Cobalt Electrolyte/Dye Interactions in Dye-Sensitized Solar Cells: A Combined Computational and Experimental Study Edoardo Mosconi, † Jun-Ho Yum, ‡ Florian Kessler, ‡ Carlos J. Gó mez García, § Cristiano Zuccaccia, ⊥ Antonio Cinti, †,⊥ Mohammad K. Nazeeruddin, ‡ Michael Gra ̈ tzel, ‡ and Filippo De Angelis* ,† † Computational Laboratory for Hybrid and Organic Photovoltaics, Istituto CNR di Scienze e Tecnologie Molecolari, via Elce di Sotto 8, 06123 Perugia, Italy ‡ Laboratory for Photonics and Interfaces, Institution of Chemical Sciences and Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland § Instituto de Ciencia Molecular Parque Científico, Universidad de Valencia, C/Jose ́ Beltra ́ n, 2 46980 Paterna (Valencia), Spain ⊥ Dipartimento di Chimica, Universita ̀ degli Studi di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy * S Supporting Information ABSTRACT: We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO 2 surfaces sensitized by ruthenium and organic dyes, and their impact on the performance of the corresponding dye-sensitized solar cells (DSSCs). We focus on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with cobalt electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with long alkoxy chains, show no significant change in the electron lifetime regardless of employed electrolyte and deliver a high photovoltaic efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of a complex between the cobalt electrolyte and the surface-adsorbed ruthenium dye, which brings the [Co(bpy) 3 ] 3+ species into contact with the TiO 2 surface. This translates into a high probability of intercepting TiO 2 -injected electrons by the oxidized [Co(bpy) 3 ] 3+ species, lying close to the N719-sensitized TiO 2 surface. Investigation of the dye regeneration mechanism by the cobalt electrolyte in the Marcus theory framework led to substantially different reorganization energies for the high-spin (HS) and low- spin (LS) reaction pathways. Our calculated reorganization energies for the LS pathways are in excellent agreement with recent data for a series of cobalt complexes, lending support to the proposed regeneration pathway. Finally, we systematically investigate a series of Co(II)/Co(III) complexes to gauge the impact of ligand substitution and of metal coordination (tris-bidentate vs bis- tridentate) on the HS/LS energy difference and reorganization energies. Our results allow us to trace structure/property relations required for further development of cobalt electrolytes for DSSCs. 1. INTRODUCTION Dye-sensitized solar cells (DSSCs) are promising hybrid/ organic photovoltaic devices for high-efficiency, low-cost solar energy conversion. 1-5 In typical DSSCs, a mesoporous film of TiO 2 nanoparticles is sensitized with light-harvesting dyes, either organic or metallorganic, which in most applications are surrounded by a redox mediator 6-8 in an organic solvent, typically acetonitrile. 9,10 The classic redox mediator in DSSCs is the I 3 - /I - redox couple, which in conjunction with Ru(II) dyes delivers certified efficiencies of 11.4%. 11 Fully organic dyes are also nicely performing with I 3 - /I - electrolytes, with efficiencies exceeding 10%. 12 The I 3 - /I - redox couple, however, is also known to have limitations due to its complex redox chemistry and to its corrosive nature, which complicates large-scale DSSCs production. 11-16 In recent reports, organic redox mediators, such as disulfide/ thiolate, 17 McMT/BM, 18 and TMFDS 2+ /TMTU, 19,20 have been investigated as alternatives to the conventional I 3 - /I - redox couple. These systems, however, are similar to I 3 - /I - in complexity because they involve the transfer of two electrons in their overall redox reactions. In contrast, monoelectronic metallorganic redox couples such as ferrocene/ferrocenium, 21 Ni(III)/Ni(IV), 22 and Co(II)/Co(III) complexes 23-39 usually show simpler kinetics and may require a smaller energy expenditure for the dye regeneration process, reducing the associated loss of open-circuit voltage. Among these alternative redox mediators, tris-chelated cobalt(II)/(III) complexes, such as [Co(bpy) 3 ] 2+/3+ , [Co(phen) 3 ] 2+/3+ , or [Co(py-pz) 3 ] 2+/3+ (bpy = bipyridine, phen = phenanthroline, py-pz = pyridine- pyrazole), or bis-chelated complexes, such as [Co(bpy- pz) 2 ] 2+/3+ or [Co(tpy) 2 ] 2+/3+ (bpy-pz = bipyridine-pyrazole, Received: August 8, 2012 Article pubs.acs.org/JACS © XXXX American Chemical Society A dx.doi.org/10.1021/ja3079016 | J. Am. Chem. Soc. XXXX, XXX, XXX-XXX