Original Research PaperNon-aqueous preparation of anatase TiO2 hollow microspheres for efficient dye-sensitized solar cells
Graphical abstract
Introduction
Dye-sensitized solar cell (DSSC) is a new type of thin film solar cell which has been considered a promising renewable photovoltaic technology [1], [2], [3]. The main advantages of DSSC are low cost, simple preparation process and easy production [2]. Until now, the record power conversion efficiency (PCE) of DSSC has been reported to be over 14% under irradiance of 100 mW cm−2 [4]. More strikingly, under indoor light, the PCE of DSSC has reached 28.9% by combining two judiciously designed sensitizers with the copper complex as a redox shuttle [5]. The key component of DSSC is a semiconductor oxide photoanode, which can affect the adsorption of dye sensitizer and the electron transport. Among various oxide semiconductors, TiO2 has been regarded as the most favorable electrode material because of its good stability, low cost and pollution-free [6], [7], [8]. Researchers have applied different morphology TiO2 electrodes to DSSCs, such as nanosheet, nanosphere, nanotube, nanoflower and nanowire [9], [10], [11], [12], [13]. No matter which morphology is used, high specific surface area, fast electron transmission and excellent light scattering ability are effective factors to improve the PCE of DSSCs [14], [15], [16], [17].
However, various morphology TiO2 electrodes have their own defects. In order to make best use of the advantages and bypass the disadvantages, the graded electrode has been fabricated by using TiO2 material with different sizes and morphologies. For instance, Wu et al. [18] demonstrated a trilayered TiO2 electrode consisting of one-dimensional TiO2 nanotubes, three-dimensional TiO2 hierarchical microspheres and zero-dimensional TiO2 nanoparticles, which showed a prominent improvement in PCE (9.10%). Qiao et al. [19] fabricated DSSCs based on TiO2 submicrospheres/nanoparticles composite photoanode, which displayed a higher PCE than pure P25 nanoparticle photoanode. Wang et al. [20] prepared a double-layered TiO2 photoanode with graded structure of nanosheet/nanoparticle, which showed excellent PCE in DSSCs. Among various photoanodes, the optimized performance of DSSCs reflected that the light scattering layer have a significant positive effect, because it confines the incident light within a photoanode so as to increase the opportunity of the photons to be absorbed by the dyes, and thus enhances the photocurrent [21].
TiO2 hollow spheres (TiO2-HSs) have a high specific surface area to absorb a large number of dye molecules, and can effectively scatter the incident photon and lead photon to occur multiple reflections within the cavities, which significantly improve light utilization efficiency. Since Koo et al. [22] firstly introduced TiO2-HSs into DSSCs, many researchers have focused attention on building TiO2-HSs. Park et al. [23] synthesized TiO2-HSs using polystyrene as the template, and the PCE of DSSCs based on TiO2 nanoparticles/TiO2-HSs was determined to 7.10%. Chen et al. [24] synthesized TiO2-HSs using polystyrene colloidal crystals as sacrificial templates. The double-layered photoanode derived from TiO2-HSs as light scattering layer exhibited a 46% increment of efficiency compared to P25 photoanode. Zhao et al. [25] synthesized mono-dispersive TiO2-HSs using SiO2 spheres as hard-templates. The DSSC based on 600 nm TiO2-HSs as light scattering centers in photoanode exhibited the conversion efficiency of 5.84%, which is 68.8% higher than that based on P25 TiO2 photoanode. The DSSCs using TiO2-HSs as light scattering layer obviously exhibited an improvement in photoelectric properties.
In the last few years, many efforts have been made to seek the preparative methods of TiO2-HSs including templating method, sol-gel method, spray techniques, hydrothermal/solvothermal reaction, etc. [15], [24], [25], [26], [27], [28]. Although many methods have been developed for preparing TiO2-HSs, some of them are often high-cost, time-consuming and tedious. For instance, templating method would inevitably accompany complex procedures, high cost, structural collapse and environmental pollution in the preparation process. Sol-gel process is usually combined with surfactant templating method, which is considered as a complex and time-consuming method. Thus, exploring simple and efficient preparative route is still important to design TiO2-HSs with unique properties.
In this paper, TiO2-HSs were fabricated by a template-free approach in a mixed solution containing isopropanol and tetrabutyl titanate with the assistance of oxalic acid. This synthetic approach requires short reaction time, simple instruments and procedures, which is efficient, cost-saving. The as-prepared TiO2-HSs have high anatase crystallinity, high surface area and superior light-scattering ability. Furthermore, we assembled the DSSC based on a photoanode of the bilayer structure (TiO2-HSs as a scattering layer on the underlayer composed of P25 nanoparticles), and the cell demonstrated 7.90% solar energy conversion efficiency, which increases by 31.67% compared with the DSSC base on pure P25 nanoparticles photoanode with the same film thickness.
Section snippets
Materials
Oxalic acid was purchased from Yong Da Chemical Reagent Co., Ltd (Tianjin, China). Tetrabutyl titanate was obtained from Fuchen Chemical Reagent Co., Ltd (Tianjin, China). Isopropanol and absolute ethanol were purchased from Jinhuada Chemical Reagent Co., Ltd (Guangzhou, China). The N719 dye (Ruthenizer535bis-TBA) was purchased from Solaronix SA (Switzerland). Transparent fluorine-doped tin oxide (FTO) conducting substrates were purchased from Nippon Sheet Glass Co., Ltd.
Preparation of TiO2-HSs
TiO2-HSs were prepared
Results and discussion
The as-prepared TiO2-HSs at 180 °C for 6 h were characterized by XRD analysis. The result presented in Fig. 1a shows the peaks at 25.4°, 37.9°, 48.1°, 54.0°, 55.2° and 62.9°, corresponding respectively to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) planes, which can be indexed to the anatase TiO2 phase (JCPD: 73-1764). In addition, no other phases or impurities are observed, which proves that TiO2-HSs sample has high crystallinity and purity. The average crystallite size of 12.3 nm
Conclusions
In summary, this study presented a cost-effective and template-free method for preparation of anatase TiO2-HSs with high specific surface area and appropriated size of particles, which are in favor of dye loading as well as light-scattering effects. In addition, we fabricated a bilayer structure using TiO2-HSs as a scattering layer and P25 nanoparticles as underlayer, which demonstrated higher photocurrent density up to 13.58 mA cm−2 and superior solar energy conversion efficiency of 7.90%
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (21561010), the Science and Research Key Project of Universities of Hainan Province (HNky2019ZD-16), the Natural Science Foundation of Hainan Province (20162023, 2017CXTD007), and the Key Science and Technology Program of Haikou City (2017042).
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