Improved efficiency of TiO2 nanotubes in dye sensitized solar cells by decoration with TiO2 nanoparticles

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Abstract

In the present work we investigate the effect of TiCl4 treatments on the photoconversion efficiency of TiO2 arrays used in dye sensitized solar cell. The results clearly show that by an appropriate treatment the decoration of the TiO2 nanotube arrays with TiO2 nanocrystallites of a typical size of 3 nm can be achieved. These particles can be converted to mixture of anatase and rutile phase by annealing in air. This decoration of the TiO2 nanotubes leads to a significantly higher specific dye loading and, for certain annealing treatments, to a doubling of the solar cell efficiency (in our case from 1.9% to 3.8% of AM 1.5 conditions) can be achieved.

Introduction

In 1991 Grätzel and O’Reagan established with TiO2 based dye-sensitized solar cells (DSSC), a cheap and versatile technology for large scale production of solar energy conversion devices [1]. The active electrode in DSSCs is usually composed of TiO2 mesoporous films covered by an organic dye [2], [3]. Recently, 1-D oxide morphologies such as, nanowires [4], nanorods [5], [6], [7], or nanotubes [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] have been considered for efficiency enhancement as facilitated electron transport can be expected from these structures due to the suppression of random walk phenomena. Over the past few years particularly self-ordered arrays of TiO2 nanotubes have attracted considerable interest due to various potential applications [11], [19]. In the first report on their dye sensitization [11] only very low visible light conversion efficiencies were reported. Meanwhile the efficiencies are steadily increasing from photoelectrochemical as well as solar cell constructions up to approximately 3% AM 1.5 light illumination [13], [14], [15], [16], [17], [20]. However, recent comprehensive work on nanotubes shows clearly that the limiting factor in efficiency is still the amount of dye that can be deposited per unit length of such TiO2 nanotubes [18].

For nanoparticulate layers, to increase the surface area of the TiO2 solar cells, approaches to decorate the structure with an extra layer of TiO2 nanoparticles is frequently attempted [21], [22], [23], [24], [25], [26], [27], [28], [29]. Different hypotheses for the effect of these TiCl4 treatment has been proposed. These include increased surface area, improved electron transport, light scattering and dye anchoring [23], [24], [25], [26], [27], [28], [29]. In previous work we showed how the hydrolysis of WCl6 precursor can be used for decorating TiO2 nanotubes with WO3 nanocrystallites [30]. In the present work we investigate the effect of TiCl4 treatments on the TiO2 nanotube morphology and the resulting effect on solar cell performance.

Section snippets

Experimental

Ti foils (99.6% purity, Advent) of 0.1 mm thickness were used for the anodic growth of TiO2 nanotubes. Sample pretreatment and anodization was carried out as previously described [31] in an aged ethylene glycol with 0.2 M HF electrolyte for 2 h 20 min to grow 20 μm long tubes.

The as-prepared nanotubes are amorphous in nature but can be converted to anatase phase by annealing. In the present work we used either 350 or 450 °C for 30 min with a heating and cooling rate of 30 °C/min to achieve anatase

Results and discussion

Fig. 1 shows SEM images of the TiO2 nanotube layers before (left) and after (right) the TiCl4 treatment. Clearly the samples after the TiCl4 treatment are uniformly decorated with TiO2 nanoparticles with approximate 3 nm in diameter. From the cross section views it can be seen that in Fig. 1c,d and Fig. 1e,f after TiCl4 treatment, the wall thickness is uniformly increased by ∼25–40 nm. Most importantly, the SEM images show a drastically more corrugated surface area of tubes which allows a

Conclusion

The present work shows that the efficiency in TiO2 nanotube based DSSCs can be significantly improved by a TiCl4 treatment leading to TiO2 nanoparticle decoration on the nanotube walls if the approach is combined with an appropriate annealing treatment.

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