Anchoring stability and photovoltaic properties of new D(-π-A)2 dyes for dye-sensitized solar cell applications

This study deals with the synthesis and characterization of two new di-anchoring dyes for applications in dye-sensitized solar cells. The materials were designed with a branched D(-π-A)2 structure containing (i) a rigid alkyl-functionalized carbazole core as the donor part, (ii) one (DYE1) or two (DYE2) thiophene units as the π-bridge and (iii) a cyano-acrylic moiety as acceptor and anchoring part. Electrochemical impedance spectroscopy indicated that the injected electron lifetime is higher in the case of DYE2, probably due to the length of the π-spacer that, in combination with the alkyl chain on the carbazole unit, hampers the charge recombination with the electrolyte. Stability tests on TiO2-sensitized films revealed that the di-anchoring remarkably slows down the desorption process, which conversely is evident for classic reference dyes. The highest power conversion efficiency reaches 5.01% in the case of DYE2 with a photovoltage of 0.70 V and a photocurrent of 10.52 mA cm−2, substantially deriving from a broader absorption with respect to DYE1, as also confirmed by IPCE measurements. These results support the efforts aimed at the structural engineering of D(-π-A)2 dyes to design new, more efficient and stable organic sensitizers.