Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
The adsorption of dyes as monolayers onto titania in dye sensitized solar cells is assumed to be preferred over the formation of multilayers because the first case allows a more efficient charge transport from the dye to the titania compared to the latter. We have measured directly the concentration depth profiles of N719 adsorbed on nano-porous titania and ALD titania [1] and Z907 on nano-porous titania [2] using neutral impact collision ion scattering spectroscopy (NICISS). It was found that the both N719 and Z907 molecules adsorb as islands on titania and that the growth of the dye layer can be described as an increase of the number of islands with the adsorption isotherm bearing similarities to a Langmuir adsorption isotherm. The total coverage of the surface with dye molecules is 65 to 85% of the total titania surface area thus that even at the maximum coverage the titania is incompletely covered.[2] Impedance spectra show that recombination losses increase with increasing dye coverage which means that the dye molecules have to be considered as a major pathway for recombination.[2]. The current, as well as the efficiency, increase linearly while the open-circuit voltage slightly increases and the fill factor slightly decreases. The increase in current with the coverage of titania with dye molecules means that the performance of the DSCs follows the number of dye molecules adsorbed.
The figure shows the concentration depth profile of the sample immersed into the 0.3 mM N719 dye solution.[1] The measured profile is deconvoluted and corrected for the spherical nature of the substrate.[1] The profile shows that a fraction of the surface is covered with a monolayer of the dye (region with a thickness < 10 Å, fraction indicated with the upper arrow) and a region with multilayers (region with a thickness > 10 Å, fraction indicated with the lower arrow). The choice of 10 Å for separating mono- and multilayer is to some degree arbitrary.
[1] Ellis-Gibbings, L.; Johansson, V.; Walsh, R. B.; Kloo, L.; Quinton, J. S.; Andersson, G. G.; Formation of N719 Dye Multilayers on Dye Sensitized Solar Cell Photoelectrode Surfaces Investigated by Direct Determination of Element Concentration Depth Profiles. Langmuir, 2012, 28, 9431 - 9439. [2] Johansson, V.; Ellis-Gibbings, L.; Clarke, T.; Gorlov, M.; Andersson, G.; Kloo, L. On the Correlation between Dye Coverage and Photoelectrochemical Performance in Dye-sensitized Solar Cells. PCCP, 2014, 16, 711 - 718.
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