A Facile and Green Approach to Synthesize Mesoporous Anatase


Herein we report a simple, template-free, and green approach to synthesize MATNs consisting of interconnected ... Nanocrystalline anatase titanium dio...
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A Facile and Green Approach to Synthesize Mesoporous Anatase TiO2 Nanomaterials for Efficient Dye-sensitized and Hole-conductor-free Perovskite Solar Cells Liang Chu, Jie Zhang, Wei Liu, Rui Zhang, Jian Yang, Ruiyuan Hu, Xing’ao Li, and Wei Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00607 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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A Facile and Green Approach to Synthesize Mesoporous Anatase TiO2 Nanomaterials for Efficient Dye-sensitized and Hole-conductor-free Perovskite Solar Cells

Liang Chu a, b, Jie Zhang b, Wei Liu b, Rui Zhang b, Jian Yang b, Ruiyuan Hu b, Xing’ao Li * a, b, Wei Huang* b, d

a

New Energy Technology Engineering Laboratory of Jiangsu Provence & School of Science,

Nanjing University of Posts and Telecommunications (NUPT), 9 Weiyuan Road, Nanjing 210046, P. R. China b

Key Laboratory for Organic Electronics & Information Displays & Institute of Advanced

Materials, Jiangsu National Synergistic Innovation Center for Advanced Materials, School of Materials Science and Engineering, Nanjing University of Posts and Telecommunications (NUPT), 9 Weiyuan Road, Nanjing 210046, P. R. China c

Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU),

127 West Youyi Road, Xi’an, 710072, P. R. China

* E-mail: [email protected] & [email protected]

ABSTRACT: Mesoporous anatase TiO2 nanomaterials (MATNs) with both large specific surface areas and structural coherence are highly desirable to achieve excellent physicochemical properties for photovoltaic applications, but the existing synthesis methods either need templates or cause pollution. Herein we report a simple, template-free, and green approach to synthesize MATNs consisting of interconnected nanoparticles. The Ti-complex intermediates were first prepared using titanium 1

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isopropoxide and acetic acid in a solvothermal reaction, which went through a morphology transformation sequence of nanowires, microspheres and microflowers with prolonging reaction time. Then the Ti-complex intermediates were cracked into MATNs under annealing, which were applied in dye-sensitized solar cells (DSSCs) and hole-conductor-free perovskite solar cells (HPSCs). The mesoporous anatase TiO2 nanowires-based DSSCs achieved high power conversion efficiency (PCE) up to 7.78% because of both high dye adsorption capacity and long charge transfer channels, while the PCE based on the P25 photoelectrodes is 6.61%. The further application of mesoporous anatase TiO2 nanowires in HPSCs achieved an improved PCE of 8.52%, compared to 6.78% for cells prepared using the P25 electrodes.

KEYWORDS: Mesoporous anatase TiO2 nanomaterials; dye-sensitized solar cells; perovskite solar cells; specific surface area; charge transfer.

INTRODUCTION Nanocrystalline anatase titanium dioxide (TiO2) has attracted fervent attentions over the past few decades owing to its outstanding optical, electrical and catalytic properties. 1-3 Being low cost, environmentally friendly, easily available, and with a wide bandgap (3.2 eV), it has been extensively used in photocatalysis,

1, 4, 5

gas

sensors,6 photovoltaic cells, 7 ultraviolet detectors, 8 and electrical energy storage. 9 The morphology of the TiO2 nanostructures is one of the most crucial factors in determining the performance of these applications. TiO2 nanoparticles usually provide

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high specific surface areas, but exhibit a low electron transport rate because of a large number of surface defects and grain boundaries, while one-dimensional TiO2 nanostructures, such as nanorods,

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nanowires

11, 12

and nanotubes,

13

can facilitate

efficient electron transport, but have insufficient specific surface areas. Mesoporous anatase TiO2 nanomaterials (MATNs) can possess both the desired highly accessible specific surface areas due to internal pores and the long-range charge transfer channels, 14 making them the superior candidates for the above applications. To date, template thermolysis and hydrothermal methods have been the most common techniques to synthesize the MATNs.

15-21

In thermolysis method, the

templates are generally modified to enhance the interaction between templates and the next coated TiO2 layers, and then the templates must be removed to make pores. 22 For example, Snaith and coworkers synthesized mesoporous TiO2 single crystals using TiCl4-treated mesoporous SiO2 template, which was selectively etched in NaOH solution. Unfortunately, the preparing process is tedious, and some templates cannot be removed completely if they bind with the TiO2 layers strongly. 23 The hydrothermal method usually needs capping agents, 16-18 which might cause pollution. For example, in our previous work, mesoporous anatase TiO2 microspheres consisting of interconnected nanoparticles were synthesized by a one-step fast solvothermal process, in which urea was used as the capping agent to control phase and promote oriented growth. 24 So it is still in urgent need to develop simple, template-free, and green routes to synthesize the MATNs. Mesoscopic solar cells such as dye-sensitized solar cells (DSSCs),25-28 quantum

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dot-sensitized solar cells 29 and perovskite solar cells (PSCs), 30-31 are among the most promising candidates for the next generation photovoltaics due to their facile fabrication, low cost and high power conversion efficiency (PCE). In mesoscopic solar cells, the scaffold oxide films are indispensable, serving as both photosensitizer loading sites and electron transfer channels. Therefore, the scaffold oxide films are expected to simultaneously meet the requirements of high surface areas and long-range charge transfer channels. Organic-inorganic hybrid perovskites substituting for dye molecules in DSSCs, are extremely promising materials for the future photovoltaic applications due to their direct optical bandgap, high absorption coefficient, long carrier diffusion length and low excition binding energy.

32-34

During the past few years, the PCE of PSCs have

rapidly increased to 22.1%. 35 While the silicon-based solar cells took nearly 30 years to mature to such high PCE. Intriguingly, PSCs could still work without the costly hole transport layers because the perovskite materials have hambipolar characteristics 30

. Meanwhile, carbon materials have been the excellent electrodes for PSC due to the

suitable electronic properties, moisture resistant and perfect chemical stability. And carbon-based hole-conductor-free PSCs (HPSCs) have been successfully developed because of the low cost, simplified fabrication process, and high stability. 36 Fortunately, mesoporous oxide nanomaterials can satisfy both requirements of high surface areas and long-range charge transfer channels, and they have been successfully incorporated in DSSCs and PSCs as scaffolds. However, investigations of MATNs still remain intensive. For instance, Dai et al. synthesized mesoporous

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anatase TiO2 sub-microspheres composed of anatase granular-like nanocrystallines, which were employed as efficient scaffold layers for PSCs because of their fast charge transfer and efficient light-harvesting.

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Chen et al. synthesized the Li-doped

hierarchical TiO2 nanostructures, which exhibit superior electron transport ability with less trap states in PSCs.

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Wang et al. synthesized uniform discrete mesoporous

titania microspheres via a facile and controllable interface-directed co-assembly approach, which showed excellent performance in the application of DSSCs.

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Recently, Li et al. explored bunchy TiO2 hierarchical spheres with fast electron transport and large specific surface area, which enable the improved PCE of the DSSCs up to 7.5%, compared to 5.6% of that fabricated with the P25 photoelectrodes .40 Here, we successfully prepared the MATNs consisting of nanoparticles by a simple, template-free, and environment-friendly route, in which the Ti-complex intermediate nanowires, microspheres and microflowers were first prepared by a solvothermal reaction in the mixed titanium isopropoxide and acetic acid solution at 150 ○C for 3, 6 and 9 h, respectively. Next, the Ti-complex intermediates were annealed at 500 ○C for 3 h to decompose into the MATNs consisting of interconnected nanoparticles. Interestingly, the Ti-complex intermediate nanowires were decomposed into mesoporous anatase TiO2 nanowires. The MATNs were then used as photoelectrodes for DSSCs, showing a higher PCE than that based on the P25 photoelectrodes. Especially, the mesoporous anatase TiO2 nanowires exhibited excellent photovoltaic properties, due to the high light adsorption capacity and long

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electron transport channels, with the PCEs of DSSCs up to 7.78%, which are dramatically higher than that based on P25 electrodes (6.61%). Meanwhile, the mesoporous anatase TiO2 nanowires were also used in carbon-based HPSCs, and an improved PCE (8.52%) was observed when compared to cells prepared using P25 electrodes (6.78%). The present strategy of synthesizing the MATNs has a few clear advantages over traditional methods, such as simple, fast, green and without requirement of surfactants or templates, and it might lead to broad applications for energy conversion and storage, photocatalysis, gas and ultraviolet sensors.

EXPERIMENTAL SECTION Synthesis of MATNs. Ti-complex intermediate products were prepared by a facile solvothermal reaction. Typically, 0.5 mL titanium isopropoxide (Ti(OiPr)4) was added dropwise into 30 mL acetic acid under magnetic stirring. The mixed solution was subsequently transferred into a 50 mL stainless steel Teflon lined autoclave. Then, the autoclave was sealed, put into an electric oven, heated at 150 °C for 3, 6, 9 h, respectively, and cooled to room temperature in air. The products were collected and washed with deionized water and ethanol by centrifuge. After being dried in air at 80 ○C overnight, the Ti-complex intermediate products were obtained. To form the MATNs, the as-prepared intermediate products were annealed at 500 °C for 3 h with a heating rate of 2 °C min-1 in a muffle furnace under air condition. Preparation of DSSCs. The DSSCs were prepared as in our previous work. 24 Typically, the fluorine-doped tin oxide (FTO, ~7 Ω sq-1, transmittance ~80%) glasses were sonicated using detergent in deionized water, acetone, and ethanol in sequence. After being dried under N2 gas

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flowing, the FTO glasses were treated with ultraviolet-ozone for 10 min to remove the last organic residues. The clean FTO glasses were dipped into 40 mM TiCl4 solution at 70 °C for 30 min, which were then washed with deionized water and ethanol, and dried under air condition. Then the prepared TiO2 pastes were coated by doctor-blading method, followed by heat treatment at 125 °C for 15 min, at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 30 min in a muffle furnace. The TiO2 films were treated with 40 mM TiCl4 solution at 70 °C for 30 min again, and heated at 500 °C for 30 min in a muffle furnace. The thickness of TiO2 films was about 15 µm. To fabricate TiO2 paste, 1 g as-synthesized TiO2 powder (or Degussa P25 powder) was first mixed with 0.2 ml acetic acid, 8 ml ethanol, 0.5 g ethyl cellulose and 3 g terpineol under sufficiently magnetic stirring. After being ground with a mortar and pestle for about 30 min, the mixed paste was finally supersonic treated for 1 h. To be sensitized, the TiO2 photoelectrodes were heated to about 70 °C, immersed into 0.5 mM N719 dye (Ruthenium 535-bis TBA) solution in acetonitrile and tert-butanol with volume ratio of 1 : 1 for 18 h, then carefully washed with acetonitrile, and dried at room temperature. A Pt counter electrode was buckled on an N719-sensitized TiO2 film, between which a space was separated by polypropylene plastic and introduced with redox electrolyte. The Pt counter electrodes were deposited by magnetron sputtering on the cleaned FTO glasses, and the redox electrolyte was composed of 0.3 M 1-methyl-3-propylimidazolium iodide (99 %), 0.5 M lithium iodide, 0.05 M iodine and 0.5 M 4-tert butyl-pyridine in acetonitrile. The active area of the devices was 0.16 cm2 without mask. Preparation of HPSCs. The FTO glasses were patterned with zinc powder and 2 M HCl solution before being cleaned. A TiO2 blocking layer was spin-coated on the clean FTO glass by precursor solution prepared by mixing 0.23 M titanium isopropoxide and 0.013 M HCl in isopropyl alcohol,

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and heated in an oven at 500 ○C for 30 min. After they had cooled down to room temperature, the prepared TiO2 inks were spin-coated on the substrates, and heated at 125 °C for 15 min, at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 30 min, respectively. The thickness of mesoporous TiO2 layers was about 150 nm. To fabricate TiO2 inks, 0.01 g commercial P25 nanoparticles or mesoporous anatase TiO2 nanowires were dispersed into 10 ml ethanol under ultrasound, and then 0.5 ml acetic acid and 0.05 g ethyl cellulose were added with rigorous stirring and ultrasound again. Before the perovskite deposition, the TiO2 substrates were ultraviolet-ozone 41

treated for 10 min. CH3NH3I powder was prepared as in the previous literature . Typically, the perovskite precursor was made by mixing 462 mg PbI2 and 159 mg CH3NH3I powder in a solution of 0.64 ml anhydrous dimethylformamide and 0.16 ml dimethyl sulfoxide under stirring at 70 °C. The precursor solution was spin-coated on TiO2 substrates at 500 rpm for 3 s and 4000 rpm for 20 s in a N2-filled glovebox. During the spinning process at 10 s, 0.3 mL toluene as anti-solvent was dripped to rush the film. After being annealed at 75 °C for 10 min and 105 °C for 10 min, brilliant black perovskite films were obtained. Then, a carbon paste was doctor-bladed on the perovskite films and dried at 105 °C for 20 min in air to form carbon electrodes. The active area of the HPSCs was 0.14 cm2 with a black mask. Measurement and characterization. Morphology and structure of the samples were observed by scanning electron microscope (SEM, S-4800, Hitachi, Tokyo, Japan) and aberration corrected transmission electron microscope (TEM, FEI Titan G260-300). X-ray diffraction (XRD) patterns were investigated by X-ray diffractometer (XRD, D8 ADVANCE, Bruker, Karlsruhe, Germany) with Cu-Ka radiation (λ = 0.15405 nm). Fourier transform infrared (IR) transmittance measurement was carried out by means of an IRPrestige-21 spectrophotometer (Shimadzu) in the

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4000-400 cm-1 wavenumber range using KBr pellets. Surface area of the TiO2 powder was measured by the Brunauer-Emmett-Teller (BET, V-Sorb 2800P). Simulated sunlight was provided by a solar simulator (Newport, USA), which was calibrated to one sun (AM 1.5G, 100 mW cm-2) by a filtered Si reference solar cell. Current density-voltage characteristics were recorded by a Keithley 2400 Source Meter. The electrochemical impedance spectra (EIS) was measured by a CHI660D electrochemical workstation (Shanghai, China) under dark condition at a bias of 0.80 V with an amplitude of 10 mV in a frequency range from 100 kHz to 0.1 Hz. For desorption dye molecules, the sensitized TiO2 films were immersed in 0.1 M NaOH solution, followed to measure the UV-vis absorbance by an ultraviolet-visible spectrophotometer (Lambda 35, PerkinElmer, Massachusetts, USA). The amounts of dye molecules adsorbed on the TiO2 films were determined by Beer-Lambert law: A = ε·b·c, where ε is the absorptivity, b (1 cm) is the path length, and c is the concentration. The absorptivity of the N719 dye molecules is 1.47 × 104 L mol -1 cm -1 at 535 nm.

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Photoluminescence spectra were detected by a CCD detector (Edinburgh F900), and the

excitation wavelength was 507 nm provided by a low noise solid state laser.

RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of the samples by solvothermal reaction for 3, 6 and 9 h, respectively, in which there were no obvious peaks, indicating an unknown amorphous structure. Figure 1b shows the XRD patterns of the corresponding samples through an annealing process at 500 °C under air condition, in which the diffraction peaks at 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, 62.69° and 68.76°, can be ascribed to (101), (004), (200), (105), (211), (204) and (116) planes of anatase TiO2 phase,

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respectively (JCPDS No. 21-1272). No impurity peaks are observed, indicating high purity of the products after annealing. In addition, the enlarged image of the XRD patterns are shown in Figure 1S (Supporting Information), in which the comparison of the different half-peak widths is clearer. The average gain sizes D can be roughly calculated by the Scherrer equation: D = Kλ/(βcosθ), where K is the Scherrer constant (0.89), λ the wavelength of the X-ray (1.54 Å), β the half-peak width, and θ the diffraction angle. The calculated D values of the anatase TiO2 samples from being heated 3, 6 and 9 h samples are ca. 12.0, 16.5 and 12.5 nm, respectively. To further analyze the structures of Ti-complex products, the Fourier transform infrared (FTIR) spectrum was recorded to measure the sample being heated for 3 h, as shown in Figure 1c. The signal at 2939 cm-1 corresponds to the C-H bond stretch vibration in the -OAc and iPrOH ligands. The signals at 1753 and 1717 cm-1 are attributed to the carbonyl stretches associated with free carboxylic acid and H-bonded acid groups, respectively. The signal at 1554 cm-1 and those at 1417, 1453 cm-1 correspond to the asymmetric mode and symmetric stretching modes of carboxylate binding modes, respectively. In addition, the separation of about 100 cm-1 between these signals suggests that the signal at 1554 cm-1 is the bridging bidentate mode. The signals at 1348 and 1029 cm-1 are assigned to the C-C stretching modes of acetate bound to Ti, and the low energy signals at 757 and 658 cm-1 correspond to the bonds of Ti-O (Ti-Ac) and Ti-O (Ti-O-Pr), respectively. These analyses imply that the powder products can be inferred as Ti6(OAc)6(OiPr)6-n(OH)n. 43, 44 The time-dependent morphology of the Ti6(OAc)6(OiPr)6-n(OH)n phase was

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characterized by SEM to investigate the growth process, as shown in Figure 2. After 3 h reaction, nanowires with diameter about 40 nm were formed (Figure 2a). With prolonging reaction time to 6 h, the formed nanowires aggregated and grew into microspheres with dimension about 3 µm (Figure 2b-d). Meanwhile, the nanowires gradually developed into nanosheets as the enlarged image of Figure 2d. When the reaction time was extended to 9 h, microflowers consisting of nanosheets were formed (Figure 2e-f), and the nanosheets became thinner and wider (Figure 2f). In the above experimental processes, the formation mechanism of microflowers can be interpreted in terms of nucleation, nuclei aggregate, dissolution and regrowth. Specifically, the formed nucleus aggregated and grew into nanowires and further microspheres, to reduce the surface energy. Then the microspheres transformed into microflowers, whose constituent nanosheets gradually grew thinner and wider by the Ostwald ripening process. 45 After being annealed at 500 ○C, the morphology of the solvothermal products were largely destroyed into interconnected nanoparticles and nanoporous, and the Ti6(OAc)6(OiPr)6-n(OH)n phase were decomposed out of H2O, CO2, and TiO2 with molecular weight. Typically, after 3 h annealing, the TiO2 samples were mesoporous nanowires consisting of nanoparticles (Figure 3a). In the 6 h TiO2 samples (Figure 3b-d), the main nanosheets consisting of nanoparticles were formed; while the mesoporous structures emerged in the 9 h TiO2 samples (Figure 3e-f). Figure 4a shows a TEM image of a typical 3 h TiO2 product having a rough morphology of nanowires with a lot of cracks. The enlarged TEM image (Figure 4b)

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exhibits a great amount of mesoporous holes throughout the nanowires, indicating the formation of mesoporous nanowires consisting of nanoparticles. The high-resolution TEM image in Figure 4c shows a pure TiO2 interconnected nanoparticle, which has a size of about 10 nm and exhibits the perfect lattice plane with interplanar distance of 0.352 nm, matching well with the (101) plane of anatase TiO2. The corresponding SAED pattern (Figure 4d) indicates the nanocrystalline state of the product, and the diffraction rings correspond well with the (101), (004), (200), (105) and (204) lattice planes of anatase TiO2, respectively. Figure 5 summarizes the nitrogen adsorption isotherms and the pore-size distribution curves (inset). All the nitrogen isotherms for these three TiO2 samples belong to type IV (Brunauer-Deming-Deming-Teller classification),

46

in good

agreement with the existence of mesoporous structure. The H3-type hysteresis loops imply that the porosity is derived from the interconnected nanoparticles. Therefore, the porous structures of the TiO2 samples were observed directly by SEM and TEM. The BET specific surface areas of the 3, 6, and 9 h TiO2 samples were found to be 54.55, 43.52 and 52.96 m2 g-1, respectively. Furthermore, the pore-size distribution curves suggest the bi-modal pore structure, which can be ascribed to the small inner pores between nanoparticles and the large pores constructed by independent structures, respectively, agreeing with the XRD and SEM results very well. The thinner the Ti6(OAc)6(OiPr)6-n(OH)n nanosheets, the smaller the decomposed TiO2 nanoparticles, thus the BET specific surface area of 9 h TiO2 sample is larger than that of 6 h TiO2 sample. Since the 3 h TiO2 sample has the smallest size, it possesses largest value of

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the specific surface area. In comparison, the BET specific surface area of P25 nanoparticles is 49.35 m2 g-1 (Figure 2S in Supporting Information). These MATNs consisting of nanoparticles were used as photoelectrodes in DSSCs. For comparison, the DSSCs based on commercial P25 nanoparticles were also prepared following the same procedure. The current density-voltage (J-V) curves of these DSSCs (using P25 film; 3, 6 and 9 h TiO2 samples) under one-sun illumination (AM 1.5G, 100 mW cm-2) are plotted in Figure 6a, and the corresponding photovoltaic parameters are summarized in Table 1. The values of the open-circuit voltage (VOC) are almost the same, because VOC is mainly determined by the difference between the Fermi level of the active photoelectrode and the oxidation potential of the electrolyte.

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However, the short-circuit current densities (JSC) of the

DSSCs based on the 3 h and 9 h TiO2 samples are higher than that based on P25, while the DSSCs based on the 6 h TiO2 sample have the lowest JSC, thought its PCE is slightly higher than that based on P25. It might be due to the fact that the interconnected nanoparticles in the 6 h TiO2 sample provide long-range charge transfer channels, resulting in a higher fill factor (FF). 48 It’s worth noting that among them the DSSCs based on the 3 h TiO2 sample achieve the best photovoltaic performance with PCE of 7.78%, JSC of 15.87 mA cm-2, VOC of 0.802 V and FF of 0.611. A schematic drawing of the DSSCs based on the 3 h TiO2 sample is shown in Figure 6b, in which the mesoporous TiO2 nanowires with interconnected nanoparticles provide with both high specific surface areas for loading dye molecules and structural coherence for long-range electronic connectivity as the dashed-line

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arrows illustrate. To quantify the dye-loaded amounts, the sensitized photoelectrodes were immersed into 3 ml of 0.1 M NaOH solution to desorb the dye molecule, followed by measuring the UV-vis absorbance as shown in Figure 6c. According to Lambert-Beer’s law, the calculated values of dye-absorbed amounts were listed in Table 1, which support that a larger specific surface area has a larger dye-adsorption capacity because it can load more dye molecules to absorb sunlight and enhance JSC. The mesoporous anatase TiO2 nanowires with interconnected nanoparticles have the largest specific surface areas, resulting in a maximum JSC of 15.87 mA cm-2. On the other hand, the film based the 6 h TiO2 sample loads the least amount of dye molecules because of its smallest specific surface areas, leading to a lowest JSC of 12.67 mA cm-2. The photo-generated charge transfer is another crucial factor to photovoltaic performance, which was examined according to the EIS measurement at a bias of 0.80 V under dark condition. Figure 6d shows the Nyquist plot curves, in which there are two semicircles, one in the high-frequency region (>1 kHz) and the other in the low-frequency region (100-0.1 Hz). The equivalent circuit and the corresponding values by the Zview software are shown in Figure 3S and Table1S (Supporting Information), respectively. According to the EIS model, 49, 50 the semicircle in the low-frequency

region

characterizes

the

charge

transfer

at

the

photoelectrode/electrolyte interface. The charge-transfer resistances are 7.699, 12.55, 22.66, 13.33 Ω for the cells based on P25, 3, 6 and 9 h TiO2 samples, respectively, and the larger charge-transfer resistance means the lower charge recombination and longer

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charge transport range. The DSSCs based on P25 have the smallest charge-transfer resistance, suggesting the highest charge recombination, whereas the 6 h TiO2 sample has the lowest charge recombination and longest electron transport range, so that its PCE is higher than that of P25. The mesoporous anatase TiO2 nanowires, compared with the P25 nanoparticles, were used in carbon-based HPSCs, in which the J-V curves and photovoltaic parameters are shown in Figure 7a. The JSC, VOC, FF, and PCE are 16.80 mA/cm2, 0.943 V, 0.538, and 8.52% for the cells using the mesoporous TiO2 nanowire films as the scaffold layer, compared with 14.24 mA cm-2, 0.912 V, 0.552, and 6.78% if the scaffold layer consisting of the P25 films. The enhanced overall PCE for the TiO2 nanowires over the P25 nanoparticles is owing to their long-range charge transfer channels. To further support the enhancement of the TiO2 nanowires, the photovoltaic parameters were measured for eight devices to investigate the reliability of the HPSCs (Tables 2S and 3S, Supporting Information). Figure 7b sketches the energy band diagram of the carbon-based HPSCs.

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In these cells, after the perovskite layer

captures photons, the electron-hole pairs are generated, and the excited electrons at the conduction band minimum (CBM) at -3.86 eV are injected into the CBM of TiO2 at -4.0 eV and then are collected at the FTO substrates with the work function of 4.6 eV. Simultaneously, the excited hole at the valence band maximum (VBM) at -5.43 eV are extracted to the carbon electrode with a work function of 5.00 eV. To further investigate charge transport and recombination in the HPSCs, the EIS measurements were conducted at a bias of 0.8 V under dark condition. The Nyquist

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plot curves and the equivalent circuit model are plotted in Figure 7c and Figure 4S, respectively. The difference in arc shapes in Figure 7c indicates the variation in the charge-recombination dynamics at the interface. The charge-transfer resistance of 10734 Ω for the cells based on the mesoporous TiO2 nanowires are much larger than the value of 478 Ω for the P25 nanoparticles, suggesting that the mesoporous TiO2 nanowires provide better electron transport ability and efficiently reduce the carrier recombination than P25. 52 Furthermore, the steady-state photoluminescence (PL) was taken to investigate the electron transport between the perovskite film and the TiO2 scaffold layer as the charge extractor, since the magnitude of the PL quenching effect is proportional to the ability of electron transport range. As shown in Figure 7d, the significantly reduced PL intensity associated with the TiO2/perovskite film relative to that for the P25/perovskite film suggests a considerable quenching effect and efficient electron transport ability in the mesoporous TiO2 nanowires. 53, 54 In addition, Figure 7f indicates that there are obvious tracks of nanowires after depositing perovskite films from the top side, which help improve electron transport ability, while there are perovskite particles on the P25 film as shown Figure 7e. Therefore, the mesoporous TiO2 nanowires are outstanding scaffold to offer efficient charge transport in PSCs.

CONCLUSION In summary, we report a facile, template-free, and environment-friendly approach to fabricate mesoporous anatase TiO2 nanomaterials consisting of interconnected nanoparticles for efficient DSSCs and carbon-based HPSCs. The

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Ti6(OAc)6(OiPr)6-n(OH)n phase was first achieved by a solvothermal reaction, which was subsequently annealed at 500 ○C to be decomposed into mesoporous anatase TiO2 nanomaterials. The mesoporous anatase TiO2 nanowires demonstrate excellent photovoltaic properties, due to their high specific surface areas and long charge-transport channels. The DSSCs based on the mesoporous anatase TiO2 nanowires reached a high PCE of 7.78%, compared to 6.61% for DSSCs based on the P25 photoelectrodes. Meanwhile, the HPSCs based on the mesoporous anatase TiO2 nanowires achieved a PCE of 8.52%, compared with 6.78% for cells prepared using the standard P25 electrodes. This study opens up a better route to fabricate mesoporous anatase TiO2 nanomaterials consisting of interconnected nanoparticles, which may lead to broad applications for energy conversion and storage, photocatalysis, gas and ultraviolet sensors.

ASSOCIATED CONTENT Supporting Information Part of enlarged XRD patterns of the TiO2 products, nitrogen adsorption isotherms and pore size curves of the P25 nanoparticle powder, equivalent circuit models of Nyquist plots of DSSCs and carbon-based HPSCs, simulated values of the equivalent circuit model of Nyquist plots of DSSCs, photovoltaic parameters of different HPSCs based on P25 nanoparticles and mesoporous TiO2 nanowires.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education of China (IRT1148), the National Natural Science Foundation of China (U1732126, 51372119, 61377019, 61136003, 51173081, 51602161, 61605087), Jiangsu Synergistic Innovation Center for Advanced Materials (SICAM), the Priority Academic Program Development of Jiangsu Higher Education Institutions (YX03001), the Natural Science Foundation of Jiangsu Province (BK20150860, BK20160881), the College Postgraduate Research and Innovation Project of Jiangsu Province (KYLX_0794, KYLX15_0848), and the Natural Science Foundation of NJUPT (NY215022, NY217091).

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Figures with Caption

Figure 1. (a) XRD patterns of the Ti-complex samples prepared by a hydrothermal method for different times (3, 6 and 9 h). (b) XRD patterns of TiO2 products obtained by annealing the Ti-complex powders at 500 ○C. (c) FTIR spectrum of as-prepared Ti-complex sample by the hydrothermal reaction for 3 h.

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Figure 2. FE-SEM images of the as-prepared Ti-complex samples synthesized for different times: (a) 3 h, (b-d) 6 h, (e, f) 9 h. Fig. 2(c) is the enlarged image of (b), and (d) is the corresponding enlarged (c). Fig 2(f) is the enlarged image of (e).

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Figure 3. FE-SEM images of corresponding TiO2 powders by annealing the Ti-complex powders with different synthesized times: (a) 3 h, (b-d) 6 h, (e, f) 9 h.

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Figure 4. (a) TEM image of TiO2 sample by annealing the Ti-complex powder synthesized for 3 h, (b) the enlarged TEM image, (c) the corresponding HRTEM image and (d) the SAED pattern.

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Figure 5. Nitrogen adsorption isotherms and (inset) pore size curves of the TiO2 powders from the annealed Ti-complex powders with different synthesized times: 3, 6 and 9 h.

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Figure 6. (a) Photocurrent–voltage curves of DSSCs using the MATNs from the annealed Ti-complex powders prepared for different times (3, 6 and 9 h), and P25 as photoanodes. (b) Schematic illustration of DSSCs based on the mesoporous TiO2 nanowires with high specific surface area for loading dye molecules and structural coherence for long-range electronic connectivity. (c) UV-vis spectra of solutions containing N719 desorbed from the corresponding sensitized photoelectrodes, and the inset is the structure of the N719 molecule. Dye-adsorbed films with a area of 0.563 cm2 were used for estimating the adsorbed dye concentration. (d) Nyquist plots of DSSCs using the MATNs from the annealed Ti-complex powders prepared for different times (3, 6 and 9 h), and P25 as photoanodes, with a bias of 0.8 V under dark condition.

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Figure 7. (a) J− V curves of carbon-based HPSCs using P25 and mesoporous TiO2 nanowires as scaffold layers. (b) Energy band diagram of the carbon- and TiO2-based HPSCs. (c) Impedance spectroscopy of the corresponding devices. (d) PL emission spectra for perovskite films on P25 and mesoporous TiO2 nanowire films. (e), (f) SEM images of the perovskite films on P25 and mesoporous TiO2 nanowire films, respectively.

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Table 1 Photovoltaic parameters of DSSCs with different photoelectrodes. JSC

Dye-amount

Photoelectrodes

VOC(V)

FF

PCE (%)

2

(nmol/cm2)

(mA/cm ) P25

13.87

0.800

0.555

6.16

106.4

3h

15.87

0.802

0.611

7.78

141.6

6h

12.67

0.801

0.611

6.20

87.2

9h

15.01

0.803

0.602

7.21

133.6

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Mesoporous anatase TiO2 nanomaterials with both desired highly accessible specific surface areas and long-range charge transfer channels were synthesized by a simple, template-free, and green route, which were further applied in enhanced dye-sensitized and hole-conductor-free perovskite solar cells.

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