Efficient Sensitization of Dye-Sensitized Solar Cells by Novel Triazine- Bridged Porphyrin-Porphyrin Dyads Galateia E. Zervaki, † Mahesh S. Roy, ‡ Manas K. Panda, † Panagiotis A. Angaridis, † Emmanouel Chrissos, † Ganesh D. Sharma,* ,§ and Athanassios G. Coutsolelos* ,† † Laboratory of Bioinorganic Chemistry, Department of Chemistry, University of Crete, Voutes Campus, P.O. Box 2208, 71003 Heraklion, Crete, Greece § R&D Center for Engineering and Science, JEC Group of Colleges, Jaipur Engineering College, Kukas, Jaipur (Raj.) 303101, India ‡ Defence Laboratory, Jodhpur (Raj.) 342011, India * S Supporting Information ABSTRACT: Two novel porphyrin-porphyrin dyads, the sym- metrical Zn[Porph]-Zn[Porph] (2) and unsymmetrical Zn- [Porph]-H 2 [Porph] (4), where Zn[Porph] and H 2 [Porph] are the metalated and free-base forms of 5-(4-aminophenyl)-10,15,20- triphenylporphyrin, respectively, in which two porphyrin units are covalently bridged by 1,3,5-triazine, have been synthesized via the stepwise amination of cyanuric chloride. The dyads are also functionalized by a terminal carboxylic acid group of a glycine moiety attached to the triazine group. Photophysical measurements of 2 and 4 showed broaden and strengthened absorptions in their visible spectra, while electrochemistry experiments and density functional theory calculations revealed negligible interaction between the two porphyrin units in their ground states but appropriate frontier orbital energy levels for use in dye-sensitized solar cells (DSSCs). The 2- and 4-based solar cells have been fabricated and found to exhibit power conversion efficiencies (PCEs) of 3.61% and 4.46%, respectively (under an illumination intensity of 100 mW/cm 2 with TiO 2 films of 10 μm thickness). The higher PCE value of the 4-based DSSC, as revealed by photovoltaic measurements (J-V curves) and incident photon-to-current conversion efficiency (IPCE) spectra of the two cells, is attributed to its enhanced short-circuit current (J sc ) under illumination, high open-circuit voltage (V oc ), and fill factor (FF) values. Electrochemical impedance spectra demonstrated shorter electron-transport time (τ d ), longer electron lifetime (τ e ), and high charge recombination resistance for the 4-based cell, as well as larger dye loading onto TiO 2 . ■ INTRODUCTION Dye-sensitized solar cells (DSSCs) are currently attracting considerable attention as alternatives to conventional amor- phous silicon-based solar cells because of their low production cost, simplicity of fabrication, and high power conversion efficiency (PCE). In these devices, a thin layer of a sensitizer (a chromophore), chemically bound to nanoporous TiO 2 , is photoexcited and ultimately provides the photocurrent. 1 Among the most efficient DSSC sensitizers are ruthenium polypyridyl complexes, which exhibit intense and wide-range absorption from the visible to near-IR regime 2 and attain cell efficiencies above 11%. 3 However, because of their high cost and environmental concerns, their wide application is limited. Metal-free organic dyes have also been used as sensitizers, 4 but the most efficient ones result in cell efficiencies in the range of 9-10%. 5 A lot of effort is currently devoted to the development of new, efficient chromophores that exhibit strong and panchromatic absorption and good light-harvesting efficiency and are suitable for practical use. 6 Porphyrins, owing to their light-harvesting potential (exemplified by their role in photosynthesis) and their physicochemical properties, is a class of compounds that stimulated significant research interest as artificial antennae in solar cells. 7 Porphyrin derivatives exhibit intense absorption in the visible regime (a strong Soret band in the 450-500 nm region and moderate Q bands in the 550-650 nm region). 8 In addition, their frontier orbital energy levels allow for efficient electron injection into the TiO 2 band and regeneration of the oxidized dye by the electrolyte in a solar cell. Furthermore, by appropriate functionalization of the porphyrin ring, either in the meso or β positions, their spectral and redox properties can be tuned, allowing control of the solar cell efficiency. Over the last years, a large number of reports on porphyrin-based solar cells with remarkable performances appeared in the literature. 9 Campbell et al. were the first to report in 2007 a DSSC based on a porphyrin with a conjugated, dicarboxylic acid anchoring Received: March 28, 2013 Article pubs.acs.org/IC © XXXX American Chemical Society A dx.doi.org/10.1021/ic400774p | Inorg. Chem. XXXX, XXX, XXX-XXX