?-Substituted ZnII porphyrins as dyes for DSSC: A possible approach to photovoltaic windows

The development of building integrated photovoltaic (BIPV) technology and its implementation in construction of the building envelop provide aesthetical, economical and technical solutions toward the zero-energy building. In this perspective dye-sensitized solar cells (DSSCs), which can be obtained in transparent form and with tunable different colors, offer not only an alternative to the traditional silicon solar cells to be applied in particular to decorative effects on windows and glass integrated façades, but also to indoor structures (and furnishings) in order to recapture the energy spent for the inner lighting, thanks to their peculiar ability of operating in diffuse light condition. In this context, porphyrin-based molecules have an immense potential as light harvesting component of dye-sensitized nanocrystalline TiO2 solar cells, reaching now efficiencies up to about 13%. However the multistep synthesis of the best performing porphyrin dyes, showing a meso substitution pattern, is characterized by very low overall yields compromising their possible applicative development for instance in large photovoltaic (PV) glass modules in competition with the actual commercial PV glass modules based on CuInGaSe2 or CdTe thin voltaic films. In this review the renewed interest in the role of the b-substituted ZnII porphyrins for PV application, less studied than the meso substituted ones, is highlighted. Indeed they can rely on a more accessible synthetic procedure since their tetraaryl porphyrinic core can be easily obtained by a one pot reaction between pyrrole and the appropriate aryl aldehyde. Moreover, their remarkable light harvesting properties in the visible range as well as their peculiar steric hindrance, which strongly opposes to the charge recombination process at the photoanode/dye/electrolyte interface, make this kind of cost-effective porphyrinic dyes more promising for application in new PV glass modules based on DSSC technology, to be applied BIPV.