(Alkylsulfanyl)bithiophene-alt-Fluorene: π-Conjugated Polymers for Organic Solar Cells

We describe the synthesis of alternating bithiophene–fluorenecopolymers (P1–P3) with different regiochemistry of thebithienyl unit and different alkylsulfanyl chain lengths. Thestructural, electrochemical and photophysical properties ofthese polymers were investigated by gel permeationchromatography (GPC), differential scanning calorimetry(DSC), NMR, UV/Vis and photoluminescence (PL) spectroscopyand cyclic voltammetry (CV), and the polymerswere used to assemble organic solar cells (OSCs), in combi-IntroductionOrganic conjugated materials are attracting increased attentiondue to their potential in the fields of chemical sensors,[1] electroluminescent devices,[1c,2] field-effect transistors[3] and OSCs.[4] The last is one of the most appealingapplications of conjugated polymers because they have thepotential to be a low cost alternative to conventional inorganicsemiconductors.[5] The major desirable features ofOSCs are mechanical flexibility, very high speed of processingand low cost for large area fabrication.[6] The mostefficient device architecture of polymeric solar cells is basedon the bulk heterojunction concept (BHJ),[7] in which theactive layer, sandwiched between two electrodes with differentwork functions, consists of a blend of electron donatingmaterial, e.g. a p-type conjugated polymer, and an electronaccepting material (n-type), such as fullerene derivatives.For application in OSCs, conjugated polymers should[a] Dipartimento di Chimica, Università di Modena e ReggioEmilia,Via G. Campi 183, 41125 Modena, ItalyFax: +39-059373543E-mail: adele.mucci@unimore.it[b] ENEA, C.R. Portici,Piazzale E. Fermi 1, 80055 Portici (NA), ItalyE-mail: pasquale.morvillo@enea.it[c] Dipartimento di Chimica Industriale e dei Materiali, Universitàdi Bologna,V.le del Risorgimento 4, 40136 Bologna, ItalySupporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejoc.201100738.Eur. J. Org. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1nation with the fullerene derivative methyl [6,6]-phenyl-C61-butyrate (PCBM). P3, with a head-to-head bithienyl unit,shows a broader absorption and a lower band gap with respectto P1 and P2, which have tail-to-tail bithienyl units.The PL intensity of P1–P3 is dramatically quenched in thepresence of PCBM, demonstrating that an efficient chargetransfer between donor and acceptor occurs. The best OSCdevice was obtained with P3.