A new series of N-heterocyclic carbene (NHC)-pyridine ruthenium complexes incorporating a carbene unit as an ancillary ligand were designed and successfully synthesized by using simple synthetic methods. The photophysical, electrochemical and photovoltaic properties of these NHC-pyridine based ruthenium complexes were investigated. These complexes showed photoelectric conversion efficiencies in the range of 6.43 ∼ 7.24% under the illumination of AM 1.5 (100 mW cm(-2)). Interestingly, the modifications on the ancillary ligand of these sensitizers by removal of an alkoxyl group and replacement of the octyl chain with a 3,5-difluorobenzyl group showed a 13% increase in the conversion efficiency for the CifPR dye. These results demonstrated that structural modifications on the NHC-pyridine ancillary ligand of ruthenium complexes results in dye-sensitized solar cells exhibiting a comparable cell performance to that obtained using the standard N719 dye.

Dye-sensitized solar cells (DSSCs) are being investigated extensively for their use in renewable energy technologies because of their low cost and high light-to-electrical energy conversion efficiency.[1] Since their initial report in 1991 by ORegan and Grätzel, DSSCs have attracted much attention from researchers, resulting in the preparation of thousands of ruthenium polypyridine complexes, non-ruthenium organometallic dyes, and metal-free organic sensitizers. [2,3] Organic dyes exhibiting high molar absorption coefficients and presenting a variety of specific functional groups, allowing fine tuning of the absorption spectra, have provided respectable incident photon-to-current conversion efficiencies (IPCEs).[2] Coordination complexes of ruthenium, including N3, [4,5] N719, [6,7] and black dyes, [8][9][10] have also received significant attention because their photophysical and photochemical properties provide DSSCs with excellent photoelectric conversion efficiencies. The most common inorganic dyes exploited in DSSCs are typically ruthenium(II) polypyridine complexes.[3] Several attempts have been made to enhance the efficiency and long-term stability of the dyes, including increasing the conjugation length of the anchoring or ancillary ligand using thiophene, [11,12] carbazole, [13] or other [14][15][16][17] substituents. Long-term stability can be achieved by modifying the architecture of amphiphilic bis(bipyridyl) Ru II dyes featuring alkyl, [18][19][20] alkoxy, [16] or other [21] substituent groups. Grätzel and co-workers replaced the NCS ligand of a ruthenium polypyridine complex with an anionic carbon atom as an alternative approach to structural modification. [22,23] Herein, we report a set of N-heterocyclic carbene (NHC)/pyridine ruthenium complexes, in which one of the nitrogen atoms in the traditional bipyridine framework has been replaced by a carbon atom, for use in DSSCs exhibiting enhanced solar cell performance.Although there is much research on the preparation of metal-carbene complexes and their roles as catalytic reagents and luminescent emitters, their photovoltaic characteristics remain relatively unknown. NHC-pyridine-based ligands, with their unique set of electronic properties, are an exceptional class of donors; therefore, we expected them to make excellent ancillary ligands. The essential requirement of a dye that provides a DSSC with high efficiency is for the energy level of the lowest unoccupied molecular orbital (LUMO) of the sensitizer to be sufficiently high for efficient charge injection into the TiO 2 electrode, while the energy level of the highest occupied molecular orbital (HOMO) must be sufficiently low for efficient regeneration of the oxidized dye by the hole-transport material. Several methods have been reported to lower the HOMO energy level through tuning of the ancillary ligands and their substituents. [3,11,13] The use of ruthenium complexes bearing ancillary ligands functionalized with NHC-pyridine units might be an alternative approach toward tuning the frontier o...

We report the synthesis, characterization, and photovoltaic properties of four ruthenium complexes (CI101, CBTR, CB111, and CB108) having various N-heterocyclic carbene ancillary ligands, pyridine-imidazole, -benzimidazole, -dithienobenzimidazole, and -phenanthroimidazole, respectively. These complexes were designed to investigate the effect of extended conjugation ordained from ring fusion on the power conversion efficiencies of the solar cells. The device sensitized by CB108, the pyridine-phenanthroimidazole conjugated complex, showed an improved efficiency (9.89%) compared to those of pyridine-benzimidazole conjugated system (CBTR, 9.72%) and the parent unfused ring system (CI101, 6.24%). Surprisingly, the sulfur-incorporated pyridine-dithienobenzimidazole system (CB111, 9.24%) exhibited a little lower efficiency than that of N719 (9.41%). The enhanced photovoltaic performance of CB108 was mainly attributed to the increase in electron lifetime and diffusion length confirmed by the electrochemical impedance spectroscopy.

Dye-sensitized solar cells (DSSCs) have recently attracted intensive interest because of their simple construction, low cost, and high efficiency of sunlight to electricity conversion. Usually, a mesoporous titanium oxide (TiO 2 ) film deposited on a transparent electrode is used as a photoanode on which the ruthenium complex dye molecules adsorb. The aim of this work is to prepare pure anatase phase TiO 2 nanoparticles with high surface area for DSSCs application. We used the titanium (IV) n-butoxide as the precursor and acetic acid as a peptizer, after the hydrothermal treatment at 200 • C, the anatase TiO 2 (a-TiO 2 ) can be obtained. The effect of hydrothermal duration on the properties of a-TiO 2 was studied. The TiO 2 products were characterized by XRD, TEM, and BET. The a-TiO 2 particles showed two distinct particle features: the irregular polyhedron and the elongated shaped as observed by TEM. Increasing the hydrothermal reaction time increased the average particle size for both cases and the surface area decreased as well. The a-TiO 2 -based DSSCs were prepared with different thickness for the photovoltaic characteristics study. The best efficiency of ∼6.61% was obtained for thickness of 24 µm (V oc = 0.77 V, J sc = 16.48 mA/cm 2 , FF = 0.56).

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