DFT and TDDFT study on the electronic structure and photoelectrochemical properties of dyes derived from cochineal and lac insects as photosensitizer for dye-sensitized solar cells

Essential parameters related to the photoelectrochemical properties, such as ground state geometries, electronic structures, oxidation potential and electron driving force, of cochineal insect dyes were investigated by DFT and TDDFT at the B3LYP/6-31+G(d,p) level of the theory. The results show that the major charge flow dynamic for all dyes is the HOMO -> LUMO transition. The bi-coordinated binding mode, in which the dye uses one carboxyl- and hydroxyl oxygen bound to Ti(IV), is found for all dye-TiO2 systems. Additionally, the doubly bi-coordinated binding mode in which the dye used both carboxyl groups bound to two Ti(IV) is also possible due to high energy distribution occupied at anchoring groups. This study highlights that most of these insect dyes can be good photosensitizers in dye-sensitized solar cells based on their strong binding to the TiO2 surface, good computed excited state oxidation potential and thermodynamically favored electron driving force.