Optoelectronic devices : synthesis, analysis, and application of functional materials

Suhr, Kristin Jeannette
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The field of optoelectronics is booming, as it involves important, functional electrical-to-optical and optical-to-electrical transducers. This dissertation contains efforts in three important areas of optoelectronics – electrode modification, light-emitting electrochemical cells (LEECs), and solar cells. Two ubiquitous metal oxide electrodes – indium tin oxide (ITO) and titanium dioxide (TiO₂) – have been chemically modified using the electrochemical reduction of iodonium salts to enhance interfacial charge transfer properties in devices. The surface coverage was controllable and monolayer to multilayer growth was achieved, resulting in passivation of the electrochemical reactivity of the electrode from 9 to 73%. X-ray photoelectron spectroscopy confirmed the presence of modifiers on the electrode that were either bound to surface hydroxyl groups or strongly physisorbed.

LEECs have been fabricated from a series of ionic iridium(III) complexes to examine the effect of bulky, hydrophobic phenyl substituents. The modified complexes displayed band gap tuning and increased quantum yields. Devices doped with LiPF₆ displayed reduced response times, modest lifetimes, and peak luminances as high as 5425 cd/m², 50% higher than the corresponding salt-free device and nearly double the luminance of the parent complex. The best complex benefited from the increased stability and limited self-quenching imparted by the phenyl substituents, while maintaining a large band gap and nearly planar ligand torsion angles, which further improves emission efficiency. Additionally, it was found that the coordination strength and identity of the counteranion can have a significant effect on the emission wavelength in the solid state.

Hybrid materials were fabricated with a conductive, organic zinc metallopolymer and inorganic semiconducting ZnO nanoparticles (NPs) for photovoltaic applications. Electropolymerization allowed for tunability of the metallopolymer thickness and zinc seed points were evenly dispersed throughout, providing NP nucleation sites. ZnO NPs were grown directly on the conducting polymer backbone and all throughout the film, affording improved physical and electronic contact between the materials. Transmission electron microscopy confirmed that increasing the number of ZnO growth cycles from 4 to 6 resulted in an increase of the average NP diameter from 2.7 to 3.7 nm. Additionally, electrochemical studies revealed that as the diameter increased, the NP band gap decreased, following the quantum confinement effect.