/TNT arrays double-layer films with different anodization time under AM 1.5 illuminations.

Figure 8 :
Figure 8: EIS spectra of the DSSCs based on P25/TNT arrays double-layer films with different anodization time.(a) Nyquist plots.(b) Bode phase plots.

Figure 8 (
Figure 8(b)  shows the Bode phase plots of EIS spectra, which display the frequency peaks of different chargetransfer process.The electron lifetime (  ) of DSSCs at different anodization time can be calculated by using the following equation:   = 1/2 max[44], where  max is the maximum frequency of the low-frequency peak.As is shown in Table2, the electron lifetime (  ) of DSSCs with anodization time of 18 h is longer than other films.Having a longer electron lifetime to ensure the occurrence of electronic cycle is another reason why TiO 2 composite film at 18 h has the highest conversion efficiency.

Table 1 :
Comparisons of the photovoltaic performances of DSSCs based on P25/TNT arrays double-layer films with different anodization time.

Table 2 :
Parameters obtained by fitting the impedance spectra of DSSCs using the equivalent circuit in Figure8(a)., from which we can obtain R s ,  ct , and R w by using Z-view software.R s consisted of the contact resistance of TiO 2 /FTO and the plane resistance of FTO. ct is the chargetransfer resistance of redox electrolyte/Pt electrode interface.R w is associated with the electron transfer within TiO 2 film and the interfaces of TiO 2 /redox electrolyte.R s ,  ct , and R w data with different anodization time are listed in Table 2. Obviously, from Table 2, we can see that TiO 2 composite film with anodization time of 18 h has the lowest values for  ct and R w resistance, which are 26.63Ω and 26.45 Ω, respectively.These results indicate that the TiO 2 /electrolyte interfaces and the electrolyte/Pt electrode interfaces of composite film with anodization time of 18 h have more efficient electron transport.