Duva Giuliano, Mann Alexander, Pithan Linus, Beyer Paul, Hagenlocher Jan, Gerlach Alexander, Hinderhofer Alexander, Schreiber Frank
Institute for Applied Physics , University of Tübingen , Auf der Morgenstelle 10 , 72076 Tübingen , Germany.
ESRF The European Synchrotron , 71, Avenue des Martyrs , 38000 Grenoble , France.
J Phys Chem Lett. 2019 Mar 7;10(5):1031-1036. doi: 10.1021/acs.jpclett.9b00304. Epub 2019 Feb 20.
Many organic semiconductors (OSCs) feature strong optical anisotropy that can be exploited to increase the efficiency of optoelectronic devices. We demonstrate that for the technologically relevant, rod-like OSCs diindenoperylene (DIP), pentacene (PEN), and α-sexithiophene (6T) deposited on silicon oxide surfaces it is possible to prepare polycrystalline thin films in which the long molecular axis is oriented parallel to the substrate plane in a template-free fashion. In films grown by organic molecular beam deposition at room temperature or higher, the molecules are oriented upright standing (σ-orientation). Instead, the here-presented growth at low temperatures followed by slow annealing up to a temperature near molecular desorption has the effect of "freezing" the molecules with their long axis oriented parallel to the substrate plane (λ-orientation) while conferring them crystalline long-range order. We discuss the huge impact on the optical anisotropy of the films observed as a consequence of the orientation transition. Finally, we propose a mechanism for explaining the achieved λ-orientation, which is stable under environmental conditions.
许多有机半导体（OSC）具有很强的光学各向异性，可利用这一特性提高光电器件的效率。我们证明，对于技术上相关的、棒状的OSC二茚并苝（DIP）、并五苯（PEN）和α-六噻吩（6T）沉积在氧化硅表面的情况，可以制备多晶薄膜，其中长分子轴以无模板方式平行于衬底平面取向。在室温或更高温度下通过有机分子束沉积生长的薄膜中，分子呈直立取向（σ取向）。相反，这里提出的低温生长然后缓慢退火至接近分子解吸的温度，具有“冻结”分子的效果，使其长轴平行于衬底平面取向（λ取向），同时赋予它们晶体的长程有序性。我们讨论了由于取向转变而观察到的对薄膜光学各向异性的巨大影响。最后，我们提出了一种机制来解释所实现的λ取向，这种取向在环境条件下是稳定的。
