Bis(9,9-dihexyl-9H-fluorene-7-yl)amine (BDFA) as a new donor for porphyrin-sensitized solar cells

•Two novel zinc porphyrin dyes with a bis(9,9-dihexyl-9H-fluorene-7-yl)amine as an electron donor were synthesized.•Dyes displayed broader absorption spectra with an onset ∼700 nm and ∼28 nm red-shift compared to YD2 dye.•Moving long alkyl chains from the porphyrin ring to donor groups increased charge resistance and electron life times.•ZZX-N1 having two 3,5-di-tert-butylphenyl groups in the porphyrin ring gave energy conversion efficiency of 5.78%.

Two novel zinc porphyrin dyes (coded as ZZX-N1 and ZZX-N2) with a bis(9,9-dihexyl-9H-fluorene-7-yl)amine (BDFA) as an electron donor and a benzoic acid as an acceptor were designed and synthesized in a donor–π–acceptor configuration for dye-sensitized solar cells. ZZX-N1 with two small 3,5-di-tert-butylphenyl groups on the meso positions of porphyrin ring and ZZX-N2 with two large 2,6-dioctoxylphenyl groups displayed similar absorption spectra with an onset ∼700 nm, and ∼28 nm red-shift was observed when compared to YD2 because of stronger electron donating ability of bis(9,9-dihexyl-9H-fluorene-7-yl)amine (BDFA) than bis(4-hexylphenyl)amine in YD2. ZZX-N1-sensitized cells exhibited higher energy conversion efficiency than ZZX-N2-sensitized device (5.78% vs. 3.61%). The electrochemical impedance study showed higher electron recombination resistance in the interface of TiO2/dye/electrolyte in ZZX-N1-sensitized cell than in ZZX-N2-sensitized cell. The transient decay measurements showed the longer electron lifetime of former than the later. The density functional theory calculations suggested that this could be due to small voids among ZZX-N1 dye molecules on the TiO2 surface, preventing the charge recombination with the redox couple. The results demonstrated the potential of BDFA as an excellent functional group for high efficiency solar cells.

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