Twisted coumarin dyes for dye-sensitized solar cells with high photovoltage: adjustment of optical, electrochemical, and photovoltaic properties by the molecular structure

Abstract

The general drawback of coumarin dye dye-sensitized solar cells (DSSCs) is their relatively low photovoltage. According to our previous experience, the twisted molecular structure is beneficial for the prevention of π-aggregation. Thus, in order to further individually evaluate the effect of twisted and curved structures on optical, electrochemical, and photovoltaic properties, herein, rational molecular design has been performed to develop three simple coumarin dyes, coded as CS-3, CS-4, and CS-5. For CS-3, overlarge dihedral angles seriously twist the molecular skeleton and affect the intramolecular charge transfer process, leading to the poorest light-harvesting capability among the three dyes. More importantly, with a highly twisted structure, the charge recombination rate of CS-3 is obviously accelerated. In contrast, with an appropriate twisted and curved structure, CS-4 shows better light-harvesting capability than CS-3, as well as a better prevention effect of charge recombination. As a result, a high photovoltage of 704 mV is obtained by CS-4 based DSSCs even without a co-adsorbent. Accordingly, our finding demonstrates that although the breakage of molecular coplanarity may weaken the light-harvesting capability and decrease the photocurrent, an appropriate twisted and curved molecular structure is still greatly favorable for the improvement of photovoltage, providing a powerful strategy for the future development of organic sensitizers with high photovoltage.