Title

Synthesis and Electro-optical Properties of Novel Materials for Application in Organic Light-Emitting Diodes

Date of Award

2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Pavel Anzenbacher

Abstract

Organic light-emitting diodes (OLEDs) could become the leading lighting technology for fabrication of full color flat panel displays and general illumination purposes in the near future. To meet this goal, operational hurdles still need to be addressed in electroluminescent devices. In particular, long-term operation and efficient performance are the main challenges to be tackled.

Energy transfer processes play a significant role in the improvement of electroluminescence efficiency by utilizing all of the excited states generated during the electron-hole recombination, in particular non-emissive triplet excitons. In this regard, the advantageous energy transfer features displayed by molecular photonic wires could be of practical importance for the fabrication of efficient OLEDs.

In the present work, donor-bridge-acceptor triads with appropriate triplet energy alignment of the components were studied. The systems consisted of materials already successfully used in OLEDs. Specifically, aluminum (III) tris(8-quinolinolate) was used as a triplet energy donor (3Alq3 = 2.17 eV), fluorene oligomers as the connecting bridge (3OF1-4 = 2.86-2.18 eV), and platinum (II) tetraphenylporphyrin as an energy acceptor (3PtTPP = 1.91 eV). The molecular photonic wire behavior of these triads was investigated and OLEDs were fabricated.

A rationale for tuning the excited-state energies of Alq3was introduced. The synthetic preparation of tunable derivatives bearing conjugated aryl spacers was realized. Red, green, and blue electroluminescence was obtained from OLEDs fabricated using the Alq3-based materials.

Dyad systems Alq3-oligofluorene (n=1-9) were synthesized. Strong electronic coupling was observed for materials comprising short oligofluorene fragments (n=1, 3). These systems were used as components for the construction of triads to study molecular photonic wire behavior.

Donor-bridge-acceptor triads with a small number of fluorene units (n=1-4) were prepared. In these systems efficient singlet and triplet energy transfer were observed. Furthermore, improved OLED output was obtained for triads having longer oligofluorene bridges showing better alignment of triplet energy levels despite the longer donor-acceptor distance. This constitutes the first example of molecular photonic wire behavior demonstrated to take place in both solid state and in functional OLEDs.