New photoactive oligo- and poly-alkylthiophenes Massimiliano Lanzi * , Luisa Paganin, Daniele Caretti Department of Industrial and Materials Chemistry, University of Bologna, Viale del Risorgimento, 4 I-40136 Bologna, Italy article info Article history: Received 6 May 2008 Received in revised form 4 September 2008 Accepted 5 September 2008 Available online 24 September 2008 Keywords: Conjugated polymers Oligothiophene derivatives Heterojunction polymer solar cells abstract A new type of u-methoxy-functionalized oligo-(OCT) and poly-(PQ) hexylthiophene characterized by a tetrameric repeating unit was synthesized and characterized. The configurational regularity and the mean degree of functionalization per thiophenic ring, lower than in commonly synthesized PATs, permitted the obtainment of more ordered chain conformations especially in the solid (film) state. Solar cells based on OCT and PQ films (as p-donor polymeric layer) mixed with Single-walled Carbon Nano- tubes (SWCNTs, used as electron-acceptor system) were prepared and investigated. A power conversion efficiency of 0.52% with a fill factor of 0.42, an open-circuit voltage of 0.48 V and a short-circuit current of 1.93 mA/cm 2 under 70 mW/cm 2 white light illumination is reported for the polymeric sample. The obtained performance is comparable with that of devices made with regioregular P3HT but the easiness of the monomer synthesis and polymer preparation, giving OCT and PQ with good yields, as well as the enhanced workability and filmability of the latters from diluted solutions of common organic solvents together with their low sensitivity to the environmental conditions (air oxygen, moisture, temperature) makes the synthesized materials very promising for the set-up of polymer photovoltaic cells. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Organic polymers have recently received great interest as semi- conducting materials since they have band gaps that range from 1.5 to 3 eV, which make them very suitable for opto-electronic applications in the visible range. In this field, thiophene-based materials are a good example of compounds [1,2], with tuneable functional properties by suitable molecular engineering reactions. 3-substituted poly- thiophenes (PATs) have given relevant results in photovoltaic applica- tions and electrooptical devices, where it is essential to reach high conductivities and high charge mobilities [3–5]. It is in fact well known that, to fabricate high-efficiency solar cells, the promising polymer should possess lower bandgap, in order to enlarge its absorption spectrum range, higher charge carrier mobility to reduce charge recombination and increase the photocurrent, and higher stability. The aim of this work is the synthesis and characterization of new thiophene derivatives bearing a u-methoxy-functionalized hexamethylene side chain, able to enhance the solubility of the final polymer in common organic solvents [6,7]. This feature is particularly important for obtaining highly homogeneous thin films and must be carefully considered in the preparation of the polymer dealt with in this paper, having a low content of plastifying side chains and a high degree of regioregularity. In fact, the obtained poly(methoxyhexyl)thiophenes possess a degree of functionalization lower than conventional PATs (50 vs 100%), in order to improve their stability in the neutral state, pre- venting the oxidation exerted by the atmospheric oxygen [8]. More- over, they are completely regioregular, in order to enhance the electronic properties deriving from the mean conjugation length of the polymeric backbone. On one hand, constitutional regularity is a fundamental prerequisite for long-range order and crystallization of the macromolecules to be achieved but, on the other hand, the pres- ence of more packed main chains strongly lowers the solubility of the final polymer. Therefore an accurate control of the polymerization reactions has been ensured in order to enhance the polymeric soluble fraction content. An oligothiophenic derivative was also obtained, and its structural, optical and electrical properties were compared to those shown by the derivatives with higher molecular weight. Even if few works have been reported on thiophene oligomers, these p-conju- gated systems are suitable for making light-based devices [9,10] and can be used directly to prepare the active layer of photovoltaic cells [11], or may constitute the repeating units of a polymeric material generally obtainable with easier synthetic procedures and showing better mechanical and thermal properties than those deriving from the polymerization of a monothiophenic monomer [12]. 2. Experimental section 2.1. Materials and instrumentation All the solvents and reagents used for the synthesis were purchased from Aldrich Chemical Company. NMR spectra were * Corresponding author. Tel.: þ39 51 2093689; fax: þ39 51 2093669. E-mail address: massimiliano.lanzi@unibo.it (M. Lanzi). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2008.09.013 Polymer 49 (2008) 4942–4948