Effect of asymmetric solubility of diketopyrrolopyrrole-based polymers and PC 71 BMs in a binary solvent system on the performance of bulk heterojunction solar cells Seon Kyoung Son a,b,1 , Hyo-Sang Lee a,b,c,1 , Jae Seung Ha b , Kyung Hwan Kim b , Hae Jung Son a , Min Jae Ko a , Honggon Kim a , Doh-Kwon Lee a , Jin Young Kim a , Wonmok Lee d , Sungnam Park b , Dong Hoon Choi b,n , BongSoo Kim a,c,e,nn a Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea b Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-701, Republic of Korea c Green School, Korea University, Seoul 136-701, Republic of Korea d Department of Chemistry, Sejong University, Gunja-dong, Gwangjin-gu, Seoul 143-747, Republic of Korea e Nanomaterials Science and Engineering, University of Science & Technology (UST), Daejeon 305-350, Republic of Korea article info Article history: Received 3 December 2013 Received in revised form 6 February 2014 Accepted 8 February 2014 Available online 3 March 2014 Keywords: Organic photovoltaics Power conversion efficiency Binary solvent system Morphology Hole mobility Polymer solar cell abstract In this study, we demonstrated the effective morphological control of polymer:fullerene blends using three separate solvent systems: chloroform (CF), CF:1,8-diiodooctane (DIO), and CF:o-dichlorobenzene (ODCB). The polymer:fullerene blends are composed of two diketopyrrolopyrrole (DPP)-based polymers of P(DPP-alt-QT) and P(DPP-alt-DTBSe) and a fullerene derivative of [6,6]-phenyl-C71-butyric acid methyl ester (PC 71 BM), i.e., P(DPP-alt-QT):PC 71 BM or P(DPP-alt-DTBSe):PC 71 BM. The CF:ODCB binary solvent exhibited the best photovoltaic performance among the three solvent systems for both polymer-based devices, although the CF:DIO also exhibited an improved performance compared to the CF system. By examining film morphology of the blend films, we found that the CF:ODCB enabled the most optimal nanoscale phase separation and the morphological features were strongly affected by the solubility of each material in the high boiling-point (BP) solvent. Specifically, the polymers have limited but slightly higher solubility in ODCB than in DIO, while the PC 71 BM molecules have a high solubility in both DIO and ODCB. Therefore, this work highlights that the optimally asymmetric solubility of each photoactive component in the high BP solvent is a critical factor to form the nanoscale, bicontinuous domains in the blend films and thereby to determine the performance of photovoltaic devices. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Organic photovoltaics (OPVs) have attracted much attention as promising renewable energy resources because of their advan- tages in low-cost, printable, light-weight, large-area, and flexible applications [1–13]. Recently, power conversion efficiencies (PCEs) have exceeded 8% from polymer:fullerene bulk heterojunction (BHJ) solar cell devices employing low bandgap (LBG) polymers. These LBG polymers were typically organized by an alternating arrangement of electron-rich units and electron-deficient units [14–18]. While the PCEs of OPVs have increased rapidly, it is very important to control the nanoscale morphology of the BHJ-type photoactive layers that are composed of conjugated polymers and fullerenes (typically [6,6]-phenyl-C71-butyric acid methyl ester (PC 71 BM)). In order to achieve a bicontinuous nanophase-segregated morphology of photoactive layers, they are often treated with thermal annealing [19,20] and solvent annealing processes [21,22]. Alterna- tively, for high-performance LBG polymer:fullerene blends, the film morphology was effectively controlled by using binary solvent systems where one is a good solvent with a low boiling point (BP) for LBG polymers and fullerenes, and the other is a poor solvent with a high BP [23–26]. The solubility of LBG polymers and fullerenes in each solvent plays a significant role in the formation of bicontin- uous nanophase-separated domains of polymers and fullerenes. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.02.011 0927-0248 & 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ82 2 3290 3140; fax: þ82 2 925 4284. nn Corresponding author at: Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea. Tel.: þ82 2 958 5516; fax: þ82 2 958 6649. E-mail addresses: dhchoi8803@korea.ac.kr (D.H. Choi), bongsoo@kist.re.kr (B. Kim). 1 S.-K. Son and H.-S. Lee equally contributed to this work. Solar Energy Materials & Solar Cells 124 (2014) 232–240