We present a first-principles computational investigation on the adsorption mode and electronic structure of the highly efficient heteroleptic ruthenium dye C101, [NaRu(4,4′-bis(5-hexylthiophene-2-yl)-2,2′-bipyridine)(4carboxylic acid-4′-carboxylate-2,2′-bipyridine)(NCS) 2 ], on anatase TiO 2 models exposing the (001) and ( 101) surfaces. The electronic structure of the TiO 2 models shows a conduction band energy upshift for the (001)-surface ranging between ∼50 and ∼110 meV compared with the (101) surface, in agreement with previous interfacial impedance and recent spectro-electrochemical data. TDDFT excited-state calculations provided the same optical band gap, within 0.01 eV, for the (001)-and (101) models. Two dominant adsorption modes for C101 dye adsorption on the (001) and (101) surfaces were found, which differ by the binding of the dye carboxylic groups to the TiO 2 surfaces (bridged bidentate vs monodentate), leading to sizably different tilting of the anchoring bipyridine plane with respect to the TiO 2 surface. The different adsorption mode leads to a smaller dye coverage on the (001) surface, as experimentally found, due to partial contact of the thiophene and alkyl bipyridine substituents with the TiO 2 surface. For the energetically favored adsorption modes, we calculate a larger average spatial separation, by 1.3 Å, between the dye-based HOMO and the semiconductor surface in (001) and (101) TiO 2 models. In terms of simple nonadiabatic electron-transfer considerations, our model predicts a retardation of the charge recombination kinetics, in agreement with the experimental observations.

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We report a combined spectro-photometric and computational investigation of the acid-base equilibria of the N3 solar cell sensitizer [Ru(dcbpyH(2))(2)(NCS)(2)] (dcbpyH(2) = 4,4'-dicarboxyl-2,2' bipyridine) in aqueous/ethanol solutions. The absorption spectra of N3 recorded at various pH values were analyzed by Single Value Decomposition techniques, followed by Global Fitting procedures, allowing us to identify four separate acid-base equilibria and their corresponding ground state pK(a) values. DFT/TDDFT calculations were performed for the N3 dye in solution, investigating the possible relevant species obtained by sequential deprotonation of the four dye carboxylic groups. TDDFT excited state calculations provided UV-vis absorption spectra which nicely agree with the experimental spectral shapes at various pH values. The calculated pK(a) values are also in good agreement with experimental data, within <1 pK(a) unit. Based on the calculated energy differences a tentative assignment of the N3 deprotonation pathway is reported.

A new Ru(II) dye, Ru(L1)(L2) (NCS)2, L1 = (4-(5-hexylthiophen-2-yl)-4'(4-carboxyl-phenyl 2,2'-bipyridine) and L2 = (4-4'-dicarboxy-2,2'-bipyridine), labelled MC112, based on a dissymmetric bipyridine ligand for improved interfacial and optical properties, was synthesized and used in DSCs, yielding photovoltaic efficiencies of 7.6% under standard AM 1.5 sunlight and an excellent device stability. Increased light harvesting and IPCE maximum were observed with MC112 compared to the prototypical homoleptic N719 dye, due to the functionalized bipyridyne ligand acting as an antenna. In addition, the mixed bipyridyne ligand allowed MC112 binding to TiO2 to occur via three anchoring carboxylic groups, thus exhibiting similar interfacial properties to those of the N719 dye. DFT/TDDFT calculations were performed on the new dye, both in solution and adsorbed on a TiO2 surface model, revealing that the peculiar photovoltaic properties of the MC112 dye are related to its anchoring mode. The new design rule thus allows us to engineer both light-harvesting and interfacial properties in the same dye.

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