Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Influence of Rutile on the Efficiency of Dye Sensitized Solar Cells
Titanium dioxide (TiO2) is one of the most extensively investigated material for a wide range of applications due to the ease of production, and suitable physical and chemical properties. The three main crystalline forms of TiO2 are rutile, anatase and brookite, but the latter has been poorly studied because is transformed into rutile at low temperatures. Rutile is the thermodynamically most stable phase showing a band-gap of about 3 eV. However, anatase has been widely used in energy conversion and heterogeneous catalysis due to the higher band-gap, electron mobility, higher adsorption capability and lower electron-hole recombination rate. Typically, higher catalytic activity has been found in mixed-phase materials rather than in the pure rutile or anatase[1]. This is assigned to the presence of trapping states in the band gap, leading to more efficient charge separation processes [2]. Despite the efforts made so far, the role of the rutile/anatase interface on the photoinduced charge-separation process, the key process involved in the conversion of light into electricity in DSSCs and photocatalysis, continue to draw our attention.
In this work, we describe a careful and systematic study that is been carried out using rutile/anatase mixtures in order to understand the key processes involved in the enhancement of solar cell performance. The light to electricity conversion efficiency showed a tendency of decay as a function of the percentage of rutile in mesoporous TiO2 films. This can be attributed to the decrease in the amount of N719 dye adsorbed on the surface, thus lowering the number of electrons injected in the conduction band and the short-circuit current. However, that tendency was halted when the mesoporous films were prepared with mixtures containing 85 to 75% and 70 to 60% of anatase, probably due to synergic effects. Thus, impedance spectroscopy measurements were carried out to understand the reasons in more detail. We found out that the impedance for charge transfer tends to increase as a function of the percentage of rutile, probably due to the increasing diffusion and recombination resistances in the mesoporous TiO2 film. This is reflected on the higher lifetime and higher free mean path of electrons in that range, as confirmed by data obtained by fitting the impedance spectra using a suitable equivalent circuit.
Figure 1: LEFT: Overall solar-to-electricity conversion efficiency versus %anatase in mixed rutile/anatase mesoporous TiO2 films. RIGHT: Nyquist spectrum of cells prepared with TiO2 films containing 100, 95, 90, 85, 80, 75, 70, 60 and 50% of anatase.
[1] Bickley, R.I.; Gonzalez-Carreno, T.; Lees, J.S.; Palmisano, L.; Tilley, R.J.D. A structural investigation of titanium dioxide photocatalysts. Journal of Solid State Chemistry 1991, 1, 178-190. [2] Hurum, D.C.; Agrios, A.G.; Gray, K.A.; Rajh, T.; Thurnauer, M.C. Explaining the Enhanced Photocatalytic Activity of Degussa P25 Mixed-Phase TiO2 Using EPR. The Journal of Physical Chemistry B, 2003, 19, 4545-4549.
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