5802 Chem. Commun., 2013, 49, 5802--5804 This journal is c The Royal Society of Chemistry 2013 Cite this: Chem. Commun., 2013, 49, 5802 Non-fullerene acceptors containing fluoranthene-fused imides for solution-processed inverted organic solar cells† Yan Zhou, Ya-Zhong Dai, Yu-Qing Zheng, Xiao-Ye Wang, Jie-Yu Wang* and Jian Pei* Six fluoranthene-fused imide derivatives were employed as acceptors in solution processed inverted BHJ solar cells with P3HT as the donor. The PCEs of all devices vary from 2.14% to 2.89%. All acceptors are in their amorphous state with low electron mobility, but achieving high PCEs. The power conversion efficiencies (PCEs) of organic bulk hetero- junction (BHJ) solar cells have exceeded 10%. 1 In the road map of the development of BHJ solar cells, significant efforts have been devoted to the development of various polymeric donors. 2–5 For acceptors, however, the structures are rather limited, with fullerene derivatives being the most investigated and the most successful species. 6 Recently, soluble non-fullerene electron acceptors, with the possibilities of broadening absorption, tunable energy levels, and facial derivatization and functionalization as compared with fullerene derivatives, have received considerable attention. 6–8 Hence, developing non-fullerene acceptors will enrich the diversity of acceptors to match the present high-performance donors, which may ultimately lead to higher PCEs via proper selection and combination of donors and acceptors. To date, with P3HT as the donor, only a few BHJ solar cells containing non-fullerene small-molecule acceptors exhibited PCEs higher than 1.5%, 9–14 and fewer achieved PCEs higher than 2.5%. 13 Recently, we used a fluoranthene-fused imide based molecule (FFI-1, Th-CN) as a non-fullerene electron acceptor, and achieved the highest PCE of 1.86% when blended with the P3HT donor in BHJ solar cells. 9 To achieve higher PCEs, systemic investigation of the relationship between acceptor structure and device performance is necessary. Herein, we further modify the acceptor structure by changing the substituents from thienyl to five other aryl groups perpendicular to the backbone of the acceptors. 9,15 These acceptors provide us a platform to understand the structure–property relation- ship and to screen out better acceptors based on fluoranthene-fused imides. Through optimization of the process of device-fabrication, we achieve PCEs of over 2.15% for all devices using these six acceptors blended with P3HT, and the highest PCE is up to 2.89%. Fig. 1 illustrates the chemical structures of the six acceptors based on fluoranthene-fused imide units, which were prepared following our previous report. 15 The different aryl groups perpendi- cular to the backbone of the acceptors are expected to improve the device performance for their different sizes and electron-donating properties. All new compounds have been characterized using 1 H and 13 C NMR, high resolution mass spectroscopy (HRMS), and elemental analysis (see ESI†). The electrochemical and photophysi- cal properties of all acceptors in the solid state including absorption, emission, photoluminescent quantum efficiency (PLQE), fluores- cence lifetime, and lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels are shown in Table 1. All six acceptors show identical absorption features both in solution and in the solid state, which indicates that the introduction of the bulky aromatic substituents effectively suppresses the aggre- gation of these acceptors in the solid state. The optical band gaps of the six acceptors are measured to be in the range of 2.69 to 2.83 eV from their absorption spectra on ZnO substrates, which are shown in Fig. S1 (ESI†). The measurement of the electrochemical behavior of all six acceptors indicates that the LUMO levels of the six acceptors are in the range of À3.40 to À3.48 eV, indicating higher open circuit voltage ( V OC ) than that of PCBM system when blending with the Fig. 1 Chemical structures of the acceptors. Beijing National Laboratory for Molecular Sciences, The Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. E-mail: jianpei@pku.edu.cn, jieyuwang@pku.edu.cn † Electronic supplementary information (ESI) available: Experimental and sup- plementary data (Table S1 and Fig. S1–S4). See DOI: 10.1039/c3cc41803k Received 10th March 2013, Accepted 4th April 2013 DOI: 10.1039/c3cc41803k www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 05 April 2013. Downloaded by Beijing University on 10/06/2013 14:54:02. View Article Online View Journal | View Issue