DOI: 10.1002/cphc.200900941 Photovoltaic Universal Joints: Ball-and-Socket Interfaces in Molecular Photovoltaic Cells Noah J. Tremblay, [a] Alon A. Gorodetsky, [a] Marshall P. Cox, [b] Theanne Schiros, [c] Bumjung Kim, [a] Rachel Steiner, [d] Zachary Bullard, [d] Aaron Sattler, [a] Woo-Young So, [e] Yoshimitsu Itoh, [a] Michael F. Toney, [f] Hirohito Ogasawara, [f] Arthur P. Ramirez, [g] Ioannis Kymissis,* [b] Michael L. Steigerwald, [a] and Colin Nuckolls* [a] Herein, we detail how to grow one crystalline organic semicon- ductor on another epitaxially and thereby provide a method to tune the electronic nature of the p–n junction in organic pho- tovoltaics (OPVs). While OPVs are attractive as materials for conversion of sunlight into electrical energy, [1] higher conver- sion efficiencies [2] are needed for OPVs to become a viable technology. [3–6] Regardless of the type of OPV, either a bilayer [7] or bulk-heterojunction (BHJ) [4] (Figure 1 A), the interface be- tween the hole and electron transporting films is the critical locus for exciton formation and dissociation. [8–11] In inorganic materials, the interface between two semiconductors is crucial- ly important in determining and controlling the electrical prop- erties of these materials and is controlled by a heteroepitaxial growth of one crystalline material on another. We show here that p-type and n-type organic semiconductors can be de- signed to have nested shapes that create an epitaxial growth that achieves higher conversion efficiencies and open circuit voltages in these devices to within 10 % of the theoretical limit. We utilize the class of molecules known as contorted hexabenzocoronenes (HBCs, Figure 1 B) because they are es- tablished p-type semiconductors [12–14] and are also photocon- ductive. [15, 16] This HBC has an unusual shape in that it is con- torted and doubly-concave. [12] The size and shape of this mole- cule are complementary to buckminsterfullerene (C 60 ), which is a well-known n-type semiconductor (Figure 1 C). It is this po- tential for shape and electronic complementarity between these two molecular structures that led us to investigate them in the context of heteroepitaxial growth. We first focused on whether HBC and C 60 formed co-crystal- line, supramolecular assemblies. Two experiments, one from solution (Figure 2 A) and one from the gas phase (Figure 2 B) show that the materials form co-crystals. Large purple-gray crystals were produced from a saturated solution of C 60 and HBC in chlorobenzene. The molecular structure determined from the solution- grown crystals reveals that HBC and C 60 spontaneously formed an interdigitated supramolecular complex (complex 1). The three-dimensional structure of HBC comprises two opposing concave aromatic faces, wherein a C 60 had nestled into each face (Figure 2 A). It is important to note that a number of or- ganic molecules have been specifically designed to form com- plementary interactions with C 60 and have yielded co-crys- tals. [17–20] However, few of these molecules are suitable candi- dates for the formation of a p–n junction. [17] The crystal of 1 comprises C 60 , HBC, and chlorobenzene (2:1:1), wherein HBC and C 60 organize into a repeating pattern of ABAABA as shown in Figure 2 A. Each HBC has two C 60 near- est neighbors, and each C 60 has one HBC nearest neighbor and one C 60 nearest neighbor. The C 60 is centered over one of the Figure 1. A) Depiction of ball-and-socket interfaces in bilayer and bulk heter- ojunction devices. B) Chemical structure of the contorted-HBC. C) Correlation between depiction (top) and molecular structure from the co-crystal of HBC and C 60 (bottom). [a] N. J. Tremblay, Dr. A. A. Gorodetsky, B. Kim, A. Sattler, Dr. Y. Itoh, Dr. M. L. Steigerwald, Prof. C. Nuckolls Department of Chemistry and The Center for Electron Transport in Molecular Nanostructures Columbia University, New York, NY 10027 (USA) Fax: (+ 1) 212-932-1289 E-mail : cn37@columbia.edu [b] M. P. Cox, Dr. I. Kymissis Department of Electrical Engineering Columbia University, New York, NY 10027 (USA) Fax: (+ 1) 212-932-9421 E-mail : johnkym@ee.columbia.edu [c] Dr. T. Schiros The Center for Electron Transport in Molecular Nanostructures Current Affiliation: Columbia Energy Frontier Research Center (EFRC) Columbia University, New York, NY 10027 (USA) [d] R. Steiner, Z. Bullard Department of Materials Science and Engineering The Center for Electron Transport in Molecular Nanostructures Columbia University, New York, NY 10027 (USA) [e] Dr. W.-Y. So Department of Applied Physics and Applied Mathematics Columbia University, New York, NY 10027 (USA) [f] Dr. M.F. Toney, Dr. H. Ogasawara Stanford Synchrotron Radiation Lightsource Menlo Park, CA 94035 (USA) [g] Prof. A. P. Ramirez The Jack Baskin School of Engineering University of California - Santa Cruz Santa Cruz, CA 95064 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200900941. ChemPhysChem 2010, 11, 799 – 803  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 799