Versatile dyes based on benzothiadiazole and benzoselenadiazole chromophores have been developed that perform efficiently in dye-sensitized solar cells. Power conversion efficiency of 3.77% is realized for a dye in which charge recombination is probably hindered by the nonplanar charge-separated structure.

No abstract

Dye-sensitized solar cells (DSCs) have been explored for photovoltaic applications because of their low cost and impressive conversion efficiency.[1] A DSC with 10 % efficiency was first demonstrated by Grätzel and co-workers using N3 (cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4-dicarboxylate) ruthenium(II)) as a sensitizer.[2] Progress in optimizing ruthenium-based sensitizers for DSCs has been focused primarily on enhancing the light-harvesting ability, and redshifting the metal-to-ligand charge transfer (MLCT) band. [3][4][5][6][7][8][9][10] These results can be achieved by extending the conjugation length of the anchoring or ancillary ligand. [11][12][13][14][15] Furthermore, retaining the photoinduced interfacial charge separation between the dye molecules and TiO 2 is also a crucial strategy to enhance the performance of DSCs. [16] This strategy is beautifully demonstrated by adding a hole-transport segment on the dye molecule in all-solid-state DSCs. [17,18] However, this concept could not be applied to liquid-state DSCs, [19,20] probably because the ruthenium sensitizers have relatively low light-harvesting capacity and large molecular size.We have shown [21,22] that thiophene-derived units are the good candidates for increasing the conjugation length of the ancillary ligand to increase the light-harvesting ability and red-shift the MLCT band of a ruthenium complex. Herein we reveal that thiophene-derived species can be functionalized easily with a alkyl-substituted hole-transport moiety, such as bis(heptyl)carbazole. Ruthenium complexes with ligands functionalized by thiophene, carbazole, and alkyl chains can be regarded as supersensitizers. The efficiency of a liquidstate DSC based on one of these supersensitizers is 9.72 %, which is 1.2 % higher than that (8.51 %) of the N3-based cell at the same fabrication and efficiency measuring conditions. This is the first demonstration that a carbazole moiety in the dye can enhance the performance of a liquid-state DSC. Furthermore, the terminal alkyl chains on the ancillary ligand were also modulated to explore the impact of the sensitizer size on the cell performance in the DSC. In situ photoelectrochemical measurements were used for the first time to study the intramolecular electron-transfer processes of the oxidized dye.The structures of the supersensitizers CYC-B6S and CYC-B6L are depicted in Figure 1. The electronic absorption spectra of these supersensitizers and N3 measured in DMF are displayed in Figure 2, and the optical data are summarized in Table 1. The absorption spectra of the supersensitizers show that the band centered at around 550 nm (which is the characteristic metal-to-ligand charge-transfer (MLCT) transition) is stronger and more red-shifted than that for N3. These results indicate that the spectral response of ruthenium

New organic dyes that contain variable lengths of conjugation featuring alternating thiophene and fluorene segments have been synthesized and efficient nano-crystalline TiO2 based dye-sensitized solar cells were fabricated using these molecules as light-harvesting sensitizers.