Optical Enhancement in Heteroleptic Ru(II) Polypyridyl Complexes Using Electron-Donor Ancillary Ligands Rui Dong, † Arrigo Calzolari, ‡ Rosa di Felice, § Ahmed El-Shafei, ∥ Maqbool Hussain, ∥ and Marco Buongiorno Nardelli* ,⊥ † Department of Physics, North Carolina State University, Raleigh, North Carolina 27607, United States ‡ CNR-NANO Istituto Nanoscienze, Centro S3 41125 Modena, Italy and Department of Physics, University of North Texas, Denton, Texas 76203, United States § CNR-NANO Istituto Nanoscienze, Centro S3 41125 Modena, Italy and Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089, United States ∥ Polymer and Color Chemistry Program, North Carolina State University, Raleigh, North Carolina 27695, United States ⊥ Department of Physic and Department of Chemistry, University of North Texas, Denton, Texas 76203, United States and CSMD, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: Organic dyes are a viable alternative to silicon for energy conversion. Using simulations from first-principles, we show that chemical manipulation is a powerful tool for tuning the optical absorption spectra of a special class of dyes in a way that is convenient for exploitation in dye- sensitized solar cells. Specifically, we have carried out density functional theory calculations on three Ru(II) polypyridyl complexes with electron- donor ancillary ligands. These complexes were recently developed to study how different electron-donor ancillary ligands affect the photophysical and electrochemical properties of these dyes for light harvesting and photon-to- electron conversion efficiency. We found that the electron-donor ancillary ligands significantly enhance the light harvesting in the visible and the near- infrared regions relative to the reference dye N3. Furthermore, we detected a decrease in the ionization potential, which improves the energy alignment with the redox potentials of the electrolyte. These findings demonstrated that better organic materials for energy applications were developed. 1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have been intensively investigated in both academia and industry in recent years for their potential of low cost and high efficiency. 1−5 A percolating mixture of a molecular dye and a metal-oxide semiconductor nanoparticle (e.g., TiO 2 , ZnO) is the key optical part of DSSCs, where photons are absorbed by the dye and the excited electrons are injected into the conduction band of the semiconductor and collected at the external leads. An ionic electrolyte, typically I − /I 3 − , donates electrons back to the dye to complete the cycle. We investigate in this work novel dye sensitizers (Figure 1). A good dye sensitizer should have the following properties: 6 (1) strong absorption over the entire visible and near-IR (NIR) range, in the challenging search for panchromatic dyes; 7−10 (2) the presence of an anchoring group to favor the attachment to the oxide surface and the proper band alignment of all components, i.e., the LUMO of the dye should be higher than the conduction band of the semiconductor host substrate (e.g., TiO 2 ) to allow for thermodynamically favorable electron injection, and the redox potential of the electrolyte should be higher than the HOMO of the dye for effective dye regeneration (hole replenishment). The last requirement ensures optimal performance over ∼10 8 turnover cycles (about 20 years). A scheme of the alignment of our target dyes is illustrated in Figure 2. The N3 dye [bis(2,2′-bipyridine-4,4′-dicarboxylate)-Ru(II)], labeled as 1 in this work, satisfies all such requirements and has become a benchmark since it was introduced in 1993. 11 Its strong absorptivity is due to a metal-to-ligand charge transfer (MLCT) transition, in which an electron is transferred from the t 2g orbital of the Ru(II) center to the π* orbital of a polypyridyl ligand. This mechanism enhances charge separation and reduces electron−hole recombination. However, the high ionization potential of N3 (0.85 V vs saturated calomel electrode, SCE), generates a potential drop with respect to the redox level of the I − /I 3 − couple used in the electrolyte (0.15 V vs SCE), which in turn makes the dye regeneration process Received: September 30, 2013 Revised: March 31, 2014 Published: April 3, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 8747 dx.doi.org/10.1021/jp409733a | J. Phys. Chem. C 2014, 118, 8747−8755