Benzooxadiazole-based donor/acceptor copolymers imparting bulk-heterojunction solar cells with high open-circuit voltages Jian-Ming Jiang, Po-An Yang, Shang-Che Lan, Chia-Ming Yu, Kung-Hwa Wei * Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan article info Article history: Received 21 August 2012 Received in revised form 2 November 2012 Accepted 14 November 2012 Available online 20 November 2012 Keywords: Polymer solar cell Suzuki coupling Donor/acceptor conjugated polymers abstract In this study we used Suzuki cross-coupling to synthesize three new donor/acceptor copolymersdPFTBO, PAFTBO, and PCTBOdfeaturing soluble alkoxy-modified 2,1,3-benzooxadiazole (BO) moieties as acceptor units and electron-rich building blocksddialkyl fluorene (F), alkylidene flu- orene (AF), and carbazole (C), respectivelydas donor units. These polymers, which we characterized using gel permeation chromatography, thermogravimetric analysis, NMR spectroscopy, UVeVis absorption spectroscopy, and electrochemical cyclic voltammetry, exhibited good solubility, low-lying energy levels for their highest occupied molecular orbitals, excellent thermal stability, and air stability. Using these polymers, we fabricated bulk-heterojunction solar cell devices having the structure indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/polymer:[6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) (1:1, w/w)/Ca/Al. Under AM 1.5G illumination (100 mW cm 2 ), the solar cell incorporating PFTBO exhibited a high value of V oc of 1.04 V and that based on PCTBO provided a power conversion efficiency of 4.1% without the need for any post treatment. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Polymer solar cells (PSCs) are attracting growing interest as a potential renewable energy technology because they can be manufactured at low cost with the capability of being used in flexible large-area devices [1e3]. To date, bulk-heterojunctions (BHJs), in which the active layer consists of a blend of electron- donating conjugated polymers and electron-accepting fullerene derivatives, have been the most prevalent active layer structures in polymer solar cells exhibiting high power conversion efficiencies (PCEs). Several conjugated polymers have been developed featuring electron donor/acceptor (D/A) units in main chaine conjugated configurations [4e15] and side chaineattached archi- tectures [16e20]. Recently, BHJ solar cells based on blends of some D/A low-band gap polymers and [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) or PC 71 BM have been investigated exten- sively, providing PCEs as high as 7% [21e27]. The PCE of a solar cell device is essentially determined by short- circuit current density (J sc ), the fill factor, and the open-circuit voltage (V oc ). The relatively low open-circuit voltage (ca. 0.6 V) obtained in some thiophene-polymer based BHJ devices will limit their PCEs. In a BHJ-structured active layer, the open-circuit voltage is typically proportional to the difference in energy between the highest occupied molecular orbital (HOMO) of the polymer and the lowest unoccupied molecular orbital (LUMO) of the fullerene, although some other characteristics of the device structure (e.g., the type of cathode material, the active layer morphology, or exciton non-radiative recombination) can also affect the values of V oc of BHJ PSCs [28e31]. Therefore, the value of V oc can be increased either by elevating the LUMO energy level of the fullerene or depressing the HOMO energy level of the polymer while keeping its counterpart unchanged. Low-band gap polymers that provide efficient absorp- tion of the solar spectrum, however, tend to have high-lying HOMOs and low-lying LUMOs; the difference in the energy levels between the low-lying LUMOs of the polymers and the LUMO of the fullerene frequently result in inefficient charge separation, leading to a smaller enhancement of J sc . On the other hand, the combination of a high-lying HOMO in a low-band gap polymer and a fixed LUMO in fullerene will also provide a lower value of V oc . Therefore, fine tuning of the band gap and the energy levels such as lowering the HOMO and LUMO of the polymer simultaneously but with a larger decrease in the LUMO while maintaining its value 0.3 eV above that of the fullerene is required to obtain BHJ PSCs with high values of V oc and J sc [32e34]. Currently, the highest open-circuit voltages obtained from BHJ PSCs (ca. 1 V) have required polymers possessing medium-sized band gaps (ca. 2 eV) [35e38]. * Corresponding author. E-mail address: khwei@mail.nctu.edu.tw (K.-H. Wei). Contents lists available at SciVerse ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.11.034 Polymer 54 (2013) 155e161