Grazing Incidence X-ray Diffraction of a Photoaligned Nematic Semiconductor

Abstract

Grazing incidence X-ray diffraction is used to find the thin film morphology of an extended molecule with an irregular alternating fluorene-thiophene structure, which is used to obtain linearly polarized electroluminescence and the photovoltaic effect. The material has a room temperature nematic glassy phase and is uniaxially aligned in the plane of the film using photoalignment techniques. Two distinct intermolecular separations of 0.45 and 1.5 nm are identified showing that the molecules are lamellar. The lamellae stack with only local order and the two short axes of the lamellae have no preferred orientation at the surface or bulk of the film. Neighboring molecules show a wide range of longitudinal displacements along the axis of the director, as expected for a nematic liquid crystal with no positional order. There is, however, a dominant feature corresponding to a longitudinal offset of 0.51 nm. Unlike some other fluorene-containing semiconductors where microphase separation of the side chains inhibits close packing of neighboring molecules, the lamellar structure and 0.45 intermolecular spacing found here allows pi-pi intermolecular interactions for efficient carrier transport. We obtain a room temperature hole mobility up to 3.4 x 10-3 cm2 V-1 s-1 using a time-of-flight technique.

One-Step Photoembossing for Submicrometer Surface Relief Structures in Liquid Crystal Semiconductors

“…1 is hole-transporting with a mobility of 2 × 10 −5 cm 2 V −1 s −1 at room temperature . A similar reactive mesogen with an extended aromatic core has a mobility >3 × 10 −3 cm 2 V −1 s −1 …”

“…25 A similar reactive mesogen with an extended aromatic core has a mobility Ͼ3 ϫ 10 Ϫ3 cm 2 V Ϫ1 s Ϫ1 . 26 1 emits green light peaking at a wavelength of 510 nm with a solid-state photoluminescence quantum efficiency of 27%. An OLED incorporating 1 produced 380 cd m Ϫ2 at a voltage of 5 V with an efficacy of 1.6 cd A Ϫ1 .…”

Liquid crystals in photovoltaics: a new generation of organic photovoltaics

“…140 In a nematic reactive mesogens, a planar alignment with the long molecular axis aligned on the substrate plane can be achieved upon crosslinking. 141 The performance of the device is limited by the charge-carrier mobility of the acceptor material, which can be enhanced by the electron-donating layer formation by mesogens.…”

Solution-processed bilayer photovoltaic devices with nematic liquid crystals

“…Thin films can be formed by solution processing, Nematic reactive mesogens adopt a planar alignment with the long molecular axis aligned in the substrate plane. [12] They can be uniaxially aligned using rubbing or photoalignment techniques resulting in very high order parameters particularly for materials with extended molecular cores. [13][14][15] Polymerization and crosslinking occur either by the thermal or photoinduced generation of free radicals using ultraviolet light or by ionic photoinitation resulting in insoluble thin films.…”