Out-of-Plane Orientation Control of Solution-Processed Small-Molecule Organic Semiconductor Crystals

We demonstrated the ability to control the out-of-plane orientation of triisopropylsilethynyl (TIPS)-derivatized organic semiconductor crystals by tuning the molecular structure and substrate surface energy. For TIPS-pentacene crystals, molecules invariably oriented in a horizontal manner, with the long axis of the conjugated core parallel to the substrate. For TIPS-dibenzopyrene, anthanthrene and pyanthrene crystals, a vertical orientation, with the long axis of the conjugated core perpendicular to substrate, was observed. By varying the surface energy of the substrate, the ratio of the populations of horizontal versus vertical orientations in TIPS-pyranthrene crystals could be tuned. As observed by polarizing optical microscopy, orientation-dependent linear birefringence and linear dichroism were observed. Furthermore, compared to crystals adopting the horizontal orientation, crystals adopting the vertical orientation displayed a 42-fold improvement in out-of-plane hole mobility measured via conductive atomic force microscopy. Because out-of-plane charge transport is necessary for a range of optoelectronic applications, understanding structure-property relationships between molecular structure and orientation is critical to improving device performance.