A dye-sensitized solar cell (DSSC) using Ru complexes as a photosensitizer was first reported by ORegan and Grätzel in 1991.[1] The low-cost, easy preparation make DSSC one of the most promising photovoltaic cells for conversion of sunlight to electricity. Numerous sensitizers have been prepared, and their performance has been tested. [2][3][4][5][6][7][8][9][10] A conversion efficiency of up to 11 % was achieved by using cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) (N3) as a photosensitizer. [11][12] However, the conversion efficiency of DSSCs is still lower than that of the silicon-based photovoltaic cells. To obtain a high conversion efficiency, optimization of the short-circuit photocurrent (I sc ) and open-circuit potential (V oc ) of the cell is essential. The value of V oc depends on the edge of conduction band in TiO 2 and the redox potential of I À / I 3 À , otherwise I sc is related to the interaction between TiO 2 and the sensitizer as well as the absorption coefficient of the sensitizer. The conduction band of TiO 2 was known to have a Nernstian dependence on pH. [13][14] Thus, the molecular engineering of the ruthenium complexes for achieving the highest efficiency was attempted to increase the molar absorption coefficient and reduce the number of protons on the complexes. 4,4'-Dicarboxylic acid-2,2'-bipyridine (dcbpy) has been considered as the best anchoring ligand in Ru sensitizers.[15] Finding new metal-complex sensitizers with higher conversion efficiency was achieved by modifying one of the anchoring ligands. Replacement of one of the dcbpy anchoring ligands with a highly conjugated ancillary ligand represents a molecular engineering approach for increasing the absorption coefficient and therefore the photocurrent density of the sensitizers as reported by Grätzel and coworkers. [16][17][18][19][20] Herein, we report a new ruthenium photosensitizer CYC-B1 in which one of the dcbpy ligands in N3 was replaced with abtpy, a bipyridine ligand substituted with alkyl bithiophene groups. CYC-B1 has the highest absorption coefficient among the Ru-based photosensitizers used in DSSCs, and its power-conversion efficiency is 10 % higher than that of N3 under the same cell fabrication and measuring procedures carried out in our laboratory.CYC-B1 was prepared in a typical one-pot synthesis, [20] and its structure (Scheme 1) was identified from NMR spectroscopy, mass spectrometry, and elemental analysis. The electronic absorption spectra of the free dcbpy and abtpy ligands, CYC-B1, and N3 in DMF are displayed in Figure 1, and the optical data are summarized in Table 1. The absorption maximum (l max ) assigned to the p-p* transition for dcbpy and abtpy are 299 nm and 375 nm, respectively. The absorption maximum of abtpy is red-shifted by 76 nm, which is attributed to its longer conjugation length, compared to that of dcbpy. The absorption spectrum of CYC-B1 shows three bands centered at 553 nm, 400 nm, and 312 nm. Based on comparison with the free abtpy ligand and the homoleptic compl...

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Dye-sensitized solar cells (DSCs) have been explored for more than a decade for realistic photovoltaic applications owing to their high conversion efficiency and low cost.[1] Molecular engineering of the sensitizers to achieve high photovoltaic performance and long-term device stability is one of the critical strategies. Since the first high-efficiency ruthenium-based sensitizer, cis-di(thiocyanato)-bis-(2,2′-bipyridyl)-4,4′-dicarboxylate ruthenium(II) (N3), reported by Grätzel and coworkers in 1993, [2] various structural modifications have been performed to improve the molar extinction coefficient of the sensitizers. It was found that elongating the conjugation length of the anchoring or ancillary ligand is the best route, [3][4][5] although it may come up against the problem of solubility, which is not only a critical point for dye preparation, purification, and identification but also one of the crucial factors for the photovoltaic performance of DSCs.[6] Nevertheless, it is essential to enhance the light-harvesting capacity and at the same time maintain the desirable solubility of dyes to be used in DSCs. Here we report the synthesis and performance of a new well-designed ruthenium complex, SJW-E1 (cis-di(thiocyanato)-4,4′-di(octylethylenedioxythienyl)-2,2′-bipyridine-4,4′-dicarboxylate-2,2′-bipyridine ruthenium(II)), which showed high light-harvesting capacity and good solubility in organic solvents. Another new ruthenium complex, denoted CYC-B3 (cis-di(thiocyanato)-4,4′-di(octylthienyl)-2,2′-bipyridine-4,4′-dicarboxylate-2,2′-bipyridine ruthenium(II)), was also prepared not only to explore the effect of thiophene moieties but also for comparison with SJW-E1 to investigate the impact of the ethylenedioxy groups on the physicochemical properties and performance of the dye molecules.The structures of SJW-E1 and CYC-B3, which incorporate a-octyl-ethylene-dioxythiophene (O-EDOT) and octyl-thiophene-substituted bipyridine, respectively, as an ancillary ligand, are shown in Figure 1. The synthetic details and structure characterizations are provided in the Supporting Information. The frontier orbitals of SJW-E1 and CYC-B3 obtained with a semiempirical [7] calculation method (ZINDO/1) were illustrated in Figure 2. The results showed that both the highest-occupied molecular orbitals (HOMOs) and lowest-unoccupied molecular orbitals (LUMOs) of SJW-E1 and CYC-B3 have similar localizations: The HOMOs and LUMOs are contributed from the metal center with the NCS ligands and the anchoring ligand (4,4′-dicarboxylate-2,2′-bipyridine), respectively. In other words, the two dyes have a similar metal-to-ligand charge transfer (MLCT) excitation. In addition, the same anchoring group in the two dyes provides a comparable interfacial electron transfer process. Therefore the efficiency of the dyes will depend primarily on the absorption coefficients of their MLCT bands. The electronic absorption spectra of the dye molecules measured in dimethylformamide (DMF) (Fig. 3a) display that the lower energy MLCT band for SJW-E1 is centered...

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Two new ruthenium complexes [Ru(dcbpy)(L)(NCS)2], where dcbpy is 4,4′‐dicarboxylic acid‐2,2′‐bipyridine and L is 3,8‐bis(4‐octylthiophen‐2‐yl)‐1,10‐phenanthroline (CYC‐P1) or 3,8‐bis(4‐octyl‐5‐(4‐octylthiophen‐2‐yl)thiophen‐2‐yl)‐1,10‐phenanthroline (CYC‐P2), are synthesized, characterized by physicochemical and semiempirical computational methods, and used as photosensitizers in nanocrystalline dye‐sensitized solar cells. It was found that the difference in light‐harvesting ability between CYC‐P1 and CYC‐P2 is associated mainly with the location of the frontier orbitals, in particular the highest occupied molecular orbital (HOMO). Increasing the conjugation length of the ancillary ligand decreases the energy of the metal‐to‐ligand charge transfer (MLCT) transition, but at the same time reduces the molar absorption coefficient, owing to the HOMO located partially on the ancillary ligand of the ruthenium complex. The incident photon‐to‐current conversion efficiency curves of the devices are consistent with the MLCT band of the complexes. Therefore, the overall efficiencies of CYC‐P1 and CYC‐P2 sensitized cells are 6.01 and 3.42 %, respectively, compared to a cis‐di(thiocyanato)‐bis(2,2′‐bipyridyl)‐4,4′‐dicarboxylate ruthenium(II)‐sensitized device, which is 7.70 % using the same device‐fabrication process and measuring parameters.