Panchromatic Ru(II) sensitizers TF-30-TF-33 bearing a new class of 6-quinolin-8-yl-2,2'-bipyridine anchor were synthesized and tested under AM1.5 G simulated solar irradiation. Their increased π conjugation relative to that of the traditional 2,2':6',2''-terpyridine-based anchor led to a remarkable improvement in absorptivity across the whole UV-Vis-NIR spectral regime. Furthermore, the introduction of a bulky tert-butyl substituent on the quinolinyl fragment not only led to an increase in the JSC value owing to the suppression of dye aggregation, but remarkably also resulted in no loss in VOC in comparison with the reference sensitizer containing a tricarboxyterpyridine anchor. The champion sensitizer in DSC devices was found to be TF-32 with a performance of JSC =19.2 mA cm(-2) , VOC =740 mV, FF=0.72, and η=10.19 %. This 6-quinolin-8-yl-2,2'-bipyridine anchor thus serves as a prototype for the next generation of Ru(II) sensitizers with any tridentate ancillary.
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.
Two Ru(II) sensitizers TCR-1 and TCR-2 bearing four carboxy anchoring groups were prepared using 4,4',5,5'-tetraethoxycarbonyl-2,2'-bipyridine chelate and 4-(5-hexylthien-2-yl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine and 6-t-butyl-1-(3-trifluoromethyl-1H-pyrazol-5-yl)isoquinoline, respectively. Dissolution of these sensitizers in DMF solution afforded a light green solution up to 10(-5) M, for which their color gradually turned red upon further dilution and deposition on the surface of a TiO2 photoanode due to the spontaneous deprotonation of carboxylic acid groups. These sensitizers were characterized using electrochemical means and structural analysis time-dependent density functional theory (TDDFT) simulation and were also subjected to actual device fabrication. The as-fabricated DSC devices showed overall efficiencies η = 6.16% and 6.23% versus their 4,4'-dicarboxy counterparts TFRS-2 and TFRS-52 with higher efficiencies of 7.57% and 8.09%, using electrolyte with 0.2 M LiI additive. Their inferior efficiencies are possibly caused by the combination of blue-shifted absorption on TiO2, inadequate dye loading, and the perpendicularly oriented central carboxy groups.
Systematically changing the ancillary chelate from L3 to L6, together with addition of a 3,4-ethylenedioxythiophene (EDOT) appendage, boosts the overall efficiencies of a fabricated DSC device.
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