ARTICLE pubs.acs.org/JPCC
Insights into Working Principles of Ruthenium Polypyridyl Dye-Sensitized Solar Cells from First Principles Modeling Frederic Labat,†,* Ilaria Ciofini,† Hrant P. Hratchian,‡ Michael J. Frisch,‡ Krishnan Raghavachari,§ and Carlo Adamo†,* †
Laboratoire d’Electrochimie, Chimie des Interfaces et Modelisation pour l’Energie, CNRS UMR-7575, Ecole Nationale Superieure de Chimie de Paris Chimie-ParisTech, 11 rue P. et M. Curie, F-75231 Paris, Cedex 05, France ‡ Gaussian, Inc., 340 Quinnipiac Street, Building 40, Wallingford, Connecticut 06492, United States § Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States
bS Supporting Information ABSTRACT: With the aim to better describe the phenomena taking place at the dye/semiconductor interface in dye-sensitized solar cells and to understand the interfacial electron transfer from the excited dye to the semiconductor, the cis-[Ru(4,40 -COOH-2,20 -bpy)2(NCS)2]/TiO2 system has been investigated using density functional theory (DFT) in conjunction with a periodic approach and a hybrid functional. At this level of theory, the interplay of the electronic and geometrical coupling between the semiconductor and the dye has been analyzed, and the feasibility of the interfacial electronic transfer has been discussed. Our results show that the electronic transfer is highly favorable from a thermodynamic point of view, the LUMO of the dye being significantly higher in energy than the conduction band of the semiconductor. In addition, the theoretical injection time, computed using the Newns-Anderson model, is in fair agreement with the observed value.
1. INTRODUCTION Conversion of solar radiation to electricity is among the most difficult challenges for energy generation from natural resources. Today, photovoltaic technology is a rapidly growing field with solar cell applications being used at many levels, ranging from consumer electronics to large power plants. In comparison with conventional fossil sources, operation without noise and toxic and greenhouse gas emissions are key advantages.1 Although intense research activity has been devoted to this field (including many modeling studies), the high cost of conventional siliconbased solar cells, originating from the high purity of the silicon required for efficient operation of the cell, remains a strongly limiting factor for a larger utilization of such a technology.2 Today, cost reductions of this conventional approach are mostly foreseen by economies of scale, thin films being the most promising technology.3,4 A valuable alternative is represented by dye-sensitized solar cells (DSSC) in which the optical response of a large-band-gap semiconductor is shifted from the UV to the visible region by dye sensitization (for a recent review of the latest developing progress of DSSC, see ref 5). These systems present pleasing advantages over traditional inorganic compounds, such as flexibility and easier spectral tunability obtained by chemical modification of the dye. In addition to its inexpensiveness and nontoxicity, TiO2 is the most commonly used semiconductor material since it can be made highly porous, exposing a large surface area for dye r 2011 American Chemical Society
binding, thus significantly increasing the number of dye molecules that can contribute to current generation.6 Combined with ruthenium polypyridyl complexes such as cis-[Ru(4,40 -COOH2,20 -bpy)2(NCS)2], the N3 dye, chemically bound to anatase TiO2 nanoparticles by carboxylic acid anchoring groups, the conversion efficiencies (around 11%) obtained in the early 1990s7 are still very close to that using the most advanced recent developments.4,5 In the commonly accepted model, the basic working principles of N3-based DSSC are relatively well-defined: upon photoexcitation of the dye, an ultrafast interfacial electron transfer (
