Synthesis, Characterization And Photoresponse Properties Of Ruthenium-Based Dye Sensitizers For Solar Cells: Computational And Experimental Studies

ABSTRACT

Dye-Sensitized Solar Cells (DSSCs) are promising third-generation photovoltaic devices that offer the conversion of light energy into electricity at a lower cost. At the heart of this device is a sensitizer (dye) adsorbed on TiO2-photoanode, which is responsible for the harvesting of visible light energy. However, the currently used sensitizers offer little improvement to the overall cell efficiency and this constitutes the main challenge to this technology. This work reports a combined experimental and computational study for the synthesis and characterization of the designed ruthenium-based dye complex (S2*) as a potential sensitizer for use in DSSCs. Initially, a series of ligands (S1, S2, S3 and S4) and their corresponding ruthenium-based dye complexes (S1*, S2*, S3* and S4*) were designed. Then, selection criteria based on the photoresponse properties, HOMO-LUMO energy gaps and redox properties of these dye complexes were considered to select the potential sensitizer. The structures of the ligands and corresponding ruthenium complexes were fully optimized using B3LYP variant of the density functional theory (DFT) in conjunction with the 6-31G(d,p) basis set. The Stuttgart-Dresden (SDD) effective core potential was used to represent the inner orbitals of ruthenium (Ru). Subsequently, excitation energies were computed at the optimized geometries of the ligands and complexes using time-dependent DFT. Aided by computed results, the potential dye (S2*) was synthesized and characterized using spectroscopic methods of Fourier Transform Infrared (FTIR), UV-Vis and fluorescence. Detailed theoretical study of S2* was performed using computational approaches such as DFT and TD-DFT at the B3LYP/6-31G(d,p) level. Full geometry optimization followed by frequency calculations suggested the structure of S2* as a genuine minimum characterized with Hessian index of zero (0). The optimized iii geometry of S2* was used to compute excitation energies as well as to investigate solvation effects using the Polarizable Continuum Model (PCM). Both theoretical and experimental absorption spectra indicate that S2* displays a wellenhanced metal-to-ligand charge transfer band (MLCT) in the visible region. This character is defined by an absorption maximum of 452 nm experimentally, which is in good agreement with 483 nm obtained computationally. Solvatochromic studies of S2* shows that acetonitrile is the best choice of solvent in terms of photocatalytic enhancement and tetrahydrofuran is the best solvent in terms of bathochromic effect, with a broad low intense red-shifted band. S2* shows excellent photoresponse properties which makes it to be a potential candidate for DSSCs.