Phenothiazine conjugated bipyridine as ancillary ligand in Ru(II)-complexes for application in dye sensitized solar cell

A new high molar extinction coefficient ruthenium(II)-bipyridine complex “cis-Ru(4,4′-bis((E)-2-(10-decyl-10H-phenothiazin-3-yl)vinyl)-2,2′-bipyridine)(4,4′-dicarboxylic acid-2,2′-bipyridine)(NCS)2 PTZ1″ was synthesized through conjugation of phenothiazine unit with bipyridine and characterized by FT-IR, 1H-NMR and ESI-MASS spectroscopes. Absorption measurements and time dependent-density functional theory (TD-DFT) calculations show increased spectral response for the ancillary ligand and the corresponding complex. The dye upon anchoring onto mesoporous nanocrystalline TiO2 solar cells exhibited solar-to-electric energy conversion efficiency (η) of 3.77% short-circuit photocurrent density (JSC) = 7.79 mA/cm2, open-circuit voltage (VOC) = 640 mV, fill factor = 0.750) under air mass 1.5 sunlight, the reference Z907 and HRS1sensitized solar cells, fabricated and evaluated under identical conditions exhibited η-value of 7.02% (JSC = 15.25 mA/cm2, VOC = 650 mV, fill factor = 0.705) and 3.05% (JSC = 8.20 mA/cm2, VOC = 610 mV, fill factor = 0.620) respectively. The lower film absorption of PTZ1on TiO2 surface could be probably due to larger molecular diameter and planarity of phenothiazine prone to aggregate in solution as well as on TiO2 surface. The DFT calculations show that the first three HOMOs of PTZ1 have t2g character as observed in case of Z907, while HOMO-4 and HOMO-5 have π-orbitals with major component on phenothiazine moieties of L1.