DOI: 10.1002/adma.200502540 High-Efficiency Organic-Dye-Sensitized Solar Cells Controlled by Nanocrystalline-TiO 2 Electrode Thickness** By Seigo Ito, Shaik M. Zakeeruddin, Robin Humphry-Baker , Paul Liska, Raphaël Charvet, Pascal Comte, Mohammad K. Nazeeruddin, Peter Péchy , Masakazu Takata, Hidetoshi Miura, Satoshi Uchida, and Michael Grätzel* The dye-sensitized solar cell (DSC) [1,2] is an attractive candidate for a new renewable energy source because of the low-cost materials and the facile manufacturing used in its production. [3] This cell has been intensively studied in the last decade. The photovoltaic effect in a DSC originates at the in- terface between a redox electrolyte containing iodide and triiodide (I – /I 3 – ) ions and a dye-derivatized mesoscopic TiO 2 electrode. For outdoor applications, ionic liquids containing I – and I 3 – ions are the medium of choice because of their high thermal stability, non-flammability, negligible vapor pressure, and low toxicity. [4] Since the diffusion of I – and I 3 – ions in ionic liquids is slow owing to their high viscosity, [5] a thin nanocrys- talline TiO 2 film is required to reach a high conversion effi- ciency. In order to compensate for the lower optical depth of such thin, porous electrodes, a high-extinction-coefficient dye should be used. A similar strategy is applied to other types of viscous electrolytes [6] and organic hole conductors. [7] The pres- ent study reports, for the first time, on a DSC combining a high-extinction-coefficient organic sensitizer with an ionic-liq- uid electrolyte. An indoline dye was employed (referred to as D149 below, Fig. 1), [8] which has an extinction coefficient (68 700 mol –1 cm –1 at 526 nm) that is five times higher than that of the conventional high-efficiency Ru dye (N719, 13 900 mol –1 cm –1 at 541 nm) [9] and reaches 8 % conversion efficiency with a volatile organic electrolyte. A screen-printed double layer, consisting of a nanocrystalline TiO 2 film and a scattering layer, constitute the mesoporous working electrode that supports the organic sensitizer (Fig. 1b). Below, we scru- tinize the effect of the film thickness of nanocrystalline TiO 2 films on the photovoltaic performance of ionic-liquid- and acetonitrile (AcCN)-based electrolytes. Results obtained under 100 % sun irradiation are shown in Figure 2. The open-circuit photovoltage (V OC , Fig. 2a) de- creases for both the AcCN- and ionic-liquid-based electro- lytes with increasing film thickness, due to augmentation of the surface area, which provides additional charge-recombina- tion sites [10–12] and enhances the dark current. [13] Moreover, for thick films the outer TiO 2 particle layers do not contribute significantly to the photogeneration of conduction-band elec- trons, owing to the filtering of light by the dyed particles lo- cated close to the fluorinated tin oxide (FTO) glass. The shar- ing of photoinjected conduction-band electrons by these particles lowers their quasi-Fermi level and hence the V OC . In contrast to the V oc , the short-circuit photocurrent density (J SC ), fill factor (FF), and overall solar-energy-conversion effi- ciency (g) obtained with the two electrolytes are affected in a different way by the film thickness. For the AcCN solvent (Fig. 2b), J SC increases continuously with film thickness, reaching a plateau value of close to 20 mA cm –2 at 14 lm, while g reaches a maximum of 9 % at a thickness of 12.6 lm. In contrast, for the ionic-liquid-based device, both J SC and g peaked at 6.3 lm, the values being 16 mA cm –2 and 6.7 %, re- spectively. The FF remained constant for the volatile AcCN- based electrolyte, while the ionic-liquid-based system showed a decline in FF , which became more prominent as the film thickness increased beyond 9 lm (Fig. 2c). The different be- havior observed when using the different electrolytes is COMMUNICATIONS 1202 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18, 1202–1205 – [*] Prof. M. Grätzel, Dr. S. Ito, Dr. S. M. Zakeeruddin, Dr. R. Humphry-Baker, Dr. P. Liska, Dr. R. Charvet,Dr. P. Comte, Dr. M. K. Nazeeruddin, Dr. P. Péchy Laboratoire de Photonique et Interfaces École Polytechnique Fédérale de Lausanne 1015 Lausanne (Switzerland) E-mail: michael.graetzel@epfl.ch Prof. M. Takata, Dr. H. Miura Chemicrea Inc. Quattro Muromachi Bldg. 9F 4-16 Nihonbashi Muromachi Chuoh-ku, Tokyo 103-0022 (Japan) Prof. S. Uchida Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University 1-1 Katahira 2-Chome, Aoba-ku, Sendai 980-8577 (Japan) [**] This work was supported by a grant from the Swiss Federal Energy Office (OFEN). Figure 1. Molecular structure of the dye D149 and the configuration of the DSC.