Highly Efficient N‐Heterocyclic Carbene/Pyridine‐Based Ruthenium Sensitizers: Complexes for Dye‐Sensitized Solar Cells

Abstract

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(NHC)/pyridine ruthenium complexes, in which one of the nitrogen atoms in the traditional bipyridine framework has been replaced by a carbon atom, for use in DSSCs exhibiting enhanced solar cell performance.Although there is much research on the preparation of metal-carbene complexes and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...

Tetracarboxylate Bis‐Bipyridine Ruthenium Dyes: Synthesis, Structural and Electronic Characterisation

“…To achieve complete hydrolysis of all eight ester functionalities in [Ru( 3 ) 2 (Cl) 2 ] or the remaining six ester functionalities in the already partially hydrolysed complex, a range of reagent mixtures were trialled (Scheme ), including KOH/acetone/water, KOH/DMF/water, lithium hydroxide, TBAOH/water/methanol and TBAOH/acetone . Hydrolysis reactions were then conducted in the most effective mixtures of acetone/TBAOH, and product formation was readily assessed by observing changes in the aromatic ( δ =10.5–6.5 ppm) and ester ( δ =4–3.5 ppm) regions of the 1 H NMR spectra; individual hydrolysis reactions are presented in Figure ).…”

Synthesis and electron-transfer properties of benzimidazole-functionalized ruthenium complexes for highly efficient dye-sensitized solar cells

“…The IPCE action spectra of the devices account for the order of J SC to be RD5 > N719 > RD10 > RD1 > RD11. Note that the molecular structure of RD10 is similar to a highly efficient Ru sensitizer (CBTR) recently reported, 12 but the latter benzimidazyl ligand was coordinated with the N-C atoms involving more synthetic steps. We emphasize here that the ease of synthesis is an important factor to be considered in making a highly efficient sensitizer for future commercialization.…”

Dye-Sensitized Solar Cells: Fundamentals and Current Status

“…Thus, a number of metal dyes, metal-free organic dyes, and natural dyes have been synthesized till today. Many other dyes like K51 [ 288 ], K60 [ 289 ], K68 [ 290 ]; D5, D6 (containing oligophenylenevinylene π-conjugated backbones, each with one N , N -dibutylamino moiety) [ 291 ]; K77 [ 292 ]; SJW-E1 [ 293 ]; S8 [ 294 ]; JK91 and JK92 [ 295 ]; CBTR, CfBTR, CiPoR, CifPoR, and CifPR [ 296 , 297 ]; Complexes A1, A2, and A3 [ 298 ]; T18 [ 299 ]; A597 [ 300 ]; YS-1–YS-5 [ 301 ]; YE05 [ 302 ]; and TFRS-1–3 [ 303 ] were developed and applied as sensitizers in DSSCs.…”