A Ruthenium Complex with Superhigh Light‐Harvesting Capacity for Dye‐Sensitized Solar Cells

Abstract

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...

Visible Light Generation of Iodine Atoms and I−I Bonds: Sensitized I− Oxidation and I3− Photodissociation

“…[52][53][54][55][56][57] It is interesting that this enhancement is observed even though the excited state is localized on an adjacent bipyrazine ligand. The weak temperature dependence of excited state relaxation was examined to establish the activated decay pathways of Ru III (bpz -)(bpz)(deeb) 2+* .…”

DFT/TD-DFT molecular design of porphyrin analogues for use in dye-sensitized solar cells

“…Since then, considerable research has been undertaken to find suitable sensitizers for increasing DSSC efficiency. The majority of this work has centered on ruthenium polypyridyl complexes, [2][3][4][5][6][7][8] where the greatest performance attained in solar-to-electric conversion efficiency has been 11%. 3,4 However, the cost of Ru is high and is likely to increase as the demand for raw Ru materials increases.…”

Increasing the Efficiency of Solar Cells by Combining Silicon- and Dye Sensitized Devices

“…Furthermore, synthetic dyes like ruthenium complexes would increase the power output of the DSSCs [10]. But the costs of the solar cells would definitely increase, too.…”