Efficient molecular solar cells processed from green solvent mixtures† Mahmoud E. Farahat, abc Packiyaraj Perumal, ad Widhya Budiawan, abe Yang-Fang Chen, d Chih-Hao Lee * e and Chih-Wei Chu * bf Cyclopentyl methyl ether (CPME), a green solvent, can be used to replace toxic halogenated solvents in the production of efficient molecular solar cells. With CPME alone as the processing solvent, a power conversion efficiency (PCE) of 3.13% was achieved when using a two-dimensional conjugated small molecule (SMPV1) as the donor and [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) as the acceptor to form the bulk heterojunction (BHJ) organic photovoltaic (OPV) device. This low PCE arose from the low solubility of PC 61 BM in this green solvent. Accordingly, toluene (Tol) was introduced in various amounts as a co-solvent for CPME. The greater solubility of PC 61 BM in these mixtures led to significant improvements in the short-circuit current density (J sc ) and fill factor (FF) of the device, achieving a PCE of 7% after processing in the optimized green solvent mixture of CPME : Tol (60 : 40). Furthermore, thermal annealing (TA), at 80  C for 10 min, of the active layers processed from the 60 : 40 green solvent mixture enhanced the PCE to 8.10%—the highest ever reported for a molecular solar cell processed from a green solvent mixture. Large-area devices fabricated this way, having areas of 1 and 5.5 cm 2 , exhibited PCEs of 6.20 and 3.73%, respectively. The morphological changes that occurred when applying the co-solvent and TA played key roles in achieving such high PCEs for molecular solar cells processed from green solvent mixtures—a promising step toward the upscaling of OPVs. Introduction Because of their light weight, low cost, exibility, and solution- processability, organic photovoltaics (OPVs) are among the most attractive candidates for use in clean and cost-effective energy production technologies. 1,2 Over the past decade, research in OPVs has developed steadily such that they have reached the threshold power conversion efficiency (PCE) needed for commercialization. Both polymer- and molecule- based solar cells have achieved PCEs greater than 10% for single-junction cells. 3–6 Unfortunately, most of the high effi- ciencies reported for both polymer and molecular OPVs have been achieved aer processing using toxic halogenated solvents (e.g., chlorobenzene 3,7 and chloroform 4,8–10 ). The need for hazardous processing solvents remains a big obstacle affecting the upscaling of OPVs prepared through large-scale coating techniques (e.g. roll-to-roll coating). These solvents are not favored for sustainable development because of the undesirable procedures necessary for their production, use, and waste pro- cessing. Moreover, they are strictly banned for mass production in industrialized countries having strict environmental health and safety regulations. 11 Therefore, nding greener alternatives for the efficient processing of OPVs, similar to the efficiencies obtained using halogenated counterparts, remains a challenge for the industrial commercialization of OPVs. 12 In addition, solvents having medium-temperature boiling points are favored to simplify device fabrication. 13 In this regard, great efforts have been devoted to developing efficient OPVs processed from eco- friendly solvents. Although halogen-free solvents have been re- ported as reliable replacements for toxic solvents in the production of polymer solar cells, 13–17 few such studies have been reported for molecular OPV counterparts. 11,18–20 Recently, the environmentally friendly solvent 2-methyl- anisole was used to achieve one of the highest PCEs reported for a single-solvent polymer solar cell: 9.6%. 21 Combining common hydrocarbon solvents with 1-phenylnaphthalene as a solvent additive has led to the highest reported PCE (11.7%) for a poly- mer solar cell fabricated from halogen-free solvents. 22 The use of several other halogen-free solvents, including anisole, 23 xylenes, 15 toluene (Tol), 16 derivatives of N-methyl-2-pyrrolidone a Nanoscience and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taiwan b Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. E-mail: gchu@gate.sinica.edu.tw c Central Metallurgical Research and Development Institute (CMRDI), P.O. Box: 87, Helwan, Cairo 11421, Egypt d Department of Physics, National Taiwan University, Taipei 106, Taiwan e Department of Engineering and System Science, National Tsing-Hua University, Hsinchu 30013, Taiwan. E-mail: chlee@mx.nthu.edu.tw f College of Engineering, Chang Gung University, Tao-Yuan 333, Taiwan † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta09626c Cite this: J. Mater. Chem. A, 2017, 5, 571 Received 7th November 2016 Accepted 17th November 2016 DOI: 10.1039/c6ta09626c www.rsc.org/MaterialsA This journal is © The Royal Society of Chemistry 2017 J. Mater. Chem. A, 2017, 5, 571–582 | 571 Journal of Materials Chemistry A PAPER Published on 17 November 2016. Downloaded by Academia Sinica - Taipei on 07/04/2017 11:15:25. View Article Online View Journal | View Issue