Molecular design of organic semiconductors for electronic devices and their application in flexible light emitting diodes.

摘要

The focus of the studies investigated in this work has been two-fold. In the first part, a structure-property study of three novel two-dimensional conjugated phenylenevinylene (PV) based isomers containing oxadiazole moieties for optoelectronic applications has been carried out. The compounds have been designed to serve as a model to study charge delocalization and transport as well as molecular symmetry in organic semiconductor molecules. These molecules are tetra-substituted at the central phenyl ring with two PV based arms and two oxadiazole derivatized PV (OXAPPV) arms. In the three molecules, termed p-, o-, and m-OXA-X, the OXAPPV arms are positioned in a para-, ortho- or meta-position, respectively, in relation to each other. Comparing these molecules, the role of symmetry and charge delocalization in this class of compounds has been explored. Despite having different linear segments, they have nearly identical photophysical properties suggesting a similar charge delocalization mechanism. However, in a LED with the molecules as emissive layers between aluminum and ITO electrodes and with poly(3,4-ethylenedioxythiophene) with poly(styrenesulfonate) (PEDOT:PSS) and lithium fluoride to aid in hole and electron injection, respectively, o-OXA-X exhibited the highest external quantum, luminance and power efficiencies. We explain these differences based on the changes in the film morphology between the three molecules. Supporting data from cyclic voltammetry and morphological data from atomic force microscopy, NMR studies, thermal characterization and x-ray diffraction studies are also reported.; In the second part, in an effort to construct stable and high efficiency devices with a single emissive layer and with a stable cathode, flexible LEDs have been fabricated and tested in simple device configurations. Devices on flexible substrates have been made using either the 2D conjugated molecules or a soluble poly(phenylenevinylene) (PPV) and oxadiazole substituted PPV derivatized random copolymer, RC30, as the emissive layer and higher work function aluminum cathodes and compared with control devices on glass substrates. In all devices PEDOT:TSS has been used as the hole transporting layer and a thin layer of cesium fluoride or lithium fluoride has been employed at the polymer/cathode interface to aid in electron injection. For the RC30 polymer, devices on plastic substrates with a lithium fluoride interlayer performed the best, exhibiting up to 1% external quantum efficiency and an average of 0.8% and luminance of 1600 cd/m2 at 40 mA/cm2 (7.8 V). Stability of this device and morphology of the emissive film have also been investigated.