Shorter nanotubes and finer nanoparticles of TiO2 for increased performance in dye-sensitized solar cells

Electrochimica Acta 63 (2012) 375–380

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Shorter nanotubes and finer nanoparticles of TiO2 for increased performance in dye-sensitized solar cells E.V.A. Premalal a , N. Dematage a , G.R.A. Kumara b , R.M.G. Rajapakse b,c,∗ , K. Murakami b , A. Konno a a b c

Graduate School of Science and Technology, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8561, Japan Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8011, Japan Department of Chemistry, University of Peradeniya, Peradeniya, Sri Lanka

a r t i c l e

i n f o

Article history: Received 15 August 2011 Received in revised form 9 November 2011 Accepted 30 December 2011 Available online 5 January 2012 Keywords: TiO2 nanotubes TiO2 nanoparticle-loading Dye-sensitized solar cells

a b s t r a c t Vertically aligned, reasonably dense, about 500 nm long TiO2 nanotubes (NTs) are prepared on transparent conducting fluorine-doped tin oxide (FTO) surfaces by a wet chemical procedure. The dye-sensitized solar cells (DSCs) fabricated using such active electrodes and N719 dye with usual I− /I3 − electrolyte yield 2.46% solar-to-electricity conversion efficiency () without the TiCl4 treatment and 3.40% with the TiCl4 treatment, both at 1000 W m−2 simulated AM 1.5 irradiation. These values are higher and impressive for shorter NT arrays of ∼500 nm length. The successive introduction of TiO2 nanoparticles (NPs) by spraying into the NTs to form NT–NP composite films results in a linear increase of dye coverage. The variation of  as a result of NT–NP composite structure (with TiCl4 treatment) shows a gradual increase up to 8.53% at 1:2.2 NT:NP mass ratio, beyond which it slowly decreases. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction The dye-sensitized nanocrystalline solar cell (DSC) invented by O’Reagan and Gràtzel [1] is a cheap and versatile alternative to silicon based solar cells and has already achieved solar-to-electricity conversion efficiency as high as 11.5% [2]. In this device, electrons moving along interconnected nanoparticles (NPs) of TiO2 in a “random walk” path undergo recombination thus lowering the efficiency [3]. The maximum diffusion length of electrons in the TiO2 matrix is ∼15 ␮m [4]. Although technologies have been developed to fabricate DSCs based on perfectly transparent TiO2 nanoparticle matrices allowing longer light absorption lengths [5–7], increasing the thickness of the DSC to absorb more light is thus not possible due to the limitations imposed by the maximum diffusion length of electrons. A way round this problem is to use 1-D nanomaterials such as nanowires, nanorods, nanobelts and nanotubes instead of nanoparticles to avoid the random walk imposed by the 3-D network structure. The electrons injected to the CB of a 1-D nanomaterial find an “expressway” as opposed to “random walk” in NP based DSCs. Jennings et al. [8] have studied the electron transport, trapping, and back transfer in titania nanotube-based cells with different lengths of nanotubes (5 ␮m, 10 ␮m and 20 ␮m) by several complementary techniques such as charge-extraction

∗ Corresponding author at: Department of Chemistry, University of Peradeniya, Peradeniya, Sri Lanka. Tel.: +94 81 239 4442; fax: +94 81 2388018. E-mail address: [email protected] (R.M.G. Rajapakse). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.127

experiments and transient current and voltage measurements and shown that the electron diffusion length is of the order of 100 ␮m for nanotube-based cells [8]. The collection efficiency for photoinjected electrons is thus shown to be close to 100%. In 1991, Zwilling et al. reported that when titanium metal is subjected to anodization in fluoride-containing electrolytes, its surfaces become porous [9]. A decade later, Grimes and coworkers first reported the formation of uniform titania nanotube (NT) arrays via anodic oxidation of Ti in an HF electrolyte [10]. Since then, numerous methods have been developed to prepare 1-D titania nanomaterials. These include the preparation by depositing into a nanoporous alumina template [11,12], sol–gel techniques [13,14], hydrothermal processes [15,16], seeded growth [17,18] and anodization of titanium metal in fluoride-based baths [10,19–23]. The DSCs have been fabricated using 1-D titania nanomaterials synthesized by all these methods and their performance has been studied under various conditions. The performance of the DSCs based on virgin nanotubes has been rather low in many cases [15,16,24–28]. Such poor performance observed in virgin NT based solar cells has been attributed to the poor dye adsorption due to lower surface area and hence lower charge injection compared to that of interconnected NPs where the surface area available for dye adsorption is high. The composite films made of mixtures of NPs and NTs have shown better performance compared to either pristine NT or pristine NP-based DSCs as a result of synergistic effects of advantages associated with NT and NP systems [29,30]. In many cases, the superior performance over pristine NT or pristine NP systems has been reported when a composite is used though the highest  reported hardly exceeded 6.5%. In light of these observations, we

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have investigated the DSC performance of TiO2 NT based DSCs with and without TiCl4 (aq) treatment and as a function of amount of NP in a fixed amount of NT arrays formed on FTO electrodes. We also compared the performance of NT and NP based DSCs with those of NP-based solar cells having approximately same total TiO2 mass and have shown that much improved efficiencies could be obtained with much shorter NTs. In this publication, we reveal, in addition to solar cell characteristics of NT and NT–NP based DSCs, the effect of TiCl4 treatment. 2. Experimental All chemicals used in this study were of high purity which were purchased from Wako Chemicals (Japan) and were used without further purification unless otherwise stated. I–V characteristics of the solar cells were measured using JASCO, CEP-25BX set up and the XRD, FT-IR, SEM and EDS characterizations were done using Rigaku Miniflex X-ray Diffractometer, JASCO FT-IR-460 plus and JSM-6320F Scanning Electron Microscope respectively. 2.1. Preparation of TiO2 NTs The TiO2 NTs used in this study were prepared by the replacement of seed-assisted grown sacrificial ZnO nanorods by TiO2 NTs using a literature method that was introduced by Rattanavoravipa et al. [31]. The method was modified to enable the growth of sufficiently long NTs. In brief, pieces of 1 cm × 2 cm FTO plates (sheet resistance 10  cm−2 ) were cleaned thoroughly by first wiping with ethanol-wetted tissue papers followed by ulta-sonication for 20 min each in isopropanol and acetone to remove any grease layers adherent on the surface. The plates were finally washed with ethanol and were immersed in an ethanolic solution of zinc acetate (0.005 mol dm−3 ) for 10 min. The plates were then subjected to the heat-treatment programme at 130 ◦ C for 2 h, 180 ◦ C for 1 h and at 280 ◦ C for 2 h. The transition from one temperature to other was effected at 3 ◦ C min−1 in a programmable furnace (KDF 007). The plates were then allowed to cool down to RT in the furnace and then immersed in a hot aqueous solution which was prepared by mixing 50 mL each of 0.04 mol dm−3 zinc nitrate and 0.8 mol dm−3 NaOH for varying time periods. With time, the white deposit formed on the FTO surface was found to thicken and the required time period was chosen so as to have the required thickness in subsequent preparations after studying the film thickness versus time of deposition. The plates were then removed, washed by dipping in DW and dried using hot air stream. These plates were then placed in an aqueous solution containing 0.05 mol dm−3 ammonium hexafluorotitanate and 0.15 mol dm−3 boric acid at RT for 3 h. The dissolution of white ZnO deposit followed by the formation of another white deposit was clearly visible during the reaction period. When the deposit was completed the plates were removed and washed in distilled water and allowed to dry under ambient laboratory conditions. These dried plates were sintered at 500 ◦ C for 30 min. The materials formed were characterized by SEM, EDS, XRD and FT-IR. 2.2. Fabrication of solar cells The DSCs were fabricated using TiO2 NT deposited on FTO (working electrode WE) by the usual procedure of dye-adsorption, sandwiching the electrolyte [0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.1 M LiI, 0.05 M I2 , 0.5 M tert-butylpyridine in acetonitrile] between this electrode and a lightly platinized FTO counter electrode. The TiO2 NT containing FTO plates was heated at 450 ◦ C for 30 min and then allowed to cool down to ca. 80 ◦ C and transferred into a 1.00 × 10−4 M N-719 (cis-diisothiocyanato-bis

(2,2 -bipyridyl-4,4 -dicarboxylato) ruthenium(II) bis (tetrabutylammonium) dye solution (1:1 volume ratio of acetonitrile and t-butanol) to allow for dye adsorption over 15 h. Parallel experiments were performed using the WEs made by treating the TiO2 NT layer with 0.04 M TiCl4 (aq) solution for 30 min at 70 ◦ C and then rinsing with DW and allowing to dry (first 2 M TiCl4 (aq) solution was prepared at 0 ◦ C and then diluted to 0.04 M TiCl4 (aq)). The introduction of NPs was done by spraying varying amounts of 3–5 nm size TiO2 NP solution (TKC-302) on to TiCl4 -treated TiO2 NT containing FTO electrodes using a purpose-made spray gun. After heat-treating the electrodes at 500 ◦ C for half an hour and allowing to cool down to RT, the dry masses of the electrodes were determined to evaluate the mass of the NP introduced. The amount of dye adsorbed in each case was determined by the usual dye-desorption method in NaOH solution. 3. Results and discussion The SEM pictures of TiO2 NTs grown are shown in Fig. 1. Most of the NTs are straight and vertical though occasionally a few nonvertical NTs can also be observed. The texture of the NTs is uniform, and reasonably dense though there are ample voids between the tubes. The tubes have an average diameter of ∼120 nm (Fig. 1a shows the top view of the NTs at 70,000 magnification) and an average length of ∼500 nm (Fig. 1b). In most of the NTs, the top surfaces are covered by a cap and most of the NTs are, therefore, topend-closed NTs where the dye adsorption could only take place on their outer surfaces (surface area of one NT = 2rh + r2 , r – radius and h – height of the NT). The average surface area of an NT is therefore ∼2 × 10−13 m2 and the number of NTs in 1 cm2 cell is estimated from their SEM to be 4.10 × 109 . The total surface area available for dye adsorption is, therefore, 8.19 cm2 in 1 cm2 cell. The geometric volume of 1 cm2 cell containing 500 nm high NTs is 5 × 10−5 cm3 and the volume occupied by the NTs in 1 cm2 cell is 2.32 × 10−5 cm−3 . This gives us an estimation of the total pore volume to be 2.68 × 10−5 cm3 . A very few open end nanotubes are also clearly visible in some parts of the film as revealed by their SEM pictures though their presence has been justifiably neglected in the previous calculation. From the top-end-open NTs, the inner diameters of the tubes calculated are around 45 nm giving rise to a thickness of the tube walls to be around 38 nm. The EDS experiments performed show the presence of mainly Ti and to a very small extent Zn with approximately 99:1 atomic ratio and the Zn atoms being located at the top surfaces of the tubes (Fig. 2). In the present study, efforts were not taken to remove traces of Zn present in these NTs though a very recent publication shows an enhancement of performances of DSCs when TiO2 NTs are pure [32]. They have used 10–11 ␮m long TiO2 NTs and after removal of top-ends, the DSCs fabricated gave an efficiency of 3.6% at AM 1.5 illumination. The XRD spectra of both ZnO (nanorods) NRs and TiO2 NTs are shown in Fig. 3. The clear sharp peaks obtained at 2 values (◦ ) 34.26, 36.36, and 47.78 for ZnO NRs show that these NRs are mainly composed of zincite crystals (JCPDS 36-1451). The peaks at 2 (◦ ) 25.42 and 48.12 for TiO2 show that the NTs have anatase structure (JCPDS 21-1272). The FT-IR spectra of TiO2 NTs with and without TiCl4 -treatment do not give any clear difference in the wave number range between 1600 cm−1 and 3400 cm−1 . 3.1. Solar cell characteristics Fig. 4 shows the I–V curves for the DSCs made using pristine TiO2 NTs without TiCl4 -treatment (curve a), pristine TiO2 NTs with the TiCl4 -treatment (curve b), composite films of NT (0.5 mg in each case) with increased amounts of TiO2 NPs with the TiCl4 -treatment

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Fig. 1. SEM images of (a and b) pristine TiO2 nanotubes and (c) nanotube and nanoparticle composite film. Table 1 Nanoparticle-loading and dye adsorption together with photovoltaic parameters of the cells (a)–(g) under the simulated sunlight irradiation (AM 1.5, 100 mW cm−2 ). Cell

Mass of NT (mg cm−2 )

Mass of NP (mg cm−2 )

JSC (mA cm−2 )

VOC (mV)

FF

 (%)

Dye adsorption (mol cm−2 )

a b c d e f g

0.5 0.5 0.5 0.5 0.5 0.5 0

0 0 0.1 0.9 1.1 1.6 1.4

5.11 7.45 12.87 17.90 18.00 17.10 15.86

720 753 755 731 740 712 725

0.670 0.607 0.613 0.615 0.640 0.669 0.620

2.46 3.40 5.96 8.04 8.53 8.14 7.13

5.69 × 1015 7.44 × 1015 2.23 × 1016 6.23 × 1016 6.41 × 1016 8.19 × 1016 3.58 × 1016

(curves c–h) and using NP alone with the TiCl4 -treatment (curve i). The masses of NT and NP used in each case together with solar cell performance characteristics (JSC , VOC , FF and ) and the number of dye molecules in 1 cm2 cell in each case are given in Table 1.

The comparison of the curves a and b and the corresponding data extracted from these curves, given in rows a and b in Table 1, clearly demonstrate the effect of TiCl4 -treatment on TiO2 NTs. The TiCl4 -treatment has enhanced the dye coverage by 30.7% and

Fig. 2. EDS spectrum of pristine TiO2 nanotubes.

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dye adsorption has resulted in approximately same (38%) increase in . The partial hydrolysis of TiCl4 in water at 0 ◦ C produces TiOCl2 and HCl and a 2 M TiOCl2 solution, therefore, contains 4 M HCl and at such a low pH the TiO2+ ion is the most stable state of the oxides of Ti(IV) and the aqueous TiCl4 solution is, therefore, an aqueous solution of TiOCl2 in HCl. TiCl4 + H2 O → TiOCl2 (aq) + 2H+ (aq) + 2Cl− (aq)

Fig. 3. XRD spectra of (a) ZnO nanorods and (b) TiO2 nanotubes.

consequently the solar-to-electricity conversion efficiency  has been increased by 38.2%. The changes in contributing factors for the change in  are as follows: JSC has been increased by 45.8% and the VOC by 4.6% but the FF has been decreased by 10.4%. The TiCl4 treatment is a method used to enhance the performance of DSCs. O’Regan et al. [33] have investigated the influence of the TiCl4 treatment on nanocrystalline TiO2 films in DSCs by studying charge density, band edge shifts and quantification of recombination losses at short circuit. They have concluded that the main effects of this treatment are to bring down the bottom edge of the CB of TiO2 by 80 mV and a 20-fold decrease in the electron–electrolyte recombination rate constant. In their study, the lowering of the VOC due to the lowering of the bottom of the CB has been compensated by the rising of the Fermi level due to the increased accumulation of electrons in the CB as a result of reduction in recombination. They found that there is only a little effect on the transport kinetics of electrons due to the TiCl4 -treatment [33]. Roy et al. have reported the uniform decoration of the cylindrical surfaces of the TiO2 nanotubes by 3 nm particles upon TiCl4 -treatment [28]. This has doubled the dye adsorption and consequently  has also been doubled from 1.9% without TiCl4 -treatment to 3.8% with the treatment. Our data matches well with both these explanations where 30% increase in

Fig. 4. I–V curves for the DSCs made using pristine TiO2 NTs without TiCl4 -treatment (curve a), pristine TiO2 NTs with TiCl4 -treatment (curve b), composite films of nanotubes with increased amounts of TiO2 nanoparticles with TiCl4 -treatment (curves c–f) and using nanoparticle alone with TiCl4 -treatment (curve g).

(1)

Further hydrolysis of TiOCl2 (aq) gives tine TiO2 particles which can effectively block the recombination centers and consequently suppresses the recombination of electrons injected to the CB with the solution species [28,33]. This is in accordance with O’Regen’s results [33] on TiO2 NPs where the recombination rate constant has been decreased by 20-fold due to TiCl4 treatment. The decrease in recombination allows for the accumulation of a greater number of electrons in the CB thus offsetting the effect due to decreases in the CB edge to result in virtually unchanged VOC for the DSCs made with and without TiCl4 treatment. However, they have argued that this exact balancing of the two opposing effects imposed on VOC is purely a coincidence and one effect could dominate over the other, giving rise to lower or higher VOC values due to TiCl4 treatment. They have not ruled out the possibility for improved electron transport. The enhanced dye adsorption increases the electron density of the CB in combination with the decrease in recombination, fills the CB well beyond its bottom thus shifting the Fermi level in the negative direction. A net 32 mV rise in VOC of our system upon TiCl4 treatment may be an indication of compensation of 80 mV downward shift of the bottom of CB [33] by 112 mV upward shift of the Fermi level. Obviously the enhanced charge injection and protection from recombination can effectively transport injected electrons towards the CTO surface through the “fast track” available in the 1-D nanomaterials [8]. As a result, the JSC shows 45.8% enhancement upon TiCl4 treatment. The TiCl4 treatment is known to interconnect 3-D nanoparticulate system used in DSCs. However, in a 1-D system, the transport of electrons is different to that in the 3-D system. In the 1-D system, the interconnection of nanomaterial is not required for electron transport as the electrons injected to individual discrete NTs carry them towards their bottom surface. Therefore, the significant enhancement of JSC in 1-D system as opposed to 3-D system is an indication of significant enhancement of electron transport rate constant. The SEM photograph shown in Fig. 1c reveals that the voids between the NTs are filled with NPs which have been interconnected by the calcinations at 450 ◦ C to have interconnected nanocomposite matrices containing TiO2 NTs and NPs. The increase in NP concentration drastically increases the dye adsorption in a linear manner (Supplementary Fig. 1) owing to the increase in surface area available for this purpose. From the gradient of the straight line, it can be estimated that 4.63 × 1016 dye molecules per milligram of TiO2 NPs are introduced upon NP spraying whereas the dye absorption is only 1.50 × 1016 molecules per milligram of NTs (after TiCl4 -treatment) suggesting that, assuming the full surface coverage by the dye (which is justifiable as the dye concentration is increasing linearly with the amount of NPs introduced), the surface area per milligram of NPs is 3 times higher than that of NTs. As the NP concentration increases, the solar cell efficiency gradually improves up to a maximum value of 8.53% at the 0.5 mg NT:1.1 mg NP composite film, beyond which it gradually decreases. In parallel with the linear increase in dye-loading in these cells, the variation of  is, however, not linear and is somewhat less than that demanded by the linear increase. This is due to recombination centers introduced by the interconnected NPs. As the NP concentration increases, the collection efficiency shows an initial sharp increase as the considerable amount of NTs present could effectively transport electrons towards the FTO surface. It is

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Fig. 5. Schematic illustration of charge transport process of nanotube-nanoparticle composite film based dye-sensitized solar cells under illumination.

very likely that, at the beginning when the NP concentration is not too great, the expressway electron transport along the NTs is dominant by effectively collecting photoinjected electrons collectively by both NTs and NPs but transferring them to NTs for transport in the least resistant pathway. However, when the NP concentration increases, the random walk is also going to be a means of electron transport and in which case opportunities for recombination enhances and hence a leveling of  is observed as the NP concentration is increased. The further increase in NP concentration has a somewhat detrimental effect due to two reasons, viz., increased recombination and decreased porosity for poor electrolyte penetration As a result, the performance of the DSC then decreases. The schematic illustration shown in Fig. 5 clearly shows these ideas in a pictorial form. Alivov et al. have studied the efficiency of DSCs based on TiO2 NTs filled with nanoparticles and found that NP filling in NTs enhances the  significantly and the enhancement depends on the diameter of the NTs. The enhancements are 17.4% for the tubes with diameter 40 nm, 55.6% for those with 80 nm and 131% for 160 nm tubes. The highest efficiency they have observed is 5.94% for NTs with diameter 80 nm and length 5 ␮m filled with NPs [34]. Roy et al. have decorated TiO2 NTs in DSC with TiO2 NPs by TiOCl2 treatment. Their SEM pictures clearly show the presence of ∼3 nm NPs on NTs [28]. Qu et al. have calcined hydrothermally prepared TiO2 NTs at different temperatures in the range 400–700 ◦ C in air and found that they could obtain TiO2 nanotubes, anatase nanorods, and anatase nanoparticles by these treatments [35]. The DSC with anatase nanorods calcined at 600 ◦ C shows much better photoelectrochemical performance than those using other samples, with a photovoltaic conversion efficiency of 7.71%. Wang et al. have prepared DSCs with TiO2 NTs with diameter ∼12 nm and length of several nanometers. They found that better performance could be achieved when NT and NP are in combination with only 5% NT and they have reported the incident photon to current efficiency of about 63.1% which is an 13.8% increment compared to that using full P25 under the same conditions [36]. Yip et al. reported a DSC based on TiO2 nanobelt and porous NP layer mixed morphology and found that the porous layer could enhance the  from 1.6% to 2.2% for 5 ␮m long belts [37]. Asagve et al. have shown that nanowire (NW) based DSC had 1.33% efficiency and P-25 NP based DSC 5.59% efficiency. However, 10% NW doped in NP showed 6.17% efficiency [38]. Parasupre have shown that pure P-25 based DSC to have 5.82% efficiency while a mixture of NRs 10–20 diameter, 10–200 nm length and NPs 5–10 nm diameter to have 7.12% efficiency [39].

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All these reports clearly indicate that the combination of 1-D nanomaterials which have superior electron transport characteristics, with highly porous 3-D interconnected NP matrices show superior performance to their parent counterparts. The performance of the DSCs depends on the length of 1-D nanomaterials and pore diameters and so on. In our study, we used ca. 500 nm long NTs and effectively tied them up to the FTO surface and increased their surface area by the TiCl4 treatment. We have also introduced systematically a progressive amount of NPs and we could obtain efficiencies as high as 8.53%. These results clearly show the usefulness of composite films and the possibility for further enhancement of efficiencies by using longer NTs. The independent measurement of SEM cross-sections of the NT films and TiCl4 treated NT films shows that there is no difference in the thickness but the introduction of NPs gradually increases the thickness of the film. The increase in thickness gives increased amounts of dye absorption and hence increased electron injection. This may account for increase in  when NP loading is increased. Of the sticking interest is to compare the performance of the DSCs (e) and (i) given in Fig. 4 (the corresponding cell performance data are available in Table 1) which have the same total mass of 1.4 mg of TiO2 but the former has 0.5 mg NT and 0.9 mg NP whereas the latter contains 1.4 mg of NP alone. Surprisingly, the dye coverage is 1.74 times higher in the NT–NP composite film which gives a very good performance in DSC characteristics than the NP alone system. Whether it is due to 1.74 times increased surface area in the NT–NP system compared to NP alone system is a question worthy of investigation. The DSC characteristics of the former are JSC = 17.9 mA cm−2 , VOC = 731 mV, FF = 0.62 and  = 8.04% whereas those of the latter are JSC = 15.8 mA cm−2 , VOC = 725 mV, FF = 0.62 and  = 7.14%. The NT–NP composite system certainly gives improved DSC characteristics over NP alone system at the same TiO2 mass. Efforts are being undertaken to grow increased amounts of NTs by this method to further enhance the DSC performance. Considering the easiness, mildness and the environmentally friendlierness in the synthesis of NTs by the wet chemical route to give vertically aligned array of TiO2 NTs, we believe that further fine-tuning synthetic procedures, DSCS with even better performance could be obtained and the future of DSC research could certainly be geared in the direction of 1-D nanomaterials which provide ‘expressway’ electron transport track to improve DSC kinetics.

4. Conclusions We have shown that TiO2 nanotubes (NTs) prepared by chemical bath produces uniform vertical NTs with approximately 500 nm length, 80 nm thickness and 45 nm pore diameter. There are ample free spaces available between the tubes. The DSC made using N719 dye gives 2.46% efficiency at AM 1.5 irradiation but upon TiCl4 treatment, the efficiency is increased to 3.48% due to 30% increase in the dye coverage. The systematic introduction of TiO2 nanoparticles into the voids and calcinations at 500 ◦ C to result in interconnected nanoporous matrix, systematically increases the dye coverage and the collection efficiency and a maximum value of 8.53% could be obtained at NT:NP mass ratio of 0.5 mg cm−2 :1.1 mg cm−2 . When the active electrode is made entirely by TiO2 NPs with the same mass as the sum of masses of NPs and NTs, the collection efficiency is lowered. The enhanced collection efficiency in the presence of NTs is due to the fast 1-D electron transport along defect-free NTs. The collection efficiencies reported herein are much higher than those reported for NTs with similar lengths.

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2011.12.127.

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