First-principles study of nickel complex with 1,3-dithiole-2-thione-4,5-dithiolate ligands as model photosensitizers

Abstract

Dye-sensitized solar cells have become in one important and promising technology in the photovoltaic field. The ability for a sensitizer to harvest light photons and inject the excited electrons into a photoanode, typically a metal oxide, determines the performance and operation range of the solar cell. Metal complexes with 1,3-dithiole-2-thione-4,5-dithiolate (dmit) ligands, which are an important class of functional materials, have received extensive attention due to their intriguing chemical and physical properties. The electronic and molecular properties of isolated and adsorbed nickel complexes with dmit ligands have been investigated using first-principles calculations based on the density functional theory (DFT). Adsorption energies of metal complexes supported on the anatase TiO2(101) surface were calculated for three different configurations, linked by sulfur atom of Sthione, Sthiole–Sthiolate, and planar. The most stable adsorption configurations found in this study are the Sthiole–Sthiolate and the planar forms for the nickel complex. TD-DFT molecular calculations reveal that the lowest energy transition in ultraviolet visible near-infrared (UV–Vis-NIR) mainly corresponds to the HOMO–LUMO π–π* excitation for the nickel complex. The effect of the TiO2 (101) surface on the absorption spectra of the nickel complex is practically limited to a red shift of about 0.1–0.3 eV. The analysis of the density of states for the dmit/TiO2 (101) system shows that the LUMO of the metal complex lies at the edge of the TiO2 conduction band indicating, therefore, that electron injection from the complex excited state into the semiconductor surface is unlikely.