Synthetic Metals 161 (2011) 1838–1844 Contents lists available at ScienceDirect Synthetic Metals j o ur nal homep ag e: www.elsevier.com/locate/synmet Synthesis and characterization of dithienothiophene/benzothiadiazole based low band gap donor–acceptor copolymers for bulk hetero junction photovoltaic cells Abasaheb V. Patil a , Woo-Hyung Lee b , Kyuri Kim a , Youn-Sik Lee a , In-Nam Kang b,∗ , Soo-Hyoung Lee a,∗ a School of Semiconductor and Chemical Engineering, Chonbuk National University, Duckjin-dong 664-14, Jeonju 561-756, Republic of Korea b Department of Chemistry, The Catholic University, 43-1, Yeokaok2-dong, Wonmi-gu, Buchen-si, Gyeonggi-do 420-743, Republic of Korea a r t i c l e i n f o Article history: Received 22 January 2011 Received in revised form 4 June 2011 Accepted 9 June 2011 Available online 29 July 2011 Keywords: Low band gap -Conjugated copolymer Donor–acceptor Photovoltaic solar cell a b s t r a c t Two newly designed low band gap copolymers P2TDTT-BT and P2TDTT-DTBT, comprised of a substituted dithienyl-dithieno[3,2-b:2 ′ ,3 ′ -d]thiophene derivative as a donor and a benzothiadiazole or dithienyl ben- zothiadiazole based acceptor, respectively, were synthesized by Stille polymerization. The UV–visible absorption of P2TDTT-BT and P2TDTT-DTBT films ranged from the UV to NIR regions. The HOMO/LUMO energy levels of P2TDTT-BT and P2TDTT-DTBT were estimated to be -5.41/-3.74 eV and -5.34/-3.65 eV, corresponding to energy band gaps of 1.67 and 1.69 eV, respectively. Bulk heterojunction photovoltaic cells were fabricated using a blend of P2TDTT-BT or P2TDTT-DTBT and PC 61 BM in a 1:1 ratio and delivered power conversion efficiencies of 0.31% and 0.92%, respectively. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Polymer solar cells (PSCs) are a focus of worldwide research interest because of their potential utility as low-cost, lightweight, flexible, and easy to fabricate solar energy-to-electric power con- version devices [1–3]. Bulk hetero junction (BHJ) devices [1,3], in which a donor polymer (p-type) is blended with a fullerene derivative or other acceptor (n-type) material, have emerged as the most efficient polymer solar cells to date. Among donor polymers in fullerene-based BHJ solar cells, regioregular poly (3- hexylthiophene) (P3HT) has been the most widely investigated, and its use in solar cells has yielded some of the highest power conversion efficiencies (3–5%) [2–5]. However, P3HT only harvests photons with wavelengths below 650 nm, while a majority of solar photon energy is observed at much lower wavelengths (around 700 nm) [6]. Therefore, polymer materials with low band gaps are needed to harvest solar photons with longer wavelengths, particu- larly in the NIR region. Various design strategies have been pursued to fulfill this requirement. One popular approach is to synthesize copolymers containing alternating monomer units with electron rich donor (D) and electron poor acceptor (A) character in the conjugated molecular backbone. Judiciously chosen D and A groups are partic- ularly desirable for low-band gap polymers due to the significant ∗ Corresponding authors. Tel.: +82 632 702 435; fax: +82 632 702 306. E-mail addresses: inamkang@catholic.ac.kr (I.-N. Kang), shlee66@jbnu.ac.kr, shlee66@chonbuk.ac.kr (S.-H. Lee). enhancement of intra-molecular charge transfer (ICT) intensity and conjugation length, which lead to greater extended absorption and a higher absorption coefficient. An extended, rigid -conjugation with quinoidal character in the polymer backbone facilitates inter- molecular interactions between the polymer chains and increases the charge mobility of the polymer [7]. Fused thiophene ring sys- tems stabilize the quinoidal structure, reduce the band gap, and enhance – stacking [8,9]. Recently, several D–A copolymer sys- tems have achieved better efficiencies (up to approximately 5%) by tuning the HOMO energy of the polymer through modifica- tions of the monomer structures based on known thienopyrazine or benzothiadiazole acceptor groups with electron-rich thiophene donor groups [10–12]. In these studies, considerable attention was focused on D–A conjugated polymers whose optical and electronic properties involving intra-molecular charge transfer (ICT) from the D to the A were tunable. Taking these results into account, in our survey of electron-rich monomers, we became interested in the dithieno[3,2-b:2 ′ ,3 ′ - d]thiophene (DTT) unit [13], an important building block for a wide variety of functional organic materials. The planarity and S–S inter- actions of fused DTT structures promote highly ordered -stacking [14,15] and elevated hole mobility [15], which are predictors of pronounced charge transport in devices [16]. Several groups have reported the synthesis of DTT derivatives for applications in organic thin film transistors (OTFTs) [14,17–22]. It is interesting to note that despite all of these promising features, to the best of our knowledge, there have been only a few reports on the photovoltaic properties of DTT-containing D–A type copolymers [23–25,20]. Zhan and co- workers reported DTT-based donor–acceptor polymers consisting 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.016