Epitaxial van der Waals (vdW) heterostructures of organic and layered materials are demonstrated to create high-performance organic electronic devices. High-quality rubrene films with large single-crystalline domains are grown on h-BN dielectric layers via vdW epitaxy. In addition, high carrier mobility comparable to free-standing single-crystal counterparts is achieved by forming interfacial electrical contacts with graphene electrodes.

A new approach toward higher efficiency organic photovoltaic devices (OPVs) is described. Complementarity in shape between the donor (contorted hexabenzocoronene, see picture) and acceptor (buckminsterfullerene) molecules results in OPVs that perform surprisingly well. This exploitation of host–guest chemistry at the organic/organic interface demonstrates a new direction for OPV device design.

We report the optical reflectivity and Raman scattering of few layer (L) graphene exposed to K and Rb vapors. Samples many tens of layers thick show the reflectivity and Raman spectra of the stage 1 bulk alkali intercalation compounds (GICs) KC(8) and RbC(8). However, these bulk optical and Raman properties only begin to appear in samples more than about 15 graphene layers thick. The 1 L to 4 L alkali exposed graphene Raman spectra are profoundly different than the Breit-Wigner-Fano (BWF) spectra of the bulk stage 1 compounds. Samples less than 10 layers thick show Drude-like plasma edge reflectivity dip in the visible; alkali exposed few layer graphenes are significantly more transparent than intrinsic graphene. Simulations show the in-plane free electron density is lower than in the bulk stage 1 GICs. In few layer graphenes, alkalis both intercalate between layers and adsorb on the graphene surfaces. Charge transfer electrically dopes the graphene sheets to densities near and above 10(+14) electrons/cm(2). New intrinsic Raman modes at 1128 and 1264 cm(-1) are activated by in-plane graphene zone folding caused by strongly interacting, locally crystalline alkali adlayers. The K Raman spectra are independent of thickness for L = 1-4, indicating that charge transfer from adsorbed and intercalated K layers are similar. The Raman G mode is downshifted and significantly broadened from intrinsic graphene. In contrast, the Rb spectra vary strongly with L and show increased doping by intercalated alkali as L increases. Rb adlayers appear to be disordered liquids, while intercalated layers are locally crystalline solids. A significant intramolecular G mode electronic resonance Raman enhancement is observed in K exposed graphene, as compared with intrinsic graphene.

Though both the crystal structure and molecular orientation of organic semiconductors are known to impact charge transport in thin-film devices, separately accessing different polymorphs and varying the out-of-plane molecular orientation is challenging, typically requiring stringent control over film deposition conditions, film thickness, and substrate chemistry. Here we demonstrate independent tuning of the crystalline polymorph and molecular orientation in thin films of contorted hexabenzocoronene, c-HBC, during post-deposition processing without the need to adjust deposition conditions. Three polymorphs are observed, two of which have not been previously reported. Using our ability to independently tune the crystal structure and out-of-plane molecular orientation in thin films of c-HBC, we have decoupled and evaluated the effects that molecular packing and orientation have on device performance in thin-film transistors (TFTs). In the case of TFTs comprising c-HBC, polymorphism and molecular orientation are equally important; independently changing either one affects the field-effect mobility by an order of magnitude.