Enhancing dye-sensitized solar cell performances by molecular engineering: highly efficient π-extended organic sensitizers.

This study deals with the synthesis and characterization of two π-extended organic sensitizers (G1 and G2) for applications in dye-sensitized solar cells. The materials are designed with a D-A-π-A structure constituted by i) a triarylamine group as the donor part, ii) a dithienyl-benzothiadiazole chromophore followed by iii) a further ethynylene-thiophene (G1) or ethynylene-benzene (G2) π-spacer and iv) a cyano-acrylic moiety as acceptor and anchoring part. An unusual structural extension of the π-bridge characterizes these structures. The so-configured sensitizers exhibit a broad absorption profile, the origin of which is supported by density functional theory. The absence of hypsochromic shifts as a consequence of deprotonation as well as notable optical and electrochemical stabilities are also observed. Concerning the performances in devices, electrochemical impedance spectroscopy indicates that the structural modification of the π-spacer mainly increases the electron lifetime of G2 with respect to G1. In devices, this feature translates into a superior power conversion efficiency of G2, reaching 8.1%. These results are comparable to those recorded for N719 and are higher with respect to literature congeners, supporting further structural engineering of the π-bridge extension in the search for better performing π-extended organic sensitizers.