Tailoring side chains of low band gap polymers for high efficiency polymer solar cells Weiwei Li a, b , Ruiping Qin c , Yi Zhou b , Mattias Andersson b , Fenghong Li b , Chi Zhang a , Binsong Li a , Zhengping Liu c , Zhishan Bo a, * , Fengling Zhang b, ** a Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b Department of Physics, Chemistry and Biology, Linköping University, SE-58183 Linköping, Sweden c College of Chemistry, Beijing Normal University, Beijing 100875, China article info Article history: Received 17 February 2010 Received in revised form 22 April 2010 Accepted 9 May 2010 Available online 15 May 2010 Keywords: Organic solar cell Side chains Low band gap abstract High efficiency organic solar cells (OSCs) require conjugated polymers with a low band gap, broad absorption in visible and IR region, high carrier mobility, and relatively high molecular weight as p-type donor materials. Flexible side chains on the rigid polymer backbone are crucial for the solubility of conjugated polymers. In this work, four polymers with the main chain structure of fluorene-thiophene- benzothiadiazole-thiophene and flexible side chains located on fluorene, thiophene, and benzothiadiazole moiety, respectively, have been synthesized by SuzukieMiyauraeSchlüter polycondensation. Photovoltaic device measurements with a device configuration of ITO/polymer:PC 71 BM blends/LiF/Al show that P1 carrying octyloxy chains on benzothiadiazole rings gives the best performance, with a power conversion efficiency of 3.1%. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In the past two decades, plastic solar cells as an alternative to inorganic photovoltaic devices have been investigated due to their flexibility and low cost of processing [1,2]. Organic solar cells (OSC) based on regioregular poly(3-hexylthiophene) (P3HT) donor and a soluble fullerene derivative acceptor, with a bulk heterojunction structure, have reached power conversion efficiencies (PCE) of 5% [3,4]. Until now, more and more conjugated polymers have been developed and used for OSCs. Benefiting from their low band gap, broad absorption in the visible and infrared (IR) region, crystalli- zation, and/or high carrier mobility, PCEs of 5%e7.5% have been achieved [5e11]. Several groups have devoted to designing new optimized conjugated polymers, improving efficiency, and decreasing the cost of processing for device fabrication [12e14]. Low band gap conju- gated polymers with main chain alternating donoreacceptor (DeA) structure are promising, which can broaden the absorption to the red and even to the near IR region via the so-called internal charge transfer and result in higher PCE [15]. Functional groups such as fluorene [16e20], carbazole [6,9,21,22], silafluorene [7,23], cyclopentadithiophene [8,24e26], N-substituted dithienopyrrole [27,28] and dithienosilole [10,29], have been used as donor moieties; whereas, functional groups such as benzothiadiazole [12,30], quinoxaline [26,31,32], diketopyrrolopyrrole [33,34], thienopyrazine [35,36], fluoranthene [37] and so on were usually used as acceptor moieties. The rigid conjugated polymers are usually required to carry flexible side chains to ensure that polymers have certain solubility in organic solvents. The lengths of alkyl chains play an important role in molecular weights, energy levels of conjugated polymers, morphologies of blend films, and therefore the photovoltaic performance of devices [19,38,39]. Moreover, the position of alkyl chain on each DeA repeat unit is also a key point in tailoring polymer structures [9,35,40e43]. Our previous work about a copolymer named poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2-yl) benzo[c][1,2,5]thiadiazole-7-yl) thiophene-2-yl)-9-octyl-9H-carba zole) (HXS-1), with two octyloxy chains on benzothiadiazole moiety demonstrated that a planar structure was formed due to the low steric hinderance of octyloxy chain, and a PCE of 5.4% was achieved [9]. Janssen et al. have synthesized a series of conjugated polymers based on the thiophene-benzothiadiazole-thiophene structure, among which the polymer with octyl substituents on the 3-position of thiophene bears much more steric hinderance than that with octyl substituents on the 4-position of thiophene, and results in different optical and electrochemical properties for these two polymers [41,42]. * Corresponding author. Tel./fax: þ86 10 82618587. ** Corresponding author. Tel.: þ46 13281257; fax: þ46 13288969. E-mail addresses: zsbo@iccas.ac.cn (Z. Bo), fenzh@ifm.liu.se (F. Zhang). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.05.015 Polymer 51 (2010) 3031e3038