Single-Layer TiO2 Film Composed of Mesoporous Spheres for High

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A Single-Layer TiO2 Film Composed of Mesoporous Spheres for High-Efficiency and Stable Dye-Sensitized Solar Cells Yuewu Huang, Haigang Wu, Qingjiang Yu, Jianan Wang, Cuiling Yu, Jinzhong Wang, Shiyong Gao, Shujie Jiao, Xitian Zhang, and Peng Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03626 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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ACS Sustainable Chemistry & Engineering

A

Single-Layer

TiO2

Film

Composed

of

Mesoporous

Spheres

for

High-Efficiency and Stable Dye-Sensitized Solar Cells Yuewu Huang,a,‡ Haigang Wu,a,‡ Qingjiang Yu,*ab Jianan Wang,ac Cuiling Yu,d Jinzhong Wang,*a Shiyong Gao,a Shujie Jiao,a Xitian Zhang,b and Peng Wang*c a

Department of Opto-electronic Information Science, School of Materials Science and Engineering,

Harbin Institute of Technology, Harbin 150001, China. b

Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin

Normal University, Harbin, 150025, China. c

Center for Chemistry of Novel & High-Performance Materials, Department of Chemistry, Zhejiang

University, Hangzhou, 310028, China. d

Department of Physics, Harbin Institute of Technology, Harbin, 150001, China.

‡

The authors equally contributed to this work.

Corresponding Authors *E-mail: [email protected] (Q.Y.); [email protected] (J.W.); [email protected] (P. W.)

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ABSTRACT: Mesoporous TiO2 spheres with high crystallinity and large specific surface area were facilely prepared via sol-gel and solvothermal methods. By tuning the time and temperature of the solvothermal process, the specific surface area and pore size of TiO2 spheres were fine-controlled without any additives. Compared with the dye-sensitized solar cell (DSC) based on the commercial P25 nanoparticles, the dye-sensitized solar cell employing optimized TiO2 spheres achieves an excellent efficiency of 10.3%, due to the outstanding dye loading and light scattering abilities as well as attenuated charge recombination. Furthermore, an impressive efficiency of 11.2% can be obtained after TiCl4 post-treatment. More importantly, in combination with a single-layer TiO2 film composed of mesoporous spheres and low-volatility electrolyte, the DSC exhibits high a efficiency and long-term durability.

KEYWORDS: Titanium dioxide, Mesoporous spheres, Solvothermal process, Dye-sensitized solar cell, Charge recombination

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INTRODUCTION The dye-sensitized solar cell (DSC), as a promising alternative to silicon photovoltaics, has caused widespread interest owing to its sustainable energy and impressive power conversion efficiency (PCE).1 Over the past two decades, considerable efforts for improved PCEs of DSCs have been attained by virtue of the design of more efficient dyes and optimized photoanodes.2−11 As the crucial part of a DSC, the photoanode plays two vital roles in the carrier of dyes and the transfer medium for photoinjected electrons. Normally, for a DSC, photoanodes composed of anatase TiO2 nanocrystals with sizes of ∼20 nm are satisfied with high specific surface for loading large amounts of dye molecules.12 However, such a photoanode

exhibits negligible light

scattering, leading to relatively poor light

harvesting.13,14 In order to extend the optical path and compensate the lack of light absorption, a light-scattering layer with large TiO2 solid particles is normally deposited on the transparent TiO2 nanocrystal layer, which constructs a double-layered photoanode. Recently, various hierarchical TiO2 nanostructures, such as mesoporous spheres,15−20 hollow spheres,21−24 hierarchical flowers,25,26 ellipsoids,27 and aggregates,28 have been synthesized and employed as photoanodes for efficient DSCs. Among them, mesoporous microspheres as photoanodes have been demonstrated to possess efficient dye loading and high light-harvesting ability owing to their high specific surface area and strong light scattering by combining the merits of primary nanocrystal building blocks and submicrometer-sized assembles. Caruso et al. first designed the mesoporous TiO2 beads with high surface area and the pore sizes of beads were controlled by adding different amounts of ammonia.15 Compared with the Degussa P25 TiO2 photoanode, the DSC based on 3



mesoporous TiO2 beads exhibits an improved diffusion property because of the stacking and interaction of grains within the TiO2 beads, yielding a PCE of higher than 10%.16 To the best of our knowledge, over 11% PCEs of DSCs are normally achieved by employing double-layered photoanodes while it is very difficult to reach such high PCEs for DSCs based on a single-layered film. In addition, it is well known that the long-term stability is an important parameter for the practical application of DSCs. However, stable DSCs using mesoporous TiO2 spheres were rarely investigated. In this paper, mesoporous TiO2 spheres with large specific surface areas were facilely prepared via sol-gel and solvothermal methods. By simply changing the reaction time and temperature, the specific surface areas and pore sizes of TiO2 spheres can be regulated without any additives, and a possible formation mechanism of mesoporous TiO2 spheres was proposed. A remarkable PCE of 11.2% can be achieved for the DSC based on the optimized TiO2 spheres and TiCl4 post-treatment. Additionally, with the aid of a low-volatility electrolyte, the TiO2 sphere based DSC can show an impressive PCE of 10.1% and an outstanding stability under the long-term thermal and light-soaking dual stress.

RESULTS AND DISCUSSION Morphological and structural characterizations of TiO2 spheres prepared under the solvothermal condition of 160 οC for 15 h are presented in Figure 1. The low-resolution field-emission scanning electron microscope (FESEM) image (Figure 1a) shows that the obtained products are monodisperse TiO2 spheres with an average diameter of ∼ 620 nm. These spheres possess rough surfaces and consist of numerous interconnected TiO2 nanoparticles ∼ 14.3 nm in size (Figure 1b). More detailed nanostructures of TiO2 spheres 4


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were further investigated by transmission electron microscope (TEM) and high-resolution TEM (HRTEM). As shown in Figure 1c and S1, the TiO2 sphere exhibits a porous nanostructure, which is advantageous to the dye loading and electrolyte penetration for DSCs. The lattice fringes of nanoparticles in the TiO2 sphere can be clearly distinguished from the HRTEM image (Figure 1d), indicating that these nanoparticles were well crystallized. The interplanar distance is about 0.353 nm, corresponding to the (101) plane of anatase TiO2. To explore the evolution process of mesoporous TiO2 spheres, the TiO2 spheres were prepared under solvothermal conditions of 160 οC for various reaction times. The spherical precursors prepared via a sol-gel process have smooth surfaces without obvious granular features (Figure S2). It is apparent that these spherical precursors are amorphous from the x-ray power diffraction (XRD) pattern (Figure 3a) When these spherical precursors were treated under the solvothermal condition of 160 οC for 1 h (Figure 2a and b), the obtained products are still solid spheres (sample S-1h), and some smaller particles (∼ 7.1 nm in diameter) appear on the surfaces of the obtained spheres. Extending the reaction time (2 h), these nanoparticles become bigger (Figure 2c), resulting in the relatively rough surfaces of the obtained spheres (sample S-2h). Moreover, the porous structure of the sphere surface can be observed in Figure 2d. When the reaction time is 6 h, it is clearly observed that these spheres (sample S-6h) contain numerous nanoparticles with an average diameter of ∼ 11.6 nm and pores (Figure 2e and f). After further prolonging the reaction time (15 h), uniform TiO2 spheres with abundant mesopores (sample S-15h) were achieved, as shown in Figure 1 and S1. As the reaction time increases to 24 h, the distances among some adjacent nanoparticles 5



in the spheres (sample S-24h) are enlarged and the average size of these nanoparticles increases to ∼ 16.5 nm, as presented in Figure 2g and h. In addition, the XRD patterns of the TiO2 spheres prepared under solvothermal conditions of 160 οC for different reaction times were also characterized. As confirmed by the XRD patterns in Figure 3a, these obtained TiO2 spheres can be matched with the anatase structure (JCPDS card no. 21-1272). It is found that the diffraction peaks of these spheres become more intensive with increasing the reaction time, indicating the gradual crystallization and crystal growth of TiO2. Thermogravimetric analysis (TGA) was applied to investigate the thermal properties of the spherical precursors and time-dependence of TiO2 spheres after the solvothermal treatment, as shown in Figure 3b. For the spherical precursors, it presents a first weight loss of 8.3% around 200 οC, which is ascribed to the dehydration from the precursor condensation and the removal of physically adsorbed water. The second weight loss of 13.1% at ∼ 275 οC may mainly be associated with the decomposition and desorption of the hexadecylamine (HDA) molecules. In addition, it is found that the weight loss of all the spheres via the solvothermal treatment is less than that of the spherical precursors below 275 οC, especially for the spheres via the solvothermal treatment over 2 h, indicating that the spheres gradually become less hydrated and the content of HDA in spheres reduces with increasing the solvothermal time. The weight loss between 275 and 500 οC is due to the oxidation of organics and TiO2 phase transformation from amorphous to anatase.29 The last weight loss at temperature higher than 500 οC is very little, which confirms that the organics could be entirely removed after sintering at 500 οC. The specific surface areas and pore size distributions of these TiO2 spheres were also 6


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measured by N2 adsorption–desorption. As presented in Figure 3c, for the shorter reaction time, both samples S-1h and S-2h have small specific surface area and poor porosity. The corresponding specific surface areas are 6.1 and 14.3 m2 g−1, respectively. With further prolonging the reaction time over 6 h, type IV isotherms with sharp capillary condensation steps and H2 type hysteresis loops are observed for the samples S-6h, S-15h, and S-24h, suggesting that they have relatively good porosity. As listed in Table S1, the sample S-15h has the largest specific surface area of 95.6 m2 g−1. The TiO2 nanoparticles in spheres grew up gradually with increasing the solvothermal reaction time, resulting in the corresponding enlargement in pore size from 12.2 to 18.3 nm. The formation mechanism of mesoporous TiO2 spheres during the solvothermal process was concluded according to the above results. In the sol-gel synthesis, HDA plays a structure-oriented role in the morphology and monodispersity control.30 The spherical precursors are formed via a cooperative assembly process involving long-chain alkylamine (such as HDA) and Ti(OCH(CH3)2)4-x(OH)x species/oligomers.30,31 During the solvothermal process, the spherical precursors are first attacked externally by chemicals such as ethanol and water. The surface of spherical precursors is crystallized while HDA molecules are released from the composites, resulting in the formation of initial TiO2 nanoparticles on the sphere surface. With increasing the reaction time, the chemicals can further penetrate through the voids exposed to the exterior. Along with the releasing of HDA and crystallization of TiO2, porous TiO2 frameworks are gradually generated from the surface to the interior of spheres. Thus, mesoporous TiO2 spheres are ultimately produced. To elucidate the effect of the solvothermal temperature on TiO2 spheres, we changed the 7



solvothermal temperatures while keeping a constant reaction time of 15 h. When the solvothermal temperature is 120 οC, some sparse nanoparticles with an average size of ∼ 8.5 nm are observed on the surface of obtained spheres (sample S-120C), as shown in Figure 4a and b. Moreover, this sample has no obvious porous characteristic and a low specific surface area (Figure S3 and Table S1). After the solvothermal treatment of 140 οC, the spheres (sample S-140C) exhibit a distinct porous structure (Figure 4c, d and S3) and their specific surface area is increased to 83.2 m2 g−1 (Table S1). Further increasing the temperature to 180 C, the specific surface area of the spheres (sample S-180C) is slightly decreased to 90.3 m2

ο

g−1 (Table S1) compared to the mesoporous spheres prepared at 160 οC, which is attributed to the larger nanoparticles (∼15.2 nm in diameter) in the spheres, as shown in Figure 4e and f. Hence, the solvothermal temperature is a key factor in the preparation of mesoporous spheres with various specific surface areas. To explore the performance of TiO2 spheres as photoanodes for DSCs, we first fabricated the DSCs based on the TiO2 spheres prepared under solvothermal conditions of 160 οC for different reaction times. The thickness of TiO2 photoanodes is about 14.4 µm (Figure S4). The typical photocurrent density−photovoltage (J−V) curves of the assembled DSCs are shown in Figure 5a, and the detailed photovoltaic parameters are listed in Table 1. For the DSC with the sample S-1h, the short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), and fill factor (FF) are 5.03 mA cm−2, 867 mV and 0.736, respectively, generating a low PCE of 3.2%. However, for the DSCs based on the TiO2 spheres prepared under solvothermal conditions for longer reaction times, the Jsc values obviously increase from 5.03 to 17.38 mA cm−2 and then reduce to 15.87 mA cm−2 whereas the Voc values gradually reduce with increasing the reaction time 8


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of samples. A maximum PCE of 10.3% is achieved for the DSC with the sample S-15h. To clarify the Jsc variation of these DSCs, the external quantum efficiency (EQE) is first measured, and the results are in agreement with the Jsc variation (Figure 5b). The EQE value of the DSC with the sample S-1h is ∼ 31% at 560 nm. By prolonging the reaction time of samples, the EQE values of their corresponding DSCs significantly enhance in the wavelength range from 400 to 750 nm, suggesting greatly improved light-harvesting abilities. The DSC based on the sample S-15h exhibits the highest EQE value of ∼ 87% at 560 nm. Moreover, UV-visible absorption spectra of dye desorbed from the above TiO2 sphere photoanodes were employed to examine the influence of the reaction time on dye-loading ability, and the corresponding dye-uptake amounts are listed in Table 1. It is found that the highest amounts of absorbed dye is observed at the reaction time of 15 h and then a small value decrease occurred, ascribing to the augmentation or reduction of the specific surface areas of corresponding TiO2 spheres. The improved dye-uptake capabilities can enhance the amount of photogenerated electrons and thereby lead to a higher Jsc of DSCs. Further, the electrochemical impedance spectra (EIS) of these DSCs were carried out to explore the dynamics of electron transport and recombination. Figure 5c displays the Nyquist plots of DSCs with various photoanodes. The sheet resistance (Rs), charge transfer resistance (Rct1 and Rct2), and the corresponding constant phase element (CPE) in DSCs can be estimated by fitting the Nyquist plots using an equivalent circuit (inset in Figure 5c). From left to right, the first small arc represents the resistance with respect to the reduction reaction of I3− ions in the electrolyte (Rct1), and the second arc reflects the resistance related to the charge recombination process at the TiO2/electrolyte interface (Rct2).32−34 Since the same FTO and Pt electrodes are used in DSCs, the impact of Rs and Rct1 is negligible. Thus, the Rct2 and electron lifetime (τr) are the main concern on 9



the interfacial charge recombination process, and their fitting results are summarized in Table S2. It is found that both Rct2 and τr values gradually decrease with prolonging the solvothermal reaction time, indicating that there is a faster charge recombination process within the DSC based on the TiO2 sphere prepared for a longer reaction time. This may be attributed to the fact that the spaces between the adjacent nanoparticles in TiO2 spheres become larger with the increase in the solvothermal reaction time, which resulted in excess I3− accumulation around the nanoparticles in spheres, leading to a reduced Voc. For comparison, a commercial P25 nanoparticles based photoanode with a similar thickness, counterpart to the aforementioned photoanode composed of the sample S-15h, was used to assemble a DSC. As a consequent, the Jsc and Voc values of the P25 based DSC are 13.96 mA cm−2 and 755 mV, respectively, which are obviously inferior to that of the TiO2 sphere based DSC with the sample S-15h, leading to a relatively lower PCE (8.0%). By contrast the maximum EQE value of the P25 based DSC is only 76% at the same wavelength. The normalized EQE spectra (Figure S5) clearly shows the strong effect of the mesoporous TiO2 spheres on the long wavelength harvesting of sunlight in the DSCs. With respect to the enhanced light harvesting ability, light-scattering property also plays a decisive role in addition to the influence of dye-loading amount. As shown in Figure S6, the TiO2 sphere based film composed of the Sample S-15h exhibits a higher diffuse reflection due to the superior light scattering effect of Sample S-15h compared to P25. Therefore, the Jsc increment for the TiO2 spheres based DSC with the sample S-15h can be resulted from the excellent dye-uptake amounts and light scattering ability compared to the P25 nanoparticle based DSC. According to the EIS analysis (Table S2), the DSC based on the sample S-15h shows a longer electron lifetime with respect to the P25 particle based DSC, which may be because there is a better 10


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electron transport in the densely interconnected network of TiO2 spheres, leading to a high Voc. In addition, we also fabricated the DSCs based on the TiO2 spheres prepared at various solvothermal temperatures for 15 h. Figure 6a presents their J−V curves, and the detailed photovoltaic parameters are tabulated in Table S3. With increasing the solvothermal temperatures from 140 to 180 οC, the Jsc values of corresponding DSCs first improve and then slightly decrease and the maximum Jsc reaches 17.37 mA cm−2 based on the TiO2 spheres (sample S-15h) prepared at the solvothermal temperature of 160 οC (Table 1), which has a good agreement with their EQE spectra (Figure 5b and 6b). The dye-uptake amounts on these TiO2 sphere photoanodes increase from 0.54×10−7 to 2.26×10−7 mol cm−2 then decrease to 2.13×10−7 mol cm−2 with enhancing the solvothermal temperature (Table 1 and S3), resulting in the variation of Jsc. Moreover, the reduction in Voc is mainly originated from the faster charge recombination (Figure 6c and Table S4), due to the bigger spaces between the adjacent nanoparticles in spheres. To optimize the DSC performance, the TiO2 photoanode based on the sample S-15h was subjected to the post-treatment of TiCl4 aqueous solution (0.1 M) at 70 οC for 30 min. Figure 7 shows the J−V curve of the DSC assembled with the TiO2 photoanode after TiCl4 post-treatment. The DSC possesses a Jsc of 18.53 mA cm−2, a Voc of 778 mV, and a FF of 0.774, yielding an impressive PCE of 11.2%, whereas the DSC without TiCl4 post-treatment shows a relatively lower PCE of 10.3%. The obtained result is similar to the result reported by the Grätzel et al.16 However, the effect mechanism of TiCl4 post-treatment is not completely clear to date. According to the EIS analysis from the Figure S7, both Rct2 (139.1 Ω) and τr (0.283 s) values of the DSC with TiCl4 post-treatment are higher than that of the DSC without TiCl4 post-treatment (Table S2), which may be because TiCl4 post-treatment can improve the electronic connection of adjacent TiO2 spheres and 11



depress the charge recombination at the TiO2/electrolyte interface,16 resulting in a high-efficiency DSC. More impressively, the performance of DSC based on the sample S-15h is superior to that used the Dyesol paste (18NR-T, Figure S8). In addition, we also attempted to fabricate perovskite solar cells (PSCs) using the sample S-15h as the electron-transport layer. The EQE spectrum of the PSC exhibited a strong absorption in a broad wavelength range (Figure S9a) and the highest EQE value reaches ~90%. The PCEs of this PSC are of 19.1 and 15.1 % by a reverse scan mode and forward scan mode, respectively (Figure S9b). It is found that there is a significant J–V hysteresis for this PSC. Recently, Liu et al.35 have demnostrated that the J–V hysteresis in PSCs can be effectively suppressed by surface modification of the electon-tranport layer using a ionic-liquid and obtained an impressive PCE of 19.62%. Moreover, they also employed a solid-state inoic-liquid as a electron-transport material to eliminate the hysteresis and achieved the champion efficiency for flexible PSCs.36 Accroding to their novel ideas, we will further restrain the J–V hysteresis of the PSCs based on mesoporous TiO2 spheres using an effective ionic-liquid and improve their efficiencies and stability. For any photovoltaic devices, long-term stability is still the main factor affecting its commercialization. Degradation of the solvent proved to be highly detrimental for the DSCs employing the acetonitrile electrolyte, in stark contrast to the 3-methoxypropinitrile (MPN) -based DCSs that presented very good durability. The excellent properties of MPN in terms of low volatility and good photochemical stability give promise for real application. In combination with the sample S-15h and the low-volatility electrolyte E2 (Supporting Information), the J−V curve of the DSC with the TiCl4 post-treatment is shown in Figure S10. The photovoltaic parameters (Jsc, Voc, FF, and PCE) of this DSC under the standard AM 1.5G full sunlight are 16.96 mA cm−2, 772 12


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mV, 0.768, and 10.1%, respectively. It is well known that UV light may induce band-gap excitation of TiO2, leading to triiodide reduction in DSCs. Moreover, the total contribution of UV photons to the standard AM 1.5G solar emission is relatively small. Therefore, UV light was removed with a UV absorbing polymer film in our light-soaking experiments. Under both the thermal and light-soaking stress for 1,000 h, this DSC exhibits rather good stability, retaining 93% of its initial efficiency (Figure 8). To the best of our knowledge, this is for the first time reported that the over 10% DSC based on a single-layer film has passed a long-term stability test.

CONCLUSIONS To sum up, highly crystallized mesoporous TiO2 spheres were successfully synthesized by a combined sol-gel and solvothermal method. The specific surface area and pore size of spheres can be controlled without any additives via varying the time and temperature of solvothermal process. The formation mechanism of mesoporous TiO2 spheres was discussed from the angle of crystallization and morphology. As a photoanode, the optimal TiO2 sphere based DSC has achieved a PCE of 10.3%, which is significantly higher than that of the DSC based on commercial P25 nanoparticles due to the high dye-loading capacity, superior light-scattering property, and suppressed charge recombination of the former. After TiCl4 post treatment, the efficiency of this TiO2 sphere based DSC can be further improved to 11.2%. In addition, combined with the low-volatility MPN based electrolyte, it is for the first time demonstrated that the over 10% DSC based on a single-layer TiO2 film can retain a rather good long-term durability.

ASSOCIATED CONTENT Supporting Information 13



Experimental section and additional information related to this article can be found via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Q.Y.); [email protected] (J.W.); [email protected] (P. W.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51502056 and 51673165), the National Science and Technology Support Program (No. 2015BAI01B05), the National 973 Program (No. 2015CB932204), the Natural Science Foundation of Heilongjiang Province of China (No. F2016013), the Fundamental Research Funds for the Central Universities (Nos. HIT.BRETIII.201403 and 2017FZA3007), and the Open Project Program of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, China.

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(16) Sauvage, F.; Chen, D.; P. Huang, Comte. F.; Heiniger, L.-P.; Cheng, Y.-B.; R. Caruso, A.; Grätzel, M. Dye-sensitized solar cells employing a single film of mesoporous TiO2 beads achieve power conversion efficiencies over 10%. ACS Nano 2010, 4, 4420–4425. (17) Yang, W.-G.; Wan, F.-R.; Chen, Q.-W.; Li, J.-J.; Xu, D.-S. Controlling synthesis of well-crystallized mesoporous TiO2 microspheres with ultra high surface area for high-performance dye-sensitized solar cells. J. Mater. Chem. 2010, 20, 2870–2876. (18) Li, Z.-Q.; Que, Y.-P.; Mo, L.-E.; Ding, W.-C.; Ma, Y.-M.; Jiang, L.; Hu, L.-H.; Dai, S.-Y. One-pot synthesis of mesoporous TiO2 micropheres and its application for high-efficiency dye-sensitized solar cells. ACS Appl. Mater. Interfaces 2015, 7,10928–10934. (19) Peng, J.-D.; Tseng, C.-M.; Vittal, R.; Ho, K.-C. Mesoporous anatase-TiO2 spheres consisting of nanosheets of exposed (001)-facets for [Co(byp)3]2+/3+ based dye-sensitized solar cells. Nano Energy 2016, 22, 136–148. (20) Zhao, T.; Luo, W.; Deng, Y.; Luo, Y.; Xu, P.; Liu, Y.; Wang, L.; Ren, Y.; Jiang, W. Monodisperse mesoporous TiO2 microspheres for dye sensitized solar cells. Nano Energy 2016, 26, 16–25. (21) Koo, H. J.; Kim, Y. J.; Lee, Y. H.; Lee, W. I.; Kim, K.; Park, N. G. Nano-embossed hollow spherical TiO2 as bifunctional material for high-efficiency dye-sensitized solar cells. Adv. Mater. 2008, 20,195–199. (22) Park, J. H.; Jung, S. Y.; Kim, R.; Park, N. G.; Kim, J.; Lee, S-S. Nanostructured photoelectrode consisting of TiO2 hollow spheres for non-volatile electrolyte-based dye-sensitized solar cells. J. Power Sources 2009, 194, 574–579. (23) Pan, J. H.; Wang, X. Z.; Huang, Q.; Shen, C.; Koh, Z. Y.; Wang, Q.; Engle, Bahnemann, A. D. W. Large-scale synthesis of urchin-like mesoporousTiO2 hollow spheres by targeted etching and their photoelectrochemical properties. Adv. Funt. Mater. 2014, 24, 95–104.

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Figure 1. Morphological and structural characterizations of TiO2 spheres prepared under the solvothermal conditions of 160 οC for 15 h: (a) low-resolution FESEM image, (b) high-resolution FESEM image, (c) TEM image, and (d) HRTEM image.

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Figure 2. FESEM and TEM images of TiO2 spheres prepared under solvothermal conditions of 160 ο

C for different times: (a, b) 1 h, (c, d) 2 h, (e, f) 6 h, and (g, h) 24 h.

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Figure 3. (a) XRD and (b) TGA patterns of the spherical precursors and TiO2 spheres prepared under

solvothermal

conditions

of

160 οC

for

various

reaction

times,

(c)

Nitrogen

adsorption-desorption isotherms of TiO2 spheres prepared under solvothermal conditions of 160 οC for various reaction times.

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Figure 4. FESEM and TEM images of TiO2 spheres prepared under solvothermal conditions of different temperatures for 15 h: (a, b) 120 οC, (c, d) 140 οC, and (e, f) 180 οC.

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Figure 5. (a) J−V characteristics, (b) EQE spectra and (c) Nyquist plots of EIS data of DSCs based on the TiO2 spheres prepared under solvothermal conditions of 160 οC for various reaction times. The inset in Figure 5c is the equivalent circuit applied to fit the impedance data.

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Figure 6. (a) J−V characteristics, (b) EQE spectra and (c) Nyquist plots of EIS data of DSCs based on the TiO2 spheres prepared under solvothermal conditions of different temperatures for 15 h. The inset in Figure 6c is the equivalent circuit applied to fit the impedance data.

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Figure 7. J−V characteristic and EQE spectrum of the DSC based on a TiO2 sphere photoanode with TiCl4 post-treatment. The TiO2 spheres were prepared under the solvothermal condition of 160 ο

C for 15 h.

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Figure 8. Photovoltaic parameters measured under the irradiance of 100 mW cm−2 simulated AM 1.5G sunlight for the DSC based on a single-layered TiO2 film of mesoporous spheres and a low-volatility electrolyte during successive full sunlight soaking at 60 οC. The TiO2 spheres were prepared under the solvothermal condition of 160 οC for 15 h.

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Table 1. Detailed photovoltaic parameters of DSCs based on the TiO2 spheres prepared under solvothermal conditions of 160 oC for various reaction times. smaple S-1h S-2h S-6h S-15h S-24h P25

Jsc (mA cm−2) 5.03 11.31 16.62 17.38 15.87 13.96

Voc (mV) 867 802 777 770 759 755

FF 0.736 0.742 0.747 0.766 0.763 0.754

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PCE (%) 3.2 6.7 8.9 10.3 9.1 8.0

Dye loading (10−7mol cm−2) 0.35 0.81 2.04 2.26 1.89 1.55

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For Table of Contents Use Only

Mesoporous TiO2 spheres with large specific surface area were facilely prepared via sol-gel and hydrothermal methods. An impressive efficiency of 11.2% is obtained for the DSC based on a single-layer TiO2 film composed of mesoporous spheres. In combination with a low-volatility electrolyte, the TiO2 sphere based DSC exhibits an excellent thermal and light-soaking stability.

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Single-Layer TiO2 Film Composed of Mesoporous Spheres for High

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