Nitrogen-Containing Heterocycles’ Interaction with Ru Dye in Dye-Sensitized Solar Cells

Abstract

Interactions of N-containing heterocycles such as pyrazole, imidazole, pyridine, pyrimidine, pyrazine, and 4-t-butylpyridine (TBP) with the neutral and oxidized Ru(II)-polypyridyl dye (N719) have been studied using density functional theory (DFT). All of the heterocycles formed two hydrogen bonds with N719; one via the N atom with a lone pair of heterocycles and the H atom of the carboxyl group for the dye ligand, and the other via the H atom adjacent to the N atom forming the other hydrogen bond and the O atom with a lone pair of carboxyl groups. Additionally, the heterocycles were examined as an additive in an I−/I3− redox electrolyte solution of a dye-sensitized TiO2 solar cell. As the positive atomic charge on the S atom of the NCS ligand in the oxidized state decreased via interaction with the heterocycles, it became difficult for the oxidized dye to interact with I−, which prevented the dye from regenerating and led to a decrease in the short-circuit photocurrent density (Jsc) of the dye-sensitized solar cell. The DFT results also indicated that the heterocycles moved the energy of both HOMO and LUMO levels up, as well as raised the Fermi level of TiO2. This produced lower electron injection efficiency from photoexcited dye into the TiO2 conduction band and thereby decreased the Jsc value.