Use of poly(3-methylthio)thiophene blends for direct laser tracing and bulk heterojunction solar cells Massimiliano Lanzi a,⇑ , Luisa Paganin a , Filippo Pierini b , Francesco Errani a , Francesco Paolo Di-Nicola a a Dipartimento di Chimica Industriale ‘‘Toso Montanari’’, Università di Bologna, Viale del Risorgimento, 4 I-40136 Bologna, Italy b Dipartimento di Chimica ‘‘G. Ciamician’’, Università di Bologna, Via Selmi, 2 I-40126 Bologna, Italy article info Article history: Received 5 August 2013 Received in revised form 6 June 2014 Accepted 5 July 2014 Available online 17 July 2014 Keywords: Electrical conductivity Laser tracing Bulk heterojunction polymeric solar cells Regioregular polyalkylthiophenes Polymer blends abstract In this article we demonstrate the use of a blend made of two regioregular polythiophenic derivatives, namely poly(3-methylthio)thiophene and poly(3-hexyl)thiophene, to obtain conductive traces by the simple laser exposure of their thin films to a suitable laser source. The polymeric blend was also tested as a photoactive layer for BHJ solar cells, showing an improved surface morphology and a wider absorp- tion spectrum, thus resulting in an enhanced photovoltaic performance. In the standard condition nor- mally used for the cell preparation, we obtained a 3.16% power conversion efficiency. The device showed good reproducibility and stability over time. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction The need for flexible, lightweight, easily processable new mate- rials for electronics has pushed research toward the synthesis of organic materials able to compete with the ‘‘classic’’ inorganic semiconductors in their own application fields. Organic semicon- ductors, such as conjugated polymers, are promising candidates for this replacement, as demonstrated both by the number of pat- ent applications in 2013 (more than 100 on polythiophenes) and by the growing number of newly launched companies and existing manufacturers of materials or devices that have added organic photovoltaics to their portfolio. In this context, polythiophenes are intriguing materials which have now been studied for a long time thanks to their electric and electronic properties. In fact, in the charged (doped) state they are very effective conductors of electricity, while in the neutral (undoped) state they are mainly employed in optoelectronic devices – for the construction of organic light emitting diodes (OLEDs), low-voltage field effect transistors and bulk heterojunc- tion (BHJ) solar cells – and in devices such as optical filters, signal modulators and polarization rotators. Alkyl-substituted polythio- phenes (PATs) are very soluble in common organic solvents and can be easily filmed and processed using a number of different techniques, e.g. spincoating [1], doctor blading [2], screen printing [3], inkjet printing [4] and roll-to-roll methods [5], making them very interesting materials from an industrial standpoint. Structurally, PATs belong to the class of conjugated polymers the precursor of which is polyacetylene (PAc). PAc shows electronic characteristics similar to those of PATs; recently some researchers have reported that PAcs functionalized with a thioalkylic group (PAc-SR) are photosensitive polymers [6]. In fact, thin insulating films of PAc-SR can be traced by laser operating at suitable speed, power, and wavelength leading to electroconductive patterns on the film surface [7]. This approach is particularly intriguing since it makes it technically feasible to obtain high resolution patterns for electronic circuits while strongly limiting the number of pro- cessing steps and chemicals required. In fact, the line patterning of an insulating polymeric matrix by a laser-induced photopyroly- sis process makes it possible to rapidly develop custom conductive circuits from a schematic drawing, by using simple CAD (computer aided design) systems to drive the laser source [8]. Various advan- tages are evident: over conventional inorganic conductors (metals, oxides, silicon), since no multiple etching and lithographic steps are required to obtain the final circuit, and over the most synthe- sized conjugated polymers, as no redox processes are necessary to make the polymer conductive, thus avoiding the well-known problems of time and environmental stability. But conjugated polymers are also intensively studied for other technological applications. In fact, in recent years, conjugated poly- mers-fullerene mixtures have been widely investigated for use in organic solar cells. Bulk heterojunction (BHJ) polymer solar cells http://dx.doi.org/10.1016/j.reactfunctpolym.2014.07.007 1381-5148/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +39 051 2093689; fax: +39 051 2093669. E-mail address: massimiliano.lanzi@unibo.it (M. Lanzi). Reactive & Functional Polymers 83 (2014) 33–41 Contents lists available at ScienceDirect Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react