Modification and Potential Applications of Organic–Inorganic Non-Siliceous Hybrid Materials - Mesoporous Organic-Inorganic Non-Siliceous Hybrid Materials - page 86

Fig. 5.10 Ruthenium dye
functionalized by both
phosphonic and carboxylic
acids was anchored on TiO 2
substrate. Reprinted with
permission from Ref. [ 52 ].
Copyright 2013, American
Chemical Society
to the tetrahedral phosphorus center and loss of conjugation [ 50 , 51 ]. Combining
the superior binding stability of phosphonate and the good electron injection
efficiency of carboxylate has resulted in a feasible method (Fig. 5.10 ) [ 52 ]. A
bis(tridentate)-ruthenium complex containing phosphonic and carboxylic acids
was elaborated [ 51 ]. The underlying basis of this strategy was that the carboxylate
moiety only needed to be positioned on the ligand that was involved in the charge
transfer to the TiO 2 , while the phosphonate moieties could be installed on the
opposing ligand that did not need to participate directly in the injection process.
This led to interesting electron injection properties from the dye into the semi-
conductor and a good stability in aqueous media. The absolute power conversion
efficiencies (PCEs) were not remarkable, which was expected because of the poor
spectral coverage of the dyes, but the trends in the data provide indirect informa-
tion about how charge collection is affected by the dye structure.
In the context of solar energy conversion, quantum-dot-sensitized solar cells
(QDSSCs) are a promising alternative to existing photovoltaic technologies due
to the tunable band gap and promise of stable, low-cost performance [ 53 ]. In
addition, the QDs open up a way to utilize hot electrons and to generate multi-
ple electron-hole pairs with a single photon through impact ionization. The use
of organic linkers between the QDs and titania provides a means of eliminating
recombination and leads to an increased conversion efficiency and improved sta-
bility [ 53 , 54 ]. Ardalan et al. [ 55 ] investigated the effects of self-assembled mon-
olayers with phosphonic acid head groups on the bonding and the performance of
cadmium sulfide (CdS) SSCs. Several organophosphonic acids with different tail
groups (-NH 2 , -COOH and -CH 3 ) were taken as the linkers. It was demonstrated
that the nature of the tail group does not significantly affect the uptake of CdS
quantum dots on TiO 2 nanocrystallites nor their optical properties, but the pres-
ence of the phosphonic-based linkers had a significant effect on the photovoltaic
device performance. The PCEs in devices made with phosphonic acids were up to
about 3 times higher than those without any anchoring agent, which might be due
to the organic linkers acting as recombination barriers or the passivated defects at
the TiO 2 surface (Fig. 5.11 ) [ 55 ]. However, the electrical measurements showed