Characterization and optimization of novel materials and interfaces in organic electronic devices

Abstract

The research embodied in this text involves the characterization and optimization of novel materials and interfaces critical to the performance of organic electronic devices: Thrust 1. A scheme for elucidating the relative significance of energy vs. charge transfer (ET vs. CT) routes to guest dye emission in organic light-emitting diodes (OLEDs) was developed using near-IR emissive phthalocyanine (Pc) or naphthalocyanine (NPc) dopants. CT processes were observed to occur by an oxidized guest mechanism, which may be predicted from the solution electrochemistry of these materials. Additionally, a new series of quinacridone(QA)-based guest dyes were developed/characterized, modified at the N,N' positions to form four generations of Frechet dendrimers and their t-butyl-terminated analogues. As generation number increases, they exhibited decreased aggregation and increased luminescence efficiencies in the condensed phase. In solution, sharply declining rates of heterogeneous electron transfer were observed. Despite this trend, when these new molecules were doped into OLEDs, ET and CT routes to QA emission were not significantly inhibited. Thrust 2. A series of hybrid metal cluster-organic materials containing between one and seven [Re6(mu3-Se)8] 2+ clusters per molecule were electrochemically characterized. Dimer, trimer, and tetramer assemblies yielded uncoupled cluster oxidations. For cluster 7-mer dendrimers, uncoupled oxidations were observed for unconjugated linking groups, while coupled oxidations were observed with a conjugated linker, suggesting through-bond electrical polarization of the interior cluster. Thrust 3. The first UV-Vis attenuated total reflectance (ATR) method for the simultaneous determination of molecular tilt and azimuthal rotation for ultra-thin films of molecules with circularly-polarized electronic transition dipoles was developed, where a priori assumptions of the mean molecular orientation of either angle are not necessary in order to recover both mean angles. A model Pc-based system was evaluated, where this material is known to form in-plane molecular columns. Assuming a delta distribution in the orientation of each angle, it was determined that the mean tilt of the molecular plane away from the substrate surface is 80 ± 3°, and the mean azimuthal rotation of the molecular plane away from the column direction is 56.9 ± 0.8°. The dependence of molecular orientation on environmental exposure conditions and surface premodifiers was also evaluated.