Small organic molecules engineering for the optimization of components in new-generation photovoltaics

This Ph.D. work involves the design, synthesis, and characterization of some new organic materials as components for new generation photovoltaics. Among new generation photovoltaics, dye-sensitized solar cells (DSSC) stand out for their simple manufacturing and peculiar working mechanism while and perovskite solar cells (PSCs) display remarkable high efficiencies. For these reasons these kind of devices have been the matter of great attention in recent years, aiming to optimize their components in order to improve efficiencies. The first part of this work is related on the synthesis and characterization of a new organic photosensitizer designed for improving Light Harvesting Efficiency (LHE) in DSSC. In particular we focused on a metal-free antenna (donor) - sensitizer (acceptor) covalent adduct (dyad), aiming to obtain a molecular system characterized by high molar attenuation coefficient and broad absorption spectrum in the visible region. Our target was that of exploiting Fӧrster Resonance Energy Transfer (FRET) as a mean to transfer energy within the new antenna-acceptor system. To do that two molecular fragments with complementary light absorption properties were designed and prepared to be joined together using a “click” reaction as late-stage connection. The molecular fragments and the final dyad were spectroscopically characterized using absorption and emission spectroscopy techniques, and it was possible to prove that energy transfer occurred between the two units. In the second part of the work the fabrication and testing of DSSC devices during a period of visiting of Prof. Marina Freitag’s laboratory at Newcastle University is discussed. Three different sensitizers, developed by our research group, were employed together with three copper-based redox mediators, which are particularly suitable for indoor applications. Co-sensitization with commercially available organic sensitizers was also tested as a complementary method to expand absorption. Such experiments showed how the dyes were suitable for application in DSSCs in combination with Cu complex-based liquid electrolytes, one of them reaching a remarkable efficiency. In the third part of this Ph.D. project two new Hole Transporting Materials (HTM) for PSC were prepared. The project was carried out in collaboration with the group of Prof. Aldo Di Carlo at C.H.O.S.E., University of Rome Tor Vergata, were the work of fabrication and characterization of the devices was carried out. The HTMs were characterized by a dimeric structure featuring two different phenothiazine (PTZ) cores, in order to induce a different geometric orientation of the donor units, in turn influencing the compounds packing in the solid phase and possibly affecting the performances of the devices. Before carrying out the synthesis, detailed computational study were performed in collaboration with Prof. Adalgisa Sinicropi, University of Siena. The two compounds were prepared efficiently using Pd-catalyzed couplings, a family of catalytic reactions which represent one of the most straightforward and general methods for the preparation of photovoltai materials and provided very good efficiencies when used to prepare PSC with “inverted” dopant free p-i-n structure, a cell architecture which has been identified as a promising strategy to yield devices with high intrinsic stability.