A new intramolecular donor–acceptor polyfluorene copolymer for bulk heterojunction solar cells Jung Feng Lee a , Steve Lien Chung Hsu a,n , Po I Lee a , Hung Yi Chuang a , Ming Lun Yang a , Jen Sue Chen a , Wei Yang Chou b a Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701-01, Taiwan, ROC b Institute of Electro-Optical Science and Engineering, National Cheng-Kung University, Tainan 701-01, Taiwan, ROC article info Article history: Received 17 December 2009 Received in revised form 26 February 2010 Accepted 6 March 2010 Available online 29 March 2010 Keywords: Synthesis Conjugated polymer Copolymerization Solar cells Low bandgap abstract We have synthesized a new low bandgap alternating polyfluorene copolymer (PFNAP) based on dioctylfluorene and a donor–acceptor monomer with an electron-withdrawing moiety as a side chain, via a Suzuki polymerization reaction. The resulting copolymer has low optical and electrochemical bandgaps. The optical bandgap and the electrochemical bandgap of PFNAP are 1.82 and 1.89 eV, respectively. The bulk heterojunction polymer solar cells were fabricated with the conjugated polymer as the electron donor and 6,6-phenyl C 61 -butyric acid methyl ester (PCBM) as the electron acceptor. The power conversion efficiencies (PCE) of the solar cells based on PFNAP:PCBM (1:3) and PFNAP:PCBM (1:4) are 0.61% and 0.67%, respectively, under the illumination of AM 1.5 G, 100 mW/cm 2 . & 2010 Elsevier B.V. All rights reserved. 1. Introduction Polymer solar cells have attracted a lot of attention in recent years due to their potential use for new generation renewable energy sources [1–4]. They have the advantages of low cost, light weight, flexibility, and easy manufacturing [5–8]. The most commonly used polymer solar cell device structure is the bulk heterojunction (BHJ) structure due to its high efficiency [9–14]. In this structure, an electron donating conjugated polymer blends with an electron acceptor, such as PCBM (6,6-phenyl C 61 -butyric acid methyl ester), as the active layer. After photoexcitation, electrons transfer from the excited conjugated polymer chains to the electron acceptor molecules (PCBM). The donor–acceptor (D–A) BHJ structure has been considered ideal for efficient and fast exciton dissociation, and charge transport can be enhanced due to the increase in interfacial area between the donor and acceptor [15–17]. The D–A BHJ solar cells give power conversion efficiencies in the range of 4–6% [18–20]. Compared to inorganic solar cells, the PCE of polymer solar cells is still too low for commercial application. For the conjugated polymer part, further improvement of PCE demands the development of new materials with high carrier mobility and broader absorption of the solar spectrum [21–23]. Polyfluorene (PF) copolymers are well known for their high charge carrier mobility [24,25], good processability, and high absorption coefficients [26,27]. However, they have large band- gaps and blue shifted absorptions, which do not match the solar spectrum for solar cell applications. The bandgaps of polyfluor- enes can be reduced through copolymerization of donor–acceptor units. A regular alternation of donor and acceptor groups should lead to the broadening of the valence and conduction bands, and thus reduce the bandgaps [28–31]. Intramolecular donor–acceptor systems typically consist of electron donating groups (electron rich segments) and electron acceptors (electron deficient segments) [32–35]. The incorpora- tion of electron-withdrawing moieties as side chains of a conjugated polymer can widen the absorption spectrum. After photoexcitation, the charge separation occurs through transfer of electrons from the main chains to the side chains and then to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Electrons can transfer at PCBM by hopping, and holes can transfer at the main chains of the polymer by hopping and delocalization. Therefore, conjugated polymers containing electron-withdrawing acceptors as side chains not only exhibit enhanced charge transfer ability, but also absorb light more effectively [36]. In this paper, we report the synthesis of a new donor–acceptor monomer with an electron-withdrawing moiety as a side chain followed by a Suzuki polymerization reaction to prepare a new low bandgap copolymer with a dioctylfluorene monomer. In the donor–acceptor monomer, the donor is the electron rich ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.03.002 n Corresponding author. Tel.: + 886 6 2757575x62904; fax: + 886 6 2346290. E-mail address: lchsu@mail.ncku.edu.tw (S.L.C. Hsu). Solar Energy Materials & Solar Cells 94 (2010) 1166–1172