Structure-property relationship of novel monosubstituted Ru (II) complexes for high photocurrent and high efficiency DSSCs: Influence of donor versus acceptor ancillary ligand on DSSCs performance

Abstract

Two novel high molar extinction coefficient monosubstituted-bipy Ru (II) complexes, IA-5 and IA-6, based on D-D-π and π-A-π-A ancillary ligands were synthesized with the aid of Knoevenagel reaction, to study the influence of the electron donor and electron acceptor ancillary ligand and number of anchoring group (COOH) on the light harvesting efficiency (LHE), ground and excited state oxidation potentials, incident-photon-to-current conversion efficiency (IPCE), short-circuit photocurrent density (J), and total solar-to-electric conversion efficiency (%η) for DSSCs, and their device performances were studied and showed a maximum of PCE of 7.81% (JSC = 17.61 mA cm−2, VOC = 0.69 V and FF = 64.05%) for dye IA-6 compared to PCE of 7.74% (JSC = 15.83 mA cm−2, VOC = 0.74 V and FF = 65.37%) for N719 dye. The photophysical and photoelectrochemical properties discussed herein addressed the significant impact of the electron donor and electron acceptor ancillary ligand and the number of anchoring groups on JSC and %η in DSSCs. The molecular structures of IA-5 and IA-6 were characterized using UV–Vis, emission spectrophotometry, FT-IR, ESI-MS, and 1H NMR.To probe the interrelationship between the chemical structure, photophysical and photoelectrochemical properties, molecular modeling studies, implemented in Gaussian, were employed. DFT/TD-DFT calculations were used to calculate the thermodynamics and electronic properties of IA-5 and IA-6 including HOMO and LUMO isosurfaces, lowest singlet-singlet electronic transitions (E0-0), ground and excited states oxidation potentials, GSOP and ESOP, which were in excellent agreement with the experimental results. Surprisingly, the insertion of the strong electron acceptor benzodithiazole in the ancillary ligand of IA-5 showed that the frontier LUMO shifted by 100% from 2,2′-bipyridyl-4,4′-dicarboxlic acid to the ancillary ligand containing benzodithiazole with electron injection accomplished from the anchoring group tethered to benzodithiazole. This new finding of relocating the LUMO from bipy-dicarboxylic acid to the other ancillary ligand would open the door for the molecular engineering of better light harvesting and more efficient Ru (II) complexes for DSSCs.