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Among the third-generation photovoltaic devices, much attention is being paid to the so-called Quantum Dot sensitized Solar Cells (QDSCs). The currently poor performance of QDSCs seems to be efficiently patched by the ZnS treatment, increasing the output parameters of the devices, albeit its function remains rather unclear. Here new insights into the role of the ZnS layer on the QDSC performance are provided, revealing simultaneously the most active recombination pathways. Optical and AFM characterization confirms that the ZnS deposit covers, at least partially, both the TiO(2) nanoparticles and the QDs (CdSe). Photoanodes submitted to the ZnS treatment before and/or after the introduction of colloidal CdSe QDs were studied by electrochemical impedance spectroscopy, cyclic voltammetry and photocurrent experiments. The corresponding results prove that the passivation of the CdSe QDs rather than the blockage of the TiO(2) surface is the main factor leading to the efficiency improvement. In addition, a study of the ultrafast carrier dynamics by means of the Lens-Free Heterodyne Detection Transient Grating technique indicates that the ZnS shell also increases the rate of electron transfer. The dual role of the ZnS layer should be kept in mind in the quest for new modifiers for enhancing the performance of QDSCs.

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Self-assembled monolayers (SAMs) of 11-amino-1-undecanethiol (AUT) have been prepared on polycrystalline Au by immersion of the corresponding surfaces in an AUT 1 mM solution in pure ethanol. The films were studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), and quartz crystal microbalance (QCM). Results of CV and EIS experiments in NaClO 4 solution agree with the fast formation of a well-packed film with low current density (hundreds of nA/cm 2 ) and capacitance values around 2 µF/cm 2 . The films were stable between -0.7 and 0.7 V (vs Ag/AgCl/NaCl sat.). Average values of 1.6 nm and 26°were obtained for the film thickness and the tilt chain angle in the potential region under study. The kinetic analysis of the adsorption process, monitored in situ by the QCM technique, showed that it occurs in two stages: a fast Langmuir type adsorption step (k 1 ) 0.1047 min -1 ), followed by a much slower process of molecular rearrangement (k 2 ) 0.0020 min -1 ). AFM operated in tapping mode did not reveal any morphological changes on the surface after immersion in AUT discarding multilayer growth. The electron transfer (ET) of Fe(CN) 6 -3 and Ru(NH 3 ) 6 +3 species in solution through the AUT layer was investigated by CV and EIS. A mechanism of selective permeation of the electroactive species across the monolayer, controlled by the nature of the electrostatic interactions established at the SAM-solution interface, explains the experimental data obtained and previously reported in the literature for ET processes through substituted SAMs. Analysis of the film structure according to theoretical models (commonly used for SAM characterization) to our experimental data, led to contradictory results clearly affected by the nature of the unconsidered electrostatic interactions (values of θ ) 0.80 and 0.99 were obtained in the presence of Fe(CN) 6 -3 and Ru(NH 3 ) 6 +3 , respectively, in KCl electrolyte using the Amatore model).

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