Cyclometalated Ruthenium(II) Complexes as Near‐IR Sensitizers for High Efficiency Dye‐Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO 2 films have attracted considerable attention because of their great potential in terms of low fabrication costs and high solar-light-to-electricity conversion efficiency. [1] Extensive efforts have been focused on the development of new, highly efficient sensitizers, as they play a critical role in cell performance. Sensitizers exhibiting absorption over a wide range of the solar spectrum and a high molecular extinction coefficient have been investigated for improving the conversion efficiency of DSSCs. Among the most successful of the various sensitizers are complex N3, [Ru(dcbpy) 2 (NCS) 2 ] (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), [2] and complexes of the type [Ru(dcbpy)(L1)(NCS) 2 ], where L1 is a 2,2'-bipyridine with a highly conjugated ancillary group. [3] However, these sensitizers show insufficient light-harvesting efficiencies in the near-IR region. As the solar spectrum has a large photon flux in the near-IR region above 800 nm, the synthesis of efficient near-IR sensitizers is currently one of the most important issues in the development of solar cells.The absorption properties of Ru II complexes can be tuned by careful consideration of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. [4] The absorption band can be extended into a longer wavelength region by either destabilizing the metal t 2g orbital using a strong s-donating ligand or by introducing a ligand with a low-lying p*-level molecular orbital. Complex N749, (TBA) 3 [Ru(tctpy)(NCS) 3 ] (tctpy = 4,4',4''-tricarboxy-2,2':6',2''-terpyridine; TBA = tetra-n-butylammonium), [5] and complexes of type PRT-11-14, (TBA)[Ru(L2)(NCS) 3 ], where L2 is a 4,4'-dicarboxy-2,2':6',2''-terpyridine derivative with a highly conjugated ancillary group, [6] have been reported to exhibit panchromatic sensitization up to 900 nm. Although the introduction of the tctpy ligand improves near-IR sensitization, the main drawbacks of N749 are the inferior incident-photon-to-current conversion efficiency (IPCE) in the shorter wavelength region, and the presence of three NCS ligands. The former problem arises predominantly from the lack of an effective chromophore, whereas the latter is caused by two factors: 1) The linkage isomers of the NCS ligand cause a decrease in the synthetic yield. [5,7] 2) The stability of the complex decreases owing to dye decomposition by weak Ru-NCS bonding. Although NCS-free Ru II complexes with a conversion efficiency of up to 10 % have been reported, [8] these dyes also show relatively low light-harvesting properties over 800 nm.We [9] and others [10] have examined terpyridyl Ru II complexes of the type [Ru(tctpy)(L3)(NCS)] z , where L3 is a bidentate ligand and z = 0 or + 1, in an attempt to optimize near-IR sensitizers. The role of the NCS ligand is to regenerate the sensitizers from the iodine redox. [10a, 11] Among these complexes, cyclometalated Ru II complexes show superior light-harvesting properties in the...

Excited state decay of cyclometalated polypyridine ruthenium complexes: insight from theory and experiment

“…16,17,19,[107][108][109] Additionally, cyclometalated bridging ligands enhance the electronic coupling between the redox centers in mixed-valent Ru/Ru complexes [110][111][112] and Ru/organic hybrid structures. [113][114][115][116] Despite the large variety of cyclometalated polypyridine ruthenium complexes synthesized up-to-date, however, no phosphorescence comparable to [Ru(bpy) 3 ] 2+ has yet been achieved.…”

Terpyridine and Quaterpyridine Complexes as Sensitizers for Photovoltaic Applications

“…Funaki investigated the possibility to maintain the same terpyridine ligand of Black Dye, tctpy, substituting two thiocyanates with a series of C ^ N bidentate ligands ( 30 in Figure 24) [116,117,118]. These complexes were designed in order to utilize ancillary ligands with stronger donor properties with respect to thiocyanates in order to destabilize the t 2 g HOMO orbital, to reduce the band gap and to harness lower energy regions of the solar spectrum.…”

Applications of Five-Membered Ring Products in Cyclometalation Reactions for Other Purposes

“…Furthermore, in 2012, Funaki et al [ 82 ] reported on a cyclometalated ruthenium compound of 2-phenylpyrimidine containing terpyridine and NCS ligands as ancillary ligands 9.32 as dye-sensitized solar cells. The dye-sensitized solar cells sensitized with the 2-phenylpyrimidine compound 9.32 show 10.7 % effi ciency, which is higher than the N749 ( 9.40 ) benchmark (10.1 %) as shown in Fig.…”