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

► Novel Ru(II) sensitizer for application in dye sensitised solar cells. ► Phenothiazine is an excellent electron donating chromophore. ► N-alkyl phenothiazine conjugated bipyridine, a new class of ancillary ligand for Ru(II) sensitizers. ► Multiple performance improving structural features incorporated for increased efficiency. 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 TiO 2 solar cells exhibited solar-to-electric energy conversion efficiency ( η) of 3.77% short-circuit photocurrent density ( J SC) = 7.79 mA/cm 2, open-circuit voltage ( V OC) = 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% ( J SC = 15.25 mA/cm 2, V OC = 650 mV, fill factor = 0.705) and 3.05% ( J SC = 8.20 mA/cm 2, V OC = 610 mV, fill factor = 0.620) respectively. The lower film absorption of PTZ1on TiO 2 surface could be probably due to larger molecular diameter and planarity of phenothiazine prone to aggregate in solution as well as on TiO 2 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.