Studies on the synthesis and the transport properties of organic materials

Abstract

A series of photo-crosslinkable arylamine-based hole-transport copolymers has been synthesized. The synthetic methodology employed allows for the redox potential of the polymer to be tuned by the incorporation of electron-donating or withdrawing moieties. Upon exposure to ultraviolet radiation, the copolymers become insoluble, as evidenced by ultraviolet-visible absorption spectroscopy. The ability to switch the solubility of the polymers enable one to photo-pattern the materials, a feature that is useful for the fabrication of multilayer organic light-emitting diodes (OLEDs) by solution processing techniques. OLEDs using the methacrylate based hole-transport polymers have been fabricated and the performance of the devices has been evaluated. 2,7-Bis(p-methoxyphenyl- m'-tolylamino)-9,9-dimethylfluorene, 2,7-bis(phenyl- m'-tolylamino)-9,9-dimethylfluorene, and 2,7-bis( p-fluorophenyl-m'-tolylamino)-9,9-dimethylfluorene have been synthesized using palladium-catalyzed reaction of the appropriate diarylamines with 2,7-dibromo-9,9-dimethylfluorene. These molecules have glass-transition temperatures 15-20°C higher than those for their biphenyl-bridged analogues, and are 0.11-0.14 V more easily oxidized. The hole mobilities of the three fluorene derivatives (blended with polystyrene) have been measured by the time-of-flight technique and are lower than the corresponding biphenyl-bridged analogues. Analysis of transport data according to the disorder formalism yields parameters similar to those for the biphenyl species, but with lower zero-field mobility values. Density functional theory based calculations suggest that the enforced planarization of the fluorene bridge leads to a larger reorganization energy, due to increased vibration contributions in the bridge for the neutral/cation electron-exchange reaction relative to the analogous biphenyl-bridged system. OLEDs have been fabricated using blends of the fluorene-bridged compounds with polystyrene as the hole-transport layer and Alq₃ as electron-transport layer and lumophore. Device performance shows a correlation with the ionization potential of the amine materials paralleling that seen in biphenyl-based systems. Monomers based on the fluorene bridged arylamine-based hole transport materials were also synthesized. Monomers having methacrylate, styrene, and norbornene moieties have been synthesized and their hole mobilities measured. Adding a methacrylate or norbornene moiety had no effect on the hole mobility. The mobility of the norbornene polymer was significantly higher than the methacrylate polymer.