Enhancement of power conversion efficiency of dye-sensitized solar cells by co-sensitization of zinc-porphyrin and thiocyanate-free ruthenium(II)-terpyridine dyes and graphene modified TiO2 photoanode

A combination of a meso substituted zinc-porphyrin with two pyridyl groups and two benzoate groups at cis positions to each other (POR) with a mononuclear ruthenium(II) complex containing a 2,6-bis(1-methylbenzimidazol-2-yl)pyridine ligand and a carboxylic acid functionalized terpyridine ligand (SPS-G3) has been used as co-sensitizing system for the fabrication of dye-sensitized solar cells (DSSCs). The POR/SPS-G3 co-sensitized solar cell with a TiO2 photoanode displays enhanced short circuit current, open circuit voltage, and fill factor values (J(sc) = 16.18 mA cm (-2), V-oc = 0.64 V, FF = 0.71, respectively), resulting in an overall power conversion efficiency (PCE) of 7.35%, which is superior to that for the DSSCs based on the individual dyes POR and SPS-G3. This is attributed to the improved light harvesting efficiency of the POR/SPS-G3 co-sensitizing system, which is evidenced by the incident photon-to-current efficiency (IPCE) spectrum of the co-sensitized solar cell. Further improvement of the PCE value of the co-sensitized solar cell (up to 8.15%) has been achieved by using a graphene modified TiO2 (G-TiO2) photoanode, instead of a pure TiO2 photoanode. Based on dark current measurements and electrochemical impedance spectra (EIS), the increased PCE value of the solar cell with the G-TiO2 photoanode can be attributed to suppression of charge recombination at the photoanode/dye/electrolyte interface and enhancement of electron transport in the photoanode. These results are in accordance with the longer electron lifetime exhibited by the G-TiO2 photoanode.