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1.1 hybrid interfaces for photovoltaics 7by the liquid-solid dye sensitized solar cell (DSSC), wherean organic dye is used for absorption of light and injectionof the photoexcited electron into a TiO 2 mesoporoussubstrate. In 1991, Grätzel proposed a DSSC with 7% efficiencyusing a Ru-complex dye, a nanocrystalline TiO 2mesoporous film [29] and liquid redox electrolyte (usuallythe I − 3 /I− system) acting as hole transporting layer [30].In the solid state dye-sensitized solar cell (SDSSC) [31,32], the holes are transferred to a solid organic hole transportermaterial (HTM) infiltrated within the substrate[33].High efficiency for the SDDSC systems has been obtainedusing as HTM the spiro-OMETAD[31], a small optically inactivemolecule forming a solid amorphous network. Alsoin this case the principal limit in the improvement of thesolid state DSSC efficiencies is the high rate of recombinationbetween photogenerated electrons and the holes [33].By replacing the HTM with a polarisable liquid electrolyte,the screening of the holes makes possible to reach efficienciesas high as 12%[34], though reducing the long-term stabilityof the cell[35, 32]. Novel strategies are to use inorganicinterlayers (e.g. ZrO 2 [36]) to separate the HTM andthe metal oxide. The recombination can be reduced but inthis case the charge injection to the semiconductor is affectedtoo.Hybrid polymer/metal oxide systems can be seen as aparticular case of SDSSC, where the polymer combines thefunctions of light-absorption and charge transport in thesame material so replacing both the dye and the hole transportingmaterial [22]. Although, in principle, there are noreasons for which the solid state technology should havepoorer efficiencies than in DSSCs, however polymer/metaloxide efficiencies are still well below DSSC.The above scenario and the technological potential ofpolymer/metal oxide systems require the optimization ofthe polymer, a better fundamental understanding and accuratetheoretical investigations.