ar ticle

Copyright © 2014 by American Scientific Publishers All rights reserved. Printed in the United States of America

Science of Advanced Materials Vol. 6, pp. 1–10, 2014 (www.aspbs.com/sam)

Completely Different Performances of the Dye-Sensitized Solar Cells Based on Potassium-Tungsten-Oxide and -Bronze Nanobranches Jingfang Qin1, 2, ∗ , Gengmin Zhang1 , Jia Liang1 , Huarong Xia1 , Yingjie Xing1 , and Wentao Sun1 1

Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing, 100871, P. R. China 2 Academy for Advanced Interdisciplinary Studies, Peking University, Beijing, 100871, P. R. China

ABSTRACT

1. INTRODUCTION Recently, dye-sensitized solar cells (DSCs) have become regarded as the most promising alternatives to siliconbased photovoltaic devices because of their ease of fabrication and cost-effectiveness. Intensive studies have been conducted on DSCs based on TiO2 nanomaterials, such as nanotubes, nanowires, and nanoparticles,1–3 because these were found to perform much better in DSCs than many other nanomaterials. The highest power-conversion efficiency of TiO2 -based DSCs with a YD2-o-C8 sensitizer design has so far reached 12.3% under a simulated air mass of 1.5 (AM 1.5) sunlight.3 Other metal oxides were also studied as components of DSCs, of which the most notable is ZnO. DSCs based on ZnO achieved efficiencies ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: xx Xxxx xxxx Accepted: xx Xxxx xxxx

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as high as 5%,4–7 mainly because ZnO has very similar energy band structures and comparable electron-injection dynamics to those of TiO2 8 There are also many reports on DSCs based on other metal oxides, such as Nb2 O5 ,9 10 SnO2 ,11–13 and WO3 14 15 Though these have received less attention than TiO2 and ZnO, they are helpful to the understanding of the electric mechanism within DSC systems and to the solution of various problems. To date, DSCs based on porous WO3 nanoparticles covered with a layer of TiO2 have achieved efficiencies as high as 1.46%.14 Nanoparticles have larger surface-to-volume ratios than those of nanowires and nanoplates, and the surface area of a film of a particular thickness increases with the decreasing nanoparticle size. However, the smallest possible size of nanoparticles is finite, and the necks between these tiny particles, even after sintering, noticeably reduce a DSC’s photocurrent.16 In addition, it is difficult to prepare nanoparticles of uniform size, and the particles interspersed with the larger ones result in fewer

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doi:10.1166/sam.2014.1693

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K2 W8 O25 nanobranches were successfully obtained on fluorine-doped tin oxide substrates by annealing K03 WO3 nanobranches in air that had been prepared by using an improved chemical vapor deposition method. The products were examined by scanning electron microscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. For the first time, a dye-sensitized solar cell based on a K2 W8 O25 nanobranch film of 5.05 m thick was fabricated, and the efficiency achieved was 0.447%, much larger than that of K2 W8 O25 nanowires-based cells. Though K2 W8 O25 and K03 WO3 have similar crystal structures, unexpectedly, K03 WO3 -based cells barely generated electricity. TiCl4 -treated K2 W8 O25 nanobranch films were also applied in dye-sensitized solar cells, and achieved an efficiency of 0.785%. The performances of these dye-sensitized solar cells demonstrate that: the nanobranch structure has a large surface-to-volume ratio; although the band gap of K2 W8 O25 is narrow, K2 W8 O25 still may be applied in dye-sensitized solar cells after appropriate surface modification; and electrons that move freely in K03 WO3 prevent the export of photogenerated electrons in dye-sensitized solar cell systems. KEYWORDS:

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Completely Different Performances of the Dye-Sensitized Solar Cells Based on Potassium-Tungsten-Oxide and -Bronze Nanobranches

necks that connect these particles, which generates many traps that retard electron transport. Nanoporous-walled WO3 nanotubes have been prepared successfully, and they were found to perform better than WO3 nanoparticles in DSCs,15 probably because of the superior connections between the nanoparticles that compose a nanotube. However, the diameters of these nanotubes are more than 300 nm,15 which produces a decreased surface area for a particular volume. Though the recombination current density increases superlinearly with increasing surface area of a film of specific thickness and electrons move slower with increasing roughness factor,17 more photoinjected electrons are collected because more dye is loaded on DSC photoanodes and therefore more photons absorbed. One report also concluded that enlarging the surface areas was very important to increasing the efficiencies of WO3 -based DSCs.18 The crystal structures of hexagonal alkali metal tungsten bronze (Mx WO3 , 0 < x < 1/3) and alkali metal tungsten oxides (Mx WO3+x/2 , nonstoichiometric) are similar to those based on WO3 . WO3 crystals are generally formed by corner or edge sharing of WO6 octahedra, and Mx WO3 and Mx WO3+x/2 have M ions positioned at the centers of these octahedral perovskite units,19 which lead to a light insulator-to-metal transition of WO3 after the intercalation of M atoms. For n-type WO3 semiconductors, electrical conduction relies on a significant concentration of free electrons being present in the conduction bands.20 The free-electron concentration in such materials is determined mainly by the concentration of stoichiometric defects, such as oxygen vacancies.20 A relative high electron mobility of 6.5 cm2 V−1 S−1 was reported for WO3 ,21 while for a single W18 O49 nanowire a value as high as 40 cm2 V−1 S−1 was measured.22 Therefore, it is believed that the oxygen vacancies and alkali-metal dopants that exist in Mx WO3 and Mx WO3+x/2 largely improve free-electron concentrations and thus increase electron mobility. In DSC devices, high electron mobility enables photogenerated electrons to move more rapidly to the conducting layers of photoanodes and combine with I3− through an external circuit. After comparing Mx WO3 and Mx WO3+x/2 with WO3 , the authors believe that Mx WO3 and Mx WO3+x/2 have the potential to be used as the photoanode in a DSC, and Figure 1 shows the schematic mechanism of charge transfer for a DSC. Many properties of alkali metal tungsten oxides and bronzes have been studied, such as the chromic behavior, Drude-type optical performance, and superconducting properties,23–25 but their performances as components of DSCs have not been studied to date (to the best of the authors’ knowledge). In this article we describe the preparation of regular Kx WO3+x/2 nanobranches with large surface areas by using an improved chemical vapor deposition (CVD) method, as well as their properties as components of DSCs. 2

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Fig. 1. Schematic mechanism of charge transfer for a Mx WO3+x/2 based DSC. Dashed lines show the electron-transfer route for the DSC based on TiO2 -coated Mx WO3+x/2 . ECB is the conduction band energy of Mx WO3+x/2 or TiO2 versus a normal hydrogen electrode (NHE) potential. LUMO and HOMO are the lowest unoccupied and highest occupied molecular orbitals of dye, respectively.

2. EXPERIMENTAL DETAILS Potassium tungsten oxide (Kx WO3+x/2 nanobranches were prepared by annealing potassium tungsten bronze (Kx WO3 nanobranches above 400 C in air, and Kx WO3 nanobranches were synthesized by using an improved CVD method, which involved both a high reaction temperature and a rapid heating process. Before sample preparation, tungsten wires were fixed to two graphite electrodes, and a cleaned fluorine-doped tin oxide (FTO) plate with potassium bromide (KBr) solids around it was placed on a platform located about 8 mm below the tungsten wires. The chamber was exhausted by a rotary-vane pump and molecular pump, and H2 was introduced with a 3 cm3 /min flux immediately after the molecular pump was turned off. When the chamber pressure rose to 26 Pa, a voltage was applied to the two electrodes and the tungsten wires were heated to approximately 1600 C after a few seconds. This temperature was maintained for less than 3 min, and then the power supply was switched off with H2 continuing to flow until the FTO plate had cooled to room temperature. The FTO plate was now covered with a layer of Kx WO3 nanowires. We then placed a sample made in this manner in the chamber and repeated the above process to produce a film of Kx WO3 nanobranches. Both the nanowire and the nanobranch samples were annealed at 480 C in air to give a single potassium tungsten oxide crystal. Then the two Kx WO3+x/2 samples described above were treated with TiCl4 and annealed at 480 C in air to give a layer of anatase TiO2 ,14 and then all the Kx WO3+x/2 samples with and without TiCl4 treatment were applied as photoanodes in DSCs. Before the TiCl4 solution was made, both the TiCl4 stock and the deionized water were kept in a refrigerator for several hours at nearly 5 C because TiCl4 can be hydrolyzed easily at room temperature. The two prepared samples were immersed in a 40 mM aqueous TiCl4 solution at 70 C for 1 h. Sci. Adv. Mater., 6, 1–10, 2014

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3. RESULTS AND DISCUSSION Potassium tungsten oxide nanowires and nanobranches were synthesized on FTO plates on a large scale. The surface of the as-prepared films was blue initially, but became light yellow after annealing treatment. Figure 2 shows SEM images of the potassium tungsten oxides, which reveal the morphologies of the nanowires and the nanobranches. We can see that the diameters of the nanowires (inset of Fig. 2(a)) and the nanobranches (inset of Fig. 2(b)) are around 80 nm and 40 nm, respectively. The nanowires align regularly and vertically on the substrates, and the nanobranches grow compactly on each nanowire. The cross-sectional views in the insets of Figures 2(a) and (b) Sci. Adv. Mater., 6, 1–10, 2014

Fig. 2. (a) SEM images of nanowires with one deposition. (b) SEM images of nanobranches with two depositions. Cross-sectional views (insets of (a) and (b)) show the thickness of potassium tungsten oxide films.

show that the films comprised two layers: one layer was of compact nanorods attached directly to the FTO substrate; the other layer was of regular nanowires or nanobranches with a thickness of about 1 m, and each nanowire was covered completely with nanobranches (Fig. 2(b) inset). The thickness of a nanobranch film was 3.33 m and that of a nanowire film 2.81 m. The thicknesses did not increase much after a second deposition, mainly because the nanobranches growing on the nanowires were very short compared to the nanowires. Figure 3 shows XRD spectra of the potassium tungsten oxide nanowires and nanobranches without any treatment and annealed at 480 C in air, and of those treated with TiCl4 after annealing at 480 C in air. The peaks of the nanostructures before and after annealing are mostly indexed to hexagonal K03 WO3 (ICDD no. 49-0541) and orthorhombic K2 W8 O25 (ICDD no. 31-1114), respectively. Several peaks cannot be indexed to K03 WO3 or K2 W8 O25 , most probably because the annealing temperature of 480 C is not suitable for the formation of a stoichiometric potassium tungsten oxide. The impure peaks in K03 WO3 XRD patterns can be indexed to orthorhombic 3

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The samples were then washed in ethanol and annealed at 480 C for 30 min in a tube furnace. To sensitize both the nanowires and the nanobranches of Kx WO3+x/2 before and after TiCl4 treatment, the four samples were immersed in a 0.3 mM cis-diisothiocyanato-bis(2,20-bipyridyl-4, 40-dicarboxylato)ruthenium(II) bis(tetrabutylammonium) solution (N719, Dalian HeptaChroma SolarTech, China), and maintained for 48 h for the samples treated with TiCl4 and for 96 h for those with no treatment, respectively. The counter electrode was composed of Pt-coated FTO glass. One small hole was drilled through the counter-electrode substrate to let liquid electrolyte in. After dye sensitization, the potassium tungsten oxide electrode and a platinumcoated FTO counter electrode, separated by a 25 m thick hot-melt gasket, were sandwiched together. The gasket had a central cut-out area of around 0.09 cm2 . The electrolyte (DHS-E23, Dalian HeptaChroma SolarTech, China) was introduced into the cell via the hole in the counter electrode. Before and after TiCl4 treatment the films of potassium tungsten oxide nanowires and nanobranches were applied as photoanodes in DSCs. The products’ morphologies were observed by scanning electron microscopy (SEM; Quanta FEG, USA). The crystal structures of the products were analyzed by X-ray powder diffraction (XRD; DMAX 2400, Rigaku Corp., Japan). A high-resolution transmission electron microscope (HRTEM; FEI Tecnai F30, USA) equipped with an energy-dispersive X-ray (EDX) spectrometer was used to investigate the structures and composition of the samples. An X-ray photoelectron spectroscope (XPS; Axis Ultra, Kratos Analytical Ltd., UK) was used to determine the valencies of the elements of the products. Ultravioletvisible (UV-vis) diffuse reflectance spectra were measured with a UV-vis Cintra spectrophotometer (CARY 500, GBC, Australia). The performances of the DSCs were characterized with a custom-made test station. The test station’s light source was fitted with an AM 1.5 filter, and the illumination power density on the surface of the cells was 100 mW/cm2 . The current density–voltage (J –V ) characteristics were acquired.

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Fig. 3. XRD spectra of the nanowires (a) and nanobranches (b). The black lines correspond to hexagonal K03 WO3 (ICDD no. 49-0541), and the dark-gray lines and gray lines correspond to orthorhombic K2 W8 O25 (ICDD no. 31-1114). The impure peaks labled with an asterisk (∗ ) and a plus sign (+ ) can be indexed to orthorhombic K2 W8 O25 and orthorhombic WO3 , respectively.

K2 W8 O25 , and those in K2 W8 O25 XRD patterns can be indexed to orthorhombic WO3 . As shown in Figure 3, peaks of anatase TiO2 do not appear in the XRD spectra of TiCl4 -treated samples compared with those of untreated ones, possibly because peaks are weak and overwhelmed by the relatively strong peaks of K2 W8 O25 . TEM and HRTEM images of the K03 WO3 and K2 W8 O25 nanomaterials are shown in Figure 4. According to the interplanar spacing of the products before (Figs. 4(a) and (b)) and after (Figs. 4(c) and (d)) annealing and TiCl4 treatment, we confirmed that the products with no treatment were hexagonal K03 WO3 and grew along the [002] crystal axis, while those with the two-step treatment were orthorhombic K2 W8 O25 and also grew along the [002] crystal axis. The [002] peaks in the XRD spectra are also the strongest ones for both K03 WO3 and K2 W8 O25 . EDX spectra are shown in Figure 4(e). K, W, and O are the chemical elements of the nanomaterials, and Cu and C are from micro-gates that support the products. Comparing the chemical elements in the EDX spectra, we propose that amorphous layers covering the nanowires (inset of Fig. 4(c)) and nanobranches (inset of Fig. 4(d)) after TiCl4 4

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treatment are titanium oxides. The amorphousness of titanium oxides is also probably responsible for the absence of titanium oxide peaks in the XRD spectra. On the one hand, K03 WO3 contains both W6+ and W5+ 5+ states, and the formula can be written as K03 W6+ 07 W03 O3 ; on the other hand, K2 W8 O25 has a formula of K2 O · 8WO3 , which means only the W6+ state exists within it. XPS was used to examine the oxidation states in the nanomaterials before and after annealing. As shown in Figure 5(a), peaks at binding energies that correspond to K, W, and O were detected. Detailed information on the chemical state of core level tungsten (W4f) is shown in Figures 5(b)–(e). The curves can be fitted to two spin-orbit doublets (W4f7/2 and W4f5/2 with an interval of 2.1 eV. The K03 WO3 nanowires and nanobranches exhibit four peaks at 34.5 and 36.6, and 35.8 and 37.9 eV (Figs. 5(b) and (c)), which can be assigned to W6+ and W5+ , respectively. For K2 W8 O25 samples, two peaks at 34.5 and 36.6 eV are attributed to W6+ (Figs. 5(d) and (e)). Peaks at a binding energy that correspond to Ti2p were also detected in the XPS spectra of TiCl4 -treated nanowires and nanobranches (Figs. 5(a), (f), and (g)). The Ti2p spectra show two peaks at 459.1 and 464.8 eV, which correspond to Ti2p3/2 and Ti2p1/2 states of stoichiometric TiO2 , respectively. All these peaks agree with the reported values.26–28 From the above analysis, the nanowires and the nanobranches of K03 WO3 and K2 W8 O25 show the same crystal structures and elemental compositions, and therefore we infer that they follow the same growth mechanism. When the tungsten wires were heated to 1600 C, thermally evaporated tungsten reacted with leaked air and generated tungsten oxides (WOy , described by Eq. (1). Under the high radiant heat, KBr solids vaporized and changed to potassium oxide (K2 O) and hydrogen bromide (HBr) according to Eq. (2). The WOy molecules moved downward to the vicinity of the FTO substrate and reacted with K2 O and H2 molecules as in Eq. (3), and potassium tungsten bronze (hexagonal Kx WO3 , 0 < x < 1) was produced. If the temperature was above a particular value, such as 1600 C, the amount of hydrogen gas near the FTO surface decreased and potassium tungsten oxides (hexagonal Kx WO3+x/2 , 0 < x < 1) were generated as in Eq. (4). Increasing numbers of molecules of potassium tungsten oxide cumulated on the surface of FTO and small drops of potassium tungsten oxide liquid formed. Solid potassium tungsten bronzes separated out and grew preferentially in one direction when the drops were supersaturated, and therefore the Kx WO3 nanowires formed. Nanobranches grow on nanowires and maintain smaller lengths and diameters than nanowires, most probably because the liquid drops that generate the nanobranches are smaller than those that create the nanowires. When the K03 WO3 nanowires and nanobranches were annealed under air, W5+ was oxidized to W6+ and blue K03 WO3 changed to white K2 W8 O25 , as described by Eq. (5). The oxidation of W5+ Sci. Adv. Mater., 6, 1–10, 2014

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ARTICLE Fig. 4. TEM and HRTEM images of the potassium tungsten oxide nanowires and nanobranches. (a) Nanowires without any treatment, (b) nanobranches without any treatment, (c) nanowires after TiCl4 treatment and annealing in air (480 C, 0.5 h), (d) nanobranches after TiCl4 treatment and annealing in air (480 C, 0.5 h), and (e) EDX spectra of the nanowires before (2) and after (4) the two-step treatment, and of the nanobranches before (1) and after (3) the two-step treatment.

is demonstrated by the variation of the valence shown in the XPS (Fig. 5). Though potassium tungsten bronze changed to potassium tungsten oxide, the morphologies did not vary. W + y/2O2 = WOy

(1)

2KBr + H2 + 1/2O2 = K2 O + 2HBr ↑

(2)

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WOy + x/2K2 O + y + x/2 − 3H2 = Kx WO3 + y + x/2 − 3H2 O

0 < x < 1

(3)

0 < x < 1

(4)

WOy + x/2K2 O + y − 3H2 = Kx WO3+x/2 + y − 3H2 O Kx WO3 + x/4O2 = Kx WO3+x/2

0 < x < 1

(5) 5

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Fig. 5. (a) XPS spectra of the nanowires and nanobranches before and after TiCl4 treatment. W4f core-level high-resolution XPS spectra of (b) K03 WO3 nanowires, (c) K03 WO3 nanobranches, (d) K2 W8 O25 nanowires, and (e) K2 W8 O25 nanobranches. Ti2p high-resolution XPS spectra of TiO2 covering on (f) nanowires and (g) nanobranches.

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Table I. Performances of DSCs based on WO3 potassium tungsten oxides before and after TiCl4 treatment. Cell type K2 W8 O25 nanowire DSC K2 W8 O25 nanodranch DSC TiCl4 -treated K2 W8 O25 nanowire DSC TiCl4 -treated K2 W8 O25 nanobranch DSC K2 W6 O19 nanobranch DSC K2 W6 O19 nanobranch DSC WO3 nanoparticle DSC14 WO3 nanoparticle DSC14

Figure 6 shows the J –V and incident photon conversion efficiency (IPCE) curves of the DSCs based on the K2 W8 O25 films before and after TiCl4 treatment, and the cell performances are summarized in Table I. The K2 W8 O25 films were applied in DSCs. The efficiency of the DSC based on a 5.05 m thick K2 W8 O25 nanobranch Sci. Adv. Mater., 6, 1–10, 2014

396

381

1.52

0.304

0.176

505

450

3.60

0.276

0.447

395

421

3.18

0.341

0.456

476

474

4.74

0.350

0.785

350

341

2.65

0.311

0.281

384

476

3.20

0.253

0.386

450

380

3.29

0.413

0.573

1200

390

4.61

0.375

0.746

film reached 0.447%, more than twice that of the cell based on a 3.96 m thick K2 W8 O25 nanowire film. The Jsc (short circuit current density) also improved greatly to 1.52 mA/cm2 and 3.60 mA/cm2 for the nanowire and nanobranch DSCs, respectively. The K2 W8 O25 nanowires and nanobranches have the same crystal structure, chemical compositions, and band gap (1.6 eV, Fig. 7). The surface area is believed to be the main reason for the performance improvement in DSCs, because a film with a larger surface area absorbs more dye molecules and so absorbs more solar photons. The Voc (open-circuit voltage) of the nanobranch-based DSC was 450 mV while that of the nanowire-based DSC was 381 mV. Different values of the Voc of DSCs generally result from differences between the Femi level of the rough semiconductor layer and the redox potential of the electrolyte.30 For DSCs based on the same nanomaterial film, the Voc is usually verified by the sealing process, the film thickness, and the nanomaterial connection with substrates, which determine the internal resistance of a DSC system. For the above two DSCs, the electrolyte was the same and thus the redox potential was the same. The nanobranch film was thicker than the nanowire film, and it is not possible to increase the cell’s Voc . The sealing process could cause a difference between them, but was unlikely to alter the values that much. Therefore, the Femi level of the nanobranches was probably higher than that of the nanowires during the DSC-testing process, as they share the same lowest conduction band. When the K2 W8 O25 -nanobranch DSC system was tested, the photogenerated electrons diffusing to the FTO substrate were retarded by traps in the connections between nanowires and nanobranches, accumulated in the conduction band of the nanobranches, and finally caused a higher Femi level. The fill factor (FF) of the nanobranchbased DSC was similar to that of the nanowire-based one, 7

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Fig. 6. (a) J –V characteristic curves of DSCs based on K2 W8 O25 nanowires and nanobranches before and after TiCl4 treatment; (b) IPCEs for the DSCs described in (a); (c) J –V characteristic curves of a K033 WO3 -based DSC illuminated from the front and from the back, and the dark current curve.

Film Jsc Fill factor thickness Voc (m) (mV) (mA/cm2 ) (FF) (%)

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Fig. 7. (a) UV-vis absorption spectra and (b) Tauc plot ((h1/2 vs. h29 of nanowires and nanobranches before (K03 WO3 ) and after (K2 W8 O25 ) annealing at 480 C for 30 min. From this data, we calculated that the band gap of K2 W8 O25 was 1.6 eV.

which demonstrates that the traps generated by the connections created little internal resistance. Considering the thickness of the K2 W8 O25 films described here, the photovoltaic performance of K2 W8 O25 is not worse than that of WO3 , though the band gap of K2 W8 O25 is much narrower (1.6 eV, Fig. 7) than that of WO3 (2.6–3.1 eV).14 The Voc and Jsc of a K2 W8 O25 -nanobranch-based DSCs was also larger than those of a WO3 -nanoparticle-based DSC (Table I), which reveals that the nanobranches produce a comparable surface area to that of the nanoparticles. The smaller FF of the K2 W8 O25 -nanobranch DSC, compared to that of the WO3 -nanoparticle DSC, probably results from the more positive conduction band of K2 W8 O25 . There was some WO3 impurity in the K2 W8 O25 nanomaterials, as shown in the XRD patterns, which is possibly another reason for the small FF. The impurity disturbed the periodic potential field in the K2 W8 O25 crystal, and so raised the electron-scattering rate, increased the internal resistance, and thus decreased the FF for the DSC. Therefore, it is necessary to acquire a purer potassium tungsten oxide crystal or modify the surface of K2 W8 O25 . The DSCs based on the K2 W6 O19 (annealing at 660 C for 30 min in air) and K03 WO3 (without annealing) were also tested. The performances of the K2 W6 O19 DSCs are also summarized in Table I, and there is not much 8

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difference to those of the K2 W8 O25 DSCs. However, the K03 WO3 DSCs performed completely differently to the K2 W8 O25 DSCs. There was nearly no photocurrent in the DSCs, as shown in Figure 6(c). The K03 WO3 and K2 W8 O25 nanomaterials have the same chemical compositions, morphologies, and band gap (1.6 eV, Fig. 7), but different colors and potassium positions in crystal structures. To exclude the influence of the dark color of K03 WO3 films, we tested K03 WO3 -based DSCs with light illuminated, respectively, from the front and back, but there was no change in performance. To make sure dye molecules were absorbed on the dark-blue K03 WO3 , a sensitized K03 WO3 sample was immersed in 8 mg/ml NaOH solution (240 mg NaOH, 15 ml deionized water, and 15 ml C2 H6 O) for several hours, and the solution’s UV-vis spectrum was measured. A sensitized K2 W8 O25 sample was also treated in the same manner. As shown in Figure 8, the dye absorbed on a K03 WO3 sample of a unit area was calculated as 963 × 10−9 mol/cm2 , similar to the amount absorbed on the unit area of a K2 W8 O25 (858 × 10−9 mol/cm2 sample. Though there was some K2 W8 O25 impurity in the K03 WO3 nanomaterials, as shown in the XRD spectra, the impurity could not worsen the K03 WO3 performance in the DSC, because the K2 W8 O25 performed well in DSCs, and it was not possible for the periodic potential disturbance to retard fully the electron transfer

Fig. 8. (a) UV-vis absorption spectra of dye desorbed from sensitized K2 W8 O25 (gray line) and K03 WO3 (black line) nanowires; (b) an absorption characteristic curve of the solution desorbing dye under an illumination of 520 nm. Sci. Adv. Mater., 6, 1–10, 2014

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4. CONCLUSIONS We studied the performances of DSCs based on potassium tungsten oxides and potassium tungsten bronzes. K03 WO3 nanowires and nanobranches were prepared successfully using an improved CVD method. K2 W8 O25 nanowires and nanobranches were achieved by annealing the K03 WO3 nanomaterials in air at 480 C. The DSC based on a 5.05 m thick K2 W8 O25 -nanobranch film generated electricity with an efficiency of 0.447%, mainly attributed to the large surface area of the nanobranches. The Voc and Jsc values obtained were 450 mV and 3.6 mA/cm2 , respectively, larger than those of a 4.5 m thick WO3 nanoparticle DSC. The K03 WO3 nanomaterials were also applied in DSCs. Unexpectedly, the DSCs based on K03 WO3 generated no electricity, probably because of the flexible lattice positions of potassium ions in the channels composed of perovskite octahedral units. While keeping the K2 W8 O25 nanobranch structure as the main electrontransfer path, TiO2 was coated on the K2 W8 O25 films, and a great improvement was achieved across the board. The efficiency achieved 0.785%, still much poorer than that of the TiO2 -based DSCs. However, the authors believe Sci. Adv. Mater., 6, 1–10, 2014

that multiple-branched nanostructures can increase the surface area further by taking full advantage of the limited space in the photoanode films, while producing fewer traps that retard electron transfer. Further study is currently being conducted to achieve this success. We aim to decrease the compact nanorod layer between the nanowires or nanobranches and FTO substrates, and so, to some extent, reduce the internal resistance of the DSCs. We expect to be able to reduce the sizes of the nanowires and nanobranches, currently 80 nm and 40 nm, respectively, and so improve their performances in DSCs. We believe potassium tungsten oxide nanobranches have great potential in their application as components of DSCs. Acknowledgment: This work was supported by the National Natural Science Foundation of China (Nos. 61171023, 61076057, 61271050 and 61072025).

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in the K03 WO3 DSC. As described in the introduction of this paper, K03 WO3 has potassium ions positioned at the centers of the octahedral perovskite units, while K2 W8 O25 is proposed to have potassium and oxygen ions positioned at the centers.19 31 32 Therefore, we propose that potassium ions in K03 WO3 may move freely in the channel composed of perovskite octahedral units, which may lead to the recombination of negative electrons on the K03 WO3 conduction band and positive holes for dye molecules in the ground states. Similar to the approach Zheng et al. used, we coated the K2 W8 O25 with a layer of TiO2 through TiCl4 treatment.14 The efficiency of the DSC based on the TiCl4 -treated nanobranches reached 0.785%, which is almost twice that of the DSC based on the same film without TiCl4 treatment. Voc and Jsc also improved greatly. Though the performance was much improved by using TiCl4 treated K2 W8 O25 nanobranch films as photoanodes, it was still much poorer than that of TiO2 -based DSCs.1–3 The K2 W8 O25 nanomaterials absorb fewer dye molecules as a result of their more positive conduction band, which increases the interface between K2 W8 O25 and the electrolyte, and therefore increases the recombination probability of electrons in the K2 W8 O25 conduction band and I3− in the electrolyte.30 The sandwiched layer (insets of Fig. 2) between FTO and the K2 W8 O25 nanomaterials may also increase the internal resistance of cells. Therefore, modifying the K2 W8 O25 surface, decreasing the harmful layer, and improving the potassium tungsten oxide connection with substrates are important aspects to be studied to develop K2 W8 O25 -based DSCs.


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