Anthraquinone sulfonate modified, layered double hydroxide nanosheets for dye-sensitized solar cells.

Dye-sensitized solar cells (DSCs) have been extensively investigated for solar energy conversion by using various combinations of inorganic semiconductors and organic sensitizers because of their low cost, easy production, and high efficiency. For efficient visible-light absorption, various organic dyes have been intensively exploited because of their advantages, such as their high molar extinction coefficients and tunable optical bands, in which they are anchored on mesoscopic TiO2 semiconductors. Recently, inorganic semiconducting materials, such as quantum dots (CdSe, CdS,) and organometal perovskites, have been proposed as inorganic sensitizers in photovoltaic cells to overcome some drawbacks of the organic dyes such as their relatively low heat stability and narrow absorption bands. One possible approach to improving the inherent light-harvesting ability of the organic dyes is to hybridize them with nanosized multifunctional inorganic materials such as layered double hydroxides (LDHs), which can provide a stable chemical environment, higher heat or photostability, and are environmentally friendly. The LDHs, also known as anionic or hydrotalcite-like clays, are useful in new multifunctional systems such as biological carriers, catalysts, and hybrid optical layers. Recently, Duan et al. reported an ultrathin hybrid film consisting of LDH nanosheets and luminescent polyanions, in which the LDH nanosheets induced a welldefined photoluminescence of polymer monolayers that are individually separated by the exfoliated LDH nanosheets. Moreover, the LDHs provide a stable chemical environment to increase the photochemical function and thermal stability of the intercalated organic photochromic dyes. In this study, LDH nanosheets are suggested as the inorganic matrix in an attempt to induce an intense photochromic function of the organic photochromic dyes, anthraquinone sulfonate anion (AQS), that are chemically immobilized on the surface of the LDH nanosheets. Herein, we report a new hybrid light sensitizer for DSCs, in which the AQS anion is selected as the organic sensitizer and the LDH nanosheets as the inorganic host. This is believed to be the first example of hybrid LDH/organic nanosheets used as a light sensitizer in photovoltaic devices. The chemical structure and photochromic behavior of the LDH AQS nanosheets in formamide are shown in Figure 1a. The powder X-ray diffraction (XRD) pattern of LDH AQS microcrystals indicated a well-crystallized rhombohedral hydrotalcite-like, 3R1 phase with lattice parameters of a=3.05 and c=60.0 . An antiparallel arrangement of the AQS would be the best model by assuming that the length of the AQS anion was 12.9 . Notably, a transparent solution was obtained by ultrasound treatment for 10 min, indicating the successful exfoliation of the platelike LDH AQS microcrystals. Typical Tyndall light scattering of the resulting solution demonstrated the presence of exfoliated LDH nanosheets as shown in Figure 1a. Interestingly, the suspension showed a strong photoinduced coloration that was not seen in the AQS–formamide solution. In (4) of Figure 1b, the UV/Vis absorption spectra for the irradiated suspension of the LDH AQS nanosheets show extremely enhanced absorption bands in the range of 400–600 nm, where the absorption bands at 435 and 525 nm are characteristic signals for the reduction state of anthraquinone sulfonate (AQS ) that have a long-term stability in a high pH condition, whereas formamide (the solvent) might be oxidized during the photoreaction. Under continuous irradiation of the exfoliated LDH AQS solution shown in (2) of Figure 1b, the band intensities gradually increased as a function of the irradiation time and then decreased after the light was cut off. The rate constants for photocoloration and decay were 0.072 and 0.059 min , respectively. This may be [a] Dr. J. H. Lee, J. Chang, J.-H. Cha, Prof. D.-Y. Jung, S. S. Kim, Prof. J. M. Kim Department of Chemistry-BK21 and Sungkyunkwan Advanced Institute of Nanotechnology Institute of Basic Sciences, Sungkyunkwan University Suwon, 440-746 (Korea) Fax: (+82)31-290-7075 E-mail : [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000703.