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A series of pushepull triarylamine organic dyes containing the benzo[1,2-b:4,5-b0]dithiophene unit as a spacer and bearing alkyl chains in different positions of the molecule were synthesized. The new dyes 1 e4 were characterized by optical and electrochemical measurements and density functional theory calculations and used as sensitizers in liquid dye-sensitized solar cells. The effect of alkyl chain position on dye properties and performances was investigated and dye 1, bearing two 3,7-dimethyloctyl groups on the benzodithiophene core, exhibited the best behaviour in term of light absorption and cell performance (PCE of 6.6%, to be compared with a PCE of 8.1% for N719-based device). The efficiency of the DSSC was highly dependent on the solvent used for the dye-sensitizing bath, which affects the position of the COOH/COO equilibrium. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction In recent years metal-free organic dyes for DSSC [1e4] have been the object of a growing interest by organic chemists, because a great number of diverse structures can be easily prepared using well established synthetic methodologies and, through appropriate molecular design, their optical, electronic and electrochemical properties can be finely modulated [5e7]. The most efficient metalfree dyes for DSSC are characterized by a pushepull structure in which an

Finding viable alternatives to silicon-based photovoltaics, through low-cost solution processable materials, is crucial, facing as we are, a complex transition out of the fossil fuelled civilization. In this scenario, the utilization in nanostructured solid-state solar cells of an underexplored eclectic class of materials, the hybrid halide perovskites, has represented a field breakthrough, allowing novel device architectures leading to record device performances up to 15%,[1]thus holding the promise of cost effective solar energy production. Among the first results of the pioneer reports on perovskites-based solar cells, probably the most intriguing discover concerned the application of a iodide/chloride mixed-halide perovskite CH3NH3PbI3- xClx in a so called "meso-superstructured" Solid State Solar Cell, where the perovskite is concomitantly capable of both absorbing light and transporting charge within a mesopourus network.[2] This mixed system has recently been compared to the I-based perovskite CH3NH3PbI3 via spectroscopical investigation and interesting differences have been reported, some of them leading to hypothesizing a better charge transport within CH3NH3PbI3- xClx.[3]However, an investigation on the exact materials composition and structure is still missing. Here[4] we report a detailed investigation on Cl/I mixed halide self-assembling perovskites, and study the relation between the I:Cl ratio in the material and the solar cell characteristics, aiming at optimizing device performances through composition tuning. We found out that the exact composition of the final compound does not reproduce the stoichiometry of the precursor solution. We demonstrated, both experimentally and theoretically, that the formation of continuous solid phase MAPbI3-xClx is actually not allowed and that chloride incorporation into MAPbI3 is possible only at relatively low concentration (3-4%), so that it could be classified as a dopant agent. However, even if the material band-gap remains substantially unchanged, the Cl doping dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material. [1] Liu M.; Johnston M. B.; Snaith H. J.; Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-8. [2] Lee M.M.; Teuscher J.; Miyasaka T.; Murakami T.N.; Snaith H.J.; Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. [3] Stranks S.D.; Eperon G.E.; Grancini G.; Menelaou C.; Alcocer M.J.P.; Leijtens T.; Herz L.M.; Petrozza A.; Snaith H.J.; Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341. [4] Colella S.; Mosconi E.; Fedeli P.; Listorti A.; Gazza F.; Orlandi F.; Ferro P.; Besagni T.; Rizzo A.; Calestani G.; Gigli G.; De Angelis F.; Mosca R. MAPbI3-xClx mixed halide perovskite for hybrid solar cel ls: the role of chloride as dopant on the transport and structural properties. Chemistry of Materials 2013, 25, 4613.