Molecular hole transporting materials for organic light-emitting diodes (OLEDs)

Organic light-emitting devices are rapidly becoming viable contenders in the display market. One of the major obstacles to the commercial viability of OLEDs is device stability and lifetime. Device stability has been partially, if not mostly, attributed to thermal instability of the organic charge transport layers. Some characteristics of good hole transporters for OLEDs are reversible oxidation, high thermal stability, and the ability to form amorphous films upon vacuum deposition. The main objective of this research was to design novel hole transporting materials with improved thermal stability, while retaining favorable electronic properties. Molecular structure can have drastic effects on the properties of the thin organic films of the hole transporting layers. Hole transporters with increased molecular asymmetry and rigidity have been demonstrated to give amorphous materials with higher glass transition temperatures. While asymmetry provides materials that are more thermally stable, the asymmetry must not be to an extent that would cause electronic asymmetry. Electronic asymmetry resulting from dipoles can act as local charge traps thus hindering hole transport. Increasing molecular rigidity provides materials with increased thermal stability, as well as improved hole mobility. The increased mobility is due in part to the better conjugation with materials held in a planar orientation. A direct correlation has been found between the thermal stability of a OLED and the glass transition of the hole transporting material used. Structural design, synthesis and characterization of novel hole transporting materials will be reported, as well as their performance in electroluminescent devices. Device operation and architecture will also be discussed, including the use of hole transporters as host for emitting materials.