Fluorine substituent effect on organic dyes for sensitized solar cells

Two sets of organic dyes containing a stilbene backbone with fluorine substituents were designed for a study on the quantum efficiency of dye-sensitized solar cells (DSSCs). The results revealed that adding a fluorine substituent on the phenyl group ortho to the cyanoacrylate can enhance the light-harvesting performance in comparison with the unsubstituted one. However, when the two ortho-positions were both substituted by fluorine atoms, the performance of DSSCs was substantially reduced. The reason was mainly ascribed to a distortion from a planar geometry caused by steric hindrance. The π-conjugation was therefore disturbed, and the result led to a substantial reduction of the short-circuit photocurrent density (Jsc). Another effect was found that the open-circuit photovoltage (Voc) of the doubly substituted derivative was lower than that of the mono-substituted one. The more flexible conformation of the difluoro-substituted dyes induced an undesired nonradiative decay, therefore led to a reduction of open-circuit photovoltage. The phenomenon can be verified by electrochemical impendence spectrum. The non-planar geometry was realized by a computation using the density function theory (DFT) model. The slight blue shift of absorption band was also consistent with the calculated transition energy by a time dependent DFT model.

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► Dyes containing a stilbene backbone with fluorine substituents were designed for a study on the quantum efficiency of DSSCs.
► Adding a fluorine substituent on the phenyl group ortho to the cyanoacrylate can enhance the light-harvesting performance in comparison with the unsubstituted one.
► When the two ortho-positions were both substituted by fluorine atoms, the performance of DSSCs was substantially reduced.
► A distortion from a planar geometry caused by steric hindrance led to a substantial reduction of the short-circuit photocurrent density.