PhD, University of Cincinnati, 2015, Arts and Sciences: Physics

In this thesis, the optical properties of tris (8-hydroxyquinoline) aluminum (Alq3) and 3,5,9,10-perylentetracarboxylic dianhydride (PTCDA) organic films, PTCDA/ Alq3 multilayers and plasmonic Alq3-metal waveguides are investigated. The organic films and heterostructures used for this work were fabricated by organic molecular beam deposition (OMBD). We investigated the quenching of the light emission in Alq3 films grown on a Si substrate as a function of cw laser excitation intensity at varying temperatures from 15 to 300 K. The saturation of the singlet-singlet annihilation coefficient was measured with spectrally-integrated (SI) photoluminescence (PL) using a photodiode. The bimolecular quenching coefficient was further studied with time-resolved (TR) PL as a function of 100 fs pulse fluences. The PL quenching is attributed to the annihilation of trapped excitons at Alq3 nanocrystal grain boundaries. The saturation is explained by the limited density of available trapping states at the grain boundaries. Our interpretation is supported by structural investigations of ultrathin Alq3 films with atomic force microscopy (AFM), scanning electron microscopy (SEM) and by comparing the experimental data with calculations using a coupled rate equation model. The wavelength dispersion of the refractive indices of PTCDA and Alq3 layers and of PTCDA/Alq3 multilayer waveguides grown on Pyrex substrates was investigated. The m-line technique, an evanescent prism coupling technique, was used to determine the layers' thickness and the in-plane (TE modes) and normal (TM modes) refractive indices. The potential for controlling the refractive index dispersion and anisotropy by tailored organic multilayer waveguides is discussed.

Committee: Hans Peter Wagner Ph.D. (Committee Chair); Young Kim Ph.D. (Committee Member); Brian Meadows Ph.D. (Committee Member); Rostislav Serota Ph.D. (Committee Member) Subjects: Condensation

PhD, University of Cincinnati, 2012, Arts and Sciences: Physics

In this thesis, linear and non-linear optical properties of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and of tris(8-hydroxyquinolinato)aluminum (Alq3)films have been investigated using various experimental methods. The films were grown using the technique of organic molecular beam deposition (OMBD). The dispersion of the in-plane and normal refractive index in PTCDA waveguides has been studied using the m-line technique. The PTCDA waveguides were grown on Pyrex substrates. TE and TM mode coupling at excitation wavelengths ranging from 633 to 910 nm has been accomplished using a Rutile prism. The derived refractive index values are in good agreement with existing ellipsometric data, which emphasizes the high structural quality of our waveguides. The nonlinear absorption (two-photon absorption) of PTCDA and Alq3 films has been investigated using the z-scan technique. The films were grown on Pyrex substrates and a high repetition rate (80 MHz) laser was used as excitation source. Various methods have been utilized to minimize laser induced damaging of the soft organic films and to improve the signal-to-noise ratio. In addition, nonlinear fluorescence (two-photon induced fluorescence) measurements have been performed on Alq3 films to further investigate nonlinear absorption processes in this material. The singlet-singlet annihilation (nonlinear bimolecular quenching) of excitons in disordered quasi-amorphous Alq3 films has been investigated using both time-resolved and continuous wave (cw) spectroscopic techniques at temperatures ranging from 15 K to 300 K. A significant decrease of the PL efficiency with increasing exciton density (excitation intensity), especially at low temperatures, has been observed which is attributed to bimolecular quenching of excitons that are funneled into low-energy traps. This effect is different from the known diffusion based singlet-singlet annihilation in Alq3. To explain both the intensity and temperature dependence (open full item for complete abstract)

Committee: Hans Peter Wagner PhD (Committee Chair); Young Kim PhD (Committee Member); Leigh Smith PhD (Committee Member); L.C.R. Wijewardhana PhD (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2008, Arts and Sciences : Physics

This thesis contains optical investigations on 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) and tris (8-hydroxy) quinoline aluminum (Alq3) films as well as on PTCDA/Alq3 multilayers and co-deposited films. The organic layers are grown on Si and Pyrex substrates using the technique of organic molecular beam deposition (OMBD). In temperature dependent (10 - 300 K) transmission measurements the exciton absorption in PTCDA⁄Alq3 multilayers and co-deposited films is shifted to higher energies with respect to the pure PTCDA films. In comparison with a recently developed theoretical model for α PTCDA the observed energy shift is explained by Coulomb screening. In strain dependent photoluminescence (PL) measurements of PTCDA film uniaxial pressure is applied along the molecular stacking direction using a specially designed pressure cell. With increasing pressure exciton emission channels are shifted to lower energy and the integrated PL intensity is quenched. When the pressure is released, the PL spectrum and the total PL intensity partially recover, indicating a reversibility of the strain effects to a large extent. The experimental results are compared with recent total energy calculations. The assignment of different emission channels in PTCDA single crystals and polycrystalline films is validated by photoluminescence excitation studies. From the excitation energy dependence of the spectral position of different emission bands the transition energies of free and self trapped excitons are deduced. The obtained transition energies are compared with theoretically predicted values. The anisotropy of the refractive index in PTCDA waveguides is investigated using the m-line technique. The observed effective refractive index values of TE and TM modes are used to determine the waveguide thickness as well as the inplane and perpendicular bulk refractive index values. The recombination dynamics of excitonic states in thin Alq3 films is studied using temperature and time (open full item for complete abstract)

Committee: Hans-Peter Wagner PhD (Committee Chair); Kay Kinoshita PhD (Committee Member); Michael Ma PhD (Committee Member); Leigh M Smith PhD (Committee Member) Subjects: Molecules; Optics; Physics

PhD, University of Cincinnati, 2006, Engineering : Electrical Engineering

Hybrid device approaches have been utilized in this research to study the Alq3-based [tris-(8-hydroxyquinoline) aluminum] organic light emitting diode (OLED) structures and europium complex doping in OLEDs. Bright, efficient Alq3-based hybrid polymer/small-molecule (OLED) structures that incorporate hemispherical lenses for increased output power efficiency were engineered. The 6-layer hybrid OLED structure contains 2 spin-coated polymer layers and 4 thermally evaporated small molecule layers. This structure results in balanced charge injection, thus leading to a bright and efficient device. The use of index-matched transparent lenses resulted in luminous and external quantum efficiency of 7.5 lm/W and 8%, respectively. Hybrid electroluminescent/photoluminescent device structure were also fabricated, in which high quantum yield (>90%) lumophores were incorporated with the conventional OLED structure as a color conversion material (CCM) to achieve high efficient devices in various colors. Saturated yellow, orange and red devices with external quantum efficiencies as high ~4% were obtained by using CCM lensed approach. Two Eu complexes, Eu(Brittany) and Eu(Pigment) were evaporated under ultra high vacuum to incorporate into OLED structure as emitting centers. Electroluminescence from Eu3+ was obtained for both Eu complexes with dominant emission peak of 612 nm as a result of 5D0-7F2 transition. A voltage-induced red to blue color tunable device was demonstrated by using a coevaporated emissive layer of NPB [(N,N'-bis(naphthalene-1-yl)-N,N'-bis(phenyl)benzidine)] and Eu(pigment) complex. As a result of a minimized energy transfer from the blue emitter (NPB) to the red emitter (europium complex), the hybrid molecular/atomic (NPB/Eu3+) lumophores are able to yield chromaticity coordinates range with increasing applied bias of CIEx = 0.61-0.17 and CIEy = 0.31-0.12, respectively.

Committee: Dr. Andrew Steckl (Advisor) Subjects:

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

In this work, optical properties of organic nanostructures are investigated. In temperature dependent (10 – 300 K) absorption studies of PTCDA and PTCDA/Alq3 multilayers a red shift and a line narrowing of vibrational Frenkel exciton bands is observed when the temperature is decreased. The reduced transition energy is explained by a thermal contraction along the PTCDA molecular stacks that cause an increased inter-molecular overlap between PTCDA molecules leading to an enhanced environmental shift. The reduction of the inhomogeneous broadening of the bands is explained by a reduced population of internal and external vibrational levels of the electronic ground state. The reduced temperature shift in multilayer is attributed to a reduced thermal contraction in the PTCDA crystallites due to adjacent Alq3 interlayers that possess a smaller thermal contraction than PTCDA. The exciton emission in PTCDA thin films, PTCDA/Alq3 multilayers and co-deposited PTCDA/Alq3 layers is studied by temperature dependent (10 – 300 K) PL measurements. The different recombination channels arising from Frenkel excitons and self-trapped excitons (charge-transfer excitons CT1-nr, CT1, CT2 and excimers) that were observed earlier in PTCDA single crystals also appear in films, multilayers and in co-deposited layers. In PTCDA/Alq3 multilayers, an unknown low energy line dominates the emission spectrum up to 200 K. In accordance to investigations using X-ray diffraction, FTIR absorption and strain dependent PL measurements the new channel is attributed to a modified CT2 transition due the compressive strain between stacked molecules. Temperature dependent (10 – 300 K) PL measurements of Alq3 layers are performed. An exciton trapping model which includes the formation of self-trapped excitons is proposed to explain the observed temperature dependent PL intensity enhancement and the spectral red-shift of the PL spectrum (at ~ 180 K). Alq3 based OLED structures are fabricated and electro-optical m (open full item for complete abstract)

Committee: Dr. Hans-Peter Wagner (Advisor) Subjects: Physics, Condensed Matter

MS, University of Cincinnati, 2005, Engineering : Electrical Engineering

This thesis presents the fabrication procedures and experimental results of organic light emitting diodes (OLED) in an effort to produce a fully functional Al/LaS bilayer cathode. An introduction to OLED technology and its promising market outlook against competing display technologies is described. A comparison of commonly used cathode, small molecule, and polymer organics is performed along with their applications within OLED structures. Our motivation for researching OLEDs and the potential benefits of incorporating an LaS cathode are expressed. A chronological presentation of our experimental OLED designs is given along with the benefits and reasons for the elected patterns. Procedures and schematics of laboratory fabrication and test equipment necessary for OLED processing are described in detail. These processes include LaS processing, substrate handling, thermal evaporation, pulsed laser deposition (PLD), optical and electrical measurement instruments for device characterization. A summary of OLED optical and electrical performance for each of the adopted device designs is presented. The implemented innovated approaches leading to improved brightness, smoother thin film surfaces, and longer operating lifetimes are discussed. Much of the success is attributed to the use of an energetic cluster evaporation technique (ECE) and superior electrical and optical test arrangements. A comparison among the different Al and Al/LaS bilayer cathode OLEDs is presented, and suggestions for future improvements are offered.

Committee: Dr. Marc Cahay (Advisor) Subjects:

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2010, Photochemical Sciences

Organic light-emitting diodes (OLEDs) are solid state lighting devices which offer many advantages in respect to current lighting and displays technologies. OLEDs offer low power consumption and a wide viewing angle, which make them a perfect replacement for liquid crystal displays (LCD). Since OLEDs emit light due to the process of electroluminescence, they do not need extra light sources to be used in small electronic displays or as stand alone pixels for television sets. In this past decade, OLEDs have also been speculated as replacements for light-bulbs and fluorescent tubes. Many reports have been published describing how to obtain white light from OLEDs (WOLEDs). However, to obtain pure white light and efficient lighting devices, researchers in organic electronics have studied many ways to properly utilize all primary colors (red, green and blue) that organic dyes can achieve. Since white light requires the combination of several colors, we have studied how to simply manipulate the highest occupied molecular orbitals in fluorescence aluminum(III) complexes. By introducing electro withdrawing groups on position five of 2-methyl-8-hydroxy-quinoline, we show that is possible to obtain roughly 40 nm blue-shifted emission (454 nm) from our complexes in respect to the emission of the parent 2-methyl-8-hydroxy-quinolinolate aluminum(III) complex (495 nm), achieving in this way true-blue fluorescent emitters that can be used for OLEDs. Another approach to obtain more colors and higher efficiency in OLEDs from organic compounds is to utilize phosphorescent dyes doped in appropriate hosts (PhOLEDs). Most deep blue (~ 420- 450 nm) emitters have high triplet-energies, rendering difficult the reward for hosts with at least 0.2 eV higher triplet energy, which could avoid back energy transfer and in consequence obtain higher external quantum efficiency from the OLEDs. We show that is possible to design hosts with high triplet energies and high electron mobilities, by introd (open full item for complete abstract)

Committee: Pavel Anzenbacher Jr. (Advisor); Felix Castellano (Committee Member); Thomas Kinstle (Committee Member); Michael Geusz (Committee Member) Subjects: Chemistry; Materials Science