Design of Ru(II) sensitizers endowed by three anchoring units for adsorption mode and light harvesting optimization Maria Grazia Lobello a , Simona Fantacci a , Norberto Manfredi b , Carmine Coluccini b , Alessandro Abbotto b, ⁎, Mohammed K. Nazeeruddin c, ⁎⁎, Filippo De Angelis a, ⁎⁎⁎ a Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari, Via elce di Sotto 8, I-06213 Perugia, Italy b Department of Materials Science and Milano-Bicocca Solar Energy Research Center-MIB-Solar, University of Milano-Bicocca and INSTM, Via Cozzi 53, I-20125 Milano, Italy c Laboratory for Photonics and Interfaces, Station 6, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland abstract article info Available online xxxx Keywords: Ru(II) dyes Density functional calculations UV/vis spectroscopy Dye adsorption on TiO 2 We report the design, synthesis and computational investigation of a class of Ru(II)-dyes based on mixed bipyridine ligands for use in dye-sensitized solar cells. These dyes are designed to preserve the optimal anchoring mode of the prototypical N719 sensitizer by three carboxylic groups, yet allowing for tunable optimization of their electronic and optical properties by selective substitution at one of the 4-4′ positions of a single bipyridine ligand with π-excessive heteroaromatic groups. We used Density Functional Theory/Time Dependent Density Functional Theory calculations to analyze the electronic structure and optical properties of the dye and to inves- tigate the dye adsorption mode on a TiO 2 nanoparticle model. Our results show that we are effectively able to introduce three carboxylic anchoring units into the dye and achieve at the same time an enhanced dye light harvesting, demonstrating the design concept. As a drawback of this type of dyes, the synthesis leads to a mixture of dye isomers, which are rather tedious to separate. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Dye-sensitized solar cells (DSCs) are promising alternatives to con- ventional photovoltaics for the direct conversion of solar energy into electricity at low cost and with high efficiency [1–6]. In DSCs, a dye sen- sitizer, adsorbed on the surface of a mesoporous nanostructured semi- conductor film, usually made of titanium dioxide (TiO 2 ), absorbs the solar radiation then transferring a photoexcited electron to the semicon- ductor conduction band. The concomitant charge hole which is created on the dye is transferred to a liquid electrolyte or to a solid substrate functioning as hole conductor [7,8]. Ruthenium(II) complexes are widely employed as dye sensitizers [9–11], delivering record efficiencies in DSCs devices [12–14]. The Ru(NCS) 2 (dcbpyH 2 ) 2 (dcbpyH 2 = 4,4′-dicarboxyl- 2,2′ bipyridine) dye and its doubly deprotonated tetrabutylammonium salt, N3 and N719, respectively, have maintained a clear lead in DSCs technology, with efficiencies exceeding 11% [13,15]. In these complexes, the thiocyanate ligands ensure fast regeneration of the photo-oxidized dye by the redox mediator, while the two equivalent 2,2′-bipyridine (bpy) ligands functionalized in their 4,4′ posi- tions by carboxylic groups ensure stable anchoring to the TiO 2 surface, allowing at the same time for the strong electronic coupling required for efficient excited state charge injection [13,16,17]. For further prog- ress, however, higher conversion efficiencies need to be achieved. To this end, sensitizers and a deeper understanding of the interaction be- tween the dye and the TiO 2 nanoparticle are essential. A problem with the otherwise highly optimized homoleptic N3/N719 dyes is that their absorption is mainly centered in the blue and green spectral regions, substantially missing harvesting of photons in the red or near infrared region of the spectrum. Heteroleptic sensitizers have been therefore devised in which one of the two bpys is specifically func- tionalized to obtain increased DSCs' performances, in particular their light harvesting capability [18–24]. Quite unexpectedly, however, ex- periments have shown that the photovoltaic performances of DSCs employing such heteroleptic dyes are significantly lower compared to those observed using the parent homoleptic dyes [13,18–20]. For this family of dyes, it was found that the photocurrent obtained from TiO 2 -sensitized films decreased when the fully pro- tonated to fully deprotonated dyes were used to sensitize the semi- conductor film [25]. The DSCs' open circuit voltage, on the other hand, showed an opposite trend, increasing from the fully protonat- ed to the fully deprotonated dye. An optimal product of photocurrent and open circuit voltage was found for an intermediate number of protons, which allowed further device optimization leading to effi- ciency exceeding 11% [13]. Thin Solid Films xxx (2013) xxx–xxx ⁎ Correspondence to: A. Abbotto, Department of Materials Science and Milano-Bicocca Solar Energy Research Center-MIB-Solar, University of Milano-Bicocca, Via Cozzi 53, I- 20125, Milano, Italy. ⁎⁎ Corresponding author. ⁎⁎⁎ Corresponding author. Tel.: +39 0755855523. E-mail addresses: alessandro.abbotto@unimib.it (A. Abbotto), mdkhaja.nazeeruddin@epfl.ch (M.K. Nazeeruddin), filippo@thch.unipg.it (F. De Angelis). TSF-32540; No of Pages 8 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.08.112 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Please cite this article as: M.G. Lobello, et al., Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.08.112