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Display technology has become a vital and ubiquitous part of our daily life. Undoubtedly, today’s technologically minded society is living in the era of the digital image. After high resolution and efficiency could successfully be realized, the major trends in display technology now aim towards achieving high color purity for natural looking display colors. Organic light-emitting diodes (OLEDs), as one strong contender for high performance displays and lighting, have been undergoing tremendous industrial and commercial development. Despite the great progress, though, there is still space for improvement, especially in the case of blue light emitting devices. Blue OLEDs are always challenging, since they traditionally suffer from low efficiencies and lifetimes. Both, novel materials and device architectures, are driving ongoing developments while still always aiming to lower the overall costs. In a continual effort to search for robust materials for blue devices, small molecules (SMs) and polymers, are shown to be promising candidates. In this thesis is presented the results of the detailed study of photophysical and electroluminescence (EL) properties in the case of thin films based on blends of the conjugated polymer Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and the of novel SMs; 4,4'-(anthracene-9,10-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TPAA) and 4,4'-(pyrene-1,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TPAP). Finally, devices based on these systems are optimized step by step as a solution processable emissive layer (EML), for applications in sky blue OLEDs.

Since the single-molecule diode was first envisioned by Aviram and Ratner in 1974, researchers have investigated how the electronic properties of molecules might be designed to achieve a variety of device functionality. However, although electron-phonon and electron-photon interactions have been studied in systems where the molecule is poorly electronically coupled to the environment, only a few experimental modalities exist for studying inelastic transport in two-terminal single-molecule junctions. Furthermore, each phenomena typically has a few possible mechanisms which must be distinguished. The objective of this dissertation is to expand the experimental tools available for probing inelastic transport in single-molecule junctions, with special attention to electron-photon interactions. Throughout the dissertation, we utilize the scanning tunneling microscope break-junction technique to form either tunnel junctions or single-molecule junctions. By repeatedly pushing and pulling a Au STM tip into a Au-coated mica substrate, a variety of junction geometries are sampled to give a distribution of device performances. Transport and optical measurements are made while controlling the electrode displacement and junction bias independently, which permits flexible experimental design. The body of the dissertation is divided into three chapters, each chapter exploring a different phenomenon. In the first chapter, I study light emission from tunnel junctions driven at high bias. It was shown previously that electroluminescence from tunnel junctions can have photon energies exceeding the classical limit, so-called overbias emission. Multi-electron processes and blackbody radiation have been proposed as possible explanations for this extraordinary result. We demonstrate that the intensity of the overbias emission depends superlinearly on the junction conductance even at room temperature, which strongly supports the theory from multi-electron processes. Additionally, we show that blackbody radiation-like effects can be produced by multi-electron processes. In the second chapter, I demonstrate experimentally the enhanced conductance of single-molecule junctions under illumination. Again, we consider two mechanisms for enhancement, namely photon-assisted tunneling and hot-electron distributions. By carefully comparing the two theories, we find that their steady-state signatures are nearly identical, but that the contribution from hot-electron distributions is larger in our system. This is confirmed by measuring a conductance enhancement at a polarization where photon-assisted tunneling is negligible. In the third chapter, I explore both junction rupture and nonlinear transport phenomena in single-molecule junctions around the resonant tunneling regime. Importantly, we develop nonlinear regression curve-fitting to allow straightforward extraction of key transport parameters on individual single-molecule junctions. We observe a strong correlation between the bias at which the junction ruptures and the level alignment of the dominant transport orbital, which suggests that, in the resonant tunneling regime, the tunneling electrons interact strongly with the nuclear degrees of freedom. However, we also find that not all junctions rupture and those that sustain display negative differential resistance and hysteresis. We hypothesize that this nonlinear behavior is due to a change in the charge state of the molecule. We study the stability of this charge state and find that the dynamics of charging and discharging occur on millsecond timescales. Although the blocking-state and polaron models each predict parts of our data, neither are fully consistent with the experiments in their entirety. This reveals opportunities for further experimental and theoretical investigations into transport in the resonant tunneling regime.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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This thesis reports the spectroscopic and computational studies of two classes of compounds, which have applications in new optoelectronic materials technology. Substituted ligands of dipyrido-[3,2a:2�,3�c]phenazine (dppz), and their Cu(I), Re(I) and Ru(II) complexes have utility in organic electroluminescent devices. A series of Zn(II) tetraphenylporphyrins with conjugated functional groups at the β-position have been used with success in liquid heterojunction dye-sensitized solar cells. The vibrational spectra and optoelectronic properties of the two classes were investigated using Raman, resonance Raman and transient resonance Raman spectroscopy, in conjunction with density functional theory methods. Density functional theory frequency calculations were used to aid vibrational mode assignments for the dppz compounds, and show close agreement with the experimental non-resonance Raman spectra. The enhancement of modes which are localized on differing sections of the ligand was identified. The nature of the absorbing chromophores for the dppz ligands and complexes was established using resonance Raman spectroscopy in concert with vibrational assignments from calculations. Transient resonance Raman spectra of the ligands provided spectral signatures for the triplet ligand-centred state; these features were observed in the TR� spectra of the metal complexes, along with other features attributable to MLCT states. Electroluminescent devices were fabricated using the dppz ligands and complexes as emissive dopants, and their properties investigated. The optoelectronic behaviour of the devices was found to be influenced by the mechanism of exciton formation on the dopant. The device properties were also dependent on the dopant concentration, the concentrations of other components and the driving voltage. The electronic structure of the porphyrin compounds was investigated using time-dependent density functional theory methods. Comparison of calculated optical transitions with experimental data shows that the calculations predict trends in the optical absorption spectra with change of functional group and with increase in conjugation chain length. The calculations suggest that the electron-withdrawing substituent decreases the configuration interaction effect by breaking the degeneracy of the two lowest unoccupied MOs, and other configuration interaction effects come into play involving other frontier MOs. Interrupting the conjugation of the functional group is shown to mitigate the breakdown of the configuration interaction. The perturbation of the normal electronic structure of the porphyrin by the substituent was also investigated using resonance Raman spectroscopy. Vibrational analysis identified bands due to the substituent, implying coupling between the porphyrin and substituent chromophores. Changes in frequency of porphyrin core modes due to the differing substituents and different metal centres were reproduced by density functional theory calculations. This project has allowed the spectroscopic investigation of the active optical states in a number of polypyridyl and porphyrin compounds, and determined the efficacy of DFT and TDDFT calculations to predict the properties of these compounds.

Graduation date: 2004

Time-resolved electroluminescence has been primarily and widely applied in the field of optoelectronic devices and phosphor materials. Here we use the method to measure the response time and phase difference of green light-emitting diode. In our experiment, which is essentially dominated by pulsed signal from function generator¡Bhigh spatial resolution from laser scanning microscopic and phase-lock characteristic from lock-in amplifier to complete our experimental result. We are capable to detect relative carrier information within any point or any surface region from epi-layer of green light-emitting diode. Otherwise, the carrier transport of the sample is able to be observed through time-resolved characteristic measurement. Laser cutting would be beneficial for obvious phase-difference observation and mobility acquirement.

Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references (p. 81-89). Also available in print.