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High-Efficiency Dye-Sensitized Solar Cells Based on the Composite Photoanodes of SnO2, Notas de estudo de Engenharia Elétrica

High-Efficiency Dye-Sensitized Solar Cells Based on the Composite Photoanodes of SnO2

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2010

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Baixe High-Efficiency Dye-Sensitized Solar Cells Based on the Composite Photoanodes of SnO2 e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! High-Efficiency Dye-Sensitized Solar Cells Based on the Composite Photoanodes of SnO2 Nanoparticles/ZnO Nanotetrapods† Wei Chen,‡ Yongcai Qiu,‡ Yongchun Zhong,§ Kam Sing Wong,§ and Shihe Yang*,‡ Departments of Chemistry and Physics, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ReceiVed: September 10, 2009; ReVised Manuscript ReceiVed: NoVember 9, 2009 We have devised dye-sensitized solar cells (DSSCs) with >6% efficiency by employing composite photoanodes of SnO2 nanoparticles/ZnO nanotetrapods. Benefiting from material advantages of both constituents, the composite photoanodes exhibit extremely large roughness factors, good charge collection, and tunable light scattering properties. Among the three composite photoanodes with widely differing compositions tested, the best performance (efficiency ) 6.31%) was obtained with a weight ratio of SnO2/ZnO 2:1 mainly due to the highest saturated Jsc achieved at a thinnest film thickness. Charge collection losses in composite films with more ZnO nanotetrapods content and thus necessarily larger film thicknesses appear to be a main limiting factor on IPCE and therefore Jsc, which undermines the gain from their favorable light scattering ability. An ultrathin layer of ZnO spontaneously shelled on SnO2 nanoparticles is found to enhance Voc primarily by lifting the band edges rather than by suppressing recombination. Finally, by intensity modulated photocurrent/ photovoltage spectroscopy (IMPS/IMVS), we have identified that recombination in SnO2/ZnO composite films is mainly determined by the ZnO shell condition on SnO2, whereas electron transport is greatly influenced by the morphologies and sizes of the ZnO crystalline additives. In particular, ZnO nanotetrapods have proved to be superior in electron transport and therefore charge collection over ZnO particles additives in the SnO2/ ZnO composite-based DSSCs. 1. Introduction Dye-sensitized solar cell (DSSC), which accomplishes ef- ficient solar-to-electric power conversion by virtue of nano- structures,1-3 is regarded to be a potentially low cost alternative to traditional silicon-based photovoltaics. A certified DSSC efficiency record has been pushed to >11% with a maximum IPCE up to ∼85%4,5 because of the introduction of a highly porous TiO2 nanoparticles photoanode by O’Regan and Gratzel in 1991.1 Many other semiconductors besides TiO2 have also been tested as photoanode materials, and some of them, for example, SnO2-ZnO composite (15 nm SnO2 nanoparticles/1 µm ZnO particles 1:1 by weight),6-8 SnO2-MgO core-shell nanoparticles,9 Zn2SnO4,10 and so on, have achieved promising performance.6-13 In parallel to the exploration of new photo- anode materials, novel 1D nanostructures,11,14-18 including ZnO nanowires19,20/nanorods21,22 and TiO2 nanorods23,24/nanotubes arrays,25,26 have also attracted serious attention. Although their overall performance is still far from satisfactory as photoanodes because of various practical problems, they are generally superior in certain aspects of efficient DSSCs. Examples include facilitated electron transport in 1D materials such as perpen- dicularly aligned ZnO nanorods arrays21,22 and TiO2 nanotubes/ nanowires arrays25 as well as composites of nanowires/nanotubes and nanoparticles.27,28 Because the photoanodes of DSSCs are nanostructured by nature for effectively ferrying electrons and holes, it is of great importance to explore new materials with different architectures on the nanoscale. This cannot only enhance our understanding of the inherent working principles of DSSCs but also help to improve the cell performance,29,30 especially in emerging frontiers such as flexible DSSCs31 and solid-state DSSCs.32 Most recently, our group reported a DSSC efficiency of 3.2% based on a network structured photoanode assembled from building blocks of ZnO nanotetra- pods.33 Such kind of photoanode, albeit prepared without calcina- tion, has demonstrated incredibly long effective electron diffusion lengths. This success encouraged us to blend apposite nanoparticles into the nanotetrapods to improve further the cell performance and to elucidate the working mechanism of the composite photoanodes in the hope to realize ultimately practicable and highly efficient flexible DSSCs. We had in mind that the ZnO nanotetrapods network acts as a global electron transport highway, while the nanoparticles enclosed in and attached to the network serve to increase the roughness factor and take part in the local charge transport.27,28,30 Our survey experiments have shown that blending ZnO, TiO2, or SnO2 nanoparticles with ZnO nanotetrapods generally leads to greatly different performance features of the corresponding DSSCs. It is believed that a proper combination of the electronic structures of the constituents, for example, conduction band edge positions, which for TiO2 is close to that of ZnO but ∼500 mV higher than that of SnO2,34 distribution of accepting states in the conduction bands, which for SnO2 and ZnO consist of s orbital contrary to d orbital for TiO2,30 is critical to the performance of composite photoanodes. In practice, different surface chemistry of the constituents has to be duly considered. In this work, SnO2 nanoparticles were chosen on the basis of two considerations. First, good cell performance records have already been achieved for SnO2/ZnO composite photoanodes but with very different nanostructures.6-8 It is tempting to see whether a similar or even higher cell efficiency can be achieved by blending SnO2 nanoparticles with ZnO nanotetrapods. † Part of the “Benoît Soep Festschrift”. * Corresponding author. E-mail: chsyang@ust.hk. ‡ Department of Chemistry. § Department of Physics. J. Phys. Chem. A XXXX, xxx, 000 A 10.1021/jp908747z  XXXX American Chemical Society Second, the precise functions of each component in the composite film and their synergy are yet to be elucidated.7,35 This study aims to provide guiding principles for the search of efficient photoanodes based on binary or even ternary nano- materials systems. Indeed, our DSSCs based on the composite photoanodes of SnO2 nanoparticles/ZnO nanotetrapods have reaped efficiencies comparable to or higher than the preceding record of SnO2/ ZnO composite photoanodes. The maximum photocurrent densities that can be extracted from the composite photoanodes vary only slightly when the ratio of SnO2 nanoparticles to ZnO nanotetrapods is changed in a wide range, manifesting the outstanding charge transport property of the composite photo- anodes. The structural forms and the functional roles of the ZnO component in the composite films have been carefully studied by the IMVS and IMPS methods, lending direct evidence of the origins of the Voc enhancement and the Jsc variation of the DSSCs arising from the nanoscale blending. Moreover, ZnO nanotetrapods have proved to be excellent in electron transport (charge collection) in the composite photoanodes in comparison to particle forms of ZnO additives. 2. Experimental Section 2.1. Synthesis of SnO2 Nanoparticles and ZnO Nanotet- rapods. SnO2 ultrafine nanoparticles were synthesized via a modified hydrothermal method from literature.10 In a typical synthesis, 6 g SnCl4 ·5H2O was dissolved in 80 mL of deionized water at 353 K. To this hot solution, diethylenetriamine was added dropwise under vigorous stirring until its pH reached 9.2. After the stirring was maintained at this temperature for 2 h, the initially formed white precipitates were completely dissolved, and a nearly transparent, light-yellow sol was obtained. The sol was transferred to a 100 mL Teflon-lined stainless steel autoclave, which was then heated to 453 K and kept at this temperature for 48 h. A dark-yellow gel was formed with a thin liquid layer on the top. The SnO2 product was separated by repeated centrifugation and washing with excessive ethanol, stored in residual ethanol with the solid content of 40 wt % for later use. Details of the controlled synthesis of ZnO nanotet- rapods by the metal vapor transport-oxidation method can be found elsewhere.33,36 2.2. Fabrication of Composite Films. For the preparation of pastes with three different SnO2/ZnO weight ratios of 2:1, 1:1, and 1:2, 1, 0.5, and 0.25 g SnO2 wet products (solid content ) 40 wt %) were, respectively, mixed with 0.2 g ZnO nanotetrapods powder. To each mixture was added 6 mL of an ethanol/H2O (5:1, volume ratio) solvent mixture to obtain a sol. After ultrasonic treatment of the sols for 10 min, 0.015 g PEO 2 000 000 and 0.1 g block copolymer Pluronic P123 were added with thorough stirring, forming homogeneous viscous pastes. Pure SnO2 paste was prepared in the same way. Doctor blade technique was employed to spread the as-prepared pastes onto conductive glass substrates (FTO-coated glass, 14 Ω/square, Nippon sheet glass, Japan). The films were calcined at 450 °C in air for 30 min and then treated in 0.03 M ammonia for 30 min. After being soaked in excessive deionized water and ethanol, they were finally calcined at 450 °C for 30 min for a second time. The ammonia treatment is critical to the formation of a homogeneous ultrathin ZnO shell on SnO2 nanoparticles (Supporting Information, Figure S1), which, as will be revealed below, plays an important role in determining the charge transport/recombination kinetics in composite photoanode-based DSSCs. For the sake of comparative studies, some films were treated with a solution of acetic acid/H2O/ethanol 1:33:66 by volume for 5 or 30 min, which are hereafter referred to as HAc5 and HAc30, respectively. 2.3. General Materials Characterization. Morphologies of the nanomaterials were directly examined by SEM using a JEOL 6700F at an accelerating voltage of 5 kV. TEM observations were carried out on a JEOL 2010F microscope operating at 200 kV. BET surface areas and pore size distributions of the film samples were characterized using a Coulter SA 3100 surface area analyzer. The samples for BET characterization were obtained by scratching the as-prepared films off the conductive glass substrates. XPS measurements of the film samples on glass slides were performed with Physical Electronics surface analysis equipment (model PHI 5600). XRD of the film samples on glass slides was characterized using a Philips high resolution X-ray diffraction system (model PW1825). Diffused reflectance spectra were carried out on the same film samples using a Perkin-Elmer UV/vis spectrophotometer (model Lambda 20). The film thick- ness was determined by a Tencor Alpha-Step 200 surface profiler system. 2.4. Solar Cell Assembly and Photoelectrochemical Char- acterization. The calcined films were immersed in a mixed acetonitrile/tert-butyl alcohol solution (1:1 in volume) containing 5 × 10-4 M N719 dye (cis-bis(isothiocyanato)bis(2,2′-bipyridyl- 4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium, Suzhou Chemsolarism, China) for 4 h. These dye-coated electrodes were assembled to make solar cells with Pt-sputtered FTO counter electrodes and acetonitrile/valeronitrile (85:15, volume ratio) electrolyte containing 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.03 M I2, 0.1 M GuSCN, and 0.5 M 4-tert-butylpyridine. We determined the adsorbed dye amount by immersing the films in 1 mM NaOH solution (water/ethanol, 1:1, volume ratio) and monitoring the desorbed dye concentration by UV-vis spec- troscopy. J-V characteristic curves were recorded using a CHI 600A electrochemical analyzer. The light source (Oriel solar simulator, 450 W Xe lamp, AM 1.5 global filter) was calibrated to 1 sun (100 mW cm-2) using an optical power meter (Newport, model 1916-C) equipped with a Newport 818P thermopile detector. The photoanode film area for the DSSC performance test was typically 0.25 cm2. IPCE was measured on the basis of a Jobin-Yvon Triax 190 monochromator. Intensity-modulated photocurrent/photovoltage spectra (IMPS/IMVS) were taken by the Zahner Zennium C-IMPS system based on an IM6x electrochemical workstation, using green light-emitting diode (525 nm, LED) as the light source. 3. Results and Discussion 3.1. Nanostructural Characterizations. Shown in Figure 1A,B are TEM images of the as-synthesized SnO2 nanoparticles. The nanoparticles are well dispersed without serious aggregation (Figure 1A), a feature that is important for film fabrication. High-resolution image and the corresponding selective area diffraction pattern (Figure 1B) show that the nanoparticles are well-crystallized with sizes in the range of 6-10 nm, much smaller than the commercial SnO2 nanoparticles (15 nm in size) normally used in the literatures.6,8 A typical SEM image in Figure 1C reveals a good uniformity and the unique symmetrical branching morphology (40 nm in arm diameter and 500 nm in length) of the ZnO nanotetrapods. Importantly, the branching trait of the ZnO nanotetrapots make them apt to forming 3D interpenetrating networks imbedded with small SnO2 nanopar- ticles. The ZnO nanotetrapods feature four arms growing symmetrically from a common center along the 〈0001〉 direction (Figure 1D,E); such 1D arms not only offer a mechanically robust scaffold but also provide rapid electron transport pathways due to the minimized grain boundaries. B J. Phys. Chem. A, Vol. xxx, No. xx, XXXX Chen et al. in the range of 3-6 nm as the ZnO content is increased in the composite films, which may be partially responsible for the abnormal increase in BET surface area. One of the reasons for the formation of the small pores may be hindered hierarchical aggregation of the ∼8 nm SnO2 nanoparticles as a result of the increasing incorporation of the ZnO nanotetrapods such that the pore size is mainly determined by the interspaces of the primary nanoparticle assembly. Moreover, the roughening of the ZnO and SnO2 surfaces together with the formation of the ZnO ultrafine nanoparticles in composite films with an increasing ZnO content should also increase the BET surface area. The difference in pore structure has a direct influence on the light scattering property of the composite films, as can be clearly perceived from their diffuse reflectance spectra in Figure 5. It should be pointed out that the dramatic reduction of reflection below 380 nm (except the pure SnO2 film) and below 340 nm (except the pure ZnO film) is caused by the band gap transitions of ZnO (Eg ) 3.2 eV) and SnO2 (Eg ) 3.6 eV), respectively. Overall, the reflectance spectra appear to abide by the Mie scattering theory, which states that pores with a comparable size to the light wavelength can act as effective light scattering centers.38,39 Understandably, light scattering in the pure SnO2 nanoparticles film is nearly negligible because the pores inside the film are too small (<20 nm) to act as effective scattering centers. However, the diffuse reflectance of the composite films increases dramatically and monotonously with the increase in the ZnO content, with the utmost for the pure ZnO nanotetrapods films. Notably, a substantial fraction of visible light is scattered back. This is not surprising because more ZnO nanotetrapods blended with SnO2 nanoparticles will generate more pores with sizes closer to ideal for light scattering, namely, 50-100 nm, as suggested by Hore et al.38 Similar diffuse reflectance spectra arising from the scattering of voids (pores) were previously reported.16,40 3.2. Performance Appraisal of the Dye-Sensitized Solar Cells. Presented in Figure 6 are the performance test results of the DSSCs based on the three series of composite films under 1 Sun AM 1.5G simulated solar light. The corresponding data for the pure SnO2 nanoparticles film and for the pure ZnO nanotetrapods film33 are also included for comparison. The dependence of roughness factor (RF) on film thickness for the different series of films is plotted in Figure 6A. RF was calculated by multiplying a BET surface area by the weight of a film peeled off from the glass substrate, which is then divided by the area of the film. Significantly, the RF values for the three series of composite films can exceed 1600, much higher than that of the pure ZnO nanotetrapods film and also comparable to those of the most efficient TiO2 nanoparticles photoanodes.41 The dependence of dye-adsorbing amount on film thickness shows roughly similar increasing trends as those of RF (Figure 6A). It is noticed that the dye-adsorbing amount per RF for the pure SnO2 film is obviously lower than those of composite films probably because of the more basic surfaces of the latter, that is, the ZnO ultrathin layer modified SnO2 surfaces.8 More to the point, because the dye-adsorbing time of 4 h in this work was optimized with respect to the composite films, it is probably too short for saturated dye adsorption on the pure SnO2 nanoparticles film. The dependence of short-circuit photocurrent density, Jsc, on film thickness exhibits obviously nonlinear arc-shaped trends (Figure 6B) for all except the ZnO nanotetrapods films. For the three series of composite films with SnO2/ZnO 2:1, 1:1, and 1:2 compositions, Jsc become saturated at about 7, 10, and 15 µm, respectively. At the thicknesses of the saturated Jsc, the RF and dye-adsorbing amount are nearly the same, which are ∼1600 and in the range of (2.0-2.4) × 10-7 mol · cm-2, respectively. The nonlinear trends of Jsc versus RF (dye adsorbing amount) and their saturated positions should be primarily due to the Beer’s Law, which is related to the light harvesting efficiency of a photoanode through the equation LHE(λ) ) 1- 10-εΓ,42 where Γ is the surface coverage of dye (mol cm-2) and ε is the dye’s molar absorption coefficient (mol-1 cm2) at wavelength λ. Using ε ) 1.41 × 107 mol-1 cm2 for N719 dye at 515 nm,43 we find that Γ with a value of 7.1 × 10-8 mol cm-2 is sufficient to secure LHE(515 nm) as high as 90%. Further increase in the dye-adsorbing amount to (2.0-2.4) × 10-7 mol cm-2 associated with the saturated Jsc will mainly promote harvesting of the near-infrared light, resulting in reduced increasing rate of overall light harvest and therefore reduced increasing rate of Jsc with the increase in RF. This is one of the reasons why the Jsc dependences on film thickness (or RF and dye adsorbing amount) exhibit nonlinear arc-shaped trends. Additionally, light scattering effect as revealed by diffuse reflectance spectra of the three composite films also needs be considered; the light scattering effect could cause deviation from the Beer’s law42 and thus may be partially responsible for different arc shapes of the Jsc thickness plots. Voc values (600-660 mV) of the three composite films (Figure 6C) are found to lie between those of the pure SnO2 nanopar- Figure 4. Pore size distributions of pure SnO2 nanoparticles film and three SnO2 nanoparticles/ZnO nanotetrapods composite films. The inset is the corresponding nitrogen adsorption-desorption isotherms. Black, 9: SnO2 nanoparticles film; red, b: SnO2/ZnO 2:1 composite film; green, 2: SnO2/ZnO ) 1:1 composite film; blue, 1: SnO2/ZnO ) 1:2 composite film. Figure 5. Diffuse reflectance spectra of pure SnO2 nanoparticles film, pure ZnO nanotetrapods film, and three SnO2 nanoparticles/ZnO nanotetrapods composite films. All films are prepared with the same thicknesses of ∼6 µm on glass slides. High-Efficiency Dye-Sensitized Solar Cells J. Phys. Chem. A, Vol. xxx, No. xx, XXXX E ticles film and the pure ZnO nanotetrapods film but much closer to the latter. The origin of the Voc enhancement of SnO2/ZnO composite photoanodes in comparison with pure SnO2 has been debated since the first report on this subject.6 We will resolve this point in latter sections of this article after presenting our new IMVS results. The Voc dependences for the three composite films on thickness are similar (Figure 6D): first increase and then decrease, which seems to be at odd with the previously reported monotonically decreasing trend of Voc with increasing film thickness.10,27 The common decreasing trend of Voc with film thickness is usually attributed to the increase in recombina- tion loss, which can be qualitatively appreciated from the equation of Voc ) (kBT/q) ln(Jinj/Jdark),44 where kB, q, and T are the Boltzmann constant, electron charge, and absolute temper- ature, respectively, and Jinj is injected photocurrent density from dye to the semiconductor and Jdark is dark current due to the recombination at semiconductor/electrolyte interface. The dark reaction can take place at two interfaces:45 (1) the interface between electrolyte and bare FTO glass substrate uncovered by nanomaterials, whereat recombination is important when the active film is relatively thin; (2) the highly porous film/ electrolyte interface, which may mainly be the recombination site when the roughness factor is large. Therefore, the Jdark can be portioned into Jdark,FTO and Jdark,porousfilm, where the first term represents the dark current loss through FTO and the second is dark current through porous film. It is well documented that Jdark,FTO can be abated by coating a blocking layer between FTO and active film.46,47 Because we have not exploited an adequate blocking layer yet, the dark current loss through the bare FTO substrate may play an important role in the small thickness regime of our composite-film-based photoanodes. In this small thickness regime, as the film thickness increases, Jinj and Figure 6. Thickness-dependent characteristics of the SnO2 nanoparticles/ZnO nanotetrapods composite films as well as pure SnO2 nanoparticles film and pure ZnO nanotetrapods film. (A) Roughness factor, RF, (9) and dye adsorbing amount (0); (B) short-circuit photocurrent density, Jsc; (C) open-circuit photovoltage, Voc; (D) blown-up region highlights, Voc, of the three composite films; (E) fill factor, FF; and (F) overall energy conversion efficiency η. F J. Phys. Chem. A, Vol. xxx, No. xx, XXXX Chen et al. Jdark,porousfilm both increase with thickness but more for the former, and the nearly constant Jdark,FTO means that the ratio of Jinj/Jdark and therefore Voc increase with film thickness. In the thick film regime, however, as the thickness increases, Jdark,porousfilm becomes predominant over Jdark,FTO and increases more rapidly than Jinj because of the reduced light harvest, leading to the gradual decline of the ratio of Jinj/Jdark and thus Voc. Fill factors (FFs) for the three composite films vary within the range of 0.55 to 0.7 (Figure 6E) depending on the thickness, which are generally higher than that of pure SnO2 nanoparticles films and comparable to that of pure ZnO nanotetrapods films. Although we have not been able to account for the detailed FF dependences on film thickness at present, complications such as those arising from the peculiar nanostructures and their combinations as well as dye corrosion on ZnO in the composite films may play a part. Maximum cell efficiencies for the three composite films obtained from Figure 6F are listed in Table 1. The efficiencies up to 6.31% are a dramatic improvement over the best SnO2- based DSSCs (∼2%)48 and rank among the highest level for SnO2/ZnO composite photoanode-based DSSCs.6,8 The Voc enhancement over that of the pure SnO2 film is mainly responsible for the significant improvement in the overall efficiency of the SnO2/ZnO composite-film-based DSSCs be- cause pure SnO2 photoanode itself can also yield high photo- current.48 On the other hand, the difference in maximum efficiency among the three composite films is within 15%, which is mainly ascribed to the difference in Jsc. As another noteworthy feature, our composite photoanodes achieve the best perfor- mance at the weight ratio of SnO2/ZnO 2:1, and this contrasts with the work of Tennakone et al. on ZnO big particles/SnO2 nanoparticles composite films, which obtained the maximum efficiency strictly at the ZnO/SnO2 weight ratio of nearly 1:1.6,7 Such a difference is largely a result from the peculiar structure of our nanotetrapods, which possess three-dimensionally dis- tributed long arms and can be embedded in the SnO2 nanopar- ticles matrix without occupying much space in contrast with the space-filling, micrometer-sized ZnO particles used by Tennakone et al. 3.3. Relationship between Jsc and IPCE. IPCEs of solar cells based on the three composite photoanodes are shown in Figure 7 and also selectively listed in Table 1. The maximum IPCE value for the three composite films at ∼520 nm decreases from 74.1, to 71.5, to 67.6% with increasing content of ZnO nanotetrapods. As expected, this decreasing trend of IPCE for the composite films is naturally in keeping with the gradual drop of the maximum Jsc from 16.3, to 15.1, to 14.5 mA cm-2 with increasing ZnO content. It is also important to recognize from Figure 7 that with the increase in the ZnO content, the IPCE value in the long wavelength tail increases but suffers a comparable or higher decrease in the short wavelength region. A similar observation has also been reported on IPCE of a film in which a light scattering material was directly blended with TiO2 nanoparticles.43 With respect to the IPCE variations of the three composite films, we will discuss their origins in terms of light-harvesting efficiency (LHE(λ)), electron injection yield from the dye excited state into semiconductor (Φinj), and charge collection efficiency at the front conductive glass substrate (ηcc).49 The LHE of a cell mainly depends on intrinsic properties of the dye, for example, the wavelength-dependent molar absorp- tion coefficient and its adsorbing density on porous photoanodes, which is governed by the Beer’s law.42 For a practical solar cell such as our ZnO nanotetrapod-based DSSCs, the LHE equation should accommodate the scattering effect by introduc- ing the concept of effective optical path length within the photoanodes. The effective optical path length can be increased by the addition of relatively large particles to small nanoparticles matrix to induce light scattering of the photoanodes, which is also called the light-trapping effect.4,41 Such light-trapping effect leading to enhancement on IPCE especially in the near-infrared wavelength range is very important for DSSCs because the widely used Ru-pyridine-based dyes absorb light weakly in this region. In our work, the broad pore size distribution resulted in relatively strong light scattering. As reflected in Figure 5, the optical path length in the three composite films is gradually enhanced in the sequence of SnO2/ZnO 2:1, 1:1, and 1:2. Consequently, IPCE on the red side of the spectra is gradually enhanced in this sequence, as revealed in Figure 7. To examine the difference in electron injection yield (Φinj), we employed the time-resolved fluorescence decay method to probe the nanosecond time scale emission decays of N719 dye on the three composite films with nearly the same dye-adsorbing amount. It is well known that the fluorescence emission of N719 dye can be strongly quenched by electron injection to semi- conductors with a suitable conduction band edge position such as TiO2, SnO2, and ZnO.30,50 Therefore, by comparing the emission decays of the dye on the composite films, one can draw at least qualitative conclusions about the variation of injection efficiency with the film compsotion.51,52 The emission decay profiles for the three composite films show almost the same initial intensity and a similar decreasing trend (Supporting Information, Figure S4), a biexponential decay that can be fitted by two exponential functions. The fact that the maximum difference on the integrated areas of the fitted curves is only ∼10% implies that the charge injection efficiencies are nearly the same for the three composite films because Φinj is determined by the integrated area of an active film divided by that of a reference (e.g., dye on ZrO2 or Al2O3 film51,52). The similar electron injection kinetics for the three composite films lends further evidence of the existence of an ultrathin, homogeneous ZnO shell on the SnO2 nanoparticles, which, in combination TABLE 1: Summary of the Performance Parameters of DSSCs Based on SnO2 Nanoparticles/ZnO Nanotetrapods Composite Photoanodes sample thickness (µm) Voc (mV) Jsc (mA cm-2) FF IPCEmax η (%) SnO2/ZnO 2:1 6.6 656 16.3 0.59 74.1% 6.31 SnO2/ZnO 1:1 10.9 639 15.1 0.64 71.5% 6.18 SnO2/ZnO 1:2 13.5 648 14.5 0.62 67.6% 5.83 Figure 7. IPCE of DSSCs based on the three typical composite photoanodes with different weight ratios, as listed in Table 1. High-Efficiency Dye-Sensitized Solar Cells J. Phys. Chem. A, Vol. xxx, No. xx, XXXX G nanoparticles film (HAc30 film), although the electron transport time is relatively long, the electron lifetime is similarly long, leading to the insensitivity of charge collection efficiency on the HAc treatment. This explains why Jsc for the SnO2/ZnO 2:1 composite films with nearly the same dye-adsorbing amount (Figure S7, Supporting Information) has almost no change after HAc treatments (Figure 10A). Similarly, nearly no change occurs to the charge collection efficiency for the SnO2/ZnO 1:1 composite film (∼90% at the highest light intensity), presumably for the same reason discussed above. However, there is a pronounced decrease in Jsc, which for the HAc30 film is ∼20% lower than that of the original SnO2/ZnO 1:1 composite film (Figure 10B). Because charge collection remains a nearly constant efficiency as described above, the decrease in Jsc is mostly ascribable to the reduced dye-adsorbing amount, from (2.0 to 1.6) × 10-7 mol cm-2 according to our dye-desorption measurements, which is induced by the corrosion of a much higher ZnO nanotetrapods content than that in the SnO2/ZnO 2:1 composite film. The reduction of Jsc from the original SnO2/ ZnO 1:1 composite film to its corresponding HAc30 film implies that the electrons from ZnO nanotetrapods contribute to the total Jsc of the composite film; namely, photoinjected electrons from the dye adsorbed on ZnO nanotetrapods can flow fluently to the SnO2 nanoparticles network. 3.4.4. Effects of Film Composition. As mentioned above, for the SnO2/ZnO 1:1 composite film, the increase in τt and τr due to HAc treatment is much more significant than those of the SnO2/ZnO 2:1 composite film (see arcs 1 and 2 in Figure 11 for the different magnitudes). A larger change of τt (solid line arc 2 in Figure 11B) for the SnO2/ZnO 1:1 composite film because of HAc treatment than that of the SnO2/ZnO 2:1 film (solid line arc 1 in Figure 11A) is believed to arise from the removal of more ZnO nanotetrapods in the former sample, leading to a less continuous film with a smaller connectivity and therefore a more tortuous electron diffusion pathway in it. A higher porosity means that more nanoparticles are suspended with less neighboring nanoparticles in the SnO2/ZnO 1:1 HAc30 film than those in the SnO2/ZnO 2:1 HAc30 film. The reduced interparticles communication leaves an increased possibility for the recombination of photoinjected electrons. Therefore, the change in τr (dotted line arc 2 in Figure 11B) for the SnO2/ ZnO 1:1 HAc film is obviously larger than that for the SnO2/ ZnO 2:1 HAc film (dotted line arc 1 in Figure 11A). A similar argument about the effect of porosity (or number of neighboring nanoparticles) of TiO2 nanoparticles films on the electron transport and recombination kinetics has been previously put forward.61 To come to the point, the changes in τt and τr caused by the dissolution of ZnO nanotetrapods prove that the ZnO nanotetrapods play an import role in the charge transport and recombination kinetics of our composite films. 3.5. More on the Electric Role of ZnO Nanotetrapods in the Composite Films. From the discussion above, it is still unclear as to what extent ZnO nanotetrapods separately impinge on electron transport and recombination kinetics irrespective of the ZnO shell thickness on SnO2 nanoparticles because both the corrosion of ZnO nanotetrapods and the decrease in the ZnO shell thickness occur simultaneously during the HAc treatment. As a further question, what are the benefits associated with the structural advantages of ZnO nanotetrapods over ZnO particles used previously in SnO2/ZnO composite films?6,8 With these questions in mind, we opted to study two reference composite films (5.0 ( 0.2 µm thick) prepared by blending our SnO2 nanoparticles separately with two types of ZnO particles, one with a size of ∼40 nm (synthesized according to the literature62) and another with a size of about 500 nm (commercially available). The weight ratios of the two reference films were controlled at SnO2/ZnO 2:1 for the sake of comparison with the SnO2/ZnO nanotetrapods 2:1 composite film on transport and recombination kinetics. The large particles are similar to those that were employed in the literature,6,8 whereas the small particles have a BET surface area of 25.6 m2 g-1, which is comparable to that of the ZnO nanotetrapods (BET ) 18.9 m2 g-1), for which a similar dye-adsorbing amount can be expected. Advantageously, because the ultrathin ZnO shells on SnO2 are similar in all of these films because of the similar preparation procedures used (this can also be appreciated from their similarly enhanced Voc, all above 600 mV), the variation in transport kinetics can then be exclusively attributed to the changes of the ZnO particles or nanotetrapods in the composite films. To facilitate comparison, diffusion coefficient Dn as an intrinsic material property will be used in place of τt. It is calculated by Dn ≈ d2/2.35τt,25,63 where d is the film thickness. According to the relationship between Dn and τt, the dependence of Dn on light intensity (I0) can also be described by a power law expression Dn ∝(I0)1-R.63 Dn and τr dependences on the incident photon flux (light intensity, I0) of the composite films are shown in Figure 12A,B side by side. Taken as a whole, Figure 12A highlights the Figure 11. Incident light-intensity-dependent transport and recombination time constants. (A) Original SnO2/ZnO nanotetrapods 2:1 composite film and the corresponding HAc5 and HAc30 films and (B) original SnO2/ZnO nanotetrapods 1:1 composite film and the corresponding HAc30 film. Arcs 1 and 2 highlight the differences in electron transport time (solid arc) and recombination time (dotted arc) caused by HAc treatment for the two different composite photoanodes, respectively. The straight lines represent power-law fits. R values are calculated from the logarithmic slopes (R-1) by least-squares fitting. J J. Phys. Chem. A, Vol. xxx, No. xx, XXXX Chen et al. importance of morphologies of the ZnO additives in modulating the transport kinetics given the similar ZnO shelled SnO2 nanoparticles for all of the composite films, whereas Figure 12B repudiates any substantial influence of such morphologies on the recombination kinetics, leaving behind the ultrathin ZnO shells on the SnO2 nanoparticles as the main determinant of recombination kinetics in the composite films. In getting into more detail, we attend to a few important features in Figure 12A. First, Dn of the ZnO nanotetrapod-derived composite film is larger than those of the two reference films, especially that of the small ZnO particles reference film by a factor of 2.18 to 3.72 in the tested light intensity range. Second, the logarithmic slope (1-R) of Dn dependence on I0 for the ZnO nanotetrapods derived composite film (R ) 0.11) is much larger than those for the reference composite films (R ) 0.27 for the small ZnO particles reference film; R ) 0.41 for the big ZnO particles reference film, R ) 0.41). Third, Dn for the small ZnO particles reference film is smaller than that of its corresponding HAc30 film, in marked contrast with the fact that Dn for the ZnO nanotetrapod-derived composite film is mostly larger than that of its corresponding HAc30 film. The larger Dn value and the larger logarithmic slope of Dn dependence on light intensity for the ZnO nanotetrapod-derived composite film are clear advantages for charge collection over the two ZnO particles reference films, which become even more apparent under close-to-one sun light illumination (the highest light intensity employed in our IMPS/IMVS study was only ∼1/20 sun). This is consistent with our experimental observation that under similar film fabrication and solar cell test conditions the cell performance for the ZnO-particle-derived composite films proved to be inferior to that for our ZnO nanotetrapods/ SnO2 nanoparticles composite films. The sharp contrast between the HAc-induced increase in Dn for the small ZnO particles reference film and the HAc-induced decrease in Dn for the ZnO nanotetrapod-derived composite film connotes a “relay transport” mechanism in the composite films. In a “relay transport” scenario, photoinjected electrons are free to move from SnO2 nanoparticle regions to ZnO additive regions and vice versa and finally reach the conductive glass substrate, contributing to the overall Jsc of the solar cell. In fact, we have already demonstrated above the injection of electrons generated by the photoexcited dyes adsorbed on ZnO nanotetrapods into SnO2 nanoparticles matrix and the attendant contribution to the overall Jsc. Now the question is whether the opposite electron injection, for example, from SnO2 nanoparticles to ZnO nan- otetrapods, is possible. We argue that if only the ZnO-to-SnO2 electron injection was allowed but the reverse was forbidden, then Dn values of the composite films should not have shifted toward the opposite directions upon the HAc treatment just because of different morphologies of the ZnO additives. In addition, the feasible electron injection from SnO2 to ZnO is consistent with the ZnO shell-induced up-shift of the conduction band edge of the SnO2 nanoparticles. We suspect that the opposing effects of small ZnO particles and ZnO nanotetrapod- derived composite films on electron transport can be attributed to the more numerous grain boundaries between small ZnO particles and SnO2 nanoparticles; their lattice mismatch could engender electron traps,30 impeding fast electron transport in the composite network. This result, in turn, highlights the structural benefit of our ZnO nanotetrapods in facilitating electron transport in the composite films because of the reduction of heteroparticle grain boundaries. The recombination kinetics are mostly decided by their similar ZnO shells on SnO2 instead of the morphology of ZnO crystalline additives because the amount of dye taken up by ZnO crystalline additives is much smaller than that by SnO2 nanoparticles for all three composite films. Furthermore, for recombination, photoinjected electrons only need to diffuse to the outer surfaces of nanoparticles (particles) or nanotetrapods and hence the effect of the grain boundaries mentioned above is much less consequential, as evidenced by the similar recombination kinetics in Figure 12B for the different ZnO/ SnO2 composite films. 4. Conclusions By capitalizing on the material advantages of both ZnO nanotetrapods and SnO2 nanoparticles, we have achieved energy conversion efficiencies well >6% for DSSCs based on composite photoanodes encompassing the two nanomaterials. Whereas ultrafine SnO2 nanoparticles promise a significant high roughness factor of composite films, ZnO nanotetrapods with special structural features afford fast electron transport for photoanodes. In this work, three SnO2 nanoparticles/ZnO nanotetrapod (SnO2/ ZnO 2:1, 1:1, and 1:2) composite films have been tested, among which the SnO2/ZnO 2:1 composite film has attained the best efficiency of 6.31% mainly due to its highest saturated Jsc achieved at thinnest thickness. For composite films with higher ZnO nanotetrapods content, because of the requirement of higher film thicknesses, charge collection losses appear to be a main limiting factor that compromises the benefit from their favorable light scattering ability. We have shown that a ZnO source, be Figure 12. Incident light-intensity-dependent (A) electron diffusion coefficient Dn and (B) electron lifetime τr of the SnO2/ZnO nanotetrapods 2:1 composite film and the two reference films with the ZnO nanotetrapods being replaced by ZnO small particles (40 nm) or ZnO big particles (500 nm). The straight lines represent power-law fits. R values are calculated from the logarithmic slopes (1-R) by least-squares fitting. High-Efficiency Dye-Sensitized Solar Cells J. Phys. Chem. A, Vol. xxx, No. xx, XXXX K it from particles or nanotetrapods, tends to form a generally similar ultrathin ZnO shell on SnO2 nanoparticles. By compari- son, for studies using the IMPS/IMVS technique, the functional roles of the ultrathin ZnO shell and the ZnO nanotetrapods in the SnO2/ZnO composite film have been further elucidated for the first time: (1) The ultrathin ZnO shell mainly determines the recombination kinetics rather than the transport rate in the composite films and can elevate the conduction band edge position of SnO2, which is responsible for the higher Voc of DSSCs based on SnO2/ZnO composite films; (2) ZnO nanotet- rapods do contribute to electron transport in a “relay transport” manner, and because of the minimal-to-moderate heteroparticle grain boundaries, electron transport kinetics is facilitated in the ZnO nanotetrapod-derived composite film, accentuating the structural and thus transport supremacy of our branched nano- structure over particles additives in the SnO2/ZnO composite photoanodes. More generally, the motif of branched structure and ultrathin shelled bicontinuous network holds a great potential in future developments of nanostructured solar cells. Acknowledgment. This work was supported by the Hong Kong Research Grants Council (RGC) General Research Funds (GRF) no. HKUST 604206 and 604608. Supporting Information Available: Optical images of dye- adsorbed thick composite films before and after ammonia treatment, XRD patterns of composite film samples, TEM-EDX characterization on composite film, time-resolved emission decays of dye-sensitized three composite films, and XRD, TEM, XPS, and UV-vis absorption characterizations on a standard composite film before and after HAc treatment. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Oregan, B.; Gratzel, M. Nature 1991, 353, 737. (2) Bai, Y.; Cao, Y. M.; Zhang, J.; Wang, M.; Li, R. Z.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M. Nat. Mater. 2008, 7, 626. 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