Aqueous Solution-Processed Multifunctional SnO2 Aggregates for

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Materials and Interfaces

Aqueous Solution-Processed Multifunctional SnO2 Aggregates for Highly Efficient Dye-Sensitized Solar Cells Dongting Wang, Shangheng Liu, Mingfa Shao, Qiuyi Li, Yukun Gu, Jinghan Zhao, Xianxi Zhang, Jinsheng Zhao, and Yuzhen Fang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00039 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Aqueous Solution-Processed Multifunctional SnO2 Aggregates for Highly Efficient Dye-Sensitized Solar Cells Dongting Wang*, Shangheng Liu, Mingfa Shao, Qiuyi Li, Yukun Gu, Jinghan Zhao, Xianxi Zhang, Jinsheng Zhao, Yuzhen Fang* Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059, PR China

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Abstract: A room temperature based environmental friendly aqueous solution synthesis route was developed to fabricate highly dispersed small SnO2 nanoparticles (3-5 nm) without the application of any pressurized reaction vessel or organic solvents. The subsequent treatments, i.e., dialyzing and freezing, allow for the acquision of dual-functions nanostructures with sheetlike feature and large aspect ratio except for the ordinary irregular aggregates. Dye-sensitized solar cells (DSCs) constructed with the resultant multifunctional SnO2 showed an outstanding photovoltaic conversion efficiency (PCE) of 6.92% and an unexpected JSC of 19.5 mA cm−2 at an optimized thickness of 14.1 µm. The excellent performance can be ascribed to the effective coordination of the favorable features (i.e., strong light scattering, large dye loading capability, and fast electron transport) via rational film thickness control as indicated by diffused reflectance spectra, UV-vis absorption spectra, and electrochemical impedance spectroscopy (EIS).



1. Introduction Dye-sensitized solar cells (DSCs) composed of a dyed porous photoanode, an electrolyte and a counter electrode have been proven to be effective photovoltaic devices for direct generation of electricity owing to the advantages of low production cost and high photovoltaic conversion efficiency (PCE).1-5 Since the pioneering work by O’Regan and Grätzel,6 TiO2-based DSCs has been thoroughly studied over the last decades, and impressive conversion efficiencies exceeding 13% have been achieved.7,8 Currently, the development of SnO2 is particularly attractive and has already been regarded as a potential breakthrough to overcome the bottleneck of most-widely studied TiO2 due to its intrinsic higher electron mobility and larger band gap.9-11 However, the conversion efficiency of SnO2 is still much lower than that of TiO2, thereby making them less competitive. The inferior performance of SnO2 could be attributed to the faster interfacial electron recombination and less adsorption amount of acidic dyes due to its more positive conduction-band edge and lower isoelectric point.12,13 Although post treatment of the SnO2 photoelectrodes with an isolating oxide precursor, especially TiCl4, could partially solve these problems, the development of bifunctional or multifunctional photoelectrode materials by microstructure control and synthesis routes innovation could be the fundamental approach to effectively improve the conversion efficiency of SnO2-based DSCs. It is now well-established that a high specific surface area accessible to the dye molecules is indispensable for efficiency enhancement of the DSCs. However, so far, the conventional method of preparing SnO2 photoanodes material has been generally based on high-temperature treatment of SnO2 precursors for morphology control and optimization.14,15 Though annealing treatment enables the effective removal of organic compounds, the required thermal means adversely leads to increased tendency of forming large particles and additional consumption of


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energy, thus giving rise to reduced specific surface area and intensified environmental pollution.16,17 Developing low-temperature based environmentally friendly synthesis routes is therefore necessary for both specific surface area retention and environmental protection. Recently, some efforts have been devoted to the synthesis of SnO2 products via low-temperature routes.18-20 However, among these studies, it should be noted that the organic compounds acting as solvents or reactant are still used during the synthesis process and, in particular, the hydrothermal treatment with the temperature more than 100 °C is also employed. Although the adopted treatment could ensure the crystallinity of the obtained products, it also increases the possibility of creating large particles, thereby sacrificing the accessible surface for dye molecules uptakes. In this context, room temperature based aqueous solution-processed approach is now greatly desired with respect to the advantages of effective structural and morphological properties preservation as well as energy conservation, which, to the best of our knowledge, has yet to be reported. Apart from high surface area, the architecture of the photoanode materials is another key factor that should be considered for efficient light harvesting and electron transport.21-23 Among various SnO2 nanostructures reported, the regular and irregular 3D SnO2 architectures assembled with 0D nanocrystals have already been the research hotspot to achieve high efficiency DSCs since the purpose-built SnO2 could effectively promote light harvesting by enhancing light scattering capability without sacrificing dye loading property of the electrodes.24,25 However, even in the photoanodes constructed with 3D SnO2, electron transport is often retarded because of the presence of zigzag electron transport characteristic along the 3D structure thus leading to electron recombination within the resultant film to some extent. Actually, it is hard to simultaneously gain the aforementioned three features for the photoanode based on single



nanostructure.26,27 As such, from a materials point of view, fabricating hybrid photoanode nanomaterials was proposed to combine the desirable characteristics of each nanostructure, and this established strategy rapidly became an effective approach to drive the efficiency towards higher level. As an example, Tao et al reported a novel integrated ZnO nanostructure consisting of 3D ZnO aggregates (NAs) and surrounded 2D ZnO nanosheets (NSs), termed ZnO NA/NS, that yielded a much higher PCE (i.e., 7.35%) than that based on single ZnO NAs (i.e., 4.06%) because of enlarged surface area and accelerated electron transport arising from the introduced ZnO NSs as well as retained relatively strong light-scattering property of ZnO NAs.28 Independently, Wang et al. devised a highly efficient photoanode composed of 0D TiO2 nanoparticles and 2D TiO2 nanosheets, in which TiO2 nanosheets played the key roles of effectively speeding up electron transport and improving light scattering. Consequently, a remarkably enhanced cell efficiency of 10.1% was obtained as a result of the synergetic effect of light scattering, larger surface area and lower charge recombination.29 All aforementioned researches have proved the synergy resulting from different specific nanostructures and, most importantly, offer the inspiration of realizing remarkable PCE by finely modulating the composition and/or structure. Nevertheless, the exploration of SnO2 hybrid photoanode for high efficiency DSCs has yet to be reported and it is still challenging to design and synthesize multifunctional SnO2 composite via a facile approach rather than the common one by one strategy. Herein, we report the design of the unique multifunctional SnO2 composite architecture mainly consisting of irregularly hierarchical nanostructures via a simple and environmental friendly aqueous solution process under an eco-efficient condition. In this method, extremely small SnO2 nanocrystals are first fabricated via an oxidation-crystallization-dispersion process at room


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temperature without any assistance of organic solvents or compounds (i.e., surfactant, ligand, or capping agent). The subsequent dialysis and freezing treatments lead to the formation of SnO2 aggregates with specific characteristics. The synthesized SnO2 composite are then employed as photoanodes in DSCs, and the thickness of the innovative multifunctional SnO2 film is rationally controlled to maximize the cell performance. Consequently, a strikingly high conversion efficiency of 6.92% is achieved for the photoanode film with thickness of approximate 14.1 nm, which is among the top efficiencies of SnO2 based DSCs. Various measurements revealed that such an excellent performance of the intentionally designed photoanode was attributed to the coordination of fast electron transport and strong light-scattering capabilities as well as the retention of large surface area. 2. Experimental section 2.1. Materials. Anhydrous lithium iodide (LiI), iodide (I2), tert-butylpyridine (t-BPy), 2,3dimethyl-1-propyl imidazolium iodide (DMPII), 3-methoxypropionitrile, acetonitrile, and H2PtCl6 were obtained from Sigma. SnCl2·2H2O, Na2CO3, H2O2 TiCl4, ethanol were all obtained from commercial sources and used without further purification or treatment. Ru-based N719 dye [cis-bis(isothiocyanato)bis-(2,2-bipyridyl-4,4-dicarboxylato)

ruthenium

(II)

bis(tetrabutyl-

ammonium)] was received from Solaronix (Aubonne, Switzerland). 2.2. Preparation of SnO2 Nanoparticles. Extremely small SnO2 nanoparticles were first synthesized through a facile room temperature route. In a typical synthesis, 1.25 mM Na2CO3 was slowly added into SnCl2 aqueous solution (5 mM, 40 ml) under magnetic stirring, then the white suspension was continuously stirred at room temperature for 7 days. Afterwards, the obtained homogeneous SnO2 solution was transferred into a pre-treated semipermeable



membrane and maintained for 3 days. The collected jelly like SnO2 product was then refrigerated for 10 h, washed with deionized water for several times, and finally dried at 80 °C in air for 4 h. 2.3. Preparation of multifunctional SnO2 photoanode. Prior to photoanodes fabrication, a vicious SnO2 paste was first prepared by means of adding a defined amount of SnO2 powder (0.2 g) into a mixture of ethanol (2 ml), terpineol (0.6 g), ethyl cellulose (0.1 g) and acetic acid (0.04 ml), followed by grounding for about 60 min. Thereafter, it was spread uniformly on pre-cleaned FTO substrate (NSG, 7 Ω/sq) using the doctor-blading technique. After drying at 80 °C for 2 h, the as-prepared film was gradually heated in a programmed procedure, i.e., at 325 °C for 5 min, at 375 °C for 5 min, then at 450 °C for 15 min, and finally at 500 °C for 15 min to remove the organic components. The coating of TiO2 was subsequently performed by impregnating the synthesized SnO2 film in 0.4 M TiCl4 aqueous solution at 77 °C for 50 min, thoroughly rinsing with water and ethanol to remove unanchored Ti source, and subsequently annealing at 450 °C for 2h. In the present study, the thickness of SnO2 films was controlled by the layers of the adhesive tape, and the SnO2 photoanodes with thickness of 5.1, 10.8, 14.1, and 18.6 µm were fabricated and labeled as MST1, MST2, MST3, and MST4, respectively. For comparison, the pure SnO2 photoanode without TiCl4 treatment, termed MS, was also prepared according the same process mentioned above. 2.4. Solar cell fabrication. For dye sensitization, the resultant SnO2 electrodes were immediately soaked into 5.0 ×10-4 M ethanolic solution of commercial dye N719 for 24 h at room temperature. The Pt coated photocathode were prepared by dropping H2PtCl6 solution (0.35 mM) on the FTO plate and annealing at 400 °C for 30 min under air flow condition. Then, the dye sensitized photoanode, with an active area of 0.16 cm2, and the counter electrode were bonded together to form a two-electrode sandwich cell with a hot-melt 25-µm-thick Surlyn film


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spacer (Dupont). The electrolyte, composed of 1.0 M DMPII, 0.12 M I2, 0.1 M LiI, and 0.5 M tBPy dissolved in 3-methoxypropionitrile, was injected from one side and driven into the whole sealed region of the assembled cell by vacuum backfilling. 2.5. Material characterization and photoelectrochemical measurements. The crystal structure and morphology of the as-synthesized SnO2 products were obtained from a D/MAX-rA diffractmeter (Rigaku) equipped with Cu Kα radiation (λ= 0.15406 nm) and a high-resolution transmission electron microscopy (HR-TEM, JEOL-2010) at an accelerating voltage of 200 kV. The surface morphologies of the resultant films were observed by scanning electron microscope (SEM, JEOL-6701F, FEI). Nitrogen adsorption–desorption isotherms were measured for Brunauer–Emmett–Teller (BET) data analyses of the SnO2 samples using an Autosorb iQ-XR Analyzer (Quantachrome instruments). The analysis of the dye loading amount in the SnO2 electrodes was performed by desorbing the absorbed dye from sensitized photoanode into 1.0 M NaOH in water/ethanol (50:50, V/V) solution, followed by quantitatively detecting the concentration of each NaOH/dye solution using a UV–vis spectrophotometer (UV 2550, Shimadzu). The reflectance spectra of the samples were recorded as a function of excitation wavelength in a spectral range of 400−800 nm with a PV measurements QEX10 instrument. The photocurrent density-voltage (J−V) characteristics measurements were carried out with a Keithley Model 2400 source meter under simulated AM 1.5 G one sun (100 mW·cm−2) illumination provided by a solar simulator (Newport Corporation) with the sweep direction from forward to reverse. Before device test, the light intensity was first calibrated by a NREL calibrated Si solar cell (PV Measurements, Inc.). The EIS analysis of the assembled cells were measured using an electrochemical workstation (CHI760, CH Instruments) at open-circuit



voltage with an AC amplitude value of 10 mV and a frequency range of 0.1 Hz-106 Hz, respectively. 3. Results and discussion 3.1. XRD analysis. The crystallographic structures of the products synthesized at different times were identified by the X-ray diffraction (XRD) pattern (see Figure 1). By stirring at room temperature for 45 min, white turbid solution is formed primarily (see Figure S1). The weak XRD patterns shown in Figure 1a indicate the amorphous characteristic of the as-prepared product in the initial reaction stage. After continuously stirring for 1 day, the diffraction peaks corresponding to rutile SnO2 become stronger (Figure 1b), which clearly demonstrates the strong transformation tendency from amorphous phase to crystalline structure. Extending the stirring time to 7 days leads to further crystallization of SnO2 product (see Figure 1c). These broad peaks in XRD pattern indicate the small crystal size of the as-prepared individual SnO2 particle. The average crystallite size calculation result deduced from the FWHM of the (110) peak at 26.3 reveals that the mean size of the as-prepared SnO2 is about 4 nm. Apart from the materials obtained under low pH condition (i.e., pH = 1.9), the product synthesized under a higher reaction pH value (i.e., pH = 10.0) is also characterized. Apparently, all the strong diffraction peaks in the XRD patterns shown in Figure 1d are similar to those given in Figure 1c, revealing the negligible influence of pH values on the transformation from amorphous SnO2 to the stable crystalline state. 3.2. Morphological characterization of the as-synthesized SnO2. In order to illustrate the morphology evolution process during the synthesis of SnO2 nanostructure, transmission electron microscopy (TEM) analysis were performed. As displayed in Figure S1a, after reacting 45 minutes, white slurry-like products is formed. TEM image in Figure 2a clearly reveals that the


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obtained products consist of tightly attached nanoparticles. Figure 2b exhibits the morphology of the obtained particles after stirring for 1 day. Notably, although the precipitation is still comprised of closely attached nanoparticles, the boundaries and pores between nanoparticles are clearly visible. The aforementioned morphology and composition variations are also reflected by the color change of the aqueous solution shown in Figure S1a. When the reaction time is increased to 7 day, the morphology of the acquired product alters evidently. Highly dispersed nanoparticles with diameter domains of 3−5 nm are inspected for the newly synthesized aqueous solution (Figure 2c). The well dispersion of extremely small SnO2 in water eventually results in the formation of homogeneous transparent solution, as shown in the supporting information of Figure S1a. In comparison to the synthesis of SnO2 nanoparticles in low pH value (i.e., pH = 1.9), the preparation of SnO2 under high pH condition (i.e., pH = 10.0) was also carried out, and the images of the resultant solutions at various stages are shown in Figure S1b. The obvious distinction suggests the necessity of appropriate reaction management for the production of highly dispersible SnO2 with small crystal size. According to the TEM and XRD characterizations of the products collected at different reaction times, the possible oxidation-crystallization-dispersion mechanism is proposed and depicted in Figure 3. In the early stage, the added SnCl2 was first hydrolyzed and then oxidized to Sn4+ due to the presence of the dissolved oxygen molecules in the aqueous solution. However, because of the low concentration of O2 molecules in the precursor solution only amorphous product was obtained as shown in Figure 1a. As the reaction proceeded, composition evolution and morphology variation occur spontaneously, and highly dispersed crystalline SnO2 is ultimately obtained (see Figure 1c). The formation of highly dispersive SnO2 is most likely associated with gradual change of surface charge related to the aqueous solution pH value. It has



been reported that the alkaline condition facilitates the formation of aggregated Sn species, while the suitable acidic environment may give rise to the generation of positive charged surface.30 The acidic condition eventually leads to the formation of electrostatic repulsion forces between different nanoparticles, and appropriate pH value finally results in the establishment of dynamic equilibrium between aggregation and dispersion (see Figure S2). After dialyzing in deionized water for 3 days, however, notable phase change took place for the as-synthesized transparent SnO2 solution, giving rise to the formation of jelly like products (inset in Figure 4a). HRTEM examination (Figure 4a) shows that the dialyzed SnO2 nanoparticles tend to cluster together irregularly probably due to the cross-linking and bonds sharing among the nanoparticles.31 Further observation reveals that plenty of mesopores and macropores are visible in the dialyzed SnO2, which are believed to facilitate the dye absorption and electrolytes penetrability for the resultant photoanode. Afterwards, the obtained products were freezed for 10 h, and then taken out to be heated under natural condition. During the freezing treatment, the connected SnO2 nanoparticles tend to aggregate more tightly, and SnO2 aggregates with various morphologies and sizes are obtained by grounding the dried product for 20 min (see Figure 4b). The key to generate SnO2 aggregates could be stemmed from the spontaneous process of lowering the Gibbs free energy.32 More interestingly, it is worthy to note that apart from the irregular 3D structures other unique nanostructures, i.e., sheet-like morphology with larger thickness and aggregates with large aspect ratio, are clearly observed (see Figure 4b and Figure S3). As a result, it is supposed that the embedded special structures will provide more oriented ways for electron transfer by effectively connecting the adjacent aggregates, thus leading to a significant reduction in the charge recombination.33


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3.3. N2 adsorption–desorption isotherms. To get an insight into the potential dye loading ability and interior porous structure of the resulting SnO2 samples, N2 absorption/desorption isotherms and the corresponding pore size distribution curves were measured and presented in Figure 5. As can be seen from Figure 5a, obvious N2 adsorption at the low pressure region (P/P0 < 0.3) is observed for the dialyzed material, revealing the presence of micropore and its nonnegligible contribution to the total pore volume.35,36 Moreover, when the pressure is further increased, a hysteresis hoop is found at a higher pressure approaching P/P0 =1, indicating the adsorption of N2 in macropores. The conclusion agrees well with Barrett–Joyner–Halenda (BJH) pore size distributions (Figure 5 inset) and microscopy image findings in Figure S3. The high porosity in combination with extremely small particle size ultimately results in a strikingly high specific surface area of 192.8 m2g–1 for the dialyzed SnO2. To the best of our knowledge, rare achievement has been reported for such a high BET surface area regarding to pure SnO2 aggregates. Figure 5b compares the N2 adsorption-desorption isotherms for all the photoanode samples with and without TiCl4 treatment, i.e., MS and MST1-4. As one can see, the N2 adsorptiondesorption isotherms of these materials peeled off from various photoanode films almost display the same variation trends in the whole relative pressure region, namely, first gradual increase in the relative low pressure region followed by an abruptly increase part. According to IUPAC nomenclature, it can be inferred that the obtained adsorption characteristic curve is a type IV isotherm with a representative H3 hysteresis loop, indicating the presence of mesopores in the constructed SnO2 film. In contrast to the appearance of a sharp capillary condensation step at a high relative pressure (P/P0; 0.75) for MS, a clear shift of the hysteresis loop towards the lower pressure is observed for the MST samples. This implies the size variation of the mesoporous



structure after TiCl4 treatment,37 which hence results in the change of Brunauer-Emmett-Teller (BET) specific surface area and pore characteristics of the resulting SnO2 products. As illustrated in Table 1, SnO2 still possesses a quite high BET surface area of 46.2 m2g–1 even though it has been suffered from two times of calcination treatments, which most likely derives from the small primary nanoparticle size and large pores resulted from nanoparticle gathering. Post treatment of SnO2 films with TiCl4, however, leads to the reduction of the surface area to some extent. Typically, although almost possessing the same film thickness, the MTS3 sample prepared under identical condition just presents a BET surface area of 43.6 m2g–1. Pore size distribution measurements of MS and MTS3 reveal that MS has an average pore size of 56 nm while MST3 displays smaller pores with mean diameters of 22 nm and 34 nm, which further reveals the influence of TiCl4 post treatment on SnO2-based film electrodes. As for MTS1, MTS2, and MTS4, the variation of BET values seems rational in terms of the change of SnO2 surface coverage with the increase of film thickness. 3.4. Morphological characterization of SnO2 photoanode films. To investigate the morphology and microstructure differences of the as-formed SnO2 photoanodes, SEM and TEM characterizations were carried out. As shown in Figure 6a, the resultant MS films display similar morphology to that of dialyzed product; irregular shape and size range regardless of heat treatment. However, the primary SnO2 nanocrystallites constituting the aggregate become bigger in comparison with that of the dialyzed SnO2. Additionally, the calcination at elevated temperature also significantly affected the interconnection between aggregates probably due to grain growth and induced coalescence during the calcination step.38 As a result, smaller pores and gaps between SnO2 aggregates are observed for the resultant SnO2 film as compared to the uncalcined sample. TiO2 coating layer was introduced by impregnating the fabricated SnO2 films


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into high concentration TiCl4 solution (0.4 M) followed by a programmed calcination procedure. As can be seen in Figure 6c-f, all MST products almost maintain the similar unique structure to that of pure SnO2 in spite of the TiCl4 treatment. However, it also can be clearly seen that the surface of the MSTs is much rougher and the size becomes greater as compared with pure SnO2. High-magnification SEM and TEM images of MST3 reveal that the TiCl4-derived outer layer consists of short thorn-shaped branches with length in the range of 50-100 nm. Further the XRD and EDS characterizations demonstrate that the outer coating layer is composed of anatase TiO2 (see Figure S4). The coating of TiO2 layer on the surface allows the physicochemical characteristics modulation of SnO2 and hence gives rise to increased dye uptakes and enhanced electrons injection efficiency from the excited dye into the semiconductor.39 In addition, the special feature of the TiO2 outer layer could be expected to improve the interconnectivity between the SnO2 structures and thus provides direct routes for electron transport within the film. Furthermore, in view of the orientation randomness for special aggregates, the presence of partially oriented aggregates is therefore worth expecting. As shown in Figure S5, the crosssectional SEM image confirms the existence of irregular aggregates with beneficial orientation within the resultant film, thus supplying direct pathways for fast electron transport throughout the photoanode. 3.5. J-V curves. The typical photocurrent versus voltage (J–V) characteristics of the DSCs based on MS and MST (i.e., MST1, MST2, MST3, and MST4) films are shown in Figure 7 and the derived photovoltaic parameters are listed in Table 1. It can be found that the JSC and VOC of SnO2 are 10.8 mA cm−2 and 383 mV, respectively, yielding only a relatively low PCE of 1.88%. After TiCl4 treatment, all the devices (i.e., MST-series) display a simultaneous augment in JSC, VOC, and PCE in comparison with that of MS. The dramatic increment of the photovoltaic



performance can be attributed to the increased isoelectric points and suppressed interfacial charge recombination arising from the inherent physical property difference between the materials.40 In addition, by adjusting film thickness, the photovoltaic performance of the MST based cells can be effectively tuned and optimized. Specifically, the JSC evolves with a gradual rising trend from 11.9 to 19.5 mA cm−2 for MST1 to 3 along with increasing the film thickness from 5.1 to 14.1 µm, whereas further thickness increase conversely results in a sharp drop of JSC to 14.6 mA cm−2. In contrast to JSC, the VOC decreases successively with the gradual film thickness increment from MST1 to MST4. As reported previously, TiO2 possesses a higher quasi-Fermi level than SnO2, and the coating of TiO2 could shift quasi-Fermi to a higher level, thus leading to enhanced Voc. However, the attempt to increase film thickness progressively will generate insufficient surface coverage of SnO2, which consequently limits quasi-Fermi level shift to some extent and slows the photo-generated electron injection rate from excited dye to SnO2. Moreover, the increase of film thickness implies the addition of the charge-recombination sites within the film, resulting in unwanted recombination of the oxidized dye with the photogenerated electrons.41 3.6. Dye absorption and diffuse reflectivity. To explore the origination of the high photovoltaic performance of MST based DSCs, dye uptakes in all the photoanode films was first investigated using desorption test. As one can see, compared with MS (0.83 × 10−7 mol cm−2), all of the MST-based films display much better dye adsorption abilities with the estimated dye content increased to 1.08, 1.86, 2.25 and 2.56 ×10−7 mol cm−2 for MST1, MST2, MST3 and MST4, respectively, showing the thickness-dependent dye absorbing capability. The above observation can be explained as follows: On one hand, in view of the lower isoelectric point, SnO2 film electrode without TiCl4 treatment cannot adsorb dye effectively, which consequently


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leads to limited dye absorption amount. On the other hand, for MST series, the gradual argument in dye anchoring could be due to the enhancement of overall surface area defined by per unit area (cm2) of the active film. Provided that the MSTs films have the same density and microstructure, the above surface areas of MST 2-4 show 110, 180, and 270-percent increase, respectively, in comparison with MST1. However, it can be observed from Table 1 or the data given above that the actual dye attachment of MST 2-4 is only about 1.7, 2.1, and 2.4 times higher than that of MST1. Therefore, the dye loading ability of the active films actually decreases upon gradually increasing the thickness from 5.1 to 18.6 µm, which can be presumably attributed to the concurrent change of microstructure of the film and the coverage of SnO2 surface with TiO2. More dye anchoring will consequently result in sufficient incident photons absorption, and, therefore, substantially contributes to robust photocurrent densities. Figure 8 show UV−vis diffused reflectance spectra of all five films. Apparently, the diffuse reflectance characteristics of all MST films studied are enhanced successively from MST1 to MST4 over the entire visible wavelength range with the increment of film thickness, indicating the gradually improved light harvesting capabilities of the resultant photoelectrodes. However, it should be noted that the MS film generates even higher reflectivity than MST4 within the whole wavelength range of 400−800 nm. This is understandable because high reflection and scattering are likely to be related not only to the thickness of the films, but also to particularly morphological features such as the particle size, morphology, and interspace between the hierarchical structures.42 For MST samples, although TiCl4 post-treatment lead to size increment, the pore sizes between SnO2 aggregates decrease at the same time. Therefore, the diminution of the reflectance for MST samples can be mainly given rise by their reduced pore size despite of the possible positive contribution from particle size augment.



3.7. EIS analysis. As indicated in Figure 9, all Nyquist plots show three well-defined semicircles in a wide frequency range, which, in the order of frequency increasing, correspond to the Warburg diffusion in the electrolyte, electron transfer at the dye-absorbed SnO2/electrolyte interface, and electrochemical reaction at Pt/electrolyte interface, respectively. In particular, the largest one in the middle frequency region of 101-103 Hz has a vital influence on the photovoltaic performance of the cells, and the extent of charge transport resistance (Rct) can be deduced from the corresponding diameter by fitting the EIS spectra.1,43 As listed in Table 1, it can be clearly observed that the Rct decreases appreciably after introducing TiO2 layer. Such a phenomenon is expected since the presence of TiO2 could effectively accelerate electrons injection from the excited dye to SnO2 due to its less positive conduction band edge position. For the MST samples, it is obvious that the Rct reduces in the following order: MST3 (14.5 Ω) < MST2 (16.1 Ω) < MST1 (17.5 Ω) < MST4 (22.3 Ω), implying that the MST3 has the fastest electron transport rate among all the cells. This observation is explainable because the total resistance could be attributed to the additive of the more conductive SnO2 and less conductive TiO2 out layer. However, it can be easily conjectured that the content or coverage of TiO2 would change along with the variation of the thickness. Specifically, the surface of the thin SnO2 film could be fully covered or covered with a large extent under otherwise the same conditions (i.e., TiCl4 concentration, hydrolysis time and temperature), while the surface of the thick SnO2 film cannot be fully covered or only coated with a small portion.44 The increase of TiO2 amount implies the less conductivity of the resulting film. On the other hand, as revealed by the characterizations in Figure 6 and S4, the formation of the thorn-shaped TiO2 would lead to the improved interconnections between SnO2 nanostructures within the film, thus providing more effective electronic conduction route. However, further increase of the film thickness would inevitably


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give rise to the increase of Rct since there might be only a small portion of TiO2 coating layer accessible to the interior of the SnO2 film, leading to more SnO2 structures directly exposed in electrolyte and hence augmenting charge recombination rate. 3.8. IPCE spectra. The typical IPCE spectra of various DSCs fabricated with MST photoelectrodes were recorded. As shown in Figure S6, the IPCE value first increases and then degrades with the increase of film thickness, which coincides with the observed variation trend of JSC as shown in Figure 3. Moreover, the IPCE of the MST film-based DSCs increases gradually in the lower-energy range from MST1 to 4. To better reveal the optical characteristic difference among the photoanodes, normalized IPCE characterization was conducted by normalizing the obtained IPCE to the highest efficiency at 530 nm. As displayed in Figure 10, a robust enhancement in the wavelength range of 550–750 nm is clearly visualized, revealing the impact of film thickness on the light conversion efficiency. Considering the successive augment of dye loading amount and reflectivity from MST1 to 4, the achievement of the best IPCE performance for MST3 must be attributed to coordination of light harvesting efficiency and electron transfer property, which further confirms the necessity of film thickness control for performance optimization. 4. Conclusion In summary, we presented a novel and eco-friendly aqueous solution route to construct multifunctional SnO2 architecture consisting of irregular aggregates. In this report, highly dispersed small SnO2 nanoparticles were successfully synthesized by an oxidationcrystallization-dispersion process at room temperature without applying any organic compounds. The acquired SnO2 could further gather together to form aggregates with various size and shapes, especially morphologies with sheet-like feature and large aspect ratio, via a simple dialysis and



freezing treatments. The systematic investigations revealed the significant influence of the resulting SnO2 film thickness on photovoltaic parameters, particularly for JSC and PCE. At an optimal thickness of 14.1 µm, the SnO2 composite photoanode delivered a remarkable shortcircuit current density of 19.5 mA cm–2 and a comparable efficiency of 6.92%. The outstanding performance may have been attributed to the combined effects of large surface area, effective light scattering, and fast electron transport. This novel method provides a facile route to construct semiconductor metal oxide composite and could be likely introduced to design other nanostructures composite for effective energy conversion in a wide range of applications. ASSOCIATED CONTENT Supporting Information The evolution of SnO2 during stirring, the influence of pH on SnO2 dispersion, TEM and SEM images of sheet-like SnO2, TEM and SEM images of MST3, EDX spectrum of MST3, XRD patterns of MS and MST3, the cross-section SEM of MST3 and the IPCE spectra of MST samples. AUTHOR INFORMATION Corresponding Authors *Dongting Wang. E-mail: [email protected]. *Yuzhen Fang. E-mail: [email protected]. ORCID Dongting Wang: 0000-0002-5350-6027 Yuzhen Fang: 0000-0003-0210-5714 ACKNOWLEDGMENT


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Table 1. Characteristics of the Various SnO2 Photoanodes together with the Corresponding Photovoltaic Parameters.

Sample Thickness

Adsorbed dye

Voc

Jsc (mA/cm²)

（V）

FF

PCE

[×10-7 mol·cm-2] Rct

SBET (m2/g)

MS

12.8

10.8

0.383

45.7

1.88

0.83

35.8

46.2

MTS1

5.1

11.9

0.678

60.2

4.86

1.08

17.5

37.6

MTS2

10.8

17.3

0.603

59.9

6.26

1.86

16.1

39.1

MTS3

14.1

19.5

0.594

59.7

6.92

2.25

14.5

43.6

MTS4

18.6

14.6

0.578

58.9

4.96

2.56

22.3

36.7


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Figure 1. XRD patterns of SnO2 products with different reaction times: (a) 45 min, (b) 1 day, and (c) 7 days. (d) XRD pattern of SnO2 obtained at high pH condition (pH = 10) after stirring for 7 days. Vertical bars below represent the standard XRD pattern of SnO2.

Figure 2. TEM images of SnO2 with different morphologies obtained after various reaction times: (a) 45 min, (b) 1 day, and (c) 7 days.



Figure 3. Schematic representation for morphological evolution of the SnO2 nanostructures with the change of reaction time.

Figure 4. TEM images of SnO2 with the treatments of (a) dialysis and (b) dialysis followed by freezing. (c) High-resolution TEM image of the as-prepared SnO2.


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Figure 5. The N2 adsorption and desorption isotherms of a) dialyzed SnO2 and (b) MS and MSTs samples. The inset shows the pore size distribution of the dialyzed SnO2.



Figure 6. Representative SEM images of (a,b) MS with different modifications, (c) MST1, (d) MST2, (e) MST3, and (f) MST4, respectively.


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Figure 7. J−V characteristic curves of various DSCs devices with and without TiCl4 treatment.

Figure 8. Diffuse reflectance spectra of the five photoanode films based on MS and MSTs series.



Figure 9. Nyquist plots of various SnO2-based DSCs.

Figure 10. Normalized IPCE of DSCs based on MST1, MST2, MST3, and MST4, respectively.


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Aqueous Solution-Processed Multifunctional SnO2 Aggregates for

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