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Design of SnO2@[email protected]2 hierarchical urchin-like double-hollow nanospheres for high performance dye-sensitized solar cells
Solar Energy 189 (2019) 412–420
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Design of SnO2@Air@TiO2 hierarchical urchin-like double-hollow nanospheres for high performance dye-sensitized solar cells
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Yinglin Wang, Chao Ma, Chen Wang, Pengfei Cheng , Luping Xu, Li Lv, Hua Zhang School of Aerospace Science and Technology, Xidian University, Xi'an 710126, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Double-hollow TiO2 SnO2 DSSC PCE
The hierarchical double-hollow SnO2@Air@TiO2 microspheres (SATS) have been successfully synthesized by coating of SiO2 and TiO2 on the SnO2 templates layer by layer, and its detailed performances for dye-sensitized solar cells (DSSCs) application also have been studied. SATS microspheres have an average diameter of about 600 nm while the sizes of SnO2 hollow spheres are about 250 nm. The urchin-like SnO2/TiO2 composite microspheres with a large specific surface area of 96.1 m2/g promote dye adsorption and efficient electron transport so as to increase short-circuit photocurrent density to 13.69 mA/cm2. Compared to the traditional simple hollow structure, the double-hollow structure can ulteriorly improve light scattering ability. Hence, it reveals that the DSSC made from SATS nanospheres and P25 exhibits a higher photo conversion efficiency of 6.77%, a 33% improvement compared to the DSSC made from pure P25. The results indicate that the improved photovoltaic performance of SATS DSSCs can mainly attribute to the unique urchin-like double-hollow microstructure and the synergistic effect of SnO2/TiO2 composite material.
1. Introduction During recent years, the energy crisis has greatly affected social development and economic growth. Solar energy resources with the advantages of environmental protection and renewable, have gradually attracted people's attention. Solar cells is an important method to utilize solar energy effectively. In the field of solar cells, dye-sensitized solar cells (DSSCs) were presented for the first time by Grätzel and O'Regan in 1991 and have been the subject of discussion among experts due to the simple fabrication, low cost and relatively high theoretical efficiency (Grätzel, 2001; Hagfeldt et al., 2010; Koo et al., 2008). Up to now, the highest photovoltaic conversion efficiency of DSSCs has exceeded 9% (He et al., 2019). A typical DSSC consists of a dye-sensitized photoanode, an electrolyte and a counter electrode (Park et al., 2000; Liao et al., 2012; Jung et al., 2010; Liu et al., 2012). In which, the photoanode plays a significant role in light harvesting and electron transport, for the dye molecules adsorbed by photoanode can absorb light effectively (Bhatti et al., 2019; Zhu et al., 2019; Younas et al., 2019). Based on the most advantageous photoanode structure type at present, researchers investigate many structures to further improve the efficiency. For example, nanoarrays can be fabricated to improve the electronic transmission capacity; traditional hollow structure is constructed by template method to improve the photon scattering ability;
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compared to the traditional simple hollow structure, the double-hollow structure can ulteriorly improve light scattering ability. Strong acids or alkali can corrode the surface topography of the material and increase the specific surface area (Zhang et al., 2017; Zhao et al., 2016a,b; Cheng et al., 2013). Meanwhile, hierarchical nanostructured materials generally have the advantages of large specific surface area, strong light scattering ability and fast electron transmission, which can effectively improve the photoelectric conversion efficiency (PCE) of DSSC. Manikandan et al. have presented a Eosin-Y sensitized core-shell TiO2-ZnO nanostructure, promising an improvement in the photoelectric conversion efficiency by 57% in comparison with that of the DSSCs based on the pristine TiO2 (Manikandan et al., 2018). And Chen et al. have designed a double-layered structure composed of a TiO2 nanorod overlayer and TiO2 nanoparticle-embedded ZnO nanoflower underlayer, and the results of a series of tests indicate that a promising power conversion efficiency of 8.01% is determined on the DSSC (Chen et al., 2018). Moreover, composite photoanode of SnO2-ZnO and composite photoanode of SnO2-TiO2 provide more possibilities for composite semiconductor oxides as DSSCs photoanode materials (Zinab et al., 2019). Recently, researchers have devoted continuously to study the chemical synthesis, microstructure and photovoltaic characterization of TiO2 materials, since photovoltaic conversion efficiency of TiO2-based
Corresponding author. Tel.: +86 029 81891034; fax: +86 029 81891034. E-mail address: [email protected] (P. Cheng).
https://doi.org/10.1016/j.solener.2019.07.082 Received 29 April 2019; Received in revised form 15 July 2019; Accepted 26 July 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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HCl aqueous solution was added into 33 mL mixed solution (the volume ratio of ethanol to water, 10:1) under continually stirring for 5 min. Then 0.263 g of SnCl2·2H2O were ultrasonically dispersed in the mixed solution. After continual stirring for 1 h, the mixture was transferred and sealed in a 45 mL Teflon-lined autoclave which was then kept at 200 °C for 12 h. The product centrifugation and washed with absolute ethyl alcohol and deionized water three times, respectively. At last, the obtained sample was dried at 60 °C overnight. Preparation of SnO2@SiO2 microspheres: The SnO2@SiO2 microspheres were synthesized by means of an improved Stöber method (Liu et al., 2012). In a typical deposition of SiO2 nanoparticles, 100 mg of asprepared SnO2 were dispersed in a mixed solution of absolute ethyl alcohol (140 mL) and deionized water (35 mL) under vigorously stirring for 30 min. Afterward, 26 wt% ammonia (2 mL) and TEOS (0.3 mL) were added drop by drop, respectively. After ultra-sonication for 10 min, the above solution underwent continuous stirring for 8 h at room temperature. After centrifugation and washing, the precipitates were dried at 60 °C overnight and subsequently annealed at 300 °C for 2 h. Preparation of SnO2@SiO2@TiO2 microspheres (Chen et al., 2011): the DETA (0.05 mL) dispersed in isopropanol (40 mL) and then 100 mg of the SnO2@SiO2 microspheres were added into the above solution under vigorous stirring. To research the influence of precursor TIP on the morphology and property of the product, the dose of TIP was set to 0.1 mL, 0.3 mL, 0.5 mL and 0.7 mL, while the corresponding products were denoted as SATS-1, SATS-2, SATS-3 and SATS-4, respectively. Hence, X mL isopropyl titanate (TIP) was added into the mixed solution and stirred until form a homogeneous solution and then kept at 200 °C for 24 h in a Teflon-lined autoclave. The resulting samples were washed with ethanol and subsequently dried at 80 °C overnight. Preparation of SnO2@Air@TiO2 double-shell microspheres: The SnO2@SiO2@TiO2 microspheres (100 mg) dispersed in NaOH aqueous solution (120 mL, 1.0 M) under continuously stirring and kept at 50 °C for 1 h in a water-bath heater. The products were washed with dilute nitric acid and deionized water several times until a neutral solution, respectively. After dried at 60 °C for 12 h, the SATS-1, SATS-2, SATS-3 and SATS-4 were calcined at 450 °C for 2 h with a heating rate of 3 °C/ min in air.
DSSCs exceeded 10% (Nazeeruddin et al., 2001; Kartini et al., 2004; Zukalova et al., 2005; Shim et al., 2015; Chai et al., 2015; Jung et al., 2015). Although single TiO2 material of DSSCs has good photosensitive property, it is easy to generate electron recombination. Therefore, many researchers have explored other alternative metal oxides such as ZnO, SnO2, Fe2O3 and so on. These metal oxides can replace TiO2 or combine with TiO2 to form composite materials. Among these, SnO2 has two advantages compared to TiO2 for DSSC applications. On the one hand, SnO2 possesses higher electron mobility (100–200 cm2 V−1 S−1) than TiO2 (0.1–1.0 cm2 V−1 S−1), suggesting more efficient transport of photo-induced electrons in SnO2 than in TiO2. On the other hand, SnO2 has a larger band gap (3.5 eV) than TiO2 (3.2 eV), which would create fewer oxidative holes in the valence band, thereby promoting stability of DSSCs (Javed et al., 2019; Bakr et al., 2018; Steffy et al., 2017; Wali et al., 2016; Zhao et al., 2016a,b). However, a 300 mV positive shift of the conduction-band edge of SnO2 with respect to that of TiO2 affects the properties of SnO2 photo-electrodes, such as weaker photon capture ability and less adsorption of the dyes with acidic carboxyl groups (Gao et al., 2014; Pang et al., 2014). Fabrication composites of SnO2 and TiO2, which combine the respective advantages of SnO2 and TiO2, can effectively enhance the power conversion efficiency of DSSCs. In addition, microstructures of materials used in photoanode also have great influence on the photovoltaic performance of DSSCs, such as hollow, urchin-like, core-shell, nanorod, nanowire, nanotube, nanofiber and nanoarray structure. For example, Motlak et al. have presented a Cddoped TiO2 nanofibers structure, and the DSSC based on Cd-doped TiO2 nanofibers has a photoelectron conversion efficiency (2.945%) in comparison to that of pristine TiO2 nanofibers; Hu et al. have designed novel double-layered photoanodes based on porous-hollow TiO2 microspheres and La(OH)3: Yb3+/Er3+ for DSSCs, indicating the photoelectron conversion efficiency of 8.89%; Yu et al. have presented a hierarchical core-shell structure of hollow TiO2 microspheres for DSSCs, showing a high photoelectron conversion efficiency of 7.7% (Motlak et al., 2019; Hu et al., 2019; Yu et al., 2015). Moreover, Wu et al. synthesized the shell-in-shell TiO2 hollow spheres for DSSCs, which showed higher photoelectron conversion efficiency than simple hollow ones (Wu et al. 2011; Jia et al. 2014). Therefore, the rational selection of materials and the construction of reasonable structure can effectively improve the photoelectron conversion efficiency of the battery. In this study, a facile continuous hydrothermal/sol–gel/solvothermal strategy has been adopt for preparing a series materials of SnO2, SnO2@SiO2, SnO2@SiO2@TiO2, and SnO2@Air@TiO2 urchinlike double-hollow nanospheres (SATS). The formed SnO2/TiO2 doublehollow structure can further increase the reflection path of photons and promote the absorption of photons by the dye. The obtained SATS composite material not only shows excellent light scattering ability and dye adsorption ability (contributed by TiO2), but also has fast electron transport rate (contributed by SnO2). This kind of multifunctional composited semiconductor oxide material applied in DSSCs reflects the enhanced performance than that pure P25 based, showing the largest PCE of 6.77% among the comparative DSSCs.
2.2. Fabrication of photoanodes and DSSCs A series of paste were prepared to fabricate SATS and P25 composite photoanode. 0.3 g of powders (SATS-1, SATS-2, SATS-3 or SATS-4 or pure P25 NPs) were dissolved in ethanol under continuously stirring, respectively, and then 1 g of terpinol was dripped into the suspension. 1 g of ethyl cellulose was weighed and slowly poured into ethanol with fully stirred. The two turbid solutions were mixed and stirred for 24 h at room temperature. The preparation method of SATS paste is similar to that of P25 paste. For the preparation of a compact TiO2 layer, the cleaned fluorine doped tin oxide glass substrates (FTO) were immersed into 40 mM TiCl4 aqueous solution in a closed vessel for 30 min at 70 °C. Two kinds of resulting paste (SATS and P25) were successively coated onto the compact layer by the doctor blade method, followed by annealing at 125 °C for 15 min. After immersed into TiCl4 aqueous solution, the asprepared films were then annealed at 450 °C for 30 min in air to remove the organic ingredients and improve the thermal stability (Ito et al., 2008). After cooling to 80 °C, the thick film with double layer structure were sensitized by dye (0.4 mM N-719 ethanol solution) for 24 h at room temperature. The dye-sensitized photoanodes were assembled with the Pt-coated FTO counter electrode and the electrolyte, consisting of 0.05 M LiI, 0.05 M I2, 0.5 M 4-tert-butylpyridine (Aldrich) and 0.6 M 1-propyl-3-methylimidazolium iodide (PMII) in 3-methoxypropionitrile, was then injected into the cell from the edges by capillarity. All of the DSSCs are fabricated following the same process and the effective area of the DSSCs is 0.2 cm2
2. Experiments 2.1. Materials preparation All chemicals including stannous chloride dehydrate (≥98.0%), tetraethyl orthosilicate (TEOS, ≥98%), titanium isopropoxide (TIP, ≥97%), sodium hydroxide (NaOH, ≥96%), hydrochloric acid (36 wt% in water), isopropanol (≥99.7%), ammonia (25 wt% in water), diethylenetriamine (DETA), and absolute ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Company. All the reagents in the experiment were analytical grade and used without further purification. Preparation of SnO2 hollow microspheres: Typically, SnO2 hollow spheres were synthesized by a modified hydrothermal method. 0.8 mL 413
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2.3. Measurements of materials and DSSCs Morphologies and microstructures were examined by field-emission scanning electron microscopy (FESEM, JEOL JSM-7500F, operated at 15 kV) and transmission electron microscopy (TEM, JEOL EM-2100, operated at an accelerating voltage of 200 kV). The images of element distribution could also be measured by SEM. The crystalline structures of the powder were analyzed by X-ray diffraction (XRD) measurement on a diffractometer (Rigaku TTRIII, with Cu Kα1 radiation). N2 adsorption-desorption isotherms were obtained on a Brunner-EmmetTeller (BET) apparatus (Micromeritics Gemini VII apparatus). Photocurrent density-voltage (J-V) characteristics and the incident photon-to-current efficiency (IPCE) spectra of the DSSCs were respectively measured using a Keithley 2450 Source Meter and a Spectral Product Zolix DSC300PA, under one sun AM 1.5G (100 mW/cm2) illumination with a solar light simulator (Newport, Model: 94023A). The reflectance and absorbance spectra were measured on N719-sensitized films and unsensitized films on a Perkin-Elmer UV–Vis spectrophotometer (SHIMADZU 2550). The electrochemical impedance spectroscopy (EIS) were recorded on an electrochemical workstation (Solartron SI1287) with a frequency ranging from 0.1 to 100 kHz at open-circuit voltage. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) measurements were carried out on an electron lifetime with a diode laser light source with variable intensity at 620 nm (PSL-100, EKOJapan). 3. Results and discussion In order to explore the crystal structure of the samples, the X-ray diffraction pattern of SnO2 microspheres and SATS microspheres is characterized, as shown in Fig. 1a. The results reveal that all diffraction peaks of SnO2 microspheres can be perfectly indexed to an cassiterite SnO2 (JCPDS card No. 1-625), and the diffraction peaks are distinguishable and well arranged, indicating the highly crystallinity of SnO2 microspheres. The peaks at 26.9°, 34.2° and 52.1° can be assigned to (1 1 0), (1 0 1) and (2 1 1) crystal planes of cassiterite SnO2, respectively. In addition, the diffraction peak of SATS microsphere not only matches the diffraction peak of SnO2 (JCPDS card No. 1–625), but also matches the characteristic peak of standard anatase TiO2 (JCPDS card No. 71-1168), and this indicates that SnO2 exists in the samples and are not affected by subsequent chemical reactions. The diffraction peaks of SATS microspheres at 25.5° and 48.0° are well consistent with (1 0 1) and (2 0 0) crystal planes of anatase TiO2. In particular, the main peak corresponds to the (1 0 1) lattice plane of standard anatase TiO2, and the diffraction peak is very sharp, indicating a high degree of crystallinity. Besides, no other impurities can be found in the XRD spectrum. In order to have a deeper understanding of the specific surface area of SATS-1, SATS-2, SATS-3 and SATS-4 materials, nitrogen adsorption desorption technology has been used to measure relevant parameters. Fig. 1b shows the isothermal curves of four different materials. The type IV isothermal curves of the four materials can be obviously observed, indicating that the outer layer of the four materials has mesoporous structure. In addition, the isothermal curves of SATS-1, SATS-2, SATS-3 and SATS-4 all have H3 type hysteresis loops, indicating that four materials have narrow pores assembled by a large number of nanosheets. Brunauer-Emmett-Teller (BET) method has been used to calculate the specific surface area of SATS-1, SATS-2, SATS-3 and SATS-4 are respective 69.1, 76.3, 96.1 and 78.3 m2/g. The scanning electron microscopy (SEM) on the products of each step has been utilized to characterize the structure and morphology of SATS microspheres in detail. As depicted in Fig. 2a, SnO2 microspheres have good dispersion, and are not approximately uniform. The SnO2 microspheres act as good spherical template for subsequent sol-gel and hydrothermal experiments. Moreover, the surface of SnO2 microspheres is not very smooth. As shown in Fig. 2b, SiO2 nanoparticles are
Fig. 1. (a) XRD patterns, Anatase TiO2; ● Cassiterite SnO2, (b) N2 adsorption and desorption isotherms of the SATS-1, SATS-2, SATS-3 and SATS-4.
uniformly coated on the surface of SnO2 microspheres by sol-gel method. Meanwhile, the diameter of obtained SnO2@SiO2 microspheres increases a lot. In addition, SiO2 nanoparticles have good light transmission (Li et al., 2019), the SiO2 layer is visible in Fig. 2b and the thickness of SiO2 layer is about 80 nm. The as-prepared microspheres still present good dispersion and very smooth surface. The magnified image in Fig. 2c suggests that the surface of SnO2@SiO2@TiO2 microspheres is assembled by densely packed nanosheets of TiO2 with about 100 nm in length. The short nanosheets of different SATS microspheres are intertwined and connected with each other, which is conducive to the transmission of electrons between SATS microspheres. Fig. 2d shows a broken SATS microsphere and its internal and external microstructures. The location of the fracture shows a hollow layer about 80 nm thick, which matches with the thickness of SiO2 layer, and a spherical core with rough surface indicates that NaOH aqueous solution washes away SiO2 layer successfully and roughens the SnO2 surface. In order to better understand the synthesis details of SnO2@Air@TiO2 microspheres, the specific synthesis process of SnO2@Air@TiO2 microsphere has been illustrated in Fig. 3. The structure characteristics of SATS-1 SATS-2 SATS-3 and SATS-4 microspheres have been further studied, and the TEM image of the samples has been carried out, as shown in Fig. 4a–d. It reveals that the SnO2 microspheres in the core position present dark 1color, however,
1 The photo of the transmission electron microscope is a black and white picture, and the color depth of the core position indicates the thickness of the
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Fig. 2. SEM images of as prepared (a) SnO2 microspheres, (b) SnO2@SiO2 microspheres and (c, d) SnO2@Air@TiO2 double–shell microspheres.
Fig. 3. Schematic formation process of the SATS structure.
SATS. The nanocrystalline structure of SATS microspheres also has been studied by high resolution TEM characterization. Fig. 4e clearly shows the lattice fringes of TiO2 nanoparticles and SnO2 nanoparticles, the lattice distances of 0.238 nm and 0.336 nm well correspond to the (0 0 4) facets of anatase TiO2 and the (1 1 0) facets of cassiterite SnO2, respectively. Additionally, the element mapping image measurement based on energy dispersive X-ray detector reveals the presence and distribution of O, Ti, Sn in a single SATS microsphere, as can be seen from Fig. 4f–h. It has been presented that Ti elements are distributed in the outer shell of SATS microsphere, Sn elements are mainly distributed in the inner shell of SATS microsphere, and O elements are distributed in the whole SATS microsphere. According to the dispersion of elements such as: O, Ti and Sn, the double-hollow structure can be concluded as a composition of SnO2 (inner shell part) and TiO2 (outer shell part). From
their central position show some light transmittance, which indicates the hollow structures of the SnO2 microspheres. Although the SnO2 hollow microspheres are not uniform in size, their relatively smooth surfaces are similar. As revealed in each TEM Fig. 4a–d, there is a hollow layer between the SnO2 hollow microsphere and the outer layer of TiO2. And the SnO2 hollow microspheres can move freely within the inner space of the shell layer of TiO2, indicating that the SiO2 layer coated on the surface of the SnO2 hollow microspheres has been washed away by alkali without damaging the double-shell and urchin-like structures. In addition, as can be seen from Fig. 4a–d, with the increase of TiO2 content, the color of TiO2 layer gradually exhibits darker and thicker, which also confirms the successfully growth of TiO2 crystal on
(footnote continued) core. 415
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Fig. 4. TEM images of the (a) SATS-1, (b) SATS-2, (c) SATS-3 and (d) SATS-4 microspheres, (e) HRTEM image of the SATS-0.5 microspheres, (f-h) Elemental mapping of individual SATS.
the figures above we can see that TiO2 random wire web on SnO2 microspheres. This TiO2 random wire web leads to a higher specific surface area of SATSs, and this is consistent with the results of Fig. 1b. As UV–Vis diffuse reflectance spectrum can reflect photon scattering ability of photoanode film, the UV–Vis diffuse reflectance spectrum has been carried out in Fig. 5a. As shown in the UV–Vis diffuse reflectance spectra, the five different photoanode films under the condition of nonimpregnated dye have been recorded. Because all the sizes of SATS microspheres are at the nanometer level, the highest photon reflectivity has been achieved in the small wavelength range of 400–450 nm (Huang et al., 2010). However, when the wavelength exceeds 450 nm,
the photon scattering ability gradually weakens, indicating that the long-wavelength photon has a weak scattering ability in the small-sized microspheres. Furthermore, SATS-4 film shows the strongest photon reflectance in the wavelength range of 400–450 nm, indicating that the TiO2 outer layer with special nanosheets morphology may help improve the photon scattering ability. It can be seen from Fig. 6a that photons reflected through the FTO conducting glass in the double-hollow structure of SATS microspheres for many times, which can prolong the routine of photons in SATS microspheres and enable the dye molecules to fully absorb photons. It is not difficult to find that the reflection path of photons in double-hollow structure is longer than that in traditional simple hollow structure. Fig. 6b shows that FTO, N719, I−/I3−, SnO2 and TiO2 have different energy level positions and forbidden band widths. It can be found that the N719 dyes on the surface of TiO2 absorb photons, change from the ground state to the excited state, and then release photogenerated electrons. Electrons travel along the conduction band from the highenergy material to the low-energy material. Meanwhile, the electrons pass through TiO2 and SnO2 successively and finally arrive at FTO. In order to explore the influence of the thickness of TiO2 layer on photoelectric performance of SATS microspheres, 0.1 mL, 0.3 mL, 0.5 mL and 0.7 mL of isopropanol titanium have been used for coating SnO2@SiO2 microspheres as outer layers, which worked as comparative experiments. Fig. 7 shows the J-V curves of SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 DSSCs measured under sun illumination (AM 1.5 G, 100 mWcm−2). As shown in Table 1, the four critical parameters of J-V curves have been listed such as: open circuit voltage, short circuit current density, filling factor and photoelectric conversion efficiency, which can effectively reflect the photoelectric performance of these DSSCs. The performance parameters in Table 1 can be calculated from the five J-V curves. For the five cells, the Jsc and the η of SATS-3 DSSC
Fig. 5. UV-Vis reflectance spectra of the five films without N719 dye loading. 416
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Fig.7. J-V curves of SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 DSSCs measured under sun illumination (AM 1.5 G, 100 mW cm−2).
SnO2@Air@TiO2 DSSCs for the maintenance the Voc are (i) higher dyeloading on account of its higher specific surface area than the P25 and (ii) SnO2@Air@TiO2 micro-structure improved charge transport characteristics compared with the P25 based devices. High specific surface area photoanode materials lead to high dye adsorption amount, therefore SATS film, which has a surface area of 96.1 m2 g−1, may be an excellent photoanode material. The amount of N719 dye adsorbed on these four films was investigated by thorough desorption in 1 mM NaOH solution and then measuring the UV–Vis absorption spectra of the resulting solutions. The dye loading of SATS-3 was 1.6, 1.4, 1.3 and 1.2 times higher than that of P25, SATS-1, SATS-2 and SATS-4 (see Table 1), respectively. The higher dye loading of SATS3 was ascribed to the larger specific surface area of the film. In fact, a larger amount of dye adsorption can capture more photons, resulting in more photogenerated electrons, which provides the possibility of a larger photocurrent. However, photogenerated electrons are trapped by defects during transmission, and whether the photogenerated electrons can be smoothly derived is determined by the electron transport characteristics of the photoanode. Therefore, the photocurrent is affected by the combination of the amount of dye adsorption (capacity to capture photons) and the electron transport capability of the photoanode. Fig. 8 shows normalized IPCE spectra of the obtained DSSCs. All the five DSSCs photoanode films use the same N719 dye. Affected by the highest absorption peak of N719 dye, the most powerful absorption peak of the five cells is displayed at the position of 525 nm of light wavelength, so the highest value of IPCE is obtained at 525 nm for all the five cells. Compared with pure P25 DSSC, SATS DSSCs have higher IPCE values in most of spectral range mainly due to the improvements of photon scattering ability, electron transmission rate and specific surface area. The IPCE value of SATS-3 DSSC reaches the highest value of 70% when the incident wavelength is 525 nm. However, excessive TiO2 of SATS-4 DSSC leads to the decrease of IPCE because of electron recombination. To further understand the possible reasons for the enhanced photoelectric performance, the electrochemical impedance spectrum (EIS) has been measured in the dark circumstance under a forward bias of −0.8 V to investigate the charge-transfer process (Wang et al., 2015). The Nyquist impedance spectra and equivalent circuit of SATS-1, SATS2, SATS-3, SATS-4 and pure P25 DSSCs are shown in Fig. 9, and the corresponding parameters are shown in Table 1, respectively. The smaller and larger semicircles in the Nyquist impedance spectra are attributed to the charge transfer at the counter electrode/electrolyte interface and the photoanode material/dye/electrolyte interface, respectively. The sheet resistance (Rs) of substrate, charge transfer resistance of the counter electrode (R1), and recombination resistance
Fig. 6. (a) Reflection mechanism diagram of photons inside SATS microspheres, (b) FTO, N719, I−/I3−, SnO2 and TiO2 energy levels and electron transfer diagrams.
present the largest value. The reason is that the double-hollow structures of SATS-3 greatly increases the propagation path of photons and the suitable specific surface area promotes the absorption of photons by dyes, both of these two factors improve the photoelectric performance of the DSSC (Shao et al., 2012; Naveen et al, 2012a,b). Additionally, both the Jsc and the η of SATS-1 DSSC are lower than that of P25 DSSC. In SATS-1 microspheres, the content of TiO2 is much lower than that of SnO2. Compared with TiO2, SnO2 has better conductivity but weaker photosensitivity. Therefore, the tiny amounts of TiO2 with the relatively excessive SnO2 will reveal the weak photoelectric performance of the battery (Fig. S1, Table S1). In the case of SATS-4 DSSC, the lower Jsc and η compared with SATS-3 DSSC can be ascribed to the excessive TiO2 accelerates electron recombination. The open circuit voltage of P25 DSSC is higher than that of SATS DSSCs, which is much related with the 300 mV positive displacement of energy band in SnO2 relative to TiO2. Furthermore, The Voc values obtained using double-hollow SnO2@Air@TiO2 DSSCs are as high as P25 (Pure TiO2), similar or higher Voc values were previously obtained with surface modifications by TiO2 (Gubbala et al, 2008; Dou et al, 2011). Two possible reasons to 417
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Table 1 Specific photovoltaic parameters and EIS parameters of the DSSCs based on SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 photoanodes. DSSC
Jsc(mAcm−2)
Voc(V)
FF(%)
RS(Ω)
R1(Ω)
R2(Ω)
η(%)
Dye adsorption(10−7mol cm−2)
P25 SATS-1 SATS-2 SATS-3 SATS-4
10.55 10.25 10.99 13.69 11.15
0.737 0.725 0.725 0.726 0.730
65.5 67.6 64.5 68.1 64.9
16.0 15.8 15.6 15.9 16.1
9.8 9.6 9.4 9.5 9.8
62.1 90.5 98.6 115.5 107.0
5.09 5.02 5.14 6.77 5.28
0.80 0.96 1.05 1.33 1.08
Fig. 8. Normalized IPCE spectra of SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 DSSCs.
Fig. 10. (a) Electron diffusion coefficient and (b) electron lifetime as a function of applied voltage for the five cells.
electron lifetime (τe) of the SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 DSSCs. The photoelectric performances of the five DSSCs have been further explored. As the light intensity increases, open circuit voltage also increases, and the electron diffusion coefficients curves show an increasing trend (Fig. 10a), while the electron lifetime curves show a decreasing trend (Fig. 10b). What's more important is that the electron diffusion coefficients are affected by the micro-morphology of membrane, and the electron lifetime is related to the structure of membrane. As depicted in the electron diffusion coefficient curves, the Dn curves of the four SATS DSSCs are above the pure P25 DSSC, and all the Dn values of SATS-3 DSSC are the maximum, which indicates that SATS DSSCs have better electron diffusion characteristics. The contiguous microspheres have better interfacial conductivity and high crystallinity, which will effectively suppress electron recombination and the dispersion of P25 nanoparticles inhibits electron transport. What's more, the SnO2 microspheres in the core position will also facilitate the electron diffusion and transport due to their higher electron mobility. As a result, the electron lifetime curves reveals that the τe curves of SATS
Fig. 9. Nyquist plots from EIS of the five cells. The inset illustrates the equivalent circuit simulated to fit the impedance spectra.
(R2) were analyzed by Z-view software using an equivalent circuit containing a constant phase element (CPE) and resistances (R; Fig. 9, inset). As the SATS DSSCs and pure P25 DSSC are based on the same electrolyte, N719 dye, FTO conductive glass and Pt electrode, R1 and Rs in the equivalent circuit of the five cells are the same, as presented in Table 1. And SATS-3 DSSC presents the largest R2 of 115.5 Ω, which is higher than those of other SATS DSSCs and pure P25 DSSC, revealing a lower charge recombination at the dye/electrolyte interface (Wang et al., 2005; Buonsanti et al., 2011; Ahn et al., 2014). As shown in Fig. 10, a laser emitter with adjustable intensity and constant wavelength of 620 nm is used as the light source, and the open circuit voltage (Voc) is controlled by changing the light intensity (by PSL-100, EKO-Japan, stepwise light intensity from small to large, in the black box), which affects the electron diffusion coefficients (Dn) and 418
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DSSCs are above the pure P25 DSSCs, and all the τe values of SATS-3 DSSCs are the maximum, and the reason is that the electron recombination probability of SATS DSSCs is smaller than that of P25 DSSC.
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4. Conclusions In summary, the SnO2@Air@TiO2 nanospheres (SATS) have been successfully prepared via a facile hydrothermal treatment using a layerby-layer construction strategy. The SATS microspheres have combined two advantages of SnO2/TiO2 composite material and double-hollow structure. On the one hand, SnO2 hollow spheres can improve the electron transfer rate, and TiO2 nanosheets of the outer shell can promote the adsorption of dyes. On the other hand, the double-hollow structure can increase the number of reflections of photogenerated electrons in the inner and outer hollow layer to improve the outstanding light scattering ability. An overall PCE of 6.77% has been achieved for the DSSC based on the SATS-3 photoanode and is far superior to that of P25 based DSSC (5.09%). The enhanced photoelectric performance is mainly due to the synergistic effect of the SnO2/TiO2 composite and double-hollow microstructure. Therefore, SATS presents a superior performance and ample potential to serve as decent alternatives to replace the traditional P25 NPs in the highly efficient DSSCs. Acknowledgements This work was supported by the National Nature Science Foundation of China (Nos. 61604116 and 61803289), China Postdoctoral Science Foundation (No. 2017 M623120 and 2019 M653552). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.07.082. References Grätzel, M., 2001. Photo electrochemical cells. Nature 414, 338–344. Hagfeldt, A., Boschloo, G., Sun, L., Kloo, L., Pettersson, H., 2010. Dye-Sensitized Solar Cells. Chem. Rev. 110, 6595–6663. Koo, H.J., Kim, Y.J., Lee, Y.H., Lee, W.I., Kim, K., Park, N.G., 2008. Nano-embossed hollow spherical TiO2 as bifunctional material for high-efficiency dye-sensitized solar cells. Adv. Mater. 20, 195–203. He, X., Guo, Y., Liu, J.H., 2019. Fabrication of peanut-like TiO2 microarchitecture with enhanced surface light trapping and high specific surface area for high-efficiency dye sensitized solar cells. J. Power Sour. 423, 236–245. Park, N.G., Lagemaat, J.V., Frank, A.J., 2000. Influence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline TiO2 solar cells: A study by electrical impedance and optical modulation techniques. J. Phys. Chem. B. 104, 2044–2052. Liao, J.Y., He, J.W., Xu, H., Kuang, D.B., Su, C.Y., 2012. Effect of TiO2 morphology on photovoltaic performance of dye-sensitized solar cells: nanoparticles, nanofibers, hierarchical spheres and ellipsoid spheres. J. Mater. Chem. 22, 7910–7918. Jung, H.G., Kang, Y.S., Sun, Y.K., 2010. Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells. Electrochim. Acta. 55, 4637–4641. Bhatti, K.A., Khan, M.I., Saleem, M., 2019. Analysis of multilayer based TiO2 and ZnO photoanodes for dye-sensitized solar cells. Mater. Res. Exp. 6, 075902. Zhu, M.Y., Dong, Y.Z., Xu, J.L., 2019. Enhanced photovoltaic performance of dye-sensitized solar cells (DSSCs) using graphdiyne-doped TiO2 photoanode. J. Mater. Sci. 54, 4893–4904. Younas, H., Gondal, M.A., Dastageer, M.A., 2019. Fabrication of cost effective and efficient dye sensitized solar cells with WO3-TiO2 nanocomposites as photoanode and MWCNT as Pt-free counter electrode. Ceram. Int. 45, 936–947. Zhang, Y.H., Cai, J.G., Ma, Y.R., 2017. Mesocrystalline TiO2 nanosheet arrays with exposed 001 facets: Synthesis via topotactic transformation and applications in dyesensitized solar cells. Nano. Res. 10, 2610–2625. Zhao, J.H., Yang, Y.N., Cui, C., 2016a. TiO2 hollow spheres as light scattering centers in TiO2 photoanodes for dye-sensitized solar cells: the effect of sphere diameter. J. Alloys Compd. 663, 211–216. Cheng, P.F., Du, S.S., Cai, Y.X., 2013. Tripartite layered photoanode from hierarchical anatase TiO2 urchin-like spheres and P25: a candidate for enhanced efficiency dye
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Gubbala, S., Chakrapani, V., Kumar, V., Sunkara, M.K., 2008. Band-edge engineered hybrid structures for dye-sensitized solar cells based on SnO2 nanowires. Adv. Funct. Mater. 18, 2411–2418. Dou, X., Sabba, D., Mathews, N., Wong, L.H., Lam, Y.M., Mhaisalkar, S., 2011. Hydrothermal synthesis of high electron mobility Zn-doped SnO2 nanoflowers as photoanode material for efficient dye-sensitized solar cells. Chem. Mater. 23, 3938–3945. Wang, Y.F., Li, X.F., Li, D.J., Zhang, X.X., 2015. Controllable synthesis of hierarchical SnO2 microspheres for dye-sensitized solar cells. J. Power Sour. 280, 476–482.
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Design of SnO2@Air@TiO2 hierarchical urchin-like double-hollow nanospheres for high performance dye-sensitized solar cells
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Yinglin Wang, Chao Ma, Chen Wang, Pengfei Cheng , Luping Xu, Li Lv, Hua Zhang School of Aerospace Science and Technology, Xidian University, Xi'an 710126, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Double-hollow TiO2 SnO2 DSSC PCE
The hierarchical double-hollow SnO2@Air@TiO2 microspheres (SATS) have been successfully synthesized by coating of SiO2 and TiO2 on the SnO2 templates layer by layer, and its detailed performances for dye-sensitized solar cells (DSSCs) application also have been studied. SATS microspheres have an average diameter of about 600 nm while the sizes of SnO2 hollow spheres are about 250 nm. The urchin-like SnO2/TiO2 composite microspheres with a large specific surface area of 96.1 m2/g promote dye adsorption and efficient electron transport so as to increase short-circuit photocurrent density to 13.69 mA/cm2. Compared to the traditional simple hollow structure, the double-hollow structure can ulteriorly improve light scattering ability. Hence, it reveals that the DSSC made from SATS nanospheres and P25 exhibits a higher photo conversion efficiency of 6.77%, a 33% improvement compared to the DSSC made from pure P25. The results indicate that the improved photovoltaic performance of SATS DSSCs can mainly attribute to the unique urchin-like double-hollow microstructure and the synergistic effect of SnO2/TiO2 composite material.
1. Introduction During recent years, the energy crisis has greatly affected social development and economic growth. Solar energy resources with the advantages of environmental protection and renewable, have gradually attracted people's attention. Solar cells is an important method to utilize solar energy effectively. In the field of solar cells, dye-sensitized solar cells (DSSCs) were presented for the first time by Grätzel and O'Regan in 1991 and have been the subject of discussion among experts due to the simple fabrication, low cost and relatively high theoretical efficiency (Grätzel, 2001; Hagfeldt et al., 2010; Koo et al., 2008). Up to now, the highest photovoltaic conversion efficiency of DSSCs has exceeded 9% (He et al., 2019). A typical DSSC consists of a dye-sensitized photoanode, an electrolyte and a counter electrode (Park et al., 2000; Liao et al., 2012; Jung et al., 2010; Liu et al., 2012). In which, the photoanode plays a significant role in light harvesting and electron transport, for the dye molecules adsorbed by photoanode can absorb light effectively (Bhatti et al., 2019; Zhu et al., 2019; Younas et al., 2019). Based on the most advantageous photoanode structure type at present, researchers investigate many structures to further improve the efficiency. For example, nanoarrays can be fabricated to improve the electronic transmission capacity; traditional hollow structure is constructed by template method to improve the photon scattering ability;
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compared to the traditional simple hollow structure, the double-hollow structure can ulteriorly improve light scattering ability. Strong acids or alkali can corrode the surface topography of the material and increase the specific surface area (Zhang et al., 2017; Zhao et al., 2016a,b; Cheng et al., 2013). Meanwhile, hierarchical nanostructured materials generally have the advantages of large specific surface area, strong light scattering ability and fast electron transmission, which can effectively improve the photoelectric conversion efficiency (PCE) of DSSC. Manikandan et al. have presented a Eosin-Y sensitized core-shell TiO2-ZnO nanostructure, promising an improvement in the photoelectric conversion efficiency by 57% in comparison with that of the DSSCs based on the pristine TiO2 (Manikandan et al., 2018). And Chen et al. have designed a double-layered structure composed of a TiO2 nanorod overlayer and TiO2 nanoparticle-embedded ZnO nanoflower underlayer, and the results of a series of tests indicate that a promising power conversion efficiency of 8.01% is determined on the DSSC (Chen et al., 2018). Moreover, composite photoanode of SnO2-ZnO and composite photoanode of SnO2-TiO2 provide more possibilities for composite semiconductor oxides as DSSCs photoanode materials (Zinab et al., 2019). Recently, researchers have devoted continuously to study the chemical synthesis, microstructure and photovoltaic characterization of TiO2 materials, since photovoltaic conversion efficiency of TiO2-based
Corresponding author. Tel.: +86 029 81891034; fax: +86 029 81891034. E-mail address: [email protected] (P. Cheng).
https://doi.org/10.1016/j.solener.2019.07.082 Received 29 April 2019; Received in revised form 15 July 2019; Accepted 26 July 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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HCl aqueous solution was added into 33 mL mixed solution (the volume ratio of ethanol to water, 10:1) under continually stirring for 5 min. Then 0.263 g of SnCl2·2H2O were ultrasonically dispersed in the mixed solution. After continual stirring for 1 h, the mixture was transferred and sealed in a 45 mL Teflon-lined autoclave which was then kept at 200 °C for 12 h. The product centrifugation and washed with absolute ethyl alcohol and deionized water three times, respectively. At last, the obtained sample was dried at 60 °C overnight. Preparation of SnO2@SiO2 microspheres: The SnO2@SiO2 microspheres were synthesized by means of an improved Stöber method (Liu et al., 2012). In a typical deposition of SiO2 nanoparticles, 100 mg of asprepared SnO2 were dispersed in a mixed solution of absolute ethyl alcohol (140 mL) and deionized water (35 mL) under vigorously stirring for 30 min. Afterward, 26 wt% ammonia (2 mL) and TEOS (0.3 mL) were added drop by drop, respectively. After ultra-sonication for 10 min, the above solution underwent continuous stirring for 8 h at room temperature. After centrifugation and washing, the precipitates were dried at 60 °C overnight and subsequently annealed at 300 °C for 2 h. Preparation of SnO2@SiO2@TiO2 microspheres (Chen et al., 2011): the DETA (0.05 mL) dispersed in isopropanol (40 mL) and then 100 mg of the SnO2@SiO2 microspheres were added into the above solution under vigorous stirring. To research the influence of precursor TIP on the morphology and property of the product, the dose of TIP was set to 0.1 mL, 0.3 mL, 0.5 mL and 0.7 mL, while the corresponding products were denoted as SATS-1, SATS-2, SATS-3 and SATS-4, respectively. Hence, X mL isopropyl titanate (TIP) was added into the mixed solution and stirred until form a homogeneous solution and then kept at 200 °C for 24 h in a Teflon-lined autoclave. The resulting samples were washed with ethanol and subsequently dried at 80 °C overnight. Preparation of SnO2@Air@TiO2 double-shell microspheres: The SnO2@SiO2@TiO2 microspheres (100 mg) dispersed in NaOH aqueous solution (120 mL, 1.0 M) under continuously stirring and kept at 50 °C for 1 h in a water-bath heater. The products were washed with dilute nitric acid and deionized water several times until a neutral solution, respectively. After dried at 60 °C for 12 h, the SATS-1, SATS-2, SATS-3 and SATS-4 were calcined at 450 °C for 2 h with a heating rate of 3 °C/ min in air.
DSSCs exceeded 10% (Nazeeruddin et al., 2001; Kartini et al., 2004; Zukalova et al., 2005; Shim et al., 2015; Chai et al., 2015; Jung et al., 2015). Although single TiO2 material of DSSCs has good photosensitive property, it is easy to generate electron recombination. Therefore, many researchers have explored other alternative metal oxides such as ZnO, SnO2, Fe2O3 and so on. These metal oxides can replace TiO2 or combine with TiO2 to form composite materials. Among these, SnO2 has two advantages compared to TiO2 for DSSC applications. On the one hand, SnO2 possesses higher electron mobility (100–200 cm2 V−1 S−1) than TiO2 (0.1–1.0 cm2 V−1 S−1), suggesting more efficient transport of photo-induced electrons in SnO2 than in TiO2. On the other hand, SnO2 has a larger band gap (3.5 eV) than TiO2 (3.2 eV), which would create fewer oxidative holes in the valence band, thereby promoting stability of DSSCs (Javed et al., 2019; Bakr et al., 2018; Steffy et al., 2017; Wali et al., 2016; Zhao et al., 2016a,b). However, a 300 mV positive shift of the conduction-band edge of SnO2 with respect to that of TiO2 affects the properties of SnO2 photo-electrodes, such as weaker photon capture ability and less adsorption of the dyes with acidic carboxyl groups (Gao et al., 2014; Pang et al., 2014). Fabrication composites of SnO2 and TiO2, which combine the respective advantages of SnO2 and TiO2, can effectively enhance the power conversion efficiency of DSSCs. In addition, microstructures of materials used in photoanode also have great influence on the photovoltaic performance of DSSCs, such as hollow, urchin-like, core-shell, nanorod, nanowire, nanotube, nanofiber and nanoarray structure. For example, Motlak et al. have presented a Cddoped TiO2 nanofibers structure, and the DSSC based on Cd-doped TiO2 nanofibers has a photoelectron conversion efficiency (2.945%) in comparison to that of pristine TiO2 nanofibers; Hu et al. have designed novel double-layered photoanodes based on porous-hollow TiO2 microspheres and La(OH)3: Yb3+/Er3+ for DSSCs, indicating the photoelectron conversion efficiency of 8.89%; Yu et al. have presented a hierarchical core-shell structure of hollow TiO2 microspheres for DSSCs, showing a high photoelectron conversion efficiency of 7.7% (Motlak et al., 2019; Hu et al., 2019; Yu et al., 2015). Moreover, Wu et al. synthesized the shell-in-shell TiO2 hollow spheres for DSSCs, which showed higher photoelectron conversion efficiency than simple hollow ones (Wu et al. 2011; Jia et al. 2014). Therefore, the rational selection of materials and the construction of reasonable structure can effectively improve the photoelectron conversion efficiency of the battery. In this study, a facile continuous hydrothermal/sol–gel/solvothermal strategy has been adopt for preparing a series materials of SnO2, SnO2@SiO2, SnO2@SiO2@TiO2, and SnO2@Air@TiO2 urchinlike double-hollow nanospheres (SATS). The formed SnO2/TiO2 doublehollow structure can further increase the reflection path of photons and promote the absorption of photons by the dye. The obtained SATS composite material not only shows excellent light scattering ability and dye adsorption ability (contributed by TiO2), but also has fast electron transport rate (contributed by SnO2). This kind of multifunctional composited semiconductor oxide material applied in DSSCs reflects the enhanced performance than that pure P25 based, showing the largest PCE of 6.77% among the comparative DSSCs.
2.2. Fabrication of photoanodes and DSSCs A series of paste were prepared to fabricate SATS and P25 composite photoanode. 0.3 g of powders (SATS-1, SATS-2, SATS-3 or SATS-4 or pure P25 NPs) were dissolved in ethanol under continuously stirring, respectively, and then 1 g of terpinol was dripped into the suspension. 1 g of ethyl cellulose was weighed and slowly poured into ethanol with fully stirred. The two turbid solutions were mixed and stirred for 24 h at room temperature. The preparation method of SATS paste is similar to that of P25 paste. For the preparation of a compact TiO2 layer, the cleaned fluorine doped tin oxide glass substrates (FTO) were immersed into 40 mM TiCl4 aqueous solution in a closed vessel for 30 min at 70 °C. Two kinds of resulting paste (SATS and P25) were successively coated onto the compact layer by the doctor blade method, followed by annealing at 125 °C for 15 min. After immersed into TiCl4 aqueous solution, the asprepared films were then annealed at 450 °C for 30 min in air to remove the organic ingredients and improve the thermal stability (Ito et al., 2008). After cooling to 80 °C, the thick film with double layer structure were sensitized by dye (0.4 mM N-719 ethanol solution) for 24 h at room temperature. The dye-sensitized photoanodes were assembled with the Pt-coated FTO counter electrode and the electrolyte, consisting of 0.05 M LiI, 0.05 M I2, 0.5 M 4-tert-butylpyridine (Aldrich) and 0.6 M 1-propyl-3-methylimidazolium iodide (PMII) in 3-methoxypropionitrile, was then injected into the cell from the edges by capillarity. All of the DSSCs are fabricated following the same process and the effective area of the DSSCs is 0.2 cm2
2. Experiments 2.1. Materials preparation All chemicals including stannous chloride dehydrate (≥98.0%), tetraethyl orthosilicate (TEOS, ≥98%), titanium isopropoxide (TIP, ≥97%), sodium hydroxide (NaOH, ≥96%), hydrochloric acid (36 wt% in water), isopropanol (≥99.7%), ammonia (25 wt% in water), diethylenetriamine (DETA), and absolute ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Company. All the reagents in the experiment were analytical grade and used without further purification. Preparation of SnO2 hollow microspheres: Typically, SnO2 hollow spheres were synthesized by a modified hydrothermal method. 0.8 mL 413
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2.3. Measurements of materials and DSSCs Morphologies and microstructures were examined by field-emission scanning electron microscopy (FESEM, JEOL JSM-7500F, operated at 15 kV) and transmission electron microscopy (TEM, JEOL EM-2100, operated at an accelerating voltage of 200 kV). The images of element distribution could also be measured by SEM. The crystalline structures of the powder were analyzed by X-ray diffraction (XRD) measurement on a diffractometer (Rigaku TTRIII, with Cu Kα1 radiation). N2 adsorption-desorption isotherms were obtained on a Brunner-EmmetTeller (BET) apparatus (Micromeritics Gemini VII apparatus). Photocurrent density-voltage (J-V) characteristics and the incident photon-to-current efficiency (IPCE) spectra of the DSSCs were respectively measured using a Keithley 2450 Source Meter and a Spectral Product Zolix DSC300PA, under one sun AM 1.5G (100 mW/cm2) illumination with a solar light simulator (Newport, Model: 94023A). The reflectance and absorbance spectra were measured on N719-sensitized films and unsensitized films on a Perkin-Elmer UV–Vis spectrophotometer (SHIMADZU 2550). The electrochemical impedance spectroscopy (EIS) were recorded on an electrochemical workstation (Solartron SI1287) with a frequency ranging from 0.1 to 100 kHz at open-circuit voltage. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) measurements were carried out on an electron lifetime with a diode laser light source with variable intensity at 620 nm (PSL-100, EKOJapan). 3. Results and discussion In order to explore the crystal structure of the samples, the X-ray diffraction pattern of SnO2 microspheres and SATS microspheres is characterized, as shown in Fig. 1a. The results reveal that all diffraction peaks of SnO2 microspheres can be perfectly indexed to an cassiterite SnO2 (JCPDS card No. 1-625), and the diffraction peaks are distinguishable and well arranged, indicating the highly crystallinity of SnO2 microspheres. The peaks at 26.9°, 34.2° and 52.1° can be assigned to (1 1 0), (1 0 1) and (2 1 1) crystal planes of cassiterite SnO2, respectively. In addition, the diffraction peak of SATS microsphere not only matches the diffraction peak of SnO2 (JCPDS card No. 1–625), but also matches the characteristic peak of standard anatase TiO2 (JCPDS card No. 71-1168), and this indicates that SnO2 exists in the samples and are not affected by subsequent chemical reactions. The diffraction peaks of SATS microspheres at 25.5° and 48.0° are well consistent with (1 0 1) and (2 0 0) crystal planes of anatase TiO2. In particular, the main peak corresponds to the (1 0 1) lattice plane of standard anatase TiO2, and the diffraction peak is very sharp, indicating a high degree of crystallinity. Besides, no other impurities can be found in the XRD spectrum. In order to have a deeper understanding of the specific surface area of SATS-1, SATS-2, SATS-3 and SATS-4 materials, nitrogen adsorption desorption technology has been used to measure relevant parameters. Fig. 1b shows the isothermal curves of four different materials. The type IV isothermal curves of the four materials can be obviously observed, indicating that the outer layer of the four materials has mesoporous structure. In addition, the isothermal curves of SATS-1, SATS-2, SATS-3 and SATS-4 all have H3 type hysteresis loops, indicating that four materials have narrow pores assembled by a large number of nanosheets. Brunauer-Emmett-Teller (BET) method has been used to calculate the specific surface area of SATS-1, SATS-2, SATS-3 and SATS-4 are respective 69.1, 76.3, 96.1 and 78.3 m2/g. The scanning electron microscopy (SEM) on the products of each step has been utilized to characterize the structure and morphology of SATS microspheres in detail. As depicted in Fig. 2a, SnO2 microspheres have good dispersion, and are not approximately uniform. The SnO2 microspheres act as good spherical template for subsequent sol-gel and hydrothermal experiments. Moreover, the surface of SnO2 microspheres is not very smooth. As shown in Fig. 2b, SiO2 nanoparticles are
Fig. 1. (a) XRD patterns, Anatase TiO2; ● Cassiterite SnO2, (b) N2 adsorption and desorption isotherms of the SATS-1, SATS-2, SATS-3 and SATS-4.
uniformly coated on the surface of SnO2 microspheres by sol-gel method. Meanwhile, the diameter of obtained SnO2@SiO2 microspheres increases a lot. In addition, SiO2 nanoparticles have good light transmission (Li et al., 2019), the SiO2 layer is visible in Fig. 2b and the thickness of SiO2 layer is about 80 nm. The as-prepared microspheres still present good dispersion and very smooth surface. The magnified image in Fig. 2c suggests that the surface of SnO2@SiO2@TiO2 microspheres is assembled by densely packed nanosheets of TiO2 with about 100 nm in length. The short nanosheets of different SATS microspheres are intertwined and connected with each other, which is conducive to the transmission of electrons between SATS microspheres. Fig. 2d shows a broken SATS microsphere and its internal and external microstructures. The location of the fracture shows a hollow layer about 80 nm thick, which matches with the thickness of SiO2 layer, and a spherical core with rough surface indicates that NaOH aqueous solution washes away SiO2 layer successfully and roughens the SnO2 surface. In order to better understand the synthesis details of SnO2@Air@TiO2 microspheres, the specific synthesis process of SnO2@Air@TiO2 microsphere has been illustrated in Fig. 3. The structure characteristics of SATS-1 SATS-2 SATS-3 and SATS-4 microspheres have been further studied, and the TEM image of the samples has been carried out, as shown in Fig. 4a–d. It reveals that the SnO2 microspheres in the core position present dark 1color, however,
1 The photo of the transmission electron microscope is a black and white picture, and the color depth of the core position indicates the thickness of the
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Fig. 2. SEM images of as prepared (a) SnO2 microspheres, (b) SnO2@SiO2 microspheres and (c, d) SnO2@Air@TiO2 double–shell microspheres.
Fig. 3. Schematic formation process of the SATS structure.
SATS. The nanocrystalline structure of SATS microspheres also has been studied by high resolution TEM characterization. Fig. 4e clearly shows the lattice fringes of TiO2 nanoparticles and SnO2 nanoparticles, the lattice distances of 0.238 nm and 0.336 nm well correspond to the (0 0 4) facets of anatase TiO2 and the (1 1 0) facets of cassiterite SnO2, respectively. Additionally, the element mapping image measurement based on energy dispersive X-ray detector reveals the presence and distribution of O, Ti, Sn in a single SATS microsphere, as can be seen from Fig. 4f–h. It has been presented that Ti elements are distributed in the outer shell of SATS microsphere, Sn elements are mainly distributed in the inner shell of SATS microsphere, and O elements are distributed in the whole SATS microsphere. According to the dispersion of elements such as: O, Ti and Sn, the double-hollow structure can be concluded as a composition of SnO2 (inner shell part) and TiO2 (outer shell part). From
their central position show some light transmittance, which indicates the hollow structures of the SnO2 microspheres. Although the SnO2 hollow microspheres are not uniform in size, their relatively smooth surfaces are similar. As revealed in each TEM Fig. 4a–d, there is a hollow layer between the SnO2 hollow microsphere and the outer layer of TiO2. And the SnO2 hollow microspheres can move freely within the inner space of the shell layer of TiO2, indicating that the SiO2 layer coated on the surface of the SnO2 hollow microspheres has been washed away by alkali without damaging the double-shell and urchin-like structures. In addition, as can be seen from Fig. 4a–d, with the increase of TiO2 content, the color of TiO2 layer gradually exhibits darker and thicker, which also confirms the successfully growth of TiO2 crystal on
(footnote continued) core. 415
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Fig. 4. TEM images of the (a) SATS-1, (b) SATS-2, (c) SATS-3 and (d) SATS-4 microspheres, (e) HRTEM image of the SATS-0.5 microspheres, (f-h) Elemental mapping of individual SATS.
the figures above we can see that TiO2 random wire web on SnO2 microspheres. This TiO2 random wire web leads to a higher specific surface area of SATSs, and this is consistent with the results of Fig. 1b. As UV–Vis diffuse reflectance spectrum can reflect photon scattering ability of photoanode film, the UV–Vis diffuse reflectance spectrum has been carried out in Fig. 5a. As shown in the UV–Vis diffuse reflectance spectra, the five different photoanode films under the condition of nonimpregnated dye have been recorded. Because all the sizes of SATS microspheres are at the nanometer level, the highest photon reflectivity has been achieved in the small wavelength range of 400–450 nm (Huang et al., 2010). However, when the wavelength exceeds 450 nm,
the photon scattering ability gradually weakens, indicating that the long-wavelength photon has a weak scattering ability in the small-sized microspheres. Furthermore, SATS-4 film shows the strongest photon reflectance in the wavelength range of 400–450 nm, indicating that the TiO2 outer layer with special nanosheets morphology may help improve the photon scattering ability. It can be seen from Fig. 6a that photons reflected through the FTO conducting glass in the double-hollow structure of SATS microspheres for many times, which can prolong the routine of photons in SATS microspheres and enable the dye molecules to fully absorb photons. It is not difficult to find that the reflection path of photons in double-hollow structure is longer than that in traditional simple hollow structure. Fig. 6b shows that FTO, N719, I−/I3−, SnO2 and TiO2 have different energy level positions and forbidden band widths. It can be found that the N719 dyes on the surface of TiO2 absorb photons, change from the ground state to the excited state, and then release photogenerated electrons. Electrons travel along the conduction band from the highenergy material to the low-energy material. Meanwhile, the electrons pass through TiO2 and SnO2 successively and finally arrive at FTO. In order to explore the influence of the thickness of TiO2 layer on photoelectric performance of SATS microspheres, 0.1 mL, 0.3 mL, 0.5 mL and 0.7 mL of isopropanol titanium have been used for coating SnO2@SiO2 microspheres as outer layers, which worked as comparative experiments. Fig. 7 shows the J-V curves of SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 DSSCs measured under sun illumination (AM 1.5 G, 100 mWcm−2). As shown in Table 1, the four critical parameters of J-V curves have been listed such as: open circuit voltage, short circuit current density, filling factor and photoelectric conversion efficiency, which can effectively reflect the photoelectric performance of these DSSCs. The performance parameters in Table 1 can be calculated from the five J-V curves. For the five cells, the Jsc and the η of SATS-3 DSSC
Fig. 5. UV-Vis reflectance spectra of the five films without N719 dye loading. 416
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Fig.7. J-V curves of SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 DSSCs measured under sun illumination (AM 1.5 G, 100 mW cm−2).
SnO2@Air@TiO2 DSSCs for the maintenance the Voc are (i) higher dyeloading on account of its higher specific surface area than the P25 and (ii) SnO2@Air@TiO2 micro-structure improved charge transport characteristics compared with the P25 based devices. High specific surface area photoanode materials lead to high dye adsorption amount, therefore SATS film, which has a surface area of 96.1 m2 g−1, may be an excellent photoanode material. The amount of N719 dye adsorbed on these four films was investigated by thorough desorption in 1 mM NaOH solution and then measuring the UV–Vis absorption spectra of the resulting solutions. The dye loading of SATS-3 was 1.6, 1.4, 1.3 and 1.2 times higher than that of P25, SATS-1, SATS-2 and SATS-4 (see Table 1), respectively. The higher dye loading of SATS3 was ascribed to the larger specific surface area of the film. In fact, a larger amount of dye adsorption can capture more photons, resulting in more photogenerated electrons, which provides the possibility of a larger photocurrent. However, photogenerated electrons are trapped by defects during transmission, and whether the photogenerated electrons can be smoothly derived is determined by the electron transport characteristics of the photoanode. Therefore, the photocurrent is affected by the combination of the amount of dye adsorption (capacity to capture photons) and the electron transport capability of the photoanode. Fig. 8 shows normalized IPCE spectra of the obtained DSSCs. All the five DSSCs photoanode films use the same N719 dye. Affected by the highest absorption peak of N719 dye, the most powerful absorption peak of the five cells is displayed at the position of 525 nm of light wavelength, so the highest value of IPCE is obtained at 525 nm for all the five cells. Compared with pure P25 DSSC, SATS DSSCs have higher IPCE values in most of spectral range mainly due to the improvements of photon scattering ability, electron transmission rate and specific surface area. The IPCE value of SATS-3 DSSC reaches the highest value of 70% when the incident wavelength is 525 nm. However, excessive TiO2 of SATS-4 DSSC leads to the decrease of IPCE because of electron recombination. To further understand the possible reasons for the enhanced photoelectric performance, the electrochemical impedance spectrum (EIS) has been measured in the dark circumstance under a forward bias of −0.8 V to investigate the charge-transfer process (Wang et al., 2015). The Nyquist impedance spectra and equivalent circuit of SATS-1, SATS2, SATS-3, SATS-4 and pure P25 DSSCs are shown in Fig. 9, and the corresponding parameters are shown in Table 1, respectively. The smaller and larger semicircles in the Nyquist impedance spectra are attributed to the charge transfer at the counter electrode/electrolyte interface and the photoanode material/dye/electrolyte interface, respectively. The sheet resistance (Rs) of substrate, charge transfer resistance of the counter electrode (R1), and recombination resistance
Fig. 6. (a) Reflection mechanism diagram of photons inside SATS microspheres, (b) FTO, N719, I−/I3−, SnO2 and TiO2 energy levels and electron transfer diagrams.
present the largest value. The reason is that the double-hollow structures of SATS-3 greatly increases the propagation path of photons and the suitable specific surface area promotes the absorption of photons by dyes, both of these two factors improve the photoelectric performance of the DSSC (Shao et al., 2012; Naveen et al, 2012a,b). Additionally, both the Jsc and the η of SATS-1 DSSC are lower than that of P25 DSSC. In SATS-1 microspheres, the content of TiO2 is much lower than that of SnO2. Compared with TiO2, SnO2 has better conductivity but weaker photosensitivity. Therefore, the tiny amounts of TiO2 with the relatively excessive SnO2 will reveal the weak photoelectric performance of the battery (Fig. S1, Table S1). In the case of SATS-4 DSSC, the lower Jsc and η compared with SATS-3 DSSC can be ascribed to the excessive TiO2 accelerates electron recombination. The open circuit voltage of P25 DSSC is higher than that of SATS DSSCs, which is much related with the 300 mV positive displacement of energy band in SnO2 relative to TiO2. Furthermore, The Voc values obtained using double-hollow SnO2@Air@TiO2 DSSCs are as high as P25 (Pure TiO2), similar or higher Voc values were previously obtained with surface modifications by TiO2 (Gubbala et al, 2008; Dou et al, 2011). Two possible reasons to 417
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Table 1 Specific photovoltaic parameters and EIS parameters of the DSSCs based on SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 photoanodes. DSSC
Jsc(mAcm−2)
Voc(V)
FF(%)
RS(Ω)
R1(Ω)
R2(Ω)
η(%)
Dye adsorption(10−7mol cm−2)
P25 SATS-1 SATS-2 SATS-3 SATS-4
10.55 10.25 10.99 13.69 11.15
0.737 0.725 0.725 0.726 0.730
65.5 67.6 64.5 68.1 64.9
16.0 15.8 15.6 15.9 16.1
9.8 9.6 9.4 9.5 9.8
62.1 90.5 98.6 115.5 107.0
5.09 5.02 5.14 6.77 5.28
0.80 0.96 1.05 1.33 1.08
Fig. 8. Normalized IPCE spectra of SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 DSSCs.
Fig. 10. (a) Electron diffusion coefficient and (b) electron lifetime as a function of applied voltage for the five cells.
electron lifetime (τe) of the SATS-1, SATS-2, SATS-3, SATS-4 and pure P25 DSSCs. The photoelectric performances of the five DSSCs have been further explored. As the light intensity increases, open circuit voltage also increases, and the electron diffusion coefficients curves show an increasing trend (Fig. 10a), while the electron lifetime curves show a decreasing trend (Fig. 10b). What's more important is that the electron diffusion coefficients are affected by the micro-morphology of membrane, and the electron lifetime is related to the structure of membrane. As depicted in the electron diffusion coefficient curves, the Dn curves of the four SATS DSSCs are above the pure P25 DSSC, and all the Dn values of SATS-3 DSSC are the maximum, which indicates that SATS DSSCs have better electron diffusion characteristics. The contiguous microspheres have better interfacial conductivity and high crystallinity, which will effectively suppress electron recombination and the dispersion of P25 nanoparticles inhibits electron transport. What's more, the SnO2 microspheres in the core position will also facilitate the electron diffusion and transport due to their higher electron mobility. As a result, the electron lifetime curves reveals that the τe curves of SATS
Fig. 9. Nyquist plots from EIS of the five cells. The inset illustrates the equivalent circuit simulated to fit the impedance spectra.
(R2) were analyzed by Z-view software using an equivalent circuit containing a constant phase element (CPE) and resistances (R; Fig. 9, inset). As the SATS DSSCs and pure P25 DSSC are based on the same electrolyte, N719 dye, FTO conductive glass and Pt electrode, R1 and Rs in the equivalent circuit of the five cells are the same, as presented in Table 1. And SATS-3 DSSC presents the largest R2 of 115.5 Ω, which is higher than those of other SATS DSSCs and pure P25 DSSC, revealing a lower charge recombination at the dye/electrolyte interface (Wang et al., 2005; Buonsanti et al., 2011; Ahn et al., 2014). As shown in Fig. 10, a laser emitter with adjustable intensity and constant wavelength of 620 nm is used as the light source, and the open circuit voltage (Voc) is controlled by changing the light intensity (by PSL-100, EKO-Japan, stepwise light intensity from small to large, in the black box), which affects the electron diffusion coefficients (Dn) and 418
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DSSCs are above the pure P25 DSSCs, and all the τe values of SATS-3 DSSCs are the maximum, and the reason is that the electron recombination probability of SATS DSSCs is smaller than that of P25 DSSC.
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4. Conclusions In summary, the SnO2@Air@TiO2 nanospheres (SATS) have been successfully prepared via a facile hydrothermal treatment using a layerby-layer construction strategy. The SATS microspheres have combined two advantages of SnO2/TiO2 composite material and double-hollow structure. On the one hand, SnO2 hollow spheres can improve the electron transfer rate, and TiO2 nanosheets of the outer shell can promote the adsorption of dyes. On the other hand, the double-hollow structure can increase the number of reflections of photogenerated electrons in the inner and outer hollow layer to improve the outstanding light scattering ability. An overall PCE of 6.77% has been achieved for the DSSC based on the SATS-3 photoanode and is far superior to that of P25 based DSSC (5.09%). The enhanced photoelectric performance is mainly due to the synergistic effect of the SnO2/TiO2 composite and double-hollow microstructure. Therefore, SATS presents a superior performance and ample potential to serve as decent alternatives to replace the traditional P25 NPs in the highly efficient DSSCs. Acknowledgements This work was supported by the National Nature Science Foundation of China (Nos. 61604116 and 61803289), China Postdoctoral Science Foundation (No. 2017 M623120 and 2019 M653552). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.07.082. References Grätzel, M., 2001. Photo electrochemical cells. Nature 414, 338–344. Hagfeldt, A., Boschloo, G., Sun, L., Kloo, L., Pettersson, H., 2010. Dye-Sensitized Solar Cells. Chem. Rev. 110, 6595–6663. Koo, H.J., Kim, Y.J., Lee, Y.H., Lee, W.I., Kim, K., Park, N.G., 2008. Nano-embossed hollow spherical TiO2 as bifunctional material for high-efficiency dye-sensitized solar cells. Adv. Mater. 20, 195–203. He, X., Guo, Y., Liu, J.H., 2019. Fabrication of peanut-like TiO2 microarchitecture with enhanced surface light trapping and high specific surface area for high-efficiency dye sensitized solar cells. J. Power Sour. 423, 236–245. Park, N.G., Lagemaat, J.V., Frank, A.J., 2000. Influence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline TiO2 solar cells: A study by electrical impedance and optical modulation techniques. J. Phys. Chem. B. 104, 2044–2052. Liao, J.Y., He, J.W., Xu, H., Kuang, D.B., Su, C.Y., 2012. Effect of TiO2 morphology on photovoltaic performance of dye-sensitized solar cells: nanoparticles, nanofibers, hierarchical spheres and ellipsoid spheres. J. Mater. Chem. 22, 7910–7918. Jung, H.G., Kang, Y.S., Sun, Y.K., 2010. Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells. Electrochim. Acta. 55, 4637–4641. Bhatti, K.A., Khan, M.I., Saleem, M., 2019. Analysis of multilayer based TiO2 and ZnO photoanodes for dye-sensitized solar cells. Mater. Res. Exp. 6, 075902. Zhu, M.Y., Dong, Y.Z., Xu, J.L., 2019. Enhanced photovoltaic performance of dye-sensitized solar cells (DSSCs) using graphdiyne-doped TiO2 photoanode. J. Mater. Sci. 54, 4893–4904. Younas, H., Gondal, M.A., Dastageer, M.A., 2019. Fabrication of cost effective and efficient dye sensitized solar cells with WO3-TiO2 nanocomposites as photoanode and MWCNT as Pt-free counter electrode. Ceram. Int. 45, 936–947. Zhang, Y.H., Cai, J.G., Ma, Y.R., 2017. Mesocrystalline TiO2 nanosheet arrays with exposed 001 facets: Synthesis via topotactic transformation and applications in dyesensitized solar cells. Nano. Res. 10, 2610–2625. Zhao, J.H., Yang, Y.N., Cui, C., 2016a. TiO2 hollow spheres as light scattering centers in TiO2 photoanodes for dye-sensitized solar cells: the effect of sphere diameter. J. Alloys Compd. 663, 211–216. Cheng, P.F., Du, S.S., Cai, Y.X., 2013. Tripartite layered photoanode from hierarchical anatase TiO2 urchin-like spheres and P25: a candidate for enhanced efficiency dye
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