Synthesis of new methoxy-functionalized polythiophenes for charge transport in organic solar cells Massimiliano Lanzi ⇑ , Luisa Paganin, Daniele Caretti, Leonardo Setti, Francesco Errani Dipartimento di Chimica Industriale e dei Materiali, Università di Bologna, Viale del Risorgimento, 4 I-40136 Bologna, Italy article info Article history: Received 5 February 2011 Received in revised form 8 April 2011 Accepted 15 April 2011 Available online 28 April 2011 Keywords: Bulk heterojunction polymeric solar cells Chromism Conducting polymers Functionalized polythiophenes abstract The performance of bulk heterojunction organic solar cells based on poly(3-hexylthiophene) and PCBM can be enhanced by the employment of suitably tailored partially substituted poly(oligothiophene)s. In view of this, three new polythiophenes with a different content in a hexamethylenic side chain ending with a methoxy group have been prepared and characterized completely using NMR and FT-IR tech- niques. The optical features of the synthesized polymers have been compared by registering their UV–Vis spectra both in different solvent–non solvent systems and in film, and their thermal stability, electrical conductivities, and photovoltaic properties have been carefully analyzed. In particular, the two samples with low side-chain content showed very promising electrical performances even if their use as the active layer in polymer-based organic photovoltaic (OPV) solar cells has not yet been fully optimized. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Over the past few years, a number of academic and industrial research projects have focused on the synthesis, optimization, and comprehension of the electroconduction mechanism of inher- ently conjugated polymers (ICP) [1–3]. The employment of conduc- tive polymers in sensors, energy storage, smart windows, light emitting diodes [4–6], and also for photovoltaic applications is being extensively studied [7], and, recently, solar cells made with poly(p-phenylenevinylene) and poly(alkylthiophene) thin films have shown remarkable efficiency under illumination with AM 1.5 light [8]. Since the discovery of the efficient transfer between fullerene and conjugated polymers in bulk-heterojunction (BHJ) organic photovoltaic devices, much attention has been focused on engi- neering the properties of these systems. Poly(3-alkylthiophenes) (PAT)s are indeed the most studied and also the most promising class of ICPs thanks to their high solubility, filmability, thermal resistance, and structural versatility. Their constitutional and con- figurational regularity are very important parameters, driving the backbone planarity and then directly influencing the energy gap (E g ) between the valence and conduction bands. In fact, the exclu- sive presence both of a-a 0 linkages among the thiophenic rings and of HT dyads in the polymer main chains can enhance the optical and electronic properties of the polythiophene derivative as a con- sequence of the reduced band gap, while also giving rise to inter- chain interactions which are stronger than in non-regioregular samples, as well as to more extended crystalline domains. The polymer structure is also able to influence other features, such as the tendency of the macromolecules to be effectively solvated by a wide number of organic solvents to give, after spin-coating or casting of the prepared solution, thick, self-consistent, homoge- neous films which can be directly used on practical devices. In view of this, high regioregularity is not always a convenient feature, since the presence of ordered and planar main and side chains generally increases the degree of crystallinity, thus giving rise to brittle, inhomogeneous, and spotted (due to the presence of micro- aggregates) polymeric films [9]. Moreover, in some cases even the presence of a low bandgap may be useless for practical uses, by making the polymer too sensitive to the air-induced oxidation [10]. This drawback can be avoided by making less-substituted polythiophenic structures, while sacrificing the overall polymer solubility and, sometimes, the chain planarity as well. It is evident that the final efficiency of a BHJ solar cell is influ- enced by many factors which are mainly related to the composi- tion and morphology of the photoactive blend. The considerable progress in this field leads to a power conversion efficiency (PCE) of up to 5% using a mixture of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) [11], while en- hanced stability and optimization in the production of the organic photovoltaic devices are the new challenges to be met [12]. Indeed, the final properties of PATs can be effectively modulated, being the result of a very delicate balance among a number of characteristics which are directly connected with both the polymer structure and the subsequent interactions with its surrounding environment. 1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2011.04.004 ⇑ Corresponding author. Tel.: +39 051 2093689; fax: +39 051 2093669. E-mail address: massimiliano.lanzi@unibo.it (M. Lanzi). Reactive & Functional Polymers 71 (2011) 745–755 Contents lists available at ScienceDirect Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react