Hydrothermal Synthesis of High Electron Mobility Zn-doped SnO2

ARTICLE pubs.acs.org/cm

Hydrothermal Synthesis of High Electron Mobility Zn-doped SnO2 Nanoflowers as Photoanode Material for Efficient Dye-Sensitized Solar Cells Xincun Dou,* Dharani Sabba, Nripan Mathews, Lydia Helena Wong, Yeng Ming Lam,* and Subodh Mhaisalkar School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 ABSTRACT: This paper demonstrates the potential of a new photoanode material Zn-doped SnO2 nanoflower for efficient dyesensitized solar cells. The nanoflower structure is synthesized using a hydrothermal method and is shown to have electron mobility higher than that of the conventional titania photoanode. The overall power conversion efficiency for the Zn-doped SnO2 nanoflower dye-sensitized solar cell reaches 3.00% with a Voc of 0.78 V and increases to 6.78% after TiCl4 treatment. Electrochemical impedance spectroscopy measurement showed that the Zn-doped SnO2 nanoflower film has a large intrinsic electron mobility that favors the fast charge transport. This work shows that Zn-doped SnO2 nanoflower material is a most interesting material and has good potential for application in solar cells. KEYWORDS: Zn doped SnO2, nanoflower, dye-sensitized solar cells, photoanode, electron mobility, EIS

’ INTRODUCTION Considerable research has been devoted to the study of mesoscopic dye-sensitized solar cells (DSSCs) in the past decade due to its potential to be a low cost solar cell option.1
8 Nanocrystalline TiO2, ZnO, and SnO2 have been studied extensively for use as photoanode materials to develop high performance DSSCs.5,9
13 Various nanomorphologies of these photoanode materials have been synthesized with the aim of improving the dye adsorption properties and hence increase light absorption.3,11,14,15 SnO2 with its wide bandgap (Eg = 3.6 eV) has characteristics make it highly suitable to be used as the photoanode in DSSC. It is a chemically stable oxide and has an electron mobility (∼100
200 cm2V
1s
1)16 much higher than that of anatase TiO2 (0.1
1 cm2V
1s
1)11 or porous TiO2 (∼10
2 cm2V
1s
1).17 It has a conduction band edge Ec, of about 0.3 eV lower than that of anatase TiO2. Therefore, it can be used in combination with dyes with low-lying LUMOs that inject poorly into TiO2, such as some perylene sensitizers.18 However, in comparison to TiO2-based DSSC and other solar cells, SnO2-based DSSC has suffered from the low overall power conversion efficiency (PCE). The best reported efficiency for SnO2 DSSCs is 2.8%19 which were attained for photoanodes sensitized with the organic dye D149 (N719 gave 1.2% in the same study). Another limitation of SnO2 based DSSCs is the low Voc when coupled with iodide/triiodide redox couple (best about 0. 45 V).18 A novel coral-like porous SnO2 hollow architecture has been synthesized for DSSCs which attained Voc values of 0.52 V.20 A significant improvement in Voc (0.55
0.7 V) and efficiency (5.2%) can be achieved by coating the mesoporous r 2011 American Chemical Society

SnO2 with a very thin shell of another metal oxide, such as ZnO, MgO, and Al2O321 or by mixing with another oxide powder, such as ZnO.22,23 The best efficiencies so far have been obtained using ZnO-coated SnO2 with efficiencies up to 6.3%.21 However, the chemically unstable ZnO in the composite still poses a challenge that needs to be resolved. Zn-doped SnO2 might be a good choice that incorporate both the advantages of SnO2 and ZnO. It is still SnO2 and thus is a chemically stable ternary oxide (will also be discussed further). This ternary oxide has fast electron transport property24 and has been studied extensively.25,26 It is now well-accepted that for a high-efficiency DSSC, the photoanode not only requires a high surface area for light harvesting but also requires a densely packed microstructure for fast electron transport.11 To satisfy these conflicting requirements, much effort has been focused on the development of bifunctional oxide materials, which consist of a thin surface layer of nanocrystalline particles to effectively adsorb the dye and a mesoporous microstructure to confine the incident light within the electrode and also enhances the electron conduction. It has also been shown that this coating improved the charge collection by strongly inhibiting recombination.5 Some attempts to address these factors have been nanoembossed TiO2 hollow spheres,27 hierarchically structured ZnO spheres,28,29 multilayered SnO2 hollow microspheres,5 ZnO and TiO2 nanowires.30 They have Received: May 13, 2011 Revised: July 26, 2011 Published: August 08, 2011 3938

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Figure 1. Characterization of Zn-doped SnO2 nanoflowers. (a) SEM image, (b) XRD (reference to tetragonal SnO2 (JCPDS, 41-1445)), (c) HRTEM and SAED pattern of the rod, (d) EDS by TEM EDS attachment, (e) UV-vis absorbance spectra and the optical bandgap calculation, and (f) PL comparison with pure SnO2 nanocrystals and pure SnO2 hollow nanospheres synthesized by the same method.

been demonstrated to be good materials for improved power conversion efficiency in DSSCs.5,31 As discussed, Zn-doped SnO2 will combine good mobility with good chemical stability. As such, in this paper, a novel Zndoped SnO2 nanoflower structure was synthesized using a hydrothermal method and for the first time used as the photoanode in DSSC. By TiCl4 treatment of the photoanode, a thin layer of TiO2 nanoparticles were coated which enhances both the dye adsorption property and the photocurrent. To the best of our knowledge, this is the first report of the use of Zn-doped SnO2 nanoflower as the photoanode of solar cell and showed remarkable PCE of close to 7%.

’ EXPERIMENTAL SECTION Materials and Chemicals. FTO (Resistance 15 Ω/0, 2.2 mm thickness), Iodolyte AN-50 electrolyte in acetonitrile solvent (solaronix), N719 dye(Ruthenizer 535-bisTBA, solaronix), Surlyn (Meltonix 1170
25 Series, solaronix). Zinc acetate dihydrate, Tin(IV) chloride, Sodium hydroxide, chloroplatinic acid, tert-butanol, acetonitrile, ethyl cellulose powders (nos. 46070

and 46080), R-terpineol (All bought from Sigma-Aldrich and used as received). Zn-doped SnO2 Nanoflower Synthesis. The Zn-doped SnO2 nanoflowerwas prepared by a hydrothermal method. Typically, a 30 mL mixture solution of equal amount of ethanol and DI water contains 0.02 mmol Zn(CH3COO)2 3 2H2O, 1.0 mmol SnCl4, and 15.0 mmol NaOH was transferred into a 50 mL Teflon-lined autoclave and maintained at 180 °C for 24 h. The precipitate was collected by centrifugation after being washed with distilled water and ethanol several times. Finally, the product was dried completely at 70 °C. Electrode Preparation and DSSC Making. The typical procedure of the paste makingand DSSC assembling are according to the TiO2standard DSSC making procedures.32 The photoelectrodes were made by screen printing the pastes on FTO glass to the desired thickness. The TiCl4 treatment for the Zn-doped SnO2 nanoflower film was performed by immersing the Zn-doped SnO2 nanoflower photoelectrode into a 0.2 M TiCl4 aqueous solution for 60 min at 80 °C, while the TiCl4 treatment for the P25 film was performed by immersing the P25 photoelectrode into a 0.04 M TiCl4 aqueous solution for 30 min 3939

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Chemistry of Materials at 70 °C. After that, the electrodes were washed with distilled water and ethanol to remove residual TiCl4, and then sintered at 500 °C in air for 30 min. Finally, the photoelectrodes were soaked in a mixture solution of equal amount of tert-butanol and acetonitrile containing 0.5 mM N719 dye for 20
24hrs at room temperature and then washed with acetonitrile and dried in air. For the preparation of counter electrode, the perforated sheet was washed with H2O as well as with a 0.01 M HCl solution in ethanol and cleaned by ultrasound in an acetone bath for 10 min. After removing residual organic contaminants by heating in air for15 min at 400 °C, the Pt catalyst was deposited on the FTO glass by coating with a drop of H2PtCl6 3 6H2O solution (8 mM in isopropanol) and then with the heat treatment at 400 °C for15 min. Iodide based low viscosity electrolyte with 50 mM of tri-iodide in acetonitrile (AN-50, Solaronix) was used as the electrolyte. Characterization. XRD measurements were carried out using powder X-ray diffraction (Bruker D8 Advance, with Cu
KR radiation operating at 40 kV and 40 mA, scanning from 2θ = 20 to 80°). Field emission scanning electron microscopy (FESEM; JEOL JSM-7600F) and transmission electron microscope(JEOL 2100 TEM, 200 kV) equipped with an X-ray energy-dispersive spectrometer (EDS) were used to characterize the morphology, the crystallinity and the chemical composition of the samples. Nitrogen adsorption
desorption isotherms for surface area and pore analyses were measured using an Nova 3200e (Quantachrome instruments). UV
vis absorbance spectra is measured by a Shinmadzu 3600 UV
vis Spectrophotometers. X-ray photoelectron spectroscopy (XPS) (VG ESCA 220i-XL) was employed to analyze the zinc oxidation state. Monochromatic Al KR X-ray (hν = 1486.6 eV) is used for the analysis with photoelectron takeoff angle of 90° to the surface plane. Charge compensation was performed by means of low-energy electron flooding and further correction was made based on adventitious C1s at 285.0 eV using the manufacturer’s standard software. The error of binding energy is estimated to be within (0.2 eV. The amount of adsorbed dye was measured by desorbing the dye into 20 mM NaOH solution with equal amount of deionized water and ethanol and by absorption measurement of the solution using the absorption peak intensity of N719 at 511 nm. The current
voltage test of DSSCs were performed under one sun condition using a solar light simulator (Oriel, 91160, AM 1.5 globe,100 mW/cm2). The incident-light intensity was calibrated using a radiant power/energy meter (Oriel, 70260) before each experiment. EIS experiments were performed under illumination provided by a red LED (λ) 627 nm, 19.2 nmfwhm) while the cell was biased at the VOC induced by the illumination.

’ RESULTS AND DISCUSSION Zn-doped SnO2 nanoflower was prepared by a one step hydrothermal method at 180 °C for 24 h. Figure 1a shows the novel flower structure obtained from the synthesis. The welldispersed nanoflower has a fairly uniform size of about 1 μm. XRD pattern of the Zn-doped SnO2 nanoflower corresponds well to the tetragonal SnO2 (JCPDS, 41-1445) (Figure 1b). It can be seen that all peaks can be indexed to SnO2 and no other extra peaks were found. All the peaks showed a slight shift to smaller angle, which is the result of Zn doping in SnO2. The high resolution transmission electron microscopy (HRTEM) image clearly shows the lattice fringes along three directions (Figure 1c), the corresponding SAED (inset in Figure 1c)

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Figure 2. XPS analysis of Zn-doped SnO2 nanoflower and pure SnO2 powder (Aldrich, 325 mesh, 99.9% trace metals basis). (a) Sn 3d, (b) O 1s, and (c) Zn 2p peaks.

showed that the nanorod is single crystalline and grow along [112] direction. The EDS shows the average atomic ratio of Zn to Sn is 0.17:1 (Figure 1d). The Zn-doped SnO2 nanoflowers shows a strong absorption under 356 nm (Figure 1e), which corresponds to an optical bandgap of 3.48 eV (inset in Figure 1e). From the photoluminescence (PL) investigation (Figure 1f), one can see the broad emission at around 609 nm, which is much stronger than the band edge emission situated at 351.3 nm (3.53 eV). Compared to the PL spectra of the pure SnO2 nanocrystals or the PL spectra of pure SnO2 hollow nanospheres synthesized by the same method in our lab, the broad emission is also much stronger, which implies more defects states exist in the Zn-doped SnO2 nanoflowers. More concrete proof for Zn doping in SnO2 can be obtained by looking at the binding energy of Zn. XPS measurements were conducted on the Zn-doped SnO2 nanoflower and compared with the pure SnO2 powder (Aldrich, 325 mesh, 99.9% trace metals basis). A comparison of the Sn 3d5/2 and 3d3/2 transition peaks can be seen in Figure 2a. The binding energy of Sn in Zndoped SnO2 nanoflower decreased by 0.9 eV as compared to the pure SnO2 powder (from 486.8 to 485.9 eV for Sn 3d5/2 and from 495.2 to 494.3 eV respectively). Zn doping decreases the 3940

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Chemistry of Materials binding energy of Sn and this can be attributed to oxygen deficiency.33 The O 1s transition peak also shifted from 530.7 to 529.6 eV upon Zn doping (Figure 2b). The shoulder peak could be ascribed to Sn
O
Zn coordination.33 A strong Zn 2p3/2 and Zn 2p1/2 spectrum (Figure 2c) with corresponding binding energies of 1021 and 1044 eV confirms the presence of Zn in the doped SnO2 system. To elucidate the possible doping process and the morphology evolution of the Zn-doped SnO2 nanoflower structure, the reaction was terminated at 3, 12, and 15 h. The corresponding morphologies are shown in Figure 3a
c. Figure 4 shows the XRD pattern of the samples. The composition is a combination of cubic ZnSn(OH)6 (JCPDS, 74
1825), tetragonal SnO2 (JCPDS, 41-1445) and face-centered Zn2SnO4 (JCPDS, 241470). The possible growth process is illustrated in Scheme 1.

Figure 3. SEM observation of the evolution process of Zn-doped SnO2 nanoflowers, the SEM images of the products obtained at a reaction time of (a) 3 h, (b) 12 h, (c) 15 h, and (d) 24 h.

Figure 4. XRD characterization of the products obtained at a reaction time of (a) 3 h, (b) 12 h, and (c) 15 h.

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There are four stages of growth: (I) In the initial stage, ZnSn(OH)6 bipyramids are formed due to hydrolysis reaction under high temperature and pressure, and during this the Sn(OH)62
also exists in the solution due to the excess Sn4+ and OH
ions; (II) Due to the etching action of OH
, ZnSn(OH)6 bipyramids are decomposed into Zn(OH)42
and Sn(OH)62
along the bipyramid surface and went into the solution and further formed numerous Zn-doped SnO2 nanocrystals, and the residual structure decomposed into Zn-doped SnO2 and Zn-poor Zn2SnO4; (III) Zn-doped SnO2 nanocrystals in the solution attached onto the surface of the plates which are composed of Zn-doped SnO2 nanocrystals and grew on the surface, and on the surface of these plates, the Zn-doped SnO2 nanocrystals were also aligned to nanowires with rough surfaces; (IV) Zn-poor Zn2SnO4 further react with Sn(OH)62
and form Zn-doped SnO2 nanocrystals, the rough nanowires further grow and smoothen into the thorns of the nanoflower. The pore-size distribution and the Brunauer
Emmett
Teller (BET) surface area of this nanoflower structure are determined using nitrogen adsorption and desorption isotherms. It was found that the Zn-doped SnO2 nanoflower structure has a surface area of ∼10 m2/g. The surface area is only one-fifth of the surface area of P25 (∼50 m2/g), which implies that the surface area can still be improved for higher dye adsorption when this material is used as the photoanode of DSSC. Next, we evaluate the performances of these nanoflowers in DSSC. Zn-doped SnO2 nanoflowers were made into a paste by a standard procedure and the photoanode prepared by screenprinting the paste on fluorine-doped tin oxide (FTO) glass.32 To compare the DSSC performance of the flower structure, TiO2 (Degussa P25) cells were also made using the same procedure. Three kinds of DSSCs with thickness of either 8 and 10 μm were fabricated (I) Zn-doped SnO2 nanoflower photoanode (denoted as Zn-SnO2), (II) Zn-doped SnO2 nanoflower photoanode with TiCl4 treatment (TiO2
Zn-SnO2), (III) TiO2 (P25) photoanode with TiCl4 treatment (denoted as P25). The characteristic current
voltage (I
V) curves of the three kinds of DSSCs are given in Figure 5, and their photovoltaic parameters derived from the I
V curves are summarized in Table 1. It can be seen that the open-circuit photovoltage of the Zn-doped SnO2 flower DSSC (0.70 or 0.78 V) is much higher than DSSCs based on SnO2 (0.52 V),20 TiO2 coated SnO2 (0.67 V),5 ZnO/SnO2 composites (0.67 V).23 The high Voc (0.70 or 0.78 V) obtained here is due to the larger energy level offset between the Fermi level in the present system with the reduction potential of I
/I3
since a comparable current density is achieved compared with pure SnO2. The fill factor (0.65 or 0.62) is also better than the SnO2 system. The dye desorption measurement shows that the flower system also has a much better dye adsorption property. Although the surface area of the nanoflower system is only about one-fifth

Scheme 1. Schematic Illustration of the Morphology Evolution of the Zn-Doped Nanoflower

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of the surface area of P25 (∼50 m2/g), the dye adsorbed is disproportionately higher than P25. This may be attributed to the larger pore size which allow better dye penetration into the bulk of the photoanode. The overall power conversion efficiency reaches 2.11% and 3% for the 8 and 10 μm Zn-doped SnO2 flower-based DSSC respectively, which are much higher than that of the SnO2 system.5,20 The above results strongly implies that the Zn-doped SnO2 flower system is extremely favorable as the photoanode of DSSC compared to any reported pure SnO2 systems. After TiCl4 treatment, the jsc increased 2.2 times, while Voc increased to 0.80 V and the FF dropped slightly. The dye adsorption is improved by about 50% with the TiCl4 treatment. TiCl4 treatment induced a rough TiO2 layer on the nanoflower surface which increased the surface area of the Zn-doped SnO2 photoanode. The surface morphology of the Zn-doped SnO2 photoanode before and after TiCl4 treatment are shown in Figure 6. The PCE increases to 5.16% and 6.78% for the 8 and 10 μm Zn-doped SnO2 flower based DSSCs. One can also find that the ratios of the short circuit current density to the dye molecules adsorbed are very consistent for the nanoflower samples without TiCl4 treatment and the P25 samples, which implies that the equal electron injection and collection performance of Zn-doped SnO2 flower and TiO2. However, this ratio increased by 50% for the TiCl4 treated Zndoped SnO2 flower samples. This result is surprising and suggests an inhibition of the electron recombination at the interface and a much faster electron transport in the Zn-doped SnO2 flower based photoanode. This last point will be further explained in the EIS measurement.

From the device data, it can be seen that even with reduced dye adsorption, the jsc and PCE of the cells with Zn-doped SnO2 as photoanode are comparable to the conventional TiO2 cells. It is also found that the performances of the Zn-doped SnO2 flower DSSCs are stable in the whole measurement process, which indicates that the Zn-doped SnO2 flower material is stable in the acid electrolyte. Also, the Zn-doped SnO2 flower photoanodes are stable in the alkaline environment since they are synthesized in a solution containing high concentration of NaOH and also displayed morphological stability during the dye desorption experiment. This material has a good potential as photoanodes for high efficiency DSSC. Electrochemical impedance spectroscopy (EIS) can provide a better understanding of the transport properties and charge transfer properties relating to the bulk and interfacial regions in DSSCs.34
38 To study the differences in the interfacial characteristics of these photoelectrodes, EIS spectra were collected using an Autolab potentiostat/galvanostat and evaluated using the Nova 1.5 software package. EIS experiments were performed under illumination provided by a red LED (λ) 627 nm, 19.2 nm fwhm) while the cell was biased at the VOC induced by the illumination and a frequency range studied is from 0.1 Hz to 1 MHz. Figure 7a shows the typical Nyquist plots of the DSSCs at their Voc. In all the EIS spectra, two well-defined semicircles were observed in the high frequency region (>1 kHz) and in the frequency region of 0.1
100 Hz, respectively. The first semicircle in the high-frequency region represents the redox reaction of I
/I3
at the Pt/electrolyte interface, and the other semicircle denotes the electron transfer at the oxide/dye/electrolyte

Figure 5. Photocurrent density
voltage characteristics of the DSSCs using Zn-doped SnO2 nanoflower or TiO2 (P25) with/without TiCl4 treatment under AM1.5 G simulated sunlight with a power density of 100 mW cm
2.

Figure 6. (a) SEM image of the Zn-doped SnO2 flower photoanode (a, c) without and (b, d) with TiCl4 treatment.

Table 1. Comparison of Short-Circuit Photocurrent Density (jsc), Open-Circuit Photovoltage (Voc), Fill Factor (FF), Overall Power Conversion Efficiency (PCE) Measured Under AM 1.5, 1 Sun along with the Amount of Adsorbed Dye N719 for the Films Consisting of Zn-Doped SnO2 Nanoflower or TiO2 (P25) with or without TiCl4 Treatmenta film

a

adsorbed dye (  10
7 mol/cm2)

jsc (mA)

Voc (V)

FF

PCE (%)

8 μm Zn-SnO2

0.454

4.50

0.70

65.16

2.11

8 μm TiO2
Zn-SnO2

0.667

10.02

0.80

62.91

5.16

10 μm Zn-SnO2

0.635

6.06

0.78

62.06

3.00

10 μm TiO2
Zn-SnO2

0.932

13.83

0.80

59.41

6.78

8 μm TiO2 (P25) 10 μm TiO2 (P25)

1.231 1.407

12.39 14.67

0.82 0.82

68.29 63.13

6.97 7.57

The active areas of the photoanodes are 0.2826 cm2 for all the tested cells. 3942

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Figure 7. (a) Full and (b) Enlarged Nyquist plots of the DSSCs at their Voc, the inset in (b) is the equivalent circuit used to fit the EIS spectra.

Figure 8. Characteristic cell data with a dependence on the applied voltage extracted from the EIS spectra (a) The chemical capacitance Cμ, (b) The interfacial charge recombination resistance Rrec, (c) Rrec versus Cμ, (d) The electron lifetime τn.

interface.36
38 To derive the transport characteristic of the cells, one has to look at the expanded EIS spectra shown in Figure 7b. All the EIS spectra were fitted by using ZView software with an equivalent circuit (inset in Figure 7b) which is based on the general transmission line mode.34,35 Figure 8 shows the extracted characteristic data by fitting the EIS spectra of the above five DSSCs. The chemical capacitance (Cμ) contributed by the electronic states shows an exponentially increase with the increase of the applied voltage (Figure 8a). The Cμ of the Zn-doped SnO2 DSSCs is smaller than that of the P25 cell before TiCl4 treatment, and increased to similar values after TiCl4 treatment. It is due to the increase in surface states with TiCl4 treatment. The interfacial charge recombination resistance (Rrec) of all the cells decreases exponentially with the increase of the applied voltage (Figure 8b). Before TiCl4 treatment, the Zndoped SnO2 DSSCs show a smaller or equivalent Rrec than the P25 cell. Bisquert et al. pointed out that variations in conduction band (Ec) also affect the recombination resistance values.39 In order to

remove the effect of Ec position, Rrec has to be plotted vs Vecb (the common equivalent conduction band potential). However, it is difficult to shift Cμ to overlap in Figure 8a. Wang et al.40 showed that changes in Rrec at a constant Cμ can be associated with changes in the charge transfer rate constant at the same quasiFermi level (QFL) position. As a result, in the present study, the interfacial charge recombination resistance is compared at the same density of states. Plots of Rrec versus Cμ of all the cells show that the electron recombination of the Zn-doped SnO2 DSSCs was greatly inhibited after TiCl4 treatment (Figure 8c). The electron lifetime (τn) also decreases exponentially with the increase of the applied voltage for all the cells and τn increases with TiCl4 treatment (Figure 8d). Due to an increase of Rrec and Cμ with TiCl4 treatment, the electron lifetime for the Zn-doped SnO2 flower photoanode DSSC exceeds that of P25 cell. At Voc, the electron lifetime for the TiCl4 treated Zn-doped SnO2 flower photoanode DSSC reaches 39.2 ms, which is almost 3 times of the electron lifetime of the P25 cell (13.7 ms). It is well-known that the electron lifetime in DSSC is the central quantity that 3943

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Figure 9. (a) The electron transport resistance in the photoanode material Rt, (b) The calculated the effective diffusion length Ln, (c) The electron mobility μ versus applied potential, (d) μ versus Cμ.

The competition between transport and recombination of electrons is reflected in the electron diffusion length (Ln), which can be expressed as, rffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi Rrec L n ¼ Dn τ n ¼ L ð1Þ Rt

Figure 10. Schematic diagram of the energy level and the electron transport in the TiCl4 treated Zn-doped SnO2 nanoflower photoanode DSSC.

determines the recombination dynamics in the solar cell.41 Thus, the enhancement of the electron lifetime in the TiCl4 treated Zndoped SnO2 cell shows the remarkable inhibition of the charge recombination at the oxide/dye/electrolyte interface. Since a low recombination rate is highly desirable to achieve high chargecollection efficiency and will eventually lead to high solar power conversion efficiency, there is no doubt that the longer electron lifetimes enable the TiCl4 treated Zn-doped SnO2 flower films to achieve higher photon-to-current conversion efficiencies.5 The electron transport resistance in the photoanode material (Rt) for the Zn-doped SnO2 DSSCs does not change much with the TiCl4 treatment and remains comparable to the Rt of P25 photoanode (Figure 9a). This indicates that the overall electron transport in Zn-doped SnO2 photoanode is as good as the P25 photoanode regardless the huge morphological difference. It is notable that the fitting of Rt at high applied voltages is less reliable since Rt is too small and not obvious, as can be seen in Figure 7b.

where Dn is the electron diffusion coefficient and L is the film thickness of the photoanode. The nanoflower structure has a diffusion length smaller than the P25 cell, but after TiCl4 treatment the diffusion length exceeds that of the P25 cell, especially in the lower voltage region (Figure 9b). This result indicates that the charge transport and thus the charge collection are better for the TiCl4 treated Zn-SnO2 flower photoanode compared to the P25 photoanode and the values are large enough for excellent electron collection.42 In the higher voltage region, because of the less reliable values of Rt, the diffusion length may not be accurate. The electron mobility (μ) is calculated based on the Einstein’s relation, μ¼

Dn e ¼ Dn  38:9 kB T

ð2Þ

Where e is the elementary charge of an electron, kB is Boltzmann’s constant, T is absolute temperature in Kelvin and it is taken to be the room temperature (298 K). The electron (chemical) diffusion coefficient (Dn) was calculated by Dn ¼ Ln 2 =τn ¼ L2 =Rt Cμ

ð3Þ

The electron mobility for the nanoflower photoanode is at least twice of the P25 photoanode and reaches to almost 4 times at higher voltages (Figure 9c). Plots of μ versus Cμ of all the cells (Figure 9d) further shows that the Zn-doped SnO2 nanoflower material has an intrinsic high electron mobility which will favor the electron transport for a longer distance with less diffusive 3944

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Chemistry of Materials hindrance. In this case, the TiCl4 treated Zn-doped SnO2 photoanode not only incorporates the high electron mobility of the Zn-doped SnO2, but also inhibits the charge recombination by the surface TiO2 layer (Figure 10). As a result, the TiCl4 treated Zn-doped SnO2 photoanode DSSCs showed a remarkable increase of the short circuit current density despite the low dye adsorption.

’ CONCLUSIONS In summary, a novel high electron mobility Zn-doped SnO2 nanoflower structure with fast transport properties was synthesized using a hydrothermal method and for the first time has been used as the photoanode of DSSC. The overall power conversion efficiency for the Zn-doped SnO2 nanoflower DSSC reaches to 3.00% with a Voc of 0.78 V. The SnO2 nanoflower has a very good dye adsorption property which is highly desirable for light harvesting. EIS measurement showed that the Zn-doped SnO2 nanoflower film has a large intrinsic high electron mobility. After TiCl4 treatment of the Zn-doped SnO2 nanoflower film, the current density doubled. The PCE increases to 6.78%, which is comparable to the P25 DSSCs. The reason for the great enhancement of the PCE is found to be a combination of the increase of the electron lifetime which inhibits the charge recombination and the fast electron transport of the Zn-doped SnO2 nanoflower which improves the charge collection efficiency. Therefore, through this work, it is found that Zn-doped SnO2 nanoflower material is a most interesting material and has good potential for application in solar cells. We believe the PCE can be further improved by engineering the morphology and thus the surface area and the dye adsorption of the Zn-doped SnO2. ’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (X. D.); [email protected] (Y. M. L.).

’ ACKNOWLEDGMENT This work is supported by the National Research Foundation, Singapore. We are grateful for the EIS measurement in Prof. Qing Wang’s group in Department of Materials Science and Engineering, NUSNNI-NanoCore, National University of Singapore. ’ REFERENCES (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (2) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gr€atzel, M. Nat. Mater. 2003, 2, 402–407. (3) Qiu, Y. C.; Chen, W.; Yang, S. H. Angew. Chem., Int. Ed. 2010, 49, 3675–3679. (4) Sauvage, F.; Di Fonzo, F.; Bassi, A. L.; Casari, C. S.; Russo, V.; Divitini, G.; Ducati, C.; Bottani, C. E.; Comte, P.; Gr€atzel, M. Nano Lett. 2010, 10, 2562–2567. (5) Qian, J. F.; Liu, P.; Xiao, Y.; Jiang, Y.; Cao, Y. L.; Ai, X. P.; Yang, H. X. Adv. Mater. 2009, 21, 3663–3667. (6) Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595–6663. (7) Xu, J.; Luan, C.-Y.; Tang, Y.-B.; Chen, X.; Zapien, J. A.; Zhang, W.-J.; Kwong, H.-L.; Meng, X.-M.; Lee, S.-T.; Lee, C.-S. ACS Nano 2010, 4, 6064–6070. (8) Tang, Y.-B.; Lee, C.-S.; Xu, J.; Liu, Z.-T.; Chen, Z.-H.; He, Z.; Cao, Y.-L.; Yuan, G.; Song, H.; Chen, L.; Luo, L.; Cheng, H.-M.; Zhang, W.-J.; Bello, I.; Lee, S.-T. ACS Nano 2010, 4, 3482–3488. (9) Shan, G.-B.; Demopoulos, G. P. Adv. Mater. 2010, 22, 4373–4377.

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