Anatase TiO2 hollow spheres embedded TiO2 nanocrystalline photoanode for dye-sensitized solar cells

Materials Chemistry and Physics 123 (2010) 595–600

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Anatase TiO2 hollow spheres embedded TiO2 nanocrystalline photoanode for dye-sensitized solar cells Yanwei Zhang, Jing Zhang ∗ , Peiqing Wang, Guangtao Yang, Qiang Sun, Jun Zheng, Yuejin Zhu ∗ Department of Physics, Ningbo University, Fenghua Road, Ningbo 315211, China

a r t i c l e

i n f o

Article history: Received 10 June 2009 Received in revised form 15 April 2010 Accepted 7 May 2010 Keywords: Dye-sensitized solar cells TiO2 hollow spheres Light scattering Interface recombination

a b s t r a c t The anatase TiO2 hollow spheres with diameter of 500 nm and shell thickness of 25 nm are embedded in P25 TiO2 nanocrystalline photoanodes to improve the light scattering ability of the photoanode. Photoanodes embedded with different contents of TiO2 hollow spheres are prepared and the profiles of the photoanodes are systematically characterized. It is found that the morphologies of the photoanodes were modified by the TiO2 hollow spheres. With increasing the TiO2 hollow spheres’ contents, the light scattering ability of the modified photoanodes is effectively improved, while the dye adsorption decreases. By optimizing the TiO2 hollow spheres’ content in TiO2 nanocrystalline, a higher performance of the DSSC (Jsc = 16 mA cm−2 , Voc = 0.72 V, FF = 0.648 and  = 7.59%) is obtained compared with the pure TiO2 nanocrystalline DSSC (Jsc = 13.96 mA cm−2 , Voc = 0.686 V, FF = 0.684 and  = 6.67%, measured at 98.3 mW cm−2 , AM1.5). The improved performance is mainly due to the enhanced light scattering by the TiO2 hollow spheres. The open circuit voltage of the hollow spheres modified DSSCs is higher than that of the DSSC with pure P25 photoanode, which can be attributed to the fact that TiO2 hollow spheres substitute of the TiO2 nanocrystallines and reduce the interface recombination by decreasing the surface charge trap-site density of the photoanodes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction As a credible chemical alternative to the solid-state silicon based solar to electric energy conversion device, the dye-sensitized solar cell has attracted much attention worldwide for its low cost and high efficiency since 1991 [1]. A typical DSSC consists of a highsurface area TiO2 nanocrystalline film on a transparent conducting glass, the adsorbed dye sensitizers, the electrolyte solution penetrating throughout the TiO2 film, and the counter electrode [2]. Lots of efforts have been carried out to optimize each part of the DSSC to improve the performance of the DSSCs and realize the practical applications. There are many factors limiting the cell performance, among which light-harvesting efficiency (LHE) is crucial [3]. The LHE can be improved by employing the dyes with wide absorption bands, increasing the amount of the adsorbed dye molecules or the optical path length by modifying of the TiO2 nanocrystallines [4]. The light absorption of the widely used dye (N3 and N719) remains low in the region of 600–800 nm, efficient light scattering in the photoanode to increase the optical length and the LHE

∗ Corresponding authors. Tel.: +86 574 87600770; fax: +86 574 87600744. E-mail addresses: [email protected], [email protected] (J. Zhang), [email protected] (Y. Zhu). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.05.020

becomes important. The conventional TiO2 photoanode with small TiO2 nanoparticles (10–40 nm) causes Rayleigh scattering, which is too weak to scatter the visible light backward [5,6]. Additional large particles serving as scattering centers or additional scattering layers have been adopted by several researchers to increase the light scattering of the photoanode. The enhanced light-harvesting and improved performance of the DSSCs have been achieved both experimentally and theoretically [3–5,7–9]. Besides the solid spheres TiO2 , there are other morphologies of TiO2 photoanodes capable of increasing the light scattering reported [10–12], among which TiO2 hollow spheres were highlighted recently [13–16]. TiO2 hollow spheres have many potential applications due to its unique structure [17,18]. Compared to large TiO2 particles, TiO2 hollow spheres have lower density. Multiple light diffraction and reflection could occur due to its particular hollow structure [13]. TiO2 hollow spheres serving as photoanode or light scattering layer upon the TiO2 nanocrystalline layer of the DSSC were reported [13–16]. However, the incorporation of the TiO2 hollow spheres in TiO2 nanoparticles as scattering centers remains to be explored. In this paper, we report that using anatase TiO2 hollow spheres (diameter of 500 nm and shell thickness of 25 nm) as scattering centers in P25 TiO2 nanoparticles’ photoanode, the new photoanodes effectively scatter the visible light and reduce the interface recombination by decreasing the surface charge trap-site density. Compared with the DSSC with P25 photoanode, a much higher per-

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Fig. 1. The TEM images of (a) carbon spheres and (b) the TiO2 hollow spheres.

formance of DSSC is obtained at an optimum TiO2 hollow spheres’ content in TiO2 nanocrystalline photoanode.

many) using Cu K␣ radiation at 40 kV and 20 mA in the region of 2 = 20–80◦ . The BET surface areas of samples were measured on a Micromeritics ASAP 2020 (USA) instrument.

2. Experimental 2.5. Characterizations of TiO2 hollow spheres embedded photoanodes 2.1. Main materials TiO2 nanoparticles (P25, 20–30 nm, Degussa AG, Germany); LiI (Across), I2 (Beijing Yili Chemicals, China), N3 (Ru(dcbpy)2 (NCS)2 , Dyesol, Australia); tetrabutyl titanate (TBT), glucose, triton X-100, ethanol, acetyl acetone, analytically pure (Sinopharm Chemical Reagent, China); FTO (Nippon, Japan, 14  −1 ). 2.2. Fabrication of the C spheres, C/TiO2 composite spheres and the TiO2 hollow spheres

The surface morphologies of the photoanodes were examined by scanning electron microscopy (SEM, Hitachi S-400, Japan) and atomic force microscope (AFM, Vecco, USA). UV–vis reflectance spectra of the P25 nanoparticles’ photoanodes embedded with different contents of titania hollow spheres were obtained by UV–vis spectrophotometer (Pgeneral, China) equipped with an integrating sphere. The dye adsorption of the P25 photoanodes modified with different TiO2 hollow spheres’ contents was examined by desorbing the dye molecules in a 0.1 M NaOH aqueous solution and the absorption spectra of the desorbed-dye solution were measured using the UV–vis spectrophotometer.

1 M glucose aqueous solution was sealed in a Teflon-lined autoclave and maintained at 180 ◦ C for 8 h. The afforded dark-brown carbon spheres were collected by centrifugation at 4000 rpm for 10 min, subsequently washed with distilled water and ethanol four times. Then the carbon spheres were oven-dried at 80 ◦ C for 5 h. In a typical synthesis of TiO2 hollow spheres, the as-prepared carbon spheres template was dispersed in ethanol by ultrasonification. To this homogeneous mixture, TBT was added (the weight ratio of TBT to C spheres is 5:2). The mixture was stirred vigorously in a glove box for 24 h [19]. Subsequently, the resulting product was collected and cleaned by four cycles of centrifugation in ethanol and distilled water alternatively, affording the C/TiO2 composite spheres. The process is called coating process. Next, the C/TiO2 composite particles were oven-dried at 40 ◦ C for 12 h. Anatase TiO2 hollow spheres were obtained by calcining the C/TiO2 composite particles in air at 450 ◦ C for 2 h.

The photocurrent versus voltage characteristics of the DSSC were measured on a sunlight simulator (Changtuo, China). The light intensity was 98.3 mW cm−2 (AM1.5), which was calibrated by a Si-1787 photodiode (spectral response range: 320–730 nm). A Keithley 2400 digital source meter unit (USA) was used to measure the current–voltage curves under the light and the dark environments. The active DSSC area was controlled at 0.16 cm2 by a mask. The 300 W xenon lamp (Newport, USA) gave an incident light ranging from 340 to 800 nm for the incident photo-to-current conversion efficiency (IPCE) measurements. The measurements were carried out in an ambient environment.

2.3. Assembly of DSSCs with different TiO2 hollow spheres’ contents embedded photoanodes

3. Results and discussion

The P25 TiO2 nanoparticles’ slurry for doctor-blade process was prepared according to Ref. [20]. The weight content of the P25 nanoparticles in the slurry is 20 wt%. One portion of the C/TiO2 composite spheres was made by modifying 0.2 g carbon spheres with 0.5 g TBT. To find out the optimum content of TiO2 hollow spheres in the photoanode for DSSCs, each portion of C/TiO2 composite spheres was added to 10 ml (sample A, 1:10), 15 ml (sample B, 1:15), 20 ml (sample C, 1:20) and 30 ml (sample D, 1:30) P25 slurry, respectively; then stirred vigorously for 4 h to mix the slurry uniformly. Then, the four uniform slurries were doctor-bladed respectively on the fluorine tin oxide (FTO) glass plates. After being heated at 450 ◦ C in air for 2 h, TiO2 hollow spheres embedded photoanodes were obtained (named as A–D, according to the different contents of C/TiO2 composite spheres in the slurry). For comparison, pure P25 TiO2 nanoparticles’ photoanode (named as E) was also prepared by the same method. The five electrodes were all immersed in a 5 × 10−4 M N3 ethanol solution overnight for the dye sensitization process. Then the dye-adsorbed electrodes and Pt sputtered counter electrodes were assembled and the electrolyte solution (0.05 M I2 , 0.5 M LiI, and 0.5 M 4-tertbutylpyridine in acetonitrile/propylene carbonate (1:1, v/v)) was added for the measurements. 2.4. Characterizations of the carbon spheres and the TiO2 hollow spheres The morphologies of carbon spheres and hollow spheres were examined by transmission electron microscopy (TEM, Hitachi-750, Japan). The crystal structures of hollow spheres were characterized by X-ray diffraction (XRD, Bruker AXS, Ger-

2.6. Photovoltaic measurements of TiO2 hollow spheres modified DSSCs

3.1. About C spheres and TiO2 hollow spheres The TiO2 hollow spheres embedded photoanodes were prepared by calcining the C/TiO2 composite spheres and the P25 nanocrystallines constructed films. It is necessary to know the formation process and the morphologies of the TiO2 hollow spheres. The carbon spheres were prepared by the hydrothermal method and then modified by TBT. The C/TiO2 composite spheres were then calcined to afford TiO2 hollow spheres. The TEM image of carbon spheres made from the hydrothermal treatment of 1 M glucose aqueous solution at 180 ◦ C for 8 h is presented in Fig. 1a. It indicates that the obtained carbon spheres are nearly uniform and monodispersed, with a diameter of 700 nm. The C/TiO2 composite spheres were then calcined to become titania hollow spheres and the image is shown in Fig. 1b. It indicates that the hollow spheres with smooth shells of 25 nm thick and diameter of 500 nm were uniformly distributed and 30 ◦ C smaller than the carbon spheres, implying a shrinkage during the calcination. Shrinkage is a universal phenomenon during the calcinations [21,22]. The BET surface area of the hollow spheres

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Table 1 The photovoltaic parameters of the DSSCs with different TiO2 hollow spheres’ contents modified photoanodes (A–E, the TiO2 hollow spheres’ content is decreased from photoanode A to photoanode E); the optimum thickness of the photoanodes, the roughness factors and the dye adsorption amounts of each photoanode are also indicated. Photoanode

Jsc (mA cm−2 )

Voc (V)

FF

 (%)

Thickness (␮m)

Roughness factor RMS (nm)

Adsorbed dye [×10−8 mol cm−2 ]

A B C D E

13.31 15.31 16.00 14.04 13.96

0.723 0.721 0.72 0.703 0.686

0.675 0.622 0.648 0.702 0.684

6.61 6.98 7.59 7.05 6.67

13.5 14.0 14.2 12.5 11.0

60.98 51.42 39.02 30.48 17.13

2.270 2.685 2.960 3.128 3.405

3.2. The morphologies of different TiO2 hollow spheres’ contents embedded photoanodes

Fig. 2. The X-ray diffraction pattern of the TiO2 hollow spheres made from C spheres and TBT (the weight ratio of C spheres and the TBT is 2:5).

is measured to be 35.1 m2 g−1 , smaller than that of P25 particles (55 m2 g−1 ) [23]. Fig. 2 shows the X-ray powder diffraction pattern of the hollow spheres made from 0.2 g 700 nm C spheres and 0.5 g TBT, with each peak marked for the crystal plane index. As shown in Fig. 2, the hollow spheres formed anatase phase because the characteristic diffraction peaks of anatase (major peaks: 25.4◦ , 38.0◦ , 48.0◦ , 54.7◦ , 63.1◦ ) are evident. The absence of other peaks indicated that the titania hollow spheres are of high purity. The anatase crystallinity phase formed during the calcination, and the carbon spheres templates had been removed completely.

The surface morphologies of the different contents of TiO2 hollow spheres embedded P25 TiO2 electrode were examined and the SEM patterns are indicated in Fig. 3. Fig. 3a–c is the morphologies of photoanodes A–C in large scale, which represents the universal morphologies of the photoanodes. Fig. 3d and e is the local morphologies. The morphologies indicate that the 500 nm diameter hollow spheres are surrounded tightly by the P25 nanoparticles, and some grow to bigger spheres (2–4 ␮m). Surrounding the hollow spheres are annular ravines. There are long cracks a little farther away from the hollow spheres, with a width of several hundred nanometers. The formation of ravines and cracks may be due to the non-uniform stress brought about by the burning of C spheres and the formation of hollow spheres in the film. As the TiO2 hollow spheres modified content decreases, the number of ravines and cracks on the photoanode decreases correspondingly (Fig. 3a–c). The cross-section structure of photoanode A is presented in Fig. 3f, indicating the TiO2 hollow spheres are distributed in the photoanode. The RMS (root mean square) roughness factors of the different contents of TiO2 hollow spheres modified photoanodes were measured by AFM and are indicated in Table 1. The roughness factor of the modified photoanode increases with the TiO2 hollow spheres’ content increase. The pure P25 nanoparticles constructed photoanode shows the smallest roughness factor of 17.3 nm, which means the surface of the P25 photoanode is relatively flat and smooth compared with other photoanodes; in contrast, photoanode A with the highest content of hollow spheres exhibits roughest surface as high as 60.98 nm. Such a change in the roughness factor with the TiO2 hollow spheres’ content is consistent with the SEM photographs

Fig. 3. SEM morphologies of large surface areas of photoanodes (a) A, (b) B, (c) C, (d) the exposed hollow spheres and (e) surrounded ravines on the TiO2 hollow spheres modified photoanode; (f) the cross-section structure of the hollow spheres embedded photoanode A.

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Fig. 4. The UV–vis transmittance spectra of the (a) unsensitized and (b) N3-sensitized photoanodes A–E; the diffuse reflectance spectra of the (c) sensitized and (d) N3sensitized photoanodes A–E. The arrows indicated the spectra from bottom to top are the diffuse reflectance of photoanodes E–A, in the sequence of increasing the TiO2 hollow spheres’ contents in the photoanodes.

(Fig. 3a–c), where the photoanodes with higher contents of TiO2 hollow spheres exhibits more cracks and ravines. 3.3. The light-scattering and dye adsorption abilities of the photoanodes The performances of the DSSCs with photoanodes embedded with different TiO2 hollow spheres’ contents were examined. To obtain the optimum efficiency of the DSSC, photoanodes with a series of film thickness (2–20 ␮m) were prepared. For each type DSSC, the highest performance was obtained at an optimum photoanode thickness, the detailed photovoltaic parameters of each optimized DSSC are indicated in Table 1. The DSSC with photoanode C at the optimum photoanode thickness of 14.2 ␮m exhibits the highest performance of 7.59%, much higher than the performance of DSSC with P25 photoanode of 11 ␮m (6.67%). However, the DSSC with photoanode A (the highest hollow spheres’ content) shows lower Jsc and efficiency than that of the P25 photoanode DSSC. As shown in Table 1, the incorporation of the TiO2 hollow spheres (500 nm) with an optimum content into the P25 TiO2 nanocrystalline photoanode improves the performance of the DSSCs. From the SEM morphologies, the photoanode structure was changed by the hollow spheres. The observed phenomenon is that, as the mixed C/TiO2 composite spheres’ content increases, the obtained photoanode become more and more opaque, indicating that the optical property of the TiO2 hollow spheres modified photoanode is changed compared with the pure P25 nanoparticles constructed photoanode. As shown in the transmittance spectra of hollow spheres modified photoanodes with and without dye sensitization (Fig. 4a and b), the P25 photoanode (E) is more transparent to the light compared with the TiO2 hollow spheres modified photoanode, indicating the TiO2 hollow spheres modified photoanodes can trap more light. The UV–vis

reflectance spectra of the five TiO2 photoanodes (A–E, which are of the similar thickness) before and after the dye sensitization were obtained and shown in Fig. 4c and d. From Fig. 4c, the reflectance in the whole wavelength range is increased with increasing the TiO2 hollow spheres’ contents in P25 nanocrystalline photoanodes (photoanodes A–E). According to the Mie theory, the effective scattering particles are those with the size comparable to the wavelength of light and the light scattering occurred due to the higher light trapping in the device [6,24]. Considering the size (500 nm) and the special hollow structures of the TiO2 hollow spheres which are beneficial to the multiple reflectance of the light, the light scattering and the optical path length of light in the photoanode can be effectively improved. The reflectance spectra of the dye-sensitized photoanodes (Fig. 4d) also exhibit the same trend: the valley of the curve in the wavelength range of 500–600 nm is due to the light absorption of the dye molecules [14], the diffused reflectance of the dye-sensitized TiO2 hollow sphere/nanocrystalline photoanodes is also enhanced with the increase of TiO2 hollow spheres’ content especially in the wavelength range of 550–700 nm, which indicates that the TiO2 hollow spheres effectively enhance the light scattering ability of the photoanodes compared with the pure P25 photoanode. Because the light absorption of the dye is poor in the long wavelength range, the light scattering in the long wavelength range becomes more obvious in the photoanodes embedded with higher content of TiO2 hollow spheres. As discussed above, the light scattering of the TiO2 hollow spheres may enhance the light trapping and hence the light utilization in the photoanode. However, the TiO2 hollow spheres in the photoanode would cause back-scattering near the conducting glass and result in a light loss [3–5,9], which is negative for the light utilization of the photoanode. Due to the diffused light reflectance of the photoanode is increased as the TiO2 hollow spheres’ contents increase, the light loss caused by the back-scattering near the FTO

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would be more severe in the photoanode embedded with higher content of TiO2 hollow spheres. Not only the light scattering but also the dye adsorption ability will influence the LHE of the DSSCs. The dye-adsorption tests for the TiO2 photoanodes which exhibited the best performance of each type DSSC was carried out and the dye adsorption amounts are shown in Table 1. As shown in Table 1, the amounts of dye adsorbed on the TiO2 hollow spheres modified photoanodes (A–D) are all lower than that of the P25 photoanode E (3.405 × 10−8 mol cm−2 ). The amount of the adsorbed dye of photoanode A (the highest content of TiO2 hollow spheres) is the lowest (2.270 × 10−8 mol cm−2 ) and the dye adsorption ability increases as the hollow spheres’ content decreases. The phenomenon is attributed to the fact that TiO2 hollow spheres with large size of 500 nm and lower BET surface area of 35.1 m2 g−1 substitute of lots of TiO2 nanocrystallines with higher BET of 55 m2 g−1 , which decreases the effective dye adsorption area of the photoanode. The dye adsorption ability of the photoanodes decreases with the increase of TiO2 hollow spheres’ content, which will weaken the light-harvesting ability of the photoanode. From the experiments and the discussions, there is a tradeoff in the introduction of the 500 nm hollow spheres into the nanocrystalline photoanode for the DSSCs. On the one hand, the incorporation of the TiO2 hollow spheres in the P25 nanoparticles enhances the light scattering ability, which can improve the LHE of the DSSCs; on the other hand, it causes the back-scattering of light near the FTO and decreases the dye adsorption ability of the photoanodes with the similar thickness, which bring adverse effect on the LHE of the photoanode. The detailed influence of the TiO2 hollow spheres on the performance of the DSSCs is discussed in Section 3.4. 3.4. The photovoltaic parameters of the TiO2 hollow spheres modified DSSCs For the DSSCs with photoanode modified with different TiO2 hollow spheres’ contents, the variation of the short current density (Jsc ), the open circuit voltage (Voc ) and the performance () with different photoanode thickness are indicated in Fig. 5a–c. The DSSC performance largely depends on the film thickness [3]. With increasing the film thickness, the Jsc of the five type DSSCs increases first, after reaching an optimum value, then decreases (Fig. 5a). The typical variations of the Jsc with the film thickness are due to the competitions among light-harvesting efficiency, network ohmic loss and the mass transport of the electrolyte [3,11,25]. The Voc of the five different type DSSCs indicated in Fig. 5b is inclined to decrease with increasing the film thickness, which is due to the charge recombination and mass transport limitations in the thicker films [3]. Taking into consideration of the variation of Voc and Jsc with the film thickness, the  of the each type DSSCs reached an optimum value at the thickness range of 10–16 ␮m. Compared with the performances of the DSSCs with photoanode embedded with different TiO2 hollow spheres’ contents A–E in Fig. 5a and c, the DSSC with photoanode C exhibits the highest Jsc = 16 mA cm−2 and  = 7.59% at the optimum photoanode thickness of 14.2 ␮m, which is ascribed to the balanced light scattering and dye adsorption abilities of the proper TiO2 hollow spheres’ content in the photoanode C. The LHE of the DSSC with photoanode C is considered to be the highest among the five DSSCs. Though photoanode A with the highest TiO2 hollow spheres’ content exhibited the highest light scattering ability as shown in Fig. 4, the highest TiO2 hollow spheres’ content seriously decreases the dye adsorption and would cause the back-scattering of the light near the FTO glass, which offset the light scattering effect and gave adverse effect on the LHE of the DSSC. So the performance of the DSSC with photoanode A is the worst.

Fig. 5. The variations of the Jsc , Voc and the efficiency of the dye-sensitized solar cells A–E with different photoanode thicknesses.

Fig. 6. The incident photo-to-current conversion efficiency (IPCE) of the DSSCs with photoanode C of the optimum thickness (14.2 ␮m) and P25 nanoparticles’ photoanode E (11 ␮m).

The incident photo-to-current conversion efficiencies (IPCE) of the DSSCs with photoanode C of the optimum thickness (14.2 ␮m) and P25 nanoparticles’ photoanode E (11 ␮m) were compared and shown in Fig. 6. At the short wavelength range of 340–540 nm, the IPCE performance of DSSC with photoanode C cannot compete with the P25 photoanode DSSC. This is because the dye adsorption amount of the photoanode C is smaller compared with the P25 photoanode, which lead to the decreased light absorption of the adsorbed dye. It counteracts the light scattering effect and decreases the LHE at the short wavelength range. The IPCE value

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Fig. 7. The current density–voltage characteristics of solar cell device with different TiO2 hollow spheres modified photoanodes A–E in dark environment.

of the DSSC with TiO2 hollow spheres modified photoanode C is enhanced at the wavelength range of 540–800 nm, verifying that incorporation of the 500 nm TiO2 hollow spheres in the TiO2 nanoparticles effectively scatters the light and enhances the light absorption in the long wavelength region. Another phenomenon should be addressed is that the open circuit voltage (Voc ) of the DSSC with P25 photoanode is always lower than the TiO2 hollow spheres modified DSSCs in the whole thickness range of photoanodes (Fig. 5b). Voc , which is determined by the difference between the quasi-Fermi level of TiO2 and the potential of the redox couple in the electrolyte, is strongly dependent on both the recombination rate and the band-edge position of TiO2 [26,27]. Because of the large surface area of the nanoporous films, there are lots of surface charge trap-sites on the TiO2 nanocrystallines, which become electron recombination centers, decreasing the Voc and the performance of the DSSCs [28]. When the TiO2 hollow spheres were incorporated in the photoanode, the 500 nm sized spheres substituted the place of lots of TiO2 nanocrystallines, which decreased the surface charge trap-site density and suppressed the electron recombination between the photoanode and the redox electrolyte. So the Voc is enhanced. The similar Voc enhanced phenomenon was observed in the DSSC with microtube-network structures integrated TiO2 electrodes [24]. Though the I–V curves in the dark environment is not a good simulation of the recombination current under illumination, it can be used as an estimate of the extent of reducing of I3 − with the nanoporous TiO2 conduction band electrons [29]. The I–V curves of the five different types DSSCs in dark environment are indicated in Fig. 7. It shows that the DSSC with P25 photoanode exhibits earlier onset and the larger magnitude of the current than the TiO2 hollow spheres modified photoanode, which reflects a higher recombination rate in the P25 photoanode DSSC; whereas the TiO2 hollow spheres embedded photoanodes exhibits suppressed current density, verifying that the interface recombination between the TiO2 hollow spheres embedded photoanode and the electrolyte is decreased. The suppressed surface charge trap-site density by TiO2 hollow spheres is the reason for the reduced interface recombination and the improved Voc of the DSSCs. 4. Conclusions Anatase TiO2 hollow spheres which have uniform size and smooth surface were prepared by the hydrolysis of TBT in the presence of the carbon spheres template (weight ratio of the TBT and carbon spheres is 5:2), followed by the removal of carbon spheres

under calcination. For the application of DSSCs, different contents of the C/TiO2 composite spheres were mixed with P25 nanoparticles to afford a new structure of photonode. The formed hollow spheres act as scattering centers in the photonode, effectively improving the light scattering ability of the modified photoanode and enhancing the LHE. However, replacing the nanocrystallines by the TiO2 hollow spheres with large size decreases the dye adsorption ability and brought back-scattering of the light near the FTO, which is not beneficial for the LHE. The Voc of the hollow spheres modified DSSCs are higher than the DSSC with P25 nanocrystalline photoanode, which is ascribed to the decreased surface trap-site density (recombination centers) on the new photoanodes. By balancing the TiO2 hollow spheres’ content in the photoanode, an optimum performance is obtained: Jsc = 16 mA cm−2 , Voc = 0.72 V, FF = 0.648 and  = 7.59%, much better than the pure TiO2 nanocrystalline DSSC (Jsc = 13.96 mA cm−2 , Voc = 0.686 V, FF = 0.684 and  = 6.67%, measured at 98.3 mW cm−2 , AM1.5). Further optimizing of the TiO2 hollow spheres modified TiO2 nanocrystalline photoanode by constructing multilayer structures is being carried out to improve the performance of the TiO2 hollow spheres incorporated DSSCs. Acknowledgements The authors would like to thank Xingfei Zhou and Jinhai Zhang (Department of Physics, Ningbo University) for the AFM measurements and Prof. Xingzhong Zhao (Department of Physics, Wuhan University) for the IPCE measurements. This work is financially supported by Natural Science Foundation of Ningbo (Nos. 2009A610036, 2009A610056 and 2009A610103) and Natural Science Foundation of China (No. 10774079). Referencess [1] B. O’Regan, M. Gratzel, Nature 353 (1991) 737. [2] M. Gratzel, J. Photochem. Photobiol. C: Photochem. Rev. 4 (2003) 145. [3] Z.S. Wang, H. Kawauchi, T. Kashima, H. Arakawa, Coord. Chem. Rev. 248 (2004) 1381. [4] Y. Tachibana, K. Hara, K. Sayama, H. Arakawa, Chem. Mater. 14 (2002) 2527. [5] J. Ferber, J. Luther, Solar Energy Mater. Solar Cells 54 (1998) 265. [6] S. Hore, P. Nitz, C. Vetter, C. Prahl, M. Niggemann, R. Kern, Chem. Commun. (2005) 2011. [7] Y. Chiba, A. Islam, R. Komiya, N. Koide, L.Y. Han, Appl. Phys. Lett. 88 (2006) 22. [8] A. Usami, Chem. Phys. Lett. 277 (1997) 105. [9] G. Rothenberger, P. Comte, M. Gratzel, Solar Energy Mater. Solar Cells 58 (1999) 321. [10] L.I. Halaoui, N.M. Abrams, T.E. Mallouk, J. Phys. Chem. B 109 (2005) 6334. [11] B. Tan, Y.Y. Wu, J. Phys. Chem. B 110 (2006) 15932. [12] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [13] Y. Kondo, H. Yoshikawa, K. Awaga, M. Murayama, T. Mori, K. Sunada, S. Bandow, S. Iijima, Langmuir 24 (2008) 547. [14] H.J. Koo, Y.J. Kim, Y.H. Lee, W.I. Lee, K. Kim, N.G. Park, Adv. Mater. 20 (2008) 195. [15] S.C. Yang, D.J. Yang, J. Kim, J.M. Hong, H.G. Kim, I.D. Kim, H. Lee, Adv. Mater. 20 (2008) 1059. [16] X.X. Li, Y.J. Xiong, L.F. Zou, M.T. Wang, Y. Xie, Microporous Mesoporous Mater. 112 (2008) 641. [17] Z.Y. Zhong, Y.D. Yin, B. Gates, Y.N. Xia, Adv. Mater. 12 (2000) 206. [18] R.A. Caruso, A. Susha, F. Caruso, Chem. Mater. 13 (2001) 400. [19] M. Zheng, J. Cao, X. Chang, J. Wang, J. Liu, X. Ma, Mater. Lett. 60 (2006) 2991. [20] H.W. Han, L. Zan, J.S. Zhong, L.N. Zhang, X.Z. Zhao, Mater. Sci. Eng. B 110 (2) (2004) 227. [21] X. Sun, J. Liu, Y. Li, Chem: Eur. J. 12 (2006) 2039. [22] D. Wang, C. Song, Y. Lin, Z. Hu, Mater. Lett. 60 (2006) 77. [23] C.J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, M. Gratzel, J. Am. Chem. Soc. 80 (12) (1997) 3157. [24] Y. Zhao, J. Zhai, T.X. Wei, L. Jiang, D.B. Zhu, J. Mater. Chem. 17 (2007) 5084. [25] P. Prene, E. Lancelle-Beltran, C. Boscher, P. Belleville, P. Buvat, C. Sanchez, Adv. Mater. 18 (2006) 2579. [26] S.Y. Huang, G. Schlichthorl, A.J. Nozik, M. Gratzel, A.J. Frank, J. Phys. Chem. B 101 (1997) 2576. [27] Z. Zhang, N. Evans, S.M. Zakeeruddin, R. Humphry-Baker, M. Gratzel, J. Phys. Chem. C 111 (2007) 398. [28] J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 104 (2000) 4292. [29] B.A. Gregg, F. Pichot, S. Ferrere, C.L. Fields, J. Phys. Chem. B 105 (2001) 1422.