Dye-sensitized solar cells based on double-layered TiO2 composite films and enhanced photovoltaic performance

Electrochimica Acta 56 (2011) 6293–6298

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Dye-sensitized solar cells based on double-layered TiO2 composite films and enhanced photovoltaic performance Jiaguo Yu ∗ , Qinglin Li, Zhan Shu State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, PR China

a r t i c l e

i n f o

Article history: Received 4 March 2011 Received in revised form 12 May 2011 Accepted 15 May 2011 Available online 20 May 2011 Keywords: TiO2 Dye-sensitized solar cells Double-layered composite films Hollow spheres Nanoparticles

a b s t r a c t Dye-sensitized solar cells (DSSCs) are fabricated based on double-layered composite films of TiO2 nanoparticles and hollow spheres. The photoelectric conversion performances of DSSCs based on nanoparticles/nanoparticles (PP), hollow spheres/hollow spheres (HH), hollow spheres/nanoparticles (HP), and nanoparticles/hollow spheres (PH) double-layered films are investigated, and their photoelectric conversion efficiencies are 4.33, 4.72, 4.93 and 5.28%, respectively. The enhanced performance of TiO2 nanoparticles/hollow spheres double-layered composite film solar cells can be attributed to the combined effect of following factors. The light scattering of overlayer hollow spheres enhances harvesting light of the DSSCs and the underlayer TiO2 nanoparticle layer ensures good electronic contact between film electrode and the F-doped tin oxide (FTO) glass substrate. Furthermore, the high surface areas and pore volume of TiO2 hollow spheres are respectively beneficial to adsorption of dye molecules and transfer of electrolyte solution. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Since O’Regan and Grätzel in 1991 introduced nanocrystalline mesoporous titania with high surface area into dye-sensitized solar cells (DSSCs) and achieved an overall light-to-electric energy conversion efficiency of 7.12% at full sunlight [1], DSSCs have attracted extensive interest due to easy fabrication process, light weight, cheap cost and an increasing public concern about the shortage of energy supply [2–5]. In the past two decades, intensive research has been devoted to structural design, material fabrication, photovoltaic characterization and mechanism analysis of TiO2 nanoparticle-based solar cells. Other than TiO2 nanoparticles, TiO2 nanorods [6–8], nanowires [9–11], nanotubes [12–14], nanosheets [15] and core–shell structures [16,17] have also been prepared and investigated in DSSCs with the intension of improving the overall photovoltaic conversion efficiency. Their photovoltaic performances, however, are lower than these of nanoparticle-based solar cells. As far as the film electrode is concerned, the photovoltaic performance of DSSCs is greatly dependent on properties of TiO2 nanostructures, including phase structure, crystalline size, morphology, pore size and porosity, specific surface area and so on [18–20]. In conventional semiconductor p–n junction solar cells, the intrinsic electric field is responsible to separate photogener-

∗ Corresponding author. Tel.: +86 27 87871029; fax: +86 27 87879468. E-mail address: [email protected] (J. Yu). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.045

ated electrons and holes. Contrarily, in dye-sensitized solar cells, the formation of intrinsic electric field is unfavorable due to the presence of small nanoparticles and the penetration of electrolyte species [21]. As a result, the harvesting of light and transport of charge carriers are separated and undertaken by dye sensitizer and TiO2 mesoporous films, respectively [22–24]. Dye molecules adsorbed chemically to TiO2 particle surfaces absorb incident light causing electron-excitation from the ground state to the excited state of dye molecule, then, the excited electrons inject into the conduction band of TiO2 and move through the network of interconnected nanoparticles to FTO glass substrates. At the same time, the oxidized dye molecule is rapidly regenerated by electron transfer from iodide ions in the I3 − /I− redox electrolyte [10,25]. Of the parameters influencing the photovoltaic performance of DSSCs, the high surface area is necessary to enhance the adsorption of dye molecules. Furthermore, the incorporation of large TiO2 particles with several hundred of nanometers in size has been proposed to enhance solar-light harvesting. It has been demonstrated theoretically and experimentally that TiO2 solid spheres, with a comparable dimension to visible light wavelength, are capable of enhancing light-harvesting ability of dye complexes by extending the optical path length [20,26–33]. The dilemma, however, is that large particles capable of scattering light lead to a significantly decreased surface area; small nanoparticles having large surface area are usually lack of light-scattering effect [34,35]. The most applicable approach to this dilemma is to compromise between surface area and light-scattering ability by mixing small and large particles together in an optimal proportion in the film, so that the

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transparent layer and a HS scattering layer. In this work, the TiO2 hollow spheres with anatase phase were synthesized according to our reported template-free chemically induced self-transformation (CIST) method [37–40], then four kinds of different doublelayered TiO2 composite film electrodes were fabricated and their photovoltaic conversion performances were investigated and compared. 2. Experimental 2.1. Preparation of TiO2 hollow spheres

Fig. 1. Schematic illustration of (a) interface contact difference between HS and P25 nanoparticle based electrodes and (b) four double-layered film electrodes made from P25 nanoparticle layer and HS layer.

advantages of large surface area and sufficient scattering effect of the film can be preserved [20,25]. Based on this strategy, a greatly improved conversion efficiency of 10.23% has been reported [20]. Apart from TiO2 solid spheres, TiO2 hollow spheres (HS) was also investigated and applied in DSSCs for their unique hollow structures. For example, Kondo et al. [36] in 2008 reported preparation and enhanced photocatalytic activities and dye-sensitized solar cell performance of submicron-scale TiO2 hollow spheres. In 2009, Qian et al. [17] reported fabrication of TiO2 -coated multilayered SnO2 hollow microspheres and their application in dye-sensitized solar cells, and a photo-electric conversion efficiency of 5.65% was obtained. Furthermore, Koo et al. [34] found that nanoembossed hollow spherical TiO2 could be used as bifunctional material for the fabrication of high-efficiency dye-sensitized solar cells and 10.25% photovoltaic conversion efficiency was observed. Very recently, we reported fabrication and enhanced conversion efficiency of dye-sensitized solar cells based on anatase TiO2 hollow spheres obtained by self-transformation method [33]. It is generally believed that the scattering effect of light on the air/particle shell interface and within the hollow cavity leads to the increase of the optical path length and the enhancement of light absorption, thus resulting in an enhanced photoelectric performance. Moreover, because the efficient diffusion of I3 − /I− to regenerate the dye is important to the photovoltaic response of the solar cells, the novel structure of HS with a large pore size and high porosity is beneficial to the penetration of electrolytes, thus leading to a better conversion efficiency of the cells [33,35]. However, TiO2 HS-based solar cells exhibit somewhat unexpected poor photovoltaic performances when comparing with nanoparticle-based solar cells. One common feature of these HS-based DSSCs is if the HS layer was directly deposited on the surface of FTO glass substrate, a lower contact area (see Fig. 1a) between the hollow spheres and FTO glass substrate greatly reduces the electron collection efficiency of FTO glass substrate, thus leading to a poor photovoltaic performance. It has been proposed that the TiO2 working electrodes composed of a transparent nanoparticle layer and a scattering layer over the nanoparticle layer are promising to obtain a better photovoltaic performance [29], because a large interface contact between the nanoparticles and FTO glass substrate (see Fig. 1a) will enhance the transfer of electrons from the nanoparticle film to FTO glass substrate, meanwhile, the scattering layer over the nanoparticle film enhance the absorption of light. However, to the best of our knowledge, there are few reports focusing on the doublelayered composite film electrodes composed of a nanoparticle

All chemicals used in this study were analytical-grade without further purification and were purchased from Shanghai Chemical Reagent Factory of China without further treatment. Distilled water was used in all experiments. The anatase HS were prepared by the CIST method according to our pervious work [37–40]. In a typical preparation procedure, Ti(SO4 )2 , NH4 F and CO(NH2 )2 were sequentially added to 150 ml distilled water under vigorous stirring and their molar ratio was 1:1:2. After stirring for 30 min, the mixed solution was transferred to a 200 ml Teflon-lined autoclave. The autoclave was sealed and kept at 180 ◦ C for 12 h and then air cooled to room temperature. The white precipitates were collected and washed with distilled water and anhydrous alcohol for three times and then dried in a vacuum oven at 80 ◦ C for 12 h. 2.2. Fabrication of DSSCs To fabricate double-layered composite film electrodes, two kinds of screen-printable pastes, containing the TiO2 hollow spheres and commercial Degussa P-25 TiO2 nanoparticles (P25), respectively, were prepared by mixing 0.5 g of TiO2 HS powders (or P25 powders), 0.5 ml of acetylacetone, 0.5 ml of Triton X-100 and 5.0 ml of anhydrous ethanol in an agate mortar and then grinding for 30 min. The FTO glasses (Nippon sheet glass, 14–20  sq−1 ) were cleaned ultrasonically with acetone, ethanol and distilled water for 15 min, respectively, and then dried in air. The films were prepared by depositing the paste on FTO glass substrates using the doctor blade method. The film thickness was controlled by adhesive tapes that were previously covered on the edges of FTO glasses. After one TiO2 layer was deposited, the films were dried at room temperature and annealed at 450 ◦ C for 30 min in air. Then, another TiO2 layer was coated on the as-coated TiO2 film, dried and annealed at 450 ◦ C for 30 min again. The thickness of the calcined composite double-layered film was about 28 ± 2 ␮m. For comparison, four groups of double-layered film electrodes with different configuration were fabricated, as shown in Fig. 1b. PP and HH electrodes are composed of two same P25 and HS layers, respectively. On the contrary, HP and PH electrodes are alternately made from HS layer and P25 layer (see Fig. 1b). After calcination, the film electrodes were cooled down to 80 ◦ C and were ready for dye sensitization. The dye solution was prepared by dissolving 0.3 mM cis-bis(isothiocyanato)bis(2,2 bipyridy1-4,4 dicarboxylato) ruthenium (II) bis-tetrabutylammonium (N719, Solaronix S.A., Switzerland) in 1 L anhydrous ethanol. Dye sensitization was carried out by immersing the calcined TiO2 films in dye solution at room temperature for at least 24 h in a sealed beaker. The extent of sensitization was estimated by comparing the colors at the top and bottom of the film. The sensitization was completed after the colors at the top and bottom were the same. The sensitized films were washed with anhydrous alcohol one time, and then dried in an oven at 80 ◦ C for 2 h. A solar cell was assembled in a typical sandwich-type cell by placing a platinum-coated conducting glass on the dye-sensitized electrode separated by a ca. 50 ␮m

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Fig. 2. SEM (a) and TEM (b) images of TiO2 hollow spheres prepared by CIST method, inset of (a) showing high-magnification SEM image of the region of a single hollow sphere.

polymer spacer. The assembled cell was then clipped together as an open cell. An electrolyte was made with 0.3 M LiI (Aldrich), 0.05 M I2 (Aldrich), 0.6 M 1-propyl-3-methylimidazolium iodide (Suzhou, China), and 0.5 M tert-butylpyridine (Aldrich) in dry acetonitrile (Shanghai, China). To ensure the same batch of electrolyte was used for every sample, a larger amount of electrolyte solution was prepared at first and then divided into several smaller glass vials. All vials were sealed and stored in a vacuum desiccator. The electrolyte was injected into the open cell from the edges and a thin layer of electrolyte was attracted into the inter-electrode space by capillary forces, and the cell was tested immediately.

toelectric conversion efficiency () was calculated according to the following equation:

2.3. Characterization

Fig. 2a and b shows representative SEM and TEM images of TiO2 hollow spheres prepared by the CIST method, respectively. The sizes of the prepared hollow spheres are in the range of 800–1000 nm, as clearly shown in Fig. 2a. The corresponding TEM image (Fig. 2b) further confirms the existence of hollow cavity in the as-prepared samples. The dimension of the hollow cavity is in the range of 300–700 nm. The high magnification SEM image of a single hollow sphere (inset in Fig. 2a) shows that the surface of the hollow spheres is composed of many loosely packed nanoparticles with an average diameter of about 40 nm. The phase structures of hollow spheres and P25 samples were measured by the X-ray diffraction. All the diffraction peaks for the as-prepared hollow sphere sample in Fig. 3a can be indexed to anatase phase (JCPDS no. 21-1272, space group: I41/amd (1 4 1); ˚ c = 9.514 A) ˚ and no other impurities are observed from a = 3.785 A, the XRD patterns. After annealed at 450 ◦ C, the diffraction peaks of hollow sphere sample become sharper, indicating the enhancement of crystallization and the growth of crystallites. The XRD

VOC ISC FF × 100 Pin

The measurements were repeated three times for each sample, and the experimental error was found to be within ca. 5%. 3. Results and discussion 3.1. Microstructure and phase structure of TiO2 HS

A Relative intensity (a.u.)

The morphology observation was performed by a JSM-5610LV scanning electron microscopy (SEM, JEOL, Japan) at an accelerating voltage of 20 kV and an S-4800 field emission SEM (FESEM, Hitachi, Japan) at an accelerating rate of 10 kV. Transmission electron microscopy (TEM) analyses were carried out by a JEM-2100F electron microscope (JEOL, Japan) with an accelerating voltage of 200 kV to observe the hollow structure of the samples. X-ray diffraction (XRD) patterns were measured on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) using Cu K␣ irradiation at a scan rate (2) of 0.05◦ s−1 to determine the phase structure and the average crystalline size of as-prepared samples. The accelerating voltage and applied current were 40 kV and 80 mA, respectively. The crystallite size of TiO2 powders was quantitatively calculated using Scherrer equation (d = 0.9/B cos ␪, where d, ,  and B are crystallite size, Cu K␣ wavelength (0.15481 nm), Bragg’s diffraction angle and full width at half maximum intensity of (1 0 1) peak in radians, respectively) after correcting the instrumental broadening. Nitrogen adsorption–desorption isotherms were obtained by an ASAP2020 (Micromeritics Instruments, USA) nitrogen adsorption apparatus. The BET surface areas (SBET ) of the sintered HS and P25 powders respectively scrapped from the calcined HH and PP film electrodes were calculated by a multipoint BET method using the adsorption data in the relative pressure (P/P0 ) range of 0.05–0.25. The pore-size distribution was determined by the Barret–Joyner–Halender (BJH) method. The diffused reflection spectra of bare and dye-sensitized film electrodes were measured by a UV-visible spectrometer (UV2550, Shimadzu, Japan) equipped with an integrating sphere. BaSO4 was used as a reflectance standard, and one piece of FTO glass was used as reference to eliminate the influence of glass substrate in UV–visible diffused reflectance measurements. The photocurrent–voltage I–V characteristic curves were measured by a CHI660C electrochemical analyzer (CH Instruments, Shanghai, China) controlled by a computer. The solar light was produced by a solar simulator (Newport 91160) at 100 mW cm−2 (1 sun) intensity. The active area of DSSC was 4 mm × 4 mm. The pho-

 (%) =

A: Anatase R: Rutile

R c

A A A/R A R R A A R

b a 10

20

30

40

50

60

70

80

2 Theta (degree) Fig. 3. XRD patterns of as-prepared hollow spheres (a), 450 ◦ C-calcined hollow spheres (b) and 450 ◦ C-calcined P25 powders (c).

J. Yu et al. / Electrochimica Acta 56 (2011) 6293–6298 Table 1 Comparison of physical properties of annealed P25 and HS powders scrapped from FTO glass substrate.

250 200

Samples

Phasea

SBET (m2 g−1 )

Pore volume (cm3 g−1 )

Porosity (%)

Average crystallite sizeb (nm)

P25 HS

A/R A

43.3 88.4

0.16 0.33

37.9 54.8

24.0 (A) 12.6

3

Volume adsorbed ( cm /g, STP)

6296

100

b

a

150 1

a

10

100

Pore diamater (nm)

a

b

50 0 0.0

A and R denote anatase and rutile, respectively. Average crystalline size is determined by the broadening of anatase (1 0 1) facet diffraction peak using Scherrer equation. b

0.2

0.4

0.6

0.8

1.0

Relatvie pressure ( P/P0) Fig. 4. N2 adsorption–desorption isotherm and the corresponding pore size distribution (inset) of hollow spheres (a) and P25 powders (b) scrapped from annealed film electrodes.

pattern of annealed P25 sample indicates that P25 contains two phases of anatase and rutile, and a 450 ◦ C-calcination has no influence on the phase structures of P25. Usually, porous film electrode with higher specific surface areas and bigger pore volume are beneficial to the enhancement of photoelectric conversion performance due to more surface active sites available for the adsorption of dye molecules, ease transportation of electrolyte through the interconnected porous networks, and enhanced harvesting of light [15,32]. Therefore, BET surface areas and pore structures of as-prepared samples are investigated by the adsorption–desorption measurement. Fig. 4 shows the nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) of calcined HS and P25 film electrodes. It can be seen from Fig. 4 that the HS and P25 samples have isotherms of type IV according to Brunauer–Deming–Deming–Teller (BDDT) classification, indicating the presence of mesopores (2–50 nm) [41]. The shapes of hysteresis loops are of type H3 at a high relative pressure range of 0.8–1.0, indicating the presence of slit-like pores. The isotherms show high absorption at high relative pressure (P/P0 ) range (approaching 1.0), implying the formation of large mesopores and macropores [42,43]. Further observation shows that isotherms of HS shift up comparing with P25, suggesting HS sample with higher surface areas. The pore size distribution curves (see inset of Fig. 4) calculated from the desorption branch of the nitrogen isotherms by the BJH method show a wide range of 2–100 nm with a peak pore diameter of about 8 nm for sample HS and 48 nm for sample P25, further confirming the presence of mesopores and macropores. It should be noted that more macrop-

orous information (larger than 100 nm) of the HS film electrodes can not be directly obtained by N2 adsorption–desorption analyses, however, the macroporous structures are directly observed by cross-sectional SEM image of the film electrode (see Fig. 5), indicating that the 450 ◦ C-calcined hollow sphere film overlayer is composed of a disordered macroporous network, consisting of many large void spaces with several tens to several hundreds nm in size between hollow spheres. Also, every hollow sphere contains a spherical space of 300–700 nm in size within the hollow spheres and several to several tens nm mesopores in shell walls. These three-dimensional macroporous/mesoporous network can enhance the absorption of light and transfer of electrolyte [44,45]. Therefore, SEM, TEM and N2 adsorption analysis reveal that the prepared HS film electrodes possess a hierarchically multi-modal pore-size distribution from mesopores to macropores. The physical properties of annealed HS and P25 powders scrapped from the corresponding film electrodes are summarized in Table 1. HS sample has a larger surface area, pore volume and porosity than P25 sample. Contrarily, HS sample has a smaller crystallite size. 3.2. Diffused reflection of double-layered film electrodes To investigate the optical properties of four kinds of film electrodes, diffused reflectance spectra were measured before and after sensitization. Fig. 6a and b shows diffused reflectance spectra of four double-layered film electrodes before and after dye sensitization, respectively. In all measurements, the samples were irradiated from the side of the glass substrate on which the film was not deposited. When a beam of solar light passes FTO glass and contacts with the TiO2 film electrodes, the incident light is supposed to be reflected, scattered and transmitted within the film electrode. In general, the part of incident light scattered within a film and returned to the surface is considered to be diffused reflectance [46]. Diffused reflectance spectrum is a useful measurement of revealing light scattering ability of samples. The striking difference of diffused reflectance between HH and PP film electrodes clearly indicates that TiO2 hollow spheres have a much higher light-scattering ability

Fig. 5. Schematic illustration (left) of scattering of incident light and cross-sectional SEM image (right) of double-layered PH electrode consisting of a P25 underlayer and a HS overlayer.

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Table 2 Comparison of the I–V characteristics of double-layered DSSCs made from P25 layer and HS layer. Samples

Thickness (␮m)

Configuration

ISC (mA cm−2 )

VOC (mV)

FF

 (%)

HH PP HP PH

27.8 28.6 29.7 (20.1/9.6) 26.5 (7.3/19.2)

HS/HS P25/P25 HS/P25 P25/HS

13.2 9.24 12.6 12.4

707 753 725 732

0.506 0.622 0.540 0.582

4.72 4.33 4.93 5.28

than P25 nanoparticles (see Fig. 6a). This intense scattering ability of HS can be ascribed to at least two reasons. First, the unique hollow structure of HS is beneficial for light scattering, because refraction between the shell and air-filled cavity, and multireflection within the hollow cavity can greatly increase the number of light scattering and therefore the path length of incident light, thus resulting in an enhanced scattering intensity [6,17,36]. Second, the prepared HS has an optimal particle size (about 1 ␮m) and void volume (about 55%), which is beneficial to the enhancement of light scattering [47]. On the contrary, PP film electrode shows the lowest reflectance in the wavelength range of 400–800 nm. This is mainly caused by the ineffective scattering effect of P25 nanoparticles. It is also reported that the reflectance intensity depends on both the scattering ability of particles themselves and the direction of scattering light within the film [47]. Thus, the strong diffused reflectance of HH electrode implies that the optical path length is extended within the film, meanwhile, a certain proportion of incident light is scattered away from the electrode. This conclusion is further supported by the reflectance results of HP and PH electrodes. For example, the reflectance of HP electrode is not greatly reduced comparing with that of HH electrode, also larger than that of PP electrode. However, the reflectance of PH electrode shows a larger decrease than HP electrode in the visible wavelength region. The above results

Diffused reflection (%)

80

a

60

40

HH HP PH PP

20

0 400

500

600

700

800

indicate when the scattering HS layer is directly deposited on FTO glass substrate a large amount of light is reflected away from the film, thus producing the most intense reflectance light. When a P25 nanoparticle layer is first deposited on FTO glass substrate, as shown in Fig. 5, the scattered light from the HS layer is adsorbed by the P25 layer. Therefore, it is not surprising that a lower reflectance spectrum for PH film electrode is observed in comparison to HP and HH electrodes because the light can be multi-captured within PH electrode, The diffused reflectance spectra of four dye-sensitized film electrodes are indicated in Fig. 6b. All the samples show a significant decrease in reflectance after sensitization. Especially, the reflectance of four electrodes decreases significantly in the short wavelength range (<600 nm) due to the light adsorption of adsorbed dye molecules [34]. Further observation shows that HH film electrode shows the lowest reflectance in the wavelength of lower than 550 nm, indicating more dye molecules adsorbed on this film electrode because of its high specific surface areas, thus a higher photocurrent density is obtained (see Fig. 7 and Table 2). Moreover, the dye-sensitized HH film electrode still demonstrates a relative high reflectance in the wavelength of 550–800 nm. This also further confirms the intense light scattering of HH film electrode. 3.3. Photovoltaic performance of DSSCs It is well known that the overall conversion efficiency () of DSSCs depends on the short-circuit photocurrent density (ISC ), the open-circuit photovoltage (VOC ), the fill factor of the cell (FF) and the intensity of the incident light (Pin ) [5]. Fig. 7 presents comparison of I–V curves of four double-layered solar cells made from P25 and HS layers. Table 2 further lists the related photovoltaic parameters of four kinds of solar cells. HH solar cell made from two layers of TiO2 hollow spheres presents the highest ISC (13.2 mA cm−2 ) due to its strong light scattering ability and large specific surface area. Its photovoltage, however, is the smallest (only 0.707 V) among the four solar cells, thus, leading to a relative small conversion efficiency (4.72%), which is lower than that of PH and HP solar cells. This smallest VOC is due to the poor contact between TiO2 hollow spheres and FTO glass substrate (see Fig. 1a), resulting in the increased resis-

Wavelength (nm) 14

60 -2

Current density ( mA cm )

Diffused reflection (%)

b

40

HH HP PH PP

20

0 400

500

600

700

800

Wavelength (nm) Fig. 6. Diffused reflectance spectra of four double-layered film electrodes HH, PP, HP and PH before (a) and after (b) dye sensitization.

12

PP HH HP PH

10 8 6 4 2 0 0.0

0.2

0.4

0.6

0.8

Voltage (V) Fig. 7. Comparison of the I–V characteristics of four double-layered solar cells made from P25 and HS layers.

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tance. On the contrary, a largest VOC (0.753 V) is obtained for PP solar cells. This is not surprising because PP solar cell has the better contact between TiO2 nanoparticle film and FTO glass substrate (see Fig. 1a), which results in the lowest contact resistance and highest photovoltage. However, its short-current density decreases to the lowest (9.24 mA cm−2 ) owing to its small specific surface area and low reflectance, causing less dye molecules adsorbed and poor light-scattering ability, respectively. Consequently, the overall conversion efficiency of PP solar cell is only 4.33%, lower than that of HH solar cell. It can be concluded from the above results that the DSSCs made from two same layers (HS or P25 layers) are not promising to achieve a higher photovoltaic performance. Although the HS layer is able to enhance scattering of light, its poor contact with FTO glass substrate leads to a higher interface resistance. On the other hand, the P25 layer has a good contact with FTO glass substrate, but is lack of light-scattering effect. Therefore, it is reasonable to infer that the combination of HS layer and P25 layer within one film electrode is promising to obtain better photovoltaic conversion efficiency. After combining HS layer and P25 layer in double-layered electrodes, HP solar cell demonstrates an enhanced photovoltaic performance and an overall conversion efficiency of 4.93% obtained. Its efficiency is higher than that of HH and PP solar cells. Further investigation indicates that the deposition sequence of HS layer and P25 layer has an obvious influence on the performance of the double-layered composite film solar cells. PH film solar cell shows a similar photocurrent density with HP solar cell, but higher photovoltage (0.732 V) and photo-electric conversion efficiency (5.28%). This is because the former has better interface contact between PH film electrode and FTO glass substrate. In addition, its nanoparticle underlayer is also beneficial to recapture the scattering light from the above scattering layer (see Fig. 5) [29,48]. The hollow spheres scattering overlayer, on the other hand, facilitates the electrolyte transport within the film. 4. Conclusions In summary, TiO2 hollow sphere film electrodes not only have high specific surface areas and hierarchically porous structures, but also exhibit high light-scattering ability and good photo-electric conversion efficiency. The combination of TiO2 hollow sphere layer and P25 nanoparticle layer within one electrode is promising to enhance photovoltaic conversion efficiency of DSSCs. DSSCs based on double-layered composite films of TiO2 nanoparticles/hollow spheres (PH) exhibit the highest photo-electric conversion efficiencies mainly due to the combined effect of two factors, the high light scattering of overlayer hollow spheres enhancing harvesting light of the DSSCs and the underlayer TiO2 nanoparticle layer ensuring good electronic contact between film electrode and conducting substrate. The concept of double-layered composite film electrodes will provide new insight into fabrication and structural design of highly efficient dye-sensitized solar cells. Acknowledgements This work was partially supported by the National Natural Science Foundation of China (20877061 and 51072154) and the

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