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Spherical anatase TiO2 covered with nanospindles as dual functional scatters for dye-sensitized solar cells
Electrochimica Acta 123 (2014) 463–469
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Spherical anatase TiO2 covered with nanospindles as dual functional scatters for dye-sensitized solar cells Xiaopan Xue, Jianhua Tian, Wenming Liao, Zhongqiang Shan ∗ School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, PR China
a r t i c l e
i n f o
Article history: Received 24 October 2013 Received in revised form 21 December 2013 Accepted 12 January 2014 Available online 23 January 2014 Keywords: Anatase Dye-sensitized solar cell Scattering layer Sphere TiO2 aggregates
a b s t r a c t Spherical anatase TiO2 covered with nanospindles (SNS) were fabricated through a facile hydrothermal treatment of precursors in the presence of ammonia. The precursors were synthesized by controlling hydrolysis rate of TBT (tetrabutyl titanate) in ethanol. Organic structure directing agents and toxic reagents were avoided in the two–step process. By scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), it is confirmed that the morphology and structure of the products can be controlled by adjusting hydrothermal treatment conditions. Time dependent trails revealed the growth mechanism of SNS, which indicating that ammonia can not only retard the dissolution of precursors but also make TiO2 grow selectively along the <001> direction. Furthermore, photocurrent-potential (I-V) curves show that the solar cells fabricated with the SNS collected after 18 h hydrothermal treatment (SNS-18) exhibit the highest solar energy conversion efficiency. The efficiency is improved by 24.5% compared with that of the cells fabricated with pure P25. Based on the UV-Vis spectrum, nitrogen sorption and IPCE analysis, the improved performance can be attributed to the enhanced scattering and increased active sites for dye loading. Therefore, the dual functions of light scattering and many active sites for dye loading make SNS superior candidates for DSSCs. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction As key component of dye-sensitized solar cells, TiO2 photoanode serves as electron transporter and dye carrier. It has attracted intensive attention since significant breakthrough was made by Grätzal in 1991[1]. For DSSCs, TiO2 films composed of anatase TiO2 nanocrystals (∼20 nm) with high surface area can provide >more adsorption sites for dye molecules, which could make a great contribution in large current density and high photo-to-current conversion efficiency[2]. However, such films play a negligible role in light scattering due to the small particle size of TiO2. Thus, a considerable portion of photons would transmit through the TiO2 film without interacting with the photosensitizer, which directly leads to low light harvest efficiency. In addition, the photosensitizers such as N719 and N3 have poor photoresponse properties in the red region, leading to poor light harvest efficiency in the spectral region of 600-800 nm[3]. Therefore, photosensitizers with high light absorption, particularly in the near-IR region of solar spectrum, have been extensively studied[4,5]. On the other hand, many efforts have been made on the photoanode attempting to enhance the light harvest efficiency through controlling the structure and size of TiO2 [6,7]. According to the
∗ Corresponding author. Tel.: +86 13612032260; fax: +86 022 27890885. E-mail address: [email protected] (Z. Shan). 0013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2014.01.065
Mie theory[2], TiO2 particles used as light scatters for DSSCs should have a large size comparable to the wavelengths of visible light, and form an effective light scattering. However, dye adsorption would be much less for these scattering particles due to their much smaller surface area, which would severely lower the electron concentration and reduce the quasi Fermi level[8,9]. Thus, it is indispensable to develop photoanode materials that not only serve as light scattering layer but also offer enough dye adsorption sites. Recently, a nanostructure called oxide aggregates[10] has shown excellent potential for DSSCs application. Compared with large particles used in traditional DSSCs, the oxide aggregates possess a highly >porous structure consisting of nanosized crystallites, which can provide more active sites for dye loading when they serve as efficient light scatters. Hydrothermal treatment of amorphous titanium has been a common method to prepare TiO2 aggregates, which could facilitate crystallization >and stabilize the anatase phase of TiO2 . In addition, the morphology and structure of TiO2 could be tailored by manipulating acidity-alkalinity, solvent constitution, hydrothermal time and temperature of system. Yoo et al. [11] developed a two-step process of controlled hydrolysis and a subsequent hydrothermal treatment to prepare nanoporous spherical TiO2 , and an energy conversion efficiency of 8.44% was obtained. However, toxic substances were used during the preparation of precursors, and there was an safety issue to operate at high hydrothermal temperature of 240◦ C. A facile hydrothermal reaction of peroxotitanium complex precursors with
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the purpose of preparing hollow spheres assembled by nanospindles was reported by Wu et al. [3]. To form peroxotitanium complex precursors, P25 was dispersed in a mixture solution of hydrogen peroxide and ammonia with stirring for 24 h. When the hollow spheres were used as scattering layer, the energy conversion efficiency reached to 8.10%. But, it is difficult to keep concentration of hydrogen peroxide and ammonia during the agitation process. Chen et al. [12,13] also fabricated a complex titania nanostructure of monodisperse spiky mesoporous anatase beads. The precursors called mesostructured hydid beads were first prepared through a sol-gel process in the presence of hexadecylamine (HDA) as a structure-directing agent. After solvothermal treatment of precursor, the products were calcined at 500 ◦ C to remove remaining organic components. Then the spiky mesoporous titania beads were used to fabricate photoanode, and an energy conversion efficiency of 10.3% was obtained. In this paper, a simple and environmentally friendly fabrication process with low cost was proposed. The precursors were prepared through an optimized technology based on the previous report[14]. Toxic reagents and organic structure directing agents were not employed both in the two–step process of controlled hydrolysis and subsequent hydrothermal treatment. Moreover, the growth mechanism of the spherical TiO2 covered with nanospindles (SNS) was investigated by time-dependent trails. When SNS were collected to be used as the scattering layer of TiO2 photoanode, a solar energy conversion efficiency of 6.40% was obtained, much higher than that of the cell based on pure P25 photoanode. 2. Experimental 2.1. Preparation of precursor Amorphous precursor TiO2 solid spheres were prepared through controlling the hydrolysis rate of TBT in ethanol according to previous reports with some modification[14]. Firstly, 3 ml of TBT diluted with ethanol (1:8, v/v) was slowly dropped in a mixture of 96 ml ethanol and 0.48 ml of 0.1 M KCl aqueous solution. After being stirred for 60 min, the suspension was kept at 25 ◦ C for 24 h in a static state. Then the precursors were collected and washed with ethanol several times, and finally dried at 50 ◦ C in air. 2.2. Preparation of spheres covered with nanospindles In a typical synthesis, appropriate g precursors were dispersed in a solution composed of ethanol (20 ml), distilled water (5 ml) and a certain amount of 25 wt% ammonia solution. The mixture was sealed within a 100 ml Teflon-lined autoclave and heated at 160 ◦ C. After cooling to room temperature, the products were centrifuged and washed with ethanol five times, then dried in a vacuum oven at 50 ◦ C for 12 h. 2.3. Preparation of photoanodes To prepare the paste, ethyl cellulose and terpineol were added to the ethanol solution followed by the addition of TiO2 with being stirred for 24 h. Then, ethanol was removed from solution by rotary evaporation to form paste. TiO2 photoelectrodes for DSSCs were fabricated by Doctor Blade method, and the pasted films were heated at 150 ◦ C for 5 min, 325 ◦ C for 5 min, 375 ◦ C for 10 min, 400 ◦ C for 10 min and 500 ◦ C for 10 min, respectively. After being sintered, the films were placed in a solution of 50 mM TiCl4 at 70 ◦ C for 30 min and washed with water and ethanol. Finally, the films were kept at 500 ◦ C for 30 min. After cooling to 80 ◦ C, the films were immersed into a solution of cis-di(thiocyanato)N,N’- bis(2,2 -bipyridyl-4-carboxylicacid-4 -tetrabutylammonium
carboxylate)ruthenium(II) in the solvent of acetonitrile and butylcyanide (1:1, v/v) for 24 h. After that, the dyed TiO2 photoanode and the Pt-counter were assembled into a sandwich-type cell injected by electrolyte, which contained 0.05 M I2 , 0.5 M LiI, 0.4 M 1,3dimethylimidazolium iodide (DMPII), 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile and butylcyanide (85:15, v/v). 2.4. Characterization of spheres covered with nanospindles and DSSCs The morphology of SNS was investigated via field-emission scanning electron microscopy (FESEM; Hitachi, S-4800, 15 kV) and Transmission electron microscopy (TEM; JEOL, JEM-2100f, 200 kV). The crystal structures of SNS were characterized by X-ray diffraction (XRD, Bruker ASX, Germany) using Cu K radiation at 40 kV and 20 mA. The specific surface area of SNS-18 was calculated via the standard multi-points Brunauer-Emmett-Teller (BET) method by using a Micromeritics ASAP 2020 (USA) instrument. The pore size distribution was analyzed by using the Barrett-Joyner-Halenda (BJH) method. To measure the amount of dye on the films, the films with similar thickness were immersed into a 0.1 M NaOH solution in water and ethanol (1:1, v/v), and the concentration of desorbed dyes was measured by UV-Vis spectrophotometer (Perkin Elmer, Inc.). The reflectance spectra of the films were obtained using the UV-Vis spectrometer equipped with an integrating sphere. Photocurrent-potential (I-V) measurements were performed using a Keithley model 2400 source measure unit under a simulated sunlight (100 mW cm−2 , Zolix SS150 solar simulator) illumination provided by a Xenon lamp (Spectra-Physics) with an AM 1.5 filter. Incident photon to current conversion efficiency (IPCE) was measured by a Xe lamp and monochromeator (Newport). 3. Results and discussion 3.1. Effect of preparation conditions of SNS To find out critical factors for the formation of SNS, we adjusted hydrothermal treatment conditions, such as amount of precursors, solvent constitute, hydrothermal temperature and reaction time. It was discovered that when the hydrothermal system contained more than either 0.1350 g of precursors or 1.5 ml of 25 wt% ammonia solution, only ill-defined aggregations of nanocrystals could be found. Therefore, in order to obtain spherical anatase TiO2 covered with nanospindles (SNS), we controlled the amount of precursors (0.1350 g) and ammonia (1.5 ml) in the following experiments. As presented in Fig. 1a, SNS were successfully obtained through a facile hydrothermal treatment of precursors. In order to investigate the role of ammonia, a parallel experiment without ammonia was conducted. As shown in Fig. 1b, only nanoparticles could be obtained, demonstrating that ammonia can retard the dissolution of TiO2 and retain the morphology and structure of TiO2 . In addition, hydrothermal time was also important for fabrication of SNS. As shown in Fig. 2, morphology of TiO2 evolves along with the hydrothermal time varying from 12 h, 16 h, 18 h, 20 h, 22 h, to 24 h. 3.2. Structure and growth mechanism of SNS The time-dependent evolution of morphology revealed the growth mechanism of SNS. As shown in Fig. 2b, the precursors are relatively smooth spheres with size of about 550 nm. No diffraction peaks appear in the corresponding X-Ray Diffraction (XRD) pattern (Fig. 3), indicating that these spheres are amorphous phase[15]. After the spheres were hydrothermal treated at 160 ◦ C for 12 h (Fig. 2c), a few nanospindles were observed on their surface. As illustrated in Fig. 3, weak diffraction peaks corresponding to anatase TiO2 are indexed, which implies that the newly formed
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Fig. 1. SEM images of SNS after hydrothermal reaction at 160 ◦ C for 16 h in the presence of ammonia (a) and without adding ammonia (b).
nanospindles are anatase[16]. When the hydrothermal treatment was carried out for 16 h (Fig. 2d), the spheres with diameter of about 600 nm were composed of loose packed nanoparticles and covered with more interlaced nanospindles, which were in good agreement with TEM image (see Fig. 4a). It is indicated that nanospindles grew both in length and diameter with hydrothermal treatment
time increasing. Meanwhile, new nanospindles formed on the free space of the surface. A high-magnification TEM image (Fig. 4b) of nanospindle suggests that two sets of lattice fringes are 0.35 nm and 0.48 nm corresponding to (101) and (001) planes, respectively. And the interfacial angle of about 68.5◦ agrees well with theoretical value for the angle between (001) and (101) facets[17–19]. When
Fig. 2. SEM images of the precursor material (a), (b) and SNS obtained after hydrothermal reaction at 160 ◦ C for 12 h (c), 16 h (d), 18 h (e), 20 h (f), 22 h (g), 24 h (h), respectively.
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Fig. 3. XRD patterns of the precursor material and SNS obtained after hydrothermal reaction at 160 ◦ C for 12 h, 14 h, 16 h, 18 h, 20 h, respectively.
solvothermal time was extended to 18 h (Fig. 2e), nanospindles continued to grow. However, Fig. 3 reveals that the peak intensity stops increasing further, suggesting the completion of crystallization. With hydrothermal time being further increased to 20 h, 22 h and 24 h (Fig. 2f-h), the nanospindles began to shed off from the spheres due to no more space left to accommodate more nanospindles, so the spheres were inclined to collapse. In conclusion, owing to hot solvent and high surface free energy, dissolution of precursors would occur to release Ti species for nucleation on the surface of the spheres, which was driven by the minimization of the system free energy. Because of the presence of ammonia in the hydrothermal system, the growth of lateral facets of the anatase structure >was hindered and TiO2 particles selectively grew along the <001> direction to form nanospindles[3]. The specific surface area and pore size distribution of SNS collected at 160 ◦ C for 18 h were investigated by N2 sorption. As shown in Fig. 5, the nitrogen sorption isotherm for the products is characteristic of a combination of type I and IV isotherm (BDDT classification) with two district hysteresis loops. The one at low relative pressures between 0.4 and 0.6 is of type H2, indicating the presence of mesopores formed by particle pileup. However, the other at high relative pressures between 0.8 and 1.0 is of type H3, demonstrating the presence of narrow slit-shaped pores (> 50 nm) among nanospindles[15,20–22]. The above conclusions are also confirmed by the corresponding pore size distribution which shows two peaks of pore sizes for this sample. One peak locates at 159 nm, and the other at 19 nm. Furthermore, as displayed on the nitrogen
Fig. 5. N2 adsorption-desorption isotherm of SNS obtained at 160 ◦ C for 18 h. The inset displays the pore size distribution calculated by the BJH method.
adsorption-desorption isotherm, there is a sudden leap at high relative pressures, illustrating that macropores play a critical role in adsorption process. Using the multi-point Brunauer-Emmett-Teller (BET) method, the BET surface area of the products is calculated as ∼98.12 m2 g−1 much higher than that of P25 (∼ 55 m2 g−1 )[2]. 3.3. Photovoltaic performance In purpose of evaluating the photovoltaic performances, two different photoanode structures were studied. First, a P25 film of about 11 m >was coated on FTO glass (Fig. 6a). Then, a scattering layer of about 10 m >was coated on the obtained TiO2 film (Fig. 6b). For comparison, the other photoanode consisted of pure P25 TiO2 film with thickness of 21 m (Fig. 6c). The DSSC based on SNS18/P25 photoanode is referred to as DSSC-3 and the one based on P25 photoanode as DSSC-1. In order to further prove the superior performance of SNS-18, the solid spheres (precursors) were used to prepare the scattering layer as comparison. The DSSC based on solid spheres/P25 (SS/P25) is referred to as DSSC-2. The I-V characteristics of the cells were displayed in Fig. 7, and DSSC-3 showed the highest energy conversion efficiency. The detailed photovoltaic parameters were listed in Table 1. It is clear that the main factor for the improvement of energy conversion efficiency is the higher photocurrent density. The major factors contributing to higher Jsc of DSSC-3 and DSSC-2 are concluded as follows. First, SS and SNS with size comparable to the wavelengths of visible light can scatter the incident light backwards, thus the ability of
Fig. 4. TEM images of SNS obtained after hydrothermal reaction at 160 ◦ C for 16 h.
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Fig. 6. SEM images of different photoanode structures: a) pure P25 photoanode of about 11 m; b) spheres coated with nanospindles/P25 photoanode (SNS/P25); c) pure P25 photoanode of about 21 m.
Table 1 Values of the short-circuit current density (Jsc ), open circuit voltage (Voc ), fill factor (FF), overall conversion efficiency () for the cells based on SNS-18/P25 photoanode, SS/P25 photoanode and pure P25 photoanode. Cells
photoanode
Jsc /mA cm−2
Voc /mV
FF (%)
(%)
Absorbed dye (×10−7 mol cm−2 )
DSSC-1 DSSC-2 DSSC-3
P25 sphere/P25 SNS-18/P25
8.72 9.71 11.48
754 764 764
78 74 73
5.14 5.49 6.40
1.031 0.947 1.142
light capturing is enhanced. Consequently, the amount of electrons generated in the photoelectrode increases greatly. Secondly, the amount of dye in DSSC-3 was 1.142 × 10−7 mol cm−2 , which was much higher than dye absorption amount of 1.031 × 10−7 mol cm−2 in DSSC-1. It may result in more excited electrons and thus higher
Fig. 7. I-V curves of DSSC-1 based on pure P25 photoanode, DSSC-2 based on SS/P25 photoanode, DSSC-3 based on SNS-18/P25 photoanode, the rest are the cells based on SNS/P25 photoanode, whose light scattering material were obtained under different hydrothermal treatment time for 12 h, 16 h, 20 h, 24 h.
current density. However, the dye adsorption of the photoanode in DSSC-2 was lower than that of DSSC-1 although the Jsc of DSSC-2 was much higher than that of DSSC-1, indicating that SS just serve as scattering layer. In order to study the light scattering of SNS-18/P25, SS/P25 and pure P25 films, reflectance and transmittance measurements were carried out by UV-Vis Spectrometer. As shown in Fig. 8a, SNS-18/P25 and SS/P25 films show similar reflectance. And their reflectance are both stronger than that of P25 film in the wavelength range between 400 and 800 nm, which reveals that SNS-18/P25 and SS/P25 films can effectively scatter back the incident light due to the comparable size to the wavelengths of visible light. After dye adsorption on the films (shown in Fig. 8b), diffuse reflection capacities for SNS-18/P25 and SS/P25 films decrease drastically between 400 and 600 nm, which is similar to that of P25 film, resulting from light absorption by the dye molecules[23]. Moreover, the transmittance spectra of these films without dye sensitization were shown in Fig. 8c. Compared with P25 film, SNS18/P25 and SS/P25 films are less transparent to light especially at the long wavelength range of 700 -800 nm, indicating more capture of the radiation into the device[24]. Furthermore, the two curves for SNS-18/P25 and SS/P25 films approach to each other in 400-600 nm, which implies that the light absorption of SNS-18/P25 and SS/P25 is similar. However, after dye adsorption on the films (Fig. 8d), SS/P25 film shows much higher transmittance than that of SNS-18/P25 film, indicating more light captured by SNS-18/P25 film.
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Fig. 8. Diffused reflectance spectra of P25 films, SS/P25 films and SNS-18/P25 films (a) without and (b) with adsorbed N719 dye. Transmittance spectra of P25 films, SS/P25 films and SNS-18/P25 films (c) without and (d) with adsorbed N719 dye.
The IPCE spectra provide further evidence on the scattering effect of SNS. As shown in Fig. 9, the IPCE spectra of DSSC-3 exhibits a considerable increase over the entire wavelength range between 400 and 800 nm, which is in good agreement with the highest Jsc displayed in Table 1. The IPCE peak efficiency of DSSC-3 reached 79.3%, which is much higher than that of DSSC-1 (51.6%) and DSSC2 (61%), and the peak for the three cells are observed at 530 nm, which is in coincidence with the absorption maximum wavelength of N719 dye[25]. In the 400-600 nm, the increase in IPCE values for
DSSC-3 can be attributed to the synergistic effect of higher dye loading and the scattering effect. In the 600-800 nm range, N719 dye has a low absorption for incident light. The higher IPCE values for DSSC-3 than that of DSSC-1 can be attributed to the light-scattering ability caused by the larger size of SNS [15]. Furthermore, DSSC-2 has a higher IPCE than DSSC-1 at both short and long wavelengths although DSSC-2 has a lower dye loading capacity, which can be explained by the stronger back-scattering effect of SS/P25 film than that of P25 film.
4. Conclusions
Fig. 9. Incident-photon-to-current conversion efficiency (IPCE) spectra for DSSC3 based on SNS-18/P25 photoanode, DSSC-2 based on SS/P25 photoanode, DSSC-1 based on pure P25 photoanode.
Spherical anatase TiO2 covered with nanospindles (SNS) were prepared through a simple, environmentally friendly and low cost process. Organic structure directing agents and toxic reagents were not employed in the two–step process. During the hydrothermal treatment, precursors released Ti species for nucleation on the surface of the spheres. Ammonia made TiO2 particles selectively grow along the <001> direction, and hindered the growth of lateral facets of the anatase structure to form nanospindles as well. Furthermore, ammonia could also retard the dissolution of the precursors. When SNS-18 were employed to fabricate SNS18/P25 photoanode, a conversion efficiency of 6.40% was achieved, improved by 24.5% compared with that of P25 photoanode. The efficiency enhancement mainly resulted from the increment of photocurrent density from 8.72 mA cm−2 to 11.48 mA cm−2 . In addition, based on the UV-Vis spectrum, N2 adsorption-desorption isotherm, IPCE analysis, the Jsc increment can be attributed to the synergistic effect of high dye loading and the scattering effect. Therefore, we believe SNS-18 can offer further opportunities for
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the development of high-efficiency devices, and the synthesis of SNS-18 is beneficial to scale up for commercial manufacture. Acknowledgement This work was financially supported by the National Basic Research Program of China (2009CB220105). References [1] C. Wang, S., Chung, In Technical proceedings of the nanotechnology conference and trade show, Nanosicence and Technology Institute, Cambrige, MA, USA, 2007, P. 606. [2] Y. Liu, S. Wang, Z. Shan, X. Li, J. Tian, Y. Mei, H. Ma, K. Zhu, Electrochimica Acta 60 (2012) 422. [3] D. Wu, F. Zhu, J. Li, H. Dong, Q. Li, K. Jiang, D. Xu, J. Mater. Chem. 22 (2012) 11665. [4] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010) 6595. [5] M.K. Nazeeruddin, P. Pechy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, J. Am. Chem. Soc. 123 (2001) 1613. [6] Y. Rui, Y. Li, H. Wang, Q. Zhang, Chem Asian J. 7 (2012) 2313. [7] M. Adachi, Y. Murata, J. Takao, J. Jiu, M. Sakamoto, F. Wang, J. Am. Chem. Soc. 126 (2004) 14943.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Spherical anatase TiO2 covered with nanospindles as dual functional scatters for dye-sensitized solar cells Xiaopan Xue, Jianhua Tian, Wenming Liao, Zhongqiang Shan ∗ School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, PR China
a r t i c l e
i n f o
Article history: Received 24 October 2013 Received in revised form 21 December 2013 Accepted 12 January 2014 Available online 23 January 2014 Keywords: Anatase Dye-sensitized solar cell Scattering layer Sphere TiO2 aggregates
a b s t r a c t Spherical anatase TiO2 covered with nanospindles (SNS) were fabricated through a facile hydrothermal treatment of precursors in the presence of ammonia. The precursors were synthesized by controlling hydrolysis rate of TBT (tetrabutyl titanate) in ethanol. Organic structure directing agents and toxic reagents were avoided in the two–step process. By scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), it is confirmed that the morphology and structure of the products can be controlled by adjusting hydrothermal treatment conditions. Time dependent trails revealed the growth mechanism of SNS, which indicating that ammonia can not only retard the dissolution of precursors but also make TiO2 grow selectively along the <001> direction. Furthermore, photocurrent-potential (I-V) curves show that the solar cells fabricated with the SNS collected after 18 h hydrothermal treatment (SNS-18) exhibit the highest solar energy conversion efficiency. The efficiency is improved by 24.5% compared with that of the cells fabricated with pure P25. Based on the UV-Vis spectrum, nitrogen sorption and IPCE analysis, the improved performance can be attributed to the enhanced scattering and increased active sites for dye loading. Therefore, the dual functions of light scattering and many active sites for dye loading make SNS superior candidates for DSSCs. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction As key component of dye-sensitized solar cells, TiO2 photoanode serves as electron transporter and dye carrier. It has attracted intensive attention since significant breakthrough was made by Grätzal in 1991[1]. For DSSCs, TiO2 films composed of anatase TiO2 nanocrystals (∼20 nm) with high surface area can provide >more adsorption sites for dye molecules, which could make a great contribution in large current density and high photo-to-current conversion efficiency[2]. However, such films play a negligible role in light scattering due to the small particle size of TiO2. Thus, a considerable portion of photons would transmit through the TiO2 film without interacting with the photosensitizer, which directly leads to low light harvest efficiency. In addition, the photosensitizers such as N719 and N3 have poor photoresponse properties in the red region, leading to poor light harvest efficiency in the spectral region of 600-800 nm[3]. Therefore, photosensitizers with high light absorption, particularly in the near-IR region of solar spectrum, have been extensively studied[4,5]. On the other hand, many efforts have been made on the photoanode attempting to enhance the light harvest efficiency through controlling the structure and size of TiO2 [6,7]. According to the
∗ Corresponding author. Tel.: +86 13612032260; fax: +86 022 27890885. E-mail address: [email protected] (Z. Shan). 0013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2014.01.065
Mie theory[2], TiO2 particles used as light scatters for DSSCs should have a large size comparable to the wavelengths of visible light, and form an effective light scattering. However, dye adsorption would be much less for these scattering particles due to their much smaller surface area, which would severely lower the electron concentration and reduce the quasi Fermi level[8,9]. Thus, it is indispensable to develop photoanode materials that not only serve as light scattering layer but also offer enough dye adsorption sites. Recently, a nanostructure called oxide aggregates[10] has shown excellent potential for DSSCs application. Compared with large particles used in traditional DSSCs, the oxide aggregates possess a highly >porous structure consisting of nanosized crystallites, which can provide more active sites for dye loading when they serve as efficient light scatters. Hydrothermal treatment of amorphous titanium has been a common method to prepare TiO2 aggregates, which could facilitate crystallization >and stabilize the anatase phase of TiO2 . In addition, the morphology and structure of TiO2 could be tailored by manipulating acidity-alkalinity, solvent constitution, hydrothermal time and temperature of system. Yoo et al. [11] developed a two-step process of controlled hydrolysis and a subsequent hydrothermal treatment to prepare nanoporous spherical TiO2 , and an energy conversion efficiency of 8.44% was obtained. However, toxic substances were used during the preparation of precursors, and there was an safety issue to operate at high hydrothermal temperature of 240◦ C. A facile hydrothermal reaction of peroxotitanium complex precursors with
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the purpose of preparing hollow spheres assembled by nanospindles was reported by Wu et al. [3]. To form peroxotitanium complex precursors, P25 was dispersed in a mixture solution of hydrogen peroxide and ammonia with stirring for 24 h. When the hollow spheres were used as scattering layer, the energy conversion efficiency reached to 8.10%. But, it is difficult to keep concentration of hydrogen peroxide and ammonia during the agitation process. Chen et al. [12,13] also fabricated a complex titania nanostructure of monodisperse spiky mesoporous anatase beads. The precursors called mesostructured hydid beads were first prepared through a sol-gel process in the presence of hexadecylamine (HDA) as a structure-directing agent. After solvothermal treatment of precursor, the products were calcined at 500 ◦ C to remove remaining organic components. Then the spiky mesoporous titania beads were used to fabricate photoanode, and an energy conversion efficiency of 10.3% was obtained. In this paper, a simple and environmentally friendly fabrication process with low cost was proposed. The precursors were prepared through an optimized technology based on the previous report[14]. Toxic reagents and organic structure directing agents were not employed both in the two–step process of controlled hydrolysis and subsequent hydrothermal treatment. Moreover, the growth mechanism of the spherical TiO2 covered with nanospindles (SNS) was investigated by time-dependent trails. When SNS were collected to be used as the scattering layer of TiO2 photoanode, a solar energy conversion efficiency of 6.40% was obtained, much higher than that of the cell based on pure P25 photoanode. 2. Experimental 2.1. Preparation of precursor Amorphous precursor TiO2 solid spheres were prepared through controlling the hydrolysis rate of TBT in ethanol according to previous reports with some modification[14]. Firstly, 3 ml of TBT diluted with ethanol (1:8, v/v) was slowly dropped in a mixture of 96 ml ethanol and 0.48 ml of 0.1 M KCl aqueous solution. After being stirred for 60 min, the suspension was kept at 25 ◦ C for 24 h in a static state. Then the precursors were collected and washed with ethanol several times, and finally dried at 50 ◦ C in air. 2.2. Preparation of spheres covered with nanospindles In a typical synthesis, appropriate g precursors were dispersed in a solution composed of ethanol (20 ml), distilled water (5 ml) and a certain amount of 25 wt% ammonia solution. The mixture was sealed within a 100 ml Teflon-lined autoclave and heated at 160 ◦ C. After cooling to room temperature, the products were centrifuged and washed with ethanol five times, then dried in a vacuum oven at 50 ◦ C for 12 h. 2.3. Preparation of photoanodes To prepare the paste, ethyl cellulose and terpineol were added to the ethanol solution followed by the addition of TiO2 with being stirred for 24 h. Then, ethanol was removed from solution by rotary evaporation to form paste. TiO2 photoelectrodes for DSSCs were fabricated by Doctor Blade method, and the pasted films were heated at 150 ◦ C for 5 min, 325 ◦ C for 5 min, 375 ◦ C for 10 min, 400 ◦ C for 10 min and 500 ◦ C for 10 min, respectively. After being sintered, the films were placed in a solution of 50 mM TiCl4 at 70 ◦ C for 30 min and washed with water and ethanol. Finally, the films were kept at 500 ◦ C for 30 min. After cooling to 80 ◦ C, the films were immersed into a solution of cis-di(thiocyanato)N,N’- bis(2,2 -bipyridyl-4-carboxylicacid-4 -tetrabutylammonium
carboxylate)ruthenium(II) in the solvent of acetonitrile and butylcyanide (1:1, v/v) for 24 h. After that, the dyed TiO2 photoanode and the Pt-counter were assembled into a sandwich-type cell injected by electrolyte, which contained 0.05 M I2 , 0.5 M LiI, 0.4 M 1,3dimethylimidazolium iodide (DMPII), 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile and butylcyanide (85:15, v/v). 2.4. Characterization of spheres covered with nanospindles and DSSCs The morphology of SNS was investigated via field-emission scanning electron microscopy (FESEM; Hitachi, S-4800, 15 kV) and Transmission electron microscopy (TEM; JEOL, JEM-2100f, 200 kV). The crystal structures of SNS were characterized by X-ray diffraction (XRD, Bruker ASX, Germany) using Cu K radiation at 40 kV and 20 mA. The specific surface area of SNS-18 was calculated via the standard multi-points Brunauer-Emmett-Teller (BET) method by using a Micromeritics ASAP 2020 (USA) instrument. The pore size distribution was analyzed by using the Barrett-Joyner-Halenda (BJH) method. To measure the amount of dye on the films, the films with similar thickness were immersed into a 0.1 M NaOH solution in water and ethanol (1:1, v/v), and the concentration of desorbed dyes was measured by UV-Vis spectrophotometer (Perkin Elmer, Inc.). The reflectance spectra of the films were obtained using the UV-Vis spectrometer equipped with an integrating sphere. Photocurrent-potential (I-V) measurements were performed using a Keithley model 2400 source measure unit under a simulated sunlight (100 mW cm−2 , Zolix SS150 solar simulator) illumination provided by a Xenon lamp (Spectra-Physics) with an AM 1.5 filter. Incident photon to current conversion efficiency (IPCE) was measured by a Xe lamp and monochromeator (Newport). 3. Results and discussion 3.1. Effect of preparation conditions of SNS To find out critical factors for the formation of SNS, we adjusted hydrothermal treatment conditions, such as amount of precursors, solvent constitute, hydrothermal temperature and reaction time. It was discovered that when the hydrothermal system contained more than either 0.1350 g of precursors or 1.5 ml of 25 wt% ammonia solution, only ill-defined aggregations of nanocrystals could be found. Therefore, in order to obtain spherical anatase TiO2 covered with nanospindles (SNS), we controlled the amount of precursors (0.1350 g) and ammonia (1.5 ml) in the following experiments. As presented in Fig. 1a, SNS were successfully obtained through a facile hydrothermal treatment of precursors. In order to investigate the role of ammonia, a parallel experiment without ammonia was conducted. As shown in Fig. 1b, only nanoparticles could be obtained, demonstrating that ammonia can retard the dissolution of TiO2 and retain the morphology and structure of TiO2 . In addition, hydrothermal time was also important for fabrication of SNS. As shown in Fig. 2, morphology of TiO2 evolves along with the hydrothermal time varying from 12 h, 16 h, 18 h, 20 h, 22 h, to 24 h. 3.2. Structure and growth mechanism of SNS The time-dependent evolution of morphology revealed the growth mechanism of SNS. As shown in Fig. 2b, the precursors are relatively smooth spheres with size of about 550 nm. No diffraction peaks appear in the corresponding X-Ray Diffraction (XRD) pattern (Fig. 3), indicating that these spheres are amorphous phase[15]. After the spheres were hydrothermal treated at 160 ◦ C for 12 h (Fig. 2c), a few nanospindles were observed on their surface. As illustrated in Fig. 3, weak diffraction peaks corresponding to anatase TiO2 are indexed, which implies that the newly formed
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Fig. 1. SEM images of SNS after hydrothermal reaction at 160 ◦ C for 16 h in the presence of ammonia (a) and without adding ammonia (b).
nanospindles are anatase[16]. When the hydrothermal treatment was carried out for 16 h (Fig. 2d), the spheres with diameter of about 600 nm were composed of loose packed nanoparticles and covered with more interlaced nanospindles, which were in good agreement with TEM image (see Fig. 4a). It is indicated that nanospindles grew both in length and diameter with hydrothermal treatment
time increasing. Meanwhile, new nanospindles formed on the free space of the surface. A high-magnification TEM image (Fig. 4b) of nanospindle suggests that two sets of lattice fringes are 0.35 nm and 0.48 nm corresponding to (101) and (001) planes, respectively. And the interfacial angle of about 68.5◦ agrees well with theoretical value for the angle between (001) and (101) facets[17–19]. When
Fig. 2. SEM images of the precursor material (a), (b) and SNS obtained after hydrothermal reaction at 160 ◦ C for 12 h (c), 16 h (d), 18 h (e), 20 h (f), 22 h (g), 24 h (h), respectively.
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Fig. 3. XRD patterns of the precursor material and SNS obtained after hydrothermal reaction at 160 ◦ C for 12 h, 14 h, 16 h, 18 h, 20 h, respectively.
solvothermal time was extended to 18 h (Fig. 2e), nanospindles continued to grow. However, Fig. 3 reveals that the peak intensity stops increasing further, suggesting the completion of crystallization. With hydrothermal time being further increased to 20 h, 22 h and 24 h (Fig. 2f-h), the nanospindles began to shed off from the spheres due to no more space left to accommodate more nanospindles, so the spheres were inclined to collapse. In conclusion, owing to hot solvent and high surface free energy, dissolution of precursors would occur to release Ti species for nucleation on the surface of the spheres, which was driven by the minimization of the system free energy. Because of the presence of ammonia in the hydrothermal system, the growth of lateral facets of the anatase structure >was hindered and TiO2 particles selectively grew along the <001> direction to form nanospindles[3]. The specific surface area and pore size distribution of SNS collected at 160 ◦ C for 18 h were investigated by N2 sorption. As shown in Fig. 5, the nitrogen sorption isotherm for the products is characteristic of a combination of type I and IV isotherm (BDDT classification) with two district hysteresis loops. The one at low relative pressures between 0.4 and 0.6 is of type H2, indicating the presence of mesopores formed by particle pileup. However, the other at high relative pressures between 0.8 and 1.0 is of type H3, demonstrating the presence of narrow slit-shaped pores (> 50 nm) among nanospindles[15,20–22]. The above conclusions are also confirmed by the corresponding pore size distribution which shows two peaks of pore sizes for this sample. One peak locates at 159 nm, and the other at 19 nm. Furthermore, as displayed on the nitrogen
Fig. 5. N2 adsorption-desorption isotherm of SNS obtained at 160 ◦ C for 18 h. The inset displays the pore size distribution calculated by the BJH method.
adsorption-desorption isotherm, there is a sudden leap at high relative pressures, illustrating that macropores play a critical role in adsorption process. Using the multi-point Brunauer-Emmett-Teller (BET) method, the BET surface area of the products is calculated as ∼98.12 m2 g−1 much higher than that of P25 (∼ 55 m2 g−1 )[2]. 3.3. Photovoltaic performance In purpose of evaluating the photovoltaic performances, two different photoanode structures were studied. First, a P25 film of about 11 m >was coated on FTO glass (Fig. 6a). Then, a scattering layer of about 10 m >was coated on the obtained TiO2 film (Fig. 6b). For comparison, the other photoanode consisted of pure P25 TiO2 film with thickness of 21 m (Fig. 6c). The DSSC based on SNS18/P25 photoanode is referred to as DSSC-3 and the one based on P25 photoanode as DSSC-1. In order to further prove the superior performance of SNS-18, the solid spheres (precursors) were used to prepare the scattering layer as comparison. The DSSC based on solid spheres/P25 (SS/P25) is referred to as DSSC-2. The I-V characteristics of the cells were displayed in Fig. 7, and DSSC-3 showed the highest energy conversion efficiency. The detailed photovoltaic parameters were listed in Table 1. It is clear that the main factor for the improvement of energy conversion efficiency is the higher photocurrent density. The major factors contributing to higher Jsc of DSSC-3 and DSSC-2 are concluded as follows. First, SS and SNS with size comparable to the wavelengths of visible light can scatter the incident light backwards, thus the ability of
Fig. 4. TEM images of SNS obtained after hydrothermal reaction at 160 ◦ C for 16 h.
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Fig. 6. SEM images of different photoanode structures: a) pure P25 photoanode of about 11 m; b) spheres coated with nanospindles/P25 photoanode (SNS/P25); c) pure P25 photoanode of about 21 m.
Table 1 Values of the short-circuit current density (Jsc ), open circuit voltage (Voc ), fill factor (FF), overall conversion efficiency () for the cells based on SNS-18/P25 photoanode, SS/P25 photoanode and pure P25 photoanode. Cells
photoanode
Jsc /mA cm−2
Voc /mV
FF (%)
(%)
Absorbed dye (×10−7 mol cm−2 )
DSSC-1 DSSC-2 DSSC-3
P25 sphere/P25 SNS-18/P25
8.72 9.71 11.48
754 764 764
78 74 73
5.14 5.49 6.40
1.031 0.947 1.142
light capturing is enhanced. Consequently, the amount of electrons generated in the photoelectrode increases greatly. Secondly, the amount of dye in DSSC-3 was 1.142 × 10−7 mol cm−2 , which was much higher than dye absorption amount of 1.031 × 10−7 mol cm−2 in DSSC-1. It may result in more excited electrons and thus higher
Fig. 7. I-V curves of DSSC-1 based on pure P25 photoanode, DSSC-2 based on SS/P25 photoanode, DSSC-3 based on SNS-18/P25 photoanode, the rest are the cells based on SNS/P25 photoanode, whose light scattering material were obtained under different hydrothermal treatment time for 12 h, 16 h, 20 h, 24 h.
current density. However, the dye adsorption of the photoanode in DSSC-2 was lower than that of DSSC-1 although the Jsc of DSSC-2 was much higher than that of DSSC-1, indicating that SS just serve as scattering layer. In order to study the light scattering of SNS-18/P25, SS/P25 and pure P25 films, reflectance and transmittance measurements were carried out by UV-Vis Spectrometer. As shown in Fig. 8a, SNS-18/P25 and SS/P25 films show similar reflectance. And their reflectance are both stronger than that of P25 film in the wavelength range between 400 and 800 nm, which reveals that SNS-18/P25 and SS/P25 films can effectively scatter back the incident light due to the comparable size to the wavelengths of visible light. After dye adsorption on the films (shown in Fig. 8b), diffuse reflection capacities for SNS-18/P25 and SS/P25 films decrease drastically between 400 and 600 nm, which is similar to that of P25 film, resulting from light absorption by the dye molecules[23]. Moreover, the transmittance spectra of these films without dye sensitization were shown in Fig. 8c. Compared with P25 film, SNS18/P25 and SS/P25 films are less transparent to light especially at the long wavelength range of 700 -800 nm, indicating more capture of the radiation into the device[24]. Furthermore, the two curves for SNS-18/P25 and SS/P25 films approach to each other in 400-600 nm, which implies that the light absorption of SNS-18/P25 and SS/P25 is similar. However, after dye adsorption on the films (Fig. 8d), SS/P25 film shows much higher transmittance than that of SNS-18/P25 film, indicating more light captured by SNS-18/P25 film.
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Fig. 8. Diffused reflectance spectra of P25 films, SS/P25 films and SNS-18/P25 films (a) without and (b) with adsorbed N719 dye. Transmittance spectra of P25 films, SS/P25 films and SNS-18/P25 films (c) without and (d) with adsorbed N719 dye.
The IPCE spectra provide further evidence on the scattering effect of SNS. As shown in Fig. 9, the IPCE spectra of DSSC-3 exhibits a considerable increase over the entire wavelength range between 400 and 800 nm, which is in good agreement with the highest Jsc displayed in Table 1. The IPCE peak efficiency of DSSC-3 reached 79.3%, which is much higher than that of DSSC-1 (51.6%) and DSSC2 (61%), and the peak for the three cells are observed at 530 nm, which is in coincidence with the absorption maximum wavelength of N719 dye[25]. In the 400-600 nm, the increase in IPCE values for
DSSC-3 can be attributed to the synergistic effect of higher dye loading and the scattering effect. In the 600-800 nm range, N719 dye has a low absorption for incident light. The higher IPCE values for DSSC-3 than that of DSSC-1 can be attributed to the light-scattering ability caused by the larger size of SNS [15]. Furthermore, DSSC-2 has a higher IPCE than DSSC-1 at both short and long wavelengths although DSSC-2 has a lower dye loading capacity, which can be explained by the stronger back-scattering effect of SS/P25 film than that of P25 film.
4. Conclusions
Fig. 9. Incident-photon-to-current conversion efficiency (IPCE) spectra for DSSC3 based on SNS-18/P25 photoanode, DSSC-2 based on SS/P25 photoanode, DSSC-1 based on pure P25 photoanode.
Spherical anatase TiO2 covered with nanospindles (SNS) were prepared through a simple, environmentally friendly and low cost process. Organic structure directing agents and toxic reagents were not employed in the two–step process. During the hydrothermal treatment, precursors released Ti species for nucleation on the surface of the spheres. Ammonia made TiO2 particles selectively grow along the <001> direction, and hindered the growth of lateral facets of the anatase structure to form nanospindles as well. Furthermore, ammonia could also retard the dissolution of the precursors. When SNS-18 were employed to fabricate SNS18/P25 photoanode, a conversion efficiency of 6.40% was achieved, improved by 24.5% compared with that of P25 photoanode. The efficiency enhancement mainly resulted from the increment of photocurrent density from 8.72 mA cm−2 to 11.48 mA cm−2 . In addition, based on the UV-Vis spectrum, N2 adsorption-desorption isotherm, IPCE analysis, the Jsc increment can be attributed to the synergistic effect of high dye loading and the scattering effect. Therefore, we believe SNS-18 can offer further opportunities for
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