Tuning the electronic structures and related properties of Ruthenium-based dye sensitizers by ligands: A theoretical study and design

Abstract

The geometries, electronic structures, and absorption properties of three heteroleptic ruthenium complexes for dye-sensitized solar cells (DSCs), including Ru(L)(L1)(NCS)2 [L=4,4′-dicarboxylic acid-2,2′-bipyridine, Ru-T1: L1=(E)-2-(4′- methyl-2,2′-bipyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile, Ru-T2: L2=(E)-3-(5′- hexyl-2,2′-bithiophen-5-yl)-2-(4′-methyl-2,2′-bipyridin-4-yl)acrylonitrile, and Ru-T3: L3=(E)-3-(5″-hexyl-2,2′:5′, 2″-terthiophen-5-yl)-2-(4′-methyl-2,2′-bipyridin-4-yl) acrylonitrile)], were investigated by using density functional theory (DFT) and time dependent DFT. The absorption of Ru-T1 in visible region mixed the character of metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT), while the absorption of Ru-T2 and Ru-T3 mixed MLCT, LLCT, and local excitation. Through the introduction of the 3-(5-hexyloligothiophen-5-yl) acrylonitrile group extended the conjugation length of the bipyridine donor ligand of Ru-complexes, the overlap between the ground state and excited states were enhanced, and their molar absorption coefficients were improved. However, the smaller shorter-circuit current densities of Ru-T2 and Ru-T3 were resulted from the excited states with the character of not-effective charge transfer. The 11 kinds of Ru-complexes were designed by removing CN group, substituting of COOH with cyanoacetic acid, and increasing thiophene units. The analysis of excited states and oxidation potential suggest that several designed dyes may have good performance in DSCs.