Triphenylamine-based organic sensitizers with π-spacer structural engineering for dye-sensitized solar cells: Synthesis, theoretical calculations, molecular spectroscopy and structure-property-performance relationships

Abstract

The heteroaromatic units play a key role in the D-π-A organic sensitizers for dye-sensitized solar cells (DSSCs), not only as π-spacers for tuning the optical properties and energy levels, but also as π-bridges for efficient electron transfer from donor to acceptor unit. The enhancement of π-spacer donating and accepting abilities is one of essential requirements to improve the molar absorption coefficient and red-shift the absorption band in the D-π-A organic sensitizer, which is beneficial to light harvesting ability. In this study, the effect of π-spacers in the triphenylamine (TPA)-based sensitizers, such as phenyl, pyridine, thiophene, thienothiophene and benzothiadiazole-thiophene, on photovoltaic performances was systematically investigated. Time-dependent density functional theoretical calculations were also performed to unravel the nature of the absorption induced electronic excitations. Lippert-Mataga analysis shows the characteristics of intramolecular charge transfer on the change of dipole moment to provide a further understanding of charge separation in the organic sensitizer. As the electron richness of π-spacer is increased, the absorption band and photocurrent density are enhanced in the order of the π-spacer capability. The DSSC based on TPA-based sensitizer containing thienothiophene exhibits the better power conversion efficiency of 7.40%, Jsc of 14.38 mA cm−2, Voc of 694 mV and fill factor of 0.74. These established structure-property-performance relationships give the deep insight on the impact of various π-spacers in organic sensitizers on device performances and can give molecular design criteria on D-π-A structured organic sensitizers for high efficiency DSSCs.