This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 15963–15974 15963 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 15963–15974 Adsorption of organic dyes on TiO 2 surfaces in dye-sensitized solar cells: interplay of theory and experiment Chiara Anselmi, Edoardo Mosconi, Mariachiara Pastore, Enrico Ronca and Filippo De Angelis* Received 28th August 2012, Accepted 11th October 2012 DOI: 10.1039/c2cp43006a First-principles computer simulations can contribute to a deeper understanding of the dye/ semiconductor interface lying at the heart of Dye-sensitized Solar Cells (DSCs). Here, we present the results of simulation of dye adsorption onto TiO 2 surfaces, and of their implications for the functioning of the corresponding solar cells. We propose an integrated strategy which combines FT-IR measurements with DFT calculations to individuate the energetically favorable TiO 2 adsorption mode of acetic acid, as a meaningful model for realistic organic dyes. Although we found a sizable variability in the relative stability of the considered adsorption modes with the model system and the method, a bridged bidentate structure was found to closely match the FT-IR frequency pattern, also being calculated as the most stable adsorption mode by calculations in solution. This adsorption mode was found to be the most stable binding also for realistic organic dyes bearing cyanoacrylic anchoring groups, while for a rhodanine-3-acetic acid anchoring group, an undissociated monodentate adsorption mode was found to be of comparable stability. The structural differences induced by the different anchoring groups were related to the different electron injection/recombination with oxidized dye properties which were experimentally assessed for the two classes of dyes. A stronger coupling and a possibly faster electron injection were also calculated for the bridged bidentate mode. We then investigated the adsorption mode and I 2 binding of prototype organic dyes. Car–Parrinello molecular dynamics and geometry optimizations were performed for two coumarin dyes differing by the length of the p-bridge separating the donor and acceptor moieties. We related the decreasing distance of the carbonylic oxygen from the titania to an increased I 2 concentration in proximity of the oxide surface, which might account for the different observed photovoltaic performances. The interplay between theory/simulation and experiments appears to be the key to further DSCs progress, both concerning the design of new dye sensitizers and their interaction with the semiconductor and with the solution environment and/or an electrolyte upon adsorption onto the semiconductor. 1. Introduction The answer to the growing demand of environmentally sustainable energy resources at the planetary level might lie in the ability to capture and utilize solar energy for a sustainable development on a large scale. In this scenario, dye-sensitized solar cells (DSCs) represent a particularly promising approach to the direct conversion of sunlight into electrical energy at low cost and with high efficiency. 1–4 In DSCs, a dye sensitizer absorbs the solar radiation and transfers the photoexcited electron to a wide band-gap semi- conductor electrode consisting of a mesoporous oxide layer, usually TiO 2 , composed of nanometer-sized particles. The charge hole which is concomitantly created in the dye after excited state charge injection is then transferred to a redox couple in a liquid electrolyte or to a solid hole conductor (see Scheme 1). Of the three main DSCs constituting materials, i.e. the dye, the semiconductor oxide and the redox shuttle, the chemical nature and structure of the dye is by far the subject which has been more vastly investigated, with a general target of increasing the dye molar extinction coefficient and shifting the dye absorption towards the near-IR region, thus enhancing the overlap between the solar emission and the dye absorption spectrum and eventually achieving higher DSCs photocurrents. A further important sensitizer’s feature is the matching of ground and excited state oxidation potentials with the redox shuttle potential and the semiconductor conduction band, respectively. 5,6 These energetic Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, I-06123, Perugia, Italy. E-mail: filippo@thch.unipg.it; Fax: +39 075 585 5606; Tel: +39 075 585 5523 PCCP Dynamic Article Links www.rsc.org/pccp PERSPECTIVE Published on 11 October 2012. Downloaded by University of Perugia on 03/06/2014 11:37:16. View Article Online / Journal Homepage / Table of Contents for this issue