Elsevier

Journal of Power Sources

Volume 270, 15 December 2014, Pages 42-52
Journal of Power Sources

Synthesis of hierarchical TiO2 flower-rod and application in CdSe/CdS co-sensitized solar cell

https://doi.org/10.1016/j.jpowsour.2014.07.091Get rights and content

Highlights

  • Double-layered TiO2 flower-rod structure was directly grown on FTO glass.

  • A possible formation mechanism of this TiO2 flower-rod architecture was proposed.

  • This TiO2 architecture was used in CdSe/CdS co-sensitized solar cell.

  • The efficiency was significantly enhanced with CdSe/CdS/TiO2 flower-rod electrode.

Abstract

A hierarchical double-layered TiO2 flower-rod structure composed of three-dimensional (3D) TiO2 flowers and one-dimensional (1D) nanorods on transparent fluorine-doped tin oxide (FTO) conducting glass has been synthesized by a facile hydrothermal method. The possible formation mechanism of the hierarchical architecture is also proposed. When used in CdSe/CdS quantum dots co-sensitized solar cells (QDSSCs), the 1D ordered rutile nanorods at bottom can accelerate the electron transfer rate by providing direct electrical pathway for photogenerated electrons, while the 3D flowers formed on the top of nanorods can increase the adsorption of QDs due to the enlarged areas, and can also be used as a scattering layer. The performance of the CdSe/CdS/TiO2 flower-rod solar cell can achieve a short-circuit current density (Jsc) of 13.46 mA cm−2, and a open-circuit voltage (Voc) of 0.42 V, with a maximum power conversion efficiency of 2.31% under one sun illumination (AM 1.5 G, 100 mW cm−2), which is greatly higher than that of CdSe/CdS/TiO2 nanorod solar cell (1.63%).

Introduction

Nanostructured titanium dioxide (TiO2) is an important material which is widely applied in areas related to photoelectrolysis, photocatalysis, gas sensing, photovoltaic devices, etc [1], [2], [3], [4], [5]. Nanostructured TiO2 was usually used as photoanode substrate for quantum dots sensitized solar cells (QDSSCs) because of its several advantages, such as suitable conduction band position, stable chemical and physical properties, and inexpensive cost [6], [7]. Up to now, nanostructured TiO2 with various morphologies has been synthesized, such as nanoparticles, nanorods, nanotubes, and so on [8], [9], [10]. Recently, several works have been done on the 3D structural TiO2 which based on 1D rutile nanorod [11], [12], but seldom researches have been reported on the formation of double-layered hierarchical TiO2 nanostructures, which consisted of 1D rutile TiO2 nanorods and 3D TiO2 flowers. The photoelectrochemical properties of TiO2, especially in sensitized-solar cells, can be improved with formation of double-layered TiO2 structure. Previous research work reported by Ho et al. [13] have revealed that an enhanced performance of dye-sensitized solar cell can be obtained with double-layered metal-based flexible TiO2 photoanode which consisted of TiO2 nanotubes and TiO2 nanoparticles as the underlayer and overlayer, respectively. In addition, our group has reported that free-standing TiO2 nanowire/nanotube (NW/NT) structure applied in QDSSCs is also favorable for the enhancement of photovoltaic performance [14]. However, multiple steps are involved in the fabrication of this kind of free-standing TiO2 NW/NT photoanode, which increases the complexity of the operation. Moreover, hierarchical TiO2 flower-rod structure which prepared by one-step hydrothermal, recently reported by our group, as photoanode in Mn-doped CdS QDSSCs has an important effect on the improvement of solar cell performance [15]. Hence, the hierarchical TiO2 flower-rod film on transparent FTO glass which prepared with easy fabrication process may show potential value in CdSe/CdS co-sensitized solar cell. In QDSSCs based on hierarchical TiO2 flower-rod architecture, 1D nanorods can offer direct electronic pathway, accelerating the electrons transfer rate and reducing recombination of electrons and holes [16], [17], [18], [19]; meanwhile 3D TiO2 flowers can provide the increased surface areas, leading to more adsorption of QDs and more light absorption. Furthermore, 3D TiO2 flowers can also be used as a scattering layer to enhance the light harvesting and improve the performance of QDSSCs.

Various narrow-band-gap semiconductors quantum dots such as CdS, CdSe, PbS, PbSe and InP are commonly used as sensitizers in QDSSCs [20], [21], [22], [23] due to their several attractive characteristics, such as the tunable band gap, high extinction coefficients, large intrinsic dipole moments, multiple excitons generation (MEG), and hot electron injection [24], [25], [26]. Among these semiconductor QDs, CdS and CdSe have been paid much attention due to their appropriate band gap of 2.25 eV and 1.70 eV in bulk, respectively, which can allow the extension of absorption band to the visible region and show better performance in light harvesting under visible region [27]. Comparing these two kinds of QDs, CdS has a higher conduction band edge with respect to TiO2 [28], which is advantageous to the injection of excited electrons from conduction band of CdS, resulting in high charges separation efficiency. Nevertheless, the band gap of CdS (2.25 eV in bulk) determines its maximum absorption range can only reach to approximately 550 nm, which means that a small portion of visible light can be utilized to produce photoexcited electrons. On the contrary, expanding the absorption edge close to 730 nm can be obtained with CdSe, but with a lower electron injection efficiency than CdS due to its conduction band edge is located below the conduction band of TiO2 [28]. To incorporate both the advantages of these two kinds of materials, CdS and CdSe were usually used as co-sensitizers. The co-sensitized structure of CdS and CdSe has been proved to be advantageous over single CdS or CdSe in QDSSCs [29]. Currently, three methods are commonly used in the deposition of QDs on TiO2: (i) in situ growth of QDs by chemical bath deposition (CBD) or successive ionic layer adsorption and reaction (SILAR) [30], (ii) assemble of presynthesized colloidal QDs with linker molecules [31], (iii) deposition of QDs by electrochemical atomic layer deposition (EC-ALD) method [32], [33]. Among these methods, the CBD is the most widely used method due to the direct contact, high coverage of QDs on TiO2 surface, and facile preparation process. Therefore, the CBD approach has been adopted to sensitize CdSe/CdS QDs on TiO2 film in this work.

In this study, a hierarchical double-layered TiO2 flower-rod structure film on FTO glass was successfully fabricated by a facile hydrothermal method and was decorated with coupled QDs (CdS and CdSe) through in situ chemical bath deposition (CBD) to form a photoanode which can be directly illuminated in QDSSCs. The CdSe/CdS/TiO2 flower-rod solar cell exhibited much better performance than the QDSSCs based on CdSe/CdS/TiO2 nanorod, CdS/TiO2 flower-rod, and CdSe/TiO2 flower-rod photoanodes. This kind of CdSe/CdS co-sensitized solar cell based on double-layered TiO2 flower-rod substrate provides a novel strategy to construct QDSSCs.

Section snippets

Materials

Titanium butoxide (Ti(OC4H9)4), concentrated hydrochloric acid (HCl, 36.5–38 wt%), sodium chloride (NaCl), cadmium nitrate (Cd(NO3)2·4H2O), sodium sulfide (Na2S·9H2O), selenium powder (Se), sodium sulfite (Na2SO3), sulfur powder (S), potassium chloride (KCl) and chloroplatinic acid (H2PtCl6·6H2O) were purchased from Tianjin Chemical Reagents Co. Ltd. All the chemicals are of analytic grade and used directly in experiments without further purification. Deionized water (DI water, resistivity of

The formation of hierarchical TiO2 flower-rod structure

The hierarchical double-layered TiO2 flower-rod film directly grown on FTO glass has been synthesized using a facile hydrothermal method with the conductive side facing up. Morphologies of this TiO2 flower-rod have been recorded by field-emission scanning electron microscope (FE-SEM). Fig. 1a and b are FE-SEM images of the FTO glass and the TiO2 architecture grown at 150 °C for 4 h, respectively. It can be clearly seen that the surface of the FTO glass is covered sparsely with TiO2 nanorods

Conclusion

In summary, the hierarchical double-layered TiO2 flower-rod film directly grown on FTO glass has been successfully synthesized by a facile hydrothermal method. The possible formation mechanism of this hierarchical architecture is also proposed based on random aggregation of regrowth nanorods. The hierarchical double-layered TiO2 flower-rod film as substrate has been applied in CdSe/CdS co-sensitized solar cells. The effects of CdS and CdSe CBD cycles on the performance of QDSSCs have been

Acknowledgment

The authors gratefully acknowledge the support for this work from the Key Project of Tianjin Sci-Tech Support Program (no 08ZCKFSH01400).

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