Elsevier

Chemical Physics

Volume 533, 1 May 2020, 110744
Chemical Physics

Composite electrode of TiO2 particles with three kinds of crystal phases for significantly improved performance of dye-sensitized solar cells

https://doi.org/10.1016/j.chemphys.2020.110744Get rights and content

Highlights

  • High-quality brookite TiO2 nanocubes are hydrothermally prepared.

  • Brookite nanocubes enhance VOC of DSSCs fabricated with P25 photoanode.

  • The synergistic effect of the three phases promote interfacial electron transfer.

  • Composite electrode’s efficiency is improved by 40% compared to the P25 one.

Abstract

A novel composite electrode of TiO2 particles with three kinds of crystal phases is fabricated for improving the dye-sensitized solar cell's performance. The photoelectric conversion efficiency of solar cell with the content of 30 wt% brookite nanocubes and 70 wt% P25 could reach 7.40%. It is obvious that brookite nanocubes are advantageous in terms of reduced charge recombination and higher voltage. On the other hand, P25 gives advantages of high surface area for dye loading and higher charge collection efficiency. This work shows a new photoelectrode design for enhanced energy conversion of DSSCs.

Introduction

According to the increment of demands for renewable energy, solar energy attracts more and more attention [1], [2]. Dye-sensitized solar cells (DSSCs) can convert solar energy into electricity with low cost, eco-friendly and durable properties. [3], [4]. Among them, photoanode is the key component of DSSCs. TiO2 nanostructured material is most widely used and efficient material for the photoanode [5], [6]. Among three most commonly encountered crystalline polymorphs of TiO2, brookite is thermodynamic metastable and difficult to synthesize so that seldom studied [7], [8]. Nevertheless, the development of synthesis procedures for brookite makes it as an interesting candidate in photoelectrochemistry applications.

In the laboratory conditions, brookite nanoparticles can be obtained by a variety of different synthetic techniques, such as hydrothermal, template-assisted method, and aqueous precipitation/sol–gel [9], [10], [11], [12], [13]. If pure phase brookite want to be synthesized, synthesis conditions should be carefully adjusted. Recently, more and more literature about synthesize brookite TiO2 particles with high phase purity has been reported. Li et al. synthesized brookite through the hydrothermal treatment with titanium trichloride (TiCl3) as the titanium source, hydrogen peroxide and ammonium peroxodisulfate as the oxidants [14]. Pottier et al. synthesized brookite by thermolysis of TiCl4 depending on the acidity conditions [15]. To optimize the hydrothermal conditions and the Na+/OH concentrations, flower-like brookite TiO2 particle can be obtained [7]. Nevertheless, it is not common to apply in DSSC application in spite of its outstanding open circuit potential and reduced charge recombination compared with the other phases [16]. In our previous research, we have proven that the conduction band edge potential of brookite is more cathodic than anatase phase which leads to higher open circuit potential [17], [18], [19]. Based on above reason, it is necessary to exploit some new routes to take advantage of the most negative conduction band (CB) of the brookite so as to further improve the photovoltaic performance of the brookite-based DSSCs.

However, the photoelectrochemistry activity of single polymorphs usually cannot compare with the performance of mixed TiO2 phases [20], [21], [22]. As already demonstrated by other systems, the coupling of different polymorphs allows the vectorial displacement of electrons from one polymorph to another, which can promote separate electron/hole and thereby improve photoelectrochemistry activity [23], [24], [25]. On account of lower density of anchoring sites for dye on the brookite surface, anatase TiO2 can make up for the deficiency of dye loading [26], [27]. The dye loading of anatase is significantly more than that of brookite photoelectrode despite similar porosity and crystallite size. Combine the advantages of brookite and anatase, the conversion efficiency of the DSSCs can be further improved.

Hence, in this work, we prepared two different TiO2 nanoparticles to improve the performance of TiO2-based DSSCs. The composite electrode of TiO2 particles with three kinds of crystal phases containing brookite TiO2 and P25 is fabricated for improving the dye-sensitized solar cell's performance. High-quality brookite TiO2 quasi nanocubes with four {2 1 0} and two {0 0 1} exposed crystal facets as well as mean particle size of ~50 nm was synthesized through a hydrothermal method [17]. Optimizing the proportion of brookite TiO2 and P25, the composite TiO2 electrode DSSCs can achieve photovoltaic conversion efficiency up to 7.40%. There is an effective improvement of 104% or 40% compared to the single brookite TiO2 nanocubes or P25 film-based solar cell, respectively. Our results deepen the understanding of the functioning of the composite electrode containing different crystal phases.

Section snippets

Synthesis of brookite TiO2 quasi nanocubes

The brookite TiO2 quasi nanocubes are prepared via a hydrothermal method according to our previous report [17]. In this synthesis a fixed amount of TiCl4 was added dropwise to 40 mL of deionized water cooled by an ice-water bath, and then, 5.0 g urea and 5.0 mL of sodium lactate liquor (60%) was mixed and dissolved in this solution with agitation. The clarified liquid was then subjected to hydrothermal treatment at 200 °C for 20 h. The obtained solid product was centrifuged, washed and calcined

Crystal phase and morphology analyses

As shown in Fig. 1a, a set of well-defined diffraction peaks can be seen in the XRD pattern of as-prepared TiO2 quasi nanocubes (denoted as BTN). It could be clearly obtained that clear and sharp Bragg diffraction peaks with 2θ at 25.3°, 25.7°, 30.8°, 36.2°, 37. 3°, 40.1°, 42.4°, 46.0°, 48.0° and 49.1° corresponds to the (2 1 0), (1 1 1), (2 1 1), (1 0 2), (0 2 1), (2 0 2), (2 2 1), (3 0 2), (3 2 1) and (3 1 2) planes in the standard pattern, respectively (JCPDS No. 65-2448) [17], [18], [19].

Conclusion

Brookite TiO2 quasi nanocubes with high phase purity and thermal stability were prepared by a simple hydrothermal method. To take advantage of the three kinds of TiO2 crystal phases, a novel composite electrode containing brookite TiO2 nanocubes and P25 is fabricated to further improve the photovoltaic performance of the TiO2-based dye-sensitized solar cells. The higher Fermi level, increased electron concentration and lower surface reactivity as compared to P25 lead to the higher VOC of the

CRediT authorship contribution statement

Tingxin Peng: Data curation, Writing - original draft. Jinlei Xu: Conceptualization, Methodology, Investigation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21805213), the Natural Science Foundation of Hubei Province (2017CFB337), Hubei Provincial Department of Education (Q20171504).

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