A near-infrared dithieno[2,3-a:3',2'-c]phenazine-based organic co-sensitizer for highly efficient and stable quasi-solid-state dye-sensitized solar cells

A novel near-infrared (NIR) organic sensitizer FNE53 with a strong electron-withdrawing unit, dithieno[2,3-a:3',2'-c]phenazine, has been designed and synthesized for quasi-solid-state dye-sensitized solar cells (DSSCs). By simply fusing the two thiophene rings on quinoxaline unit in sensitizer FNE48, the intramolecular charge transfer (ICT) band bathochromically shifts from 542 nm for FNE48 to 629 nm for FNE53 in toluene solution. The absorption spectrum of sensitizer FNE53 covers the whole visible region and extends to the NIR region, which exhibits complementary absorption profile to another organic dye FNE46 based on quinoxaline. When FNE46 and FNE53 are used as cosensitizers for metal-free cocktail-type quasi-solid-state DSSCs, sensitizer FNE53 not only extends the photoresponse range but also suppresses the intermolecular interactions among the dye molecules. Therefore, the cocktail-type quasi-solid-state DSSC displays much higher IPCE value compared with that for the DSSC sensitizer based on FNE53 and a broader IPCE response in comparison to that for the DSSC sensitizers based on FNE46, respectively. After the molar ratio between the two cocktail dyes is optimized, the highest energy conversion efficiency of 8.04% is achieved in a metal-free quasi-solid-state DSSC cosensitized with FNE46 and FNE53, which exhibits good long-term stability after continuous light soaking for 1000 h.