Influence of the dye molecular structure on the TiO 2 conduction band in dye-sensitized solar cells: disentangling charge transfer and electrostatic effects† Enrico Ronca, ab Mariachiara Pastore, a Leonardo Belpassi, a Francesco Tarantelli ab and Filippo De Angelis * a We report a thorough theoretical and computational investigation of the effect of dye adsorption on the TiO 2 conduction band energy in dye-sensitized solar cells that is aimed at assessing the origin of the shifts induced by surface adsorbed species in the position of the TiO 2 conduction band. We thus investigate a series of working dye sensitizers and prototypical surface adsorbers and apply an innovative approach to disentangle electrostatic and charge-transfer effects occurring at the crucial dye–TiO 2 interface. We clearly demonstrate that an extensive charge rearrangement accompanies the dye–TiO 2 interaction, which amounts to transfer of up to 0.3–0.4 electrons from the dyes bound in a dissociative mode to the semiconductor. Molecular monodentate adsorption leads to a much smaller CT. We also find that the amount of CT is modulated by the dye donor groups, with the coumarin dyes showing a stronger CT. A subtle modulation of the semiconductor conduction band edge energy is found by varying the nature of the dye, in line with the experimental data from the literature obtained by capacitance and open circuit voltage measurements. We then decompose the total conduction band shift into contributions directly related to the sensitizer properties, considering the effect of the electric field generated by the dye on the semiconductor conduction band. This effect, which amounts to ca. 40% of the total shift, shows a linear correlation with the TiO 2 conduction band shifts. A direct correlation between the dye dipole and the observed conduction band shift is retrieved only for dyes of similar structure and dimensions. We finally found a near-exact proportionality between the amount of charge transfer and the residual contribution to the conduction band shift, which may be as large as 60% of the total shift. The present findings constitute the basis for obtaining a deeper understanding of the crucial interactions taking place at the dye–semiconductor interface, and establish new design rules for dyes with improved DSC functionality. Broader context The effect of surface-adsorbed species on the TiO 2 conduction band energy is a highly debated issue in the eld of dye-sensitized solar cells. The possible modulation of the position of the TiO 2 conduction band appears to be a viable way to obtain higher cell open circuit voltages, and thus higher solar energy conversion efficiency. By applying rst principles computational modeling, we investigate a series of working dyes and co-adsorbers to disentangle electrostatic and charge-transfer effects occurring at the dye–TiO 2 interface. We clearly demonstrate that an extensive charge rearrangement accompanies the dye–TiO 2 interaction. A subtle modulation of the TiO 2 conduction band is found by varying the nature of the dye, in line with available experimental data. Such conduction band shis are decomposed into contributions directly related to the sensitizer properties. A linear correlation is found between the dye electrostatic potential and the conduction band shi, which is proportional to the dye dipole for dyes of similar structure and dimensions. We also found a near-exact proportionality between the charge transfer and the residual contribution to the conduction band shi. The present ndings constitute the basis for a deeper understanding of dye–sensitized semiconductors, with possible implications for the functioning of a wide range of optoelectronic devices. 1 Introduction Dye-sensitized solar cells (DSCs) 1–4 have attracted signicant attention as low-cost alternatives to conventional photovoltaic devices for the conversion of sunlight into electricity, with a highest certied efficiency of 11.4%. 5 In DSCs, a dye sensitizer, anchored to the surface of a mesoporous oxide layer (usually TiO 2 ), absorbs the solar radiation and transfers a photoexcited a 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: lippo@thch.unipg.it b Dipartimento di Chimica, Universit` a degli Studi di Perugia, via Elce di Sotto 8, I- 06123, Perugia, Italy † Electronic supplementary information (ESI) available: CD curves, PDOS curves, and additional ts. See DOI: 10.1039/c2ee23170k Cite this: DOI: 10.1039/c2ee23170k Received 13th August 2012 Accepted 18th October 2012 DOI: 10.1039/c2ee23170k www.rsc.org/ees This journal is ª The Royal Society of Chemistry 2012 Energy Environ. Sci. Energy & Environmental Science PAPER Downloaded by University of Perugia on 08 November 2012 Published on 19 October 2012 on http://pubs.rsc.org | doi:10.1039/C2EE23170K View Online / Journal Homepage