- Email: [email protected]
Facile synthesis of SnO2 coated urchin-like TiO2 hollow microspheres as efficient scattering layer for dye-sensitized solar cells
Journal of Power Sources 336 (2016) 143e149
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile synthesis of SnO2 coated urchin-like TiO2 hollow microspheres as efficient scattering layer for dye-sensitized solar cells Fengyan Xie a, b, Yafeng Li a, b, **, Jie Dou a, b, Junxiu Wu a, b, Mingdeng Wei a, b, * a b
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China Institute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SnO2 coated urchin-like TiO2 hollow microspheres were prepared via a facile method. The microspheres showed superior light scattering ability and electron diffusibility. Cell based on the microspheres scattering layer achieved an efficiency of 8.33%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2016 Received in revised form 12 October 2016 Accepted 19 October 2016
SnO2 coated urchin-like TiO2 hollow microspheres are prepared via a facile one-step hydrothermal method by using titanium tetrabutoxide (TBOT) as titanium source. The synthesized products are characterized by XRD, SEM and TEM measurements. It's found that the as-prepared microspheres with a diameter of 500e800 nm are consisted of densely interconnected nanowires and possessed a high specific surface area of 134.92 m2 g1. Moreover, HRTEM and element mapping results show that the surface of urchin-like microsphere is coated by lots of SnO2 nanoparticles. When used as scattering layer for dye-sensitized solar cells, the microspheres show good dye adsorption capability, superior light scattering and electron diffusibility, leading to a higher photovoltaic conversion efficiency of 8.33%, which is a 28.4% enhancement comparable to that of bare nanocrystalline TiO2 (Dyesol 18NR-T, 6.49%). © 2016 Elsevier B.V. All rights reserved.
Keywords: TiO2/SnO2 microspheres Hydrothermal route Light scattering layer Dye-sensitized solar cells Efficiency Photovoltaic property
1. Introduction Dye-sensitized solar cells (DSSCs) have attracted extensive research since the remarkable breakthrough reported by M. Gr€ atzel
* Corresponding author. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China. ** Corresponding author. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China. E-mail addresses: [email protected] (Y. Li), [email protected] (M. Wei). http://dx.doi.org/10.1016/j.jpowsour.2016.10.061 0378-7753/© 2016 Elsevier B.V. All rights reserved.
in 1991 [1] owing to its low cost, high efficiency and simple fabrication. In general, DSSCs are composed of a nanocrystalline semiconducting oxide film as the photoanode, a sensitizing dye, an 3þ 2þ electrolyte containing an redox couple, such as I/I 3 , Co /Co , etc., and a counter electrode [1e4]. A lot of researchers focus their attention on the photoanode materials since photoanode plays an important role in improving efficiency of DSSCs, and various metal oxides, such as TiO2, SnO2, ZnO, WO3, Nb2O5, etc., have been used as the electrode film for dye attachment and carrier transport [3e13]. Of these metal oxides, TiO2 has been the most promising oxide so
144
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
far owing to its suitable band alignment with various organic and inorganic dyes, controllable various morphology and stability. However, the further use of TiO2 is also limited due to the unfavorable influence on performance caused by the crystal structure, crystalline size and morphology of the TiO2, as well as the rapid combination of photo-generated electrons and oxidative holes. Thus, extensive efforts have been made to improve the photovoltaic performance of TiO2-based DSSCs. One promising strategy is to composite TiO2 with other metal oxides such as ZnO, SnO2 and Nb2O5, to increase the electron transport rate of photoanodes [14e17]. Among those composites, TiO2/SnO2 composite draw more attention because of the higher electron mobility of SnO2 (100e200 cm2 V1$s1) than TiO2 (0.1e1 cm2 V1$s1) and a relative larger band gap (3.6 eV) than TiO2 (3.2 eV) [18,19]. The higher electron mobility of SnO2 can facilitates electron injection from photo-excited dye molecules to a transparent conductive oxide current collector, which will increase the electron transport rate and improve device performance. Furthermore, in the case of SnO2, its larger band gap (3.6 eV) can suppress the production of oxidative holes in the valence band of SnO2 under UV illumination, thereby minimizing the dye degradation rate and improving the long-term stability of DSSCs [18,19]. On the other hand, films based on nanocrystalline TiO2 results in negligible light scattering, which led to the low photon absorption [20,21]. Therefore, the scattering materials are designed to improve the light harvesting ability of photoanode. The introduction of scattering materials could improve the light harvesting via increasing the optical path length and reabsorb of light irradiating into the TiO2 film. There are several ways to incorporate the light scattering materials. For example, a certain proportion of 1D or 2D nanostructures, such as nanowires, nanorods and nanosheets could mix with the nanocrystalline TiO2 to increase the light scattering, or they can simply be deposited on the top of TiO2 film as a scattering layer [22e24]. Moreover, the 3D TiO2 nanostructures, such as mesoporous TiO2 beads, nanoporous TiO2 spheres or aggregates and hollow microspheres were widely constructed to play the role of scattering layer [25e28]. However, the lower surface area as well as the inefficient electron transport ability of these scattering materials limits their further applications in DSSCs. Therefore, it is greatly desired to design unique nanostructures that can provide large surface area and efficient electron transport pathways, as an efficient light scattering layer. In this work, a novel TiO2/SnO2 composite was first prepared by a facile method and applied in DSSCs as efficient scattering layer. Owing to the unique SnO2 nanoparticles coated nanowires/microsphere structure, the urchin-like TiO2 hollow sphere exhibits lager specific surface area, superior light scattering capacity and electron diffusibility, resulting in significant improvement of efficiency when applied in DSSCs as efficient scattering layer. 2. Experimental section 2.1. Synthesis and characterization of the samples All reagents were of analytical grade and were used without further purification. In a typical process, 1 mmol K2SnO3$3H2O was added to a solution of 20 mL isopropyl alcohol and 5 mL acetylacetone (acac) under vigorous stirring. Then, 1 mL titanium tetrabutoxide (TBOT) was dropped into the solution. After stirring for 20 min at ambient condition, the suspension yellowish mixture solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and kept in an oven at 200 C for 12 h. After cooling to room temperature, the light yellow precipitate was collected by centrifugation, and washed with ethanol, and then dried at 70 C
overnight. The TiO2/SnO2 microspheres were obtained after annealing at 500 C for 2 h. TiO2 microspheres had been synthesized by the above method except without adding K2SnO3$3H2O for comparison. In addition, SnO2 microspheres were also synthesized according to the previous report [16] as a reference sample. X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV using the Cu Ka radiation (1.5418 Å). SEM images were presented using an S4800 instrument (Hitachi). TEM images, Energy disperse spectrum (EDS) and elemental mapping were performed by a Tecnai G2 F20 instrument (FEI, USA). N2 adsorptiondesorption was measured on a Micromeritics ASAP 2020 instrument (Micromeritics, Norcross, GA, USA). UV-Vis spectra were investigated using a Lambda-950 (Perkin-Elmer). 2.2. Fabrication and measurement of DSSCs Different pastes of each material for the screen printing were prepared according to our previous works [29,30]. Typically, the mixture composed of powder of TiO2/SnO2 microspheres (TiO2 microspheres or SnO2 blocks), ethyl cellulose (10 wt%, Kanto Chemical Co.), a-terpineol (5 wt%) and ethanol was suitably dispersed using a homogenizer (Ultraturrax T25) to obtain a homogeneous paste. Then the paste was concentrated using an evaporator to a final concentration of 20 wt%. A transparent layer of ~10 mm and a scattering layer of ~4 mm were obtained by multiple screen printing on FTO with the Dyesol 18NR-T (denoted as DSL-TiO2) paste and TiO2/SnO2 microspheres (TiO2 microspheres or SnO2 microspheres) paste, and followed by calcined at 525 C for 2 h in air. After cooling to the room temperature, a 50 mM aqueous solution TiCl4 was dropped onto the film and kept at 70 C for more than 30 min. After that, the films were calcined at 450 C for 30 min. The prepared films were immersed into a solution containing 0.5 mM N719 dye in acetonitrile and isopropyl alcohol (v/v ¼ 1:1) for 24 h, and then were incorporated into a thinlayer, sandwiched solar cell with an active area of 0.25 cm2. The Pt-coated FTO glass as a counter electrode was prepared by dropping H2PtCl6 (5 mM) solution on the FTO glass followed by heating at 400 C for 30 min. A polyethylene spacer (ca.40 mm thickness) was used to prevent the cell from short-circuiting. The electrolyte was composed of 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide, 0.05 M I2, and 0.1 M LiI in acetonitrile with 0.5 M 4-tertbutylpyridine. Current-Voltage curves (I-V) of solar cells were carried out using a source meter (Keithley 2400). A PEC-L11 AM 1.5 solar simulator (Peccell, with a 1000 W Xe lamp and an AM 1.5 filter) was used as the light source (100 mWcm2). The spectra of IPCE were collected using a PEC-S20 (Peccell, Technology Co. Ltd.) Electrochemical Impedance Spectroscopy (EIS) were conducted using an IM6 (Zahner). And the intensity of the modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) were also conducted using IM6 with light-emitting diodes (LED, l ¼ 457 nm). The small amplitude is 5% of the applied voltage, and the frequency range was from 100 mHz to 3 kHz. 3. Results and discussion Fig. 1a shows the XRD patterns of the samples of TiO2/SnO2 microspheres and TiO2 microspheres. The results reveal that all diffraction peaks of TiO2 microspheres can be indexed to an anatase TiO2 (JCPDS 84e1285), while the additional XRD diffraction peaks of TiO2/SnO2 microspheres in comparison to TiO2 was caused the existence of cassiterite phase SnO2 (JCPDS 72e1147), indicating the TiO2/SnO2 nanocomposites were successfully prepared. The higher surface area of materials used in DSSCs are favorable
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
145
Fig. 1. (a) XRD patterns, (b) N2 adsorption and desorption isotherms and pore-size distribution (inset) of TiO2/SnO2 microspheres and TiO2 microspheres.
for the dye adsorption, thus the surface area of fabricated TiO2/SnO2 microspheres was investigated. The Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the TiO2/SnO2 microspheres and TiO2 microspheres were determined using nitrogen adsorption and desorption isotherms. As shown in Fig. 1b, the BET surface area of TiO2/SnO2 microspheres was 134.92 m2 g1, about three times of that of TiO2 microspheres (42 m2 g1) obtained from the same condition. The pore size distribution of TiO2/SnO2 microspheres and TiO2 microspheres are also shown in inset of Fig. 1b. The results denote the TiO2/SnO2 microspheres have a smaller average pore size distribution (8 nm) than that of TiO2 microspheres (10 nm), which might be caused by the densely interconnected TiO2 nanowires. This mesoporous microspheres structure could form passageways to facilitate the electrolyte diffusion and transport during the oxidation/reduction process of electrolyte. Fig. 2 shows SEM images of the as prepared TiO2/SnO2 microspheres. As depicted in Fig. 2a, urchin-like microspheres with a diameter range of 500e800 nm are clearly observed. The magnified image in Fig. 2b suggests that these microspheres are assembled by densely packed nanowires. The morphology of as prepared TiO2 microspheres and SnO2 microspheres are shown in Fig. S1 (ESI). As can be seen from it, TiO2 microspheres almost maintain spherical with rough surface composed of nanoparticles, and SnO2 microspheres show an aggregated spherical morphology composed of SnO2 nanoparticles. As previously reported, mesoporous microspheres with a diameter of 550 ± 50 and 830 ± 40 nm possessing high scattering properties, which is ideal for scattering layer in DSSCs [31]. Hence, the urchin-like TiO2/SnO2 microspheres with a diameter range of 500e800 nm would be expected to exhibit a superior scattering ability applied in DSSCs.
To confirm the morphology and component of the SnO2 coated TiO2 microspheres, TEM measurements were carried out. Fig. 3a clearly indicates these urchin-like spheres have hollow structure and the shell thickness is around 200 nm. At the same time the diameter of hollow spheres is found to be in the range of 500e700 nm, which is in good agreement with the SEM results and makes it suitable for scattering visible light. A TEM image taken from the shell of a microsphere in Fig. 3b reveals that the shell of the microsphere could be divided into two parts, one is an inner shell consisted of nanoparticles while the other one is an outer shell consisted of nanowires. Fig. 3c, d shows the HRTEM images that obtained from two portions marked with circle in Fig. 3b, respectively. Fig. 3c clearly shows the lattice fringes of nanoparticles deposited on nanowires were about 0.337 nm, corresponding to the (110) facets of SnO2. The lattice fringe of nanowires in Fig. 3d was around 0.355 nm, which could be ascribed to the (101) facets of TiO2. Furthermore, the selected area electron diffraction (SAED) pattern of a microsphere (inset in Fig. 3d) could be indexed to (101), (200) and (211), respectively, indicating a highly crystalline anatase phase of TiO2, which is consistent with the XRD analysis discussed above. In addition, the elemental mapping measurement in Fig. 3e reveals the presence and uniform distribution of O, Ti, Sn in TiO2/ SnO2 microsphere. Two obvious peaks of Sn element present in EDS spectrum (Fig. S2, ESI) also confirm the results discussed above. Therefore, the urchin-like SnO2/TiO2 hollow microspheres have been successfully constructed. To prove the superior light scattering ability of the TiO2/SnO2 microspheres, four kinds of cells were prepared for comparison. The cells without and with SnO2, TiO2, and TiO2/SnO2 scattering layers were named as Cell 1, 2, 3, and 4, respectively. The photovoltaic performance of solar cells was investigated. IeV curves of
Fig. 2. (a, b) SEM images of as prepared TiO2/SnO2 microspheres.
146
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
Fig. 3. (a, b) TEM images, (c, d) HRTEM images and the SAED patterns (inset) of TiO2/SnO2 microspheres; (e) elemental mapping of TiO2/SnO2 microspheres.
the cells were plotted in Fig. 4a, and the corresponding parameters are displayed in Table 1. It can be found that the fill factor FF and open-circuit voltage Voc of the four different kinds of cells were similar. For DSL-TiO2 photoanode, it gave a short-circuit current Jsc of 12.39 mA cm2 and an efficiency of 6.49%. When scattering layers were employed, all the cells exhibited higher Jsc and efficiency than those of the cell based on bare DSL-TiO2. The improved efficiency was mainly caused by the increased Jsc due to the enhanced light-
harvesting ability of photoanode. When the as-prepared TiO2/SnO2 microspheres were used as scattering layer, the cell demonstrated the highest Jsc of 15.23 mA cm2, and the power conversion efficiency of 8.33% was obtained, which was an enhancement of 28.4% in comparison to that of DSL-TiO2. In fact, the superior light scattering ability can be evidenced by Incident photon-to-electron conversion efficiencies (IPCE). The IPCE spectra and normalized IPCE spectra are shown in Fig. S3 (ESI) and Fig. 4b, respectively. As
Fig. 4. (a) IeV curves and (b) normalized IPCE spectra of DSSCs based on DSL-TiO2 transparent layer with and without different scattering layers.
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
147
Table 1 Photovoltaic parameters of DSSCs based on DSL-TiO2 transparent layer with and without different scattering layers. Cell
Photoanode
Thickness [mm]
1 2 3 4
DSL-TiO2 DSL-TiO2 þ SnO2 scattering layer DSL-TiO2 þ TiO2 scattering layer DSL-TiO2 þ TiO2/SnO2 scattering layer
13.8 14.1 14.2 14.0
± ± ± ±
0.5 0.5 0.5 0.5
Jsc [mA cm2]
Voc [V]
FF
h[%]
Dye loading [107 mol/cm2]
12.39 12.90 14.37 15.23
0.76 0.76 0.76 0.77
0.69 0.71 0.69 0.71
6.49 6.99 7.59 8.33
1.23 1.00 1.13 1.45
shown in Fig. 4b, the cells based on DSL-TiO2 with scattering layers almost exhibit higher IPCE than that bare DSL-TiO2 in the wavelength range of 300e800 nm, and especially in the longer wavelength range from 450 to 800 nm. Notably, cell with TiO2/SnO2 scattering layer has similar IPCE to that cell with TiO2 scattering layer at shorter wavelength (300e600 nm) might be ascribed to the similar dye loading capacities of them. But the former exhibits the highest IPCE at longer wavelength of 600e800 nm, it might be attributed to the superior light harvesting ability induced by light scattering effect of the nanowires/microspheres structure. To further prove the superior light scattering ability of the TiO2/ SnO2 microspheres, the UV-Vis diffuse reflectance of the photoelectrodes composing DSL-TiO2 transparent layer and different scattering layers with the same thicknesses (ca. 14 mm) were investigated. The results shown in Fig. 5a denote that the TiO2/SnO2 microspheres exhibited stronger light reflectance in the wavelength range of 400e800 nm than the other two films, which was consistent with the IPCE results. And film with SnO2 scattering layer showed a lower light reflectance could be owed to the poor light scattering ability of large block morphology aggregated by SnO2 nanoparticles. Moreover, the UV-Vis absorption spectra shown in Fig. 5b indicate that owing to the higher specific surface area, the photoanode consisted of DSL-TiO2 and TiO2/SnO2 microspheres scattering layer can absorb more dye molecules than the other two films under the same thickness. In fact, the higher BET surface area of TiO2/SnO2 microspheres provided sufficient dye-loading of 1.45 107 mol cm2, which was obviously higher than the others, leading to their better light harvesting ability. Therefore, the greatly enhanced Jsc and h of DSL-TiO2 with TiO2/SnO2 microspheres scattering layer could be partly ascribed to the much improved light scattering ability and increased dye adsorption amount. To further investigate the possible reasons for the enhanced performance, the electrochemical impedance spectrum (EIS) was measured in the dark under a forward bias of 0.7 V to investigate the charge-transfer process. The Nyquist impedance spectra and equivalent circuit of the cells with a series of scattering layers are shown in Fig. 6, and the corresponding parameters are displayed in Table 2. The small semicircle in the high frequency and the larger semicircle in the middle frequency can be ascribed to the charge-
Fig. 6. Nyquist impedance spectra of DSSCs based on DSL-TiO2 transparent layer with different scattering layers.
transfer resistances at the counter electrode (Rct1) and the recombination resistance at the oxide/dye/electrolyte interface (Rct2), respectively. As demonstrated in Fig. 6 and Table 2, all the cells exhibit the similar Rct1, which can be caused by the identical condition of the counter electrodes. The cell using the scattering layer of TiO2/SnO2 microspheres presents the largest Rct2 of 156.89 U, which is higher than those of TiO2 microspheres (108.32 U) or SnO2 microspheres (87.90 U), revealing a lower charge recombination at the interfaces of TiO2/SnO2 microspheres/dye/electrolyte. In additional, the OCVD result shown in Fig. S4 (ESI) also agrees well with the largest Rct2, implying a reduced charge recombination rate in TiO2/SnO2 scattering layer-based cell. As a matter of fact, the cell with TiO2/SnO2 microspheres scattering layer indeed exhibit a litter higher Voc, as shown in Table 1. In addition to the EIS measurement, the evolution of the electron transport time (td) and the electron lifetime (tn) as a function of applied voltage were also determined using the intensitymodulated photovoltage spectroscopy (IMVS) and intensitymodulated photocurrent spectroscopy (IMPS) [32]. As shown in
Fig. 5. (a) UV-Vis diffuse reflectance and (b) UV-Vis absorption spectra of films composed of DSL-TiO2 transparent layer with different scattering layers.
148
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
Table 2 EIS parameters of DSSCs based on DSL-TiO2 transparent layer with different scattering layers. Cell
Photoanode
Rs/U
Rct1/U
CPE1/F cm2
Rct2/U
CPE2/F cm2
ZN/U cm2
2 3 4
DSL-TiO2 þ SnO2 scattering layer DSL-TiO2 þ TiO2 scattering layer DSL-TiO2 þ TiO2/SnO2 scattering layer
16.95 17.64 17.70
3.41 4.66 4.20
6.80 104 9.66 104 2.79 104
87.90 108.32 156.89
5.35 104 1.09 103 6.47 104
2.11 101 4.06 101 8.71 102
Fig. 7. (a) Electron transport time and (b) electron lifetime as a function of applied voltage for DSSCs based on DSL-TiO2 transparent layer with different scattering layers.
Fig. 7a, the electron transport time is shorter in the TiO2/SnO2 scattering layer-based film in the entire range of the applied voltage compared with another two scattering layers films-based DSSCs, suggesting the superior electron diffusion and transport ability in the TiO2/SnO2 scattering layer-based film. The electron transport time is highly related to the charge recombination rate in a DSSC. As discussed above, the densely interconnected TiO2 nanowires among microspheres could bring about the faster electron diffusion and transport process from photoanode film to FTO substrate, which will effectively suppress the electron recombination. What's more, SnO2 nanoparticles coating on the outer shell of microspheres would also facilitate the electron diffusion and transport due to its higher electron mobility [18,19]. And cell with SnO2 scattering layer result in a longer time might be ascribed to the inefficient electron transport induced by large block morphology and fast interfacial electron recombination of SnO2. The result in Fig. 7b reveals that the electrons accumulated in the TiO2/SnO2 scattering layer-based cell exhibit longer electron lifetime than those in the other two films, which agrees well with the EIS and OCVD outcomes. As well known, the photoanode play a critical role in DSSCs due to its essential functions, including the light absorption function, charge injection function and electron transport function [32,33]. Therefore, these key functions have to be considered when using semiconductor oxides as photoanode. In this work, the unique structure of SnO2 coated TiO2 urchin-like hollow microspheres endows the cell with plenty of advanced properties. As shown in Scheme 1, the incent light reflected by two ways after passing through TiO2 nanocrystallines, that is diffuse reflected on the surface and multi-reflected inside the hollow spheres. Thus, the TiO2/ SnO2 scattering layer can increase the light transmission distance as well as facilitate the light scattering effect owing to the larger specific surface area and porous shell. What's more, photogenerated electron can diffuse and transport through the TiO2 shell and the SnO2 nanoparticles. And SnO2 nanoparticles will suppress the charge recombination and facilitate the electron transport due to its higher electron mobility and larger band gap. For these reasons, improved photovoltaic performance is observed in comparison with cells based on other scattering layers.
Scheme 1. Schematic diagram of incent light scattering effect and electron transport pathway.
4. Conclusions In conclusion, a unique microstructure of SnO2 coated urchinlike TiO2 hollow microspheres were synthesized by one-step hydrothermal method. When applied as scattering layer, the microspheres exhibited a high photovoltaic conversion efficiency of 8.33%, which was a 28.4% enhancement comparable to that of bare nanocrystalline TiO2. And this result might be attributed to third factors: (1) the higher BET surface area, of TiO2/SnO2 microspheres provided sufficient dye-loading of 1.45 107 mol cm2, which was higher than that of plain DSL-TiO2, leading to their better for more light harvesting ability. At the same time, (2) the superior light-scattering ability of TiO2/SnO2 microspheres could also favor, leading to better the light-utilizing efficiency and (3) superior electron diffusibility through the nanowires of the 3D microspheres, decreasing the loss of photoexcited electrons. We believe that our synthetic approach and systematic work will pave a new route to prepare the unique structures, which with
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
excellent dye absorption and light scattering property as an efficient scattering layer for further improving the g of DSSCs. Acknowledgements We acknowledge financial support from National Natural Science Foundation of China (91433104 and 21303020) and the Program of Introducing Talents of Discipline to Universities (111 project) from Ministry of Education of China. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.10.061. References €tzel, Nature 353 (1991) 737e740. [1] B. O'Regan, M. Gra [2] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, €tzel, Science 334 (2011) E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Gra 629e634. [3] P. Wang, S.M. Zakeeruddin, J.E. Moser, M.K. Nazeeruddin, T. Sekiguchi, €tzel, Nat. Mater. 2 (2003) 402e407. M. Gra €tzel, Inorg. Chem. 44 (2005) 6841e6851. [4] M. Gra [5] M.K. Nazeeruddin, F.D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Gr€ atzel, J. Am. Chem. Soc. 127 (2005) 16835e16847. [6] M. Guo, K.Y. Xie, J. Lin, Z.H. Yong, H.T. Huang, Energy Environ. Sci. 5 (2012) 9881e9888. [7] Y.F. Wang, J.W. Li, Y.F. Hou, X.Y. Yu, C.Y. Su, D.B. Kuang, Chem. Eur. J. 16 (2010) 8620e8625. [8] Y.F. Wang, K.N. Li, C.L. Liang, Y.F. Hou, C.Y. Su, D.B. Kuang, J. Mater. Chem. 22 (2012) 21495e21501. [9] Q.F. Zhang, C.S. Dandeneau, X.Y. Zhou, G.Z. Cao, Adv. Mater. 21 (2009) 4087e4108. [10] Y.T. Shi, K. Wang, Y. Du, H. Zhang, J.F. Gu, T.L. Ma, Adv. Mater. 25 (2013)
149
4413e4419. [11] M.D.Wei, Z.M.Qi, M.Ichihara, H.S.Zhou, Acta Mater.. 56(2008) 2488e2494. [12] W.Z. Li, G.H. Jin, H.S. Hu, J. Li, Y.H. Yang, Q.Y. Chen, Electrochim. Acta 153 (2015) 499e507. [13] T.Chen G.H.Guai C.Gong W.H.Hu J.X.Zhu H.B.Yang Q.Y.Yan C.M.Li, Energy Environ. Sci. 5(2012) 6294e6298. [14] Rahman Samsuri, Salleh Umar, Mater. Lett. 160 (2015) 388e391. [15] A.Y. Pang, X. Sun, H.C. Ruan, Y.F. Li, S.Y. Dai, M.D. Wei, Nano Energy 5 (2014) 82e90. [16] C.L. Wang, J.Y. Liao, Y. Zhao, A. Manthiram, Chem. Comm. 51 (2015) 2848e2850. [17] W.W. Liu, C.X. Hong, H.G. Wang, M. Zhang, M. Guo, Appl. Surf. Sci. 364 (2016) 676e685. [18] E. Ramasamy, J. Lee, J. Phys. Chem. C 114 (2010) 22032e22037. [19] J.F. Qian, P. Liu, Y. Xiao, Y. Jiang, Y.L. Cao, X.P. Ai, H.X. Yang, Adv. Mater 21 (2009) 3663e3667. [20] D. Kuang, J. Brillet, P. Chen, M. Takata, S. Uchida, H. Miura, K. Sumioka, S.M. Zakeeruddin, M. Gr€ atzel, ACS Nano 2 (2008) 1113. [21] W. Peng, L. Han, Z. Wang, Chem. Eur. J. 20 (2014) 8483. [22] K.Pan, Y.Dong, C.Tian, W.Zhou, G.Tian, B.Zhao, H.Fu, Electrochim. Acta 54 (2009) 7350. [23] L. Liang, Y. Liu, X.-Z. Zhao, Chem. Commun. 49 (2013) 3958. [24] C.Y. Wu, L.H. Qi, Y.J. Chen, Q.Y. Ouyang, C.Y. Li, Res. Chem. Intermed. 42 (2016) 5653e5664. [25] K.Y. Yuan, Y.C. Qiu, W. Chen, M. Zhang, S.H. Yang, Energy Environ. Sci. 4 (2011) 2168. €tzel, Adv. Energy Mater. 4 (2014) [26] L.P. Heiniger, F. Giordano, T. Moehl, M. Gra 3412. [27] W.Q. Wu, Y.F. Xu, H.S. Rao, C.Y. Su, D.B. Kuang, J. Am. Chem. Soc. 136 (2014) 6437e6445. [28] Y. Ding, L. Zhou, L.E. Mo, L. Jiang, L.H. Hu, S.Y. Dai, Adv. Funct. Mater. 25 (2015) 5946e5953. [29] Y.F. Li, A.Y. Pang, X.Z. Zheng, M.D. Wei, Electrochim. Acta 56 (2011) 4902e4906. [30] M.D. Wei, K.X. Wang, M. Yanagida, H. Sugihara, M.A. Morris, J.D. Holmes, H.S. Zhou, J. Mater. Chem. 17 (2007) 3888e3893. [31] Y. Chen, F.Z. Huang, D.H. Chen, L. Cao, X.L. Zhang, Rachel A. Caruso, Y.B. Cheng, ChemSusChem. 4 (2011) 1498e1503. [32] Y.F. Wang, X.F. Li, D.J. Li, X.X. Zhang, J. Power Sources 280 (2015) 476e482. [33] H.Y. Fu, L.C. Tan, Y.W. Chen, J. Mater. Chem. C 3 (2015) 828.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile synthesis of SnO2 coated urchin-like TiO2 hollow microspheres as efficient scattering layer for dye-sensitized solar cells Fengyan Xie a, b, Yafeng Li a, b, **, Jie Dou a, b, Junxiu Wu a, b, Mingdeng Wei a, b, * a b
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China Institute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SnO2 coated urchin-like TiO2 hollow microspheres were prepared via a facile method. The microspheres showed superior light scattering ability and electron diffusibility. Cell based on the microspheres scattering layer achieved an efficiency of 8.33%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2016 Received in revised form 12 October 2016 Accepted 19 October 2016
SnO2 coated urchin-like TiO2 hollow microspheres are prepared via a facile one-step hydrothermal method by using titanium tetrabutoxide (TBOT) as titanium source. The synthesized products are characterized by XRD, SEM and TEM measurements. It's found that the as-prepared microspheres with a diameter of 500e800 nm are consisted of densely interconnected nanowires and possessed a high specific surface area of 134.92 m2 g1. Moreover, HRTEM and element mapping results show that the surface of urchin-like microsphere is coated by lots of SnO2 nanoparticles. When used as scattering layer for dye-sensitized solar cells, the microspheres show good dye adsorption capability, superior light scattering and electron diffusibility, leading to a higher photovoltaic conversion efficiency of 8.33%, which is a 28.4% enhancement comparable to that of bare nanocrystalline TiO2 (Dyesol 18NR-T, 6.49%). © 2016 Elsevier B.V. All rights reserved.
Keywords: TiO2/SnO2 microspheres Hydrothermal route Light scattering layer Dye-sensitized solar cells Efficiency Photovoltaic property
1. Introduction Dye-sensitized solar cells (DSSCs) have attracted extensive research since the remarkable breakthrough reported by M. Gr€ atzel
* Corresponding author. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China. ** Corresponding author. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China. E-mail addresses: [email protected] (Y. Li), [email protected] (M. Wei). http://dx.doi.org/10.1016/j.jpowsour.2016.10.061 0378-7753/© 2016 Elsevier B.V. All rights reserved.
in 1991 [1] owing to its low cost, high efficiency and simple fabrication. In general, DSSCs are composed of a nanocrystalline semiconducting oxide film as the photoanode, a sensitizing dye, an 3þ 2þ electrolyte containing an redox couple, such as I/I 3 , Co /Co , etc., and a counter electrode [1e4]. A lot of researchers focus their attention on the photoanode materials since photoanode plays an important role in improving efficiency of DSSCs, and various metal oxides, such as TiO2, SnO2, ZnO, WO3, Nb2O5, etc., have been used as the electrode film for dye attachment and carrier transport [3e13]. Of these metal oxides, TiO2 has been the most promising oxide so
144
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
far owing to its suitable band alignment with various organic and inorganic dyes, controllable various morphology and stability. However, the further use of TiO2 is also limited due to the unfavorable influence on performance caused by the crystal structure, crystalline size and morphology of the TiO2, as well as the rapid combination of photo-generated electrons and oxidative holes. Thus, extensive efforts have been made to improve the photovoltaic performance of TiO2-based DSSCs. One promising strategy is to composite TiO2 with other metal oxides such as ZnO, SnO2 and Nb2O5, to increase the electron transport rate of photoanodes [14e17]. Among those composites, TiO2/SnO2 composite draw more attention because of the higher electron mobility of SnO2 (100e200 cm2 V1$s1) than TiO2 (0.1e1 cm2 V1$s1) and a relative larger band gap (3.6 eV) than TiO2 (3.2 eV) [18,19]. The higher electron mobility of SnO2 can facilitates electron injection from photo-excited dye molecules to a transparent conductive oxide current collector, which will increase the electron transport rate and improve device performance. Furthermore, in the case of SnO2, its larger band gap (3.6 eV) can suppress the production of oxidative holes in the valence band of SnO2 under UV illumination, thereby minimizing the dye degradation rate and improving the long-term stability of DSSCs [18,19]. On the other hand, films based on nanocrystalline TiO2 results in negligible light scattering, which led to the low photon absorption [20,21]. Therefore, the scattering materials are designed to improve the light harvesting ability of photoanode. The introduction of scattering materials could improve the light harvesting via increasing the optical path length and reabsorb of light irradiating into the TiO2 film. There are several ways to incorporate the light scattering materials. For example, a certain proportion of 1D or 2D nanostructures, such as nanowires, nanorods and nanosheets could mix with the nanocrystalline TiO2 to increase the light scattering, or they can simply be deposited on the top of TiO2 film as a scattering layer [22e24]. Moreover, the 3D TiO2 nanostructures, such as mesoporous TiO2 beads, nanoporous TiO2 spheres or aggregates and hollow microspheres were widely constructed to play the role of scattering layer [25e28]. However, the lower surface area as well as the inefficient electron transport ability of these scattering materials limits their further applications in DSSCs. Therefore, it is greatly desired to design unique nanostructures that can provide large surface area and efficient electron transport pathways, as an efficient light scattering layer. In this work, a novel TiO2/SnO2 composite was first prepared by a facile method and applied in DSSCs as efficient scattering layer. Owing to the unique SnO2 nanoparticles coated nanowires/microsphere structure, the urchin-like TiO2 hollow sphere exhibits lager specific surface area, superior light scattering capacity and electron diffusibility, resulting in significant improvement of efficiency when applied in DSSCs as efficient scattering layer. 2. Experimental section 2.1. Synthesis and characterization of the samples All reagents were of analytical grade and were used without further purification. In a typical process, 1 mmol K2SnO3$3H2O was added to a solution of 20 mL isopropyl alcohol and 5 mL acetylacetone (acac) under vigorous stirring. Then, 1 mL titanium tetrabutoxide (TBOT) was dropped into the solution. After stirring for 20 min at ambient condition, the suspension yellowish mixture solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and kept in an oven at 200 C for 12 h. After cooling to room temperature, the light yellow precipitate was collected by centrifugation, and washed with ethanol, and then dried at 70 C
overnight. The TiO2/SnO2 microspheres were obtained after annealing at 500 C for 2 h. TiO2 microspheres had been synthesized by the above method except without adding K2SnO3$3H2O for comparison. In addition, SnO2 microspheres were also synthesized according to the previous report [16] as a reference sample. X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV using the Cu Ka radiation (1.5418 Å). SEM images were presented using an S4800 instrument (Hitachi). TEM images, Energy disperse spectrum (EDS) and elemental mapping were performed by a Tecnai G2 F20 instrument (FEI, USA). N2 adsorptiondesorption was measured on a Micromeritics ASAP 2020 instrument (Micromeritics, Norcross, GA, USA). UV-Vis spectra were investigated using a Lambda-950 (Perkin-Elmer). 2.2. Fabrication and measurement of DSSCs Different pastes of each material for the screen printing were prepared according to our previous works [29,30]. Typically, the mixture composed of powder of TiO2/SnO2 microspheres (TiO2 microspheres or SnO2 blocks), ethyl cellulose (10 wt%, Kanto Chemical Co.), a-terpineol (5 wt%) and ethanol was suitably dispersed using a homogenizer (Ultraturrax T25) to obtain a homogeneous paste. Then the paste was concentrated using an evaporator to a final concentration of 20 wt%. A transparent layer of ~10 mm and a scattering layer of ~4 mm were obtained by multiple screen printing on FTO with the Dyesol 18NR-T (denoted as DSL-TiO2) paste and TiO2/SnO2 microspheres (TiO2 microspheres or SnO2 microspheres) paste, and followed by calcined at 525 C for 2 h in air. After cooling to the room temperature, a 50 mM aqueous solution TiCl4 was dropped onto the film and kept at 70 C for more than 30 min. After that, the films were calcined at 450 C for 30 min. The prepared films were immersed into a solution containing 0.5 mM N719 dye in acetonitrile and isopropyl alcohol (v/v ¼ 1:1) for 24 h, and then were incorporated into a thinlayer, sandwiched solar cell with an active area of 0.25 cm2. The Pt-coated FTO glass as a counter electrode was prepared by dropping H2PtCl6 (5 mM) solution on the FTO glass followed by heating at 400 C for 30 min. A polyethylene spacer (ca.40 mm thickness) was used to prevent the cell from short-circuiting. The electrolyte was composed of 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide, 0.05 M I2, and 0.1 M LiI in acetonitrile with 0.5 M 4-tertbutylpyridine. Current-Voltage curves (I-V) of solar cells were carried out using a source meter (Keithley 2400). A PEC-L11 AM 1.5 solar simulator (Peccell, with a 1000 W Xe lamp and an AM 1.5 filter) was used as the light source (100 mWcm2). The spectra of IPCE were collected using a PEC-S20 (Peccell, Technology Co. Ltd.) Electrochemical Impedance Spectroscopy (EIS) were conducted using an IM6 (Zahner). And the intensity of the modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) were also conducted using IM6 with light-emitting diodes (LED, l ¼ 457 nm). The small amplitude is 5% of the applied voltage, and the frequency range was from 100 mHz to 3 kHz. 3. Results and discussion Fig. 1a shows the XRD patterns of the samples of TiO2/SnO2 microspheres and TiO2 microspheres. The results reveal that all diffraction peaks of TiO2 microspheres can be indexed to an anatase TiO2 (JCPDS 84e1285), while the additional XRD diffraction peaks of TiO2/SnO2 microspheres in comparison to TiO2 was caused the existence of cassiterite phase SnO2 (JCPDS 72e1147), indicating the TiO2/SnO2 nanocomposites were successfully prepared. The higher surface area of materials used in DSSCs are favorable
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
145
Fig. 1. (a) XRD patterns, (b) N2 adsorption and desorption isotherms and pore-size distribution (inset) of TiO2/SnO2 microspheres and TiO2 microspheres.
for the dye adsorption, thus the surface area of fabricated TiO2/SnO2 microspheres was investigated. The Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the TiO2/SnO2 microspheres and TiO2 microspheres were determined using nitrogen adsorption and desorption isotherms. As shown in Fig. 1b, the BET surface area of TiO2/SnO2 microspheres was 134.92 m2 g1, about three times of that of TiO2 microspheres (42 m2 g1) obtained from the same condition. The pore size distribution of TiO2/SnO2 microspheres and TiO2 microspheres are also shown in inset of Fig. 1b. The results denote the TiO2/SnO2 microspheres have a smaller average pore size distribution (8 nm) than that of TiO2 microspheres (10 nm), which might be caused by the densely interconnected TiO2 nanowires. This mesoporous microspheres structure could form passageways to facilitate the electrolyte diffusion and transport during the oxidation/reduction process of electrolyte. Fig. 2 shows SEM images of the as prepared TiO2/SnO2 microspheres. As depicted in Fig. 2a, urchin-like microspheres with a diameter range of 500e800 nm are clearly observed. The magnified image in Fig. 2b suggests that these microspheres are assembled by densely packed nanowires. The morphology of as prepared TiO2 microspheres and SnO2 microspheres are shown in Fig. S1 (ESI). As can be seen from it, TiO2 microspheres almost maintain spherical with rough surface composed of nanoparticles, and SnO2 microspheres show an aggregated spherical morphology composed of SnO2 nanoparticles. As previously reported, mesoporous microspheres with a diameter of 550 ± 50 and 830 ± 40 nm possessing high scattering properties, which is ideal for scattering layer in DSSCs [31]. Hence, the urchin-like TiO2/SnO2 microspheres with a diameter range of 500e800 nm would be expected to exhibit a superior scattering ability applied in DSSCs.
To confirm the morphology and component of the SnO2 coated TiO2 microspheres, TEM measurements were carried out. Fig. 3a clearly indicates these urchin-like spheres have hollow structure and the shell thickness is around 200 nm. At the same time the diameter of hollow spheres is found to be in the range of 500e700 nm, which is in good agreement with the SEM results and makes it suitable for scattering visible light. A TEM image taken from the shell of a microsphere in Fig. 3b reveals that the shell of the microsphere could be divided into two parts, one is an inner shell consisted of nanoparticles while the other one is an outer shell consisted of nanowires. Fig. 3c, d shows the HRTEM images that obtained from two portions marked with circle in Fig. 3b, respectively. Fig. 3c clearly shows the lattice fringes of nanoparticles deposited on nanowires were about 0.337 nm, corresponding to the (110) facets of SnO2. The lattice fringe of nanowires in Fig. 3d was around 0.355 nm, which could be ascribed to the (101) facets of TiO2. Furthermore, the selected area electron diffraction (SAED) pattern of a microsphere (inset in Fig. 3d) could be indexed to (101), (200) and (211), respectively, indicating a highly crystalline anatase phase of TiO2, which is consistent with the XRD analysis discussed above. In addition, the elemental mapping measurement in Fig. 3e reveals the presence and uniform distribution of O, Ti, Sn in TiO2/ SnO2 microsphere. Two obvious peaks of Sn element present in EDS spectrum (Fig. S2, ESI) also confirm the results discussed above. Therefore, the urchin-like SnO2/TiO2 hollow microspheres have been successfully constructed. To prove the superior light scattering ability of the TiO2/SnO2 microspheres, four kinds of cells were prepared for comparison. The cells without and with SnO2, TiO2, and TiO2/SnO2 scattering layers were named as Cell 1, 2, 3, and 4, respectively. The photovoltaic performance of solar cells was investigated. IeV curves of
Fig. 2. (a, b) SEM images of as prepared TiO2/SnO2 microspheres.
146
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
Fig. 3. (a, b) TEM images, (c, d) HRTEM images and the SAED patterns (inset) of TiO2/SnO2 microspheres; (e) elemental mapping of TiO2/SnO2 microspheres.
the cells were plotted in Fig. 4a, and the corresponding parameters are displayed in Table 1. It can be found that the fill factor FF and open-circuit voltage Voc of the four different kinds of cells were similar. For DSL-TiO2 photoanode, it gave a short-circuit current Jsc of 12.39 mA cm2 and an efficiency of 6.49%. When scattering layers were employed, all the cells exhibited higher Jsc and efficiency than those of the cell based on bare DSL-TiO2. The improved efficiency was mainly caused by the increased Jsc due to the enhanced light-
harvesting ability of photoanode. When the as-prepared TiO2/SnO2 microspheres were used as scattering layer, the cell demonstrated the highest Jsc of 15.23 mA cm2, and the power conversion efficiency of 8.33% was obtained, which was an enhancement of 28.4% in comparison to that of DSL-TiO2. In fact, the superior light scattering ability can be evidenced by Incident photon-to-electron conversion efficiencies (IPCE). The IPCE spectra and normalized IPCE spectra are shown in Fig. S3 (ESI) and Fig. 4b, respectively. As
Fig. 4. (a) IeV curves and (b) normalized IPCE spectra of DSSCs based on DSL-TiO2 transparent layer with and without different scattering layers.
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
147
Table 1 Photovoltaic parameters of DSSCs based on DSL-TiO2 transparent layer with and without different scattering layers. Cell
Photoanode
Thickness [mm]
1 2 3 4
DSL-TiO2 DSL-TiO2 þ SnO2 scattering layer DSL-TiO2 þ TiO2 scattering layer DSL-TiO2 þ TiO2/SnO2 scattering layer
13.8 14.1 14.2 14.0
± ± ± ±
0.5 0.5 0.5 0.5
Jsc [mA cm2]
Voc [V]
FF
h[%]
Dye loading [107 mol/cm2]
12.39 12.90 14.37 15.23
0.76 0.76 0.76 0.77
0.69 0.71 0.69 0.71
6.49 6.99 7.59 8.33
1.23 1.00 1.13 1.45
shown in Fig. 4b, the cells based on DSL-TiO2 with scattering layers almost exhibit higher IPCE than that bare DSL-TiO2 in the wavelength range of 300e800 nm, and especially in the longer wavelength range from 450 to 800 nm. Notably, cell with TiO2/SnO2 scattering layer has similar IPCE to that cell with TiO2 scattering layer at shorter wavelength (300e600 nm) might be ascribed to the similar dye loading capacities of them. But the former exhibits the highest IPCE at longer wavelength of 600e800 nm, it might be attributed to the superior light harvesting ability induced by light scattering effect of the nanowires/microspheres structure. To further prove the superior light scattering ability of the TiO2/ SnO2 microspheres, the UV-Vis diffuse reflectance of the photoelectrodes composing DSL-TiO2 transparent layer and different scattering layers with the same thicknesses (ca. 14 mm) were investigated. The results shown in Fig. 5a denote that the TiO2/SnO2 microspheres exhibited stronger light reflectance in the wavelength range of 400e800 nm than the other two films, which was consistent with the IPCE results. And film with SnO2 scattering layer showed a lower light reflectance could be owed to the poor light scattering ability of large block morphology aggregated by SnO2 nanoparticles. Moreover, the UV-Vis absorption spectra shown in Fig. 5b indicate that owing to the higher specific surface area, the photoanode consisted of DSL-TiO2 and TiO2/SnO2 microspheres scattering layer can absorb more dye molecules than the other two films under the same thickness. In fact, the higher BET surface area of TiO2/SnO2 microspheres provided sufficient dye-loading of 1.45 107 mol cm2, which was obviously higher than the others, leading to their better light harvesting ability. Therefore, the greatly enhanced Jsc and h of DSL-TiO2 with TiO2/SnO2 microspheres scattering layer could be partly ascribed to the much improved light scattering ability and increased dye adsorption amount. To further investigate the possible reasons for the enhanced performance, the electrochemical impedance spectrum (EIS) was measured in the dark under a forward bias of 0.7 V to investigate the charge-transfer process. The Nyquist impedance spectra and equivalent circuit of the cells with a series of scattering layers are shown in Fig. 6, and the corresponding parameters are displayed in Table 2. The small semicircle in the high frequency and the larger semicircle in the middle frequency can be ascribed to the charge-
Fig. 6. Nyquist impedance spectra of DSSCs based on DSL-TiO2 transparent layer with different scattering layers.
transfer resistances at the counter electrode (Rct1) and the recombination resistance at the oxide/dye/electrolyte interface (Rct2), respectively. As demonstrated in Fig. 6 and Table 2, all the cells exhibit the similar Rct1, which can be caused by the identical condition of the counter electrodes. The cell using the scattering layer of TiO2/SnO2 microspheres presents the largest Rct2 of 156.89 U, which is higher than those of TiO2 microspheres (108.32 U) or SnO2 microspheres (87.90 U), revealing a lower charge recombination at the interfaces of TiO2/SnO2 microspheres/dye/electrolyte. In additional, the OCVD result shown in Fig. S4 (ESI) also agrees well with the largest Rct2, implying a reduced charge recombination rate in TiO2/SnO2 scattering layer-based cell. As a matter of fact, the cell with TiO2/SnO2 microspheres scattering layer indeed exhibit a litter higher Voc, as shown in Table 1. In addition to the EIS measurement, the evolution of the electron transport time (td) and the electron lifetime (tn) as a function of applied voltage were also determined using the intensitymodulated photovoltage spectroscopy (IMVS) and intensitymodulated photocurrent spectroscopy (IMPS) [32]. As shown in
Fig. 5. (a) UV-Vis diffuse reflectance and (b) UV-Vis absorption spectra of films composed of DSL-TiO2 transparent layer with different scattering layers.
148
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
Table 2 EIS parameters of DSSCs based on DSL-TiO2 transparent layer with different scattering layers. Cell
Photoanode
Rs/U
Rct1/U
CPE1/F cm2
Rct2/U
CPE2/F cm2
ZN/U cm2
2 3 4
DSL-TiO2 þ SnO2 scattering layer DSL-TiO2 þ TiO2 scattering layer DSL-TiO2 þ TiO2/SnO2 scattering layer
16.95 17.64 17.70
3.41 4.66 4.20
6.80 104 9.66 104 2.79 104
87.90 108.32 156.89
5.35 104 1.09 103 6.47 104
2.11 101 4.06 101 8.71 102
Fig. 7. (a) Electron transport time and (b) electron lifetime as a function of applied voltage for DSSCs based on DSL-TiO2 transparent layer with different scattering layers.
Fig. 7a, the electron transport time is shorter in the TiO2/SnO2 scattering layer-based film in the entire range of the applied voltage compared with another two scattering layers films-based DSSCs, suggesting the superior electron diffusion and transport ability in the TiO2/SnO2 scattering layer-based film. The electron transport time is highly related to the charge recombination rate in a DSSC. As discussed above, the densely interconnected TiO2 nanowires among microspheres could bring about the faster electron diffusion and transport process from photoanode film to FTO substrate, which will effectively suppress the electron recombination. What's more, SnO2 nanoparticles coating on the outer shell of microspheres would also facilitate the electron diffusion and transport due to its higher electron mobility [18,19]. And cell with SnO2 scattering layer result in a longer time might be ascribed to the inefficient electron transport induced by large block morphology and fast interfacial electron recombination of SnO2. The result in Fig. 7b reveals that the electrons accumulated in the TiO2/SnO2 scattering layer-based cell exhibit longer electron lifetime than those in the other two films, which agrees well with the EIS and OCVD outcomes. As well known, the photoanode play a critical role in DSSCs due to its essential functions, including the light absorption function, charge injection function and electron transport function [32,33]. Therefore, these key functions have to be considered when using semiconductor oxides as photoanode. In this work, the unique structure of SnO2 coated TiO2 urchin-like hollow microspheres endows the cell with plenty of advanced properties. As shown in Scheme 1, the incent light reflected by two ways after passing through TiO2 nanocrystallines, that is diffuse reflected on the surface and multi-reflected inside the hollow spheres. Thus, the TiO2/ SnO2 scattering layer can increase the light transmission distance as well as facilitate the light scattering effect owing to the larger specific surface area and porous shell. What's more, photogenerated electron can diffuse and transport through the TiO2 shell and the SnO2 nanoparticles. And SnO2 nanoparticles will suppress the charge recombination and facilitate the electron transport due to its higher electron mobility and larger band gap. For these reasons, improved photovoltaic performance is observed in comparison with cells based on other scattering layers.
Scheme 1. Schematic diagram of incent light scattering effect and electron transport pathway.
4. Conclusions In conclusion, a unique microstructure of SnO2 coated urchinlike TiO2 hollow microspheres were synthesized by one-step hydrothermal method. When applied as scattering layer, the microspheres exhibited a high photovoltaic conversion efficiency of 8.33%, which was a 28.4% enhancement comparable to that of bare nanocrystalline TiO2. And this result might be attributed to third factors: (1) the higher BET surface area, of TiO2/SnO2 microspheres provided sufficient dye-loading of 1.45 107 mol cm2, which was higher than that of plain DSL-TiO2, leading to their better for more light harvesting ability. At the same time, (2) the superior light-scattering ability of TiO2/SnO2 microspheres could also favor, leading to better the light-utilizing efficiency and (3) superior electron diffusibility through the nanowires of the 3D microspheres, decreasing the loss of photoexcited electrons. We believe that our synthetic approach and systematic work will pave a new route to prepare the unique structures, which with
F. Xie et al. / Journal of Power Sources 336 (2016) 143e149
excellent dye absorption and light scattering property as an efficient scattering layer for further improving the g of DSSCs. Acknowledgements We acknowledge financial support from National Natural Science Foundation of China (91433104 and 21303020) and the Program of Introducing Talents of Discipline to Universities (111 project) from Ministry of Education of China. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.10.061. References €tzel, Nature 353 (1991) 737e740. [1] B. O'Regan, M. Gra [2] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, €tzel, Science 334 (2011) E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Gra 629e634. [3] P. Wang, S.M. Zakeeruddin, J.E. Moser, M.K. Nazeeruddin, T. Sekiguchi, €tzel, Nat. Mater. 2 (2003) 402e407. M. Gra €tzel, Inorg. Chem. 44 (2005) 6841e6851. [4] M. Gra [5] M.K. Nazeeruddin, F.D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Gr€ atzel, J. Am. Chem. Soc. 127 (2005) 16835e16847. [6] M. Guo, K.Y. Xie, J. Lin, Z.H. Yong, H.T. Huang, Energy Environ. Sci. 5 (2012) 9881e9888. [7] Y.F. Wang, J.W. Li, Y.F. Hou, X.Y. Yu, C.Y. Su, D.B. Kuang, Chem. Eur. J. 16 (2010) 8620e8625. [8] Y.F. Wang, K.N. Li, C.L. Liang, Y.F. Hou, C.Y. Su, D.B. Kuang, J. Mater. Chem. 22 (2012) 21495e21501. [9] Q.F. Zhang, C.S. Dandeneau, X.Y. Zhou, G.Z. Cao, Adv. Mater. 21 (2009) 4087e4108. [10] Y.T. Shi, K. Wang, Y. Du, H. Zhang, J.F. Gu, T.L. Ma, Adv. Mater. 25 (2013)
149
4413e4419. [11] M.D.Wei, Z.M.Qi, M.Ichihara, H.S.Zhou, Acta Mater.. 56(2008) 2488e2494. [12] W.Z. Li, G.H. Jin, H.S. Hu, J. Li, Y.H. Yang, Q.Y. Chen, Electrochim. Acta 153 (2015) 499e507. [13] T.Chen G.H.Guai C.Gong W.H.Hu J.X.Zhu H.B.Yang Q.Y.Yan C.M.Li, Energy Environ. Sci. 5(2012) 6294e6298. [14] Rahman Samsuri, Salleh Umar, Mater. Lett. 160 (2015) 388e391. [15] A.Y. Pang, X. Sun, H.C. Ruan, Y.F. Li, S.Y. Dai, M.D. Wei, Nano Energy 5 (2014) 82e90. [16] C.L. Wang, J.Y. Liao, Y. Zhao, A. Manthiram, Chem. Comm. 51 (2015) 2848e2850. [17] W.W. Liu, C.X. Hong, H.G. Wang, M. Zhang, M. Guo, Appl. Surf. Sci. 364 (2016) 676e685. [18] E. Ramasamy, J. Lee, J. Phys. Chem. C 114 (2010) 22032e22037. [19] J.F. Qian, P. Liu, Y. Xiao, Y. Jiang, Y.L. Cao, X.P. Ai, H.X. Yang, Adv. Mater 21 (2009) 3663e3667. [20] D. Kuang, J. Brillet, P. Chen, M. Takata, S. Uchida, H. Miura, K. Sumioka, S.M. Zakeeruddin, M. Gr€ atzel, ACS Nano 2 (2008) 1113. [21] W. Peng, L. Han, Z. Wang, Chem. Eur. J. 20 (2014) 8483. [22] K.Pan, Y.Dong, C.Tian, W.Zhou, G.Tian, B.Zhao, H.Fu, Electrochim. Acta 54 (2009) 7350. [23] L. Liang, Y. Liu, X.-Z. Zhao, Chem. Commun. 49 (2013) 3958. [24] C.Y. Wu, L.H. Qi, Y.J. Chen, Q.Y. Ouyang, C.Y. Li, Res. Chem. Intermed. 42 (2016) 5653e5664. [25] K.Y. Yuan, Y.C. Qiu, W. Chen, M. Zhang, S.H. Yang, Energy Environ. Sci. 4 (2011) 2168. €tzel, Adv. Energy Mater. 4 (2014) [26] L.P. Heiniger, F. Giordano, T. Moehl, M. Gra 3412. [27] W.Q. Wu, Y.F. Xu, H.S. Rao, C.Y. Su, D.B. Kuang, J. Am. Chem. Soc. 136 (2014) 6437e6445. [28] Y. Ding, L. Zhou, L.E. Mo, L. Jiang, L.H. Hu, S.Y. Dai, Adv. Funct. Mater. 25 (2015) 5946e5953. [29] Y.F. Li, A.Y. Pang, X.Z. Zheng, M.D. Wei, Electrochim. Acta 56 (2011) 4902e4906. [30] M.D. Wei, K.X. Wang, M. Yanagida, H. Sugihara, M.A. Morris, J.D. Holmes, H.S. Zhou, J. Mater. Chem. 17 (2007) 3888e3893. [31] Y. Chen, F.Z. Huang, D.H. Chen, L. Cao, X.L. Zhang, Rachel A. Caruso, Y.B. Cheng, ChemSusChem. 4 (2011) 1498e1503. [32] Y.F. Wang, X.F. Li, D.J. Li, X.X. Zhang, J. Power Sources 280 (2015) 476e482. [33] H.Y. Fu, L.C. Tan, Y.W. Chen, J. Mater. Chem. C 3 (2015) 828.