A regioregular head to tail thiophene based ‘‘double-cable’’ polymer with pendant anthraquinone functional groups: Preparation, spectroscopy and photovoltaic properties David K. Mohamad a , Sangita S. Chauhan b , Hunan Yi b , Ashley J. Cadby a , David G. Lidzey a,n , Ahmed Iraqi b,nn a Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK b Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK article info Article history: Received 4 October 2010 Received in revised form 24 January 2011 Accepted 28 January 2011 Available online 19 February 2011 Keywords: Solar cells Photovoltaic devices Conjugated polymers abstract We present an investigation into the electronic, photophysical and photovoltaic properties of a p-conjugated ‘‘double-cable’’ polymer based on regioregular head to tail poly(3-functionalized thiophene) with pendant anthraquinone acceptor groups (PT-AQ). This material was compared with model blends composed of a mixture of regioregular head to tail poly(3-hexylthiophene) (P3HT) as the polymer donor and anthraquinone-2-carboxylic acid octyl ester (AQ) as a small-molecule acceptor. We find that PT-AQ shows complete quenching of fluorescence whilst the donor/acceptor blend shows residual fluorescence emission and both phase separation and crystallization of the acceptor compo- nent. The operation of the double-cable based devices is however, dominated by geminate recombina- tion and poor charge transport and extraction. Nevertheless, we show that the PT-AQ devices have a higher efficiency than devices based on representative bulk-heterojunction AQ:P3HT blends as the very coarse phase separation and crystallization is suppressed. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Organic photovoltaics (OPVs) are attractive alternatives to established inorganic based technologies. They have the potential advantages of mechanical flexibility, reduced cost of manufacture and light weight [1,2]. The solution processed polymer bulk- heterojunction (BHJ) solar cell concept has received considerable research interest since it may be considered a true low cost means to generate renewable energy. OPVs based on p-conjugated polymer:PC 70 BM material systems have so far achieved power conversion efficiencies (PCEs) approaching 8% [3]. The efficient generation and extraction of charge carriers relies on the forma- tion of a favorable interpercolated network of donor and acceptor phases p-conjugated organic materials. A large number of studies have addressed the processing conditions in BHJs necessary to achieve optimal morphology for device operation [4]. The choice of solvent, donor to acceptor (D:A) stoichiometry, thermal and solvent annealing [4a] are all known to play a role in controlling film morphology. An alternative approach to the BHJ concept is to create OPV materials by covalently linking the acceptor group to the donor as a pendant sidegroup on a polymeric backbone. Such molecules have been termed ‘‘double-cable’’ polymers, as they have two different pathways (cables) for ambipolar transport [5]. Such systems potentially permit an ideal active layer morphology to be created in a single step without relying upon post-processing techniques. Typically, double-cable polymers show strongly quenched fluorescence leading to the production of long-lived charge states [6]. The advantages over a BHJ include: (i) charge separation not being limited to a geometrically flat interface between D:A rich/poor regions, (ii) the requirements on proces- sing conditions may be relaxed [7] and (iii) the flexibility of chemical synthesis offers the possibility of using materials with a high degree of crystallinity, a factor known to improve charge carrier mobility [4a,8]. Previous work on double-cable polymers has focussed on donor materials functionalised with pendant fullerene units [9]. Early studies included work on a C 60 -oligoPPV [10] although recent studies focus on regioregular polythiophenes as the back- bone moiety since they produce the most efficient OPV devices [5,7,11,12]. To date, PCEs of 0.52% have been demon- strated in devices containing polythiophenes decorated with pendant fullerenes [13]. The principle limitation in this approach lies in functionalizing the polymer to achieve a sufficient Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.01.040 n Corresponding author. Fax: +44 0 114 222 3555. nn Corresponding author. Fax: +44 0 114 222 9303. E-mail addresses: d.g.lidzey@sheffield.ac.uk (D.G. Lidzey), a.iraqi@sheffield.ac.uk (A. Iraqi). Solar Energy Materials & Solar Cells 95 (2011) 1723–1730