Design and Synthesis of Multifunctional Symmerical Squaraine Dyes for Molecular Photovoltaics by Terminal Alkyl-Chain Modifications

Novel far-red-sensitive symmetric squaraine (SQ) dyes with terminal alkyl-chain modifications were designed, synthesized and characterized aiming towards imparting multifunctionalities such as photosensitization, dye aggregation prevention and source of electrolyte components. The dye sensitizers SQ-77 and SQ-80 with the alkyl chain terminal modifications consisting of iodine and 1-methylimidazolium iodide, respectively, were designed and synthesized as new dye sensitizers for DSSCs based on the symmetric SQ-5 without any terminal modification used as reference. Upon adsorption on the mesoporous TiO2 surface, The SQ-77 depicted an enhanced dye loading and dye aggregation. On the other hand, SQ-80 demonstrated reduced dye aggregation and stronger binding on the TiO2 surface leading to enhanced durability of DSSCs. Apart from the most common photosensitization behaviour, the newly designed dye demonstrated multifunctionality such as aggregation prevention and electrolyte functionality utilizing iodine-based redox electrolyte in the presence and absence of I2 and LiI additives. In the absence of LiI and I2, a mixture of SQ-77 and SQ-80 demonstrated a photoconversion efficiency of 1.54 % under simulated solar irradiation, which was about 6 times higher as compared to reference dye SQ-5 (0.24 %) having no alkyl chai terminal modification.