Interfacial Charge Transfer in Dye-Sensitized Solar Cells Using SCN-Free Terpyridine-Coordinated Ru Complex Dye and Co Complex Redox Couples

Abstract

The efficiency of dye-sensitized solar cells (DSSCs) using Ru complex dyes and Co complex redox couples has been increased with a strategy to prevent charge recombination via the addition of bulky or lengthy peripheral units to the dyes. However, despite the efforts, most of the DSSCs are still suffering from nonunity quantum efficiency and fast recombination. We examine the effect of SCN ligand, which has been used for many Ru complex dyes and could attract positively charged Co complexes. We find that replacing the ligands with 2,6-bis(2'-(4'-trifluoromethyl)pyrazolyl)pyridine increases the quantum efficiency and electron lifetime. With the combination of the replacement of SCN ligands and the addition of bulky moiety, ∼80% external quantum efficiency is achieved. These suggest that not only the addition of a blocking effect but also the reduction of electrostatic and dispersion forces between dyes and Co complexes are essential to control the charge separation and recombination processes.

Structural Effect of the Pendant Unit in Thiocyanate‐Free RuII Sensitizers on the Dye‐Sensitized Solar Cell Performance

“…The T7 dye exhibited two oxidation potentials at 0.92 and 1.17 V vs. NHE in solution. The potential difference between the Ru 2+/3+ center and the TPA in T11 was smaller than that of T7 as a result of changing the alkyl chains to alkoxy chains at the peripheral positions of the TPA unit . T12 , with the sterically crowded TPA unit, showed a unimodal wave at 0.91 V vs. NHE, suggesting the overlapping of two oxidation potentials of the Ru center and the TPA.…”

“…The IPCE values of 400–550 nm for T11 and T12 cells reached above 85 %. The PCEs of T11 and T12 cells were higher than that of T7 . A comparison between T7 and T11 shows that the addition of branched alkoxy chains slightly increased the current density but resulted in little effect on the V oc value.…”

“…Two terpyridine‐based ligands 1 and 2 with different TPA structures were synthesized. Two TPA units were prepared by the Buchwald–Hartwig amination between 4‐iodo‐1‐(2‐ethylhexyloxy)benzene and 4‐bromoaniline or 4‐bromo‐2,6‐dimethylaniline . After the conversion of bromide to boronic acid, the TPA units were coupled with the terpyridine ligand through the Suzuki coupling reaction.…”

“…Porous TiO 2 films on the transparent conducting glass substrates were immersed into 0.2 m m dye solutions containing 20 m m chenodeoxycholic acid (CDCA) in toluene/ethanol (4:1 v/v) . The coadsorption of CDCA has been used to suppress undesired dye aggregation on the TiO 2 surface.…”

“…Recently, Wu et al reported a SCN‐free bipyridine‐based Ru complex giving 9.53 % energy conversion efficiency with a Co complex redox couple . We have recently reported that a Ru complex dye, T7 , which was designed by replacing thiocyanate ligands of a well‐known Ru complex dye, called Black dye, with a tridentate ligand and by adding a bulky TPA moiety to the terpyridine ligand of the dye, increased charge separation efficiency and thus PCE values . Ru complexes with a terpyridine ligand can give wider absorption spectra than those with a bipyridine ligand.…”