Synthetic Metals 161 (2011) 1137–1140 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Short communication A cyclopentadithiophene/thienopyrroledione-based donor–acceptor copolymer for organic solar cells Christopher M. MacNeill a,b , Eric D. Peterson b , Ronald E. Noftle a,b , David L. Carroll b , Robert C. Coffin b,∗ a Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109, United States b Center for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest University, Winston-Salem, NC 27109, United States article info Article history: Received 5 January 2011 Received in revised form 7 February 2011 Accepted 10 February 2011 Available online 22 March 2011 Keywords: Organic photovoltaics Cyclopentadithiophene Thienopyrroledione Solar cell abstract A new cyclopentadithiophene/thienopyrroledione-based donor–acceptor copolymer (P1) was syn- thesized using a microwave-assisted Stille coupling procedure and was compared to a known benzodithiophene-based copolymer using the same thienopyrroledione acceptor monomer (PBDTTPD). Cyclopentadithiophene-based copolymers have been known to exhibit lower band gaps than their cor- responding benzodithiophene-based counterparts. The polymer showed excellent solubility at room temperature in chlorinated solvents. The absorption onset for P1 is close to 740 nm as compared with ∼685 nm for PBDTTPD, corresponding to an optical band gap of 1.67 eV, which is 0.15 eV lower than PBDTTPD. The photovoltaic characteristics of the polymer were determined under AM1.5 illumination. The P1:PCBM BHJ device showed a high V oc (0.92 V) and J sc (8.02 mA/cm 2 ) as well as a good PCE (2.43%), while the best device with 2% solvent additive gave a PCE of 3.47%. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Light weight flexible devices and low cost roll-to-roll pro- cessing make organic photovoltaics (OPVs) an attractive option for future energy production [1]. However, improvements in device efficiency, stability and lifetime are necessary before OPVs become commercially viable [2]. These issues need to be addressed from two directions: from the materials design and synthesis perspective, and from the device fabrication perspective. In terms of material design, an attractive approach to making high performance p-type OPV materials is by preparing an alter- nating copolymer of electron-rich (donor) and electron-poor (acceptor) monomer units [3]. Recently, D–A copolymers based on the acceptor unit 5-octylthieno[3,4-b]pyrrole-4,6-dione-1,3-diyl (TPD) have drawn considerable attention in the literature [4]. Of particular interest, are copolymers of TPD with benzo[1,2-b:4,5- b ′ ]dithiophene (BDT). Very high efficiencies, approaching 7%, have been reported for PBDTTPD, which has been attributed to the copolymers exhibiting face-on orientation relative to the device substrate. Since this represents one of the first examples of this preferred orientation, we were interested in whether we could improve upon these results by preparing a lower bandgap TPD- containing copolymer. A reduction in bandgap in this region of the spectrum greatly increases the number of accessible photons for ∗ Corresponding author. E-mail address: coffinrc@wfu.edu (R.C. Coffin). power conversion. A copolymer of TPD with cyclopentadithiophene (CPDT) seemed to be a logical target as it has been demonstrated that for the same acceptor unit copolymers of CPDT typically exhibit optical bandgaps ∼0.2 eV lower than the corresponding BDT copolymers without a significant reduction in open-circuit voltage (V oc ) (see Fig. 1) [5]. In this communication, we report the synthesis, characterization and photovoltaic properties of poly[(4,4-bis(2-ethyl)cyclopenta-[2,1-b:3,4-b ′ ]dithiophene)-2,6- diyl-alt-(5-octylthieno[3,4-b]pyrrole-4,6-dione)-1,3-diyl] (PCPD- TTPD, P1). 2. Experimental 2.1. Materials and methods All reagents were purchased from common commercial sources and used without further purification unless otherwise noted. 4H- cyclopenta-[1,2-b:5,4-b ′ ]dithiophene was purchased from Astar Pharma. THF was dried over Na/benzophenone ketal. Flash chromatography was performed on a Biotage Isolera TM Flash Purification System using Biotage SNAP Flash Purification Cartridges as the stationary phase. Microwave assisted polymer- izations were carried out using a CEM Discover Microwave reactor. 300 and 500 MHz 1 H NMR spectra were recorded on Bruker Avance DPX-300 and DRX-500 Instruments, respectively. 13 C NMR spec- tra were recorded on a Bruker Avance DRX-500 instrument at 125.76 MHz. UV–vis absorption spectra were recorded on an Agi- lent 8453 diode-array spectrophotometer operating over a range 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.02.010