Orientation Control of Selected Organic Semiconductor Crystals Achieved by Monolayer Graphene Templates

DOI: 10.1002/admi.201600621 orientations in thin films. First of all, elucidating the effect of excited state delocalization, including singlet excitons, triplet excitons, and charge transfer states, on charge generation and recombination processes in crystalline thin films and molecular heterointerfaces requires controlled access to varied molecular orientations. A lying-down orientation provides larger out-of-plane exciton diffusion lengths and favorably increases the rate of energy transfer and charge generation and transport in molecular thin films but unfavorably increases recombination of Charge transfer (CT) states and free charges across a molecular heterointerface.[5–8] Second, the amount of total light absorbed by crystalline thin films can vary by orders of magnitude depending on the orientation of the molecular transition dipole moment relative to the electric field of incident light.[9,10] Third, the molecular orientation relative to the substrate primarily determines the energy band edge of the film.[11,12] Therefore, developing a universal platform to gain insight into the dependence of various OPV performance parameters on molecular orientations and establishing reliable and robust methods to control the molecular orientations in thin films is necessary. Aromatic molecules, with the notable exception of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) mostly adopt a standing-up orientation on weakly interacting substrates, such as oxides, which are widely used in OPVs. A lyingdown molecular orientation in thin films can be achieved with thickness greater than tens of nanometers (beyond the typical thickness for OPV active layers) by quasi-epitaxial growth, in which a templating layer induces substrate–molecule interactions that are comparable to intermolecular interactions.[13] Several templating materials have been used, such as copper iodide (CuI),[14] MoS2, alkali metal halide,[16] PTCDA,[17–19] and graphene.[4,13,20] Graphene is a promising template for OPVs due to its high transparency and electrical conductivity, and its ability to be transferred onto arbitrary substrates as compared to other templating materials.[21] However, thus far, only thin films of phthalocyanine and acene molecules have been templated with graphene.[4,21] A broader study of molecular-level orientation control in various technologically relevant organic Crystal orientation in organic thin films is one of the key parameters that determine absorption cross-section, interfacial energetics and excitonic states, and free charge properties. In this work, monolayer graphene is used to direct the crystal orientation of selected planar organic molecules. Lyingdown orientation with π-stacking normal to the surface is achieved with graphene templating. The absorption spectra of the graphene-templated films are correlated to molecular orientation. The same set of absorption features with or without graphene suggests that no vibronic states are forbidden or newly introduced. However, the light absorption with graphene templating is enhanced due to the altered relative orientation between the transition dipole moment of the constituent molecules and the electric field of incident light. The energy level of the highest occupied molecular orbital (HOMO) of graphene templated p-type films is observed at a deeper-lying value (relative to vacuum) compared to untemplated diindenoperylene films grown on indium tin oxide. In contrast, the HOMO energy levels of graphene templated n-type films are observed at higher-lying energy levels (relative to vacuum) compared to the respective untemplated films due to surface dipoles. Such a change can potentially increase the theoretical Voc expected for photovoltaic devices incorporating these templated films.