Mimicking the Heteroleptic Dyes for an Efficient 1D-ZnO Based Dye-Sensitized Solar Cell Using the Homoleptic Ruthenium(II) Dipyridophenazine Complex as a Photosensitizer

Mimicking the properties of heteroleptic ruthenium dyes, a dipyrido[3,2- a:2′,3′- c ]phenazine-11-carboxylic acid (dppz-COOH) ligand based homoleptic complex, Ru(dppz-COOH) 2 (NCS) 2, with a molar extinction coefficient of 14.3 × 10 3 M –1 cm –1 has been demonstrated as an efficient photosensitizer for the one-dimensional (1D) ZnO nanowire (NW) and ZnO nanoparticle (NP) based dye-sensitized solar cells (DSSCs) employing cobalt complexes as the redox mediators. The as-synthesized homoleptic dye achieves an intense metal-to-ligand charge transfer (MLCT) absorption band throughout the visible region due to the optimal π-conjugated dipyridophenazine ring extension and concomitantly retains an efficient dye loading tendency as confirmed by the chemisorption results. Electron density distributions of the frontier molecular orbitals obtained from density functional theory (DFT) indicated a characteristic MLCT transitions of the dye molecule as well as a favorable charge transfer feasibility from the lowest unoccupied molecular orbital of the dye to the conduction band of ZnO through the anchoring carboxylic groups. All the fabricated DSSCs were characterized by measuring the current density–voltage, incident photon-to-current conversion efficiency, and the electrochemical impedance spectroscopy (EIS). The results establish an efficient light harvesting property of the dye and explain the significance of faster charge transport through 1D ZnO NWs in enhancing the device efficiency in contrast to its ZnO NP counterpart. In consort with the interfacial charge transfer and recombination processes occurring in the DSSCs, EIS analysis likewise provides an insight into the prolonged photoinduced electron lifetime for the cells composed of ZnO NWs as compared to ZnO NPs.