Since O’Regan and Grätzel’s first report in 1991, dye-sensitized solar cells (DSSCs) appeared immediately as a promising low-cost photovoltaic technology. In fact, though being far less efficient than conventional silicon-based photovoltaics (being the maximum, lab scale prototype reported efficiency around 13%), the simple design of the device and the absence of the strict and expensive manufacturing processes needed for conventional photovoltaics make them attractive in small-power applications especially in low-light conditions, where they outperform their silicon counterparts. Nanomaterials are at the very heart of DSSC, as the success of its design is due to the use of nanostructures at both the anode and the cathode. In this review, we present the state of the art for bothn-type andp-type semiconductors used in the photoelectrodes of DSSCs, showing the evolution of the materials during the 25 years of history of this kind of devices. In the case ofp-type semiconductors, also some other energy conversion applications are touched upon.

The purpose of this work is to contribute to rare earth/3d metal thermodynamics of systems not experimentally determined prior to this research. The thermodynamics of the Ni-rich phase, Me 2 Ni 17 , in the binary systems Me-Ni, with Me ) Sm, Dy, and Yb, has been studied in the temperature range from 900 to 1200 K by a new experimental setup based on solid-state galvanic cells with CaF 2 single crystal as the solid electrolyte. This is particularly useful when the vapor pressure of one of the components of the electrochemical cell is high at the working temperature. The electromotive force method has been adopted to obtain the thermodynamic properties of the intermetallics under investigation. Their ∆ f H T θ values have been found to be -14.6 ( 0.5, -18.5 ( 0.1, and -17.0 ( 0.1 kJ/mol atoms, respectively, for Sm 2 Ni 17 and Dy 2 Ni 17 at T ) 298 K and Yb 2 Ni 17 at T ) 990 K. These values agree well with the calculated values obtained through the Miedema model. Values of the activity of Me in the Ni solid solution in thermodynamic equilibrium with the respective Me 2 Ni 17 phase were calculated at 1000 K to be 6.8 × 10 -10 , 4.8 × 10 -10 , and 1.5 × 10 -10 , respectively, for Sm, Dy, and Yb. The activity against the Me atomic volume was also investigated together with the physical meaning of the partial excess free energy change of the Ni solid solution.

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Solid solutions of the rare earth (RE) cations Pr3+, Nd3+, Sm3+, Gd3+, Er3+ and Yb3+ in anatase TiO2 have been synthesized as mesoporous beads in the concentration range 0.1–0.3% of metal atoms. The solid solutions were have been characterized by XRD, SEM, diffuse reflectance UV-Vis spectroscopy, BET and BJH surface analysis. All the solid solutions possess high specific surface areas, up to more than 100 m2/g. The amount of adsorbed dye in each photoanode has been determined spectrophotometrically. All the samples were tested as photoanodes in dye-sensitized solar cells (DSSCs) using N719 as dye and a nonvolatile, benzonitrile based electrolyte. All the cells were have been tested by conversion efficiency (J–V), quantum efficiency (IPCE), electrochemical impedance spectroscopy (EIS) and dark current measurements. While lighter RE cations (Pr3+, Nd3+) limit the performance of DSSCs compared to pure anatase mesoporous beads, cations from Sm3+ onwards enhance the performance of the devices. A maximum conversion efficiency of 8.7% for Er3+ at a concentration of 0.2% has been achieved. This is a remarkable efficiency value for a DSSC employing N719 dye without co-adsorbents and a nonvolatile electrolyte. For each RE cation the maximum performances are obtained for a concentration of 0.2% metal atoms.

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