Fabrication and photoelectrochemical properties of TiO2 films on Ti substrate for flexible dye-sensitized solar cells

Electrochimica Acta 55 (2010) 5239–5244

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Fabrication and photoelectrochemical properties of TiO2 films on Ti substrate for flexible dye-sensitized solar cells Ke Fan a , Tianyou Peng a,b,∗ , Bo Chai a , Junnian Chen a , Ke Dai a a b

College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, PR China State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 6 January 2010 Received in revised form 13 April 2010 Accepted 14 April 2010 Available online 20 April 2010 Keywords: Dye-sensitized solar cell Ti foil Passivated layer Flexible electrode Efficiency

a b s t r a c t The dye-sensitized solar cells (DSSCs) using Ti foil supporting substrate for fabricating nanocrystalline TiO2 flexible film electrodes were developed, intending to improve the photoelectrochemical properties of flexible substrate-based DSSCs. The obtained cells were characterized by electrochemical impedance spectra (EIS), open circuit voltage decay (OCVD) measurement and Tafel plots. The experimental results indicate that the most important advantage of a Ti foil-based TiO2 flexible electrode over a FTO glass-based electrode lies in its reduced sheet resistance, electron traps, and the retarded back reaction of electrons with tri-iodine ions in DSSCs. All above characteristics for the Ti substrate TiO2 films are beneficial for decreasing the charge recombination in the TiO2 electrode and prolonging the electron lifetimes for the DSSCs, as well as improvement of the overall solar conversion efficiency. The photocurrent of the cell fabricated with the Ti foil-based flexible electrode increased significantly, leading to a much higher overall solar conversion efficiency of 5.45% at 100 mW/cm2 than the cell made with FTO glass-based TiO2 electrodes. Above results demonstrate that Ti foil is a potential alternative to the conventional FTO glass substrate for the DSSCs. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Dye sensitized solar cells (DSSCs) are expected as the substitutes to conventional solid-state silicon solar cells because of their high light-to-electricity conversion efficiency (above 11%), low-cost and eco-friendly production [1–3]. Generally, transparent conducting glass is used as the substrate to support TiO2 film, and a hightemperature sintering process at 450–500 ◦ C is necessary for the film electrode of the DSSCs in order to remove the organic additives in the TiO2 colloidal suspensions, and to improve the interconnection between TiO2 nanoparticles and the substrate. However, conducting glass-based DSSCs have an inherent disadvantage of unsuitable for more extensive applications such as cellular phones, ID cards, or watches; Therefore, replacing the glass substrate with plastic or metal foil materials makes possible the fabrication of lightweight, thin, and low-cost flexible electrodes through roll-toroll mass production. Most of the flexible substrates are conductive polymer substrates, such as ITO coated polyethylene terephthalate (PET), polyester and polyethylene naphthalate (PEN) [4–7]. Therefore, many efforts have been made to develop preparation methods

∗ Corresponding author at: College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, PR China. Tel.: +86 27 8721 8474; fax: +86 27 6875 4067. E-mail address: [email protected] (T. Peng). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.04.051

to improve the performance of TiO2 films on the conductive plastic substrates via low-temperature sintering below 150 ◦ C. These methods include hydrothermal crystallization [8,9], electrophoretic deposition [10,11], compression [12], chemical vapor deposition with UV irradiation [5] and lift-off/transfer [13]. However, the low temperature sintering process does not make high-quality interconnections between the TiO2 nanoparticles, and the organic residue in the TiO2 electrode increases the resistance and the recombination sites with low dye coverage. These problems usually cause lower electron diffusion and shorten electron lifetimes. Compared with the conducting glass, the efficiencies of the plastic-based DSSCs irrespective of fabrication method are low because of the poor sintering of TiO2 particles arising from lowtemperature heat treatment below 150 ◦ C. For example, to our knowledge, the highest efficiency of flexible plastic-based DSSC under 1 sun illumination is 7.4% by using compression method [14]. However, this result is still much lower than 11% efficiency of the glass-based DSSC. To solve the low-temperature problems caused by plastic substrates, some metal foils as flexible substrates have attracted considerable attention. Unlike only low-temperature sintering process can be applied to conventional plastic substrates, some metal substrates such as Fe, Zn and Ti showed no limit for the sintering process [15–22]. Replacing the conductive plastic substrates by metal substrates usually resulted in improvement of the interconnection between TiO2 particles and substrate adhesion due

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to their high-temperature sinterability. Therefore, different metal substrates have been used to fabricate the photoanodes and/or counter electrodes. For example, Kang et al. used stainless steel foils as the photoelectrode substrate and obtained 4.2% energy conversion efficiencies after coating the TiO2 film on both ITOand SiOx -sputtered stainless steel [15]. The electrical contact area between TiO2 particles and stainless steel substrates of the DSSCs was enhanced by roughing electrochemically the stainless steel foil, and then improved the efficiency to 5.7% [16]. The stability of stainless steel and nickel foils as well as several types of conducting plastics in iodine electrolyte containing ionic liquid (1,2-dimethyl-3-propylimidazolium iodide) were also investigated [17,18]. Nanocrystalline TiO2 films have also been fabricated on the flexible Ti foil by electrophoretic deposition followed by chemical treatment and a conversion efficiency of 6.33% was achieved [19]. A flexible dye-sensitized solar cell with 7.2% efficiency has also been fabricated by using Ti foil supporting TiO2 photoanode and electrodeposited Pt counter electrode on ITO/polyethylene naphthalate (ITO/PEN) [20]. Zn and some industrial sheet metals used as supporting substrate for fabricating nanocrystalline TiO2 flexible electrodes for DSSCs have also been reported [21]. The latest maximum reported performance of DSSCs based on metal flexible sheets was 8.6% [22], which is still much lower than that of DSSCs based on FTO glass. Although using metal flexible sheets as substrates for DSSCs can overcome the problems related to low sintering temperatures compulsory for plastic substrates, flexible DSSCs with metal substrates have one major drawback: back illumination. All DSSCs with metal substrates should be illuminated through the electrolyte solution side (back illumination) due to the non-penetration of light through the metal. The major loss of this system stems from the light scattering due to the Pt layer and the light absorption due to the electrolyte. These drawbacks might lower the performance of DSSCs with metal substrates. Nevertheless, metal substrates are still promising for the flexible DSSCs due to their high-temperature sinterability. Although the sheet resistance of Ti foil is higher than that of general metals such as silver and copper, its conductivity is higher and cost is cheaper than the FTO or ITO conducting glass substrate. Furthermore, Ti foil has a steady property to resist the corrosion of I− /I3 − electrolyte for DSSCs due to the existence of passivated film in comparison with most of other metals. It is expected that the application of Ti substrate as support of the nano-sized TiO2 in DSSC can not only reduce the cost of DSSC, but also improve the performance of the cell on account of its low internal resistance. Herein, we demonstrate a highly efficient DSSC fabricated by using Ti foil supporting nanocrystalline TiO2 flexible film electrodes, taking advantage of its high temperature sinterability. The differences between the photovoltaic characteristics of DSSCs based on Ti foil and FTO glass substrates were also discussed.

2. Experimental 2.1. Preparation of porous nanocrystalline TiO2 film Titanium foil (0.1 mm thickness, 6T-10, Trinity, USA) substrate was sonicated in the distilled water, acetone and isopropanol mixture (1:1:1 volume ratio) for 15 min. FTO glass (15–20 /, Qinhuangdao, China) substrate was cleaned in detergent solution, sonicated in acetone for 15 min, and then dipped in isopropanol for 30 min successively. All the substrates were rinsed with distilled water and alcohol three times, and then dried under air flow. TiO2 paste was prepared as follows: 1.4 g of TiO2 nanoparticles (P25, Degussa, Germany), 3.4 mL of ethanol and 2.0 g of polyethylene glycol (PEG, MW 400) were mixed, and then ballmilled for 72 h to yield a homogeneous paste. The prepared TiO2

paste was dropped onto the substrates (pretreated Ti foil or FTO glass) with adhesive tape (Scotch, approximately 50 ␮m) served as spacers, and then spread on the substrates by doctor blading technique. After deposition, the obtained films were dried at room temperature and then annealed at 500 ◦ C for 1 h. After cooling to 80 ◦ C, the sintered TiO2 electrodes were soaked in a 0.3 mM of cis-bis(isothiocyanato)bis(2,2 -bipyridyl-4,4 dicarboxylato)ruthenium(II) bis(tetrabutyl-ammonium) (known as N719, Solaronix) in ethanol solution for more than 10 h. Transparent Pt counter electrodes were prepared as follows: a few drops of 3 mM hydrogen hexachloroplatinate (IV) hydratein isopropanol solution were spread on FTO conducting glass, followed by heating at 450 ◦ C for 30 min according to the previous literatures [23–25]. 2.2. Fabrication of the dye-sensitized solar cell The dye-sensitized TiO2 electrode and transparent Pt counter electrode were assembled into a sandwich type cell. The electrolyte, which was composed of 0.5 M lithium iodide, 0.05 M iodide and 0.1 M 4-tert-butylpyridine in 1:1 acetonitrile–propylene carbonate, was injected into the inter-spaces between the photoanode and the counter electrode. To investigate the effects of heat treatment on the electrochemical properties of film electrodes and its substrates, various thin-layer cells were constructed by using Ti substrate (with or without loading TiO2 film), I3 − /I− electrolyte and thermally deposited transparent Pt electrode, in which the photoanode could be the clean unfired, fired Ti substrate (in air at 500 ◦ C for 1 h) and Ti substrate coated with nanocrystalline TiO2 without dyesensitization. For comparison, a similar cell was also fabricated with FTO glass-based TiO2 electrode instead of above Ti foil-based electrode. 2.3. Photoelectrochemical properties test The electrochemical impedance spectra (EIS) measurements were carried out by applying bias of the open circuit voltage (Voc ) under the conditions without electric current and recorded over a frequency range of 0.05 Hz to 100 kHz with ac amplitude of 10 mV. For the photoinduced open circuit voltage decay (OCVD) measurements, the illumination was turned off using a shutter after the cell was first illuminated to a steady voltage, and then the OCVD curve was recorded. The above two measurements were carried out on a CHI-604C electrochemical analyzer (CH Instruments) combined with Xe lamp as the light source. Tafel plots were obtained under dark condition by applying bias from −0.7 V to 0.7 V with scan rate of 10 mV/s. The DSSC was illuminated by light with energy of a 100 mW cm−2 from 300 W AM1.5G simulated sunlight (2 × 2 beam, w/6258 lamp, Newport, USA). The light intensity was determined using a SRC-1000-TC-QZ-N Reference monocrystalline silicon cell system (Oriel, USA), which was calibrated by National Renewable Energy Laboratory, A2LA accreditation certificate 2236.01. The present Ti foil-based DSSC requires illumination of the dye-sensitized TiO2 film through the counter electrode (back illumination) [19]. A computer-controlled Keithley 2400 source meter was employed to collect the current–voltage (I–V) curves. The active areas of all the DSSCs were 5 mm × 5 mm. 3. Results and discussion 3.1. Electrochemical impedance spectra analyses The electrochemical impedance spectra (Nyquist and Bode phase plots) of DSSCs based on Ti foil and FTO glass supported

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As can be seen from Table 1, RCE shows no obvious change due to the same composite of electrolyte and counter electrode used in both of the DSSCs. Whereas the serial resistances (Rs , 4.22  cm2 ) of the Ti foil-based DSSC is significantly lower than that (10.86  cm2 ) of the FTO glass-based cell, indicating that the conductivity of the Ti foil TiO2 film is much higher than that of the FTO glass-based TiO2 film. This result is consistent with the previous observation [30]. In which, the sheet resistance of metal substrate seems to be constant after annealing at 500 ◦ C, but the FTO have a low ability to withstand the heat treatment during the annealing processes, leading to an increase in the sheet resistance [30]. Moreover, after annealing at 500 ◦ C, the passivated TiO2 thin layer, which was resulted from the oxidation by the heat treatment, was coated on the surface of Ti substrate and acted as a blocking layer on Ti foil for preventing the electron recombination. Therefore, extra blocking layer is no longer need for the present Ti foil, and this is unlike the stainless steel substrate, in which SiOx layer are coated for preventing the photocurrent leakage from stainless steel to electrolyte as described in the TiO2 /ITO/SiOx /StSt cell [15]. 3.2. Open circuit voltage decay of DSSCs

Fig. 1. Nyquist (A) and Bode phase (B) plots of DSSCs based on Ti substrate and FTO glass substrate TiO2 film electrodes.

TiO2 electrodes were shown in Fig. 1. As can be seen in Fig. 1A, the observed three semicircular arcs in both EIS spectra represent the existences of Pt/electrolyte interface (the left arc), the TiO2 /dye/electrolyte interface (the middle arc) and the Nernst diffusion in the electrolyte (the right arc) [26]. A simple equivalent electric circuit has been used to fit above EIS data to explain the transportation and recombination of the injected electrons in the nanocrystalline film [27]. According to the inserted equivalent electric circuit in Fig. 1, the fitted RCE (charge-transfer resistance at the interface of Pt counter-electrode/electrolyte), Rct (charge-transfer resistance of the charge recombination process between the electrons in the porous film and the I3 − species in the electrolyte) and Rs (the series resistance accounting for the transport resistance of the substrate) values [28] for the TiO2 electrodes with different substrates are shown in Table 1. According to the Bode phase plots in Fig. 1B, the electron lifetime ( n ) in the TiO2 film can be extracted from Eq. (1): n =

1 2f

(1)

where f is the characteristic frequency, corresponding to the peak in intermediate-frequency regime [29]. The obtained  n values are also shown in Table 1.

Table 1 Estimated resistances from photoelectrochemical measurement using an equivalent circuit model shown in Fig. 1. Substrate

Rs ( cm2 )

RCE ( cm2 )

Rct ( cm2 )

f (Hz)

 n (ms)

FTO glass Ti foil

10.86 4.22

8.09 8.58

6.32 8.78

37.56 25.68

4.24 6.20

The open-circuit voltage decay (OCVD) curves can show the main information of the interfacial recombination processes between the photoinjected electrons in the TiO2 electrode and electrolyte [31]. Under the present open-circuit and dark state conditions, the electron transport resistance in the TiO2 film does not affect the OCVD measurements because there is no current flowing through the cell; and the electron lifetime (n ) in DSSCs can change with the cell’s open-circuit voltage (Voc ) due to the shift of semiconductor’s Fermi level [31]. Therefore, the effects of the electron traps on the recombination reaction can be qualitatively explained by analyzing the shapes of n ∼Voc relation curves. The electron lifetime (n ) can be derived from the OCVD measurements according to Eq. (2): n = −

kB T e

 dV −1 oc

dt

(2)

where kB is the Boltzmann constant, T is the temperature, e is the electron charge. The obtained OCVD curves and n ∼Voc relation curves of DSSCs based on Ti foil and FTO glass substrate were shown in Fig. 2. As can be seen from Fig. 2A, both DSSCs show similar decay trends of Voc in the high voltage region, while the Ti foil-based DSSC has slower Voc decay trend in comparison with the FTO glass-based DSSC in the middle and low Voc regions. It implies that there are fewer charge recombination sites in the Ti foil-based TiO2 film in comparison with the FTO glass-based electrode. Fig. 2B shows the n ∼Voc relation curves for the DSSCs based on Ti foil- and FTO glass-based TiO2 film. In the high Voc region (>0.5 V), the n ∼Voc curves of the two kinds of DSSCs show similar behavior. According to Zaban’s suggestion [31,32], one can observe the lifetime of the free electrons in the semiconductor conduction band without interference from the trap effects in this regime, and therefore the change of the substrate has no significant influence in this region. In the middle Voc region (0.2–0.5 V), the electron lifetime dependence on the Fermi level is governed entirely by trapping and detrapping processes through bulk traps [31,33]. As can be seen from Fig. 2B, the electron lifetime increased linearly (in the loglinear representation) with decreasing Voc , and the longer electron lifetime for the Ti foil-based DSSC in the middle Voc region indicates that Ti foil-based TiO2 film can reduce effectively the bulk trapsin comparison with the FTO glass-based cell. In the lower Voc region (<0.2 V), the linear dependence turned into a curved one because the charge transfer process is mainly governed by the distribution of surface traps [32]. Similarly, the longer electron lifetimes of the

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Fig. 3. Tafel plots for cells constructed with bare Ti substrate (a), fired Ti substrate (b), Ti substrate with nanocrystalline TiO2 (c), and FTO glass substrate with nanocrystalline TiO2 (d). The electrolyte is composed of 0.5 M lithium iodide, 0.05 M iodide and 0.1 M 4-tert-butylpyridine in 1:1 acetonitrile–propylene carbonate.

reaction is given by the Butler–Volmer equation [34–36]:



jsub = j0 exp

Fig. 2. The open circuit voltage decay (A) and n ∼Voc (B) curves of DSSCs based on Ti substrate and FTO glass substrates.

Ti-foil based DSSC predicates less surface state trapping in comparison with the FTO glass-based one. Therefore, the present OCVD curves indicate the Ti substrate can efficiently retard the bulk and surface trapping, and then prolong the electron lifetimes. Moreover, the longer electron lifetime ( n , 6.20 ms) for the Ti foil-based DSSC in comparison with that (4.24 ms) of the FTO glass-based cell as shown in Table 1, is consistent with the observations from the present OCVD measurement despite the two kinds of lifetime are independently derived from EIS (under light illumination) and OCVD (under open-circuit and dark state condition) measurements. Although it is reasonable to think that the longer electron lifetimes might simply arise from the reduced recombination from the substrate due to the formed TiO2 passivated layer in the Ti foilbased cells, its larger Rct (8.78  cm2 ) in comparison with the FTO glass-based cell (6.32  cm2 ) as shown in Table 1 can be considered as the passivation signature of the bulk and surface states. Namely, the larger Rct for the present Ti foil-based DSSC in comparison with the FTO glass-based one is related to the lower recombination rates and longer electron lifetimes due to the passivation of the bulk and surface states in the interface of TiO2 /Ti foil. Whether the reduced recombination from substrate or the passivation of the bulk and surface states for the present Ti foil-based cell mainly contributes to the longer electron lifetime will be further discussed in the following Tafel plots analyses. 3.3. Tafel plots analyses In an electrode process, the rate of electron transfer to and from a conducting substrate generally is dependent on the potential. If there is no blocking layer (the substrate always behaves essentially as a non-catalytic metal) and in the absence of diffusion limitations, the total current density for a simple one electron-transfer redox

 −˛nF  RT

− exp

 (1 − ˛)nF  RT

(3)

Here, j0 is the exchange current density,  corresponds to the overpotential, ˛ the cathodic transfer coefficient, and n the number of electrons transferred. Back reaction via the substrate is important for the thin nanocrystalline TiO2 films. To further understand the effect of different substrates on the electron transfer in the TiO2 film, Tafel plots (Fig. 3) were used to analyze the electron transfer behaviors via the substrate. The right-hand branch of the Tafel plots should correspond to the oxidation of I− at the FTO or Ti foil interface, and the left-hand branch corresponds to the reduction of I3 − by electrons, I3 − + 2e− → 3I− , which may be an important back reaction that causes the electron losses. Generally, a higher current density in the left-hand branch represents larger electron losses. As can be seen from Fig. 3, the thin layer cells constructed with different Ti substrates performed different Tafel plots. Comparing with the unfired Ti foil-based thin layer cells, the left-hand branch of the Tafel plot for the fired Ti foil-based one clearly shows an obvious decrease in the current density, indicating the decreased back reaction via the substrate after heat treatment at 500 ◦ C. This may be due to the passivated TiO2 layer on the surface of Ti substrate, which can suppress the back electron transfer from Ti substrate to electrolyte. Moreover, the reduction current density in the left-hand branch of the Tafel plot for the Ti substrate with nanocrystalline TiO2 film is further decreased by orders of magnitude as shown in Fig. 3. This phenomenon differs from the experiment results reported previously [37,38], where the reduction current density of the FTO electrode coated with nanocrystalline TiO2 layer shown no obvious difference comparing with that of fired FTO electrode without TiO2 loading. Although the passivated TiO2 layer can suppress the back electron transport as discussed above, it is unlikely just to ascribe above marked decrease in the reduction current density for the Ti foil-based TiO2 film to the formed passivated TiO2 layer in the interface of the TiO2 /Ti foil because the passivated layer existed in both fired Ti substrate and the Ti foil-based TiO2 film. Therefore, some other reasons should be expected to explain above marked decrease in the reduction current density. As shown in Fig. 3c and d, the current density from reduction of I3 − electrolyte in the Ti foil-based DSSC decreased 2 orders of magnitude approximately in comparison with the FTO glass-based cell. It implies much less photoelectron losses for the Ti foil-based cell. Typically, the back reaction of electrons mainly involves three paths as shown in Scheme 1A. The electron transfers to the redox

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Fig. 4. Dark current–voltage curves of DSSCs based on Ti foil and FTO glass substrates.

Scheme 1. Schematic paths of back electron transfer in DSSC based on ordinary substrate (A) and Ti substrate with passivated layer (B).

Fig. 5. I–V curves of DSSCs based on Ti foil and FTO glass substrate photoanodes under the back illumination.

species in the electrolyte may be from: (1) conducting substrate; (2) the conduction band of nanocrystalline TiO2 ; and (3) the surface states of nanocrystalline TiO2 . Usually, the path (1) is the dominant pathway of the back electron transfer for the FTO glass substrate, rather than from the nanocrystalline TiO2 electrode [37,38]. Therefore, the reduction current density of the FTO glass-based TiO2 electrode shows no obvious difference comparing with that of the fired FTO electrode without TiO2 loading [37]. Whereas for the Ti foil-based DSSC, the back reaction path (1) is blocked due to the passivated layer, and the back reaction prefers the paths (2) and (3) from substrate to electrolyte via nanocrystalline TiO2 (Scheme 1B). At this situation, the bulk and surface state traps in the TiO2 film will become main affecting factors of the back electron reaction. By considering that it is unlikely just to ascribe the marked decrease in the current density for the Ti foil-based TiO2 film to the formed passivated TiO2 layer in the TiO2 /Ti foil interface because the passivated layer existed in both the fired Ti substrate and the Ti foil-based TiO2 film as mentioned above, the effective passivation of bulk and surface states in TiO2 /Ti foil interface (as discussed in OCVD curve analyses) might also contribute to the suppression of the back electron reaction in addition that the passivated layer can reduce the current density due to the blocking the electron transfer via path (1). Therefore, it is reasonable to think that the effective passivation of bulk and surface states and blocking of the passivated layer for the Ti foil-based TiO2 electrode can contribute the decreased current losses for the Ti foil-based DSSC in comparison with the FTO glass-based one. Moreover, those also lead to the lower dark current of the Ti-foil based cell in comparison with the FTO glass-based cell as shown in Fig. 4.

On the bases of above discussions, the electrochemical impedance spectra, the OCVD curve and Tafel plots showed that Ti foil-based DSSCs can effectively reduce the sheet resistance, retard the carrier recombination, lower the back transfer and prolong injected electron lifetimes in comparison with the FTO glass-based DSSCs. All above characteristics for the Ti foil supporting TiO2 film electrodes are beneficial for the improvement of cell’s energy conversion, which will be further discussed in the following section. 3.4. Photoelectrochemical properties of DSSCs made with different substrates Fig. 5 shows the I–V curves of DSSCs made with Ti foil and FTO glass substrates under the back illumination methods and the corresponding operation parameters are summarized in Table 2. Comparing with the photovoltaic characteristics of FTO glass-based DSSC, Ti foil-based cells show lower open circuit voltage (Voc ) and fill factor (FF), but a higher short circuit current density (Jsc ) and global conversion efficiency (). The relatively lower Voc for the Ti foil-based cell should be attributed to higher I3 − concentration because the Voc value depends logarithmically on the inverse concentration of I3 − [28]. As Table 2 Performance characteristics of DSSCs based on different substrate photoanodes. Substrate

Voc (V)

Jsc (mA/cm2 )

FF

 (%)

FTO glass Ti foil

0.77 0.72

9.88 15.13

0.53 0.50

4.02 5.45

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discussed above, the reduction of I3 − by electrons (I3 − + 2e− → 3I− ) in the present Ti foil-based cell can be effectively suppressed due to the formation of passivated layer, and this causes relatively higher I3 − concentration in comparison with the FTO glass-based cell. Although both of DSSCs were illuminated from the same back side, the lower sheet resistance for the Ti foil-based cell can lead to higher short circuit current and better overall efficiency as shown in Table 2. For example, the photocurrent density of the cell increased significantly from 9.88 to 15.13 mA/cm2 when replacing the FTO glass-based TiO2 film with the Ti foil-based flexible electrode, it leads to a much higher overall conversion efficiency of 5.45% for the FTO glass-based cell at 100 mW/cm2 in comparison with that (4.02%) of the FTO glass-based cell. On the bases of above discussion, it could be concluded that the most important advantage of the present Ti foil-based TiO2 flexible electrode over the FTO glass-based electrode lies in its reduced sheet resistance, electron traps, and the retarded back reaction of electrons with tri-iodine ions in DSSCs. All above characteristics for the Ti substrate TiO2 films are beneficial for decreasing the charge recombination in the TiO2 electrode and prolonging the electron lifetimes for the DSSCs, as well as improvement of the overall solar conversion efficiency. What is more, Onoda et al. [30] found the DSSC based on Ti substrate TiO2 film showed the higher incident monochromatic photo-to-current conversion efficiency (IPCE) in red region due to the light reflecting on the Ti substrate. A part of lost incident light due to the back illumination was compensated since the light was reflected, and this might be another reason of the better performance for the present Ti foil-based DSSC. Anyway, our experiment facts on the improved efficiency for the Ti foil-based cell in comparison with the FTO glass-based one demonstrates that Ti foil is a potential alternative to the conventional FTO glass substrate for producing the large and low cost flexible DSSCs. What should be paid attention to is the relatively lower Voc and FF for the present Ti foil-based DSSC, this may be one reason that the present efficiency is still lower than the latest maximum reported performance 8.6% of DSSCs based on metal flexible sheets [22]. It should be noted that the present performance is obtained without optimizing the DSSC’s fabrication conditions. To further improve the performance of the present Ti foil-based cell, the TiO2 paste compositions and electrolyte as well as the film’s preparation conditions must be investigated, which is under progress. 4. Conclusions

and Ti foil as the substrate instead of FTO glass could be an efficient way to improve the performance of the cell. Acknowledgements This work was supported by the Natural Science Foundation of China (20871096, 20973128), Program for New Century Excellent Talents in University (NCET-07-0637), and the National High-Tech Research and Development Program (2006AA03Z344) of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

Nanocrystalline TiO2 electrodes prepared on FTO glass and Ti foil substrate were investigated comparatively. Under back illumination, the DSSC based on Ti substrate showed better performance due to its lower sheet resistance. The overall efficiency of DSSC based on Ti foil-based TiO2 electrode are attained 5.45% with open circuit voltage 0.72 V, short circuit current density 15.13 mA/cm2 , and fill factor 0.50, which is improved by 36% in comparison with DSSC based on FTO glass-based electrode. Furthermore, the electrochemical impedance spectra, open circuit voltage decay curves and Tafel plots displayed smaller sheet resistance, longer carrier lifetimes, lower back electron transfer and fewer recombination sites in the Ti foil based DSSC in comparison with the of FTO glassbased cell. The present results indicate the superiority of titanium as the substrate for producing the large and low cost flexible DSSCs,

[28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]

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