Novel D–A–π–A coumarin dyes containing low band-gap chromophores for dye-sensitised solar cells

Coumarin dyes containing a low band-gap chromophore of benzothiadiazole, which comprises a coumarin moiety as the electron donor and a cyanoacrylic acid moiety as electron acceptor in D–A–π–A system, were developed to use in dye-sensitised solar cells (DSSCs). The introduction of the benzothiadiazole unit into the molecular framve distinctly narrowed the band-gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), so that the absorption peak was red-shifted. As a result, a solar cell based on the HKK-CM4 sensitiser showed better photovoltaic performance with a JSC of 14.3 mA cm−2, a VOC of 0.58 V, and an FF of 0.72, corresponding to an overall conversion efficiency η of 5.97% under standard AM 1.5 irradiation.

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► Synthesis of coumarin dyes with a low band-gap chromophore in D–A–π–A system.
► The narrow band-gap by introducing a benzothiadiazole unit into the molecular frame.
► A maximum photo-to-electron conversion efficiency of 5.97% for DSSC with HKK-CM4 dye.