title: Geometrical isomerism of Ru(II) dye-sensitized solar cell sensitizers and effects on photophysical properties and device performances.
abstract: To supplement our study on thiocyanate-free ruthenium sensitizers (TFRS) for dye-sensitized solar cells (DSSCs), which belong to a class of Ru(II)-based complexes coordinated by a single 4,4'-dicarboxylic acid-2,2'-bipyridine and two symmetrically arranged functionalized trans-azolate chelates, we carefully isolated and characterized the second and less-abundant stereoisomer, in which the two pyridyl azolate ancillaries are asymetrically cis-arranged to each other. Two distinctive ancillaries, namely: 5-[4-(5-hexyl-2-thienyl)-2-pyridinyl]-3-trifluoromethyl pyrazole and 5-(6-tert-butyl-1-isoquinolinyl)-3-trifluoromethyl pyrazole, were employed in this study, giving a total of four sensitizers, that is, thienyl substituted TFRS-2 a and 2 b, and isoquinolinyl substituted TFRS-52 a and 52 b, in which the suffix b indicates the cis-stereoisomers. To gain insight into their fundamental properties their photophysical, electrochemical, and spectroelectrochemical behavior was investigated by density functional theory. Upon comparison of the correspondingly fabricated DSSCs, the sensitizers TFRS-2 a and 52 a yielded significantly higher conversion efficiencies than their asymmetrical cis-counterparts, TFRS-2 b and 52 b. To rationalize the cell performances charge extraction/photovoltage decay and impedance spectroscopic measurements were carried out to compare the rates of interfacial electron recombination from the TiO2 conduction band to the electrolyte.
authors.affiliation: Department of Chemistry and Low Carbon Energy Research Center, National Tsing Hua University, Hsinchu 30013 (Taiwan).
authors.forename: Fa-Chun
authors.initials: FC
authors.lastname: Hu
authors.forename: Sheng-Wei
authors.initials: SW
authors.lastname: Wang
authors.forename: Yun
authors.initials: Y
authors.lastname: Chi
authors.forename: Neil
authors.initials: N
authors.lastname: Robertson
authors.forename: Tracy
authors.initials: T
authors.lastname: Hewat
authors.forename: Yue
authors.initials: Y
authors.lastname: Hu
authors.forename: Shih-Hung
authors.initials: SH
authors.lastname: Liu
authors.forename: Pi-Tai
authors.initials: PT
authors.lastname: Chou
authors.forename: Po-Fan
authors.initials: PF
authors.lastname: Yang
authors.forename: Hao-Wu
authors.initials: HW
authors.lastname: Lin
keywords: density functional theory; dye-sensitized solar cells; pyrazolates; ruthenium; spectroelectrochemistry