Dye-Sensitized Solar Cells Employing a Multifunctionalized

Jan 23, 2014 - Then the films coated with TiCl4 aqueous were calcined at 500 °C for another 30 ... were recorded on a BET Tristar II 3020 M instrumen...
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Dye-Sensitized Solar Cells Employing a Multifunctionalized Hierarchical SnO2 Nanoflower Structure Passivated by TiO2 Nanogranulum Haihong Niu,†,‡ Shouwei Zhang,† Renbao Wang,† Zhiqiang Guo,† Xin Shang,† Wei Gan,† Shengxian Qin,† Lei Wan,† and Jinzhang Xu†,‡,* †

Hefei University of Technology, Hefei, 230009, Anhui Province, P. R. China School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, Gansu Province, P. R. China

‡

S Supporting Information *

ABSTRACT: We investigated a facile multifunctionalized hierarchical SnO2 nanoflower photoelectrode passivated by a layer of TiO2 nanogranulum. The hierarchical SnO2 nanoflower with thin nanorod and nanosheet has a unique morphology that can afford excellent electron transport propertiesorientation overall, which results in a significant diminution in the charge diffusion route and a rapid collection in FTO substrate. The passivated photoanode not only improved the distribution of dyes in the photoelectrode and reduced the surface defects of SnO2 photoelectrode to accommodate more dyes, but also suppressed the charge recombination and prolonged electron lifetime by introducing a barrier layer. The microstructure of the sample was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface areas (SBET) and pore size distribution were detected on BET measurement. The amounts of dye were calculated from UV−vis. The interfacial charge transfer process and the charge recombination were characterized by EIS and IMPS/IMVS measurements. The DSSCs assembled with multifunctionalized photoanode exhibits favorable energy conversion efficiency. The photocurrent increased from 5.44 to 12.74 mA cm2, the photovoltage from 440 to 760 mV, and the fill factor from 43.58% to 57.58%. As a result, the cell’s conversion efficiency increased by a factor of 5.3 from 1.05% to 5.60%. The increase in efficiency originates from higher open-circuit potential and higher short-circuit current as well as from superior light scattering effect, long electron lifetime, and slower electron recombination.

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INTRODUCTION In the mid-1990s, the exploration of the third generation of dye-sensitized solar cells (DSSCs) that aim for lower costs, while maintaining high solar energy conversion efficiencies garnered significant interest, building upon the milestone discovery of solution-processed solar technologies by O’Regan and Gräetzel.1,2 Porous nanocrystalline TiO2 is commonly used as the photoelectrode of DSSCs and its power conversion efficiency (PCE) has exceeded 11% according to reports.3 Tin oxide, as an n-type semiconductor with a wide direct band gap of 3.6 eV at 300 K,4 has many advantageous features compared to TiO2 for DSSCs applications: (1) It has a higher electron mobility (∼125−250 cm2 V−1 S−1)5,6 than TiO2 (95%) was obtained from Solaronix SA, Switzerland. All the reagents of analytical purity were used as received. Synthesis of Hierarchical SnO2 Nanoflower Nanostructure. In a typical experiment, 1.0738 g (5 mmol) of SnSO4 and 4.4115 g (15 mmol) of Na3C5H6O7·2H2O were dissolved in a mixture of ethanol (10 mL) and deionized water (90 mL) under magnetic stirring for 3 h to form a homogeneous solution. The suspension was then transferred to a 100 mL Teflon-stainless steel autoclave. After sealing, the autoclave was treated at 160 °C for 12 h. After the hydrothermal procedure, the autoclave was cooled down to room temperature. The precipitates were collected by centrifugation, washed several times with deionized water and ethanol, respectively, and dried at 60 °C for 6 h. Finally, the SnO2 hierarchical nanostructures were obtained by annealing the precipitates at 500 °C for 1 h under an air atmosphere. Preparation of SnO2 Paste. SnO2 powders (0.1 g) were placed in an agate mortar, and 5.0 mL of ethanol was added dropwise into the mortar. The SnO2 powders were ground for 30 min. The ground SnO2 was then transferred to a solution of ethanol (50 mL), terpineol (4.0 g), and ethyl cellulose (0.60 g) in a 100 mL beaker under magnetic stirring. The dispersion was homogenized by means of ultrasonic and magnetical stirring for 3 h. Finally, the solvent was removed by a rotary evaporator at 45 °C until a concentration of SnO2 (14% wt) was obtained. Fabrication of DSSCs. A layer of SnO2 film was first coated on the TiCl4 aqueous (50 mM) pretreated cleaned fluorine doped tin oxide (FTO) glass plates (Nippon Sheet Glass Co., resistance = 15 Ω/□, transmittance = 90%) by using a screenprinting technique. The film was dried at 125 °C for 6 min and further sintered at 500 °C for 30 min in air to remove any organic compounds. The resulted SnO2 films were then immersed in 100 mM TiCl4 aqueous solution in a closed vessel at 70 °C for 30 min. Then the films coated with TiCl4 aqueous were calcined at 500 °C for another 30 min before dye sensitization. The electrodes with an active cell area of 0.25 cm2 were immersed in a 0.5 mM N719 sensitizer dye in absolute ethanol for 24 h. The counter-electrodes were Pt screenedprinted coated FTO, and the electrolyte was contained I−/I3− redox. The DSSCs were sealed with Surlyn sealant (Surlyn1702, Dupont). The DSSCs with TiCl4 post treatment and without TiCl4 post treatment were designed by SnO2− TiCl4(100) cell and SnO2−TiCl4(0). For comparison, a 100 mM TiCl4 treated P25 TiO2-based cell was fabricated under identical conditions and designed with P25−TiCl4(100). Characterization. X-ray diffraction (XRD) measurements were conducted on an X’Pert PROS (Philips Co.) with a radiation of Cu−Kα (λ = 1.54060 Å). Scanning electron microscopy (SEM) measurements were undertaken by using a field emission environmental scanning electron microscope (Sirion 200, FEI Co.). Transmission electron microscope (TEM) and high resolution TEM (HRTEM) were used to study the morphology and microstructure of the materials by a 3505

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Scheme 1. (a, b) Illustration of the Electron in Which the Hierarchical SnO2 Nanoflower Transfers Rapidly to the FTO Substrate and (c) Diagram of Electron Transfer (black arrow), Recombination (red Arrow), and Hole Transfer (Green Arrow) Processes in DSSCs Based on Hierarchical SnO2 Nanoflower as Well as Energy Band Diagrams of SnO2, TiO2, and N719 Dye and Energy Band Matching with Respect to the Electrochemical Scale at pH = 1

frequency range was explored from 3 MHz to 10 mHz and the ac amplitude was 10 mV. The EIS data were fit to the equivalent circuits by using Zview software (Scribner Associates). Impedance measurements were carried out under illumination from LED. IMPS/IMVS measurements were carried out with the same instrument used for EIS measurements. The LED provided both dc and ac components of the illumination. A small ac component was 10% or less than that of the dc component. The frequency range is 3 kHz to 0.1 Hz.

JEM-2100 (JEOL) instrument. N2 adsorption−desorption isotherms were recorded on a BET Tristar II 3020 M instrument (Micromeritics Co.), and the specific surface areas (SBET) were calculated using the BET equation. Desorption isotherm was used to determine the pore size distribution using the Barret−Joyner−Halender (BJH) method. The concentration of desorbed dye in film was calculated from UV−vis absorption spectra (UV-2550, Shi-madzu). The photovoltaic performance of DSSCs was measured under a solar simulator (Oriel Sol 3A Solar Simulator, 94063A, Newport Stratford Inc.), equipped with a 300 W xenon lamp (Newport) and a Keithley digital source meter (Keithley, 2400) controlled by Testpoint software. The irradiation intensity was calibrated to 100 mW·cm−2 with standard reference crystalline silicon solar cell (Newport, Stratford Inc., 91150 V). The incident monochromatic photon-to-electron conversion efficiency (IPCE) plotted as a function of excitation wavelength, were recorded on a QTest Station 1000 ADI system (Crowntech, Inc.) equipped with a 300 W Xe lamp. The monochromatic photocurrent-wavelength measurements were carried out by placing a monochromator, assisted by an automatic filter wheel, between the dye-sensitized solar cells and the light source (100 mW·cm−2). EIS measurements were studied on IM6ex (Germany, Zahner Company) using light emitting diodes (λ = 455 nm) driven by Expot (Germany, Zahner Company). The

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RESULTS AND DISCUSSION Electron Transport and Recombination Pathways. As illustrated in Scheme 1a, the hierarchical SnO2 nanoflower with thin nanorod and nanosheet has a unique morphology that can afford excellent electron transport properties. Electrons are transferred orientedly overall and randomly locally in nanorod and nanosheet, which results in a significant diminution in the charge diffusion route and a rapid collection in FTO substrate. Moreover, in Scheme 1b, the excellent transport properties of interconnected hierarchical SnO2 nanoflower should allow the electron flow to be more channeled and make fast transport of electrons to the substrate, meanwhile lowering the recombination rate. Scheme 1c shows a diagram of electron transfer (black arrow), recombination (red arrow) processes, and hole transfer processes (green arrow) in DSSCs based on 3506

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hierarchical SnO2−TiCl4(100) nanoflower as well as the energy band diagram of SnO2, TiO2, and N719 dye at pH = 1 with respect to the electrochemical scale. The photoanode based on hierarchical SnO2−TiCl4(100) nanoflower was fabricated by coating TiO2 nanogranulum with SnO2 nanoflower scaffold and was designed based on band-edge engineering. In this system, the conduction band edge of SnO2 is 0.5 V more positive than that of TiO2. Thus, the electrons injected into TiO2 from the excited dye and then efficiently injected afterward into SnO2. Thus, in the open-circuit state, photogenerated electrons would be guided from the TiO2 conduction band to the SnO2 conduction band, with suppressed recombination. Consequently, abundance electrons accumulate on the SnO2 and a few holes accumulate on the TiO2 through diffusion in the hierarchical SnO2 nanoflower nanoparticle electrode. Characterization of Hierarchical SnO2 Nanoflower Nanostructure Material. The morphology of the obtained SnO2 products is first characterized by SEM and TEM techniques. Parts a and b of Figure 1 display a typical SEM

of SnO2 nanoflower could present an oriented and shorter transit pathway for rapid charge transport as well as a slow recombination rate both in the bulk photoanode and at the photoanode/electrolyte interface. The high resolution TEM image (Figure 1c) indicates that the hierarchical SnO2 nanoflower has a lattice spacing of 0.335 nm, corresponding to the (110) lattice plane of tetragonal rutile SnO2. And the corresponding selected area electron diffraction (SAED) pattern (Figure 1f) presented indicates that the hierarchical SnO2 are polycrystalline. Consequently, it is expected that the hierarchical SnO2 nanoflower with 2D ul-thin nanosheets and 1D nanorods could possess a rapid charge transport instinct, superior light-scattering capability, and produce a multireflection of incident light in-between the hierarchical nanoflower petal, so as to improve the efficiency of light harvesting.21 The product is further characterized by nitrogen adsorption and desorption isotherms at 77 K and corresponding pore size distribution in Figure S1, Supporting Information. It is found that the overall SnO2 nanoflower have a BET surface area of 15.8 m2/g with an average Barret−Joyner−Halenda (BJH) pore diameter of 30.2 nm and a pore volume of 0.05 cm3/g. The large surface area of hierarchical SnO2 nanoflower ensures sufficient dye loading. The isotherm is characteristic of a type IV with a type H3 hysteresis loop, revealing that the material is composed of aggregates (loose assemblages) of sheet-like particles forming slit-like pores, in correspondence with our microscopy findings.22 Characterization of Hierarchical SnO2−TiO2 Multifunctionalized Photoelectrode. Coated with TiO2, the SnO2−TiO2 still maintained the nanoflower structure but had many coarse surfaces due to surface coating a layer of TiO2 nanogranulum with the size of 4−6 nm (marked by red curve). As shown by HR-TEM images in Figure 2a, the TiCl4

Figure 1. (a) SEM, (b) high-magnification (HR) SEM, (c) HRTEM and (d) high- magnification TEM (e) images of as-synthesized hierarchical SnO2 nanoflower and (f) selected area electron diffraction (SAED) pattern of SnO2 nanoparticles. The scale bars in figure represent (a) 220 nm, (b) 120 nm, (c) 5 nm, (d) 90 nm, and (e) 250 nm.

image and corresponding high-magnification of the as-prepared products. The as-prepared sample contains uniform particles with a diameter about 1 μm, and each particle consists of random-shaped nanosheets and nanorods pointing radially outward. The SnO2 nanosheets have thickness of 12−17 nm, which implies short diffusion pathways, as demonstrated in Scheme 1a. Interestingly, the morphology of the assembled nanosheets and nanorods reveals a three-dimensional as well as a nanoflower like nanostructure, which can be ascribed to the minimization of surface energy. The inherent structure is shown in a TEM image (Figure 1, parts d and e). In those pictures, the bare SnO2 nanorods are elongated, around 100 to 130 nm in length and 60 nm in diameter, with clean and welldefined surfaces. The light regions suggest planar nanosheets or thin nanorods lying on the substrate. Dark regions indicate that sheets may either lie aslant, perpendicular to the substrate or spontaneously stack during the reaction process. Scheme 1a and 1b also show that the 2D thin nanosheets and 1D nanorods

Figure 2. HR-TEM images of SnO2 nanoparticles immersed in a 100 mM TiCl4 aqueous solution after annealing at 500 °C for 30 min. The scale bars in figure represent 5 nm.

treatment would result in uniform coverage of TiO2 nanogranulum on the surface of SnO2. The TiO2 nanogranulum coating on the hierarchical SnO2 nanoflower may modify the chemical and photoelectrochemical properties of the SnO2 surface and thereby enhance dye absorption and injection efficiency of photoexcited electrons into semiconductors,21 inhibit the charge recombinations between the interfaces and lead to higher current density and efficiency. The HR-TEM images (Figure 2b) show the as-prepared SnO2−TiCl4(100) sample. Three local crystal planes turn out to belong to the {110} planes of rutile SnO2 (lattices: 0.335 nm), {101} planes 3507

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Figure 3. Diffused reflectance spectra of different films hierarchical SnO2−TiCl4(0) nanoflower, P25−TiCl4(100), and hierarchical SnO2− TiCl4(100) nanoflower: (a) without and (b) with adsorbed N719 dye.

Table 1. Comparison of Short-Circuit Photocurrent Density (Jsc), Open-Circuit Photovoltage (Voc), Fill Factor (FF), and Overall Photoconversion Efficiency (η) along with the Amount of Adsorbed Dye N719 for the Films Composed of Hierarchical SnO2−TiCl4(0) Nanoflower, P25−TiCl4(100), and Hierarchical SnO2−TiCl4(100) Nanoflower, Respectively DSSCs

TiCl4 treatment/mM

Voc (V)

Jsc (mA·cm‑2)

Pmax (mW)

FF (%)

PCE (%)

adsorbed dye (m2 g‑1 × 10‑8 mol cm‑2)

SnO2−TiCl4(0) SnO2−TiCl4(100) P25−TiCl4(100)

0 100 100

0.44 0.76 0.80

5.44 12.74 10.25

0.26 1.399 1.37

43.48 57.48 66.82

1.05 5.60 5.49

1.94 3.21 4.12

a larger size than P25−TiCl4(100) film and thus the former two films have a higher light-scattering ability than those in the P25−TiCl4(100) film.23 Those larger sized hierarchical SnO2 moieties could maximize the use of solar light and hence enhance the light harvesting efficiency and Jsc. As shown in Figure 3b, after dye adsorption on the films, the reflectance of the all films show drastic decreases and the SnO2−TiCl4(0) film resembles that of the dye-adsorbed P25−TiCl4(100) film in the short wavelength region ranging from 300 to 400 nm, which is mainly due to light absorption by the dye molecules.23 Both SnO2−TiCl4(0) film and SnO2−TiCl4(100) film show a high reflectance in the wavelength range from 580 to 800 nm. However, the SnO2−TiCl4(0) film still shows substantially the highest reflectance at 400−800 nm, revealing that the highest light-scattering effect is mainly attributed to the lowest dye adsorption of SnO2−TiCl4(0) film (Figure S3, Table 1). The improved light scattering effect of hierarcical structure with submicrometer-sized diameters can thus increase the light traveling path and may result in higher light harvesting efficiency and a corresponding higher photocurrent and the efficiency. The performances of DSSCs were strongly dependent on the amount of dye adsorption, which is related, in turn, to the surface area and porosity of the semiconductor photoelectrode. Thus, the adsorption−desorption curves of the three sample films were measured as shown in Figure S3 and are summarized in Table 1 (see the UV−vis spectra of the desorbed dye solutions). The coated TiO2 nanogranulum SnO2−TiCl4(100) film provides a much higher dye-loading of 3.21 m2 g−1 × 10−8 mol cm−2, while that for SnO2−TiCl4(0) film is only 1.94 m2 g−1 × 10−8 mol cm−2. The improved dye-loading in turn may enhance the light harvesting efficiency, the photocurrent density of the device, and then the overall efficiency. Characterization of Photovoltaic Properties of DSSCs Based on Hierarchical SnO2−TiO2 Photoelectrode. The incident photon-to-current conversion efficiency (IPCE) spectra from the devices are characterized and shown in Figure 4. The incident photon-to-current conversion efficiency are in

and {103} planes of anatase TiO2 (lattices: 0.352 and 0.243 nm). All these phenomena strongly support the TiO 2 nanogranulum coating on the surface of SnO2 nanoflower. The design photoanode structures through a band-structure matching strategy of the TiO2-based heterojunction, within which electrons could be collected from TiO2 nanoparticles and subsequently transported in a short-transit pathway. The influence of TiCl4 post treatment on the microstructure of the nanoflower is also investigated via XRD analysis. Figure S2 illustrates the typical XRD pattern of the P25−TiCl4(100), hierarchical SnO2−TiCl4(100) nanoflower, and hierarchical SnO2−TiCl4(0) nanoflower films. The peak of the hierarchical SnO2−TiCl4(0) nanoflower film can be indexed to the tetragonal rutile structure of SnO2 with lattice constants of a = 3.435 Å and c = 3.186 Å (JCPDS card 41−1445).11 No peaks are observed for other impurities, indicating the high purity of the prepared SnO2 nanostructures. After post-treated by TiCl4 aqueous solution, as shown in Figure S2, the SnO2−TiCl4(100) sample has the same XRD patterns as the SnO2−TiCl4(0) sample, and the two samples almost have the same intensity and width as those of the untreated sample. The corresponding crystallite size calculated from the Scherrer equation is 9.9 nm, which is close to that of the untreated sample. In other words, the TiCl4 post treatment did not change the size of the primary crystals considerably. While these two samples possess different morphologies, the peak intensity and peak width in their XRD patterns have no obvious differences, suggesting that the primary nanocrystals which compose the nanoflower have similar sizes, which supports the conclusion obtained from TEM observations. The evaluation of light scattering ability can be investigated by the UV−vis diffuse reflectance spectroscopy. Figure 3 shows the UV−vis diffuse reflectance spectra of different films without (Figure 3a) and with dye (Figure 3b). Clearly, in Figure 3a, the reflectance of hierarchical SnO2−TiCl4(0) nanoflower film and hierarchical SnO2−TiCl4(100) treatment nanoflower film is much higher than that of the P25−TiCl 4 (100) film, demonstrating that the particles in the former two films have 3508

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SnO2−TiCl4(100) film, which results from the well-interconnected network structure of the hierarchical SnO2−TiCl4(100) layer. This network allows increasing electron transport and reducing interfacial charge recombination loss, which will be characterized by dark current and EIS analysis. Higher cell performance in SnO2−TiCl4(100) also comes from the improved fill factor (FF) due to the facile and sufficient filtration of electrolyte into the SnO2−TiCl4(100) nanopores, as confirmed by the cross-sectional FE-SEM image in Figure 1. As a result, the cell’s conversion efficiency remarkably increased by a factor of 5.3 from 1.05% to 5.60%. Although the lower dye amounts compared with P25−TiCl4(100), a slightly higher Jsc and performance was obtained, which is a consequence of increasing light scattering.23 Furthermore, the conversion efficiency of the SnO2−TiCl4(100) cell reached 5.60%, which is one of the highest values observed for three DSSCs and approximately 1.02-fold greater than that of P25, indicating the importance of an hierarchical nanoflower structure. Therefore, the efficiency improvement is due to three factors: improved dye adsorption, promotion of light scattering ability, and increased electron transport. Figure 5b shows the darkcurrent−voltage curve of the DSSCs measured under foreward bias potential. The dark-current onset of SnO2−TiCl4(0) photoanode occurred at low forward bias, indicating faster electron-recombination kinetics. SnO2−TiCl4(100) photoanode gradually shifts the dark-current onset to a higher forward bias. This can be attributed to (1) the minimized direct contact among semiconductor and electrolyte and (2) an upward shift in the flat-band potential (VFB). The Voc of DSSCs is primarily determined by the difference between the VFB of the semiconductor photoanode and the redox potential of the electrolyte. The coating of TiO2 nanogranulum on the SnO2 framework presumably passivates the reactive sub-band-edge surface-states, thereby shifting the VFB toward a more negative value and enhancing Voc.24 Therefore, it can be reasonably concluded that the desiged hierarchical SnO2−TiCl4(100) nanoflower is crucial and complementary to the electron transport and light scattering, eventually yielding a higher energy conversion efficiency. EIS and IMVS-IMPS Spectroscopy Analysis. Electrochemical impedance spectroscopy (EIS) is a powerful tool for characterizing the interfacial charge transfer process in DSSCs. The EIS Nyquist and Bode plots for the DSSCs based on SnO2−TiCl4(0), SnO2−TiCl4(100), and P25−TiCl4(100) are shown in Figure 6. The values obtained for these parameters are summarized in Table 2. The EIS Nyquist plots (Figure 6a)

Figure 4. IPCE spectra of DSSCs with photoelectrode films composed of P25−TiCl4(100), hierarchical SnO2−TiCl4(100) nanoflower, and hierarchical SnO2−TiCl4(0) nanoflower.

the range of 40−57% and in the order of SnO2−TiCl4(0) < SnO2−TiCl4(100) < P25−TiCl4(100). The efficiency up to approximately 51% at 520 nm for the cell with SnO2− TiCl4(100) and about 40% for the cell with SnO2−TiCl4(0) under the same experimental conditions. These values are not in consistency with the corresponding PCE values (Table 1). The higher spectrum in the short wavelength region attribute to the better dye-loading of the SnO2−TiCl4(100) electrode, while the higher spectrum in the long wavelength region owe to the enhanced light scattering.15 Figure 5a presents the photocurrent as a function of the voltage for three DSSCs containing the hierarchical SnO2− TiCl4(0) nanoflower, hierarchical SnO2−TiCl4(100) treatment nanoflower, and P25−TiCl4(100) under one sun (AM 1.5 illumination) conditions. The performances of devices and the relevant data of the assembled DSSCs are summarized in Table 1. As shown in Figure 5a, although the SnO2−TiCl4(0) film has the highest light scattering ability, poor photovoltaic performance is observed, which may be derived from the lowest dyeadsorbed and the fastest photogenerated electron recombination. After a TiO2 nanogranulum passivation layer was introduced, SnO2−TiCl4(100) cell has an attractive performance with respect to all cell parameters compared with the SnO2−TiCl4(0) cell. The photocurrent increased from 5.44 to 12.74 mA cm−2, the photovoltage from 440 to 760 mV, and the fill factor from 43.48% to 57.48%. The increased short circuit current (Jsc) value is attributed to the higher isoelectric point in the TiO2 nanogranulum passivation structure,14 enabling to enhance dye molecule with acidic carboxyl groups loading. There was also an increase in the open circuit voltage (Voc) for

Figure 5. Light (a) and dark (b) current density−voltage characteristics of DSSCs with photoelectrode films composed of hierarchical SnO2− TiCl4(0) nanoflower, hierarchical SnO2−TiCl4(100) nanoflower, and P25−TiCl4(100) films. 3509

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Figure 6. Results of Nyquist plots (a) and Bode-phase plots (b) from EIS of the DSSCs based on different hierarchical SnO2−TiCl4(0) nanoflower, hierarchical SnO2−TiCl4(100) nanoflower, and P25−TiCl4(100) films; (c) the equivalent circuit model of Nyquist plots.

Table 2. Detailed EIS Parameters of Dye-Sensitized SnO2 Solar Cells Based on Different Hierarchical SnO2−TiCl4(0) Nanoflower, Hierarchical SnO2−TiCl4(100) Nanoflower, and P25−TiCl4(100) Treatment Filmsa

a

DSSCs

Rs(Ω)

R1(Ω)

R2(Ω)

Ws(Ω)

Rtotal(Ω)

f max

τ (ms)

SnO2−TiCl4(0) SnO2−TiCl4(100) P25−TiCl4(100)

2.6 2.8 3.1

6.0 8.4 18.6

23.1 100.8 211.2

42.4 2.4 3.8

74.1 114.4 236.7

6.309 9.843 63.533

3 16 25

Light intensity: 96 W m−2.

by I3− ions occurring in the recombination process at counter electrode.27 In Figure 6b, Bode plot, the frequency response plot is in the range of 1−100 Hz, which is indicative of the electron recombination between the electrolyte and SnO2 and is related to the electron lifetime (τ) in the CB of SnO2.28 The electron lifetime fitted by using Z-view soft ware for the SnO2− TiCl4(100) (16 ms) is longer than that of the SnO2−TiCl4(0) (3 ms), signifying the electron would transport through a longer distance. This means there is a reduction of electron recombination and consequently leads to an enhanced Voc for the SnO2−TiCl4(100) film compared with the SnO2−TiCl4(0) film.27 In general, the electron lifetime depends on the density of charge traps, which is ultimately related to Voc. In other word, the Voc value is quite sensitive to the electron lifetime in the conduction band of semiconductors.28 The Bode plot shows the lifetime or recombination time (τ) gradually increased from SnO2−TiCl4(0) to P25−TiCl4(100), which is allusive of the gradually enhanced open-circuit photovoltage.29 This trend is in agreement with the τ vs Voc results, implying that, relative to that of SnO2−TiCl4(0), the formation of a thicker compact layer using TiO2 granulum increases the lifetime of electrons in CB- SnO2, hence reducing the rate of charge recombination with the triiodide ions I, which corresponds well with the aforementioned J−V characterization data. The electron transport and charge recombination of the DSSCs based on three kinds of films were further characterized by intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS), which were conducted under illumination of a LED light source (λ= 455 nm) with different light intensities from 30 to 96 W m−2. Figure 7 plots τd and τr of photogenerated electrons as a function of the incident photon flux for the three DSSCs, where τd and τr are electron transit time across the photoanode films

exhibited three characteristic peaks. The electrochemical analysis reveals a counter electrode with conducting glass resistance (Rs), interfacial charge transfer resistance between the Pt counter electrode and electrolyte (R1), interfacial charge transfer resistance among oxide/dye/electrolyte interface (R2), and mass transport resistance related to the I−/I3− electrolyte redox couple, which is known as Warburg diffusion (Ws).25,26 As shown in Figure 6a, all of the resistances (except for Rw) were smallest in the DSSC with SnO2−TiCl4(0). In particular, the mass transport resistance (Ws) of the SnO2−TiCl4(100) cell was much smaller than that of other systems, indicating facile mass transport through the electrolyte.25 The Rs, R1, and R2 values of the three samples increases following the order of sample SnO 2 −TiCl 4 (0), SnO 2 −TiCl 4 (100), and P25− TiCl4(100), which indicates the fastest electron generation and transfer in SnO2−TiCl4(0) film, while the slowest was in the P25−TiCl4(100) film. However, the fastest recombination of electron in the SnO2−TiCl4(0) film and the slowest recombination in the P25−TiCl4(100) film were also observed. In particular, the fitting value of R2 corresponding to SnO2− TiCl4(100)−DSSCs (100.8 Ω) is much smaller than that of P25−TiCl4(100)−DSSCs (211.2Ω), which could be explained by the fact that the passivation of the TiO2 nanogranulum on hierarchical SnO2 nanoflower would favor electron transport over a longer distance with less diffusive hindrance. Furthermore, intersectional contact between one another of the nanoflower increases the transport channel of the injected electrons through adjacent nanoflower, avoiding the high resistance existing in the nanorod and nanosheet-based flower aggregates of hierarchical SnO2 nanoflower film.26 The result can be ascribed to the fact that unique hierarchical structure of the SnO2 flower overlayer consisting of TiO2 nanogranulum is beneficial for electrolyte penetration and electron transport, and hence leads to a slower recapture of conduction band electrons 3510

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highway to facilitate electron transport.29 Furthermore, the hierarchical nanoflower overlayer TiO2 nanogranulum films have fewer surface trapping sites for mediating recombination with I3− in the electrolyte.30 Because of the higher surface area of P25−TiCl4(100) anode, which is responsible for more abundant electron back reactions with I3− in the electrolyte, the electron lifetime of cell P25−TiCl4(100) is shorter than that of cell SnO2−TiCl4(100). The τd curve of the SnO2−TiCl4(100)derived DSSC lies higher than those of other two cells, indicating the transport time of SnO2−TiCl4(100) cell is larger than that of P25−TiCl4(100) cell and the lowest electron transport of the SnO2−TiCl4(100) cell29 which could be attributed to the presence of the more trapping sites in SnO2− TiCl4(100) hierarchical film30 and numerous grain boundaries between the SnO2 nanoflower scaffold and TiO2 nanogranulum35 according to the HRTEM results. Thus, the increased traps for the hierarchical structures allow for multiple trapping/detrapping events, which prolong the electron transport time.36 On the other hand, the electron transport rate in SnO2−TiCl4(0) and P25−TiCl4(100) anode is superior to that in SnO2−TiCl4(100) anode.30,35 Generally, electron diffusion lengths, Ln, represents the average travel distance of electrons before recombining with acceptors. This meaningful parameter is related to the range of thicknesses that is suitable for semiconductor material layers, which determines whether the injected electron could effectively transit to the external circuit in photovoltaic devices.26 The electron lifetime of the SnO2−TiCl4(100) cells was highest, but its diffusion coeffcients were lower than those in P25−TiCl4(100) and SnO2−TiCl4(0) cells. As a result, the electron diffusion lengths (Ln) of the SnO2−TiCl4(100) cells were lowest. Owing to the competition between the collection of photoinjected electrons and recombination, a photoanode with high charge-collection efficiency is highly required to improve the performance of DSSCs.37 Although the electron transport rate of SnO2− TiCl4(100) and SnO2−TiCl4(0) is slower than that of P25− TiCl4(100) nanoparticles film, the increase of the electron recombination lifetime τr of SnO2−TiCl4(100) and SnO2− TiCl4(0) compensates for the slow electron transport rate, and makes the charge-collection efficiency of SnO2−TiCl4(100) and SnO2−TiCl4(0) based DSSCs be comparable that of P25− TiCl4(100) nanoparticles based DSSCs. Another noteworthy aspect for the “SnO2−TiCl4(100) film” is that, its calculated charge collection efficiency (ηcc) equals 94% at 96 W m−2, which is lower than that of ‘‘P25−TiCl4(100) film” (ηcc = 97%), whereas its Jsc is even higher. Such an abnormality implies that there should be other factors responsible for such higher Jsc. As is well-known, the photocurrent density (Jsc) can be calculated by the following expression:38 Jsc = qηlhηin jηccI0, where q is the elementary charge, ηlh is the charge harvest efficiency of a cell, ηinj is the charge-injection efficiency, ηcc is the charge-collection efficiency, and I0 is the light flux. ηinj, here for these three cells, is assumed to be the different because of the although the CBM

Figure 7. Electron transport time and electron lifetime of the DSSCs fabricated with different hierarchical SnO2−TiCl4(0) nanoflower, hierarchical SnO2−TiCl4(100) nanoflower, and P25−TiCl4(100) films as a function of the incident light intensity.

and recombination times (lifetime) of electrons with I3− ions in the electrolyte, respectively.30 The τd and τr of the three samples can be estimated from the IMPS plots and the IMVS plots and can be calculated from the expression 1 and 2, where fd and f r is the characteristic frequency minimum of the IMPS and IMVS imaginary component, respectively, which decrease with the increasing light intensity. The electron diffusion coefficient (Dn) in TiO2 films can be obtained by fitting the frequency-dependent response using the analytical expression 3,31,32 d is the film thickness for the IMPS given originally by Kim. The electron diffusion length (Ln) and charge-collection efficiency (ηcc) can be further estimated by IMPS and IMVS measurements via the following eq 4 and 533,34 1 τd = 2πfd (1) τr =

1 2πfr

(2)

Dn =

d2 2.35τd

(3)

Ln =

Dn × τr

(4)

τd τr

(5)

η=1−

The detailed IMPS and IMVS parameters (τd, τr, Dn, Ln, and ηcc) of DSSCs measured under a light intensity of 96 W m−2 were summarized in Table 3. From Figure 7, one can notice that the SnO2−TiCl4(100)− DSSCs show longer lifetimes (τr) than SnO2−TiCl4(0)− DSSCs, even longer than P25−TiCl4(100)−DSSCs. This significant improvement of electron lifetime could be understood by the basic cause of the SnO2 nanoflower scaffold already divulged above by SEM and TEM (HR-TEM), which would boost the interior conductivity by building a sort of

Table 3. Detailed IMPS and IMVS Parameters (τd, τr, Dn, and Ln) of Dye-Sensitized SnO2 Solar Cells Based on Different Hierarchical SnO2−TiCl4(0) Nanoflower, Hierarchical SnO2−TiCl4(100) Nanoflower, and P25−TiCl4(100) Treatment Filmsa

a

DSSCs

τd (ms)

τr (ms)

Dn (μm2 s‑1)

Ln (μm)

η (%)

SnO2−TiCl4(0) SnO2−TiCl4(100) P25−TiCl4(100)

3.18 7.46 1.84

53.36 119.42 61.52

0.54 0.23 0.93

5.34 5.22 7.54

94.04 93.75 97.00

Light intensity: 96 W m−2. 3511

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The Journal of Physical Chemistry C

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(2) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414 (6861), 338−344. (3) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Grätzel, M. Combined Experimental and DFT-TDDFT Computational Study of Photoelectrochemical Cell Ruthenium Sensitizers. J. Am. Chem. Soc. 2005, 127, 16835−16847. (4) Falabretti, B.; Robertson, J. Electronic Structures and Doping of SnO2, CuAlO2, and CuInO2. J. Appl. Phys. 2007, 102, 123703− 123703−5. (5) Jarzebski, Z. M.; Marton, J. P. Physical Properties of SnO2 Materials: II . Electrical Properties. J. Electrochem. Soc. 1976, 123, 299C−310C. (6) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. Field-Effect Transistors Based on Single Semiconducting Oxide Nanobelts. J. Phys. Chem. B 2002, 107, 659−663. (7) Hendry, E.; Koeberg, M.; O’Regan, B.; Bonn, M. Local Field Effects on Electron Transport in Nanostructured TiO2 Revealed by Terahertz Spectroscopy. Nano. Lett 2006, 6, 755−759. (8) Senevirathna, M. K. I.; Pitigala, P. K. D. D. P.; Premalal, E. V. A.; Tennakone, K.; Kumara, G. R. A.; Konno, A. Stability of the SnO2/ MgO Dye-sensitized Photoelectrochemical Solar Cell. Sol. Energy Mater. Sol. C 2007, 91, 544−547. (9) Gubbala, S.; Chakrapani, V.; Kumar, V.; Sunkara, M. K. BandEdge Engineered Hybrid Structures for Dye-Sensitized Solar Cells Based on SnO2 Nanowires. Adv. Funct. Mater. 2008, 18, 2411−2418. (10) Birkel, A.; Lee, Y.-G.; Koll, D.; Meerbeek, X. V.; Frank, S.; Choi, M. J.; Kang, Y. S.; Char, K.; Tremel, W. Highly Efficient And Stable Dye-Sensitized Solar Cells Based On SnO2 Nanocrystals Prepared By Microwave-Assisted Synthesis. Energy Environ. Sci. 2012, 5, 5392− 5400. (11) Liu, J.; Luo, T.; Mouli, T. S.; Meng, F.; Sun, B.; Li, M.; Liu, J. A Novel Coral-Like Porous SnO2 Hollow Architecture: Biomimetic Swallowing Growth Mechanism And Enhanced Photovoltaic Property For Dye-Sensitized Solar Cell Application. Chem. Commun. 2010, 46, 472−474. (12) Snaith, H. J.; Ducati, C. SnO2-Based Dye-Sensitized Hybrid Solar Cells Exhibiting Near Unity Absorbed Photon-to-Electron Conversion Efficiency. Nano Lett. 2010, 10, 1259−1265. (13) Bedja, I.; Hotchandani, S.; Kamat, P. V. Preparation and Photoelectrochemical Characterization of Thin SnO2 Nanocrystalline Semiconductor Films and Their Sensitization with Bis(2,2′bipyridine)(2,2′-bipyridine-4,4′-dicarboxylic acid)ruthenium(II) Complex. J. Phys. Chem. 1994, 98, 4133−4140. (14) Kay, A.; Grätzel, M. Dye-Sensitized Core-Shell Nanocrystals: Improved Efficiency of Mesoporous Tin Oxide Electrodes Coated with a Thin Layer of an Insulating Oxide. Chem. Mater. 2002, 14, 2930− 2935. (15) Qian, J.; Liu, P.; Xiao, Y.; Jiang, Y.; Cao, Y.; Ai, X.; Yang, H. TiO2-Coated Multilayered SnO2 Hollow Microspheres for DyeSensitized Solar Cells. Adv. Mater. 2009, 21, 3663−3667. (16) Zhang, Q.; Chou, T. P.; Russo, B.; Jenekhe, S. A.; Cao, G. Aggregation of ZnO Nanocrystallites for High Conversion Efficiency in Dye-Sensitized Solar Cells. Angew. Chem. 2008, 120, 2436−2440. (17) Chen, D.; Huang, F.; Cheng, Y.-B.; Caruso, R. A. Mesoporous Anatase TiO2 Beads with High Surface Areas and Controllable Pore Sizes: A Superior Candidate for High-Performance Dye-Sensitized Solar Cells. Adv. Mater. 2009, 21, 2206−2210. (18) Hore, S.; Nitz, P.; Vetter, C.; Prahl, C.; Niggemann, M.; Kern, R. Scattering Spherical Voids In Nanocrystalline TiO2 - Enhancement Of Efficiency In Dye-Sensitized Solar Cells. Chem. Commun. 2005, 15, 2011−2013. (19) Richter, C.; Schmuttenmaer, C. A. Exciton-like Trap States Limit Electron Mobility In TiO2 Nanotubes. Nat. Nano. 2010, 5, 769− 772. (20) Wang, Y.-F.; Li, J.-W.; Hou, Y.-F.; Yu, X.-Y.; Su, C.-Y.; Kuang, D.-B. Hierarchical Tin Oxide Octahedra for Highly Efficient DyeSensitized Solar Cells. Chem.Eur.J. 2010, 16, 8620−8625.

of SnO2 is thought to lie 0.3−0.5 eV below that of anatase TiO2, a lower charge injection efficiency might be existence in SnO2 semiconductor material. ηcc is largely determined by the competition between the charge collection and recombination, the ηcc of “SnO2−TiCl4(100) film” (ηcc = 94%) is lower than that of P25−TiCl4(100) film (ηcc = 97%). ηlh is commonly determined by the amount of adsorbed dye and lightscattering of films. The amounts of dye adsorbed of SnO2−TiCl4(100) film (3.2 × 10−8 mol·cm−2) are also lower than that of P25− TiCl4(100) film (4.1 × 10−8 mol·cm−2), so it is easy to infer that the higher Jsc of SnO2−TiCl4(100) sample is mainly due to the excellent light scattering.

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CONCLUSION In summary, we demonstrated a multifunctionality hierarchical SnO2 nanoflower photoelectrode passivated by a layer of TiO2 nanogranulum. The designed TiCl4 aqueous solution could promote the formation of Ti−dye complexes during the dyeadsorbing process, which could hang on the surfaces of the SnO2 nanostructures and TiO2 nanogranulum act as an insulating layer, blocking the back-reaction pathway of photoinjected electrons from semiconductors to electrolyte. It is found that TiCl4 post treat have a more pronounced effect on passivation of surface trap sites, while retaining close interparticle contacts and thus the power conversion efficiency. As a result, SnO2−TiCl4(100) DSSCs display a significant improvement of the current density together with enhanced Voc. The Voc of such SnO2−TiCl4(100) film (Voc = 760 mV) is much higher than that of the SnO2−TiCl4(0) film (Voc = 440 mV). Hence, the superior light scattering effect, long electron lifetimes and slower electron recombination are together responsible for the higher Jsc and η of cell SnO2−TiCl4(100).

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ASSOCIATED CONTENT

S Supporting Information *

Nitrogen adsorption and desorption isotherms of SnO2 nanosheets, XRD patterns, and UV−vis absorptions of the SnO2−TiCl4(0) nanoflower film, P25−TiCl4(100) film, and hierarchical SnO2−TiCl4(100) treatment nanoflower film. This material is available free of charge via the Internet at http:// pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*(J.X.)Telephone: 86-551-62901126. Fax: 86-551-62901115. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported the National Natural Science Foundation (NSF) of China (No. 51372061 and No. 51302057), the Research Fund for the Doctoral Program of Higher Education of China (No. 20110111120002), and the N a t ura l Sc ie nc e F ou n da t io n of A n h ui Pr ovince (11040606Q45).

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