Synthesis and photovoltaic property of new kind of organic dyes containing 2,2′-bithiophene unit with three electron-donors

A series of new organic dyes containing 2,2′-bithiophene unit as a π-linker to connect three electron-donors (including carbazole, DH-45; phenothiazine, DH-46; triphenylamine, DH-47) at 3,4′,5′-positions have been designed and synthesized for dye-sensitized solar cells. The experimental results and theoretic calculations indicated that the large steric hindrance among three electron-donors and 2,2′-bithiophene ring partially destroys the whole molecular coplanarity and weakens intramolecular charge transfer (ICT), which results in the narrow, blue-shift of absorption band and decrease of short-circuit current (Jsc). However, the twisted conjugation skeleton for the dyes is favorable for the spatial charge separation that enhances the open-circuit voltage (Voc). Among them, the dye containing carbazole donating group (DH-45) shows best optimized structure for intramolecular charge transfer (ICT) and exhibits better photovoltaic performances than that of the other two dyes (DH-46 and DH-47).