Theoretical study of cyclometalated Ru(II) dyes: Implications on the open-circuit voltage of dye-sensitized solar cells

A density functional theory (DFT) study was performed to elucidate why variations in the 2-phenylpyrimidinato (L) ligand of cyclometalated Ru(H3tcterpy)L(NCS) dye affect the open-circuit photovoltage (Voc) of a dye-sensitized solar cell (DSSC) using an iodine/iodide redox couple, where tcterpy represents 4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine. Neutral dye molecules interact with I2 via the N atom of the L ligand and the S atom of the NCS ligand. Among the nine tested dyes, the intermolecular interaction strengths between the dye and I2 correspond to the reduced Voc of DSSC. Additionally, I− interacts with oxidized dye via the L ligand and via the NCS ligand, and a second I− bonds to the first I−. The interaction strengths of I− with oxidized dyes strongly correlate with the enhanced Voc. The computational results suggest that the L ligand structure influences the regeneration of oxidized dye and recombination. Consequently, the L ligand structure changes Voc.