Versatile insertion of acceptors moiety in organic photosensitizers to improve stability in DSSCs working conditions

Dye-sensitized Solar Cells (DSSC) have been considered a potential alternative for building-integrated photovoltaic devices, but their industrial development has been hampered by the efficiency still lower than the one using previous technologies, and by the short lifetime, also due to dye desorption from the semiconductor layer. The aim of this study is to design, synthesize and characterize a small library of new dyes to be used as sensitizers for TiO2 in DSSCs. The efforts have been focused on purely organic compounds and, since most of the organic dyes reported in the literature have a donor-acceptor architecture (D-?-A), we decided to concentrate on such structures, aiming to improve the anchoring moiety first. The main reaction steps to build a common D-? scaffold and to link new anchoring functions, have been performed through palladium-catalyzed cross-coupling reactions. The most efficient protocols have been assessed comparing different procedures such as Ullmann-type and Buchwald-Hartwig reactions to build new carbon-nitrogen bonds; Suzuki and Stille coupling for the carbon-carbon bond formation. Further studies concerning the use of specific ligands, microwave-assisted heating, and the use of additives have been done to establish the most advantageous setup of reaction with our specific substrates. This screening has allowed us to prepare the target dyes having regioisomeric carboxypyridine as new acceptor group. The new compounds underwent full spectroscopic, electrochemical and computational characterization, and their properties where compared with those of a reference compound endowed with a classic cyanoacrylic acid receptor. Test devices have been prepared with these new dyes as sensitizers, power conversion efficiencies corresponding to 54-63% of those obtained with the reference compound were found. Device stability tests carried out on transparent, larger area cells and determination of desorption pseudo-first order rate constants show that some of these new compounds were removed from TiO2 more slowly than the reference dye, thus suggesting a possible cooperative effect of two functional groups on semiconductor binding for isomers having the nitrogen atom close to the carboxylic moiety. Once the efficiency of the new anchoring systems had been proved we looked forward to investigate the binding mechanism and to achieve higher efficiency adding electronwithdrawing substituents. In order to maximize the electronwithdrawing effect, performances in DSSC efficiency and stability against desorption from TiO2 , two isomers have been chosen and carboxyl-N-methyl pyridinium salt derivation of the anchoring group was achieved.