Molecular engineering of pyrene carbazole dyes with a single bond and double bond as the mode of linkage

Both carbazole and pyrene are electron-rich aromatic systems and are expected to be potential donors when used in a push-pull dye architecture in the field of DSSC technology. Herein, two novel pyrene-carbazole dyes bearing single bond (PC1) and double bond (PC2) linkers and cyano-acrylic acid as an acceptor were synthesized. The dye with a double bond spacer (PC2) in the presence of CDCA achieved an improved power conversion efficiency of 6.30% with a short circuit current of 11.59 mA cm-2, open circuit potential (VOC) of 0.80 V, and a fill factor of 0.68 under standard global AM 1.5G solar conditions. Cyclic voltammetry and density functional theory studies indicate that the incorporation of two donors improved the ease of oxidation of the dyes, which resulted in a high VOC. Despite having a rigid single bond, the charge transfer of the PC1 dye is found to be poor, which affected the photovoltaic performance. The dihedral angle measured at each joint of the optimized dye indicated that PC2 exhibits excellent intramolecular charge transfer due to the near planarity in the structure. Besides, the high electron lifetime in the excited state of PC2 makes it the best performer among the three dyes studied.