Photovoltaic universal joints: ball-and-socket interfaces in molecular photovoltaic cells.

Herein, we detail how to grow one crystalline organic semiconductor on another epitaxially and thereby provide a method to tune the electronic nature of the p–n junction in organic photovoltaics (OPVs). While OPVs are attractive as materials for conversion of sunlight into electrical energy, higher conversion efficiencies are needed for OPVs to become a viable technology. Regardless of the type of OPV, either a bilayer or bulk-heterojunction (BHJ) (Figure 1A), the interface between the hole and electron transporting films is the critical locus for exciton formation and dissociation. In inorganic materials, the interface between two semiconductors is crucially important in determining and controlling the electrical properties 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 designed 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 1B) because they are established p-type semiconductors and are also photoconductive. This HBC has an unusual shape in that it is contorted and doubly-concave. The size and shape of this molecule are complementary to buckminsterfullerene (C60), which is a well-known n-type semiconductor (Figure 1C). It is this potential 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 C60 formed co-crystalline, supramolecular assemblies. Two experiments, one from solution (Figure 2A) and one from the gas phase (Figure 2B) show that the materials form co-crystals. Large purple-gray crystals were produced from a saturated solution of C60 and HBC in chlorobenzene. The molecular structure determined from the solutiongrown crystals reveals that HBC and C60 spontaneously formed an interdigitated supramolecular complex (complex 1). The three-dimensional structure of HBC comprises two opposing concave aromatic faces, wherein a C60 had nestled into each face (Figure 2A). It is important to note that a number of organic molecules have been specifically designed to form complementary interactions with C60 and have yielded co-crystals. However, few of these molecules are suitable candidates for the formation of a p–n junction. The crystal of 1 comprises C60, HBC, and chlorobenzene (2:1:1), wherein HBC and C60 organize into a repeating pattern of ABAABA as shown in Figure 2A. Each HBC has two C60 nearest neighbors, and each C60 has one HBC nearest neighbor and one C60 nearest neighbor. The C60 is centered over one of the Figure 1. A) Depiction of ball-and-socket interfaces in bilayer and bulk heterojunction devices. B) Chemical structure of the contorted-HBC. C) Correlation between depiction (top) and molecular structure from the co-crystal of HBC and C60 (bottom).