Rigid fused π-spacers in D-π-A type molecules for dye-sensitized solar cells: a computational investigation

A class of D-pi-A dyes with rigid fused pi-bridges comprising electron rich and deficient segments has been investigated by means of quantum chemical calculations. To gain a better understanding of the effects of these rigid spacers on the photophysical properties and the cell performances, the geometries, electronic structures, and absorption spectra of the dyes and dye-TiO2 complexes, and the relevant physical parameters in relation to the short-circuit photocurrent density and the open-circuit photovoltage are analyzed in detail to establish structure-property relationships. The calculated results indicate that the pi-bridges containing both the electron-deficient moiety and electron-rich moiety could improve light-absorbing capacities, modulate the levels of frontier orbitals, facilitate intramolecular electron delocalization by decreasing the aromaticity of the pi-system and destroying the coherence in possible multiple electron transfer pathways, and enhance charge injection at the dye/TiO2 interface. Such combination of molecular components with different electronic characters provides an effective approach to design novel pi-bridges and shed light on the development of high-performance dyes.