New regioregular polythiophenes functionalized with sulfur-containing substituents for bulk heterojunction solar cells Massimiliano Lanzi * , Luisa Paganin Dipartimento di Chimica Industriale e dei Materiali, Università di Bologna, Viale del Risorgimento, 4 I-40136 Bologna, Italy article info Article history: Received 10 August 2009 Received in revised form 24 February 2010 Accepted 27 February 2010 Available online 4 March 2010 Keywords: Conducting polymers Polymeric solar cells Polythiophenes Regioregularity Solvatochromism abstract Organic photovoltaic devices fabricated with polythiophenic derivatives and single-walled carbon nano- tubes (SWCNT)s have been assembled and tested. For this purpose, some new regioregular polythio- phenes with different sulfur-containing groups in the terminal position of a hexamethylenic side chain were prepared using direct and indirect polymerization routes. The optical features of the synthesized polymers were examined and compared by registering their UV–Vis spectra in different solvent/non-sol- vent systems and in film; their thermal stability, electrical conductivity, cyclic voltammetry and photo- voltaic properties were carefully analyzed. In particular, the two samples bearing the sulfinyl group in side chain showed better chromic responses, enhanced self-assembling capabilities, more extended con- jugation length, higher electrical conductivity and very promising photovoltaic performances, when com- pared to those of analogous systems reported on up to now. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In the relatively young field of bulk heterojunction solar cells (BHSC), some fundamental steps forward have been made only in very recent years. First of all, the discovery that the localized elec- tron–hole pairs photogenerated in organic semiconductors can be effectively dissociated in order to obtain free charges using bulk donor–acceptor heterojunctions [1,2]. In practice, a conjugated polymer acting as positive holes-conductor and an electron-accep- tor molecule, like C 60 and its derivatives [3,4] or single-wall and also multi-wall carbon nanotubes (SWCNTs and MWCNTs, respec- tively) [5,6] having an electron affinity (EA) larger than the poly- mer, are blended together to give a composite able both to dissociate the photogenerated singlet excitons and to collect them as electrical charges at the respective electrodes. A very low elec- tric field is sufficient to drive the unbound and dissociated elec- tron–hole pairs to the opposite electrodes and is generally obtained by the difference in the electrodes work function [7]. The use of SWCNTs as an electrode, also replacing the ITO layer, or as an electron-acceptor molecule in BHSC is well assessed and many paper have been published on this subject in the recent years [8–14]. In particular, thanks to the high tensile strength [15] and Young modulus [16] of SWCNTs, their composites with conducting polymers become mechanically stable and strongly reinforced. The resulting materials have improved absorption in the Vis–NIR re- gion and then a wide solar spectral range can be effectively harvested for photovoltaic applications. Thanks to the high surface area provided by nanotubes, the transfer of the photogenerated charges from the donor conjugated polymer to the acceptor mole- cules (SWCNTs) can be very fast as well as the electron mobility, usually five order of magnitude higher in carbon nanotubes than in polymers [17]. However, the amount of nanotubes added to the blend should be carefully adjusted. On one hand, if their concentration is high the overall electrical conductivity of the sys- tem can increase up to six orders of magnitude but, on the other hand, the absorption coefficient of the polymer in the visible region correspondly decreases [18]. Moreover, Swager recently reported the use of SWCNT/polythiophene-based sensors for the selective chemorecognition of p-xylene from a mixture of isomers [19] and also for the determination of the chemical warfare nerve agent dimethyl methyl phosphonate (DMMP) [20]. In a typical architecture the composite layer made by the elec- tron donor polymer and the acceptor molecules blended together is sandwiched between two electrodes with different work func- tions [21]. The morphology of the blend has a crucial role for the device performance, directly affecting the charges photogeneration and their transport towards the electrodes and then must be care- fully optimized [22,23]. In fact, on one hand the natural tendency of the two components to phase segregate (macrophase separa- tion) should be limited by an intimate mixing of the donor and acceptor species, enhancing the efficiency of the charge photogen- eration but on the other hand the donor and acceptor counterparts should maintain a minimum degree of microphase separation, leading to the formation of percolation paths within the two phases in order to transport to the electrodes the electrons and 1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2010.02.009 * Corresponding author. Tel.: +39 51 2093689; fax: +39 51 2093669. E-mail address: massimiliano.lanzi@unibo.it (M. Lanzi). Reactive & Functional Polymers 70 (2010) 346–360 Contents lists available at ScienceDirect Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react