Solar Cells DOI: 10.1002/anie.201203330 A Supramolecular Complex in Small-Molecule Solar Cells based on Contorted Aromatic Molecules** Seok Ju Kang, Jong Bok Kim, Chien-Yang Chiu, Seokhoon Ahn, Theanne Schiros, StephanieS. Lee, KevinG. Yager, Michael F. Toney, Yueh-Lin Loo, and Colin Nuckolls* We recently described a supramolecular complex that forms in co-crystalline solids between doubly concave aromatic molecules and fullerenes. [1–3] Supramolecular complexes of fullerenes and aromatic compounds have been known for almost as long as fullerenes themselves. [4–6] Both hexabenzo- coronenes (HBCs) [1] and dibenzotetrathienocoronenes (DBTTC) [2] form co-crystals with C 60 or C 70 fullerenes. In these co-crystals, the concave faces of the aromatic molecule associate with the fullerene to form a nested structure, in which the fullerene is surrounded by the electron-rich aromatic compound. When these materials are deposited as either bilayered or reticulated structures, they form efficient active layers in photovoltaic devices. [1, 3] In earlier studies, we found that the fullerene and the coronene derivatives are both crystalline, with co-crystals forming at the interface between these two materials. This co-crystalline region is responsible for enhanced device performance. We studied, for the first time, the solution-based self-assembly of these contorted aromatic molecules with soluble fullerene derivatives. We hypothesized and tested, whether the self-assembly from solution can create the active layer in solar cells. We found that there is self-organization between the two organic semiconductors shown in Figure 1, the hexylated DBTTC (6-DBTTC) and phenyl-C 70 -butyric acid methyl ester (PC 70 BM). These self-assembled films are active in photo- voltaic devices and provide high open circuit voltages (V OC ) and power conversion efficiencies (PCE) up to 2.7 %. The optimal conditions to form these self-assembled heterojunc- tions are very different from those of polymeric photovoltaic devices. [7–10] To test for assembly between 6-DBTTC and PC 70 BM, we mixed both compounds in varying ratios and monitored the photoluminescence emission from 6-DBTTC. We found a significant amount of fluorescence quenching (see Fig- ure S1 a in the Supporting Information). From this data we could perform a Stern–Volmer analysis, [11] and it showed a significant amount of association between the two compo- nents and an association constant of 10 5 m 1 (Figure S1 b) in o- xylene as solvent. We speculated that the mode of association has the concave face of 6-DBTTC in contact with the convex face of the PC 70 BM. This mode is similar to the organization of unsubstituted fullerenes with the contorted aromatic compounds in the solid state. [1, 2] We tested how this supramolecular complex impacts the properties of films in photovoltaic devices. We varied the ratio Figure 1. Chemical structure of 6-DBTTC and PC 70 BM. [*] Dr. S. J. Kang, [+] C.-Y. Chiu, Dr. S. Ahn, Prof. C. Nuckolls Department of Chemistry, Columbia University New York, NY 10027 (USA) E-mail: cn37@columbia.edu Homepage: http://nuckolls.chem.columbia.edu/ Dr. J. B. Kim, [+] S. S. Lee, Prof. Y.-L. Loo Department of Chemical and Biological Engineering Princeton University Princeton, NJ 08544 (USA) Dr. T. Schiros Columbia Energy Frontier Research Center (EFRC) Columbia University New York, NY 10027 (USA) Dr. K. G. Yager Center for Functional Nanomaterials Brookhaven National Laboratory Upton, NY 11973 (USA) Dr. M. F. Toney Stanford Synchrotron Radiation Lightsource SLAC National Accelerator Laboratory Menlo Park, CA 94025 (USA) [ + ] These authors contributed equally to this work. [**] This research was supported as part of the Center for Re-Defining Photovoltaic Efficiency Through Molecular-Scale Control, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under award number DE-SC0001085) and the FENA (Grant 2009-NT- 2048). J.B.K., S.S.L., and Y.L.L. also acknowledge funding by the Photovoltaics Program at ONR (N00014-11-10328) and an NSF- sponsored MRSEC through the Princeton Center for Complex Materials (DMR-0819860). Portions of this research were carried out at beamline 11-3 at SSRL, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences, and at the Center for Functional Nano-materials, and beamline X-9 at BNL, which are supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. We thank Chad Miller for assistance with data collection. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201203330. . Angewandte Communications 8594  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2012, 51, 8594 –8597