Photochemical Sciences Ph.D. Dissertations

Synthesis and Studies of Materials for Organic Light-Emitting Diodes

Date of Award

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Pavel Anzenbacher, Jr.

Second Advisor

Felix Castellano (Committee Member)

Third Advisor

Thomas Kinstle (Committee Member)

Fourth Advisor

Michael Geusz (Committee Member)

Abstract

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 introducing diphenylphosphine oxide moieties in the structure of biphenyl, fluorene, dibenzophosphole and benzene compounds. We were able to obtain triplet energies ranging between of 2.71 eV (457 nm) and 3.39 eV (365 nm) from the different fluorene, biphenyl and benzene compounds, each containing two diphenylphosphine oxide groups in their structure. We also show that is possible to use diphenylphosphine oxide in conjunction with carbazole, in order to prepare ambivalent hosts with a triplet energy of 2.86 eV. We also demonstrate that synthesizing compounds containing heavy atoms (i.e. bromine) and diphenylphosphine groups, it is possible to observe phosphorescence emission at room temperature in solutions and in solid state films (including OLEDs) without suffering from extreme quenching due to dioxygen. Finally, we show that utilizing triphenylphosphine oxide as a host in PhOLEDs, it is possible to obtain electrophosphorescence from compounds like tris(4-carbazol-9-yl-phenyl)amine (TCTA), bathocuproine (BCP), 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI), meta-biscarbazolylphenyl (m-CP), tris-(8-hydroxyquinoline) aluminum (Alq3) and other materials used regularly in OLEDs in roles other than emitters. This ability to obtain electrophosphorescence at room temperature from all-organic compounds opens the possibility to achieve white light by combining the characteristic broad phosphorescence from these compounds in a way that could cover the entire visible region.

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