Job/Unit: O20958 /KAP1 Date: 21-11-12 16:39:47 Pages: 12 FULL PAPER DOI: 10.1002/ejoc.201200958 Metal-Free Benzodithiophene-Containing Organic Dyes for Dye-Sensitized Solar Cells Elena Longhi, [a] Alberto Bossi, [b] Gabriele Di Carlo, [a] Stefano Maiorana, [a] Filippo De Angelis,* [c] Paolo Salvatori, [c] Annamaria Petrozza, [d] Maddalena Binda, [d] Vittoria Roiati, [d,e] Patrizia Romana Mussini, [a] Clara Baldoli,* [b] and Emanuela Licandro* [a] Keywords: Donor-acceptor systems / Dyes/pigments / Sensitizers / Solid state structures / Solar cells Two new metal-free organic dyes, CR29 and CR52, with high extinction coefficients in the visible spectral region between 400–650 nm, have been synthesized. The donor–acceptor structure of the dyes feature benzodithiophene moieties BDT 1 and BDT as rigid π-conjugated spacers, which have so far been very little studied for dye-sensitized solar cell (DSSC) applications. DFT/TDDFT calculations have been employed to guide the design of the chromophores as well Introduction Dye-sensitized solar cells (DSSC or Grätzel cells) are photovoltaic devices that are the object of competitive and extensive research aimed at improving efficiencies and re- ducing costs. [1] One of the key components in a DSSC is the dye, which is responsible for harvesting the sunlight and injecting electrons into the mesoporous semiconducting ox- ide, typically TiO 2 . Metal-based photosensitizers have been used over the last 20 years as the key components for DSSCs. [1e,2] So far, record efficiencies of 12.3% under AM 1.5 conditions have been reached by using Zn II porphyrin dyes and cobalt poly- [a] Dipartimento di Chimica, Università degli Studi di Milano, via C. Golgi 19, 20133 Milano, Italy Fax: +39-02-50314139 E-mail: emanuela.licandro@unimi.it Homepage: http://www.unimi.it/ [b] CNR – Istituto di Scienze e Tecnologie Molecolari, via C. Golgi 19, 20133 Milano, Italy Fax: +39-02-50314139 E-mail: clara.baldoli@istm.cnr.it Homepage: http://www.istm.cnr.it/ [c] CNR – Istituto di Scienze e Tecnologie Molecolari, via Elce di Sotto 8, 06123 Perugia, Italy Fax: +39-075-5855606 E-mail: filippo@thch.unipg.it Hoempage: http://www.istm.cnr.it/ [d] Center for Nano Science and Technology @ Polimi, Istituto Italiano di Tecnologia, via G. Pascoli 70/3, 20133 Milano, Italy [e] Dipartimento di Fisica, Politecnico di Milano, P.za Leonardo da Vinci 32, 20133 Milano, Italy Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.201200958. Eur. J. Org. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 as to shed light on their electronic and optical properties. Photophysical and electrochemical characterization studies have been carried out to gather information on the charge transfer processes occurring at the dye–semiconductor inter- faces. Under standard AM 1.5 conditions, DSSC sensitized with CR29 showed good conversion efficiencies: 5.14 % in the liquid electrolyte cell setup and 2.47 % in the solid-state DSSC. pyridyl redox electrolyte. [3] Although transition metal dyes exhibit high performances, challenging synthesis and com- plex purification steps are often required. For these reasons, in recent years, alternative metal-free organic dyes have at- tracted great attention. In fact, such chromophores can be prepared rather inexpensively by using well-established syn- thetic methodologies and they present optical, electronic, and electrochemical properties that can be modulated through appropriate molecular design. [4] The development of innovative, stable organic dyes with optical absorptions extending into red and near-IR regions of the solar spectrum is, at present, a “hot topic” of re- search in this field. The most efficient metal-free organic dyes are characterized by a donor-π-acceptor (D-π-A) structure, in which the donor group (D) is an electron-rich unit, linked through a conjugated π-bridge spacer to the electron-acceptor group (A). In the cell, unit A is bound to the semiconductor surface (TiO 2 ), usually through a carb- oxylic or cyanoacrylic acid function. The nature of the conjugated bridging segment (π) has proved to be of signifi- cant importance for controlling the light-harvesting per- formance of the dye and a number of new π-conjugated aromatic and heteroaromatic systems [4] have been investi- gated. Among these, the use of several kinds of substituted thiophene or thienothiophene derivatives as π-bridges, have been reported to give remarkable efficiencies. Benzo[1,2-b:4,3-b']dithiophene (BDT) and benzo[1,2- b:4,5-b']dithiophene (BDT 1 ; Figure 1) attracted our atten- tion because of their stable, rigid, π-conjugated condensed- polycyclic structures. [5] These features lead to unique elec-